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Energy Transfer in Dendritic Systems Having Pyrene Peripheral Groups as Donors and Different Acceptor Groups Pasquale Porcu, Mireille Vonlanthen *, Andrea Ruiu , Israel González-Méndez and Ernesto Rivera * Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Circuito Exterior Ciudad Universitaria, Ciudad de México C.P. 04510, Mexico; [email protected] (P.P.); [email protected] (A.R.); [email protected] (I.G.-M.) * Correspondence: [email protected] (M.V.); [email protected] (E.R.); Tel.: +52-55-56224733 (E.R.) Received: 29 August 2018; Accepted: 19 September 2018; Published: 25 September 2018

 

Abstract: In this feature article, a specific overview of resonance energy transfer (FRET) in dendritic molecules was performed. We focused mainly on constructs bearing peripheral pyrene groups as donor moieties using different acceptor groups, such as porphyrin, fullerene C60 , ruthenium-bipyridine complexes, and cyclen-core. We have studied the effect of all the different donor-acceptor pairs in the energy transfer efficiency (FRET). In all cases, high FRET efficiency values were observed. Keywords: pyrene; dendritic molecules; energy transfer

1. Introduction Since their discovery at the end of the 80’s, the scientific community has been very interested in dendrimers due to their well-defined nanostructures that confer them outstanding chemical and physical properties [1–3]. The special attention accorded to dendrimers is essentially due to their potential applications in diverse fields such as in catalysis, drug delivery and photoactive materials [4–10]. The investigation of the photophysical properties of these molecules is very attractive and depends in large measure on the architecture of these perfectly branched macromolecules. The incorporation of photoactive units into dendrimers can be achieved by attaching them covalently or noncovalently in three possible locations: periphery, branches, or core. Many photoactive chromophores have been linked to dendrimers through covalent bonds to produce highly luminescent materials [11,12]. Depending on the selected photoactive molecules and their position in the dendritic construct, different phenomena, such as fluorescence resonance energy transfer (FRET), excimers, and charge transfer (CT) can take place [13]. The study of those processes is very important, since it helps us to make innovations to improve the efficiency of existing photovoltaic devices [13–17]. With that aim, many scientists have focused their research on the design and study of various dendritic molecules introducing different chromophores with particular optical and photophysical properties [18–21]. Among the available chromophores, pyrene is considered the most used fluorescent dye in the study of labelled polymers [22], and there are some reviews covering the most important aspects of its fluorescent properties [23–31]. The use of pyrene as a fluorescent label to study polymers of different lengths and architectures is prevalent because of its unique photophysical behavior, mainly its tendency to form excimers. Pyrene moieties have been attached to numerous macromolecules [32,33] in order to investigate micellization, polymer, and dendrimer dynamics [34], and for the development

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of new π-conjugated and oligomers with pyrene units [35,36]. This phenomenon Polymers 2018, 10,polymers x FOR PEER REVIEW 2 of 25 has been also observed in dendritic structures bearing pyrene previously reported [37–39]. Numerous dendritic [32,33] in order to investigate micellization, polymer, and dendrimer dynamics [34], and for the architectures with photoactive groups have been reported so far, but there are only a few examples development of new π-conjugated polymers and oligomers with pyrene units [35,36]. This where pyrene has been attached the presence of anpreviously acceptor reported group to give an phenomenon hascovalently been also observed in jointly dendriticwith structures bearing pyrene efficient energy (FRET) forarchitectures light-harvesting purposes. [37–39].transfer Numerous dendritic with photoactive groups have been reported so far, but there are only a few examples where pyrene has been covalently attached jointly with the presence an acceptor group to give of anPyrene efficient energy transfer (FRET) for light-harvesting purposes. Optical andofPhotophysical Features Optical Photophysical Features Pyrene Pyrene as aandchromophore has ofoutstanding optical properties, like elevated quantum yield, long fluorescence lifetime, and excimer emission, depending itselevated local concentration [22]. That is Pyrene as a chromophore has outstanding optical properties,on like quantum yield, long fluorescence lifetime, andto excimer emission, depending local concentration [22]. That whyplace it why it is often used as a probe test dynamic processesonofitspolymers in solution thatistake in the is often used as a probe to test dynamic processes of polymers in solution that take place in the pyrene lifetime regime [22–27]. Other interesting features of pyrene are its potential use in dyads for pyrene lifetime regime [22–27]. Other interesting features of pyrene are its potential use in dyads for solar energy conversion and as its use in the exfoliation of carbon nanotubes (CNT) and graphene [40]. solar energy conversion and as its use in the exfoliation of carbon nanotubes (CNT) and graphene We have [40]. investigated the photophysical properties of various polymers and compounds bearing We have photophysical properties various polymers with and compounds pyrene [36]. Pyrene hasinvestigated been also the incorporated into variousofmacromolecules the aim to develop bearing pyrene [36]. Pyrene has been also incorporated into various macromolecules with the aim to novel photoactive materials [35,36,41–46]. Furthermore, we have designed a Fréchet-type dendron develop novel photoactive materials [35,36,41–46]. Furthermore, we have designed a Fréchet-type containingdendron an increasing number of pyrene thewith generation number. It these containing an increasing numbermoieties of pyrenewith moieties the generation number. It theseconstructs, pyrene actsconstructs, as donorpyrene in a dual way, from or excimer depending on the structure acts as donor in athe dualmonomer way, from the monomeremission, or excimer emission, depending on the The structure of the dendron. The absorption emission spectra thefirst dendron of the first of the dendron. absorption and emission spectraand of the dendron ofofthe generation (Py2 -G1OH) generation (Py2-G1OH) as well as of the dendron of the second generation (Py4-G2OH) is shown in as well as of the dendron of the second generation (Py4 -G2OH) is shown in Figure 1. Figure 1.

Figure 1. Absorption (dashed lines) and fluorescence (solid lines) spectra of first (Py2-G1OH; red

Figure 1. Absorption (dashed lines) and fluorescence (solid lines) spectra of first (Py2 -G1OH; red line) line) and second (Py4-G2OH; black line) generations pyrene-labelled dendrons in THF. [Py] = 1.25 × and second -G2OH; black line) generations pyrene-labelled dendrons in THF. [Py] = 1.25 × 10–6 –6 M;4 λ ex = 344 nm. Reprinted with permission from (Zaragoza-Galán, G., Fowler, M.A., Duhamel, 10(Py M; λex = 344 (Zaragoza-Galán, G., Fowler, M.A., Duhamel, J., J., nm. Rein, Reprinted R., Solladié,with N., permission & Rivera, E.from (2012). Synthesis and Characterization of Novel Pyrene-Dendronized Porphyrins Exhibiting Efficient Fluorescence Resonance Energy Transfer: Rein, R., Solladié, N., & Rivera, E. (2012). Synthesis and Characterization of Novel Pyrene-Dendronized and Photophysical Properties. Langmuir, 28(30), 11195–11205.). Copyright (2012) American PorphyrinsOptical Exhibiting Efficient Fluorescence Resonance Energy Transfer: Optical and Photophysical Chemical Society [47]. Properties. Langmuir, 28(30), 11195–11205.). Copyright (2012) American Chemical Society [47]. Based on the obtained results, we designed dendritic systems bearing different donor-acceptor

Basedpairs on the results, we designed dendritic systems bearing donor-acceptor thatobtained behave as molecular antennae, keeping pyrene as the donor different group, such as pairs that behave as molecular antennae, keeping pyrene as the donor group, such as pyrene-porphyrin [47], pyrene-fullerene [48], pyrene-ruthenium bipyridine complexes [49], and the pyrene-cyclen core [50].

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2. Dendritic Molecules Bearing Peripheral Pyrene Groups as Donors and a Porphyrin as an Acceptor Porphyrins belong to an important category of fluorophores, deeply investigated in materials science [51–53]. The development of novel porphyrins and multiporphyrinic systems attracted our attention because of their potential applications in various fields such as nonlinear optics (NLO), to photon absorption, molecular wires, and catalysis [54–57]. Porphyrins have been covalently functionalized with various electro- and photoactive groups with the aim to tune their opto-electronic and photophysical properties. In particular, we can modify the donor-acceptor character of porphyrins by linking them covalently with photoactive groups or by coordinating them with different metal ions [58–60]. Even though the synthesis and the study of the optical and photophysical properties of many porphyrinic compounds attached to electro- and photoactive groups like for example fullerene C60 [58] and anthracene [59] or other functionalized porphyrins [60] have been published, there are only a few reports about porphyrin–pyrene systems [61]. Moreover, none of these reports is an investigation about light harvesting or the energy transfer phenomenon. Provided that monomer and excimer emissions of the pyrene [22] are partially superimposed with the Soret absorption band of porphyrins (λ = 419 nm for tetraphenyl porphyrin (TPP) and λ = 423 nm for its Zn-metallated derivative (ZnTPP)) [62–64], we can expect a very efficient fluorescence resonance energy transfer (FRET) in structures labelled with these two chromophores. Therefore, no less than three different photophysical processes can take place in these structures at the same time. Firstly, an excimer can be generated by the pyrenyl pendant groups; secondly a FRET phenomenon can occur from an excited pyrene unit or an excimer to the porphyrin, respectively. Provided that dealing with a single photophysical process taking place in an intramolecular way in a photoactive dendritic molecule is problematical to study, dealing with three simultaneous processes is a real photophysical challenge. Fortunately, the involved photophysical processes depends in large measure on controllable molecular parameters that can be modified in order to selectively affect only one photophysical process and allow a reasonable characterization of all photophysical processes. Definitely, pyrene excimer formation and FRET are significantly dependent on the pyrene content and the donor (pyrene monomer and excimer)-acceptor (porphyrin) distance. These features were seriously considered to develop two series of dendritic molecules labelled with pyrene and porphyrin. First- and second-generation poly(aryl ether) dendrons totally functionalized in the periphery with 1-pyrenebutyl units were prepared to take advantage of the elevated excimer emission that happens with increasing the generation of the dendritic constructs containing pyrene [34,37]. Furthermore, a porphyrin unit was attached to the focal point of the pyrene-labelled dendrons through a flexible spacer. Given that in a dendron the distance from the periphery and the focal point augments at higher generations, growing the generation of the pyrene-labelled dendrons from 1 to 2 augmented the distance between pyrene units and porphyrin, thereby diminishing the efficiency of FRET. Thus, growing the generation of the dendron employed in these dendritic structures increased excimer formation thereby decreasing the FRET efficiency. The structures of these dendritic molecules are illustrated in Figure 2. The synthesis of the dendronized porphyrins as well as the effect that the structural changes induce in the optical and photophysical processes has been studied in detail [47]. The synthesis and characterization of this constructs has been reported by our research group [47]. These characterization of the molecules was carried out by FT-IR, 1 H NMR, 13 C NMR, UV–VIS spectroscopy, and MALDI-TOF MS [47]. Absorption spectra of the dendronized porphyrins (Py2 -TMEG1 and Py4 -TMEG2) are illustrated in Figure 3. Py2 -TMEG1 and Py4 -TMEG2 exhibited the maximun absorption band at λ = 344 nm, arising from the S0 → S2 transition of pyrene, followed by the Soret band of the porphyrin at 416 nm (λ = 414 nm for Py2 -TMEG1, and λ = 418 nm for Py4 -TMEG2). The porphyrin unit exhibit also four Q bands, which appear in the range between 450 and 700 nm. Since the absorption spectra of the obtained dendronized porphyrins are the sum of the absorption spectra of their individual precursors

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bands, which appear in the range between 450 and 700 nm. Since the absorption spectra of the obtained dendronized porphyrins are the sum of the absorption spectra of their individual having pyrene and porphyrin elements, we can affirm that no interaction between and precursors having pyrene and porphyrin elements, wethere can is affirm that there is no pyrene interaction porphyrin in the ground state [47]. between pyrene and porphyrin in the ground state [47].

O O O O NH N NH N HN N HN N

O O

O O O O

O O

O O NH N NH N O O

HN N HN N

O O

O O O O

O O

O O O O

Py22-TMEG1

Py44-TMEG2

Figure 2. Structure of the dendronized porphyrins containing first-generation and second-generation Fréchet-type dendrons dendrons [47]. [47]. Fréchet-type

Figure 3. 3. Absorption Figure Absorption (dashed (dashed line) line) and and fluorescence fluorescence (solid (solid line) line) spectra spectra of of Py Py222-TMEG1 -TMEG1 (red (red line), line), Py444-TMEG2 (thin (thinblack blackline), line), and fluorescence spectrum of TME (porphyrin precursor Py and fluorescence spectrum of TME (porphyrin precursor withoutwithout pyrene pyrene(thick units) (thick a low fluorescence atTHF. 651 nm) in THF. [Py 22-TMEG1] = units) black lineblack with line a lowwith fluorescence intensity at intensity 651 nm) in [Py2 -TMEG1] = 5.1 × 10–7 M; –7 M; [Py44-TMEG2] –7 M; and [TME] –6 M; λex –7 –7 –6 –7 –6 ex = 2.6 × 10 = 2.5 × 10 = 344 nm. Reprinted with 5.1 × 10 [Py4 -TMEG2] = 2.6 × 10 M; and [TME] = 2.5 × 10 M; λex = 344 nm. Reprinted with permission from permission from (Zaragoza-Galán, Fowler,J., M.A., J., Rein, Solladié, N., & Rivera, E. (Zaragoza-Galán, G., Fowler, M.A., G., Duhamel, Rein,Duhamel, R., Solladié, N., &R., Rivera, E. (2012). Synthesis (2012). Synthesis and Characterization of Novel Pyrene-Dendronized Porphyrins Exhibiting Efficient and Characterization of Novel Pyrene-Dendronized Porphyrins Exhibiting Efficient Fluorescence Fluorescence Resonance Energy Transfer: Optical and Photophysical Properties. Langmuir, 28(30), Resonance Energy Transfer: Optical and Photophysical Properties. Langmuir, 28(30), 11195–11205.). 11195–11205.). Copyright (2012) American Chemical Society [47]. Copyright (2012) American Chemical Society [47].

Fluorescence -TMEG1 and and Py Py44-TMEG2 -TMEG2 were were recorded recorded exciting exciting at at 344 344 nm nm in Fluorescence spectra spectra of of Py Py22-TMEG1 in THF THF solution solution at at room room temperature temperature (Figure (Figure 3). 3). The The obtained obtained spectra spectra were were showing showing two two emission emission bands bands at at 376 nm and 476 nm corresponding to the monomer and excimer bands of pyrene, respectively, that are 376 nm and 476 nm corresponding to the monomer and excimer bands of pyrene, respectively, that comparable withwith the emission bandsbands of theof precursor pyrene-labelled dendrons. Nevertheless, we can are comparable the emission the precursor pyrene-labelled dendrons. Nevertheless, also observe a new band at band 651 nm to the porphyrin emission.emission. The ratios between the excimer we can also observe a new at due 651 nm due to the porphyrin The ratios between the intensity/monomer intensity (I /I ) were calculated for Py -TMEG1 and Py -TMEG2. The obtained E M 2 for Py22-TMEG1 4 and Py44-TMEG2. The M) were calculated excimer intensity/monomer intensity (IEE/IM values were 0.58 were and 1.46, Those values werevalues much were lowermuch than the values for obtained values 0.58respectively. and 1.46, respectively. Those lower thanobtained the values the pyrene precursors Pyprecursors and 3.05, respectively). 2 -G1OH and 4 -G1OH 2-G1OH obtained fordendron the pyrene dendron Py2Py and(0.69 Py44-G1OH (0.69 and 3.05, respectively).

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Time-resolved fluorescence experiments revealed that the pyrene fluorescence quenching happens with an energy transfer rate constant value of 2.3 and 1.8 × 109 s–1 for Py2 -TMEG1 and Py4 -TMEG2, respectively. Such rate constants are the proof of a highly efficient quenching mechanism. Regardless the IE /IM ratio, a more than 30-fold decrease in fluorescence intensity can be seen when the dendrons and the porphyrin are covalently attached, exhibiting a very efficient FRET from an excited pyrene monomer or excimer to the porphyrin. Indeed, pyrene fluorescence in the porphyrinic construct Py2 -TMEG1 and Py4 -TMEG2 suffer a quantitative quenching (99% and 97%, respectively) as shown in Table 1. The fluorescence band observed at 651 nm, exciting at 344 nm, comes from the porphyrin unit of compounds Py2 -TMEG1 and Py4 -TMEG2. This emission is more intense than that observed after direct excitation of TME (precursor porphyrin without pyrene) in solution (Figure 3). The donor (pyrene monomer and excimer) showed a remarkable decrease in fluorescence intensity while the acceptor (porphyrin) exhibited an enhancement in fluorescence intensity, which is a clear indication that FRET is occurring in the pyrene–porphyrin dendritic constructs Py2 -TMEG1 and Py4 -TMEG2. EFRET values and quantum yields are summarized in Table 1. Table 1. Quantum yields and FRET efficiency for pyrene-labelled dendrons and pyrene dendronized porphyrins. Reprinted with permission from (Zaragoza-Galán, G., Fowler, M.A., Duhamel, J., Rein, R., Solladié, N., & Rivera, E. (2012). Synthesis and Characterization of Novel Pyrene-Dendronized Porphyrins Exhibiting Efficient Fluorescence Resonance Energy Transfer: Optical and Photophysical Properties. Langmuir, 28(30), 11195–11205.). Copyright (2012) American Chemical Society [47].

Compound

PyBuOH (±error) d TME (±error) d Py2 -G1OH (±error) d Py4 -G2OH (± error) d Py2 -TMEG1 (±error) d Py4 -TMEG2 (±error) d

Quantum Yield (Φ)

Quantum Yield (Φ)

Pyrene Units a

Porphyrin Units b

λex = 344 nm

λex = 344 nm

0.52 (0.03) 0.63 (0.02) 0.60 (0.03) 0.008 (0.001) 0.018 (0.003)

EFRET

-

-

0.0015 (0.0001)

-

-

-

-

-

0.0015 (0.00005) 0.0014 (0.0001)

c

0.99 0.97

a

All reported fluorescence quantum yields were determined using the fluorescence quantum yield of pyrene in cyclohexane as a reference which has been reported to equal 0.32 [65]. b This value is the fluorescence quantum yield of the porphyrin core having undergone FRET from an excited pyrene to the porphyrin. It is calculated by integrating the porphyrin fluorescence intensity in Figure 3 between 580 and 680 nm, after the fluorescence spectrum was correct to account for the direct excitation of porpyrin. c EFRET is the FRET efficiency, calculated using the following equation: I( Py+ Por) EFRET = 1 − (1) I( Py) where I(py+por) is the absolute fluorescence intensity of one mole of pyrenyl pendant in a dendron and I(py) is the absolute fluorescence intensity of one mole of pyrene attached to the corresponding dendron. d All experiments were conducted in triplicate.

In function of the distance pyrene-porphyrin (dPy −Por ), we can expect a decrease in FRET efficiency in Py4 -TMEG2, with respect to Py2 -TMEG1. Nevertheless, if the increase of the distance dPy −Por is not enough to overcome the Förster radius (R0 ), this change in the FRET efficiency will not be observed. As a result, very high FRET efficiencies were observed for both generations of our dendrimer leading to the conclusion that the pyrene and porphyrin units in our constructs are located well within the R0 value (5.2 nm) determined for this pair of chromophores. This assumption was confirmed by molecular

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located well within the R0 value (5.2 nm) determined for this pair of chromophores. This assumption was confirmed by molecular mechanics calculations to estimate the longest possible mechanics calculations to estimate the longest possible of pyrene-porphyrin distance in arealistic stretched pyrene-porphyrin distance in a stretched conformation the molecules [47]. It is not to conformation of the molecules [47]. It is not realistic to expect that the dendritic molecules expect that the dendritic molecules remain stretched in solution since they tend to adopt aremain coiled stretched in solution since they tenddistance to adoptina these coiledconstructs conformation. The donor-acceptor in conformation. The donor-acceptor was found to be 3.0 and distance 3.5 nm for these constructs was found to be 3.0 and 3.5 nm for Py -TMEG1 and Py -TMEG2, respectively, well 4 Py2-TMEG1 and Py4-TMEG2, respectively, well under2 the 5.2 nm R0 value, showing that efficient under the 5.2innm R0 value, showing that efficient FRET occur these dendritic molecules (Table 2).FRET occur in these dendritic molecules (Table 2). We We can can observe observe that that FRET FRET efficiency efficiency is is not not affected affected by by the the quantity quantity of of excimer excimer emission emission arising arising from the pyrene-containing dendrons since the FRET efficiency is not affected by the amount of excimer from the pyrene-containing dendrons since the FRET efficiency is not affected by the amount of formed the pyrene dendron This fact reveals thatreveals excimer formation takeswould place excimerinformed in the pyrene precursors. dendron precursors. This fact that excimerwould formation after FRET process occurs fromoccurs the monomer to emission the porphyrin andacceptor that it does takesthe place after the FRET process from theemission monomer to theacceptor porphyrin and not have a contribution to the whole FRET process. Time-resolved fluorescence experiments showed that it does not have a contribution to the whole FRET process. Time-resolved fluorescence that FRET from excited pyrene to the porphyrin coretotake morecore thantake 10 times experiments showed that FRET monomer from excited pyrene monomer the place porphyrin placefaster more than the formation of pyrene excimers in the pyrene dendrons. From these results, we can affirm than 10 times faster than the formation of pyrene excimers in the pyrene dendrons. From these that FRET the excited pyrene monomer to the porphyrin is very and occurs before the results, wefrom can affirm that FRET from the excited pyrene monomer to efficient the porphyrin is very efficient formation the excimer. and occursofbefore the formation of the excimer. A second A second series series of of dendrimers dendrimers bearing bearing first-generation first-generation Fréchet-type Fréchet-type pyrene-labelled pyrene-labelled dendrons dendrons and porphyrin as asthe thecore corewas wassynthesized synthesizedand and characterized group [66]. obtained and aa porphyrin characterized byby ourour group [66]. The The obtained free free base porphyrins were further metallated with Zn. The structure of the obtained porphyrins base porphyrins were further metallated with Zn. The structure of the obtained porphyrins was was characterized by NMR 1H NMR spectra and confirmed by MALDI-TOF spectrometry characterized by 1H spectra and confirmed by MALDI-TOF massmass spectrometry [66]. [66]. The The electrochemical properties charge transfer character suchsystems systemshas hasbeen beenalso alsostudied studied [67]. [67]. electrochemical properties andand charge transfer character ofof such The The structure structure of of this this series series of of dendrimers dendrimers is is illustrated illustrated in in Figure Figure4. 4.

Por-(Py2G1)4

Por-(Py2G1)2

Por-(Py2G1)3

Por-(Py2G1)

Figure 4. Structure of the dendrimers bearing first-generation pyrene-labelled Fréchet-type dendrons and a porphyrin core core [66]. [66].

The absorption absorption spectra spectraof ofthe theporphyrinic porphyrinicdendrimers dendrimersininTHF THF solution shown Figure 5. solution areare shown in in Figure 5. In In absorption spectra thesecompounds, compounds,we wecan cansee seethe thetypical typical SS00 → absorption band band of the → SS22 absorption thethe absorption spectra ofof these pyrene group centered 344 nm, as well as the Soret band of the porphyrin (free base or metalated), which appear at about 420 and 426 nm, respectively. respectively. Usually, Usually, the the free free porphyrin porphyrin derivatives derivatives exhibit four Q bands situated between 513 and 647 nm, whereas the zinc zinc metallated metallated porphyrins show only two Q bands in the same UV–VIS UV–VIS region. region.

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Abs(344 nm)/Abs(420 nm) Abs(344 nm)/Abs(420 nm) nm) Abs(344 nm)/Abs(420 nm) Abs(344 Abs(344 nm)/Abs(420 Abs(344nm)/Abs(420 nm)/Abs(420nm) nm) Abs(344 nm)/Abs(420 nm)

4

(A) (A) (A) (A) (A) (A) (A)

Abs(344 nm)/Abs(420 nm)

ε (M−1cm−1 ×10−3)

−1 −1 −3 −1 −3 −1cm −1×10 −3) −1 −1 −3 ε(M (M −1 −1 −3 ε (M−1εεcm ×10 (M cm ×10 cm −1 −1)×10 −3)))) cm ×10 εε(M (M cm ×10

7 of 25 800 800 700 500 800 800 800 600 700 600 600 7 of 25 600 600 600 600 800 600 700 450 600 600 600 600 700 600 800 500 500 400 600600 500 500 400 500 400 600 8 400 500 400 500 400 600 400 800 700 400 600 600 500 200 200 200 300 500 400 6 350 200 200 300 400 400 500 400 600 400 400 200 400 200 400 0000 200 600 300 500 4 300 200 100 4000400 400 410 420 430 4444 400 100 500 400410 410420 420430 43044 200 300 400 400 410 420 430 44 300 400 410 420 430 44 300 0 0 2 300 Wavelength, nm 0 300 0 250 200 Wavelength, 100 nm Wavelength, nm 500 400 40044410 420 430 44 Wavelength, nm Wavelength, 400 410 420nm 430 400400410 300 420 430 044 400 410 420 430 200 0300 200 Wavelength, nm 200 200 Wavelength, nm400 200 200 410 420 430 44 Wavelength, nm Wavelength, n 400 400 410 420 430 44 300 300 200 150 Wavelength, nm 100 200 Wavelength, nm 100 100 410 420 430 44 100 100 100 300 200 200 100 Wavelength, nm 100 00000 50 100 50 300 400 500 600 700 400 500 600 700 100 400 500 600 700 300 400 500 600 700 400 600 700 400 200 500 500 600 700 100300 300 400 500 600 700 300 400 500 600 700 400 500 600 700 400 500 600 0 0 50700 0 (nm) 0(nm) Wavelength (nm) Wavelength (nm) Wavelength Wavelength (nm) Wavelength (nm) Wavelength 300 400 500 600 700 400 500 700 Wavelength (nm) 600 Wavelength (nm) Wavelength (nm) Wavelength (nm) 100 300400 4005000 500600 600700 300 400 0 500 700 600 700 300 700 700 0 Wavelength (nm) Wavelength Wavelength (nm) Wavelength (nm) (nm) 700 700 300 400 50 300 400 500 600 700 (nm) Wavelength 300 400 500 600 700 400 500 600 700 800 600 700 800 600 0 600 500 800 600 Wavelength (nm) Wavelengt 700(nm) 800 800 600 600 500 (nm) 600 600 500 Wavelength Wavelength 500 500 600 600 300 400 500 600 700 600 500 600 800 500 700 450 600 600 500 400 600 600 800 400 400 450600 Wavelength 600 700 600 (nm) 400 400 500 500 500 600600 400 300 500 500 400 400 300 300 500 500 400 80 450 600 500 400 400 300 300 600 400 700 200 800 400 400 600 200 600 200 400 600 200 500 200 200 400 200 200 500 300 300 60 500 350 100 400 200 200 400 500 400 100 100 300 400 600 350 400 800 400 100 100 400 200 0000 200 600 400 0000 200 200 400 500 40 300 300 0 350 200 0 500 400400 100 100 400 410 420 430 44 300300 600 400 400 410 420 430 44 400410 410420 420430 43044 44 400 410 100 300 400 410420 420430 43044 44 300 400 200 400 410 420 430 44 400 420 430 44 300 0410 400 410 420 430 44 420 430 44 20 0410 0 200 400 Wavelength, nm 250 300 300 300 0 Wavelength, nm Wavelength, nm Wavelength, nm 200 0 500 400 Wavelength, nm Wavelength, nm 100 400 Wavelength, nm Wavelength, nm 400 410 420 430400 44 410 400 Wavelength, nm nm 400 410 420 430 44 420 410 430 420 44 430 100250Wavelength, 400410300420 430 44 400 0 200 250 200 300 0 Wavelength, nm 200nm 0200 200 200 Wavelength, Wavelength, nm Wavelength, n 200 200 Wavelength, nm400 410 420 400 430 410 44 420 430 44 400 300 400 410 420 430 44 150 2000 300 200 150 Wavelength, nm 100 200Wavelength, nm Wavelength, nm 400 410 420 100 100 430 44 100 100 100 150 300 100 200 Wavelength, nm 200 100 100 50 100 00000 50 400 500 600 700 400 600 700 400 200 500 500 600 700 300 400 500 600 700 100 400 600 700 300 400 500 600 700 400 500 600 700 400 500 600 0 500 100300 50700 300 400 500 600 700 300 400 500 600 700 0 0 (nm) Wavelength (nm) Wavelength (nm) 0 Wavelength Wavelength (nm) Wavelength (nm) Wavelength (nm) 400 500 600 700 300 400 500 600 700 Wavelength (nm) Wavelength (nm) Wavelength (nm) Wavelength (nm) 300 400 500 600 700 300 400 500 600 700 0 100 300 400 0 500 600 0.6 0.6 0.6 0 Wavelength (nm) Wavelength (nm) Wavelength (nm) 0.6 0.6 300 400 500 600 700 Wavelength (nm) Wavelength (nm) 300 400 50 400 500 600 700 3000.6 400 0.9 500 600 700 0 Wavelength (nm) Waveleng 0.9 (nm) 0.6 (nm) 0.5 Wavelength Wavelength 0.5 0.5 300 400 500 600 700 0.5 0.5 0.9 0.6 0.8 0.8 Wavelength (nm) 0.6 0.5 0.5 0.4 0.8 0.7 0.4 0.4 0.6 0.7 0.4 0.4 0.5 0.5 0.5 0.7 0.6 0.6 0.4 0.5 0.5 0.7 0.6 0.4 0.5 0.5 0.3 0.3 0.3 0.5 0.3 0.3 0.4 0.6 0.4 0.5 0.4 0.4 0.5 0.6 0.4 0.5 0.4 0.4 0.3 0.3 0.2 0.3 0.5 0.4 0.2 0.3 0.2 0.3 0.4 0.4 0.5 0.4 0.2 0.3 0.2 0.3 0.3 0.3 0.2 0.4 0.2 0.3 0.3 0.2 0.2 0.4 0.2 0.3 0.2 0.2 0.1 0.1 0.1 0.3 0.1 0.1 0.1 0.2 0.3 0.2 0.1 0.1 0.2 0.2 0.3 0.2 0.1 0.1 0.1 0.1 0.0 0.0 0.2 0.1 0.0 0.0 0.0 0.0 0.1 0.2 0.2 0.1 0.0 0.0 0.0 0.0 0.110 1010 1010 2222 44 4 6666 8888 0.1 00000 00000 22222 44444 66666 88888 10 10 2 6 8 10 10 0.1 10 0.0 0.0 0.044 0.1 0.0 0.0 # Py / Por ###Py Py Py Py/4//Por /Por Por Py/4///Por /Por Por 0 2 #### 0.1 0 2 # 6 8 0Py Por Py Por 02 6 24 8 0.0 46 10 68 810 # Py Por 0 2 4 10 6 8 10 Py 0.0 0.0 0.0 # Py / Por # Py / Por # Py / Por 0 2 4 0 2 4 6 8 10 # Py / Por # Py / Por 0 2 4 6 8 10 0 2 4 6 8 10 5.5.Absorption Absorption spectra ofof (black G1) (green )Por-(Py G1) 3,, 3, 0.0 spectra Figure 5. Absorption spectra of(A) (A) (black Por-(Py G1) (green )Por-(Py G1) 22G1) Figure )))))Por-(Py 44,4, 4,,(green )Por-(Py 33 Figure5. Absorption spectraof (A)(black (black Por-(Py 2G1) (green )Por-(Py 2G1) # Py / #2222G1) Py /44,,Por Figure Absorption spectra of (A) (A) (black 2G1) G1) (green )Por-(Py G1) Figure 5.5.10Absorption (A) Por-(Py (green 22G1) 33,, # Py / Por)Por-(Py # Py / Por 2 of 0 spectra 4(black 6 8Por-(Py 10 8

−1 −1 −3 −1 −3 −1 cm −1 ×10 −3) −1 −1 −3 (M −1 −1 −3 ε (M−1εεεεεcm (M cm ×10 (M cm −1×10 −1)×10 −3)))) (M cm ×10 (M cm ×10

ε (mM−1.cm−1)

ε (mM−1.cm−1)

2

2525 7777of 25 7of of of 25 of 25 Polymers 2018, 10, x FOR PEER REVIEW7 of 25

(F)

(red ) Por-(Py G1)22,, and and (blue )Por-(Py Por-(Py ;Por-(Py (B) (black ) 5.Zn-Por-(Py 2 G1) 2 G1) 2 G1) 42G1)3, of (A))Por-(Py Absorption spectra (A) ))(black )1;(B) 2G1)4,Figure (green Absorption )Por-(Py spectra (black 2G1 (red (blue G1) (black G1) 5. of Absorption spectra (A) (black ) Por-(Py 22G1) (green (red Por-(Py 22G1) 2, 2Figure and (blue Por-(Py 22G1) 11;;of (black ))))Zn-Por-(Py 22G1) 44 4and (redFigure))))5. )Por-(Py Por-(Py 2G1) , and and (blue )Por Por-(Py 2G1) 1(B) (B) (black )Zn-Por-(Py Zn-Por-(Py G1) 4,and and # Py / (red Por-(Py G1) (blue ) Por-(Py G1) (B) (black Zn-Por-(Py G1) and (red Por-(Py 22G1) 22,, and (blue ) Por-(Py 22G1) 11;; (B) (black Zn-Por-(Py 22G1) 44 and and (red ) Zn-Por-(Py2 G1)2 ; (C) (black )Absorption Py4 -TMEG2spectra and (red ) Py2 -TMEG1; Figure 5. of (A) (black ) Por-(Py 2G1)4, (green Figure 5. Absorption spectra of (A) (black 2G1) )-TMEG2 Por-(Py 2(B) G1) 4(red , (red 22G1) 342,, and ) Por-(Py (red 2)G1) ))Por-(Py Por-(Py 2G1) G1) 2, (C) (blue )) Por-(Py 2G1)1; and Zn-Por-(Py G1) and) Zn-Por-(Py (blue ) Por(red ) Zn-Por-(Py (black Py 4-TMEG2 ) ))Py Py -TMEG1; (D) ) Por-(Py 2,Py (blue and )(green Por-(Py 12;2-TMEG1; (B) 2G1)4 an (red 22G1) 22;; 2(C) 4and (red (D) (red(red Zn-Por-(Py 2(red G1) ; and (C)(black (black 4-TMEG2 and(black Py 2-TMEG1; (D) (red Zn-Por-(Py G1) (C) (black Py -TMEG2 and (red Py -TMEG1; (D) (red Zn-Por-(Py 22G1) 22;; (C) (black Py 4)4-TMEG2 and (red Py 22-TMEG1; (D) (D) (black ))))Zn-Por-(Py ) Zn-Py and (red ))))Py Zn-Py (E,F) ))plot of (black Abs(344 4 -TMEG2 2 -TMEG1; Absorption spectra of (A) (black ) Por-(Py 2G1)4, (green (red )Por-(Py 2G1)3, 2G1)2, and (blue ) Por-(Py ) Por-(Py 2G1)1; (B) (black ) Zn (red 2G1) 2, and (blue ) Por-(Py 22)G1) ;4-TMEG2 (B)(black (black )the Zn-Por-(Py 2G1) and2G1) (red ) )Por-(Py ) Zn-Por-(Py 2G1) 2; (red (C) (black Py and (red )nm)/Abs(Soret) Py Zn-Por-(Py -TMEG1; (D) 2; (C) (black ) (black ) Zn-Py Zn-Py -TMEG2 and Zn-Py -TMEG1; (E,F) plot of Abs(344 nm)/Abs(Soret) (red ) Zn-Por-(Py 2G1) ; 1(C) )Abs(344 Py 4free -TMEG2 and4 (red ) Py2-TMEG1; (D (black 44-TMEG2 and ))))Zn-Py 2-TMEG1; (E,F) plot of nm)/Abs(Soret) (black Zn-Py 4-TMEG2 and (red )Zn-Py Zn-Py 22-TMEG1; (E,F) plot of Abs(344 nm)/Abs(Soret) versus the number of(red pyrenyl units per porphyrinic construct for and metallated (black Zn-Py -TMEG2 and (red Zn-Py -TMEG1; (E,F) plot of Abs(344 nm)/Abs(Soret) (black )))Zn-Py 44-TMEG2 and (red 22-TMEG1; (E,F) plot of Abs(344 nm)/Abs(Soret) ) Por-(Py2G1)2, and (blue ) Por-(Py 2 G1) 1 ; (B) (black ) Zn-Por-(Py 2 G1) 4 and (red ) Zn-Por-(Py 2 G1) 2 ; (C) (black ) Py 4 -TMEG2 and (red versus the number of pyrenyl per porphyrinic construct for the free and metallated porphyrins, (red )number Zn-Por-(Py 2Reprinted G1) 2units ;units (C) per (black ) construct 4-TMEG2 and (red )Abs(344 Py2-TMEG1; (D) (black )of Zn-Py (black )porphyrins, Zn-Py 4-TMEG2 and (red )Py Zn-Py 2-TMEG1; (E,F) plot ofFowler, nm)/Abs(Soret) 4-TMEG2 and (red ) Zn versus the number pyrenyl porphyrinic for the free and metallated versus the of pyrenyl units per construct for the free and metallated porphyrins, porphyrins, respectively. with permission from (Zaragoza-Galán, G., M., Rein, R., of Abs(344 (black )porphyrinic Zn-Py4-TMEG2 and (red )and Zn-Py 2-TMEG1; (E,F) plot nm)/Abs(Sore versus the number of pyrenyl units per porphyrinic construct for the free metallated porphyrins, versus the number of pyrenyl units per porphyrinic construct for the free and metallated porphyrins, respectively. Reprinted with permission from (Zaragoza-Galán, G., Fowler, M., Rein, R., Solladié, N., ) Zn-Por-(Py2G1)2; (C) respectively. (black ) Py 4 -TMEG2 and (red ) Py 2 -TMEG1; (D) (black ) Zn-Py 4 -TMEG2 and (red ) Zn-Py 2 -TMEG1; (E,F) plot of Abs( versus the number of pyrenyl units per porphyrinic construct for the free versus and the metallated number porphyrins, of pyrenyl units per porphyrini Solladié, N., Duhamel, J., & Rivera, E. (2014). Fluorescence Resonance Energy Transfer in Partially Reprinted with permission from (Zaragoza-Galán, G., Fowler, M., Rein, R., Solladié, N., respectively.Reprinted Reprinted with permission fromof G., Fowler, M., Rein,R., R.,Solladié, Solladié, N., (black )Reprinted Zn-Py4-TMEG2 andthe (red )(Zaragoza-Galán, Zn-Py 2-TMEG1; (E,F) plot ofM., Abs(344 nm)/Abs(Soret) versus number pyrenyl units per porphyrinic construct for the free and metallated porphyrin respectively. with permission from (Zaragoza-Galán, G., Fowler, M., Rein, R., Solladié, N., respectively. with permission from (Zaragoza-Galán, G., Fowler, Rein, N., Duhamel, J.,J.,& &&Rivera, Rivera, E.E. (2014). Fluorescence Resonance Energy Transfer inper Partially and Fully versus the number of pyrenyl units porphyrinic construct for theR., free and(Zara metN and Fully Labelled Pyrene Dendronized Porphyrins Studied with Model Free Analysis. The Journal of respectively. Reprinted with permission from (Zaragoza-Galán, G., Fowler, respectively. M., Rein, Reprinted R., Solladié, with permission from Duhamel, J., E. (2014). Fluorescence Resonance Energy Transfer in Partially and Fully Duhamel, Rivera, (2014). Fluorescence Resonance Energy Transfer in Partially and Fully ) Zn-Py4-TMEG2 and (red ) Zn-Py 2 -TMEG1; (E,F) plot of Abs(344 nm)/Abs(Soret) versus the number of pyrenyl units per porphyrinic construct for the free and metallated porphyrins, ReprintedResonance with permission (Zaragoza-Galán, G., M., Rein, Solladié, Duhamel, J., J., & & Rivera, Rivera, E. E.respectively. (2014). Fluorescence Fluorescence Resonance Energyfrom Transfer in Partially Partially and andFowler, Fully N., Duhamel, (2014). Energy Transfer in Fully Labelled Pyrene Dendronized Porphyrins Studied with Model Free Analysis. The Journal of Physical Physical Chemistry C, 118(16), 8280–8294). Copyright (2014) American Chemical Society [66]. respectively. Reprinted with permission from (Zaragoza-Galán, G., Fowler, M., Re Duhamel, J., & Rivera, E. (2014). Fluorescence Resonance Energy Transfer Duhamel, in J., Partially & Rivera, and E. Fully (2014). Fluorescence R Labelled Pyrene Dendronized Porphyrins Studied with Model Free Analysis. The Journal of Physical Labelled Pyrene Dendronized Porphyrins Studied with Model Free Analysis. The Journal of Physical number of pyrenyl units per porphyrinic construct for thePorphyrins free and metallated porphyrins, respectively. Reprinted with permission from (Zaragoza-Galán, G., Fowler, M., Rein, R.,Energy Solladié, N., Duhamel, J., & Rivera, E. (2014). Fluorescence Resonance Transfer in Partially and Ful Labelled Pyrene Dendronized Porphyrins Studied with Model Free Analysis. TheJournal Journal Physical Labelled Pyrene Dendronized Studied with Model Free Analysis. The ofofPhysical Chemistry C,C, 118(16), 8280–8294). Copyright (2014) American Chemical Society [66]. Duhamel, J., & Rivera, E. (2014). Fluorescence Resonance Energy Transfer in Labelled Pyrene Dendronized Porphyrins Studied with Model Free Analysis. Labelled The Pyrene Journal Dendronized of Physical Porphyrins Studied Chemistry C, 118(16), 8280–8294). Copyright (2014) American Chemical Society [66]. Chemistry 118(16), 8280–8294). Copyright (2014) American Chemical Society [66]. y. Reprinted with permission from (Zaragoza-Galán, G., Fowler, M., Rein, R., Solladié, N., Duhamel, J., & Rivera, E. Labelled (2014). Fluorescence Resonance Energy Transfer Partially Pyrene Dendronized Porphyrins Studied with Model and Free Fully Analysis. The Journal of PhysicP Chemistry C,118(16), 118(16), 8280–8294). Copyright (2014) American Chemical Societyin [66]. Chemistry C, 8280–8294). Copyright (2014) American Chemical Society [66]. A slight shift C, of 118(16), the absorption band of Labelled the porphyrin moiety was observed inStudied the series ofModel the Copyright Pyrene Dendronized Porphyrins with Free Analysis. Th Chemistry 8280–8294). Copyright (2014) American Chemical Chemistry Society [66]. C, 118(16), 8280–8294). (2014) J., & Rivera, E. (2014). Fluorescence Resonance Energy Transfer inStudied Partially and Fully Labelled Pyrene Dendronized Porphyrins with Model Free Analysis. The Journal of Physical Chemistry C, 118(16), 8280–8294). Copyright (2014) American Chemical Society [66]. A slight shift of the absorption band of the porphyrin moiety was observed in the series of the porphyrinic dendrimers in function of the amount of mesityl groups present in the construct. The Soret A shift of absorption band the porphyrin moiety was observed of Aslight slight shift ofthe the absorption bandof of the porphyrin moiety was observed inthe theseries series ofthe the Chemistry C, 118(16), 8280–8294). Copyright (2014) American Chemical Society [66 A slight shift of the absorption band of the porphyrin moiety was observed in the series of the yrene Dendronized Porphyrins Studied with Model Free Analysis. The Journal of Physical A slight shift of the absorption band of the porphyrin moiety was observed in the series of the Chemistry C, 118(16), 8280–8294). Copyright (2014) American Chemical Society [66]. in

# Py / Por

porphyrinic dendrimers in shifted function of the of mesityl present thePor-(Py construct. band of the free porphyrins from 421 amount to 420, 419, and 418 groups nm when going in from G1)The to Soret band the free porphyrins shifted from 421 toto 420, 419, and 418 nm when going from Por-(Py ,shift Por-(Py Por-(Py G1). Likewise, the Soret band of the metalated porphyrins A slight shift of the absorption band of porphyrin moiety was observ porphyrinic dendrimers in function amount of mesityl porphyrinic groups present dendrimers inthe the construct. inof function Theinofthe theconstru amoun Soret the free porphyrins shifted from to 420, and 418 nm when going from Soret band of the free porphyrins shifted from 421 420, 419, and 418 nm when going from 2 G1)of 3of 2 G1) 2 , and 2of Aband slight of the absorption band ofthe the porphyrin moiety was observed in the series the porphyrinic dendrimers in421 function of419, the amount of mesityl groups present Soret band of the free porphyrins shifted from 421 to 420, 419, and 418 nm when going from Soret band of the free porphyrins shifted from 421 to 420, 419, and 418 nm when going from Por-(Py 2 G1) 4 to Por-(Py 2 G1) 3 , Por-(Py 2 G1) 2 , and Por-(Py 2 G1). Likewise, the Soret band of the shifted from 427 to 425 nm when going from Zn-Por-(Py G1) to Zn-Por-(Py G1) . porphyrinic dendrimers in function of the amount of mesityl groups present Soret band of the free porphyrins shifted from 421 to 420, Soret 419, and band 418 of nm the when free porphyrins going from shifted Por-(Py 2G1) 4the Por-(Py 2in G1) 3, band G1) , 2,and Likewise, band of the Por-(Py 2of G1) 4toto Por-(Py 2G1) 3,Por-(Py Por-(Py 2G1) and Por-(Py 24shifted G1). Likewise, the Soret band ofThe the nm when from 2 22G1). 2the hift of the absorption band moiety was in Por-(Py the series ofgroups the porphyrinic function of2observed of mesityl present in the Soret of the free porphyrins from 421 to2Soret 420,construct. 419, and 418 going Por-(Py G1) toporphyrin Por-(Py G1) Por-(Py 2G1) G1)2amount and Por-(Py 2G1). G1). Likewise, the Soret band of the Por-(Py 22G1) 44dendrimers to Por-(Py 22G1) 33,, Por-(Py 2the 22,, and Por-(Py Likewise, the Soret band of the metalated porphyrins shifted from 427 to 425 nm when going from Zn-Por-(Py 2G1) G1) 434,420, to Soret band of the free porphyrins shifted from 421 to 419, and 418 Por-(Py Por-(Py 2 G1) 4 to Por-(Py 2 G1) 3 , Por-(Py 2 G1) 2 , and Por-(Py 2 G1). Likewise, 2 G1) 4 the to Por-(Py Soret band 2 G1) of Por-(Py the 2 G1) 2 , andn metalated porphyrins shifted from 427 to 425 nm when going from Zn-Por-(Py 2 4 to metalated porphyrins shifted from 427 to 425 nm when going from Zn-Por-(Py 2 G1) to ndrimers in function of the amount of mesityl groups present in the construct. The Soret band porphyrins of the free Por-(Py porphyrins from 4213nm toPor-(Py 420, 419, 418 nmZn-Por-(Py when from 4shifted to427 Por-(Py 2425 G1) ,nm 2G1) 2and , and Por-(Py 2G1). going Likewise, metalated porphyrins shifted2G1) from 427 to 425 when going from Zn-Por-(Py G1) tothe Soret band metalated shifted from to when going from 22G1) 44 to Zn-Por-(Py 2G1) G1) 2.porphyrins . Por-(Py 2 G1) 4 to Por-(Py 2 G1) 3 , Por-(Py 2 G1) 2 , and Por-(Py 2 G1). Likewise, metalated shifted from 427 to 425 nm when metalated going from porphyrins Zn-Por-(Py shifted from 427 to 42 2 G1) 4 to Zn-Por-(Py 2 2 Zn-Por-(Py 2 G1) 2 . the free porphyrins shifted from 421 tometalated and 4182,nm when going from Por-(Py 2G1)224G1) to22..Por-(Py 2420, G1)3,419, Por-(Py 2G1) and Por-(Py 2G1). Likewise, band of the porphyrins shifted from 427 to 425the nmSoret when going from Zn-Por-(Py2th G Zn-Por-(Py G1) Zn-Por-(Py metalated porphyrins shifted from 427 to 425 nm when going from Zn-Por-(Py Zn-Por-(Py 2G1) 2. 2. o Por-(Py2G1)3, Por-(Py 2G1)2porphyrins , and Por-(Py 2G1). Likewise, bandwhen of the metalated shifted from to Soret 425 nm going from2G1) Zn-Por-(Py 2G1)4 to Zn-Por-(Py 2G1)2427 . the 2G1)2. phyrins shifted Zn-Por-(Py from 427 2G1) to 2425 nm when going Zn-Por-(Py from Zn-Por-(Py 2G1)4 to .

A slight shift ofin the absorption band of the porphyrin moiety was A slight observed shift of in thewas series of theband of the po porphyrinic dendrimers function of amount of groups present in the construct. The 2absorption 4The porphyrinic dendrimers in function ofthe the amount ofmesityl mesityl groups present in the construct. The A slightSociety shift of the absorption band of the porphyrin moiety observed in the series C, 118(16), 8280–8294). Copyright (2014) American Chemical [66]. porphyrinic dendrimers in function of the amount of mesityl groups present in the construct. porphyrinic dendrimers in function of the amount of mesityl groups present in the construct. The

)2.

Polymers 2018, 10, x FOR PEER REVIEW

8 of 25

0.1 0.0 0

ε (mM−1.cm−1)

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Abs(344 nm)/Abs(420 nm)

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(D)

Abs(344 nm)/Abs(420 nm)

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(F)(F) (F) (F) (F) (F) Abs(344 nm)/Abs(420 nm)

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Abs(344 nm)/Abs(420 nm) Abs(344 nm)/Abs(420 nm) nm) Abs(344 nm)/Abs(420 Abs(344 nm)/Abs(420 nm) Abs(344 nm)/Abs(420 nm) Abs(344 nm)/Abs(420 nm)

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4 x104 Abs(344 nm)/Abs(420 nm)x10 Absolute Fluor. Int. Absolute Fluor. Int.

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Abs(344 nm)/Abs(420 nm)

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Abs(344 nm)/Abs(420 nm)

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(E)(E) (E) (E) (E) (E)

(C)

(C)

Absolute Fluo. Int. ×104

300

Abs(344 nm)/Abs(420 nm) Abs(344 nm)/Abs(420 nm) nm) Abs(344 nm)/Abs(420 Abs(344 nm)/Abs(420 nm) Abs(344 nm)/Abs(420 nm) Abs(344 nm)/Abs(420 nm)

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Absolute Fluo. Int.

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−1.cm−1) ε (mM −1−1 ε (mM−1 )−1.cm εε.cm (mM (mM .cm−1−1) ) ε (mM−1.cm−1) −1.cm (M−1cm−1ε (mM ×10−3 ) −1)

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300

(C)(C) (C) (C) (C) (C)

Absolute 4 Absolute Fluo.Fluo. Int. Int. ×10×10

Abs(344 nm)/Abs(420 nm)

350

500

4

ε (M−1 cm−1 ×10−3)

(C)

400

1.4

Absolute Fluo. Int. ×104

450

250 1.0 700 300 700 400 500 600 700 400 500 600 800 800700 450 450 500 600 600 0 600 500 Wavelength (nm) 800 450 800 600 450 500 600 600 Wavelength (nm) Wavelength (nm) 500 500 600 600 300500 400 500 600 700 600 400 400 450 700 600 600 800 500200 700600 450 600 500 300 1.2 1.8 600 600 200 400 0.8 700 400 400 450 300 1.2 600 Wavelength (nm) 400500 Wavelength (nm) 600 400 400 500 500 600 500 500 400 400 300 300 600 500 350 350 400 400 450 600 500 400 500 400 1.6 300 400 300 350 350 1.0 700 (A) 800 400 400 450 600 600 200 200 400 250 (B) 500 600 200150 200 400 200150 300 1.0 200 500 0.6 300 300 300 350 500 350 250 400 200 200 500 1.4 100 100 300 300 600 300 350 400 400 400 800 400 0.8 600 100 100 400 400 200 200 400 0 0 200 600 0 0 200 500 250 250 300 300 300 0 5001.2 0 350 0 0 200300 500 250 100 100 250 300 200 400100 600 0.8 350 400 400 410 420 430 44 44 400 410 420 430 100 0.4 400 410 420 430 44 100 400 410 420 430 44 0.6 400 400 200 400410 410 420430 43044 44 400 300 300 0 420 400410 410 420430 430 400 4444 0 420 0 nm 200 200 200 250 250 300 300 300 Wavelength, nm Wavelength, 200 Wavelength, 0 500 200 400 Wavelength, nm 200 Wavelength, nm 250 100 3001.0 300 400 Wavelength, nm nm 400 410 420 430 410 44 42 Wavelength, nm Wavelength, nm 400 410 420 430 44 400 100 400 0.4 200 150 150 0.6 300 400 410 420 430 44 0 150 200 250 050 300 Wavelength, 0.2 nm 200 050 200 Wavelength, 200 200 150 150 200 0.8 150 nm 200 Wavelen 250 200 Wavelength, nm 400 410 420 430 44 100 400 400 410 420 430 0.2 300 44 400 410 420 430 44 100 100 150 150 100 200 300 100 00.6 100 0.4 0 200 100 150 200 Wavelength, nmWavelength, nm 200 Wavelength, nm 100 100 0 0.0 400 410 420 430 44 0 400 410 420 430 44 0.0 100 50 50 100 100 100 150 300 50 50 100 0.4 200 150 350 450 550 650 Wavelength, nm 200 350 Wavelength, 450 650 nm 550 50 0.2 100 50 100 50 100 (nm) 0 00 0 50 50 0.2 100 00 0 0Wavelength Wavelength (nm) 200 300 400 500 600 700 300 400 500 600 700 300 400 500 500 600 600100 700 700 700 300 400 300 0 400 500 600 700 300 400 500 600 700 0 100 50 300 400 500 600 700 300 400 500 0 0 0.0 0 (nm)2.0 300 0 600 0 300 0.0 50 2.5 Wavelength (nm) Wavelength Wavelength (nm) Wavelength (nm) 300 400 500 600 700 300 400 500 60 350 450 550 650 Wavelength (nm) Wavelength (nm) Wavelength (nm) (nm) 300 Wavelength 400 500300 600400 700 350 450 550 300 400 500 600 700 100 650 0 1.80 0.90 0.9 0.60 0.6 Wavelength (nm) Wavelength (nm) Wavelength (nm) Wavelength (nm) 0.90.9 0.6 0.6 Wavelength (nm) 300 400 500 600 700 Wavelength (nm) 400 Wavelen 300700 4 300 400 500 250 600 700 300 0.6 500 250 600 1.6 2.0 0.9 300 0.8 0.8 0.9 2.0 300 2.5 0 Wavelength (nm) 0.9(nm) 0.6 0.80.8 Wavelength Wavelength (nm) 0.5 0.5 400 500 600 700 300 400 600 700 0.50.5 1.4 1.8 0.9500 0.6 0.8 0.7 0.7 0.8 200 0.7 (D) (C) 2002500.8 Wavelength Wavelength (nm)0.70.9 250 (nm) 0.6 0.5 1.6 2.0 0.5 1.5 1.2 0.8 0.7 0.7 0.6 0.6 0.4 0.4 0.6 0.7 0.6 0.60.8 0.40.4 0.5 1.4 150 0.5 150 1.0 200 200 0.7 0.6 0.4 0.5 0.5 0.6 0.7 0.6 0.4 0.5 0.5 1.5 1.2 0.3 0.3 1.0 0.5 0.8 0.30.3 0.6 0.4 0.5 0.4 0.4 0.5 150 100 0.4 150 0.5 1.0 100 0.4 0.40.6 0.3 0.6 0.3 0.5 0.4 0.4 1.0 0.2 0.3 0.2 0.3 0.8 0.4 0.4 0.3 0.20.2 0.30.5 0.3 0.5 0.4 100 0.3 50100 50 0.6 0.4 0.2 0.3 0.3 0.2 0.2 0.4 0.2 0.3 0.2 0.2 0.2 0.1 0.1 0.3 0.5 0.4 0.10.1 50 0.2 0.3 0.2 50 0.1 0.1 0.2 0.2 0 0.2 0.0 0.1 0.10.3 0 0.0 0.1 0.2 0.1 350 4500.2 650 0.2 0.1 550 0.1 0.0 0.0 0.0 0.0 350 450 550 650 0.2 0.00.0 0 0.1 0.00.0 0.0 0.0 2 2 4 04 6 8 0.1 10 0.10 00 0 0 0 2550 2 10 10 4 8 1010 Wavelength (nm) 350 450 66 6 8 8 8 1010 10 4 44 650 6 66 6 8 88 0.1 0.0 0.0 350 450 Wavelength 550 2 2 (nm) 6504 4 0.10 0 0.0 2 2 0.0 0.0 # Py / Por # Py / Por #Py Py / Por #/ Py / Por0 6 0.02 8 0 (nm) # 2 #Py 4 2 0 2# # 4Por 010Py/ Por Wavelength (nm)0.1 / Por Py /Por 0 6 4 0.02 8 6 4 10 4 10 Wavelength 6 8 0.0 0.0 # Py / Por Figure 6. Spectra of the absolute fluorescence of (A) (black ) Por-(Py 2G1)4,6 (green 8 ) # Py / Por # Py / Por 0 2 0 2 4 10 # Py 0 2 4 8 10 0 2 4 6 8 10 # Py / Por 0.0 fluorescence FigureFigure 6. Spectra of the absolute (A) (black Por-(Py )6 )Por-(Py 6.Figure Spectra of the absolute fluorescence of (A)(A) (black 2G1) , Por (green ) 2 G1) Figure 5.Absorption Absorption spectra ofof (A) (black )Por-(Py Por-(Py 24G1) 4,, (green (green 2G1) 3, 3, Figure 5. Absorption spectra of (black )))Por-(Py ) #Por-(Py 24G1) 4, (green )Por-(Py 2G1) Py / Figure 5. Absorption spectra of (A) (black ) Por-(Py 2G1) (green )Por-(Py 2 G1) 3 , 5. spectra of (A) (black 2G1) 4, 4,(green )Por-(Py 2 G1) 3 , #0Py / 2Por 2G1) , and (blue ) 2 4 6 ) Por-(Py 8 2G1)1;10(B) (black# Py / Por 2 4 Por-(Py 6 2G1)3, (red 8 10 ) Por-(Py

400

500

Py4-G2OH dendron. 300

Absolute Fluo. Int. ×104

300

Fluo. 4 ×10 Absolute Absolute Fluo. Int. ×10Int.

0

ε (M−1cm−1 ×10−3)

spectra of the free-base dendronized porphyrins Py2-TMEG1 and Py4-TMEG2 8 of 24 resulted to be the sum of the absorption spectra of 1-pyrenebutanol and porphyrin, so that we can realize that negligible to no electronic interactions occur between both chromophores. The absorption spectra of the dendronized porphyrins Py2core -TMEG1 Py4 -TMEG2 The fluorescence emissions offree-base all dendrimers based on a porphyrin were and recorded and the Polymers 2018, 10, x FOR PEER REVIEW 8 of 25 resulted to be the sum of the absorption spectra of 1-pyrenebutanol and porphyrin, so absolute emission spectra are shown in Figure 6. If we compare the Y-axes on the leftthat andwe oncan the realize thatThe negligible to the no electronic occur between both Py chromophores. absorption spectra of the interactions free-base dendronized porphyrins 2-TMEG1 and Pyyield 4-TMEG2 right corresponding to porphyrinic compounds and 1-pyrenebutanol (quantum of 0.52), The fluorescence of all efficient dendrimers based on a porphyrin core1-pyrenebutoxy were so recorded resulted towe be the sum of thehow absorption spectra 1-pyrenebutanol and porphyrin, that we and can respectively, canemissions realize is of FRET from an excited to the the realize that negligible to no electronic interactions occur between both chromophores. Polymers 2018, 10, x FOR PEER REVIEW 7 of of 25 Polymers 2018, 10, x FOR PEER REVIEW 725 of 25 absolute emission spectra are shown in Figure 6. If we compare the Y-axes on the left and on the ground-statePolymers of the 2018, porphyrin unit. AREVIEW reduction in the fluorescence emission intensity of more than Polymers 2018,10, 10,x xFOR FOR PEER REVIEW PEER 7 7of 25 The fluorescence emissions of all dendrimers based on a porphyrin core were recorded and the Polymers 2018, 10, x FOR PEER REVIEW Polymersyield 2018, 10, FOR PEER REVIEW 7 of right corresponding to thewas porphyrinic compounds and 1-pyrenebutanol of x0.52), Polymers 2018, 10, x FOR PEER REVIEW (quantum two orders of magnitude observed. 700 700 on the left and on the 500 500 700 absolutewe emission spectra areefficient shown in Figure 6.Polymers If we compare the Y-axes 700 500 500 2018, 10, x FOR PEER REVIEW respectively, can realize how is FRET from an excited 1-pyrenebutoxy to the ground-state Polymers 2018, efficiency 10, x FOR PEER 7 of 25 A remarkable FRET wasREVIEW expected in these dendritic constructs that the800 distance 600 600 800 700 givenyield 450 450 500 500 of800 600 600 800 right corresponding to the porphyrinic compounds and 1-pyrenebutanol (quantum 0.52), 700 450 450 600 600 (A) (B) (A) (B) 500 500500 of theREVIEW porphyrin unit. A(A) reduction in the fluorescence emission intensity of more than two orders of ymers 2018, 10, x FOR PEER 7 of 25 600 600 (A) (B) (B) 500 between the center of the porphyrin and the center of the pyrenyl unit in a fully extended 500 600 600 800 600is FRET 600 400 400 700 450 realize how efficient 500from an excited 600the 400 400450 600 600700 respectively, 400 we can 1-pyrenebutoxy to 400 500 400 600 400 (A) (B) 450construct. (A) 500 500 magnitude was(dobserved. EXT) varies (A) (B) 300 300and 500 500the 400 400600 500 dendritic conformation Por−Py between 18 35 Å depending on The 600 600 350 350 400 400 300 450 300 500 400 500 400 ground-state of the porphyrin unit. A reduction in the fluorescence emission intensity of more than 400 400 600 800 350 350 700 450 500 200 200400 400600 600 (A) 500 200 200 A remarkable FRET efficiency was expected in these dendritic constructs given that the distance 200 200 (A) (B) 300 and 48.7 300 (B) 500 500 400 Förster 0)magnitude value waswas to observed. equal 100 51.8 ± 0.2 ± 0.3 Å for a free-base and350 a Zn-metalated 300 300 350 200 100 100 200 400800 300400 500 two orders(R of300 600 400 600radius 400 300 350 100 400 450 200 200 4000 0 400 400 200 0 0 600 200 (A) (B) 300 EXT350 between center of porphyrin and the of the pyrenyl unit in500a fully extended conformation 500the 500 0 center 0expected 250the 0 0 means 250 300 porphyrin, (Table 2). The distance dPor−Py included in Table 3400 was calculated by 100 100 300 A respectively remarkable FRET efficiency was in these dendritic constructs given that300 the distance 400 600 250 250 300 350 400 410 420 430 44 100 400 410 420 430 44 400 400 410 420 430 44 44 400 410 420 430 400 400410 410 420430 4304444 EXT ) varies between 18 and 35 Å200 400 400400 410 420430 43044 300 410 300 200 The Förster400 0 420 044 0 420 (d depending on the dendritic construct. radius 300 300 0 200 200 250 250 300 −Py Wavelength, nm between the200 center of the porphyrin and the center of the pyrenyl unit in a fully extended Wavelength, nm 200 100 Wavelength, nm Wavelength, nm ofPor molecular mechanics optimizations, the program Hyperchem (Hypercube, ink., Gainesville, 300 500 using 400 200 400 250 300 Wavelength, nm 350 Wavelength, nm 400 410 420 430 44 100 Wavelength, nm Wavelength, 400 410nm 420400 430410 44 42 400 400 410 300420 0 430 44 200 was to equal EXT 300 is Wavelength, 150 0 18± 150EXT (R ) value 51.8 ± 0.2 and 48.7 0.3 Å for a free-base and a Zn-metalated 200 200 porphyrin, 0 conformation (d Por−Py ) varies between and 35 Å depending on the dendritic construct. The 250 200 Wavelength, nm Wavelen 200 200 nm 0 FL, USA). Since d Por–Py is much smaller than R 0 for all the dendronized porphyrins, FRET 150 150 200 250 300 200Wavelength, nm 200 400 410 420 430 44 100 400 400 EXT 410 420 430 44 400 410 420 430 300 44 100 150 a was 150 by means Förster (R 0100 ) value to equal 51.8 ±150 0.2 and ±00.3 for free-base and a Zn-metalated 0 to radius 200 respectively (Table 2). The distance dPor included in Å Table 3300 was200 calculated of Wavelength, nmWavelength, −Py 100 100 expected take place on faster time scale than the48.7 formation of aexcimers 200 250 200or Wavelength, nm nm 100 10044 within a Py2-G1OH 400 410 420 50 430 44 400 410 420 430 100 100 EXT 50 100 (Table 2). porphyrin, respectively The distance dPor−Py 150 included in Table 3 was calculated means 100 by 300 200 molecular mechanics optimizations, using the Hyperchem (Hypercube, ink., Gainesville, 50 nm 50 100program 150 200 Py4-G2OH dendron. 200 Wavelength, Wavelength, nm100 100 EXT is much smaller of molecular usingthan the program Hyperchem (Hypercube, ink., 50 Gainesville, 100 all 0 00 0 FL, USA). Since dmechanics R0 for the dendronized porphyrins, FRET is 600 100 700 0 00 0 50 optimizations, 50 100 150 Por–Py 200 300 400 500 300 400 500 300 400 500 600 700 300 400 500 600 700 EXT 100 300 400 0 FRET 500 is 600 600 700 700 300 400 500 600 700 300 500 300 than 600 700 USA). Since dPor–Py0 400 is400 much smaller R0 for1.2 the dendronized 300 500 600 700 50all 1.8FL, to 0or 0 (nm) 300(nm) 50 on a fasterWavelength expected take place time scale than the formation of excimers0porphyrins, within aWavelength Py 100 (nm) Wavelength Wavelength (nm) 2 -G1OH 300 400 500 600 700 300 400 500 600 700 300 400 500 60 Wavelength (nm) Wavelength (nm) Wavelength (nm) Wavelength (nm) 100time scale than to take place on a faster of excimers within a Py2700 -G1OH or 300 300 the formation 500 600 0 400 0 400 1.6expected 7000700 5000 500 50 Py4 -G2OH dendron. (nm) 300Wavelength Wavelength Wavelength (nm) (nm) 700 700 (A)Wavelength 500 500 300 250 400 600 700 (nm)500 Wavelen (B) −3) ε (M−1−1 cm−1 −3 −1)×10 ε (M−1εcm ×10 cm ×10 ε(M (M−1−1 cm−1 ×10−3−3)) ε (M−1cm−1 ×10−3) ε (M−1cm−1 ×10−3)

The absorption Polymers 2018, 10, 1062

(F)

Por-(Py2 G1)3 , (red Figure ) Por-(Py (blue ) Por-(Py2 G1)) 1 ;Por-(Py (B) (black ) 2 G1)2 , and 5. Absorption spectra of (A) (black 2G1) Figure 4, (green 5. Absorption )Por-(Py spectra 2G1)3of , ( 2 4 2 of 2(A) Figure 4 5. )Absorption spectra of (A) )(black ) 2Por-(Py 2G1)4 Figure 5.(red Absorption spectra (black )2-TMEG2 4(red ,(red (green )Por-(Py 2G1) (red ) Por-(Py (red Por-(Py 2G1)2G1) ,2;and (blue Por-(Py 2G1)12;G1) (B) (black ))) Zn-Por-(Py G1) G1) 2,3,and 4 and (blue Zn-Por-(Py 2G1)4 and ) Zn-Por-(Py 2; (C) (black )) Py -TMEG2 and ) Por-(Py 2G1) , 4Por-(Py and (blue ) absolute Por-(Py 2G1) 1;2-TMEG1; (B) 2(black ) (red Zn-Por-(Py 2G1) 2(red ; (C) (C) (black Py 4Py and Py 2Py -TMEG1; (D) (red ) )Zn-Por-(Py 2G1) (C) (black 4-TMEG2 and (red (D) (red Zn-Por-(Py 2-TMEG2 G1) (black 4-TMEG2 and (red Py 2-TMEG1;(D) (D) (red 2G1) 2; 2;(C) (black ) ))Py 4Zn-Py -TMEG2 and (red ) ))Py 2-TMEG1; Py -TMEG1; (D) (black ) ))Zn-Por-(Py Zn-Py and (red )Py -TMEG1; The 2 4 2 Py 2 -TMEG1; (D) (black ) Zn-Py 4 -TMEG2 and (red ) Zn-Py 2 -TMEG1; The absolute Figure 5. Absorption spectra of (A) (black ) Por-(Py 2-TMEG2 G1) 4, (green )Por-(Py 2G1) , (red ) Por-(Py 23G1) 2, and (blue ) Por-(Py 2G1)1; (B) (black Py2-TMEG1; (D) (black ) Zn-Py 4 and (red ) Zn-Py 2 -TMEG1; The absolute (red ) Por-(Py 2G1)2, and 2(blue )inPor-(Py 2G1) 1; (B) (black ) Zn-Por-(Py 2G1) 4 and (red ) Zn-Por-(Py G1) 2; (C) (black ) 2Py 4-TMEG2 and (red (red )nm)/Abs(Soret) Zn-Por-(Py )nm)/Abs(Soret) Py 2-TMEG1; 2G1)2;(D) (red G1) 2; (C) (black )Abs(344 Pynm)/Abs(Soret) 4-TMEG2 and (red (C) ( (black Zn-Py 4-TMEG2 and (red Zn-Py 22-TMEG1; (E,F) plot ofAbs(344 Abs(344 (black ) Zn-Py 4-TMEG2 (red) Zn-Por-(Py )figures Zn-Py -TMEG1; (E,F) plot of fluorescence spectrum of 1-pyrenebutanol ( and )and is shown with its intensity reported (black )) Zn-Py 4-TMEG2 (red ))all Zn-Py 2-TMEG1; (E,F) plot of Abs(344 nm)/Abs(Soret) (black ) Zn-Py 4-TMEG2 )in Zn-Py 2-TMEG1; (E,F) plot of fluorescence spectrum of 1-pyrenebutanol ( ( and(red )) is allfigures figures with intensity reported fluorescence spectrum of 1-pyrenebutanol is shown shown in all its its intensity reported (red ) Por-(Py2G1) 2, and (blue ) Por-(Py 2G1)1; (B) (black ) Zn-Por-(Py 2G1) 4with and (red ) Zn-Por-(Py 2G1)2; (C) (black ) Py 4-TMEG2 and (red ) Zn-Por-(Py 2side. G1) 2;units (C) (black )construct Py -TMEG2 and (red )G., Py 2)-TMEG1; (D)plotand (black )pyrenyl Zn-Py (black Zn-Py 4-TMEG2 and (red ) 4Zn-Py 2-TMEG1; (E,F) plot of Abs(344 nm)/Abs(Soret) 4-TMEG2 (r versus thenumber number of pyrenyl per porphyrinic construct forthe thefree free andmetallated metallated porphyrins, versus number of pyrenyl units per porphyrinic construct for the free and metallated porphyrins, on the secondary axis on the right hand Reprinted with permission from (Zaragoza-Galán, (black ) Zn-Py 4-TMEG2 and (red )metallated Zn-Py 2-TMEG1; (E,F) of Ab versus the number pyrenyl units per porphyrinic construct for the free and porphyrins, versus the ofof units per porphyrinic for and porphyrins, on theonsecondary axis on on thethe right hand Reprinted with permission from (Zaragoza-Galán, the secondary axis right handside. side. Reprinted with permission from (Zaragoza-Galán, G., G., (red ) Zn-Por-(Py 2G1)2;M., (C)Rein, (black ) Py 4number -TMEG2 and (red ) Py 2-TMEG1; (D) (black ) Zn-Py 4-TMEG2 and (red ) Zn-Py 2pyrenyl -TMEG1; (E,F) versus the of pyrenyl units per porphyrinic construct for the versus free and the number metallated of porphyrins, units Fowler, R., Solladié, N., Duhamel, J., & Rivera, E. (2014). Fluorescence Resonance Energy respectively. Reprinted with permission from (Zaragoza-Galán, G., Fowler, M., Rein, R., Solladié, N., respectively. Reprinted with permission from (Zaragoza-Galán, G., Fowler, M., Rein, R., Solladié, N., (black ) Zn-Py 4Duhamel, -TMEG2 and ) Zn-Py 2-TMEG1; (E,F) plot M., ofM., Abs(344 nm)/Abs(Soret) versus thefrom number of pyrenyl units per porphyrinic construct for the free and mp respectively. Reprinted withpermission permission from (Zaragoza-Galán, G., Fowler, Rein, R., Solladié, N., respectively. Reprinted with G., Fowler, Rein, R., Solladié, N., Fowler, M., Rein, R., Solladié, N.,N., Duhamel, J., Rivera, E.(Zaragoza-Galán, (2014). Fluorescence Resonance Energy Fowler, M., Rein, R., Solladié, J.,& & (red Rivera, E. (2014). Fluorescence Resonance Energy versus the number of pyrenyl units per porphyrinic construct for the Transfer in Partially and Fully Labelled Pyrene Dendronized Porphyrins Studied with Model Free respectively. Reprinted with permission from (Zaragoza-Galán, G., respectively. Fowler, M., Rein, Reprinted R., Solladié, with permiss N., Duhamel, J., & Rivera, E. (2014). Fluorescence Resonance Energy Transfer in Partially and Fully Duhamel, J., & Rivera, E. (2014). Fluorescence Resonance Energy Transfer in Partially and Fully (black ) Zn-Py4-TMEG2 and (red )J., Zn-Py 2Labelled -TMEG1; (E,F) plot of Abs(344 nm)/Abs(Soret) the number of pyrenyl units per porphyrinic construct for the free and metallated respectively. Reprinted withStudied permission from (Zaragoza-Galán, G., Fowler, M., fR Duhamel, J., &Rivera, Rivera, (2014). Fluorescence Resonance Energy Transfer inPartially Partially andFully Fully Duhamel, &Fully E.E.(2014). Fluorescence Resonance Energy Transfer in and Transfer inversus Partially and Labelled Pyrene Dendronized Porphyrins with Model Free Transfer in Partially and Fully Pyrene Dendronized Porphyrins Studied with Model Free porphyrins, Analysis. The Journal of Physical Chemistry C, 118(16), 8280–8294). Copyright (2014) American Chemical respectively. Reprinted with permission from (Zaragoza-Galán, G., Duhamel, J., & Rivera, E. (2014). Fluorescence Resonance Energy Duhamel, Transfer J., in & Partially Rivera, and E. (2014). Fully Fl Labelled Pyrene Dendronized Porphyrins Studied with Model Free Analysis. The Journal of Physical Labelled Pyrene Dendronized Porphyrins Studied with Model Free Analysis. The Journal of Physical versus the number of pyrenyl units per porphyrinic construct for the free and metallated porphyrins, respectively. Reprinted with permission from G., Fowler, M., Rein, R., Solladié, N., Duhamel, J.,Studied & (Zaragoza-Galán, Rivera, E. (2014). Fluorescence Resonance Transfer Fo in Labelled Pyrene Dendronized Porphyrins Studied with Model Free Analysis. The Journal ofPhysical Physical Labelled Pyrene Dendronized Porphyrins with Model Free Analysis. The Journal ofEnergy [66]. Chemistry Duhamel, J., American & Rivera, E. (2014). Fluorescence Resonance Energy T Labelled Dendronized Porphyrins Studied with Model Free Labelled Analysis. Pyrene The Journal Dendronized of Analysis. Physical Porph Chemistry C, 118(16), 8280–8294). Copyright (2014) American Chemical Society [66]. Chemistry C, 118(16), 8280–8294). Copyright (2014) Chemical Society [66]. respectively. ReprintedSociety with permission from (Zaragoza-Galán, Fowler, M., Rein, R.,Resonance Solladié, N., Duhamel, J., & Pyrene Rivera, E.G., (2014). Fluorescence Energy Transfer in Partially andFree Fully Labelled Pyrene Dendronized Porphyrins Studied with Model T Chemistry 118(16), 8280–8294). Copyright (2014) American Chemical Society [66]. C,C, 118(16), 8280–8294). Copyright (2014) American Chemical Society [66]. Labelled Pyrene Dendronized Porphyrins Studied with Model Free Chemistry C, 118(16), 8280–8294). Copyright (2014) American Chemical Chemistry Society C, [66]. 118(16), 8280–8294). Cop Duhamel, J., & Rivera, E. (2014). Fluorescence Resonance Energy Chemistry Transfer in 118(16), Partially8280–8294). and Model Fully Copyright Labelled Pyrene Dendronized Porphyrins Studied with Free Analysis. Journal ofChemical Physical Society [A C, (2014)The American Aslight slight shift ofthe the absorption band ofthe the porphyrin moiety was observed inthe the series ofthe the AStudied slight shift of the absorption band of the porphyrin moiety was observed in American the series of the Chemistry C, 118(16), 8280–8294). Copyright (2014) Chemica A slight shift the absorption band of the porphyrin moiety was observed in the series the Labelled Pyrene Dendronized Porphyrins with Model Free Analysis. The Journal of Physical AChemistry shift ofof absorption band of porphyrin moiety was observed series ofof C, 118(16), 8280–8294). Copyright (2014) American Chemical Society [66]. in Por-(Py2G1)(red 3, (red Por-(Py 22G1) 22, , Figure and (blue )) Por-(Py Por-(Py 2G1) ;1;of (B) (black ) ) Zn-Por-(Py spectra (A) (black) ))Zn-Por-(Py ) Por-(Py 24G1) (green Por-(Py 2G1) 2, and (blue Por-(Py 2G1) 1; 1(B) (black Zn-Por-(Py 2G1) and )) Por-(Py G1) and (blue 2G1) (B) (black 2G1) 4 4,and #5.PyAbsorption /) Por # Py / Por (red(red ) ))Por-(Py Por-(Py 2G1) , and (blue ))Por-(Py Por-(Py 2G1) (B) (black 2G1) and (red 2G1) 2, 2and (blue 2G1) 1; 1;(B) (black 2G1) 4 4and Zn-Por-(Py 2G1) 4 and (red Zn-Por-(Py 2G1)2;; (C) 4-TMEG2 and Zn-Por-(Py )) Zn-Por-(Py (C) (black (black ) Py -TMEG2 and (red (red Zn-Por-(Py ))

Adendrimers slight shift in of the absorption band of porphyrin moiety A was slight observed shift the inconstruct. the absorption series of bt porphyrinic dendrimers in function ofthe the amount ofmesityl mesityl groups present in theof construct. The porphyrinic dendrimers in function of the of mesityl groups present in the The A slight shift ofamount thethe band of the porphyrin moiety was obser Chemistry C, 118(16), 8280–8294). Copyright (2014) American Chemical Society [66]. porphyrinic dendrimers in function the amount of mesityl groups present the construct. The porphyrinic function ofof amount ofabsorption groups present inin the construct. The

Aof slight shift the absorption band of the porphyrin moiety porphyrinic dendrimers in function of thefrom amount of mesityl groups present dendrimers in going the in construct. function Soret band ofshift thethe free porphyrins shifted from 421 to420, 420, 419, and 418418 nm when going from Soret band of free porphyrins shifted to 420, 419, and when going fromTow A slight offree the absorption band the porphyrin moiety was observed in the series offrom the porphyrinic dendrimers in421 function ofporphyrinic the amount ofnm mesityl groups presen Soret band the porphyrins shifted from 421 to 420, 419, and 418 nm when going Soret band ofof the free porphyrins shifted from 421 toof 419, and 418 nm when from porphyrinic in function of the amount ofband mesityl Soret band of Por-(Py the2G1) shifted from 421 to 420, Soret 419, band and of 418 the free when porphyrins going fro s Por-(Py 2G1) 4dendrimers to Por-(Py 2free G1) 3,porphyrins Por-(Py 2G1) 2, and Por-(Py 2G1). Likewise, the Soret band of thegrou Por-(Py 2the G1) 4 to 2function G1) 3,was Por-(Py 2the G1) 2,indendrimers and Por-(Py 2G1). Likewise, the Soret of the A slight shift of the absorption band porphyrin moiety observed the of the porphyrinic of ofseries mesityl groups present in the construct. The Soret band of free porphyrins shifted from 421 tonm 420, 419, and 418 Por-(Py 2of G1) to Por-(Py 2in G1) Por-(Py 2the G1) 2,and and Por-(Py 2G1). Likewise, the Soret band the Por-(Py 2G1) 4 4to Por-(Py 3, 3,Por-(Py 2G1) 2,amount Por-(Py 2G1). Likewise, the Soret band ofof the Soret band of the free porphyrins shifted from 421 to 420, 419, Por-(Py Por-(Py 2 G1) 4 to Por-(Py 2 G1) 3 , Por-(Py 2 G1) 2 , and Por-(Py 2 G1). Likewise, 2 G1) 4 to the Por-(Py Soret 2 G1) band 3 , Por-(P of metalated porphyrins shifted from 427 to 425 425 nm when going from Zn-Por-(Py 2G1) 4 to metalated shifted from to 425 when going from Zn-Por-(Py 2G1) 4 tott rphyrinic dendrimers in function of the amount mesityl groups present in the construct. The Soret band of porphyrins theoffree porphyrins shifted from 421 to3nm 420, 419, and 418 nm when going Por-(Py 2G1) 4 427 to427 Por-(Py 2nm G1) , when Por-(Py 2G1) 2, from and Por-(Py 2G1). Likewise, metalated porphyrins shifted from to 425 nm when going from Zn-Por-(Py 2G1) 4 toto metalated porphyrins shifted from 427 to going Zn-Por-(Py 2G1) 4 from

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Table 2. Förster radii (R0 ), pyrene monomer quantum yield (ϕPy ), and porphyrin quantum yield (ϕPor ) obtained by exciting the solutions at 344 nm. Reprinted with permission from (Zaragoza-Galán, G., Fowler, M., Rein, R., Solladié, N., Duhamel, J., & Rivera, E. (2014). Fluorescence Resonance Energy Transfer in Partially and Fully Labelled Pyrene Dendronized Porphyrins Studied with Model Free Analysis. The Journal of Physical Chemistry C, 118(16), 8280–8294). Copyright (2014) American Chemical Society [66]. Without Zn

With Zn

Compound

R0 (Å)

ϕPy (×104 )

ϕPor (×104 )

R0 (Å)

ϕPy (×104 )

ϕPor (×104 )

Por-(Py2 G1)4 Por-(Py2 G1)3 Por-(Py2 G1)2 Por-(Py2 G1)1 Py2 -TMEG1 Py4 -TMEG2

51.4 51.8 51.5 51.8 52.0 52.0

6 16 12 25 13 18

15 15 15 15 14 14

48.3

1

27

48.5

2

34

48.9 49.0

12 23

39 38

Table 3. FRET efficiency and dPor −Py EXT for porphyrinic constructs. Reprinted with permission from (Zaragoza-Galán, G., Fowler, M., Rein, R., Solladié, N., Duhamel, J., & Rivera, E. (2014). Fluorescence Resonance Energy Transfer in Partially and Fully Labelled Pyrene Dendronized Porphyrins Studied with Model Free Analysis. The Journal of Physical Chemistry C, 118(16), 8280–8294). Copyright (2014) American Chemical Society [66]. Without Zn Compound

EFRET (SS)

EFRET (SPC)

Por-(Py2 G1)4 Por-(Py2 G1)3 Por-(Py2 G1)2 Por-(Py2 G1)1 Py2 -TMEG1 Py4 -TMEG2

0.999 0.997 0.998 0.995 0.997 0.998

1.000 1.000 1.000 1.000 0.998 0.997

With Zn dPor–Py

EXT

18.2 18.2 18.2 18.2 30.2 34.9

(Å)

EFRET (SS)

EFRET (SPC)

dPor–Py EXT (Å)

1.000

1.000

1.000

0.999

0.998 0.996

0.998 0.996

17.8 17.8 17.8 17.8 29.9 34.7

This conclusion was verified experimentally by time-resolved fluorescence experiments. The fluorescence decays of the pyrene monomer and porphyrin in compounds Py2 -TMEG1 and Py4 -TMEG2 were analyzed applying the model free analysis (MFA) [66]. In these constructs an excited pyrene units transfers their excess energy so efficiently to the porphyrin core that it is deactivated before having the possibility to interact with a ground-state pyrene to give an excimer. Therefore, it can be deduced that the presence of an excimer emission band in the recorded spectra (Figure 6) for the porphyrin-cored dendritic structures is due to the small amount of impurities of pyrene derivatives. 3. Dendritic Molecules Bearing Peripheral Pyrene Groups as Donors and a Fullerene as Acceptor Recently, interest for materials based on fullerene C60 derivatives has increased since this type of materials has promising properties for the development of new materials for technological applications. Particularly, performances of fullerene-based compounds have been reported for solar energy conversion [68,69]. After this discovery [70], many strategies to functionalize the fullerene C60 were proposed [71–75] in order to generate new kinds of materials. The reason for this interest is due to its chemical and physical proprieties, such as for example its absorption in the UV region of the electromagnetic spectra [76]. The main drawback of the fullerene C60 is that it is poorly soluble in organic solvents [77,78]. Fullerene C60 can be synthetically modified in order to increase its solubility without affecting its photophysical and electrochemical properties [76,79]. Fullerene C60 derivatives possess a large variety of applications that are ranging from chemosensors [80–83] to biological probes [84–86] and photocatalyst [87–90]. A large number fullerene based dyads have been published, however, there are only a few reports about pyrene-fullerene C60 derivatives [91–94].

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probes [84–86] and photocatalyst [87–90]. A large number fullerene based dyads have been published, however, there are only a few reports about pyrene-fullerene C60 derivatives [91–94]. We have have reported reported aa new new series series of dyads, and and we we have have studied studied their their optical optical We of pyrene-fullerene pyrene-fullerene C C60 60 dyads, and photophysical properties (Figure 7) [48]. and photophysical properties (Figure 7) [48].

Figure 60 [48]. Figure 7. Structures Structures of of the the novel compounds bearing pyrene and Fullerene C60 [48].

UV–VIS analysis analysis for for the the pyrene-fullerene pyrene-fullereneCC dyadswas was performed toluene (Figure UV–VIS 6060 dyads performed in in toluene (Figure 8). 8). It It showed that series exhibited characteristic bands of pyrene the pyrene chromophore at nm 346due nm showed that thethe series exhibited the the characteristic bands of the chromophore at 346 due S2 transition. No remarkable change was observed fortransition this transition in comparison to S0 to → SS02 → transition. No remarkable change was observed for this in comparison with with 1-pyrenebutanol, the compound as model thisexperiment. experiment.Moreover, Moreover,for for all all the the 1-pyrenebutanol, the compound usedused as model in in this pyrene-fullerene C60 dyadsthe theabsorption absorptionband bandat at330 330 nm nm was was more more intense intense that that the the band band observed observed pyrene-fullerene 60 dyads in the the spectra spectra of of 1-pyrenebutanol 1-pyrenebutanol at at the the same same wavelength wavelength and and the the absorption absorption was was extended extended in in the the in visible region. region. Those Thosetwo twofeatures featureswere wereattributed attributedtotothe thefullerene fullereneCC cage. case PyFPy, visible 60 60 cage. In In thethe case of of PyFPy, a a strong band followed another intense band at 346 observed. The one firstwas one followed by by another intense band at 346 nm nm werewere observed. The first strong band at at 330330 nmnm was attributed to C the C60 cage second pyrene absorption.Contrarily, Contrarily,ininthe thecase case of of attributed to the 60 cage andand the the second oneone to to thethe pyrene absorption. PyFC12 andPy Py22FC FC1212the thepyrene pyrenebands bandsare aremore moreresolved resolveddue dueto tothe thehigher higherconcentration concentration of of pyrene pyrene in in PyFC 12and the molecules. the molecules.

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Figure compounds [48]. Figure 8. 8. Absorption Absorption spectra spectra of of pyrene-fullerene pyrene-fullerene C C60 60 compounds [48]. Figure 8. Absorption spectra of pyrene-fullerene C60 compounds [48].

The fluorescence spectra of the compounds show the typical emission of pyrene monomer at The fluorescence spectra of the compounds show the typical emission of pyrene monomer at The fluorescence spectra the compounds showPyFPy, the typical of pyrene monomer at to form 376 nm as illustrated in Figure 9. ofOnly compounds Pyemission FC12 and Py2NF are able 376 nm as376 illustrated in Figure 9. Only compounds PyFPy, Py22FC12 and Py2NF are able to form nm as illustrated in Figure 9. Only compounds PyFPy, Py 2FC12 and Py2NF are able to form excimers due to the presence of two pyrene units in the structures. In the case of PyFPy, the excimer excimers due to the of two pyrene thestructures. structures. Incase theofcase of the PyFPy, the excimer excimers duepresence to the presence of two pyreneunits units in in the In the PyFPy, excimer formation is blocked by the presence of the C60 cage that obstruct rapprochement of both pyrene is blocked the presence theCC6060 cage cage that rapprochement of both pyrene formationformation is blocked by thebypresence of ofthe thatobstruct obstruct rapprochement of both pyrene units and units in the spectra, therethere are no excimer emission (Figure 9). other On the other hand, and in the spectra, are signal no signalfor forthe the excimer emission (Figure 9). On the hand, units and in the spectra, there are no signal for the excimer emission (Figure 9). On the other hand, compounds 2FC12 Py and2 NF Py2NF presentthe the two two pyrene units that that are free the fullerene steric compounds Py2 FC12Pyand present pyrene units areoffree of the fullerene steric compounds Py2FCand, 12 and Py2NF present the two pyrene units that are free of the fullerene steric influence the excimer formation is possible. This behaviorisissupported supported by influence and, therefore,therefore, the excimer formation is possible. This behavior by the the presence influence presence and, therefore, theband excimer formation of the excimer at 470 nm (Figure 9). is possible. This behavior is supported by the of the excimer band at 470 nm (Figure 9). presence of the excimer band at 470 nm (Figure 9).

Figure 9. Emission spectra of pyrene-fullerene C60 compounds [48].

The relative quantum yield was calculated using 1-pyrenebutanol as standard. The obtained values are shown in Table 4. The emission of the malonate precursors that are not containing a

Figure 9. 9. Emission Emission spectra spectra of of pyrene-fullerene pyrene-fullerene C C60 compounds [48]. Figure 60 compounds [48].

was calculated calculated using using 1-pyrenebutanol 1-pyrenebutanol as as standard. standard. The obtained The relative quantum yield was values are shown in Table 4. The emission of the malonate precursors that are not containing values are shown in Table 4. The emission of the malonate precursors that are not containing a fullerenea unit was much higher than the emission of the pyrene-fullerene constructs. Low quantum yield values obtained for the pyrene-fullerene C60 constructs and the corresponding values of quenching corroborated that efficient FRET between pyrene and fullerene C60 chromophores is taking place [48].

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Table 4. Relative quantum yield and FRET efficiencies of the pyrene-fullerene C60 compounds [48]. Compound

Relative Quantum Yield a

% Quenching

1-Pyrenbutanol PyFC12 PyFPy Py2 FC12 Py2 NF

1 (Donor) 0.01 (Dyad) 0.01 (Dyad) (Dyad) 0.04 (Dyad)

99% 99% 99% 96%

a

Measured in degassed toluene solution exciting at λ = 344 nm. Relative QY of the dyads, with respect to the donor molecules (1-pyrenebutanol and PyMPy in the range of 360 nm–560 nm.

4. Dendritic Molecules Bearing Peripheral Pyrene Groups as Donors and an Organometallic Complex as Acceptor It is of interest to include metal ions in the structure of dendrimers in order to obtain complex molecules combining the properties of both entities; the structural ordered properties from the dendritic shell and the redox properties from metal ions [95,96]. In the literature, many examples of metal ions incorporated in dendrimers have been reported with different applications, such as drug carrier, catalysts, and enzyme mimics [97,98]. In some cases, the resulting organometallic complexes are able to accept energy transfer from a donor moiety [99–101]. The specific case of the incorporation of ruthenium bipyridine complexes into photoactive dendrimer has been studied [102–105]. Ruthenium bypiridine complexes have attracted the attention of many researcher due to chemical stability, redox properties, as well as fluorescence emission and excited state lifetime [106]. The complexes of ruthenium with bipyridine ligands are presenting typical UV–VIS spectra. The first band observed at about 280 nm corresponds to the ligand centred (LC) transition and the second band observed in a range from 450 to 480 nm corresponds to the metal to ligand charge transfer (MLCT) band. The long-lived emission occurs from the 3 MLCT state with low quantum yield [107,108]. Ruthenium bipyridine complexes are able to accept energy transfer from fluorescent donors through FRET [109]. Moreover, one application of this type of compound include the field of dye-sensitized solar cells [110,111]. Coordination complexes of ruthenium incorporating pyrene units have been reported [112]. Moreover, ruthenium bipyridine complex covalently linked to pyrene units have shown prolonged emission lifetime due to the fact that the energy levels of the 3 MLCT state of the ruthenium complex and of the pyrene triplet state can be tuned to almost match each other. Therefore, we used our pyrene-based dendrimer to design new organometallic complexes of ruthenium. The benzylic core of our dendrimer was modified to a bipyridine core in order to obtain a suitable ligand to form ruthenium complexes [49]. We designed and synthesized new organometallic complexes bearing 2, 6, and 12 pyrene units in the periphery and a bipyridine core ([Ru(Bpy)2 (Bpy-Py2)]2+ , [Ru(Bpy-Py2)3 ]2+ and [Ru(BpyG1-Py4)3 ]2+ ) (Figures 10 and 11). The obtained compounds showed absorption spectra that are reflecting the absorption properties of a pyrene moiety and of the corresponding bipyridine complex measured separately (Figure 12). This is an indication that, when the compounds are present in the ground state, both chromophores are not interacting. The absorption band of pyrene is observed at 344 nm for the S0 → S2 transition for the three complexes. The absorption bands of the ruthenium bipyridine complexes depend on the substitution of the bipyridine ligand. In the case of [Ru(Bpy)2 (Bpy-Py2)]2+ , the MLCT band was observed at 462 nm. A red shift to 480 nm is observed for the other two complexes [Ru(Bpy-Py2)3 ]2+ and [Ru(BpyG1-Py4)3 ]2+ . This is due to the stabilization effect of the oxygen atoms that are directly linked to the bipyridine moieties. The corresponding extinction coefficient are also corresponding to the respective components of the complexes. The absorption band of pyrene presents a high extinction coefficient and increases proportionally with the increasing number of pyrene units. The extinction coefficient of the MLCT transition band presents values of about 12,000 M−1 cm−1 which is comparable to the reported values for MLCT transitions of ruthenium bipyridine complexes.

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complexes. The absorption band of pyrene presents a high extinction coefficient and increases proportionally the increasing number pyrene units. Theextinction extinctioncoefficient coefficientand of the MLCT complexes. Thewith absorption band of pyreneof presents a high increases −1 cm−1 which is comparable to the reported values transition band presents values of about 12,000 M proportionally with the increasing number of pyrene units. The extinction coefficient of the MLCT for MLCT transitions of ruthenium bipyridine transition band presents values of about 12,000complexes. M−1 cm−1 which is comparable to the reported values Polymers 2018, 10, 1062 13 of 24 for MLCT transitions of ruthenium bipyridine complexes.

Figure 10. Structures of the ruthenium bipyridine complexes containing two and six pyrene units [49]. 10. Figure 10. Structures Structuresofofthe theruthenium ruthenium bipyridine complexes containing six pyrene Figure bipyridine complexes containing twotwo andand six pyrene unitsunits [49]. [49].

Figure ruthenium bipyridine bipyridine complex complex possessing possessing 12 12 pyrene pyrene units units [49]. [49]. Figure 11. 11. Structure Structure of of the the dendritic dendritic ruthenium Figure 11. Structure of the dendritic ruthenium bipyridine complex possessing 12 pyrene units [49].

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Molar Extinction Coeff. (M Molar cm ) Extinction Coeff. (M-1 cm-1)

500000

10000 20000

-1

300000

15000

5000 15000

-1

-1

400000 500000

14 of 24 14 of 25

20000

600000

-1

Molar Extinction (M Coeff. cm ) (M-1 cm-1) MolarCoeff. Extinction

600000 Polymers 2018, 10, 1062 Polymers 2018, 10, x FOR PEER REVIEW

400000 200000

0 10000

400

450

500

550

Wavelength (nm)

5000

300000 0 400

100000

450

500

550

Wavelength (nm)

200000 0 100000 250

300

350

400

450

500

550

Wavelength (nm) 0

300 trisbipyridine 350 400 complexes 450 500 Figure 12. Absorption spectra250of Ru(II) [Ru(Bpy) 2550 (Bpy-Py2)]2+ (black) in Wavelength (nm) 2+ 2+ acetonitrile and [Ru(Bpy-Py2)3] (blue) and [Ru(BpyG1-Py4)3] (red) in THF. “Reprinted from 2+ Rivera, Polymers, 99, Vonlanthen, M.; of Cevallos-Vallejo, A.; Aguilar-Ortíz, E.; Ruiu,2A.; Porcu, P.; E., Figure 12. Absorption spectra Ru(II) trisbipyridine complexes [Ru(Bpy) (Bpy-Py2)] (black) in Figure 12. Absorption spectra of Ru(II) trisbipyridine complexes [Ru(Bpy)2(Bpy-Py2)]2+ (black) in 2+ 2+ Synthesis, characterization and photophysical studies of novel pyrene labelled ruthenium (II) acetonitrile and [Ru(Bpy-Py2)3 ] 2+ (blue) and [Ru(BpyG1-Py4)3 ] 2+ (red) in THF. “Reprinted from acetonitrile and [Ru(Bpy-Py2)3] (blue) and [Ru(BpyG1-Py4)3] (red) in THF. “Reprinted from trisbipyridine complex cored 13–20,A.;Copyright (2016),E.;with permission Elsevier” Polymers, 99, Vonlanthen, M.; dendrimers, Cevallos-Vallejo, Aguilar-Ortíz, Ruiu, A.; Porcu,from P.; Rivera, E., Polymers, 99, Vonlanthen, M.; Cevallos-Vallejo, A.; Aguilar-Ortíz, E.; Ruiu, A.; Porcu, P.; Rivera, E., [49]. Synthesis, characterization and photophysical studies of novel pyrene labelled ruthenium (II) Synthesis, characterization and photophysical studies of novel pyrene labelled ruthenium (II) trisbipyridine complex cored dendrimers, 13–20, Copyright (2016), with permission from Elsevier” [49]. trisbipyridine complex cored dendrimers, 13–20, Copyright (2016), with permission from Elsevier” The obtained compounds present very efficient energy transfer from the pyrene moiety to the [49]. obtained compounds present very efficient energy transfer from the pyrene moiety to the The

metal complex core. When they are excited at the maximum of absorption of the pyrene unit at 344 metal complex core.emission When they arethe excited at the maximum of absorption the pyrenecompared unit at 344tonm, nm, weak residual from pyrene units is observed but clearlyofquenched the The obtained compounds present very efficient energy transfer from the pyrene moiety to the weak emission from Only the pyrene is observed but clearly to the free free residual bipyridine ligands. 2% units of residual emission of quenched pyrene compared was observed for metal complex core. When they are excited at the maximum of absorption of the pyrene unit at 344 2+, 1% bipyridine ligands. Only 2% of emission3]2+ of, pyrene for [Ru(Bpy) (Bpy-Py2)]2+ [Ru(Bpy)2(Bpy-Py2)] forresidual [Ru(Bpy-Py2) and 4%was forobserved [Ru(BpyG1-Py4) 3]2+ 2indicating an, nm, weak residual emission from the pyrene units is observed but clearly quenched compared to the 2+ 2+ 1% for [Ru(Bpy-Py2) and for For [Ru(BpyG1-Py4) indicating an efficient energy transfer in all efficient energy transfer all 4% cases. the three compounds, emission is observed in the red part of 3 ] ,in 3] free bipyridine ligands. Only 2% of residual emission of pyrene was observed for cases. For the three compounds, emission is observed in the red part of the spectrum between 600 and the spectrum between 600 and 700 nm corresponding to the typical emission of ruthenium [Ru(Bpy)2(Bpy-Py2)]2+, 1% for [Ru(Bpy-Py2)3]2+, and 4% for [Ru(BpyG1-Py4)3]2+ indicating an 700 nm corresponding the typical of ruthenium complexes (Figure As for bipyridine complexes to(Figure 13). emission As for the porphyrin bipyridine dendrimers described in the13). previous efficient energy transfer in all cases. For the three compounds, emission is observed in the red part of the porphyrin dendrimers in the previous section, thedonor quenching of theindication fluorescence the section, the quenching ofdescribed the fluorescence from the pyrene is a good of from efficient the spectrum between 600 and 700 nm corresponding to the typical emission of ruthenium pyrene donor isItaisgood indication of efficient FRET process. It nm is important to noteofthat excited FRET process. important to note that when excited at 344 the percentage lightwhen absorbed by bipyridine complexes (Figure 13). As for the porphyrin dendrimers described in the previous at 344 nm the percentage of light absorbed by the pyrene units increases with the increasing amount the pyrene units increases with the increasing amount of pyrene units from 91% for section, the quenching of the fluorescence from the2+pyrene donor is a good indication of efficient of pyrene2(Bpy-Py2)] units from 2+91% to 97for andcompounds 99%, respectively, for compounds [Ru(Bpy) to for 97[Ru(Bpy) and 99%, respectively, [Ru(Bpy-Py2) 3]2+ and 2 (Bpy-Py2)] FRET process. It2+is 2+important to note that 2+ when excited at 344 nm the percentage of light absorbed by [Ru(Bpy-Py2) [Ru(BpyG1-Py4) (Table 5). [Ru(BpyG1-Py4) (Table 5). 3 ] 3]and 3] the pyrene units increases with the increasing amount of pyrene units from 91% for [Ru(Bpy)2(Bpy-Py2)]2+ to 0.00030 97 and 99%, respectively, for compounds [Ru(Bpy-Py2)3]2+ and 2+ [Ru(BpyG1-Py4)3] (Table 5). Fluorescence Emission Emission Fluorescence

0.00025 0.00030 0.00020 0.00025 0.00015 0.00020 0.00010 0.00015 0.00005 0.00010 0.00000 0.00005

400

500

600

700

Wavelength (nm) 0.00000 2+ 2+ Figure spectra of the the Ru(II) Ru(II) trisbipyridine complexes [Ru(Bpy) (black) Figure 13. 13. Emission spectra of trisbipyridine complexes [Ru(Bpy) 2(Bpy-Py2)] (black) in 2 (Bpy-Py2)] 400 500 600 700 2+ 2+ in acetonitrileand and[Ru(Bpy-Py2) [Ru(Bpy-Py2) (blue)and and[Ru(BpyG1-Py4) [Ru(BpyG1-Py4) ] (red) (red)inin THF. THF. “Reprinted Wavelength (nm) 3]32+] (blue) 33 ]2+ “Reprinted from from acetonitrile Polymers, Polymers, 99, 99, Vonlanthen, Vonlanthen, M.; M.; Cevallos-Vallejo, Cevallos-Vallejo, A.; A.; Aguilar-Ortíz, Aguilar-Ortíz, E.; Ruiu, A.; Porcu, P.; Rivera, E., E., Figure 13. Emission spectra and of thephotophysical Ru(II) trisbipyridine complexes [Ru(Bpy) 2(Bpy-Py2)]2+ (black) in Synthesis, characterization studies Synthesis, characterization and photophysical studies of of novel novel pyrene pyrene labelled labelled ruthenium ruthenium (II) (II) 3]2+ (blue) and [Ru(BpyG1-Py4)3]2+ (red) in THF. “Reprinted from acetonitrile and [Ru(Bpy-Py2) trisbipyridine dendrimers, 13–20, Copyright (2016), withwith permission fromfrom Elsevier” [49]. trisbipyridinecomplex complexcored cored dendrimers, 13–20, Copyright (2016), permission Elsevier” Polymers, 99, Vonlanthen, M.; Cevallos-Vallejo, A.; Aguilar-Ortíz, E.; Ruiu, A.; Porcu, P.; Rivera, E., [49]. Synthesis, characterization and photophysical studies of novel pyrene labelled ruthenium (II) trisbipyridine complex cored dendrimers, 13–20, Copyright (2016), with permission from Elsevier” [49].

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Table 5. Absorption coefficients, quantum yields, and FRET efficiencies of the ruthenium-pyrene constructs “Reprinted from Polymers, 99, Vonlanthen, M.; Cevallos-Vallejo, A.; Aguilar-Ortíz, E.; Ruiu, A.; Porcu, P.; Rivera, E., Synthesis, characterization and photophysical studies of novel pyrene labelled ruthenium (II) trisbipyridine complex cored dendrimers, 13–20, Copyright (2016), with permission from Elsevier” [49]. Compound

λmax abs (nm) ε(M−1 cm−1 )

λmax (nm)

Φ Pyrene unit λex = 344 nm c

Φ Ru (II) unit λex = 344 nm d

Φ Ru (II) unit λex = 452 nm d

[Ru(Bpy)2

342/69’000

625

-

0.0032 a

0.0052 a

b

0.0277 b

0.004 b

0.0016 a 0.0026 b

0.0028 a 0.0053 b

-

0.0025 a

0.0029 a

0.014 b

0.0031 b

0.0050 b

(Bpy-Py2)]2+

462/10’600

[Ru(Bpy-Py2)3 ]2+

344/210’900 480/11’400

[Ru(BpyG1-Py4)3 ]2+

344/411’000 480/11’200

0.008 661

661

b

0.0174

EFRET

e

0.98

0.99

0.96

Fluorescence measurements were done in acetonitrile ([Ru(Bpy)2 (Bpy-Py2)]2+ ) or THF ([Ru(Bpy-Py2)3 ]2+ and [Ru(BpyG1-Py4)3 ]2+ ) at 0.1 OD at 344 nm. a Aerated solution; b Deaerated solution; c Quantum yields were determined relative to quinine sulfate (Φ = 0.546) in 0.05 M sulfuric acid for pyrene units. d Quantum yields were determined relative to [Ru(Bpy)3 ]2+ (Φ = 0.016) in air-equilibrated acetonitrile solution [103]. e FRET efficiencies were calculated according to the following equation: FRET = 1 − (IDA /ID ).

Furthermore, the quenching effect of oxygen was evaluated through the measurements of quantum yield for the ruthenium bipyridine complexes. When excited at 344 nm in degased solutions, quantum yields values of 0.0174, 0.0026, and 0.0031 are obtained for [Ru(Bpy)2 (Bpy-Py2)]2+ , [Ru(Bpy-Py2)3 ]2+ , and [Ru(BpyG1-Py4)3 ]2+ , respectively. The same measurements were performed without degasing the solutions and the following quantum yields were obtained: 0.0032, 0.0016, and 0.0025 (Table 5). The difference between the quantum yield in degased solution and in air-equilibrated solutions is much lower for the complex bearing three times the first generation dendron (12 pyrene units) indicating that the dendrimer branches are functioning as a protection toward oxygen quenching. For the pyrene/ruthenium bipyridine complex pair of chromophores, we have calculated that the Förster radius is in the order of 3.30 nm. As previously calculated for the pyrene-porphyrin couple the expected distance between the chromophore is within the Förster distance, confirming an efficient FRET process. 5. Dendritic Molecules Bearing Peripheral Pyrene Groups as Donors and Cyclen Core as Potential Ligands for Metal Ions As mentioned above, the incorporation of different metals in photoactive dendrimers have been performed through the inclusion of porphyrin or bipyridine ligands in the structures and this has shown potential applications in sensors, catalysis, or nanomaterials for drug delivery [95,113]. Another efficient family of ligands for metal ions is the family of aza-macrocycles. Efficient chelating effects have been reported from this kind of ligand towards metal ions [114–119]. 1,4,7,10-teraazacyclododecane (cyclen) is part of this family and has exhibited a good coordination with a wide variety of metals. Moreover it has been included in a wide range of constructs for diverse applications, such as ion sensing, ion carrier, and metal diagnosis [120–124]. The coordination of cyclen construct containing various chromophores with lanthanides has also been reported, leading to sensitized emission from the photoactive metal ions [125,126]. Only a few reports of FRET from pyrene to lanthanide photoactive ions have been reported. Using the cyclen as core of our pyrene-based dendrimer, we could obtain a construct that is able to complex lanthanide ions and, consequently, a FRET process could be observed from pyrene to the photoactive metal ion [50]. The series of dendrimers of generation zero (1), one (2) and two (3) bearing 4, 8, and 16 pyrene units, respectively, was synthesized (Figures 14 and 15). The obtained compounds are showing the typical absorption properties of the pyrene chromophore, increasing its extinction coefficient linearly

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linearly with the amount of pyrene units in the construct. The emission properties16 of those Polymers 2018, 10, x FOR PEER REVIEW of 25 compounds wereof also studied. amount of excimer emissionproperties in those dendrimers is increasing as with the amount pyrene unitsThe in the construct. The emission of those compounds were linearly with the amount of pyrene the construct. emission properties of those expected with theamount increasing number ofunits pyrene units due to The the isincreased local concentration of also studied. The of excimer emission in those dendrimers increasing as expected with the compounds were also studied. The amount of excimer emission in those dendrimers is increasing as pyrene [50]. increasing number of pyrene units due to the increased local concentration of pyrene [50]. expected with the increasing number of pyrene units due to the increased local concentration of pyrene [50].

Figure 14. Structures cyclen-basedligands ligands containing and eight pyrene units [50]. [50]. Figure 14. Structures Structures containingfour four and eight pyrene units Figure 14. of of thethe cyclen-based

Figure Structures thecyclen-based cyclen-based ligand 1616 pyrene units [50].[50]. Figure 15. 15. Structures ofofthe ligandcontaining containing pyrene units

At the time of of incorporating metalions ionsinto into those dendrimers, we some faceddifficulties. some difficulties. At the time incorporating metal those dendrimers, we faced The ligands appeared toto bebe less effective foreseen towards the complexation of lanthanide ions. This ions. Figure 15. Structures of theas cyclen-based ligand containing 16 pyrene unitsof [50]. The ligands appeared less effective as foreseen towards the complexation lanthanide to the ligand only four atoms for the coordination of metal ions This may maybebedue due to fact the that factthe that the has ligand has nitrogen only four nitrogen atoms for the coordination and is, therefore, a teradentate ligand. Lanthanides are known to form hexato octa-coordinated At the time of incorporating metal ions into those dendrimers, we faced some difficulties. of metal ions and is, therefore, a teradentate ligand. Lanthanides are known to form hexa-The to complexes andto our may not be for such metal Therefore, new ligands including ligands appeared beligand less effective as suitable foreseen theions. complexation of lanthanide ions. This octa-coordinated complexes and our ligand may towards not be suitable for such metal ions. Therefore, new may be due to the carbonyl fact that the ligand hastoonly nitrogen atoms were for the coordination of metal ions ligands including group linked the four cyclen core (DOTA) designed and synthesized (4 and is, therefore, a teradentate ligand. Lanthanides are known to form hexato octa-coordinated and 5) (Figure 16) [127]. complexes and our ligand may not be suitable for such metal ions. Therefore, new ligands including

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Polymers 2018, 10,group 1062 linked to the cyclen core (DOTA) were designed and synthesized (4 and 5) (Figure 17 of 24 carbonyl

16) [127].

Figure 16. Structures DOTA-basedligands ligands containing and eight pyrene units [127]. Figure 16. Structures of of thethe DOTA-based containingfour four and eight pyrene units [127].

synthesis performedand and the the optical optical properties dendrimers werewere The The synthesis waswas performed propertiesofofthetheobtained obtained dendrimers studied. The obtained photophysical characterization is very similar to that of dendrimers 1 and studied. The obtained photophysical characterization is very similar to that of dendrimers2.1 and Titration experiments,followed followed by by fluorescence, outout with compounds 4 and in order 2. Titration experiments, fluorescence,were werecarried carried with compounds 4 5and 5 in order 3+, Eu3+, Gd3+, Tb3+, Er3+, and Zn2+ were tested and to evaluate their potential for FRET studies. Sm 3+ 3+ 3+ 3+ 3+ 2+ to evaluate their potential for FRET studies. Sm , Eu , Gd , Tb , Er , and Zn were tested and fluorescence quenching was observed for all metals except for Zn2+2+ . Figure 17 illustrates the titration fluorescence quenching was observed for all metals except for Zn . Figure 17 illustrates the titration of compound 4 with Gd3+ [127]. This result is a primary indication that the complexes are formed of compound 4 with transfer Gd3+ [127]. result is a primary indication that the complexes are formed and that energy couldThis occur. Chelation experiments with lanthanide ions are currently and that energy transfer could occur. Chelation experiments with lanthanide ions are currently being being performed in our laboratory. Polymers x FOR PEER REVIEW 18 of 25 performed in2018, our10,laboratory.

Figure 17. Titration of compound 4 with Gd3+ in chloroform [127].

6. Conclusions

Figure 17. Titration of compound 4 with Gd3+ in chloroform [127].

Conclusions In6. our interest to study pyrene dynamics in macromolecules, we have designed dendritic to studynumber pyrene dynamics macromolecules, designed dendritic our interest molecules In bearing an increasing of pyrenein peripheral groups we andhave various acceptor moieties molecules bearing an increasing number of pyrene peripheral groups and various acceptor moieties such as porphyrin, fullerene, or ruthenium bipyridine at the core. Those molecules were expected to function as light-harvesting antennas to funnel the energy absorbed from the pyrene groups to the acceptor moiety. Thus, we reported that the FRET process occurs in a very efficient manner in all the designed constructs. When the pyrene moiety was excited at 344 nm, the energy was transferred to the ground

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such as porphyrin, fullerene, or ruthenium bipyridine at the core. Those molecules were expected to function as light-harvesting antennas to funnel the energy absorbed from the pyrene groups to the acceptor moiety. Thus, we reported that the FRET process occurs in a very efficient manner in all the designed constructs. When the pyrene moiety was excited at 344 nm, the energy was transferred to the ground state porphyrin, fullerene, or ruthenium bipyridine complex. The quenching of fluorescence of pyrene is the first indication to determine the efficiency of the FRET process. It was quantified and information about pyrene dynamics was obtained. As a result, we could determine through MF analysis that the excimer formation was much slower than the FRET phenomenon. A value of 11.2 × 107 s–1 was found for the average rate constant of intramolecular pyrene excimer formation , which is much slower than the average rate constant for FRET (1.8 × 109 s–1 ). For this reason, we observed that the FRET process is more competitive than the excimer formation, it happens first, and the formation of excimer is precluded even in the case of higher-generation dendrons. Dendritic structures with the donor-acceptor pair pyrene-porphyrn exhibited FRET values between 97–99% with a drastic quenching of the pyrene emission and increased emission of the porphyrinic moiety. In the case of the pyrene-fullerene C60 constructs, FRET efficiency values were determined in a similar range, however, unlike for the case of porphyrins, we could not observe a final emission of the fullerene C60 acceptor group in the UV–VIS range. On the other hand, ruthenium bipyridine complexes exhibited FRET values in the order between 96–98%, showing an emission of the acceptor group after FRET at 661 nm. Finally dendritic constructs bearing pyrene units as peripheral groups and a modified cyclen unit as the core also showed the FRET process after metallation with lanthanides, which was monitored by fluorescence titrations. The quenching of the emission of pyrene could be observed after the addition of the different lanthanide ions. Author Contributions: P.P. and M.V.: writing—original draft preparation and writing—review and editing; A.R.: formal analysis; I.G.-M.: data curation; E.R.: supervision, project administration, funding acquisition, and writing—review and editing. Funding: We are grateful to CONACyT (Projects 253,155 and 279380) and PAPIIT-DGAPA (IN100316) for financial support. P.P. thanks Posgrado en Ciencias Químicas UNAM and CONACYT for scholarship and financial support. Acknowledgments: We thank Gerardo Cedillo (IIM-UNAM), Miguel Canseco (IIM-UNAM) and Carmen García-Javier Pérez (IQ-UNAM) for their assistance in the characterization of the compounds. Conflicts of Interest: The authors declare no conflict of interest.

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