zinc porphyrin complex in aqueous media

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range (20–49 °C) using KinetAsyst SF-61DX2 stopped-flow spectropho- tometer (dead ... Structure representative of the investigated TAIPDI and ZnTPPS4. 133 .... above considerations in a more quantitative way, giving kinetic data of ..... Balagurusamy, P.A. Heiney, I. Schnell, A. Rapp, H.W. Spiess, S.D. Hudson, H. Duan,.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 186 (2017) 132–139

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Spectroscopic and thermodynamic studies of light harvesting perylenediimide derivative - zinc porphyrin complex in aqueous media Ahmed El-Refaey, Shaban Y. Shaban, Maged El-Kemary, Mohamed E. El-Khouly ⁎ Department of Chemistry, Faculty of Science, Kafrelsheikh University, Kafrelsheikh 33516, Egypt

a r t i c l e

i n f o

Article history: Received 2 April 2017 Received in revised form 9 June 2017 Accepted 13 June 2017 Available online 15 June 2017 Keywords: Porphyrins Perylenediimide Electron transfer Artificial photosynthesis Supramolecular chemistry

a b s t r a c t Self-assembly of perylene derivative such as N,N′-bis(2(trimethylammonium iodide) ethylene)perylene3,4,9,10-tetracarboxyldiimide (TAIPDI) can produce one-dimensional form (1D) in an aqueous media. The ability of one-dimensional TAIPDI to form light harvesting complex with water-soluble zinc porphyrin (ZnTPPS4) via the π-π and electrostatic interactions has been described. Owing to electronic interactions between the π-systems, the complex formation is accompanied by pronounced absorption spectral changes in the UV/Vis absorption bands. The formation constant of the ZnTPPS4-TAIPDI complex has been determined as 2.60 × 104 M−1 suggests a moderately stable complex. The steady-state fluorescence measurements exhibited fluorescence quenching of both the singlet TAIPDI and ZnTPPS4 because of the electron transfer process from the electron-donating ZnTPPS4 to the electron-accepting TAIPDI. Based on the picosecond time-resolved fluorescence, the rate and quantum yield of the electron transfer were found to be 2.47 × 1010 s−1 and 0.99, respectively, indicating fast and efficient electron transfer. The thermodynamic parameters of the complex formation have been determined from the stopped-flow measurements. The interaction between ZnTPPS4 and TAIPDI occurs in two steps, a fast and reversible step followed by a slow and irreversible one. The activation parameters for the complex formation (ΔH# = 22 ± 5 kJ mol−1 and ΔS# = − 123 ± 18 J K−1 mol−1), (ΔH# = 133 ± 4 kJ mol−1 and ΔS# = 167 ± 13 J mol−1 K−1) were determined from variable temperature studies for the “on” and the “off” of the first step and ΔS# = 246 ± 37.89 J mol−1 K−1 and ΔH# = 130 ± 11 kJ mol−1 for the second step. The negative and positive ΔS# values found for the interaction reactions are consistent with an associative interaction for the first step followed by dissociative mechanism for both the “off” and the second step. © 2017 Published by Elsevier B.V.

1. Introduction Photoinduced electron transfer (PET) is of a great importance in the chemical and biological processes [1–12]. In the bacterial photosynthetic reaction centers, one can see that the photo- and redox-active components are arranged via non-covalent interactions into a protein matrix [13–28]. Among the utilized non-covalent binding modes for constructing light-harvesting complexes, one can find that the π-π and ionic interactions are widely used for their ease and simplicity of construction [29]. Among other classes of functional dyes, perylenediimide have been widely used as building block for constructing light-harvesting complexes that have ability to convert the light energy into chemical energy for their strong absorption in the visible region [30–33], high electronaccepting properties, long fluorescence lifetimes, photochemical and thermal stability, light bright photoluminescence with quantum yields up to unity, and excellent n-type semiconductivity [34–41]. Most of ⁎ Corresponding author. E-mail address: [email protected] (M.E. El-Khouly).

http://dx.doi.org/10.1016/j.saa.2017.06.016 1386-1425/© 2017 Published by Elsevier B.V.

the PDI derivatives are not water-soluble because of their large πplane of the aromatic core that facilitates intermolecular π-π interactions, and those that are soluble show a strong tendency to aggregate extensively in aqueous solution. In this study, we reported the characterization and photophysical properties of the cationic perylene derivative, namely N,N′-di(2(trimethylammonium iodide)ethylene)perylenediimide (TAIPDI) in different solvents. The nanostructure morphologies were controlled by varying solvents from water to methanol, where TAIPDI molecules undergo π-stacking in water, resulting in one-dimensional nanostructures. The ability of TAIPDI to form light harvesting donor-acceptor complex in an aqueous media has been also examined by combining it with water soluble zinc porphyrin derivative, namely zinc 25,10,15,20– tetraphenyl–21H, 23H porphyrin tetra-sulfonic groups (ZnTPPS4) as an electron donor (Fig. 1). Compared with other porphyrins, the studies of water-soluble porphyrins as electron donors in the artificial photosynthetic systems are rare in the literature [42–44]. Although there are several examples of molecular and supramolecular systems for porphyrin and perylenediimide derivatives [30–33,45, 46], the interaction between them via π-π and ionic interactions in an

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Fig. 1. Structure representative of the investigated TAIPDI and ZnTPPS4.

aqueous solution are quite rare in the literature. The major absorptions of TAIPDI (~520 nm) are complementary to those of porphyrins (~420 and 500–600 nm). Thus the resulting TAIPDI-ZnTPPS4 complex has an advantage of absorbing the light over a broad range in the visible region. An aqueous medium is of choice because it provokes the self-assembly of such highly extensive π-systems and facilitates the electron-transfer processes due to high polarity. Apart from solubilizing the TAIPDI stacks in water, cationic trimethylammonium heads also enhance the ionic interactions with water soluble ZnTPPS4, bearing anionic sulfonic groups. By this way, ionic interactions strongly support the hydrophobic interactions between the aromatic cores of TAIPDI and ZnTPPS4 to form TAIPDI-ZnTPPS4 complex. The characterization and kinetic studies of the complex formation between ZnTPPS4 and TAIPDI have been examined by various techniques including transmission electron microscopy, steady-state absorption and fluorescence spectroscopy, electrochemical, and stopped-flow techniques.

drop of dispersion on carbon-coated copper grids (250 mesh). Electrochemical measurements in water were performed on potentiostate/ Galvan state/ZRA-05087 electrochemical analyzer in water containing 0.10 M Na2SO4 as a supporting electrolyte at 298 K. The measurements were taken by using carbon electrode as working electrode, silver-silver chloride (Ag/AgCl) as reference electrode and platinium (Pt) as encounter electrode. All measurements were carried out in oxygen-free solutions. Stopped-flow kinetic measurements were carried out over the range (20–49 °C) using KinetAsyst SF-61DX2 stopped-flow spectrophotometer (dead time ~ 2 ms) with an optical path length of 1 cm at 470 nm. Concentrations of TAIPDI were maintained in at least 10-fold excess over the complexes to ensure pseudo-first-order conditions. Pseudo-first-order rate constants were determined by fitting absorbance-time traces to a two-exponential function. All listed rate constants represent an average value of at least three kinetic runs under each experimental condition.

2. Materials and Methods

3. Results and Discussion

2.1. Materials

3.1. Spectral Characterization TAIPDI in Different Media

N,N′-di(2-(trimethylammoniumiodide)ethylene)perylenediimide (TAIPDI) was synthesized according to reported procedures [47–49]. Zinc (5,10,15,20-tetraphenyl-21H, 23H-porphyrin tetra-sulphonic acid) (ZnTPPS4) was synthesized from p-tert-butylbenzaldehyde and pyrrole according to a literature procedure [50] and purified by column chromatography on silica gel (CH2Cl2/Hexane 1:1). Ultrapure water was generated with a Milli-Q apparatus (Millipore). Methanol (MeOH), ethanol (EtOH), acetonitrile (ACN), dimethylformamide (DMF), and tetrahydrofuran (THF) were purchased from Aldrich and used without further purification.

Fig. 2 shows the absorption spectra of TAIPDI in different media. The absorption spectrum of TAIPDI in methanol exhibited a strong absorption in the visible spectral region with significant sharp bands at 525, 489 and 458 nm, which represent 0-0, 0-1 and 0-2 transitions, respectively. Similar observations were recorded in dimethylformamide (DMF) and acetonitrile (ACN). As seen, the 0-0 transitions have the largest intensity compared to 0-1 and 0-2 transitions. These observations indicate that the TAIPDI molecules exist in the monomer form. When changing the solvent to water, the absorption spectrum of TAIPDI showed considerably bathochromic shift of the absorption bands at 467, 503 and 537 nm. In tetrahydrofuran (THF), the spectral pattern showed ~40 nm blue shifts for both 0-0 and 0-1 transitions compared to that in water. For these absorption bands in both water and THF, the 0-1 electronic transition has the largest intensity indicating the formation of “face-to-face” stacked perylenediimide aggregates in solution [51–55]. This observation suggests the aggregation behavior of the TAIPDI molecules in water and THF via π-π electrostatic and ionic interactions of hydrophobic aromatic cores. Upon photoexcitation TAIPDI at 490 nm in methanol, the fluorescence spectrum showed two characteristic emission bands at 540 and 577 nm. Similar spectral pattern was recorded in ACN and DMF, with a small red shift in DMF. When changing solvent to water, the recorded emission spectrum showed significantly weak and broad emission bands of TAIPDI at 548 and 589 nm. In THF, the weak broad fluorescence

2.2. Instruments Optical absorption and fluorescence measurements were carried out on JASCO spectrophotometer (V-780) and JASCO spectrofluorometer (model FP-8300), respectively. The picosecond fluorescence decay profiles were measured by a single-photon counting method using FluoTime 300 (PicoQuant, Germany). Lifetimes were evaluated with software FluoFit attached to the equipment. Dynamic light scattering (DLS) measurements were carried out using a Zetasizer instrument (Malvern Instruments Ltd., Malvern, UK). Scanning electron microscopy (SEM) images of nanowires were recorded on a JEOL FE-SEM JSM-6320F instrument. SEM samples were placed on a piece of glass that was attached to gold metal. TEM samples were prepared by depositing a

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Fig. 2. (Upper) Fluorescence emission of TAIPDI is tuned in a different organic solvent. (Bottom): Absorption spectra of TAIPDI in different media (left), fluorescence spectra of TAIPDI in different media; λex = 490 nm (middle), and fluorescence decay profiles of TAIPDI in different media; λex = 470 nm (right).

bands were recorded at 534, and 571 nm, which are about 15 nm blue shifts compared to that in water. The fluorescence lifetime measurements (Fig. 2, right) showed that the lifetimes of the singlet TAIPDI were found to be 4.50 ns (MeOH, DMF, and ACN), 4.25 ns (water), and 3.90 ns (THF). Form this observation; one can see that the lifetime of TAIPDI in methanol is comparable with the monomer perylenediimide derivatives, while the aggregated forms in THF and water exhibited shorter lifetimes. These collected observations indicate that the TAIPDI exists in the monomer form when dissolving in MeOH,

DMF, and ACN, while the face-to-face stacked is dominant in both H2O and THF [47,49,56]. The aggregation of TAIPDI in water was supported by recording the absorption and fluorescence spectra in the mixed solvent (water: methanol) with different ratios. As seen from Figs. 3 and S1, the absorption spectrum with 100% water corresponds to the aggregated form as shown above. With increasing the ratio of methanol in the mixed solvent, one can see that both the 0-0 and 0-1 transitions exhibited a considerable blue shift. In addition, the ratios of 0-0/0-1 transitions getting

Fig. 3. (Upper) Fluorescence emission of TAIPDI is tuned in a series of water:methanol mixed solvent. (Bottom) Absorption spectra (left) and fluorescence spectra (right) of TAIPDI in water:methanol solvent; λex = 490 nm.

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Fig. 4. (Left) Absorption spectral changes during the titration of TAIPDI (4 × 10−5 M) with ZnTPPS4 (0–14 × 10−6 M) in water. (Right) Scatchard analysis of the absorbance plot.

higher with increasing the ratio of methanol because of TAIPDI loose its aggregated character in MeOH. The fluorescence measurements track the above considerations by recording the fluorescence spectra of TAIPDI in the mixed solvent (water: methanol). As seen from Fig. 3 (bottom), the fluorescence intensity of the singlet TAIPDI tend to increase with increasing the ratio of methanol indicating that the TAIPDI aggregated in water, but not in methanol. 3.2. Complex Formation Between TAIPDI and ZnTPPS4 in Water When ZnTPPS4 molecule is added to TAIPDI solution, electrostatic interactions between the TAIPDI aggregate and ZnTPPS4 molecule result in binding of the stacked TAIPDI with ZnTPPS4. As seen from Fig. 4 (left), the complex formation was characterized by the systematic de-

crease of the absorption band of TAIPDI (at 490 nm) that accompanied with the formation of the absorption bands of ZnTPPS4 (at 420 and 600 nm). Two clean isosbestic points were observed at 460 and 560 nm, which indicate single equilibrium between distortion of TAIPDI and formation of ZnTPPS4 band. These observations indicate that the self-assembly between the aromatic cores of TAIPDI and ZnTPPS4 has been established not only by ionic interactions but also by strong π-π interactions between the two entities. The formation constant (K) for TAIPDI-ZnTPPS4 complex was obtained from the absorption spectral data by using the Scatchard method [57] (Eq. (1)):

ΔA ¼ KΔεP0 −KΔA X

Fig. 5. (Left) Absorption spectral changes during the titration of ZnTPPS4 with addition of amounts of TAIPDI (0–2 μM) in water. (Right) Scatchard plot.

Fig. 6. (Left) Fluorescence spectral changes of ZnTPPS4 in the presence of different concentrations of TAIPDI in water. (Right) Scatchard plot; λex = 420 nm.

ð1Þ

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Fig. 7. Fluorescence quenching of TAIPDI by adding different concentrations by ZnTPPS4; λex = 490 nm.

where P and X represent ZnTPPS4 and TAIPDI, respectively, ΔA is the observed absorbance minus the initial absorbance for each point in the titration at the measuring wavelength, Po is the total concentration for the free and complexed ZnTPPS4, and Δε = εPX − εp. A plot of ΔA/[X] versus ΔA was constructed and the K value was obtained from the slope (= − K) of the line (Fig. 4, right). The calculated K value of TAIPDI–ZnTPPS4 complex was determined to be 1.66 × 104 M−1. When ZnTPPS4 titrated with the aggregated TAIPDI in water, one can see decrease of the absorption of the ZnTPPS4 Soret band at 420 nm with a considerable red shift (Fig. 5). With the decay of ZnTPPS4, the concomitant rise of the absorption bands of TAIPDI was observed. The recorded two clean isosbestic points at 400 and 460 nm suggest single equilibrium between the two entities. From the slope of this linear correlation of the Scatchard plot (Fig. 5), the binding constant (K) of TAIPDI– ZnTPPS4 complex was determined to be 2.57 × 104 M−1. To determine the stoichiometry of the complex, absorbance changes at 420 nm versus the molar ratio of added TAIPDI to the total concentration has been followed. The revealed break at ca. 0.47, indicating 1:1 stoichiometry between TAIPDI and ZnTPPS4 in the polymeric π-stacks assembled via ionic and π-π interactions (Fig. S2, SI). Furthermore, the complex formation between the TAIPDI and ZnTPPS4 was examined by the steady-state fluorescence measurements. Upon excitation of ZnTPPS4 in the presence of TAIPDI, the emission intensity of singlet-excited state of ZnTPPS4 at 604 nm was significantly

quenched (Fig. 6). It is most likely that the quenching behavior is due to the electron transfer from the singlet excited state of ZnTPPS4 to ∙– TAIPDI forming the electron transfer product ZnTPPS∙+ 4 –TAIPDI , taking into account that the energy transfer via the singlet ZnTPPS4 is excluded due to the energetic considerations. From the fluorescence measurements, the binding constant of TAIPDI-ZnTPPS4 complex was determined to be 2.7 × 104 M− 1, which is in a good agreement with the obtained K from the steady-state absorption measurements. Similar electron transfer behavior was recorded upon photoexcitation of TAIPDI at 490 nm where the fluorescence intensity of the singlet-excited state of TAIPDI at 550 nm was greatly decreased with addition of ZnTPPS4 (Fig. 7). The fluorescence quenching of TAIPDI may involve the electron-transfer process (from ZnTPPS4 to the singlet-excited state of TAIPDI) and/or the energy-transfer process (from the singlet excited state of TAIPDI to singlet ZnTPPS4). The finding that the fluorescence spectra shown in Fig. 7 showed no formation of the singlet ZnTPPS4 with the quenching of TAIPDI suggests that the electron-transfer process is dominant. The Stern–Volmer plot for quenching the singlet-excited state of TAIPDI by ZnTPPS4 revealed a higher slope, indicating the occurrence of efficient quenching (Fig. 8, left). The kSV value calculated from the linear segment of the plot was found to be 2.28 × 104 M−1, from which the quenching rate constant, was calculated to be 1.00 × 1014 M−1 s−1 by employing a fluorescence lifetime of 4.4 ns for the singlet state of TAIPDI. The finding that this value is nearly 3 orders of magnitude higher than what expected for intermolecular type diffusion controlled quenching (kdiff = 3.60 × 109 M−1 s−1) may suggests the occurrence of an intramolecular quenching of the singlet-excited state of TAIPDI by ZnTPPS4 [58,59]. The fluorescence lifetime measurements (Fig. 8, right) track the above considerations in a more quantitative way, giving kinetic data of the electron transfer process in water. The time-profile of the singletexcited state of TAIPDI control exhibited a single exponential decay with a lifetime of 4.4 ns. In the case of TAIPDI-ZnTPPS4 complex, the decay profile of the singlet TAIPDI could be satisfactorily fitted to a biexponential decay: one has a short lifetime of 40 ps (40%) which reflects the actual intramolecular deactivation of the singlet TAIPDI, and the other has a larger lifetime, which resembles that of the TAIPDI reference. Based on the lifetimes of the singlet-excited states of TAIPDI control (τf0) and the fast decay of the TAIPDI-ZnTPPS4 complex (τf), the rate and efficiency of the electron transfer process (ket) were determined to be 2.47 × 1010 and 0.99 from the following relations; ket = (1/ τf)sample − (1/τf0)reference and Φet = ket / (1/τf)sample, respectively [60]. The suggested electron transfer character from ZnTPPS4 to TAIPDI in the formed supramolecular complex has been supported by the

Fig. 8. (Left) Stern-Volmer plot for quenching of TAIPDI by ZnTPPS4. (Right) Decay time profiles of the singlet TAIPDI fluorescence at 550 nm in the absence and presence of ZnTPPS4 in water; λex = 470 nm.

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Fig. 9. (Left) UV–Vis spectra recorded during the reaction of ZnTPPS4 (6 × 10−6 M) with TAIPDI (10 × 10−4 M); a) before the reaction; b) milliseconds after mixing. Inset is the kinetic trace measured during the reaction at 420 nm. (Right) Kinetic traces measured during the interaction between ZnTPPS4 (6 × 10−6 M) and different concentrations of TAIPDI in water.

Fig. 10. (Left) Absorbance-time traces at 420 nm for the reaction of ZnTPPS4 (6.0 × 10−6 M) with TAIPDI (21 μM) in distilled water at different temperatures. (Right) Dependence of kobs for the reaction of [ZnTPPS4] vs [TAIPDI] concentrations as a function of temperatures. [ZnTPPS4] = 6.0 × 10−6 M.

electrochemical measurements of ZnTPPS4 and TAIPDI entities in water (Fig. S3, SI). The reduction potentials (Ered) of TAIPDI were recorded at − 0.52 and −0.84 V vs. Ag/AgCl, while the first oxidation potential of ZnTPPS4 was recorded at 0.65 V vs. Ag/AgCl. Based on the first oxidation potential of ZnTPPS4 and reduction potential of TAIPDI, the driving force for the electron transfer (−ΔGet) was calculated to be 0.87 and 1.08 eV via the singlet-excited states of ZnTPPS4 and TAIPDI, respectively [61].

Table 1 Rate constants and activation parameters for the “on” and “off” reactions for the first reversible interaction of TAIPDI to ZnTPPS4. T (K)

[TAIPDI] × 10−4 M

kobs × 10−3 (s−1)

296

0.5 1 2 4 0.5 1 2 4 0.5 1 2 4 0.5 1 2 4 ΔH#, kJ mol−1 ΔS#, J mol−1 K−1

14.2 23.4 41.2 74.1 88.0 110.0 121.8 186.1 131.4 135.5 225.0 290.8 244.8 277.3 326.6 391.4

302

309

315

“On” reaction kon 170 ± 3

3.3. Thermodynamic Parameters of the ZnTPPS4-TAIPDI Complex by Using the Stopped-flow Technique The interaction of TAIPDI with ZnTPPS4 was further examined by using stopped-flow technique as a function of TAIPDI concentrations at different temperatures in water at ca. 420 nm. Fig. 9 (left) shows the UV–Vis spectra recorded during the reaction of ZnTPPS4 with

“Off” reaction koff × 10−3 6 ± 0.6

269 ± 29

76 ± 6

402 ± 80

100 ± 10

408 ± 42

230 ± 9

22.75 ± 5 −123 ± 18

∙− suggest exothermic electron Such negative ΔGet of ZnTPPS∙+ 4 –TAIPDI transfer via both singlet-excited states.

133.2 ± 4 167.3 ± 13

Fig. 11. Eyring plot for kon and koff for the first reaction step of ZnTPPS4 with TAIPDI; ZnTPPS4 = 6.0 × 10−6 M.

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Fig. 12. (Left) Plot of kobsd of the second step vs. TAIPDI concentration for ZnTPPS4 in water at 296 K. Experimental conditions: [ZnTPPS4] = 6.0 × 10−6 M, λ = 420 nm. (Right) Plot of ln(kobs2/TC) vs. 1/T of TAIPDI (4.0 × 10−4 M) with zinc porphyrin (6.0 × 10−6 M) in water at different temperature. Inset is the kinetic trace measured at 420 nm.

increasing amounts of TAIPDI as well as its kinetic traces measured using stopped-flow technique. Rate constants for the reaction were determined by using total TAIPDI concentration of 0–0.4 mM, i.e., always at least in 10-fold excess over the ZnTPPS4 complex [62]. Throughout the TAIPDI concentration range it was possible to fit the absorbance/time traces to a two-exponential function by using Eq. (2):

activation parameters of the second step were determined from an Eyring plot (Fig. 12, right), and resulted in ΔS# = 246 ± 37.89 J mol− 1 K−1 and ΔH# = 130 ± 11 kJ mol− 1. The positive values for ΔS# for this step support a dissociative mechanism for this reaction step.

A ¼ a1 e−kobsd1t þ a2 e−kobsd2t þ A0

TAIPDI molecule showed aggregation behavior in water and tetrahydrofuran and unstacked (monomer form) in methanol, acetonitrile, and dimethylformamide. The interaction between TAIPDI and ZnTPPS4 molecules via π-π and ionic interactions has been reported where the resulting complex ZnTPPS4-TAIPDI is accompanied by pronounced spectra changes in the UV/Vis absorption bands and significant alterations of the fluorescence spectra. Both absorption and fluorescence studies showed moderately stable complex formation with (K = 2.6 × 104 M−1). The excited state events were monitored by both steady state and time-resolved emission techniques. The main quenching pathway involved electron transfer from the electron donating ZnTPPS4 to the electron accepting TAIPDI. The rate and efficiency of the electron transfer, calculated from the picosecond time-resolved emission studies were found to be 2.47 × 1010 and 0.99, respectively, indicating fast and efficient electron transfer character. The thermodynamic parameters of the complex formation have been determined from the stopped-flow measurements. The finding that the moderately stable ZnTPPS4-TAIPDI complex exhibited absorption in the wide range of the visible region, fast and efficient electron transfer suggests its potential to be an artificial photosynthetic model.

ð2Þ

This indicates that the overall reaction is biphasic, as shown in Fig. 9 (inset figure) for typical kinetic traces. The TAIPDI concentration dependence of the first step reaction was studied as a function of temperature in the range 23 to 42 °C and is a second order process. The first step is expected to follow pseudo-first-order kinetics, for which the observed rate constant can be expressed by Eq. (3). As shown in Fig. 10, a linear dependence of kobs on the TAIPDI concentration was obtained with its kinetic traces at different temperature. The plot of kobs vs. [TAIPDI] should be linear with a slope kon and an intercept koff (Table 1) [62–66]. kobs ¼ kon ½TAIPDI þ koff

ð3Þ

The thermal activation parameters, as a useful mechanistic tool in assigning the mechanism, for the “on” reaction were determined to be Δ H# = 22.75 ± 5 kJ mol− 1 and Δ S# = − 123 ± 18 J mol−1 K− 1 by using Eyring plot (Fig. 11). The negative value for ΔS# for “on” reaction step supports an associative mechanism. The rate constant for the “off” reaction was calculated from the intercepts of the plots in Fig. 10. From the values of koff as a function of temperature, the thermal activation parameters were determined to be ΔH# = 133.2 ± 4 kJ mol−1 and ΔS# = 167.3 ± 13 J mol−1 K−1. The positive value for ΔS# for “on” reaction step supports a dissociative mechanism. As seen from Table 1, the coordination affinity of TAIPDI to ZnTPPS4 for the first step in distilled water, K1 = kon/koff = 6009 M−1. Thus, we propose a rapid pre-equilibrium involving adduct formation between the TAIPDI and ZnTPPS4 as shown in Eq. (4): ZnTPPS4 þ TAIPDI

K on



K off

½ZnTPPS4 −TAIPDI adduct

4. Conclusion

Acknowledgments This work was supported financially by the Science and Technology Development Fund (STDF), Egypt (Grant Numbers 5537 and 12436). Appendix A. Supplementary Data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.saa.2017.06.016.

ð4Þ

The TAIPDI concentration dependence of the second interaction step was also studied as a function of temperature in the range 23 to 42 °C. As shown in Fig. 12 (left), a linear dependence of kobs on the TAIPDI concentration was obtained with K2 value of 123 ± 3 M−1 s−1. The thermal

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