Anthraquinone derivatives as electron-acceptors with ...

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Downloaded by University of Calgary on 01 September 2012 Published on 24 February 2012 on http://pubs.rsc.org | doi:10.1039/C2CP23224C

Anthraquinone derivatives as electron-acceptors with liquid crystalline propertiesw Amy E. Murschell, Wang Hay Kan, Venkataraman Thangadurai and Todd C. Sutherland* Received 12th October 2011, Accepted 8th February 2012 DOI: 10.1039/c2cp23224c Dialkoxy derivatives of anthraquinone (AQ), dicyano-anthraquinone (DCAQ) and tetracyanoanthraquinone (TCAQ) were synthesized and their associated electrochemical, optical and self-assembling properties were investigated as candidates for n-type materials. AQ shows UV absorption features, whereas both DCAQ and TCAQ exhibit bathochromic and hyperchromic electronic transitions into the visible region. The electron accepting strength of the three compounds was established by cyclic voltammetry as 1.52 V, 1.3 V and 0.9 V vs. ferrocene/ferricenium for AQ, DCAQ and TCAQ, respectively. All three quinones displayed quasireversible, two sequential one-electron transfer redox reactions. DFT calculations of DCAQ and TCAQ demonstrate structural changes upon reduction, which is supported by spectroelectrochemical experiments. Furthermore, the structural changes result in different absorption profiles and show potential as electrochromic materials. Finally, both AQ and DCAQ show liquid crystalline phases and importantly, DCAQ exhibits both a smectic liquid crystalline and a soft crystal phase between 6 1C and 85 1C, which offers promise as a self-assembling n-type material.

Introduction 1

Organic semiconductors will play a key role in the next generation of electronic devices because of their low temperature processability, flexibility, electronic and optical tunability via synthetic means, low cost and potential for large area fabrication. Despite the envisioned advantages of organic semiconductors, they lag behind traditional Si-based technologies with respect to one critical feature—charge carrying ability. P-type organic semiconductors have seen the most intensive research efforts,2 which has resulted in comparable hole-mobilities with inorganic materials. The explosion of p-type organic materials is likely due to synthetic access and ambient stability. However, research on new organic n-type,3 or electron transporting, materials has lagged behind organic p-type successes, which is critical especially when several devices depend on balanced hole and electron transport. Organic n-type materials can find applications in photovoltaics, field-effect transistors and light-emitting diodes. Most small-molecule n-type compounds attempt to find a balance between the strong pi–pi interactions in the solid-state

Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4. E-mail: [email protected] w Electronic supplementary information (ESI) available: 1H-(400 MHz) and 13C-(101 MHz) NMR spectra for quinones 1–3, IR spectra of 1–3, fluorescence spectra of 1–3, cyclic voltammograms of 1–3, FMO energies of quinones 1–3 and their radical anions and spin density maps of radical anions 1 , 2 and 3 . See DOI: 10.1039/c2cp23224c

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that leads to high electron mobility and electronically benign substitutions that enable solution processability. Liquid crystals offer a balance because the rigid acene cores are held close to each other, enabling efficient ‘crystal-like’ charge transport, which is balanced by the ‘fluid-like’ alkyl groups that enable favorable organic solubilities. The three materials commonly seen as n-type self-assembling liquid crystalline materials include: phenanthrene,4 perylene,5 and anthraquinones.6 The challenges with these materials include: solubility, electron affinity, self-assembly properties, reversible redox reactions and liquid crystalline phase stability.7 Anthraquinone (AQ) liquid crystals have seen much research because of their low cost, high temperature-stability, ambient conditions stability, reversible two-electron redox reactions and because they tend to form liquid crystalline phases with appropriate substitution patterns. Recently, our group found that hexa-alkoxy AQs show excellent discotic liquid crystalline phases that span from 0 1C to 150 1C; however the AQ core showed only modest electron affinity. Several research groups have explored the feasibility of introducing either one8 or two9,10–12 dicyanomethylene groups to the anthraquinone core for different applications including, electrical conductors, molecular rectifiers, photoinduced electron transfer processes, optoelectronic devices and electrochiroptical materials. Many reports have described the condensation of anthraquinone with malononitrile as challenging due to the steric crowding of the final product; however, use of TiCl4, as a Lewis acid and a hygroscopic agent, facilitated reasonable yields.10 To the best of our knowledge, This journal is

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Fig. 1

Summary of n-type compounds synthesized.

only one report12 of a cyano-based AQ has shown liquid crystalline properties. This report explores 2,6-anthraquinone derivatives, Fig. 1, that show reasonable electron affinities and liquid crystalline phases and are solution processable. The self-assembled liquid crystalline structures are advantageous because the acene cores are held in a near crystalline state, which may lead to efficient charge transport, yet the alkyl substitutions also maintain a degree of fluidity, which enables films on flexible substrates. This manuscript details the full optical and electrochemical properties of the small molecules, which is followed by computational investigations. Furthermore, the liquid crystalline films are assessed by polarized optical microscopy, differential scanning calorimetry and variable temperature powder X-ray diffraction.

Experimental section Materials All chemicals were purchased from Aldrich and used as received without further purification unless otherwise specified. Column chromatography was performed on SiliCycle SiliaFlash P60 silica gel (230–400 mesh). Thin-layer chromatography was carried out on Merck silica gel F-254 glass-backed TLC plates. Solvents were used as received or dried using an MBraun solvent purification system and reactions were typically carried out under a N2 atmosphere. Synthesis 2,6-Didodecyloxyanthraquinone (1). Anthraflavic acid (0.48 g, 2.02 mmol) and sodium hydroxide (0.21 g, 5.15 mmol, 2.55 eq.) were stirred in DMSO (12 mL) at room temperature (15 min) under ambient conditions. 1-Bromododecane (3 mL, 12.49 mmol, 6.18 eq.) was added and the flask was heated to 75 1C for 6 h. The reaction mixture was cooled to room temperature, and water (10 mL) was added to quench the reaction. The crude yellow precipitate was collected by vacuum filtration and rinsed with water. The product was purified by precipitation from chloroform and methanol (0.81 g, 1.40 mmol, 69.3%). 1H-NMR (400 MHz, CDCl3) d 8.23 (d, J = 8.6 Hz, 1H), 7.71 (d, J = 2.4 Hz, 1H), 7.22 (dd, J = 8.7, 2.5 Hz, 1H), 4.15 (t, J = 6.5 Hz, 2H), 1.92–1.77 (m, 2H), 1.72–1.00 (m, 18H), 0.89 (t, J = 6.6 Hz, 3H). 13C NMR (101 MHz, CDCl3) d 182.43, 164.18, 135.97, 129.77, 127.10, 121.03, 110.65, 68.95, 32.06, 29.80, 29.78, 29.73, 29.70, 29.49, 29.17, 26.09, 22.84, 14.26. Spectroscopic data are consistent with reported di-substituted anthraquinones.13 This journal is

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2,6-Didodecyloxy-11,12-dicyano-9-anthraquino-10-methane (2). Quinone 1 (0.40 g, 0.69 mmol) and malononitrile (0.23 g, 3.5 mmol, 5.07 eq.) were stirred in dichloromethane (100 mL) at 0 1C under a Nitrogen atmosphere to yield a transparent yellow solution. Titanium(IV) tetrachloride (1 mL, 9.10 mmol, 13.19 eq.) was added slowly and the solution changed to an opaque red color. Pyridine (2.2 mL, 27.3 mmol, 40 eq.) was added and the solution re-gained its yellow color. The solution was then heated to reflux for 24 h and then quenched with excess 5% hydrochloric acid. The organic layer was set aside and the aqueous layer was extracted with chloroform. The organic fractions were combined and evaporated under reduced pressure, and the crude product was subjected to silica gel column chromatography (chloroform, Rf = 0.82) to yield a bright yellow solid (0.20 g, 0.32 mmol, 46.4%). 1H NMR (400 MHz, CDCl3) d 8.23 (dd, J = 8.7, 6.5 Hz, 1H), 7.78–7.70 (m, 1H), 7.23–7.14 (m, 1H), 4.14 (td, J = 6.5, 4.2 Hz, 2H), 1.91–1.77 (m, 2H), 1.53–0.97 (m, 18H), 0.89 (t, J = 6.9 Hz, 3H). 13C NMR (101 MHz, CDCl3) d 181.35, 163.50, 163.07, 161.19, 136.04, 134.01, 130.70, 129.33, 125.67, 124.33, 119.90, 119.76, 114.73, 112.56, 112.00, 80.93, 69.27, 69.14, 32.07, 29.80, 29.78, 29.72, 29.68, 29.49, 29.45, 29.13, 29.10, 26.06, 22.84, 14.26. MS (EI HRMS) m/z calcd for C41H56N2O3 + H: 625.4324; found 625.4343. Anal. calcd for C41H56N2O3: C: 78.80, H: 9.03, N: 4.48%; found C: 78.35, H: 9.00, N: 4.63%. 2,6-Didodecyloxy-11,11,12,12-tetracyano-9,10-anthraquinodimethane (3). Quinone 1 (0.38 g, 0.66 mmol) and malononitrile (0.70 g, 10.6 mmol, 16 eq.) were stirred in dichloromethane (100 mL) at 0 1C under a Nitrogen atmosphere to yield a transparent yellow solution. Titanium(IV) tetrachloride (0.5 mL, 4.55 mmol, 7 eq.) was added slowly and the solution changed to an opaque red color. Pyridine (1.1 mL, 13.7 mmol, 21 eq.) was added and the solution re-gained its yellow color. The solution was then heated to reflux for 24 h and then quenched with excess 5% hydrochloric acid. The organic layer was set aside and the aqueous layer was extracted with chloroform. The organic fractions were combined and evaporated under reduced pressure, and the crude product was subjected to silica gel column chromatography (chloroform, Rf = 0.83) to yield an intensely yellow solid (0.28 g, 4.2 mmol, 63.63%). 1 H NMR (400 MHz, CDCl3) d 8.17 (d, J = 8.8 Hz, 1H), 7.71 (d, J = 2.5 Hz, 1H), 7.16 (dd, J = 8.8, 2.5 Hz, 1H), 4.12 (t, J = 6.5 Hz, 2H), 1.90–1.77 (m, 2H), 1.77–0.99 (m, 18H), 0.89 (t, J = 6.9 Hz, 3H). 13C NMR (101 MHz, CDCl3) d 162.32, 160.25, 132.80, 129.70, 121.85, 118.52, 113.79, 113.73, 113.27, 80.80, 69.32, 31.99, 29.72, 29.70, 29.65, 29.60, 29.42, 29.36, 28.97, 25.96, 22.76, 14.19. MS (EI HRMS) m/z calcd for C37H48N2O3: 672.4403; found 672.4435. Characterization NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer. Mass spectra were recorded with a Thermo Finnigan SSQ7000 spectrometer and elemental analyses were carried out with a Perkin Elmer Model 2400 series II C H N analyzer. FTIR spectra were recorded in transmission mode with drop-cast films on NaCl plates in a Varian FTS-7000 spectrometer. UV/Vis spectra were recorded using a Cary 5000 spectrophotometer in dual beam mode. Thermograms were Phys. Chem. Chem. Phys., 2012, 14, 4626–4634

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collected using a TA-Q200 DSC and a TA-Q50 TGA under a N2 atmosphere. Cross polarized optical microscopy was carried out on an Olympus BX-41 microscope equipped with a heating stage (Linkam T95). Powder X-ray diffraction experiments were conducted using a Bruker D8 Advance (40 kV, 40 mA) (Cu Ka) equipped with a high temperature reactor chamber (Anton Paar XRK 900).


Electrochemistry Cyclic voltammetry (CV) experiments were carried out on an Autolab PGSTAT302 potentiostat that was controlled by a PC running Autolab’s GPES v 4.9 software in a temperaturecontrolled, three-electrode cell (15 mL). The working electrode was a glassy carbon disc with area of 0.28 cm2 from BASi, which was polished after each use with 0.05 mm diamond slurry (Buehler) in an automated polisher (Buehler). The reference electrode was a silver wire in a 0.1 M AgNO3 electrolyte dissolved in CH3CN sealed in a glass tube separated by porous glass. The counter electrode was a Pt wire that was flame annealed prior to each use. All potentials were referenced to the ferrocene/ferricenium redox couple. Each CV experiment consisted of approximately 1–3 mM redox active species dissolved in 0.1 M tetrabutylammonium hexafluorophosphate in deoxygenated dichloromethane. All CV experiments were bubbled with Ar for 10 minutes prior to dissolving the redox active species and an Ar blanket was maintained during the entire experiment. Spectroelectrochemical spectra were generated with a Cary 5000 spectrophotometer linked with a CHI 650 potentiostat using a thin layer cell (ALS-Japan). The working electrode, in the beam path, was a Pt mesh, a Pt wire counter electrode and the same reference electrode, electrolyte and solvent as above were used.

Fig. 2 UV-vis absorption spectra of compounds 1 (grey, 1.1 10 5 M), 2 (blue, 1.2 10 5 M) and 3 (red, 1.1 10 5 M) in CH2Cl2 and fluorescence spectrum of 3 (red line). Table 1 Summary of electrochemical and optical properties D0 E1/2 (10 6 keta ELUMO vs. cm2 (10 3 ELUMOb/ lmaxc/ ELUMOd EHOMOd Quinone Fc/V s 1) cm s 1) eV calc./eV calc./eV eV 1 2 3

Results and discussion Optical and electrochemical properties Fig. 2 shows the electronic absorption spectra of quinones 1 to 3 in dichloromethane. Two trends are apparent from the absorption spectra in series 1 to 3—the lowest energy transition experiences both bathochromic and hyperchromic shifts with the added conjugation of one and two dicyanovinylenes. The maximum absorption peak of the lowest energy transition for quinones 1, 2 and 3 changed incrementally from 350 nm to 375 nm to 400 nm, respectively, and this peak’s energy was used to estimate the HOMO to LUMO energy difference, summarized in Table 1. The absorption profile envelope of both 1 and 2 is similar, suggesting that the same electronic transitions occur in each compound. For anthraquinodimethane 1, strong p–p* transitions are observed between 270 and 375 nm14 followed by weak, low energy n–p* transitions between 375 nm and 425 nm. For quinone 2, similar, strong p–p* transitions, between 270 and 400 nm,14 and weak n–p* transitions, between 425 nm and 500 nm, are observed. Note that quinone 3 does not show the weak n–p* transitions and this region of the spectra for 1 and 2 is expanded in the ESI.w The p–p* transitions show a red-shift commensurate with additional electron-withdrawing substituents and extending conjugation lengths. The absorption data support the notion that installation of dicyanovinylene extends the pi-conjugation leading 4628


1.52 0.1 2.03 1.27 0.08 1.51 0.89 0.07 1.46

0.7

2.88

3.6

2.82

3.8

0.4

3.13

3.3

3.38

3.4

0.3

3.51

3.1

3.74

3.3

a

Determined using the method presented by Nicholson. b Calculated from the E1/2 value compared to Fc/Fc+ at 4.4 eV versus vacuum: (E1/2 + 4.4 eV).17 c Determined from the lowest energy electronic transition in CH2Cl2. d Determined from the results of the DFT calculations.

to both longer wavelength transitions and more intense absorptions. In addition, the quinone derivatives possess only moderate absorptions in the visible region, implying that the HOMO–LUMO transition is still reasonably large, which can be important for certain material applications where n-type component absorption is not desired. Note that neither 1 nor 2 exhibited fluorescence in CH2Cl2 and quinone 3 was weakly fluorescent with an emission peak at 540 nm (excitation at 400 nm) shown in Fig. 2, yielding a Stokes shift of 6800 cm 1, which is consistent with a structural change in the excited state. The absence of fluorescence for both 1 and 2 is a consequence of a weak oscillator strength and fast intersystem crossing to triplet states and is consistent with low lying n–p* states. More details of the ground-state structure are described later. Although optical absorption measurements provide key insight into the HOMO to LUMO energy difference, the absolute energy of the HOMO and LUMO levels is needed to assess the electron acceptor strength. Electrochemical measurements, namely cyclic voltammetry (CV), can be used to provide estimates of the energy levels by carrying out redox reactions This journal is

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Fig. 3 Cyclic voltammograms of approximately 1 mM 1 (grey), 2 (blue) and 3 (red) in CH2Cl2 in 0.1 M (nBu)4PF6 supporting electrolyte, a glassy carbon working electrode, a Pt wire counter electrode and a Ag/Ag+ quasi reference electrode. All potentials are reported versus Fc/Fc+.

compared to an internal redox standard, such as ferrocene. Current normalized CVs of 1–3 are shown in Fig. 3 and the non-normalized CVs are included in the ESI.w Anthraquinone 1 shows two sequential, one-electron redox reactions at 1.52 V and 2.03 V, which is consistent with our previous reports of anthraquinone reductions.15 Interestingly, the presence of two alkoxy groups, as in 1, versus six alkoxy groups in our previous report, does not alter the reduction potential. The redox reactions for 1 are quasi-reversible with moderate electron transfer rates, shown in Table 1. Non-centrosymmetric quinone 2 shows interesting CV properties. Starting from 0 V and scanning the potential cathodically, a shoulder is seen at 1.1 V, followed by a peak at 1.4 V and a final reduction peak at 1.55 V. Upon the return anodic scanning, two shoulders are seen at 1.45 V and 1.2 V with a final peak at 1.05 V. Given the quinone structure, it is unlikely that the peaks and shoulders in the CV of 2 correspond to three, one-electron redox reactions. We speculate that the first reduction shoulder is due to a surface adsorption phenomenon, signified as a pre-peak,16 and corresponds to the reduction of the neutral quinone to its mono-anion while adsorbed to the electrode. Thus the two remaining peaks in the reduction scan are attributed to the two successive oneelectron reduction reactions of the neutral and mono-anion quinones that are in solution (not adsorbed to the electrode). The return anodic scan shows a shoulder at 1.45 V, which is attributed to the dianion being re-oxidized to the radical anion, followed by the oxidation of the mono-anion to neutral quinone in solution at 1.2 V. The final oxidation peak at 1.05 V is attributed to the mono-anion to neutral quinone that is adsorbed to the electrode surface. Since one pair of redox couples in the CV of quinone 2 is attributed to surface adsorbed quinones, the two diffusion-controlled redox potentials are 1.27 V and 1.51 V. The introduction of a single dicyanomethylene has resulted in a quinone that is 0.25 eV easier to reduce than the parent anthraquinone. Note that the second reduction peak at 1.5 V is similar to the first reduction peak of the parent quinone and could be explained as the reduction of the planar anthraquinone core. Symmetric quinone, 3, This journal is

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shows two sequential, one-electron, redox reactions at 0.89 V and 1.46 V, whereby the first reduction potential is similar to the unsubstituted TCAQ at 0.81 V vs. Fc/Fc+11 and shows the alkoxy substituents are weakly electron donating in these compounds. The second redox reaction is not reversible, as indicated by the difference in peak current heights of the forward and reverse scans and the second peak appears at the same potential as the second reduction peak of 2 and first reduction peak of 1. The first redox peak of 3, at 0.89 V, is quasi-reversible and possesses a slightly slower electron transfer rate constant than both 1 and 2. The conversion to the bis(dicyanomethylene) quinone 3 results in a compound that is 0.67 eV easier to reduce than the parent anthraquinone. The difference between the first redox and second redox waves of quinone 2 is 240 mV, whereas for quinone 3 is nearly 500 mV. The reason quinone 2 shows such close redox peaks lies in its structure. Quinone 2, when neutral, has a butterfly-shaped structure; however, upon acceptance of one electron the anthraquinone core becomes planar, resulting in better delocalization and the ability to accept a second electron. Quinone 3, on the other hand, is butterfly-shaped when neutral and as the radical anion. Only when quinone 3 is reduced to the dianion, does the anthraquinone core adopt a planar geometry, which may enable degradation pathways accounting for some of the irreversibility observed in the second redox event. Note that anthraquinone 1 is planar in its neutral, radical anion and dianionic forms. The half-wave potentials of 1–3 can be used to estimate the energy of the LUMO (ELUMO) by comparison to the ferrocene/ferrocenium redox reaction, which has a EHOMO of 4.4 eV.17 All electrochemically derived parameters are included in Table 1. Further insight into the electronic properties of the quinones was gained through computational methods. Each quinone was geometry optimized using DFT methods at the B3LYP 6-31G+(d) level with Gaussian03.18 The resultant MO energy levels are shown in Fig. 4. Through the series 1–3, the HOMO energy levels show a small decrease, whereas the LUMO energy levels show marked decrease, as is typical when incorporating electron withdrawing groups. The electrochemical experiments provide estimates of the ELUMO at 2.9 eV, 3.1 eV and 3.5 eV for 1, 2 and 3, which compare favorably with the calculated values at 2.8 eV, 3.4 eV and 3.7 eV. Furthermore, the HOMO–LUMO energy difference (between highest occupied p-orbital and lowest energy unoccupied p*-orbital) was calculated at 3.8 eV, 3.4 eV and 3.3 eV for 1, 2 and 3, respectively, which also

Fig. 4 FMO energies of quinones 1–3.


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Fig. 5 (a) Top and side views of the geometry-optimized structures obtained from DFT calculations of 1–3. (b) Side view of the optimized structures of the radical anions and dianions of 2 and 3.

compares favorably with the optically-determined HOMO–LUMO energy gaps at 3.6 eV, 3.3 eV and 3.1 eV, respectively. The similar values between theory and experiment lend support to the basis set chosen. The DFT calculations resulted in optimized geometries of quinones 1–3, shown in Fig. 5a. Quinone 1 adopts a planar acene structure whereas both 2 and 3 adopt a bent geometry when neutral. Furthermore, the structure of quinone 1 does not deviate from planarity when reduced to either the radical anion or the dianion. Conversely, Fig. 5b shows the geometry changes upon the first and second reductions of quinones 2 and 3. The bent geometry of 2 undergoes a planarization after accepting an electron and the planarity is maintained with the addition of the second electron. Bis(dicyanomethylene) 3 maintains the bent geometry after the first electron is accepted and only becomes

planar upon accepting the second electron. Spin density maps of the radical anions of 1, 2 and 3 are included in the ESIw and show that the electron density for 1 is confined to the central benzoquinone ring, whereas both quinones 2 and 3 show reasonable electron densities delocalized to the dicyanomethylenes, which rationalizes why quinone 3 remains bent after being reduced. The FMO shapes of 1–3 are shown in Fig. 6 and the compounds share many of the orbital features, which are supported by similar, yet red-shifted, absorption profiles. The HOMOs of the acene fragment in each quinone 1 to 3 show the same shape features across the series, despite the bent geometry in 2 and 3. The difference between the HOMO of 1 and either 2 or 3 is exemplified by the MO coefficient on the exocyclic vinylene carbons of the dicyanomethylene. The HOMO of 1 has a node on the carbon of the carbonyl, whereas in the HOMO of 2, there is substantial electron density in the double bond to the dicyanomethylene, which is consistent with the bathochromic and hyperchromic shift observed in the optical measurements due to increased delocalization. The same comparisons can be made between 1 and quinone 3. Similar MO shapes within the acene core are also present in the LUMOs with an East-West nodal plane through the center of the acene and the dicyanomethylenes add extra conjugation compared to the parent quinone 1. Spectroelectrochemistry is a useful tool to provide detailed electronic transitions of the intermediate radical anions. Fig. 7 shows the difference absorption spectra of quinones 2 and 3 at sufficiently negative potentials to permit the one-electron reduction reaction. The positive valued peaks indicate the radical anion species concentration increasing as a function of applied potential duration and conversely the negative valued peaks indicate the consumption of the neutral quinone. In addition, the isosbestic points are clearly visible as crossing the abscissa. The planarization of the acene core and the clear isosbestic points suggest a clean conversion to the radical anion with non-optically detectable degradation products. The one-electron reduction of quinone 2 to its radical anion shows new peaks growing in between 400 nm and 625 nm, which are red-shifted compared to the neutral species, commensurate with a decrease of the peak at 350 nm, consistent with the quinone partial reduction.

Fig. 6 FMO images of quinones 1–3 calculated using DFT methods at the B3LYP 6-31G+(d) level of theory.

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Fig. 7 Spectroelectrochemical reductions of (a) quinone 2 at 1.3 V (vs. Fc) and (b) quinone 3 at 1.0 V (vs. Fc) in CH2Cl2. Each spectrum was recorded under applied negative potentials every 4 minutes. Positive values represent increases in peak intensity. Insets are photographs of the evolution of the colored species in the spectroelectrochemical cell after 2 minutes of reduction.

The red-shifted peaks are consistent with the planarization of the acene core and the clear isosbestic points suggest a clean conversion to the radical anion. The one-electron reduction of quinone 3 results in a peak growing in between 450 nm and 575 nm, which is consistent with formation of the non-planar radical anion, and matched by a decrease between 300 nm and 475 nm, consistent with conversion of the quinone to its radical anion. The spectroelectrochemical experiment of the second reduction event was carried out, but due to the cell limitations all possible species (neutral, radical anion and dianion) are present in the excitation path leading to unclear results, with no isosbestic points and possible irreversible degradation. Photographs of the before and after reduction are shown as insets in Fig. 7 to highlight the electrochromic properties of these materials from yellow to red-purple solutions. Electrochromic behavior was also observed by Isoda, Yasuda and Kato for related compounds.12

Results and discussion Self-assembly properties Quinones 1–3 were subjected to thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), shown in the ESI.w Multiple thermal scans of the DSC were carried out and the thermograms were nearly superimposable but only the 2nd full thermal cycle is shown in the ESIw and a summary of the thermal transitions, including enthalpies, is shown in Table 2. Upon heating quinone 1 from 50 1C, a broad exothermic transition is observed at 59 1C, which is ascribed to a crystal-to-crystal transition, and a sharp endothermic transition is seen at 113 1C, which is consistent with a melting event. The return cooling thermogram of quinone 1 shows two exothermic transitions at 90 1C and 75 1C, which are assigned to a liquid to liquid-crystalline (LC) transition and an LC to a soft crystal phase (described below), respectively. The DSC thermogram of quinone 2 exhibits three reversible phase transitions in both the heating and cooling cycles. During heating, quinone 2 undergoes two small endothermic transitions at 1 1C and 64 1C, which are consistent with a crystalline to This journal is

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Table 2 Phase transition temperatures (1C) and enthalpies (kJ mol 1, in parentheses) of quinones 1, 2, and 3. (Cr) crystal, (SCr) soft crystal, (Sm) smectic phase, (I) isotropic liquid

1 2 3

Heating

Cooling

Cr 59.3 (21.3) Sm 113.0 (89.8) I Cr 1.0 (5.8) SCr 63.5 (1.1) Sm 85.4 (19.3) I Cr 27.7 (3.5) Cr 63.2 (4.9) Cr 73.3 (4.9) Cr 118.9 (49.4) I

I 90.0 (10.3) Sm 74.5 (39.9) Cr I 63.9 (21.1) Sm 49.7 (1.2) SCr 6.0 (5.3) Cr I 24.9 (3.4) Cr

soft crystal transition and a soft crystal to LC phase transition, respectively, and a sharp endothermic transition at 85 1C, consistent with an LC to isotropic liquid phase transition. The cooling cycle of quinone 2 shows the same phase transitions and energies with a moderate degree of hysteresis. Importantly, quinone 2 maintains an organized and flexible structure between 64 1C and 6 1C. Quinone 2 can be described as a ‘bent-core’ LC19,20 and is one of a few bent core LCs21 that shows mesophases below room temperature. The DSC thermogram of quinone 3 upon heating from 50 1C shows several exothermic transitions that are ascribed to crystal-to-crystal transitions that are followed by a melting transition at 120 1C. The return cooling cycle of quinone 3 only shows a small exothermic transition that is consistent with a liquid to crystalline phase. Powder X-ray diffraction (P-XRD) experiments provide detailed phase behavior and the variable temperature patterns for each quinone are shown in Fig. 8. Each of the three quinones was first melted to their isotropic liquid phase temperatures, based on DSC data, and all P-XRD traces show amorphous behavior expected for melted solids in the top traces in Fig. 8. Upon cooling to 85 1C, quinone 1 in Fig. 8a shows a single peak at 2y = 3.241, which gives a d-spacing value of 27.2 A˚ and is consistent with a smectic phase indexed to the (100) plane. Upon further cooling to 30 1C, a marked change occurs with d-spacing peaks corresponding to 19.6 A˚ and 10.3 A˚, which correspond to the (100) and (200) planes of Phys. Chem. Chem. Phys., 2012, 14, 4626–4634

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Fig. 8 Powder X-ray diffraction patterns of quinones 1 (a) 2 (b) and 3 (c) at variable temperatures under a N2 atmosphere.

Fig. 9 Cross polarized microscopy (100 magnification) images of quinones 1 and 2 on glass. (a) Quinone 1 after cooling from its isotropic liquid phase to 89 1C. (b) Quinone 2 after cooling from its isotropic liquid phase to 59 1C. (c) Further cooling of quinone 2 at 30 1C.

a layered smectic structure. Furthermore, the P-XRD pattern at 30 1C of quinone 1 shows several new peaks in the higher angle region suggesting a more crystalline structure. Upon cooling quinone 2 from its isotropic phase to 54 1C, clear peaks emerge at d-spacing values of 26.9 A˚, 14.3 A˚, 9.8 A˚ and 8.5 A˚, which do not index to either a layered smectic or a discotic phase. Quinone 2 at 30 1C shows very similar features with d-spacing values of 26.6 A˚, 14.2 A˚, 9.7 A˚ and 8.5 A˚, which do not index to either a layered smectic or a discotic phase. The small peak at 2y = 26.51 correlates to a d-spacing of 3.4 A˚, which is consistent with the distance between the rigid aromatic cores and suggests that strong p–p interactions are present. The P-XRD pattern for quinone 3 is consistent with crystalline phases at all temperatures below melting. Cross polarized optical microscopy (POM) was used to assess the textures of the liquid crystalline phases and the textures of quinones 1 and 2 are shown in Fig. 9. Upon cooling anthraquinone 1 from an isotropic liquid to 89 1C, a texture consistent with a smectic mesophase emerges, shown in Fig. 9a. Further cooling of anthraquinone 1 resulted in a soft crystal formation, which is consistent with both the DSC thermogram and P-XRD pattern shown in the bottom trace of Fig. 8a. Quinone 2 exhibits two different texture patterns, shown in Fig. 9b and c. Upon cooling anthraquinone 2 from 4632


an isotropic liquid to 59 1C, a clear conic focal pattern emerges. Others have observed conical focal patterns with bent core mesogens and the texture is consistent with a Smectic C phase.19 When further cooled to 30 1C the POM texture image changes as shown in Fig. 9c, which is not well-characterized and unclear in the P-XRD pattern. The combination of DCS, P-XRD and POM data provides evidence for the assignment of phases, which are summarized in Table 2.

Conclusions In summary, three new electron-deficient quinones have been synthesized. Anthraquinone 1 shows a large HOMO–LUMO energy difference, which is decreased by installing a dicyanomethylene to give quinone 2 and further decreased by condensing a second malononitrile to give quinone 3. Cyclic voltammograms of each quinone show a two-step, one-electron redox reaction, which occurs at progressively anodic potentials through the series 1–3. Interestingly, the dicyanoanthraquinone, 2, and the tetracyanoanthraquinone, 3, have bent geometries when neutral and in the ground state, which change to planar geometries upon electrochemical reduction, as supported by DFT calculations and spectroelectrochemical experiments. Differential scanning calorimetry experiments, in conjunction This journal is

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with cross-polarized microscopy and powder X-ray diffraction patterns, show that quinone 1 has a small smectic liquid crystalline phase that spans from 75 1C to 90 1C. Bent-core, Quinone 2, is a smectic liquid crystal phase from 49 1C to 64 1C and a soft crystal phase from 6 1C to 49 1C. Quinone 3, despite possessing the strong electron accepting properties, does not form LC phases. These organic materials were synthesized, characterized and have potential as self-assembling n-type materials, which could play roles in organic devices that require electron transport. Furthermore, the liquid crystalline thermal properties of 2 are advantageous for charge carrying applications and the exploitation of the molecular shape change upon reduction is a current pursuit.

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Acknowledgements The authors thank the Natural Sciences and Engineering Research Council (NSERC) of Canada Discovery Grants program and the Canada School of Energy and Environment (CSEE) Proof-of-Principle Grant program. AEM thanks the University of Calgary’s Program for Undergraduate Research Experience for financial support.

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