Spectroelectrochemistry of a Photochromic [2.2]Paracyclophane

Article pubs.acs.org/JPCA

Spectroelectrochemistry of a Photochromic [2.2]ParacyclophaneBridged Imidazole Dimer: Clarification of the Electrochemical Behavior of HABI Katsuya Mutoh,† Emi Nakano,† and Jiro Abe*,†,‡ †

Department of Chemistry, School of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa 252-5258, Japan ‡ CREST, Japan Science and Technology Agency (JST), 7 Gobancho, Chiyoda-ku, Tokyo 102-0076, Japan ABSTRACT: The photochromic behavior of the imidazole dimers can be attributable to the photoinduced homolytic cleavage of the C−N bond between the two imidazole rings. On the other hand, although the simultaneous formation of the imidazolyl radical and imidazole anion by the one-electron reduction of an imidazole dimer was reported, no definitive evidence for this electrochemical reaction has been demonstrated. We report the first direct evidence for the electrochemical generation of the imidazolyl radical from the radical anion of the imidazole dimer by conducting the UV−vis−NIR spectroelectrochemical analysis of the [2.2]paracyclophane-bridged imidazole dimer. photosensitized dissociation of ο-Cl-HABI was described as an electron-transfer quenching process of the excited singlet state of JAW (1JAW*) by the ground state of ο-Cl-HABI. On the other hand, Evans et al. investigated the generation of Im•s by the direct electrochemical reduction of ο-Cl-HABI.8 Krongauz and Evans concluded that Im•s are generated by the C−N bond cleavage of the HABI anion radical formed by the photochemical or electrochemical reduction (Scheme 1). Thus, HABIs are classified into the unique molecules that can generate colored species reversibly by both light and electric stimuli. However, the simultaneous detection of Im•s and imidazole anions (Im−s) generated from the HABI anion radicals has not been achieved up to now. The difficulty of the simultaneous detection of these species lies in the rapid reduction of Im•s at the potential of the reduction peak of HABI. We have recently developed the [2.2]paracyclophane ([2.2]PC)-bridged imidazole dimers that show instantaneous coloration upon UV light irradiation and rapid fading in the dark.9 The [2.2]PC-bridged imidazole dimers also show photoinduced homolytic bond cleavage affording a pair of Im•s. We have also developed several derivatives of the [2.2]PC-bridged imidazole dimers applicable to a wide variety of photoresponsive systems.10 The electronic absorption spectrum of the radical species derived from the [2.2]PCbridged imidazole dimer is characteristic in that the absorption band extends to the near-infrared (NIR) region compared with that of Im•. The absorption band at 850 nm of the radical species can be assigned to that related to the radical−radical interaction resulting from the face to face alignment of a pair of Im•s.11 The simultaneous electrochemical formation of Im−

1. INTRODUCTION Photochromic materials are a well-known class of molecules that isomerize to the colored species with different absorption spectra by photoirradiation and the colored species go back to the initial state either thermally or photochemically. On the other hand, electrochromic inorganic materials such as Prussian blue also change their color reversibly by the electrochemical oxidation or reduction. These materials have received great attention due to their potential applications as smart windows and switching devices.1 These molecules change not only their color but also other properties, such as refractive indices, dipole moments, and electronic characteristics. Recently, electrochromism and the combination systems of photochromism and electrochromism of diarylethene have received much attention because of their potential use as organic memory and conductance switching materials.2 The electrochemical reactions have noteworthy advantages that they can control the number of reaction molecules and produce an excited state identical to that produced by photoexcitation in the molecularscale region. However, the electrochromism of diarylethene is the irreversible process,3 except for the one derivative reported very recently that isomerizes reversibly by both the photochemical and the electrochemical processes.4 Photochromic hexaarylbiimidazoles (HABIs) discovered by Hayashi and Maeda in the 1960s are readily cleaved, both thermally and photochemically, into a pair of 2,4,5triphenylimidazolyl radicals (Im•s), which thermally recombine to reproduce their original dimers.5 The photochromic behavior of HABIs can be attributable to the photoinduced homolytic reversible cleavage of the C−N bond between the two imidazole rings.6 Krongauz et al. reported that ο-Cl-HABI cleaves into Im•s efficiently upon visible light irradiation in the presence of the visible light photosensitizing dye, 2,5bis[(2,3,6,7-tetrahydro1H,5H-benzo[ij]quinolizin-1-yl)methylene]cyclopentanone (JAW).7 The mechanism of the © 2012 American Chemical Society

Received: April 23, 2012 Revised: May 31, 2012 Published: June 5, 2012 6792

dx.doi.org/10.1021/jp3038973 | J. Phys. Chem. A 2012, 116, 6792−6797

The Journal of Physical Chemistry A

Article

Scheme 1. Mechanism of the Dissociation of ο-Cl-HABI by an Electron Reduction

Scheme 2. Redox Behavior of pseudogem-bisTMDPI[2.2]PC

and Im• from the imidazole dimer would be also expected for the [2.2]PC-bridged imidazole dimer, owing to the presence of the C−N bond between the two imidazole rings. The pair of Im− and Im• (Im−−Im• in Scheme 2) can be distinguished from that of Im•s (Im•−Im•) due to the absence of the absorption band in the NIR region related to the radical− radical interaction. Therefore, the spectroscopic detection of Im−−Im• generated by the one-electron reduction of the [2.2]PC-bridged imidazole dimer would be definitive evidence for the subsequent C−N bond cleavage after the one-electron reduction of imidazole dimers. We have investigated in situ UV−vis−NIR spectroelectrochemistry for the [2.2]PC-bridged imidazole dimer, thereby enabling the long-standing problem to be concluded.

2.2. Cyclic Voltammetry Measurement. All cyclic voltammetry (CV) measurements were performed in a conventional three-electrode cell. A glassy carbon electrode (0.6 cm in diameter) was employed as the working electrode after polishing with 1 μm diamond on a diamond polishing pad and then with 0.05 μm alumina on an alumina polishing pad attached to a glass plate (BAS). The electrode was rinsed with pure water and dried in air before use. A platinum wire was used as the counter electrode, and a Ag/Ag+ reference electrode (Ag wire, 0.01 M AgNO3, 0.10 M tetrabutylammonium perchlorate in acetonitrile) was employed. All CV measurements were achieved from 0.05 to 1 V/s in solutions of 0.1 or 0.2 M TBAPF6 in acetonitrile or dichloromethane at room temperature. Prior to each measurement, the solutions were deoxygenated by bubbling with nitrogen, and this atmosphere was maintained over the electrochemical solutions throughout the course of the experiment. All potentials are referenced to the reversible formal potential for the ferrocene/ferrocenium (Fc/Fc+) couple. μAutolab III potentiostat/galvanostat (MetrohmAutolab B. V.) under computer control (General Purpose Electrochemical System software) was used for the CV measurement. 2.3. Spectroelectrochemical Measurement. Quartz glass cells with a 1 mm path length and a 1 cm path length were used for the spectroelectrochemistry for the imidazole dimers and the anion species of the imidazole dimers, respectively. A standard three-electrode arrangement with a

2. EXPERIMENTAL SECTION 2.1. Chemical and Reagent. Optima grade acetonitrile from Wako Pure Chemical Industries, Ltd. and CH2Cl2 from Kanto Chemical Co., Inc. were used as received. Tetrabutylammonium hexafluorophosphate (TBAPF6) from Wako Co., Ltd. was recrystallized from ethanol and dried in vacuo for 12 h before using as the supporting electrolyte. o-Cl-HABI and tetrabutylammonium hydroxide (TBAOH) (37 wt.% in ethanol) from Tokyo Chemical Industry (TCI) Co., Ltd. and pseudogem-bis(3,3′,4,4′-tetramethoxydiphenylimidazole)[2.2]paracyclophane from Kanto Chemical Co., Inc. were used as received. 6793



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detailed redox state cannot be revealed solely by the cyclic voltammogram, this electrochemical behavior is one of the characteristic features of the bridged imidazole dimers showing the fast back-reaction of the colored species. 3.2. Spectroelectrochemistry for ο-Cl-HABI and pseudogem-bisTMDPI[2.2]PC. To elucidate the formation of Im−−Im• as a transient intermediate of the electrochemical reduction process for Dimer, the in situ UV−vis−NIR spectroelectrochemistry was applied. Though the absorption band attributable to the formation of Im• could not be observed at the potential of −1.9 V versus Fc/Fc+ in a 0.1 M TBAPF6-acetonitrile solution of o-Cl-HABI, only the growth of the absorption band for Im− was observed under the constant potential condition, as shown in Figure 2. This experimental

Pt mesh as the working electrode, a platinum wire as the counter electrode, and a Ag/Ag+ as the reference electrode was employed. A USB4000 spectrophotometer (Ocean Optics, Inc.) was used to record the absorption spectra in the range from 300 to 900 nm. All measurements were performed under a nitrogen atmosphere.

3. RESULTS AND DISCUSSION 3.1. CV for ο-Cl-HABI and pseudogem-bisTMDPI[2.2]PC. The CV for 1.8 × 10−3 M o-Cl-HABI in acetonitrile with 0.2 M TBAPF6 as the supporting electrolyte in the potential region from 0.1 to −1.9 V (versus Fc/Fc+) is shown as the dashed line in Figure 1. As reported by Evans et al., the

Figure 1. Cyclic voltammograms in acetonitrile for 1.0 × 10−3 M pseudogem-bisTMDPI[2.2]PC containing 0.1 M TBAPF6 (solid line) and 1.8 × 10−3 M ο-Cl-HABI containing 0.2 M TBAPF6 (dashed line). Potential scan rate = 1.0 V/s.

Figure 2. Spectroelectrochemistry of 1.8 × 10−3 M o-Cl-HABI under the constant potential at −1.9 V versus Fc/Fc+ in acetonitrile containing 0.1 M TBAPF6 at room temperature. Each of the spectra was recorded at 30 s intervals.

irreversible voltammetric reduction step of ο-Cl-HABI is detected at −1.75 V versus Fc/Fc+.8 They assumed that the radical anion of o-Cl-HABI would result in the formation of Im• and Im− by the fast cleavage of the C−N bond between the imidazole rings and that the resulting Im• would be readily reduced to Im− at the same redox potential. On the return scan of the CV measurement, oxidation of Im− to the corresponding Im• is observed at −0.10 V versus Fc/Fc+ as a pseudo-reversible peak. The decrease in the height of the reduction peak at −0.17 V versus Fc/Fc+ coupled to the oxidation peak at −0.10 V versus Fc/Fc+ would be caused by the decrease in the concentration of Im• due to the radical−radical coupling forming the parent imidazole dimer. The cyclic voltammogram for pseudogem-bisTMDPI[2.2]PC (Dimer) in 0.1 M TBAPF6acetonitrile measured in the potential region from 0.1 to −1.9 V versus Fc/Fc+ is similar to that for o-Cl-HABI (the solid line in Figure 1). The irreversible reduction peak at −1.8 V versus Fc/ Fc+ is supposed to overlap both the one-electron reduction step of Dimer, forming the monoanion species (Im−−Im•) via Dimer•−, and the two-electron reduction step forming the dianion species (Im−−Im−) from analogy to the electrochemical behavior of o-Cl-HABI (Scheme 2a, reactions i−iii). On the other hand, the oxidation peak at −0.54 V versus Fc/ Fc+ of Im−−Im− can be also assigned to both the one-electron oxidation step forming Im−−Im• and the subsequent twoelectron oxidation step forming Im•−Im• (Scheme 2b, reactions iv and v). Compared with the cyclic voltammogram of o-Cl-HABI, the significant decrease in the height of the reduction peak of the return scan at −0.62 V versus Fc/Fc+ is observed, attributed to the radical recombination reaction of Im•−Im• to form Dimer (Scheme 2b, reaction vi). Though the

observation supports the rapid electrochemical formation of Im− from Im•, as observed by Evans et al. Figure 3 shows the

Figure 3. Spectroelectrochemistry of 2.0 × 10−3 M pseudogembisTMDPI[2.2]PC under the constant potential at −1.8 V versus Fc/ Fc+ in acetonitrile containing 0.1 M TBAPF6 at room temperature. Each of the spectra was recorded at 5 s intervals.

vis−NIR absorption spectra of pseudogem-bisTMDPI[2.2]PC in acetonitrile recorded simultaneously under the constant potential at −1.8 V versus Fc/Fc+. As is distinct from the vis−NIR spectroelectrochemistry for o-Cl-HABI, the increase in the absorption band at around 655 nm is clearly detected. This absorption band can be attributable to the presence of the monoradical species, Im•, by comparing its shape with that of the colored species of the conventional imidazole dimer, o-Cl6794



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HABI. The formation of the biradical species, Im•−Im•, can be excluded due to the absence of the absorption band in the NIR region characteristic of the colored biradical species of the [2.2]PC-bridged imidazole dimer. We have already reported that the radical−radical interaction between the adjacent Im• induces the absorption band in the NIR region.11 Moreover, the intense absorption band that appeared at around 400 nm indicates the presence of Im−. We also measured the UV−vis− NIR absorption spectrum for Im−−Im− prepared in the basic solution of pseudogem-bisTMDPIH[2.2]PC that is the precursor of pseudogem-bisTMDPI[2.2]PC (Scheme 3 and

to give a pair of Im• and Im− by the one-electron reduction of the imidazole dimer could be readily proved for the first time by the spectroelectrochemistry for the [2.2]PC-bridged imidazole dimer. 3.3. Spectroelectrochemistry for the Dianion Species. As described above, the electrochemical oxidation of Im− forms Im•, which would result in the formation of the imidazole dimer by the radical−radical coupling. In situ vis−NIR spectroelectrochemistry for the imidazole anions was carried out to investigate the absorption spectra observed by the electrochemical reduction of pseudogem-bisTMDPI[2.2]PC. The cyclic voltammograms of Im−−Im− and MIm−Im− are shown in Figure 5. MIm−Im− is an anion of pseudogem-DPIM-

Scheme 3. Preparation of Im−−Im− and MIm−Im−

Figure 5. Cyclic voltammograms of Im−−Im− (1.0 × 10−3 M) and MIm−Im− (1.0 × 10−3 M) with potential scan rates of 1.0 and 0.1 V/s, respectively, in CH2Cl2 containing 0.004 M TBAOH.

DPIH[2.2]PC11 that has an N-methylated imidazole ring to prevent the radical−radical coupling between the adjacent Im• formed by the oxidation of Im−. Im−−Im− and MIm−Im− were prepared from pseudogem-bisTMDPIH[2.2]PC and pseudogemDPIM-DPIH[2.2]PC, respectively, with tetrabutylammonium hydroxide (TBAOH) as a basic reagent in CH2Cl2 (Scheme 3). The CV measurements for Im−−Im− (1.1 × 10−3 M) and MIm−Im− (1.0 × 10−3 M) were performed in CH2Cl2 with 0.1 M TBAPF6 as the supporting electrolyte containing 0.004 M TBAOH. As expected, the oxidation of MIm−Im− produces the monoradical species, MIm−Im•, reversibly without the decrease in the height of the reduction peak at −0.53 V versus Fc/Fc+ coupled to the oxidation peak at −0.46 V versus Fc/Fc+ because of the inhibition of the dimerization reaction of the adjacent Im•. The slow intermolecular radical−radical coupling of MIm−Im• enables the reversible behavior of the cyclic voltammogram of MIm−Im− (Scheme 4).11 To the best of our knowledge, this is the first observation of the fully reversible redox behavior for the imidazolyl radical. On the other hand, the cyclic voltammogram for Im−−Im− shows an irreversible feature due to the radical−radical coupling between the

Figure 4. UV−vis−NIR absorption spectra of Im−−Im− (1.0 × 10−5 M, solid line) and Im− (1.3 × 10−5 M, dashed line) in CH2Cl2 containing 8 equiv of TBAOH.

Scheme 4. Redox Behavior of pseudogem-DPIMDPIR[2.2]PC

Figure 4). As compared with the UV−vis−NIR absorption spectrum for Im−, we confirmed that the UV−vis−NIR absorption spectra observed by the spectroelectrochemistry are composed of those of Im−−Im• and Im−−Im−. This suggests that the electrochemically generated Im−−Im• is reduced to Im−−Im− at the same redox potential in a similar manner as the electrochemistry for o-Cl-HABI. Fortunately, the decrease in the rate constant for the reduction step of Im−−Im• forming Im−−Im− would enable the detection of Im−−Im•. It should be emphasized that the subsequent C−N bond cleavage 6795



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adjacent Im• in Im•−Im• generated by the subsequent twoelectron oxidation of Im−−Im−. The oxidation peak at around −0.64 V (versus Fc/Fc+) of the cyclic voltammogram for Im−− Im− can be attributed to the stepwise two-electron oxidation of Im−−Im− to Im•−Im• (Scheme 2b, reactions iv and v). Figure 6 shows the vis−NIR spectroelectrochemistry of Im−−Im− and

Figure 7. Evolution of the vis−NIR spectrum of the spectroelecrochemistry of Im−−Im− in the CH2Cl2 solution (0.56 M) with TBAOH at −23 °C.

clearly from that of Im•−Im•. Im•−Im• has the characteristic absorption band at 850 nm that is related to the radical−radical interaction resulting from the face to face alignment of two imidazolyl radicals, whereas Im−−Im• shows an absorption maximum at 680 nm. This feature in the absorption spectra for the radical species made it possible to clarify the electrochemical behavior of the imidazole dimer. The electrochemical behavior of the photochromic imidazole dimer will open an exciting new avenue for future development of the highperformance photochromic systems.

Figure 6. Vis−NIR absorption spectra of Im•−Im• generated from pseudogem-bisTMDPI[2.2]PC by UV light irradiation (green), Im−− Im• generated by the one-electron oxidation of Im−−Im− (red), and MIm−Im• generated by the one-electron oxidation of MIm−Im−.

MIm−Im− measured at constant potentials of −0.67 and −0.48 V versus Fc/Fc+ at room temperature, respectively. The absorption spectrum for Im•−Im• measured by the laser flash photolysis for Dimer is also shown in Figure 6. The absorption spectrum of the electrochemically oxidized state of Im−−Im− is similar to that of MIm−Im−, indicating that these absorption bands can be attributable to those for the monoradical species, Im−−Im• and MIm−Im•, respectively. Though the twoelectron oxidized species of Im−−Im− may also be formed under this electrochemical condition, the corresponding absorption spectrum for Im•−Im• with the absorption band in the NIR region relating to the radical−radical interaction was not detected due to the fast radical−radical coupling between the adjacent Im• at room temperature. Thus, we could successfully demonstrate the formation of Im−−Im• either by the electrochemical reduction of Dimer or by the electrochemical oxidation of Im−−Im−. The slight difference in the absorption spectra for Im−−Im• shown in Figures 3 and 6 results from the difference in solvent. We have also carried out the vis−NIR spectroelectrochemistry of Im−−Im− at low temperature to observe the electrochemically generated Im•− Im• by decreasing the reaction rate for the radical−radical coupling. With increasing the potential more positively at −23 °C, the appearance of the absorption band at around 850 nm characteristic to Im•−Im• was detected, as shown in Figure 7. After applying the potential at 0.03 V versus Fc/Fc+ for 30 min, the solution changed color from colorless to green with UV light irradiation, indicating the formation of Dimer by the thermal radical−radical coupling of Im•−Im• (reaction vi).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research (A) (22245025) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan and the NAIST Advanced Research Partnership Project.

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REFERENCES

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4. CONCLUSIONS We report the first direct evidence for the electrochemical generation of the imidazolyl radical from the radical anion of the imidazole dimer by performing the UV−vis−NIR spectroelectrochemical analysis of the bridged imidazole dimer, pseudogem-bisTMDPI[2.2]PC. It should be noted that the absorption spectrum of Im−−Im• can be distinguished 6796



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(5) (a) Hayashi, T.; Maeda, K. Bull. Chem. Soc. Jpn. 1960, 33, 565− 566. (b) Hayashi, T.; Maeda, K. Bull. Chem. Soc. Jpn. 1962, 35, 2057− 2058. (6) (a) Dessauer, R. Photochemistry, History and Commercial Applications of Hexaarylbiimidazoles; Elsevier: Amsterdam, The Netherlands and Boston, MA, 2006. (b) Hayashi, T.; Maeda, K. Bull. Chem. Soc. Jpn. 1970, 43, 429−438. (c) Casper, J. V.; Khudyakov, I. V.; Turro, N. J.; Weed, G. C. Macromolecules 1995, 28, 636−641. (d) Lavabre, D.; Levy, G.; Laplante, J. P.; Micheau, J. C. J. Phys. Chem. 1988, 92, 16−18. (7) (a) Qin, X. Z.; Liu, A.; Trifunac, A. D.; Krongauz, V. V. J. Phys. Chem. 1991, 95, 5822−5826. (b) Liu, A.; Trifunac, A. D.; Krongauz, V. V. J. Phys. Chem. 1992, 96, 207−211. (c) Lin, Y.; Liu, A.; Trifunac, A. D.; Krongauz, V. V. Chem. Phys. Lett. 1992, 198, 200−206. (8) (a) Oliver, E. W.; Evans, D. H.; Casper, J. V. J. Electroanal. Chem. 1996, 403, 153−158. (b) Oliver, E. W.; Evans, D. H. J. Electroanal. Chem. 1997, 432, 145−151. (9) Kishimoto, Y.; Abe, J. J. Am. Chem. Soc. 2009, 131, 4227−4229. (10) (a) Harada, Y.; Hatano, S.; Kimoto, A.; Abe, J. J. Phys. Chem. Lett. 2010, 1, 1112−1115. (b) Kimoto, A.; Tokita, A.; Horino, T.; Oshima, T.; Abe, J. Macromolecules 2010, 43, 3764−3769. (c) Mutoh, K.; Hatano, S.; Abe, J. J. Photopolym. Sci. Technol. 2010, 23, 301−306. (d) Takizawa, M.; Kimoto, A.; Abe, J. Dyes. Pigm. 2011, 89, 254−259. (e) Mutoh, K.; Abe, J. J. Phys. Chem. A 2011, 115, 4650−4656. (f) Mutoh, K.; Abe, J. Chem. Commun. 2011, 47, 8868−8870. (g) Yamashita, H.; Abe, J. J. Phys. Chem. A 2011, 115, 13332−13337. (h) Kawai, S.; Yamaguchi, T.; Kato, T.; Hatano, S.; Abe, J. Dyes. Pigm. 2012, 92, 872−876. (11) Hatano, S.; Sakai, K.; Abe, J. Org. Lett. 2010, 12, 4152−4155.

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Spectroelectrochemistry of a Photochromic [2.2]Paracyclophane

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