Photo- and Electro-Driven Molecular Switching System of Aryl-Bridged

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A: Kinetics, Dynamics, Photochemistry, and Excited States

Photo- and Electro-Driven Molecular Switching System of Aryl-Bridged Photochromic Radical Complexes Katsuya Yamamoto, Katsuya Mutoh, and Jiro Abe J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b12384 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 21, 2019

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The Journal of Physical Chemistry

Photo- and Electro-Driven Molecular Switching System

of

Aryl-Bridged

Photochromic

Radical

Complexes Katsuya Yamamoto†, Katsuya Mutoh†, 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 E-mail: [email protected] Tel.: +81-42-759-6225

1 ACS Paragon Plus Environment

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ABSTRACT Fast photochromic molecules have received much interest in the potential application as a real-time switching trigger in material and biological chemistry. Pentaarylbiimidazole (PABI) and phenoxyl– imidazolyl radical complex (PIC) are one of the fast photochromic molecules based on imidazolyl radicals. Because the photochromic reaction of these fast photochromic molecules proceeds from the optically forbidden S1 state, it is difficult to estimate the excitation energy to induce the photochromic reactions by spectroscopic techniques. In this study, we performed the electrochemical measurements for PABI and PIC to investigate the electronic properties and to determine the S0–S1 transition energies. In addition, we also revealed that the electrochemical reduction of PABI and PIC generates the radical anion which spontaneously shows the C–N bond breaking reaction to produce the radical species. The initial photochromic dimer is reproduced by the reversible oxidation of the anion species. This characteristic photochromic and electrochromic properties can be applicable to the photowritable electrochromic devices with high spatial resolution.

1. INTRODUCTION Molecular switches have been attracted considerable interest over the last half century because of their various unique properties that can be regulated in response to external stimuli. These switching molecules have been applied to smart windows,1–3 molecular motors4–6 and controllable nanodevices.7,8 Photochromic molecules are well-known class of photo-switching molecules that undergo the isomerization between the colorless and the colored isomer reversibly. Because some photochromic molecules change their structures and electronic properties by electrochemical reduction or oxidation, advanced molecular switches with the combination of photo- and electrochemical stimuli have been achieved by utilizing azobenzene,9 spiropyran10 and diarylethene.11,12 For example, Kawai et al. demonstrated that the closed-ring isomer of the photochromic terarylenes is oxidized to the radical cation, which spontaneously converts to the radical cation of the open-ring isomer. The open-ring radical cation oxidizes another neutral 2 ACS Paragon Plus Environment

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closed-ring isomer through the intermolecular electrochemical reaction in the chain reaction manner, resulting in the highly efficient cycloreversion reaction far exceeding 100 %.13 Thus, it is important to facilitate further development of advanced molecular switches responsive to multiple stimulations over the conventional switching system. Hexaarylbiimidazole (HABI) is one of the T-type photochromic molecules which is readily cleaved into a pair of the colored 2,4,5-triphenylimidazolyl radicals (Im•) by irradiation with UV light. Photogenerated Im• thermally recombines to reproduce the initial imidazole dimer (Scheme 1a).14–16 We recently developed fast switchable photochromic molecules by restricting the diffusion of the photogenerated Im•s by bridging the radicals to achieve the rapid thermal back reaction rate.17–19 Pentaarylbiimidazole (PABI) and phenoxyl-imidazolyl radical complex (PIC) are a novel type of fast switchable photochromic molecules (Scheme 1b and 1c).20,21 PIC is the first example that reversibly generates two structurally and electronically different stable radicals upon UV light irradiation. The thermal back reaction rate of the biradical forms of PABI and PIC can be tuned from tens of nanoseconds to tens of seconds by modification of their molecular frameworks. Especially, a replacement of the phenyl ring at the 2-position of the imidazole ring to the thiophene ring dramatically stabilizes the biradical form and decelerates the thermal back reaction rate (2PheTPIC, Scheme 1d).22 These fast photochromic compounds have been received much attention because of their potential applications to fluorescence switching and real-time holography.23–27 The investigation of the electronic state gives an important insight to develop high performance fast switchable photochromic molecules. The photochromic reaction of imidazole dimers proceeds via the S1 state which has a bond dissociation character.28,29 The S0–S1 transition in visible light region is assigned to the HOMO–LUMO transition. However, the S0–S1 transition is optically forbidden because the HOMO and LUMO are localized on the perpendicularly oriented two imidazole rings in PABI (or localized on the imidazole and the phenoxyl rings in PIC), resulting in the small overlap between the HOMO and the LUMO (Figures 3 and S21). Therefore, UV light irradiation is required to induce the photochromic reaction of imidazole dimers. For these reasons, it was difficult to estimate the excitation energy to induce the photochromic reaction of 3 ACS Paragon Plus Environment


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imidazole dimers by spectroscopic measurements. Electrochemical measurement is a powerful tool to estimate the HOMO–LUMO gap or the excitation energy experimentally. The determination of the excitation energy of fast photochromic molecules leads to the design of attractive fast photoswitching systems driven by visible or near-infrared light integrating the sensitizing mechanisms.30–33 Moreover, it is known that photochromic imidazole dimers also show electrochromism by electrochemical reduction.34–36 Therefore, the electrochemical study for the fast photochromic molecules also provides a new perspective for molecular switching by the combination of fast photochromism and electrochromism. Here, we investigated the electrochemical reduction process of PABI and PIC by spectroelectrochemical methods.

Scheme 1. Photochromic Reaction Schemes of (a) o-Cl-HABI, (b) PABI, (c) PIC, and (d) 2PheTPIC

Scheme 2. Redox Behaviors of (a) o-Cl-HABI, (b) PABI and (c) PIC


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2. EXPERIMENTAL SECTION 2.1 Chemical and Reagents. Optima grade acetonitrile and dichloromethane from Kanto Chemical Co, Inc. were used as received. Tetrabutylammonium hexafluorophosphate (TBAPF6) from Tokyo Chemical Industry Co., Ltd. (TCI) 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 methanol) from TCI. were used without further purification. PABI, PIC and 2PheTPIC were synthesized according to literature procedures.20–22

2.2 Voltammetry Measurements. All cyclic voltammetry (CV) and differential pulse voltammetry (DPV) measurements were performed in a conventional three-electrode cell. A glassy carbon electrode (0.6 cm in diameter) was employed as a working electrode after polishing with 1 m diamond on a diamond polishing pad and then with 0.05 m alumina on an alumina polishing pad attached to a glass plate (ALS Co., Ltd). The electrode was rinsed with pure acetone and dried in air before use. A platinum wire was used as a counter electrode, and an Ag/Ag+ reference electrode (Ag wire, 0.01 M AgNO3, 0.10 M tetrabutylammonium hexafluorophosphate in acetonitrile) was employed. All CV measurements were achieved from 0.05 to 1 V/s in solutions of 0.1 M TBAPF6 in 5 ACS Paragon Plus Environment


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acetonitrile or dichloromethane at room temperature. Prior to each experiment, the solutions were deoxygenated by bubbling with nitrogen, and the nitrogen atmosphere was maintained throughout the course of the experiments. All potentials are referenced to the reversible formal potential for the ferrocene/ferrocenium (Fc/Fc+) couple. Autolab 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 0.5 mm path length and a 1 cm path length were used for the spectroelectrochemistry for all the compounds. A standard three-electrode arrangement with a Pt mesh as a working electrode, a platinum wire as a counter electrode, and Ag/Ag+ as a reference electrode was employed. A spectrophotometer (Ocean-FX, Ocean Optics, Inc.) was used to record the absorption spectra in the range from 350 to 900 nm. All measurements were performed under a nitrogen atmosphere.

3. RESULTS AND DISCUSSION 3.1 Electrochemical Reduction Processes for o-Cl-HABI, PABI and PIC. Figure 1 shows the cyclic voltammograms for o-Cl-HABI (black line), PABI (blue line) and PIC (red line) in acetonitrile with 0.1 M TBAPF6 as a supporting electrolyte. The irreversible reduction peak of o-Cl-HABI was observed at −1.75 V. This irreversible reduction process can be described as an ECE mechanism according to Scheme 2a showing the electrochemical reduction scheme of o-Cl-HABI. The first one-electron reduction of o-Cl-HABI generates the radical anion (o-Cl-HABI•−) that causes the spontaneous C–N bond breaking between the imidazole rings, resulting in the formation of an imidazolyl radical (Im•) and an imidazole anion (Im−).34,35 The generated Im• is immediately reduced to Im− at −1.75 V because Im• has a more positive reduction potential. The imidazole anion is oxidized to Im• at −0.10 V by the return sweep of the CV measurement as discussed later. The irreversible reduction peaks of PABI and PIC were observed at −1.59 V and −2.03 V, respectively. 6 ACS Paragon Plus Environment

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These reduction peaks of PABI and PIC can be attributable to the similar ECE mechanism with that of o-Cl-HABI (Scheme 2b, 2c). One electron reduction of PABI and PIC produces the closed-ring radical anions (PABI•− and PIC•−), which spontaneously show the C–N bond cleavage reaction. The subsequent reduction of the radicals generated from the closed-ring radical anions produces the dianions (Im−–Im− and Im−–PhO−) at the same reduction potential.

Figure 1. Cyclic voltammograms of 1.8 mM o-Cl-HABI (black line), 1.8 mM PABI (blue line), and 1.3 mM PIC (red line) in acetonitrile containing 0.1 M TBAPF6 as the supporting electrolyte. Potential scan rate = 500 mV/s.

Figure 2 shows the change in the UV–vis absorption spectra of PABI and PIC in a 0.1 M TBAPF6– acetonitrile solution upon −1.8 and −1.9 V potential applications, respectively. In both cases, the new absorption bands attributable to the dianions (Im−–Im− for PABI and Im−–PhO− for PIC) appeared in the UVA region (Figure S22). Thus, the C–N bond breaking of PABI and PIC occurred not only by photo-irradiation but also by electrochemical reduction. Figure 3 shows the spatial distributions of the HOMOs and the LUMOs of PABI and PIC by the DFT calculations. Because the C–N bonds have antibonding character, PABI and PIC show the C–N bond breaking by the electron injection from the electrode to the LUMOs. The energy levels of the HOMO and the LUMO of PABI and PIC were predicted from the oxidation and reduction potentials obtained from the CV measurements. Table 1 summarizes the oxidation and reduction potentials of PABI and PIC measured by CV and the excitation energies estimated by the TDDFT calculations (Figure S24). 7 ACS Paragon Plus Environment


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Because the CV for the reduction processes of PABI and PIC are irreversible due to the ECE mechanisms, the HOMO–LUMO energy gaps of PABI and PIC were approximately experimentally estimated to be 2.5 eV and 2.8 eV, respectively, by using the reduction and the oxidation peaks. The experimentally estimated energy gap between the HOMO and the LUMO of PIC is larger than that of PABI, which is consistent with the DFT calculation results. The difference in the HOMO–LUMO energy gaps between PABI and PIC is mainly related to the LUMO levels localized on the diphenylimidazole or tert-butyl phenol units. By comparing with the reduction potentials between PABI and PIC, the LUMO level of PABI is lower than that of PIC. As mentioned in the introduction part, the optically forbidden S0–S1 transitions of PABI and PIC are the HOMO–LUMO transition, the estimated HOMO–LUMO energy gaps from the electrochemistry can be approximated as the excitation energy to the S1 state (Figures S21 and S24). According to E = hc/, the HOMO–LUMO energy gaps for PABI and PIC are consistent with the photon energy of the wavelength at 502 nm and 443 nm, respectively. Therefore, although PABI and PIC do not have any absorption bands in visible light region (Figure S21), PABI and PIC will become a good candidate for a visible-light-driven photoswitchable molecule by combining with a singlet photosensitizer.33

Figure 2. Spectroelectrochemistry of (a) 1.0 mM PABI and (b) 1.0 mM PIC 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 1.5 s intervals.


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Figure 3. Spatial distributions of the HOMOs and LUMOs of (a) PABI and (b) PIC calculated by the DFT (M052X/6-31G* level of the theory). Table 1. Electrochemical Reduction (Ered.), Oxidation (Eox.) Peaks and HOMO–LUMO Energy Gaps (Eg,elec.) Estimated by Electrochemistry, and Excitation Energies (Eex.) Estimated by TDDFT Calculations (MPW1PW91/6-31+G**//M052X/6-31G*) Ered. (V)

Eox. (V)

Eg,elec. (eV)

Eex. (eV)

PABI

−1.59

0.88

2.5

2.47

PIC

−2.03

0.80

2.8

2.88

3.2. Electrochemical Oxidation Processes of the Dianion Species. After Im− was generated by the electronic reduction of o-Cl-HABI at −1.75 V, an oxidation peak is observed at −0.10 V on the return sweep. This oxidation current is attributed to the one-electron oxidation of Im− to generate Im•. The Im• thermally recombines to the parent o-Cl-HABI. As similar with the CV of o-Cl-HABI, the irreversible oxidation peak of the Im−–Im− of PABI was observed at −0.030 V after the reduction of PABI at −1.59 V. This process can be also assigned to the formation of the biradical (Im•–Im•) by the stepwise two electron oxidation of Im−– Im−. Unlike the cyclic voltammogram of o-Cl-HABI, the reduction peak coupled with the oxidation peak of the dianion was not observed. That is, the generated Im•–Im• rapidly reproduces the initial imidazole dimer by the radical–radical recombination reaction prior to the reduction on the electrode due to the potential scan rate (500 mV/s) smaller than that of the thermal recombination reaction within tens of microseconds. In the 9 ACS Paragon Plus Environment


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same manner, the two oxidation peaks after the reduction of PIC at −2.03 V were clearly observed at −0.76 V and −0.63 V. This process can be also assigned to the formation of Im•–PhO• by the stepwise two electron oxidation of Im−–PhO−. The reduction peak coupled to the oxidation peak at −0.63 V could not be observed as similar with that of PABI due to the fast radical–radical recombination reaction of Im•–PhO•. In contrast, when the applied potential was returned from the first oxidation potential at −0.71 V to −2.1 V, the oxidation process of the dianion of PIC becomes fully reversible (Figure S25). Therefore, it is expected that the oxidation potentials of the dianion and the open-ring radical anion of PIC are separated because the redox potentials of the phenolate anion unit (PhO−) and the imidazole anion unit (Im−) are significantly different. We also measured the cyclic voltammograms of the dianion species of PABI and PIC prepared from each of lophine precursors under the basic conditions (Figure 4 and S26, Scheme S2). We observed the oxidation peaks of the dianions similar with those at the same oxidation potential on the return sweep of the CV measurements of PABI and PIC. These results clearly indicate that the C–N bond of PABI and PIC is broken by the electrochemical reduction. Notably, the two characteristic oxidation peaks are observed in the oxidation process of the dianion species of PIC, while the oxidation of that of PABI shows one oxidation peak. There are two possibilities about the structure of the open-ring radical anion of PIC, namely, Im−– PhO• and Im•–PhO− (Scheme 2c). To determine which radical anion is the predominant species, we investigated the oxidation potential of the imidazole anion unit and the phenolate anion unit in detail. We designed two reference compounds, MIm–PhO and Im–MPhO in which the NH and the OH groups are protected by a methyl group, respectively (Scheme 3). Each anion species (MIm–PhO– or Im––MPhO) is prepared in the basic solution. The oxidation of each species selectively generates the imidazolyl radical or the phenoxyl radical. Figure 5 shows the cyclic voltammograms and the differential pulse voltammograms of MIm–PhO− (pink line) and Im−– MPhO (green line) in dichloromethane with 0.1 M TBAPF6 as the supporting electrolyte containing 1.4 mM TBAOH as a basic reagent. The reversible oxidation peaks of MIm–PhO− and Im−–MPhO were detected at −0.71 V and −0.34 V, respectively. These reversible peaks are described to the 10 ACS Paragon Plus Environment

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redox process of the phenol unit and the imidazole unit. The redox potential of the phenol unit is more negative than that of the imidazole unit, indicating that PhO− is preferentially oxidized, compared with Im−. We also estimated the HOMO energy levels of MIm–PhO− and Im−–MPhO by the DFT calculation at the M052X/6-31+G** level of the theory. The HOMO energy levels of MIm–PhO− and Im−–MPhO were estimated to be −2.81 eV and −2.48 eV, respectively (Figure S29). The higher HOMO energy level of MIm–PhO− is consistent with the results of the electrochemistry. Because the oxidation potentials of MIm–PhO− and Im − –MPhO are consistent with the two oxidation peaks of Im−– PhO− in the cyclic voltammogram of PIC, the open-ring radical anion species of PIC generated by one-electron oxidation of Im−– PhO− would be described as a superposition of the phenoxyl radical and the imidazole anion substructures (Im−–PhO•).

Figure 4. Cyclic voltammograms of (a) 1.8 mM PABI (blue line), 0.97 mM PABI lophine (deep blue line) and PABI lophine under the basic condition (pale blue line), and those of (b) 1.3 mM PIC (red line), 1.1 mM PIC lophine (orange line) and PIC lophine under the basic condition (pale red line) in acetonitrile containing 0.1 M TBAPF6 as the supporting electrolyte. TBAOH (2.4 mM) was used as a basic reagent. Potential scan rate = 500 mV/s.

Scheme 3. Preparations and Redox Behaviors of MIm–PhO− and Im−–MPhO.



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Figure 5. (a) Cyclic voltammograms and (b) differential pulse voltammograms for 1.2 mM Im−– MPhO (pink line) and 1.4 mM MIm–PhO− (green line) in dichloromethane containing 0.1 M TBAPF6 as a supporting electrolyte and 1.4 mM TBAOH as a base. Potential scan rate = (a) 50 mV/s (b) 20 mV/s.

3.3 The Direct Detection of the Electrochemically Generated Biradical Species of 2PheTPIC. As described above, the electrochemical oxidation of the dianions of PABI and PIC generates the corresponding biradical species via the open-ring radical anions. However, the reduction process of the biradicals of PABI and PIC could not be detected because of the fast radical–radical recombination reactions. In our latest study, we revealed that the thermal back reaction rate of the biradical can be decelerated by changing the bridging phenyl ring to the 2-phenyl-thiophene ring 12 ACS Paragon Plus Environment

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(2PheTPIC, Scheme 1d).22 The half-life of the photogenerated biradical species of 2PheTPIC was estimated to be 33 s in benzene at room temperature. The CV measurement and spectroelectrochemistry for 2PheTPIC revealed that the electrochemical reduction mechanism of 2PheTPIC is similar with that of PIC (Scheme 4). The cyclic voltammograms for 1.0 mM 2PheTPIC (purple line) and PIC (dashed red line) in acetonitrile with 0.1 M TBAPF6 are shown in Figure 6. The irreversible reduction peak was observed at −2.0 V which can be attributable to the generation of the dianion of 2PheTPIC through the C–N bond breaking reaction and the subsequent reduction of the generated radical species. On the return scan of the CV measurement, the two reversible oxidation peaks were clearly observed at −0.74 V and −0.41 V. The first oxidation peak at −0.74 V can be attributable to the oxidation of the dianion (Im−–PhO−) to the open-ring radical anion (Im−– PhO•). As compared with the voltammogram of PIC, the oxidation potential shifted to more negative potential than that of PIC because the electron donating ability of the thiophene ring raises the HOMO energy of the anion species. The second oxidation peak attributable to the redox process between Im−–PhO• and Im•–PhO• was observed at −0.41 V. We also conducted the UV–vis–NIR spectroelectrochemistry by changing the electrode potential between −1.9 V to 0 V to confirm the formation of the biradical species by the oxidation of the dianion species. The UV–vis–NIR absorption spectrum under the potential at −1.9 V shows the increase in the absorbance at 450 nm (Figure 7a). The absorption band at 450 nm is the characteristic band of the Im−–PhO− (Figure S23). The absorption spectral change with the oxidation from Im−–PhO− to Im−–PhO• was observed by changing the electrode potential from −1.9 V to −0.50 V (Figure 7b). The absorption maximum was shifted from 450 nm to 471 nm and the new absorption bands appeared at 600 nm and 900 nm regions. These absorption bands are in good agreement with those of the calculation results for the open-ring radical anion (Im−–PhO•, Figure S30). When the potential was changed to 0 V, the absorption bands at 471 nm at 900 nm gradually disappeared and the broad absorption band at around 700 nm appeared (Figure 7c). This absorption band at 700 nm is consistent with that of the biradical species of 2PheTPIC generated upon UV light irradiation (Figure S27).22 This 13 ACS Paragon Plus Environment


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characteristic band of the biradical species of 2PheTPIC is originated from the radical–radical interaction between the imidazolyl and phenoxyl radicals. Therefore, we achieved the direct detection of the electrochemically generated biradical species by reducing the rate of the thermal back reaction.

Figure 6. Cyclic voltammograms in acetonitrile for 1.0 mM 2PheTPIC (purple line) and 2.0 mM PIC (red line) containing 0.1 M TBAPF6 as a supporting electrolyte. Potential scan rate = 500 mV/s. Scheme 4. Redox Behavior of 2PheTPIC.

Figure 7. Spectroelectrochemistry of 1.0 mM 2PheTPIC under the constant potential at (a) −1.9 V, (b) −0.5 V and (c) 0 V versus Fc/Fc+ in acetonitrile containing 0.1 M TBAPF6 at room temperature. 14 ACS Paragon Plus Environment

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Each of the spectra was recorded at 1.5 s intervals.

3.4 The Photochemically Gated Electrochromic System of 2PheTPIC. Electrochemical reaction usually takes place over the entire electrode surface, a lot of electrodes are required to achieve high spatial resolution for electrochromic displays.37 2PheTPIC generates the green biradical species with high spatial resolution upon UV light irradiation. In addition, the biradical species also shows the electrochromism from the green biradical to the yellow dianion as observed in the CV measurements for 2PheTPIC (Figure 6), in which the biradical is electrochemically reduced to the dianion under the potential at −0.79 V. Therefore, a novel electrochromic system with high spatial resolution can be constructed by the combination of the photochromic and electrochromic reactions of 2PheTPIC. The electrochemical reduction process of the photogenerated biradical species of 2PheTPIC was confirmed with the spectroelectrochemistry for 2PheTPIC immediately after the UV light irradiation. Figure 8a shows the change in the absorption spectrum of the photogenerated biradical species as a function of the applying time of the potential. UV light (ex.= 365 nm, 120 mW) is irradiated to the 2PheTPIC solution for 30 s to generate the biradical species. Immediately after turned off the UV light, the potential at −1.3 V is applied to the solution for 60 s to reduce the photogenerated biradical species to form the dianion. The new absorption band at 450 nm which can be attributable to the dianion quickly observed accompanied with the disappearance of the broad absorption band at around 700 nm. This absorption band at 450 nm was not observed only by applying the potential at −1.3 V or UV light irradiation (Figure S28). Therefore, it is expected that the color of the 2PheTPIC is drastically changed by the formation of the dianion species by the electrochemical reduction of the photogenerated biradical species. We demonstrated the partial color change on the electrode by combining the photochromism and electrochromism of 2PheTPIC (Figure 8b). We prepared the transparent glass coated with tin-doped indium oxide (ITO) as a working electrode and the ionic liquid gel of 2PheTPIC consisting of poly(dimethylaminoethyl methacrylate (PDMAEMA) and N,N,N’,N’-tetra(trifluoromethanesulfonyl)-dodecane-1,12-diamine (C12TFSA) as electrolytes to 15 ACS Paragon Plus Environment


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prevent the diffusion of the solute molecules (a gelator trial kit GK-1 purchased from Kanto Chemical Co., Inc., see Supporting Information). UV light is irradiated to the ITO glass by using a photomask, resulting in the color change from colorless to green by the photochromic reaction of 2PheTPIC. The green color was gradually changed to yellow only at the area exposed with UV light by applying the potential at −1.3 V. The yellow color was disappeared by subsequently applying the potential at 0 V (Movie S1). Thus, we succeeded to induce the electrochemical reaction at the specific area on the electrode by using the photochromic reaction of 2PheTPIC. This unique electrochromic behavior of the fast photochromic molecules shows a possibility of “writable” electrochromic displays upon light irradiation.

Figure 8. (a) Spectroelectrochemistry of 1.0 mM 2PheTPIC in acetonitrile containing 0.1 M TBAPF6 at room temperature under the UV light irradiation (ex. = 365 nm, 120 mW, for 30 s) and (b) the color change of the 2PheTPIC gel by the photochromism and electrochromism (Movie S1).

4. CONCLUSION 16 ACS Paragon Plus Environment

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In this study, we investigated the electrochromic behaviors of photochromic PABI and PIC derivatives and determined the energy level of the optically forbidden S1 state to induce the photochromic reactions from the results of the CV measurements. It was revealed that the energies of S0–S1 transitions of PABI and PIC correspond to the energy of the wavelength of 502 and 443 nm, respectively. Furthermore, we revealed the electrochemical C–N bond breaking of PABI and PIC by the electronic reduction process. The one-electron reduction of PABI and PIC generates the radical anion species that spontaneously causes the cleavage of the C–N bond, resulting in the formation of the open-ring radical anion. The open-ring radical anion is reduced to the dianion at the same reduction potential. It should be noted that the imidazole unit and the phenoxyl unit of the dianion and radical-anion species of PIC are electronically separated because the two units are oxidized in a stepwise manner in which the oxidation potentials are consistent with each individual unit. In addition, we demonstrated the spatially resolved electrochromic behavior of 2PheTPIC on the ITO coated thin glass electrode with the combination of the photochromism and electrochromism. The ITO glass including 2PheTPIC gel shows the electrochromic color change only at the part in which the biradical species are generated upon UV light irradiation. Therefore, the electrochemistry for the fast photochromic PABI and PIC provides important insights not only for the development of visible light responsive fast photoswitching systems with sensitization but also for constructing the multi-stimuli responsive switching systems such as writable electrochromic displays by light irradiation. Supporting Information Available: NMR spectra, ESI-TOF-MS spectra, additional experimental results and Movie S1. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT. This work was supported partly by the JSPS KAKENHI Grant Numbers JP18H05263, JP26107010, and JSPS KAKENHI Grant Number JP17K14475 for K.M.



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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Tel.: +81-42-759-6225 Author Contributions The manuscript was written through contributions of all authors.

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Table of Contents 65x43mm (300 x 300 DPI)


Photo- and Electro-Driven Molecular Switching System of Aryl-Bridged

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