Intensity-Dependent Photoresponse of Biphotochromic Molecule

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Intensity-Dependent Photoresponse of Biphotochromic Molecule Composed of a Negative and a Positive Photochromic Units Izumi Yonekawa, Katsuya Mutoh, Yoichi Kobayashi, and Jiro Abe J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b11673 • Publication Date (Web): 23 Dec 2017 Downloaded from http://pubs.acs.org on December 23, 2017

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Journal of the American Chemical Society

Intensity-Dependent Photoresponse of Biphotochromic Molecule Composed of a Negative and a Positive Photochromic Units Izumi Yonekawa†, Katsuya Mutoh†, Yoichi Kobayashi‡, 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 ‡ Department of Applied Chemistry, College of Life Sciences, Ritsumeikan University, 1-1-1 Noji-higashi, Kusatsu, Shiga 525-8577, Japan ABSTRACT: Light-selective multiple photochromic systems are important for advanced photoswitches of chemical reactions and biological activities. While UV light has been frequently utilized to induce photochromic reactions, visible light is energetically acceptable to avoid undesired reactions. However, many of the reported multi-photochromic systems still rely on UV light to induce at least a part of photochromic reactions. Here, we designed a bi-photochromic molecule showing intensity-dependent multiple coloration with a visible light source by incorporating two T-type photochromic units; a colored positive and a colorless negative photochromophores in a molecule. The negative photochromophore acts as a visible light sensitizer for the positive photochromic reaction. The compound shows intensity-dependent color change against visible light irradiation. The weak visible light excitation leads to the gradual decoloration from orange to yellow, whereas the intense laser excitation clearly changes the color to green. This characteristic photochromism can be achieved by the control of the photochromic reaction rates of the negative and positive photochromic reactions. The combination of negative and positive photochromic reactions gives attractive important insight to develop multi-responsive optical materials.

INTRODUCTION Realizing on-demand photoresponses among complex multiple photoresponsive systems by tuning external light stimuli is important for advanced photoswitches. Furthermore, reversible multiple photoresponse can induce different stimuli on demand. It will be also useful to develop novel techniques to reveal complex biological activities. In the past two decades, multi-photoresponsive materials have been extensively studied by incorporating more than two photochromic units in a molecule.1–13 Photochromic compounds change their optical and physical properties reversibly by light irradiation,14–18 and therefore, bi-photochromic or multi-photochromic systems are suitable for achieving reversible multiple photoresponses. However, the selective excitations of each unit were often challenging in these systems because of intramolecular energy transfers and superposition of the absorption spectra of each unit. As a few successive examples, Irie et al. have demonstrated the wavelength-selective coloration of the diarylethene dimer and the trimer.3,5 Bochet et al. demonstrated the orthogonal photoinduced deprotection by involving two different photoresponsive protecting groups working with ultraviolet (UV) and visible light, respectively.19,20 Feringa et al. recently reported the orthogonal photoswitching of two different photochromic reactions by using UV and visible light.11 UV light is frequently used to induce photochromic reactions of conventional artificial photochromic molecules. On the other hand, visible light has been utilized in photoreceptors in nature, such as the cis–trans isomerization of retinal in rhodopsin proteins.21,22 Visible light has several advantages over UV light. For example, visible light reduces the degradation of devices and mutagenesis of cells. In addition, visible light can penetrate into the inside of the sample more than UV light.

Visible-sensitive photochromic compounds have been extensively developed by using azobenzenes,23–25 diarylethenes,26 spiropyrans,27–35 Stenhouse adducts,36,37 and so on.38–40 However, many of the reported multi-photochromic systems still rely on UV light to induce at least a part of photochromic reacScheme 1. Photochromic Reaction Scheme of 1.

tions. In this study, we demonstrated intensity-dependent coloration by a visible light source. Recently, we have developed the several fast photoswitchable T-type photochromic compounds.41–48 The main idea of this study for the intensitydependent coloration with a visible light source is the utilization of a positive-type (P-) and a negative-type (N-) photo-

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chromic reactions with different thermal back reaction rates. We designed the novel bi-photochromic molecule 1 (Scheme 1) composed of thiophene-substituted phenoxyl imidazolyl radical complex (TPIC)48 as a P-photochromophore and binaphtyl-bridged PIC (BN-PIC) as a Nphotochromophore.47,49 TPIC generates the transient colored open-ring form (OF) (τ1/2 = 33 s) upon UV light irradiation. On the other hand, BN-PIC shows the decoloration reaction upon visible light irradiation. In this negative photochromic reaction, the colored form (Red) is the most stable isomer. The short-lived biradical (BR) and the transient colorless form (Yellow) (τ1/2 = 380 ns and 82 min at 298 K, respectively) were generated with UV or visible light irradiation. The initial state of 1 is the colored-closed state (Red-CF, Figure 1). Under the weak visible laser excitation to 1, the photoproduct generated by the N-photochromism of BN-PIC unit (YellowCF) is mainly observed because of the slow thermal back reaction of Yellow-CF. On the other hand, under the intense visible pulse laser excitation, another photoproduct by the Pphotochromism of TPIC (Red-OF) is clearly observed. The detailed analyses reveal that the photochromic reaction of the TPIC unit is induced upon visible light irradiation by the singlet sensitization via the excited state of the BN-PIC unit although unsubstituted TPIC is colorless and insensitive to the visible light irradiation. Moreover, the vivid color change is clearly observed depending on the excitation intensity although the simple mixture of two photochromic reactions makes the color dusty dark. This clear color change is a typical feature of the combination of N- and P-photochromic systems. It is noteworthy that studies exploring the constructive combination of the N- and the P-photochromic units are rather scarce11 because almost all of negative photochromic compounds work well only in the specific environment.27–35 Therefore, the constructive combination of the N- and the Pphotochromic units in this study is a good candidate for the biphotochromic system to realize the multiple coloration that

Figure 1. Intensity-dependent photochromism of 1 represented by the core reactions upon visible light irradiation.

responses to the excitation intensity. RESULTS AND DISCUSSION Compound 1 was synthesized according to Scheme 3 as shown in the experimental section. HPLC and ESI-TOF-MS revealed that Red-CF has four structural isomers. Because these isomers generate the same radical upon light irradiation,

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Figure 2. (a) UV−vis absorption spectra of Red-CF, Yellow-CF, BN-PIC, TPIC and the superposition of those of BN-PIC and TPIC in benzene.

Figure 3. The change in the absorption spectra of 1 (3.2 × 10−4 M) upon repeated 470-nm laser pulse irradiation (5 ns/pulse, 10 Hz) of (a) 0.4 mJ and (b) 10 mJ in benzene at 298 K. (c) The time profiles of the absorbance changes of 480 nm and 750 nm at 298 K under repeated laser pulse irradiation with several intensities.

one of the isomers was isolated and used for the spectroscopic studies. Figure 2 shows the absorption spectra of Red-CF, Yellow-CF, TPIC and BN-PIC in benzene. The absorption band at around 450 nm of Red-CF is almost superposition of those of TPIC and BN-PIC, indicating that only the BN-PIC unit in 1 has the visible light sensitivity. On the other hand, the absorption bands in UVA region are not superposition of those two spectra. Therefore, the N- and P-units are weakly coupled with each other through the π-conjugated aromatic ring. The repeated irradiation of weak nanosecond visible laser pulses of 470 nm (5 ns/pulse, 0.4 mJ, 10 Hz) induces the gradual color change of the Red-CF solution from orange to yellow as recognized by naked eyes (Figures 1 and 3a). The absorption spectrum after visible light irradiation is similar to that of Yellow of BN-PIC (Figure 2). This color change means Red-CF isomerizes to Yellow-CF upon visible light irradiation. Yellow-CF thermally returns to the initial Red-CF with the halflife of 55 min (k = 2.1 × 10−4 s−1) at 298 K (Figure S10). In contrast, the repeated irradiation of intense nanosecond visible laser pulses of 470 nm (5 ns/sec, 10 mJ, 10 Hz) shows the instantaneous color change from orange to green (Figures 1 and 3b). The absorption band at 750 nm instantaneously increased and reached a maximum under intense visible laser irradiation. In the same time, the absorbance at 480 nm rapidly decreased. The system subsequently reached a photostationary


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state (PSS) under 470-nm laser irradiation. The half-life of the green species after ceasing of visible light is estimated to be 19 s (k = 3.6 × 10−2 s−1) at 298 K (Figure S11). The absorption band at 750 nm of the green colored species (Figure 3b) is similar to that of OF of TPIC upon UV light irradiation.48 These results suggest that the concentration of OF increased upon intense 470-nm laser irradiation in spite of no photosensitivity of the TPIC unit to the 470-nm light (Figure S13). The maximum concentration of the green colored species under visible light irradiation increases with increasing the excitation laser power as shown in Figure 3c, indicating that the N- and P-photochromic features of Red-CF can be regulated only by controlling the intensity of the excitation visible light. To reveal the mechanism of the intensity-dependent color modulation of Red-CF, we carried out laser flash photolysis measurements. Figure S14 shows the time profile of the transient absorbance at 750 nm upon nanosecond visible laser irradiation. The irradiation of the 470-nm visible laser to RedCF generates two-transient species, fast and slow decay species. The slow decay species which remains until the second time scale can be attributable to OF of the P-unit. On the other hand, the fast decay species can be assigned to the intermediate short-lived BR of the N-unit because the half-life (= 0.7 µs at 298 K) is consistent with that of the photogenerated biradical of BN-PIC.47 Thus the visible light simultaneously induces the photochromic reactions of the N- and the P-units although only the N-unit has the sensitivity to visible light. It was previously reported that the LUMOs of the imidazole dimers and PIC derivatives possess the anti-bonding character revealed by the DFT calculations and the electrochemical studies.46,48,50 Therefore, the injection of an electron to the LUMO results in the bond breaking reaction of the corresponding C–N bond. As mentioned above, the S1←S0 transition of TPIC is mainly described as a HOMO–LUMO transition and is calculated at 504 nm by the TD-DFT calculation.48 However, the S1←S0 transition is optically forbidden because of the small overlap integral between the HOMO and LUMO. While the TPIC unit of Red-CF has also similar optically forbidden S1←S0 transition at 500 nm, the C–N bond cleavage can be occurred by the energy transfer from the BN-PIC unit upon the irradiation with 470 nm light. In order to elucidate the unprecedented result we investigated the following two mechanisms for the ringopening reaction of the TPIC unit upon visible light irradiation; (i) the energy transfer from the excited state of Red of the BN-PIC unit to the colorless TPIC unit, (ii) the energy transfer from the excited state of the photogenerated BR of the BN-PIC unit to the colorless TPIC unit. The excitation with a femtosecond laser pulse is an effective method to reveal the energy transfer mechanism. Because the generation of the biradical species (or OF) from imidazole dimers and PICs occurs on the time scale of subpicoseconds,10,51,52 the excitation with a femtosecond laser pulse does not prompt the further photochemical reaction of the biradical species. If the transient BR of the BN-PIC unit plays a crucial role in the energy transfer process leading to initiate the photochromic reaction of the TPIC unit, OF of the TPIC unit will not be formed upon irradiation with a femtosecond laser. The time evolution of the absorption spectra by the repeated irradiations of the femtosecond laser pulse are shown in Figure 4. The absorption band at 750 nm gradually increased with repeating the irradiation of the femtosecond laser pulse (λex. = 470 nm, 14 µJ/cm2, 1 kHz), indicating no contribution of the intermediate BR to the energy transfer process. It should be

mentioned that the power of the femtosecond laser was weak enough to prevent the simultaneous TPA. The double-pulse nanosecond laser flash photolysis also corroborated the energy transfer from the excited state of Red to CF because the excitation of the transient BR of the BN-TPIC unit does not proceed further photochemical reactions to produce OF (Figure S16). Because the spectral overlap between the Red unit and the CF unit in visible region is almost zero, the energy transfer would be explained by the Dexter mechanism.

Figure 4. Time evolution of the transient absorbance of 1 and the transient absorption spectra upon repeated femtosecond laser irradiation (λex=470 nm; FWHM, ~100 fs; pulse energy, 14 µJ/cm2) in benzene at 298 K.

Figure 5. Laser power dependence of the transient absorbance at (a) 740 nm and (b) 470 nm of the benzene solution of 1 (2.5 × 10−5 M, λex = 470 nm, pulse width: 5 ns, power: 2.6–8.5 mJ/pulse). Linear relationship between the laser power and ∆Absorbance monitored at (c) 740 nm and (d) 470 nm.

The efficiency of the conversion from Red-CF to Red-OF (Փ OF) was relatively estimated by the laser flash photolysis measurement. We assume the conversion efficiency of 1 from Red-CF to Yellow-CF (Փ Yellow) is the same as that of BN-PIC, which was previously estimated to be 0.09 by the laser actinometry.47 Փ OF can be estimated from the equation as follows:

∆


(1)


ΔOF

∆@ @

@

(2)

@

ΔYellow ∆Red ' ∆OF

ΔAbs@567 8@657

(|∆@ *∆@ |+ ,-./0@

(|@ *∆@ |+

(3)

,-./0@

@

(4)

@

where ∆Red is the amount of Red-CF reacted with 470-nm light, ∆OF and ∆Yellow are the amounts of Red-OF and Yellow-CF, respectively, generated upon 470-nm laser excitation to Red-CF, α@740 and α@470 are the slopes of the fits for the ∆Absorbance vs the irradiation power plots, and εRed-CF@470, εOF@470 and εOF@740 are the absorption coefficient of Red-CF at 470 nm, and those of the OF unit of Red-OF at 470 nm and 740 nm, respectively. Because we assume that the absorption coefficient of Red-OF (εRed-OF) is the superposition of the absorption coefficients of Red and OF of 1, the absorption coefficients of the OF unit of 1 (εOF) is the same as the previously estimated absorption coefficients of the OF of TPIC, whose values at 470 nm and 740 nm (εOF@470 and εOF@740) were estimated to be 3.3 × 103 and 1.1 × 104 M−1 cm−1, respectively. At first, we confirmed the linearity of the ∆Absorbance at 470 nm and 740 nm against the laser excitation power (Figure 5). α@740 and α@470 are the slopes of the fits for the ∆Absorbance vs the irradiation power plots, which correspond to the ∆Absorbance at 470 nm and 740 nm obtained by the 1-mJ laser excitation. The absorption coefficient of Red-CF at 470 nm (εRed-CF@470) was estimated to be 1.2 × 104 M−1 cm−1 in benzene. Because the absorption spectra of the Red and the OF units of 1 overlap with each other at 470 nm, the ∆Absorbance at 470 nm for OF (∆AbsOF@470) is subtracted from the total change in the Absorbance at 470 nm (∆Abs@470) to estimate the ∆Yellow generated upon 470-nm laser excitation (equations 3 and 4). From equations 1 to 4, the Փ OF value was estimated to be 0.005. As shown in Figures 1 and 3, the N- and P-photochromic reactions of 1 are controlled by changing the excitation intensity. The plausible overall photochromic reaction of 1 is Scheme 2. Plausible Photochromic Reaction Scheme of 1.

shown in Scheme 2. The excitation of Red-CF upon visible light irradiation generates the two isomers, Yellow-CF via short-lived BR-CF and Red-OF. The possibility of the generation of BR-OF from BR-CF upon 470-nm visible light irradiation was excluded by the femtosecond laser excitation and the double-pulse laser flash photolysis. Although the repeated excitation with visible light might proceed further isomerization from Red-OF to Yellow-OF, the efficiency of this total reaction (Red-CF → Red-OF → Yellow-OF) would be negligibly small (< 0.005 × 0.09 = 4.5 × 10−4). Thus, in order to give a qualitative explanation of the visible light intensity-

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dependent photochromic reaction, we approximated the total photochromic reaction by the simple two-equilibrium model as shown in Figures 1 and 6 where A, B and C are Red-CF, Yellow-CF and Red-OF, respectively. The rate equation of each isomer can be written as follows: d [A] = −φ AB I absA + φ BA I absB + k BA [B] − φ AC I absA + k CA [C] dt

(5)

d [B] = φ AB I absA − φ BA I absB − k BA [ B] dt

(6)

d [C] = φ AC I absA − k CA [C ] dt

(7)

I absA = ε A [A]I 0 F , I absB = ε B [B]I 0 F

(8)

(1 − 10 Abs' ) F = Abs'

(9)

Abs'= (ε A [A] + ε B [B] + ε C [C])

(10)

φ AB = φ Yellow = 0.09, φ AC = φ OF = 0.005

(11)

Figure 6. (a) Kinetic model of the biphotochromic reaction. (b) Time variation and the excitation power dependence of [Red-OF]. (c) The excitation power dependence of [Red-CF], [Red-OF] and [YellowCF] at the time of the maximum [Red-OF]. The parameters are set as follows: [A]0 = 4 × 10−4 M, [B]0 = [C]0 = 0 M, I0 = 0.2–40 mW, volume = 0.2 mL, εA = 1.2 × 104 M−1 cm−1, εB = 100 M−1 cm−1, εc = 3.3 × 103 M−1 cm−1, Փ AB = 0.09, Փ BA = 0.1, Փ AC = 0.005, kBA = 2.1 × 10−4 s−1 and kCA = 3.6 × 10−2 s−1.

where εA, εB and εC are the absorption coefficients of RedCF, Yellow-CF and Red-OF at 470 nm, kBA and kCA are the rate constants for the thermal back reaction of Yellow-CF and Red-OF, and I0 is the total amount of 470-nm light absorbed by the reaction medium. We tentatively assumed that the conversion efficiency from Yellow-CF to Red-CF (Փ BA) is approximately 0.1. These differential equations were numerically solved. Figure 6b shows the time profiles of the concentration of Red-OF ([Red-OF]) by changing the excitation light intensity. This simulation agrees well with the results of the experimental absorbance changes shown in Figure 3c. Some characteristic features in the time profiles of the transient absorbance can be observed by the experimental and simulated results; (I) The maximum value of [Red-OF] at the transient state increases with increasing the excitation intensity, (II) the concentration of Yellow-CF ([Yellow-CF]) at the time of the maximum [Red-OF] is reduced upon irradiation with high power excitation light, and (III) [Red-OF] at the PSS increases with increasing the excitation intensity. When a weak visible light (10 mW) accelerates the isomerization from Red-CF to Red-OF, and [Red-OF] instantaneously increases with the increase in the absorbance at 750 nm (Figures 3b and 6b). In addition, the isomerization from Red-CF to Yellow-CF is also accelerated, resulting in high [Yellow-CF] at the time of the maximum [Red-OF] (Figure 6c). The high [Yellow-CF] leads to the hypochromic effect of the orange color (II). [Red-OF] at the PSS also increases when the intense 470-nm light is irradiated to the solution of 1. Because Yellow-CF barely absorbs the light of 470 nm, the small amount of Red-OF is remained at the PSS via Red-CF photogenerated from Yellow-CF. The intense 470-nm light irradiation prompts the photoisomerization from Yellow-CF to Red-OF, and therefore [Red-OF] at the PSS also depends on the excitation intensity (III). The small differences in the time to reach the PSS between the experimental and simulation results might be due to the minor contribution of the photogeneration of YellowOF from Yellow-CF or Red-OF neglected in the simulation model or the degradation of compounds by the repeated laser irradiation. As the result, we can clearly recognize the color change to green by naked eyes due to the synergetic effect of the coloration by the P-photochromism and the decoloration by the N-photochromism. Therefore, the biphotochromic compound composed of the P- and N-units demonstrated the color modulation of 1 depending on the incident-light intensity by the control of the balance of the N- and P-photoconversion and the thermal back reaction rates. CONCLUSION We demonstrated the intensity-dependent multi-color photochromism by the combination of two kinds of T-type photochromic compounds, the negative and the positive photochromic units. The optically-forbidden S1←S0 transition of the positive unit is effectively used to induce both negative and positive photochromic reactions simply by irradiation of the visible light (Figure 1). The weak visible light irradiation mainly induces the negative photochromic reaction with keeping the low concentration of the green open-ring form. The color change of the solution results in orange to yellow. On the other hand, the intense light clearly changes the color of 1 to green because of the synergetic effect of the negative photochromism and the acceleration of the positive photochromic reaction rate. This intensity-dependent photoresponse of the biphotochromic molecule is explained by the two-equilibrium

model. The efficient combination of the negative and the positive photochromism realizes the intensity-dependent photochromic response originating from the isomerization ratio depending on the rate of the thermal back reaction and the incident excitation intensity. A novel functionality of the biphotochromic system demonstrated in this study will expand the versatility of biphotochromic systems and stimulate to develop further attractive photoresponsive compounds that consist of a negative and a positive photochromic units. EXPERIMENTAL SECTION Experimental Detail for Transient Absorption Spectroscopy. The transient absorption spectra and the time profiles of the transient absorbance of 1 were recorded on a USB 4000 multichannel detector (Ocean Optics). CUV-QPOD (Ocean Optics) equipped TC 125 temperature controller (QUANTUM) was used as a cuvette holder. The probe beam from a deuterium and a halogen lamps, DH-2000-BAL (Ocean Optics) were guided with a QP-600-1-SR optical fiber (Ocean Optics). Visible light (470 nm, pulse width, 5 ns) irradiation was performed by a Continuum Surelite II Q-Switched Nd:YAG coupled to a Continuum Panther EX OPO. Optical grade solvents were used for all measurements. Experimental Detail for Laser Flash Photolysis in Microsecond and Millisecond Time Scales. The laser flash photolysis experiments were carried out with a TSP-1000 time resolved spectrophotometer (Unisoku). A 10 Hz Q-switched Nd:YAG laser (Continuum Minilite II) with the third harmonic at 355 nm (pulse width, 5 ns) was employed for the excitation light. The excitation pulse at 470 nm (pulse width, 5 ns) was provided by a Continuum Surelite II Q-Switched Nd:YAG coupled to a Continuum Panther EX OPO. The probe beam from a halogen lamp (OSRAM HLX64623) was guided with an optical fiber scope to be arranged in an orientation perpendicular to the exciting laser beam. The probe beam was monitored with a photomultiplier tube (Hamamatsu R2949) through a spectrometer (UNISOKU MD200). The excitation intensity of one pulse was estimated by an energy detector (Gentec Electro-Optics QE12LP-S-MB) with an energy monitor (Gentec Electro-Optics MAESTRO). Optical grade solvents were used for all measurements. Materials and Reagents. All reactions were monitored by thin-layer chromatography carried out on 0.2 mm E. Merck silica gel plates (60F-254). Column chromatography was performed on silica gel (Silica gel 60N, Kanto Chemical Co., Inc.). NMR spectra were recorded at 400 MHz on a Bruker AVANCE III 400 NanoBay. DMSO-d6 and CDCl3 were used as deuterated solvent. ESI–TOF–MS spectra were recorded on a Bruker micrOTOF II-AGA1. Unless otherwise noted, all reagents and reaction solvents were purchased-from Tokyo Chemical Industry Co., Ltd., Wako Pure Chemical Industries, Ltd., Sigma-Aldrich Inc. and Kanto Chemical Co., Inc. and were used without further purification. Synthetic Procedure 2,6-di-tert-butyl-4-[2'-(4,4,5,5-tetramethyl-1,3-dioxolan-2yl)-[1,1'-binaphthalen]-2-yl]phenol (S1) and 2-(3,5-di-tertbutyl-4-hydroxyphenyl)-5-phenylthiophene-3-carbaldehyde (S3) were synthesized according to a literature procedure.47,48



Scheme 3. Synthetic Scheme of 1

1-(4-(2-(2'-(3,5-di-tert-butyl-4-hydroxyphenyl)-[1,1'binaphthalen]-2-yl)-5-phenyl-1H-imidazol-4-yl)phenyl)-2phenylethane-1,2-dione (S2). 4-(2'-(1,3-dioxolan-2-yl)-[1,1'binaphthalen]-2-yl)-2,6-di-tert-butylphenol (S1) (1.00 g, 1.89 mmol), 1,4-bisbenzil (645 mg, 1.88 mmol) and ammonium acetate (1.44 g, 18.8 mmol) in acetic acid (96 mL) was stirred for 20 hours at 110 °C. The reaction mixture was allowed to cool to room temperature and neutralized with aqueous ammonia. The precipitate was collected by filtration and washed with water. The crude product was purified with silica gel column chromatography twice (hexane/CH2Cl2 = 1/2 and ethyl acetate/hexane = 1/4) to give a mixture of the two structural isomers of S2 as yellow solid (458 mg, 1:0.22 mixture of the structural isomers, yield: 30 %). 1H NMR (400 MHz, DMSOd6) δ: 12.10 (s, 1H, one structural isomer), 11.92 (s, 1H, one structural isomer), 8.16 (d, J = 8.4 Hz, 1H, one structural isomer), 8.15 (d, J = 8.7 Hz, 1H, one structural isomer), 8.10– 7.78 (m, 8H, two structural isomers), 7.66–7.32 (m, 13H, two structural isomers), 7.24–7.14 (m, 3H, two structural isomers), 7.08–7.00 (m, 1H, two structural isomers), 6.57 (s, 2H, one structural isomer), 6.54 (s, 2H, one structural isomer), 6.50 (s, 1H, two structural isomers), 0.88 (s, 18H, one structural isomer), 0.85 (s, 18H, one structural isomer); HRMS (ESI-TOF) calcd for C57H48N2O3 [M+H]+, 809.3738; found, 809.3733. 2,6-di-tert-butyl-4-(2'-(4-(4-(2-(2-(3,5-di-tert-butyl-4hydroxyphenyl)-5-phenylthiophen-3-yl)-5-phenyl-1Himidazol-4-yl)phenyl)-5-phenyl-1H-imidazol-2-yl)-[1,1'binaphthalen]-2-yl)phenol (S4) and compound 1. 1-(4-(2-(2'(3,5-di-tert-butyl-4-hydroxyphenyl)-[1,1'-binaphthalen]-2-yl)5-phenyl-1H-imidazol-4-yl)phenyl)-2-phenylethane-1,2-dione (S2) (153 mg, 0.189 mmol), 2-(3,5-di-tert-butyl-4hydroxyphenyl)-5-phenylthiophene-3-carbaldehyde (S3) (77 mg, 0.20 mmol) and ammonium acetate (147 mg, 1.91 mmol) in acetic acid (12 mL) was stirred for 2 days at 110 °C. The reaction mixture was allowed to cool to room temperature, neutralized with aqueous ammonia and extracted with ethyl acetate. The organic layer was washed with water and brine, and the solvent was removed by evaporation. The crude product was purified with silica gel column chromatography (ethyl acetate/CH2Cl2=1/100) to give a mixture of the four structural

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isomers of S4 as yellow solid (145 mg, 65 %). This was used in the next step without further isomerization of the structural isomers. The 1H NMR spectrum of the mixtures of the isomers are shown in Figure S2. S4 was identified by HRMS. HRMS (ESI-TOF) calcd for C82H76N4O2S [M+H]+, 1181.5762; found, 1181.5707. The mixture of the structural isomers of S4 (61 mg, 0.0518 mmol) was dissolved in degassed benzene (12 mL). A degassed 15-mL aqueous solution of potassium ferricyanide (1.07 g, 3.26 mmol) and KOH (243 mg, 4.33 mmol) was added to the benzene solution. After vigorous stirring for 2.5 hours at room temperature, the resulting solution was extracted with benzene. The organic layer was washed with water and brine, and the solvent was removed by evaporation under reduced pressure. The residual solid was purified with silica gel column chromatography (ethyl acetate/CH2Cl2 = 1/100) to give the mixture of three structural isomers (Rf = 0.13, 36 mg, 59 %) and one of the structural isomers (isomer a, Rf = 0.03, 16 mg, 26 %) as orange solid. The isolated structural isomer was used for the spectroscopy. 1H NMR (isomer a, 400 MHz, DMSO-d6) δ: 8.22 (d, J = 8.4 Hz, 1H), 8.17 (d, J = 8.4 Hz, 1H), 8.07 (d, J = 7.6 Hz, 1H), 7.95 (s, 1H), 7.75 (d, J = 7.9 Hz, 2H), 7.50–7.06 (m, 24H), 6.64 (d, J = 8.0 Hz, 1H), 6.53 (s, 2H), 6.40 (d, J = 2.9 Hz, 1H), 6.33 (d, J = 7.3 Hz, 2H), 1.16 (s, 8H), 1.13 (s, 18H), 0.83 (s, 10H). 13C NMR (100 MHz, CDCl3): δ: 186.00, 185.13, 151.52, 151.32, 150.77, 148.91, 148.82, 148.26, 148.22, 147.70, 145.28, 142.80, 141.08, 140.62, 137.38, 136.55, 136.42, 136.32, 133.98, 133.84, 133.11, 132.78, 132.72, 130.78, 130.65, 130.62, 130.27, 130.18, 129.97, 129.91, 129.69, 129.47, 129.13, 128.77, 128.63, 128.50, 128.30, 128.13, 128.05, 127.91, 127.75, 127.26, 126.87, 126.62, 126.53, 126.44, 126.28, 126.18, 125.85, 123.91, 122.84, 118.24, 114.66, 68.46, 64.79, 35.76, 35.47, 35.03, 35.01, 29.60, 29.45, 29.00; HRMS (ESI-TOF) calcd for C82H72N4O2S [M+H]+, 1177.5449; found, 1177.5395.

ASSOCIATED CONTENT Supporting Information. NMR spectra, ESI-TOF-MS spectra, HPLC charts, additional experimental results (PDF) and Movie S1 (AVI). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We acknowledge to Dr. Okajima and Prof. Sakamoto (Aoyama Gakuin University) for the experimental support of femtosecond laser excitation. We appreciate to Dr. Rémi Métivier (PPSM, ENS Cachan, CNRS, Université Paris-Saclay) for the support of the numerical analysis. This work was supported in part by JSPS KAKENHI Grant Number JP26107010 in Scien-


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tific Research on Innovative Areas "Photosynergetics" and MEXT KAKENHI Grant Number JP17K14475 for K.M. Financial assistance for this research was also provided by the MEXT-Supported Program for the Strategic Research Foundation at Private Universities, 2013−2017.

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Table of Contents (TOC)


9


Figure 1 64x50mm (600 x 600 DPI)


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Graphical abstract 75x39mm (150 x 150 DPI)


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Intensity-Dependent Photoresponse of Biphotochromic Molecule

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