Article Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Intensity-Dependent Photoresponse of Biphotochromic Molecule Composed of a Negative and a Positive Photochromic Unit 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 S Supporting Information *
ABSTRACT: Light-selective multiple photochromic systems are important for advanced photoswitching 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 multiphotochromic systems still rely on UV light to induce at least a part of photochromic reactions. In this work, we designed a biphotochromic molecule showing intensity-dependent multiple coloration with a visible-light source by incorporating two T-type photochromic units; a colorless positive photochromophore and a colored negative photochromophore in a molecule. The negative photochromophore acts as a visible-light sensitizer for the positive photochromic reaction. The compound shows an intensitydependent color change under visible-light irradiation. The weak visible-light excitation leads to gradual decoloration from orange to yellow, whereas intense laser excitation clearly changes the color to green. This characteristic photochromism can be achieved by 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 into the development of multiresponsive optical materials.
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INTRODUCTION Realizing on-demand photoresponses among complex multiple photoresponsive systems by tuning of external light stimuli is important for advanced photoswitches. Furthermore, reversible multiple photoresponses 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, multiphotoresponsive 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 upon light irradiation,14−18 and therefore, biphotochromic or multiphotochromic systems are suitable for achieving reversible multiple photoresponses. However, selective excitations of each unit are often challenging in these systems because of intramolecular energy transfers and superposition of the absorption spectra of different units. As a few successful examples, Irie and co-workers have demonstrated wavelength-selective coloration of diarylethene dimer and trimer.3,5 Bochet19 and Blanc and Bochet20 demonstrated orthogonal photoinduced deprotection involving two different photoresponsive protecting groups working with UV and visible light, respectively. Feringa and co-workers recently reported orthogonal photoswitching of two different photochromic reactions using UV and visible light.11 UV light is frequently used to induce photochromic reactions of conventional artificial photochromic molecules. On the other © XXXX American Chemical Society
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-lightsensitive 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 multiphotochromic systems still rely on UV light to induce at least a part of the photochromic reactions. 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 positive-type (P-) and negative-type (N-) photochromic reactions with different thermal back-reaction rates. We designed the novel biphotochromic molecule 1 (Scheme 1) composed of thiophene-substituted phenoxyl imidazolyl radical complex (TPIC)48 as a P-photochromophore and binaphthyl-bridged PIC (BN-PIC) as an N-photochromophore.47,49 TPIC generates the transient colored Received: November 3, 2017 Published: December 23, 2017 A
DOI: 10.1021/jacs.7b11673 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Article
Journal of the American Chemical Society
noteworthy that studies exploring the constructive combination of N- and P-photochromic units are rather scarce11 because almost all of the negative photochromic compounds work well only in a specific environment.27−35 Therefore, the constructive combination of the N- and P-photochromic units in this study is a good candidate for a biphotochromic system to realize multiple coloration that responds to the excitation intensity.
Scheme 1. Photochromic Reaction Scheme for 1
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RESULTS AND DISCUSSION Compound 1 was synthesized according to Scheme 3 as shown in the Experimental Section. High-performance liquid chromatography (HPLC) and electrospray ionization time of flight mass spectrometry (ESI-TOF-MS) revealed that Red-CF has four structural isomers. Because these isomers generate the same radical upon light irradiation, one of the isomers was isolated and used for the spectroscopic studies. Figure 2 shows
open-ring form (OF) (τ1/2 = 33 s) upon UV 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 shortlived 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-ring form (Red-CF) (Figure 1). Figure 2. UV−vis absorption spectra of Red-CF, Yellow-CF, BN-PIC, and TPIC and the superposition of those of BN-PIC and TPIC in benzene.
the absorption spectra of Red-CF, Yellow-CF, TPIC, and BNPIC in benzene. The absorption band of Red-CF at around 450 nm is almost a superposition of those of TPIC and BN-PIC, indicating that only the BN-PIC unit in 1 has visible-light sensitivity. On the other hand, the absorption bands in the 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. Repeated irradiation with weak nanosecond visible laser pulses with a wavelength of 470 nm (5 ns/pulse, 0.4 mJ, 10 Hz) induces a gradual color change of the Red-CF solution from orange to yellow as recognized by the naked eye (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 a halflife of 55 min (k = 2.1 × 10−4 s−1) at 298 K (Figure S10). In contrast, repeated irradiation with intense nanosecond visible laser pulses with a wavelength of 470 nm (5 ns/pulse, 10 mJ, 10 Hz) results in an 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 state (PSS) under 470 nm laser irradiation. The half-life of the green species after cessation of visible-light irradiation is estimated to be 19 s (k = 3.6 × 10−2 s−1) at 298 K (Figure S11). The absorption band of the green-colored species at 750 nm (Figure 3b) is similar to that of OF of TPIC upon UV irradiation.48 These results suggest that the concentration of OF increased upon intense 470 nm laser irradiation in spite of the fact that the TPIC unit
Figure 1. Intensity-dependent photochromism of 1 represented by the core reactions upon visible-light irradiation.
Under weak visible laser excitation of 1, the photoproduct generated by the N-photochromism of the BN-PIC unit (Yellow-CF) is mainly observed because of the slow thermal back-reaction of Yellow-CF. On the other hand, under intense visible pulsed-laser excitation, another photoproduct formed by the P-photochromism of TPIC (Red-OF) is clearly observed. Detailed analyses reveal that the photochromic reaction of the TPIC unit is induced upon visible-light irradiation by singlet sensitization via the excited state of the BN-PIC unit, although unsubstituted TPIC is colorless and insensitive to the visiblelight 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 B
DOI: 10.1021/jacs.7b11673 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Article
Journal of the American Chemical Society
state of the photogenerated BR of the BN-PIC unit to the colorless TPIC unit. 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 subpicosecond time scale,10,51,52 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 initiation of 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 upon repeated irradiations with femtosecond laser pulses is shown in Figure 4. The absorption band at 750 nm gradually increased with
Figure 3. (a, b) Changes in the absorption spectra of 1 (3.2 × 10−4 M) upon repeated 470 nm laser pulse irradiation (5 ns/pulse, 10 Hz) at (a) 0.4 mJ and (b) 10 mJ in benzene at 298 K. (c) Time profiles of the absorbance changes at 480 and 750 nm at 298 K under repeated laser pulse irradiation at several intensities.
has no photosensitivity to 470 nm light (Figure S13). The maximum concentration of the green-colored species under visible-light irradiation increases with increasing excitation laser power, as shown in Figure 3c, indicating that the N- and Pphotochromic 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 Red-CF with the 470 nm visible laser generates two transient species (fast and slow decay species). The slow decay species, which remains until the second time scale, can be attributed 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, visible light simultaneously induces the photochromic reactions of the N-unit and the Punit, although only the N-unit is sensitive to visible light. It was previously reported that the LUMOs of imidazole dimers and PIC derivatives possess antibonding character, as revealed by density functional theory (DFT) calculations and electrochemical studies.46,48,50 Therefore, injection of an electron into 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 by time-dependent DFT to appear at 504 nm.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 also has a similar optically forbidden S1 ← S0 transition at 500 nm, the C−N bond cleavage can occur by energy transfer from the BNPIC unit upon irradiation with 470 nm light. In order to elucidate this unprecedented result, we investigated the following two mechanisms for the ring-opening reaction of the TPIC unit upon visible-light irradiation: (i) energy transfer from the excited state of Red of the BN-PIC unit to the colorless TPIC unit and (ii) energy transfer from the excited
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.
repeating irradiation by femtosecond laser pulses (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 two-photon absorption. Double-pulse nanosecond laser flash photolysis also corroborated the energy transfer from the excited state of Red to CF because excitation of the transient BR of the BN-PIC unit does not induce further photochemical reactions to produce OF (Figure S16). Because the spectral overlap between the Red unit and the CF unit in the visible region is almost zero, the energy transfer would be explained by the Dexter mechanism. The efficiency of the conversion from Red-CF to Red-OF (ΦOF) was relatively estimated using the laser flash photolysis measurements. We assume the conversion efficiency of 1 from Red-CF to Yellow-CF (ΦYellow) is the same as that of BN-PIC, which was previously estimated to be 0.09 by laser actinometry.47 ΦOF can be estimated from ΦYellow as follows: ΦOF = C
ΔOF ΦYellow ΔYellow
(1) DOI: 10.1021/jacs.7b11673 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Article
Journal of the American Chemical Society where ΔOF and ΔYellow are the amounts of Red-OF and Yellow-CF, respectively, generated upon 470 nm laser excitation of Red-CF, which are given by ΔOF =
ΔAbs@740 εOF @740
=
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 OF units of 1 overlap with each other at 470 nm, the ΔAbs at 470 nm for OF (ΔAbsOF@470) is subtracted from the total change in the absorbance at 470 nm (ΔAbs@470) to estimate ΔYellow generated upon 470 nm laser excitation (eqs 3 and 4). From eqs 1 to 4, the value of ΦOF 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 shown in Scheme 2. Excitation of Red-CF upon visible-light
α@740 εOF @740
(2)
and ΔYellow = ΔRed − ΔOF (|ΔAbs@470 − ΔAbsOF @470 |) = εRed‐CF @470 =
(|α@470 − ΔAbsOF @470 |) εRed‐CF @470
Scheme 2. Plausible Photochromic Reaction Scheme for 1 (3)
in which ΔRed is the amount of Red-CF reacted with 470 nm light and ΔAbsOF@470 is given by εOF @470 ΔAbsOF @470 = α@740 εOF @740 (4) where α@740 and α@470 are the slopes of the fits of the plots of the change in absorbance (ΔAbs) versus irradiation power and εRed‑CF@470, εOF@470, and εOF@740 are the absorption coefficients of Red-CF at 470 nm and the OF unit of Red-OF at 470 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 coefficient of the OF unit of 1 (εOF) is the same as the previously estimated absorption coefficient of OF of TPIC, whose values at 470 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 plots of ΔAbs at 470 and 740 nm against the laser excitation power (Figure 5). α@740 and α@470 are the slopes of the fits of the plots of ΔAbs versus irradiation power, which correspond to the ΔAbs at 470 and 740 nm obtained by the 1 mJ laser excitation. The absorption
irradiation generates the two isomers, Yellow-CF (via shortlived BR-CF) and Red-OF. The possibility of the generation of BR-OF from BR-CF upon visible-light irradiation at 470 nm was excluded by the femtosecond laser excitation and doublepulse laser flash photolysis results. Although repeated excitation with visible light might lead to further isomerization from RedOF to Yellow-OF, the efficiency of this total reaction (Red-CF → Red-OF → Yellow-OF) would be negligibly small (10 mW) accelerates the isomerization from Red-CF to Red-OF, and [Red-OF] instantaneously increases with an 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). [RedOF] at the PSS also increases when the solution of 1 is irradiated with the intense 470 nm light. Because Yellow-CF barely absorbs light at 470 nm, a small amount of Red-OF remains at the PSS via Red-CF photogenerated from YellowCF. 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, which is neglected in the simulation model, or degradation of the compounds by the repeated laser irradiation. As a result, we can clearly recognize by the naked eye the color change to green 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 color modulation of 1 depending on the incident light intensity by the control of the balance of the N- and Pphotoconversions and the thermal back-reaction rates.
and Red-OF, respectively. The rate equation for each isomer can be written as follows: d[A] = −ΦABIabsA + ΦBA IabsB + kBA[B] − ΦACIabsA dt + k CA[C]
(5)
d[B] = ΦABIabsA − ΦBA IabsB − kBA[B] dt
(6)
d[C] = ΦACIabsA − k CA[C] dt
(7)
where IabsA = εA [A]I0F ,
IabsB = εB[B]I0F
(8)
in which F=
(1 − 10 Abs ′) Abs′
(9)
with Abs′ = (εA [A] + εB[B] + εC[C])
(10)
and ΦAB = ΦYellow = 0.09,
ΦAC = ΦOF = 0.005
(11)
where εA, εB, and εC are the absorption coefficients of Red-CF, Yellow-CF, and Red-OF at 470 nm, kBA and kCA are the rate constants for the thermal back-reactions 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 (ΦBA) is approximately 0.1. These differential equations were numerically solved. Figure 6b shows the time profiles of the concentration of Red-OF ([Red-OF]) at various excitation light intensities. 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 from the experimental and simulated results: (I) the maximum value of [Red-OF] in the transient state increases with increasing excitation intensity; (II) the concentration of Yellow-CF ([Yellow-CF]) at the time of the maximum [Red-OF] is reduced upon irradiation with highpower excitation light; and (III) [Red-OF] at the PSS increases with increasing excitation intensity. When weak visible light (
