A Simple and Versatile Strategy for Rapid Color Fading and Intense

Sep 13, 2017 - Benzo-annulated chromenes, i.e., naphthopyrans, are well-known photochromic molecules that undergo photochemical ring-opening reactions...
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A Simple and Versatile Strategy for Rapid Color Fading and Intense Coloration of Photochromic Naphthopyran Families Yuki Inagaki,† Yoichi Kobayashi,‡ 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 ‡ Department of Applied Chemistry, College of Life Sciences, Ritsumeikan University, 1-1-1 Noji-higashi, Kusatsu, Shiga 525-8577, Japan S Supporting Information *

ABSTRACT: Benzo-annulated chromenes, i.e., naphthopyrans, are well-known photochromic molecules that undergo photochemical ring-opening reactions to form two colored open-ring isomers, the transoid-cis and transoid-trans forms, upon light irradiation. Though the transoid-cis form returns thermally to the uncolored closed form, the fading rate of the transoid-trans form is extremely slow because of its higher thermal stability. This slow fading behavior of the transoidtrans form is responsible for the persistence of residual color for several minutes to hours, and prevents the application of such molecules to fast photoswitching materials. We have found a new simple and versatile strategy to substantially reduce the amount of the undesirable long-lived colored transoid-trans form by introducing an alkoxy group at the 1-position of azino-fused chromenes, i.e., 8H-pyranoquinazolines. The alkoxy group effectively reduces the formation of the transoid-trans form due to C−H···O intramolecular hydrogen bonding in the transoid-cis form. Moreover, the introduction of a condensed aromatic ring at the 3-position was found to be effective to increase the photosensitivity of the ring-opening reaction. This strategy can also be applied for naphthopyran derivatives and is useful for the development of fast photoresponsive photochromic lenses and fast photoswitching applications such as dynamic holographic materials and molecular actuators.

1. INTRODUCTION Switching of the physical and chemical properties of materials by light irradiation has been the subject of considerable research. There is an increasing interest in the use of organic photochromic compounds to modulate conductivity, fluorescence, magnetism, and shape at the bulk level.1−4 Photochromism of chromenes was first reported by Becker and Michl in 1966.5 Among the chromene families, 3,3-diaryl-3Hnaphtho[2,1-b]pyran derivatives (3H-naphthopyrans, Scheme 1) have been widely studied for fundamental science and applied to industrial photochromic lenses.6−21 UV light irradiation on the closed form (CF) of naphthopyrans promotes the cleavage of the C(sp3)−O bond of the pyran ring to afford the transoid-cis (TC) and transoid-trans (TT) forms through the short-lived cisoid-cis (CC) form. The TC form photochemically isomerizes to the TT form (and vice versa) through the CC bond rotation. The TC form thermally reverts to the CF in a few seconds/minutes, whereas the thermal back reaction from the TT form to the CF is much slower (minutes/hours) because the TT form is thermally stable and must overcome a relatively large activation energy barrier to isomerize to the TC form. The residual color by the TT form and the slow thermal back reaction of the TC form have been considered as one of the inconvenient problems to © 2017 American Chemical Society

Scheme 1. Photochromic Reactions of 3,3-Diphenyl-3Hnaphtho[2,1-b]pyran and 8,8-Diphenyl-8H-pyrano[3,2f ]quinazolinea

a

The atomic label for 8H-pyranoquinazoline is shown in parentheses.

be solved for photochromic lenses and fast photoswitching applications such as dynamic holographic materials22−24 and molecular actuators.25−29 Sousa and co-workers reported that the formation of the TT form of 2,2-diaryl-2H-naphtho[1,2b]pyrans (2H-naphthopyrans) can be suppressed by bridging between the pyran ring and the naphthalenic core by a fused Received: June 16, 2017 Published: September 13, 2017 13429

DOI: 10.1021/jacs.7b06293 J. Am. Chem. Soc. 2017, 139, 13429−13441

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Journal of the American Chemical Society alkyl chain.30−33 Though this technique effectively suppressed the formation of the TT form, lots of labor and time were required to prepare these compounds. In the meantime, we reported that the substitution on the 2-position of the 3Hnaphthopyrans suppresses the formation of the TT form and drastically accelerates the thermal back reaction from the TC form to the CF from tens of minutes to μs time regions.34,35 However, the thermal back reaction rate is too fast for the human eye to recognize the color change. In this study, we report a simple strategy to reduce the formation of the TT form that is applicable to a wide variety of naphthopyran families. The photochromism of 8,8-diphenyl-8H-pyrano[3,2-f ]quinazoline (8H-pyranoquinazoline, Scheme 1) was first reported by Pozzo and co-workers in 1994.36 The significant acceleration of the thermal back reaction and the marked improvement of fatigue resistance of 8H-pyranoquinazoline derivatives were also reported.37−40 Actually, the thermal bleaching rate constants of 8H-pyranoquinazoline and 3Hnaphthopyran are 0.48 and 0.09 s−1 in toluene at 298 K, respectively.37 Though the heterocyclic effect and the role of substituents in the pyran moiety of azino-fused chromenes on their photochromic properties were studied intensively, little attention has been paid to the suppression of the formation of the TT form. This study emerged from the basic idea to destabilize the TT form by steric repulsion of the bulky substituent at the 1-position of 8H-pyranoquinazoline, which would be expected to reduce the formation of the undesirable long-lived colored TT form and to accelerate the thermal back reaction to recover the CF.

3H-naphthopyran derivatives were prepared from a 2-naphthol derivative and 1,1-diphenyl-2-propyn-1-ol (see Scheme 10). Figure 1 shows the absorption spectra for the CFs of the 8Hpyranoquinazoline derivatives and the 3H-naphthopyran

2. RESULTS AND DISCUSSION We have investigated the substituent effect for the photochromic properties of the 8H-pyranoquinazoline derivatives, PQ1−PQ9, and the 3H-naphthopyran derivatives, NP1−NP3, listed in Scheme 2. The syntheses of 8H-pyranoquinazoline Scheme 2. Chemical Structures of the 8H-Pyranoquinazoline Derivatives, PQ1−PQ9, and the 3H-Naphthopyran Derivatives, NP1−NP3

Figure 1. UV−vis absorption spectra of the CFs of (a) PQ1−PQ9 and (b) NP1−NP3 in toluene at 298 K.

derivatives were accomplished by the Claisen rearrangement protocol, in which a 1,1-diaryl-2-propyn-1-ol with a 6quinazolinol derivative in toluene containing an acidic catalyst promotes the formation of the corresponding 8H-pyranoquinazoline (see Schemes 3−9).14,18 In this study, we used a commercially available 1-phenyl-1-[4-(1-piperidinyl)phenyl]-2propyn-1-ol due to the improved reaction yields, whereas the

derivatives in toluene. By comparing the absorption spectra of PQ1, PQ4, PQ7, and PQ9, it was found that an alkoxy group at the 1-position results in a hypsochromic shift of the absorption tails of PQ4 and PQ7 by ca. 14 and 7 nm, respectively, without significant changes in the absorption coefficient, although a thiomethyl group at the same position leads to the increase in the absorption coefficient without any spectral shift. On the other hand, a bathochromic shift of about 10 nm is observed for NP3 compared with NP1. As can be found from the UV−vis absorption spectra of the CFs of PQ5, PQ6, and PQ8, the introduction of a pyrenyl group or a dithienothienyl group on the 3-position induces an intense absorption band in the UVA light region. The absorption spectrum of PQ5 is almost identical with that of PQ8 and overlapped with each other in Figure 1a. The absorption tails of these compounds are fairly long and extend to 420 nm. Steady-state UV light (365 nm, 440 mW/cm2) irradiation on the toluene solutions of PQ1−PQ9 led to purple coloration at 298 K. The solutions return to the original colorless state after the removal of the light source. In Figure 2 are shown the absorption spectra of the toluene solutions at the photostationary state (PSS) under UV light irradiation with the same 13430

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pyrene unit of PQ5 and PQ8 by the 8H-pyranoquinazoline framework seems to result in the photosensitization effect for the ring-opening reaction by the pyrene unit, because the fluorescence quantum yield of pyrene was reported to be 0.65 at 293 K in ethanol and cyclohexane.41 Although the maximum absorption peak wavelength (λmax) for the colored form generally depends on the degree of πconjugation of molecules, the λmax of the colored forms of PQ5, PQ6, and PQ8 are almost identical with that of PQ1. Therefore, the condensed aromatic rings at the 3-position of 8H-pyranoquinazolines were found to give little effect on the λmax of the colored open-ring forms. On the other hand, a methyl group or a tert-butyl group on the 1-position induces the hypsochromic shift of the λmax probably due to the deviation from the planar structure of the TC and TT forms due to the steric effect. On the other hand, the colored forms of the naphthopyran systems, NP1−NP3, show the absorption band at the wavelength between 400 and 500 nm due to the molecular framework of the unsubstituted diphenyl naphthopyran. It is known that electron donating groups in the phenyl rings at the 3-position of the 3H-naphthopyran system bring about a bathochromic shift of the absorption band of the colored form accompanied by an increase in the fading speed.18 Electron withdrawing groups cause a hypsochromic shift and slow the fading speed. As shown in Figure 3, the thermal fading curves after ceasing the light irradiation consist of an initial fast decay attributable to the thermal isomerization of the TC form to the CF and a following slow decay of the long-lived TT form. The remarkable acceleration of the thermal back reaction of the TC form of NP2 compared with NP1 or NP3 lower the value of the Δabsorbance at the PSS by nearly half.34 As will be discussed later, the degree of coloration is strongly related to the half-life of the colored isomers. The half-lives (τ1/2 at 298 K) of the TC form were determined from the fitting curves of the time variation of the absorbance using the following biexponential equation:

Figure 2. UV−vis absorption spectra of (a) PQ1−PQ9 and (b) NP1− NP3 under UV light irradiation (365 nm, 440 mW/cm2) in toluene (5.5 × 10−5 M) at 298 K.

concentration (5.5 × 10−5 M) recorded while stirring the solution at 298 K. Under the continuous light irradiation, the compounds reach the PSS quickly. The transient colored species formed upon UV light irradiation can be attributable to the formation of the colored TC and TT forms. The differences in the magnitude of the change in absorbance (Δabsorbance) at the PSS reflect the differences in the molar extinction coefficients at the excitation wavelength (365 nm), the quantum yields for the ring-opening reaction, and the rate constants for the thermal back reaction. The detailed consideration for the degree of coloration will be discussed later. Anyhow, the large molar extinction coefficients in the UVA light region of the CFs of PQ5, PQ6, and PQ8 as shown in Figure 1a, and the large values of Δabsorbance for the UV light-irradiated toluene solutions of them compared with those of PQ1 or PQ4 confirm the enhancement of the photosensitivity of the CFs due to the presence of a condensed aromatic ring at the 3-position of 8H-pyranoquinazoline. The toluene solutions of these compounds actually show bright and deep color changes under sunlight (Movie S1). The negative signs of the Δabsorbance around the wavelength region of 430 nm observed for PQ5 and PQ8 are responsible for the fluorescence from the CF. The effect of the fluorescence from the CF is also observed in the spectrum of PQ6. The fluorescence quantum yields (λex = 355 nm) of the CFs of PQ5, PQ6, and PQ8 in toluene are 0.059, 0.009, and 0.065, respectively. The efficient fluorescence quenching of the

f (t ) = A1 e−k1t + A 2 e−k 2t

(1)

where k1 is the rate constant for the thermal back reaction of the TC form and k2 is that of the TT form (Table S1). The formation ratios of the TT form were defined as A2/(A1 + A2) by assuming the molar extinction coefficients of the TC and TT forms are same. These results are summarized in Table 1. The residual colors of PQ4−PQ6, PQ8, and NP3 are significantly lower, whereas the Δabsorbance at the PSS of PQ1 is higher than those of the other compounds. A phenoxy group and a thiomethyl group are not effective to decrease the residual color as suggested by PQ7 and PQ9, respectively. Thus, we have confirmed that an alkoxy group considerably reduces the formation of the TT form. The same effect of a methoxy group is also observed for a 3H-naphthopyran derivative, NP3 (Figure 3b). By considering the results that a methyl group and a tertbutyl group cannot reduce the amount of the TT form, the steric effect of an alkoxy group is excluded. A possible explanation for the origin of the effect is a C−H···O intramolecular hydrogen bond in the TC form as shown in Figure 4. The DFT calculations (M06-2X/6-311++G(d,p)) were carried out to investigate the possibility for the C−H···O hydrogen bonding42 between the oxygen atom of the alkoxy group and the olefinic proton in the TC form. The calculated 13431

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Figure 4. Plausible C−H···O hydrogen bonding between the oxygen atom of the alkoxy group and the olefinic proton in the TC form of the 8H-pyranoquinazoline systems.

The hydrogen bond formation between the oxygen atom of the methoxy group and the olefinic proton of PQ4 was experimentally confirmed by 1H NMR spectra recorded in toluene-d8 at 200 K as shown in Figure 5c. The excitation UV

Figure 3. Time variation of the change in absorbance at λmax of (a) PQ1−PQ9, and (b) NP1−NP3 in toluene (5.5 × 10−5 M) under continuous UV irradiation (365 nm, 440 mW/cm2, 20 s) at 298 K, and the subsequent thermal fading when the irradiation was ceased.

Table 1. λmax for the Colored Form at the PSS, Half-Lives (τ1/2) of the TC Forms, and Formation Ratios of the TT Form in Toluene (5.5 × 10−5 M) at 298 K compound

λmax/nm

τ1/2/s

TT form/%

PQ1 PQ2 PQ3 PQ4 PQ4a PQ4b PQ5 PQ6 PQ7 PQ8 PQ9 NP1 NP2 NP3

560 535 520 552 560 590 565 571 557 561 557 425 419 450

0.63 0.32 3.0 0.86 0.20 0.49 0.75 0.69 0.50 0.32 0.05 6.9 0.36 9.5

51 23 30 10 5 ∼0 6 7 17 7 43 16 17 3

Figure 5. Theoretical 1H NMR spectra of the TTC and CTC forms of (a) PQ1 and (b) PQ4 obtained by the GIAO method at the DFT TPSS/6-311++G(d,p)//M06-2X/6-311++G(d,p) level of theory. (c) 1 H NMR spectra of PQ1 and PQ4 recorded in toluene-d8 at 200 K after irradiation with 365 nm light. A label of 1TTC indicates the NMR peak for the proton H1 of the TTC form; the other labels follow the same convention.

LED light (365 nm, 4.1 mW) was guided with an optical fiber into the NMR spectrometer and irradiated the NMR tube (Figure S78) at 200 K. Upon UV light irradiation for 1 h, a sufficient amount of the open-ring isomers was accumulated because the thermal back reaction of the open-ring isomers can be suppressed at 200 K. While the formation of four kinds of open-ring isomers, TTC, CTC, CTT, and TTT (Figure 6), was expected for PQ1 and PQ4 due to the dissymmetrical phenyl

a

Measured in acetonitrile. bMeasured in the mixed solvent of toluene and ethanol (v/v = 1:1).

distance between the oxygen atom and the hydrogen atom is 2.04 Å for PQ4, which is shorter than the sum of van der Waals radii, 2.70 Å for H···O.42 Thus, the short C−H···O contact is likely to form a C−H···O hydrogen bond. 13432

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Figure 6. Molecular structures of the TTC, CTC, CTT, and TTT isomers of PQ1 and PQ4.

substituent, the yields of the CTT and TTT forms were negligibly small. The protons of H1 and H2 were reported as fingerprints for the open-ring isomers of naphthopyrans by Delbaere and Vermeersch,43 and were assigned on the basis of the two-dimensional NMR (H−H COSY) spectra (Figures S79 and S80) and the DFT calculations (TPSS/6-311++G(d,p)// M06-2X/6-311++G(d,p)) by including the solvent effect (Figure 5a,b). The two doublet signals at 9.80 and 9.31 ppm of PQ1 were assigned to the proton H2 of the TTC and CTC forms, respectively. By comparing the integrated area of these signals, the existence ratio of the TTC form is found to be nearly equal to that of the CTC form. The proton H1 would be deshielded by the formation of the C−H···O hydrogen bond with the oxygen atom of the methoxy group. This prediction was evidenced by the large downfield shifts of the corresponding protons. The two doublet signals at 7.97 and 7.72 ppm assigned to the proton H1 of the CTC and TTC forms of PQ1 shift to 9.72 and 9.14 ppm, respectively, in PQ4. Thus, it is apparent that the introduction of a methoxy group causes a significant downfield shift in the proton H1, suggesting that the proton H1 is involved in the intramolecular C−H···O hydrogen bonding in the TC form as shown in Figure 4. The hydrogen bond formation between the oxygen atom of the methoxy group and the olefinic proton of the TC form of NP3 was also confirmed by the 1H NMR measurements. In contrast to the 1H NMR spectra of PQ1 and PQ4, the diphenyl-substituted naphthopyrans, NP1 and NP3, give only the TC and TT forms and show simple spectra as shown in Figure 7c. The weak signals attributable to the TT form were observed at 8.25 and 8.43 ppm in the spectrum of NP1. The formation ratio of the TT form of NP1 was 15% calculated from the integrated intensity ratio in the spectrum. The weak signals at 7.66 and 8.42 ppm in the spectrum of NP3 are due to the CF. Based on the DFT calculations (Figure 7a,b) and the H−H COSY spectra (Figures

Figure 7. Theoretical 1H NMR spectra of the TC forms of (a) NP1, and (b) NP3 obtained by the GIAO method at the DFT TPSS/6311++G(d,p)//M06-2X/6-311++G(d,p) level of theory. (c) 1H NMR spectra of NP1 and NP3 recorded in toluene-d8 at 200 K after irradiation with 365 nm light. A label of 1TC indicates the NMR peak for the proton H1 of the TC form. The other labels are the same as well.

S81 and S82), a significant downfield shift in the proton H1 was also confirmed for the TC form of NP3. The temperature dependence of the photochromic reaction was also performed to investigate the effect of the hydrogen bonding. Although the amount of the TT form was not altered in toluene over the temperature range 298−333 K, the significant increase in the component of the TT form was confirmed over the same temperature range in acetone (Figure S77). That is, the hydrogen bond is thermally broken in acetone, resulting in the formation of the TT form from the TC form upon light irradiation. Lewis and co-workers reported that the hydrogen-bonded conformer of the Z isomer of 2-(2-(2-pyridyl)ethenyl)indole does not undergo Z→E photoisomerization due to either the occurrence of rapid nonradiative decay or a hydrogen-bondingdependent barrier for twisting the corresponding CC bond.44 Therefore, the C−H···O hydrogen bonding in the TC form of 8H-pyranoquinazoline is also considered to disturb the CC bond twisting and reduces the formation of the TT form. Although the detailed mechanism of the photoisomerization process has not been cleared at the present time, the strategy for the molecular design to reduce the formation of the 13433

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with that of NP1 (6.9 s) and the larger amount of the TT form at the PSS of NP1 (16%) compared with that of NP3 (3%) explain the less durable behavior of the TT form. On the other hand, the electron-donating piperidinyl group reduces the performance of the durability. Furthermore, we investigated the photochromic behavior of the polymer-doped film. We reported that the rapid photoswitching properties of fast photochromic molecules can be achieved in a commercially available block copolymer (KURARITY LA2330, Kuraray Co., Ltd.), that is a triblock acrylic-based polymer composed of poly(methyl methacrylate) (PMMA) and poly(butyl acrylate) (PBA).45 The film of LA2330 doped with PQ5 (3 wt%) was prepared on a glass substrate by spin coating method (800 rpm, 5 s → 1500 rpm, 5 s). The film thickness is 16.5 μm measured by spectral interferometry. Although the polymer film has absorption tail at the wavelength longer than 400 nm as well as in solution (Figure S83), the film is colorless for the human eye as shown in Figure 9a. The clear and colorless film turns light blue by

undesirable long-lived colored TT form is applicable to a wide variety of naphthopyran families. Moreover, we investigated the solvent dependence on the formation ratio of the TT form at room temperature. The photochromic responses of PQ4 (5.5 × 10−5 M) in acetonitrile and binary mixed solvent of toluene and ethanol (v/v = 1:1) were measured in a similar manner as above (Figures S75 and S76). As shown in Table 1, the fading speed of the TC form slightly accelerates and the formation ratio of the TT form decreases in polar solvent. It is worth noting that the formation of the TT form is completely suppressed in the mixed solvent of toluene and ethanol. The fatigue resistance against the repeated photochromic reactions is an important issue for the practical applications such as photochromic lenses, and smart windows. We investigated the effect of the methoxy group on the fatigue resistance of the 8H-pyranoquinazoline and the 3H-naphthopyran frameworks. The durability test for the non-degassed toluene solutions of PQ1, PQ4, NP1, and NP3 was conducted by continuous recording of the Δabsorbance at the PSS under continuous irradiation of UV light maintained at 298 K. A 355 nm continuous wave UV light from an optically pumped semiconductor laser was used for the excitation light source. The laser beam was diverged through a plano concave lens to illuminate a circular area with the radius of 5 mm. To 1.6 mL of the stirred solution in a quartz cell (10 × 10 mm), 26 mW of 355 nm light was irradiated continuously. The concentration of the toluene solution of PQ1, PQ4, NP1, and NP3 were adjusted to 1.4 × 10−5, 8.9 × 10−6, 1.0 × 10−5, and 7.4 × 10−6 M, respectively, so as to be the same value (0.046) of the absorbance at 355 nm. Figure 8 shows the time courses of the

Figure 9. Photographs taken (a) under room light and (b) under irradiation with a solar simulator of a polymer film doped with PQ5.

shining with a solar simulator (AM 1.5 G) with a xenon 300 W lamp (Figure 9b, Movie S2). The temperature dependence on the coloration and the fading behavior after turning off the UV light irradiation are shown in Figure 10b. The increase in the fading speed at higher temperature results in the decrease in the coloration as was observed for conventional T-type photochromic molecules. It is noteworthy that the amount of the formation of the long-lived TT form is also reduced in the block copolymer as well as in solution. Therefore, the colorless feature under room light and the fast response to sunlight of the polymer film guarantees the applications in smart windows and ophthalmic glasses. It is known that there is a trade-off between the decoloration speed and the absorbance of a photogenerated isomer in T-type photochromic compounds. That is, when the thermal back reaction of the photochromic reaction becomes faster, the absorbance of the photogenerated open-ring isomer at the PSS usually decreases. This relationship can be analyzed by solving the rate equations of the photochromic reaction. Since the synthesized compounds in this study generate few TT forms upon UV light irradiation, we regard these systems as two-state AB systems for simplicity, where the initial state A (CF) is converted to the state B (TC form) by light irradiation with the quantum yield of ϕAB and the back reaction from the B to A occurs thermally with the rate constant of kBA (= 1/τ1/2).46 Since the lifetime of the excited state of A (A*) is usually nanosecond time scales or faster, the concentration of the A* remains low and changes little (d[A*]/dt ≈ 0). In this case, the rate equation can be written as follows:

Figure 8. Time courses of the normalized Δabsorbance monitored at λmax under the continuous irradiation of UV light (355 nm, 26 mW) on the toluene solutions of PQ1, PQ4, NP1, and NP3 at 298 K.

normalized Δabsorbance monitored at λmax under the continuous irradiation of UV light. The values of Δabsorbance gradually decreased with time due to a photodecomposition of the dyes. The methoxy-substituted derivatives, PQ4 and NP3, shows higher durability compared with their parent molecules, PQ1 and NP1. These results are suggestive in regard to thinking about the degradation process of naphthopyran families. The reduction of the amount of the TT form of PQ4 and NP3 at the PSS is presumably concerned with the higher fatigue resistance against the repeated coloration cycles. The longer half-life of the TC form of NP3 (9.5 s) compared

− 13434

d[A] = (kBA + ϕABεA I0F )[A] − kBA[A]0 dt

(2)

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cm−1, ε′B = 2.2 × 104 M−1 cm−1, I0 = 440 mW cm−2 (wavelength is set to 365 nm), ϕAB = 0.8. Figure 11 shows the absorbance at the PSS as a function of τ1/2 obtained by the kinetic analyses. εA = 5.0 × 103 M−1 cm−1

Figure 11. Simulated absorbance of the photogenerated open-ring isomer at the PSS as a function of τ1/2. We assume a condition for a photochromic film dispersed in the polymer matrix and the parameters are set as follows: l = 20 μm, c = 1.0 wt % (Mw = 500), I0 = 440 mW cm−2, εB = 1.0 × 104 M−1 cm−1, εB′ = 2.2 × 104 M−1 cm−1, and ϕAB = 0.8.

approximately corresponds to PQ1−PQ4, PQ7, and PQ9 (black curve), while εA = 4.0 × 104 M−1 cm−1 corresponds to PQ5, PQ6, and PQ8 (red curve). It shows that the absorbance at the PSS gradually saturates at τ1/2 ≈ 0.2−0.3 s with the increase in τ1/2 and the absorbance at the PSS becomes insensitive to τ1/2 for larger τ1/2. In general, commercially applicable photochromic films require the transmittance less than ∼10% upon UV light irradiation, that is, the absorbance >1. The figure suggests that the absorbance over 1 can be realized when the τ1/2 is slower than 0.15 s for large εA compounds (red curve) or the τ1/2 is slower than 0.36 s for small εA compounds (black curve). Since τ1/2 of PQ4−PQ6 and PQ8 are 0.85, 0.76, 0.66, and 0.31 s, respectively, the kinetic analyses suggest that these compounds are promising for photochromic devices which realize dense coloration and fast color fading. Moreover, the figure also suggests that the increase in εA increases the absorbance at the PSS about 10− 20%. This effect of the large εA is remarkable in the fast τ1/2 region (