Femtosecond Laser Photolysis Studies on Temperature Dependence

Article pubs.acs.org/JPCC

Femtosecond Laser Photolysis Studies on Temperature Dependence of Cyclization and Cycloreversion Reactions of a Photochromic Diarylethene Derivative Yukihide Ishibashi,†,∥ Toshiyuki Umesato,† Seiya Kobatake,‡ Masahiro Irie,*,§ and Hiroshi Miyasaka*,† †

Division of Frontier Materials Science, Graduate School of Engineering Science, Center for Quantum Science and Technology under Extreme Conditions, Osaka University, and CREST, JST, Toyonaka, Osaka 560-8531, Japan ‡ Department of Applied Chemistry, Graduate School of Engineering, Osaka City University, Sumiyoshi, Osaka 558-8585, Japan § Department of Chemistry, School of Science, Rikkyo University, Nishi-Ikebukuro, Toshima, Tokyo 171-8501 Japan ABSTRACT: Temperature dependencies of cyclization and cycloreversion processes of a photochromic diarylethene derivative, 1,2-bis(2-methyl-3-benzothienyl)perfluorocyclopentene (BT), were investigated by steady-state spectroscopy and femtosecond laser photolysis methods. Steady-state measurements revealed that the cyclization reaction quantum yield and the fraction of the conformer with C2v symmetry favorable for the cyclization (antiparallel, AP conformer) were independent of temperature in the range of 253−343 K. These results indicated that the cyclization reaction of the AP conformer in the open-ring isomer in the excited state had no apparent temperature dependence and suggested that the fate of the excited AP conformer in the open-ring isomer, such as cyclization or deactivation to the ground state, was determined at the conical intersection. On the other hand, the cycloreversion reaction was dependent on the temperature; the reaction quantum yield increased together with a decrease in the lifetime of the excited state of the closed-ring isomer with increasing temperature. On the basis of the adiabatic energy surface for the reaction profiles, it was deduced that the rapid deactivation into the ground state took place in the S1 state in competition with the activated pathways leading to the conical intersection where the cycloreversion occurred.

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INTRODUCTION Photochromism is a photoinduced reversible transformation in a chemical species between two isomers. Chemical bond reconstruction in the excited state induces the rapid change of various molecular properties, which has been investigated from viewpoints of elucidation of fundamental photochemical reactions and application to various photonic devices.1−18 Among many kinds of photochromic molecules, diarylethene derivatives3,4 are one of the representative systems undergoing photoinduced cyclization (ring-closing) and cycloreversion (ringopening) reactions between open- and closed-ring isomers. The excellent thermal stability of both isomers and high fatigue resistance have been attracting much attention and a number of investigations have been reported for the change of physical and chemical properties accompanied with photochromic reactions, such as fluorescence spectra,9 refractive indices,10 oxidation/ reduction potentials,11 chiral properties,12 fluorescence switching,13,14 multiphoton controllability of the reaction,15 one-color reversible reactions,16 and so on.17 In addition, it was revealed for several derivatives that the photochromic reaction can take place even in the crystalline phase18 leading to the morphology change of the crystals. The property has been attracting much attention also from the application to mesoscopic photodriven actuators. Because the photochromic reactions occur in a finite lifetime of electronically excited states, detailed information on the reaction dynamics is indispensable for the comprehensive © 2012 American Chemical Society

elucidation of the photoinduced property change and for the acquisition of the rational principles for the developments of advanced molecules. Along this line, many experimental and theoretical investigations on the excited-state dynamics have been accumulated for various diarylethene derivatives.15,19−31 From these results, several features have been deduced for cyclization and cycloreversion reactions. Open-ring isomers of diarylethene derivatives, in general, have two conformations in the ground state; the conformer with the two rings in mirror of Cs symmetry (parallel conformer, P) and the other one in C2v symmetry (antiparallel conformer, AP). The AP conformer has the geometry favorable for the cyclization reaction, while the P conformer cannot undergo the conrotatory cyclization reaction. NMR measurement in solution phase indicates that the ratio between the AP and P conformers is almost 1:1 for various derivatives in the ground state at room temperature.32 Time-resolved measurements revealed that many diarylethene derivatives undergo the cyclization reactions in sub-ps to several ps time regions,20−27 although several exceptional cases of the cyclization taking place in much longer time regions were reported for derivatives with charge-transfer character22 and for intramolecular dyad systems connected with an energy donor of the triplet excited state.31 Received: November 9, 2011 Revised: January 14, 2012 Published: February 1, 2012 4862

dx.doi.org/10.1021/jp2107632 | J. Phys. Chem. C 2012, 116, 4862−4869

The Journal of Physical Chemistry C

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These results pointed out that the rapid cyclization immediately after the excitation is attributable to the reaction of the AP conformer with the molecular structure favorable for the cyclization. Although the rapid reaction is responsible for the cyclization in many cases, it is worth noting that the cyclization yield of the excited AP conformer is usually smaller than the unity. For this result, it has been considered from the theoretical investigations23b,33,34 that the branching of pathways in the excited state of the open-ring isomer (AP conformer), such as the production of the closed-ring isomer and the deactivation to the open-ring isomer in the ground state, is determined at the conical intersection in the potential energy surfaces. On the other hand, it was reported for the cycloreversion reaction of diarylethene derivatives3−5 that the reaction quantum yields were in the wide range from 10−5 to 0.6 and that the lifetime of the excited state of the closed-ring isomers was in the range of sub-ps to several tens of ps at room temperature.20−27 In addition, the cycloreversion yield generally increases with an increase in the temperature.3 For these experimental results, theoretical investigations23b,33,34 deduced that the activation barrier locates in the reaction pathway from the minimum of the S1 state of the closed-ring isomer to the conical intersection, which is the same one as that for the cyclization reaction. Although this model based on the adiabatic potential energy surface has been employed for the interpretation of various experimental results for the cyclization and cycloreversion processes, the increase in the cycloreversion yield with increasing temperature strongly suggests some rapid deactivation processes being involved in the relaxation pathways in the excited state of the closed-ring isomer. That is, because the lifetime of the excited state of the closed-ring isomers is quite short compared to the fluorescence lifetime of typical organic dyes, almost all of the excited molecules could reach the conical intersection even though the activation barrier slows down the relaxation to some extent. The increase in the cycloreversion yield with increasing temperature indicates that the cycloreversion reaction takes place in competition with some other rapid processes leading to the ground state, even in the case that the same conical intersection is responsible for both of the reactions in the adiabatic potential surface. In order to experimentally clarify the reaction pathways of the photochromic process, we have studied the temperature dependencies of cyclization and cycloreversion reactions of one of the representative diarylethene derivatives, 1,2-bis(2-methyl3-benzothienyl)perfluorocyclopentene in n-hexane solution in the temperature range of 253 to 343 K, by means of steadystate and transient spectroscopies. On the basis of the experimental results on the reaction quantum yields, reaction profiles, and the fraction of the AP conformer, we will discuss the mechanism of the cycloreversion processes in the S1 potential surface.

Scheme 1

1

H NMR spectra were recorded on a BRUKER AV-300N spectrometer. The fraction of the AP and P conformations in BT in the ground state was determined by the intensity ratio of the methyl protons at ca. 2.2 and 2.5 ppm, respectively.36 A 100 W Xe lamp (LAX-101, Asahi Spectra) combined with a monochromator (77250, ORIEL) was used as a light source for the steady-state irradiation. Absorption and emission spectra in the steady state were, respectively, measured by a Hitachi U3500 spectrophotometer and a Hitachi F-850 fluorometer, by using a fused silica cell with a 1.0 cm optical length. Femtosecond laser system with noncollinear optical parametric amplifier, NOPA, was used for transient absorption spectroscopy. Details of the system were described elsewhere.15 Briefly, output of a femtosecond Ti:Sapphire laser (Tsunami, Spectra-Physics) pumped by the SHG of a cw Nd3+:YVO4 laser (Millennia Pro, Spectra-Physics) was amplified with 1 kHz repetition rate by using a regenerative amplifier (Spitfire, Spectra-Physics). The amplified pulse (802 nm, 0.9 mJ/pulse energy, 85 fs fwhm, 1 kHz) was divided into two pulses with the same energy (50%). One of the two pulses was guided into NOPA systems (TOPAS-white, Light-Conversion). The output of NOPA can cover the wavelength region of 500−780 nm with 1−40 mW output energy and 20−40 fs fwhm. In the present work, we set the excitation wavelength at 510 nm, whose pulse duration was ca. 40 fs fwhm. A white light continuum generated by focusing the fundamental light at 802 nm into a 2 mm quartz plate was used as the probe light. The relative polarization of the two pulses was set at a magic angle for all the measurements. Signal and reference lights were detected by two sets of multichannel diode array systems (PMA-10, Hamamatsu) and sent to a PC for further analysis. Chirping of the monitoring white light continuum was corrected for transient absorption spectra. The fwhm of the cross-correlation between the NOPA output and the continuum at the sample position was ca. 80 fs. A rotating sample cell with a 2 mm optical length was used, and the sample solution was circulated during the measurements. The absorbance of the sample at the excitation wavelength was set to be ca. 1. The kinetic data were analyzed by linear or nonlinear least-squares methods. All the measurements were performed under O2-free conditions.

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RESULTS AND DISCUSSION

Temperature Dependence of Cycloreversion and Cyclization Reaction Quantum Yields. Figure 1a shows steady-state absorption spectra of BT in n-hexane solution at 293 K, indicating that the open-ring isomer, BT(o), has absorption bands only in the UV region, whereas the closed-ring isomer, BT(c), has absorption bands both in UV and visible regions. The absorption maximum of BT(c) in the visible region is 517 nm. Figure 1b shows the temperature dependence of the steady-state absorption spectrum of BT in n-hexane solution. To obtain the molar absorption coefficients at each temperature, we first prepared the solution for which photostationary state (PSS) was attained by the irradiation at 316 nm at 293 K.

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EXPERIMENTAL SECTION 1,2-Bis(2-methyl-3-benzothienyl)perfluorocyclopentene (BT) was synthesized and purified.32 Photochromic reactions of BT are shown in Scheme 1. Both cyclization and cycloreversion yields of BT at room temperature are reported25,32,35 to be 0.3−0.35. n-Hexane (Wako, infinity pure grade) was used without further purification. For the control of the temperature, a thermoelectric module with a Peltiler Element (KTS, YAMAKI) was employed. 4863



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Figure 1. (a) Steady-state absorption spectrum of a diarylethene derivative, BT, in n-hexane solution at 293 K. The dotted and solid lines show the spectrum of the open-ring isomer and closed-ring isomer, respectively. The isosbestic point is 317 nm at 293 K. (b) Temperature dependence of absorption spectra of BT in n-hexane solution. The photostationary state was attained by the irradiation at 317 nm at 293 K, and the temperature was varied from 253 K to 343 K by every 10 K.

To keep the ratio between BT(c) and BT(o), the temperature of the solution was changed in the dark place. In addition, the temperature effect of the volume change of the solvent was calibrated. This figure shows that the spectral band shapes become broad with increasing temperature and the molar absorption coefficients at the absorption maxima decrease both for BT(o) and BT(c). Because thermal isomerization reactions between the open- and closed-ring isomers are negligible in this temperature range,3 these spectral changes are due to the temperature dependence of electronic absorption bands. Figure 2a shows temporal evolution of absorption spectra of BT(c) at 253 K, under the steady-state irradiation at 517 nm. With an increase in the irradiation time, the visible band due to the closed-ring isomer decreases, together with an evolution of the band shape in the UV region. On the basis of the reference spectra in Figure 1a, these spectral changes are safely assigned to the cycloreversion reaction. The isosbestic point around 316 nm in Figure 2a indicates that the interconversion between the open- and closed-ring isomers is attained without any remarkable photodegradation. The temporal evolution of the absorbance at 520 nm was analyzed by eq 1, which was derived for the spectral evolution

Figure 2. (a) Time dependence of absorption spectra of BT(c) in nhexane solution at 253 K under the steady state light irradiation at 517 nm. Absorption spectra were measured in the time range of 0 to 10 000 s by every 1000 s. (b) Temperature dependence of the time profiles of the absorbance of BT(c) in n-hexane solution under the steady-state light irradiation at 517 nm. The temperature dependence in 253− 343 K was measured every 10 K. Solid lines are curves calculated with eq 1. (c) Temperature dependence of the cycloreversion reaction quantum yield of BT in n-hexane solution.

via photochemical reaction under the steady-state light irradiation. ln(10 A(t ) − 1) − ln(10 A(0) − 1) = −ln 10ΦoεcλI0t 4864

(1)



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Here, A(t), Φo, I0, and εcλ are, respectively, the absorbance of the closed-ring isomer at time, t, the cycloreversion reaction quantum yield, the intensity of the irradiation light, and the molar absorption coefficient at the irradiation wavelength, λ. In the measurement of the cycloreversion reaction quantum yield, the sample solution was irradiated at λ = 520 nm where the open-ring isomer has no absorption. Figure 2b shows time profiles of the absorbance, indicating that the linear relationship between ln(10A(t) − 1) − ln(10A(0) − 1), and t holds for the cycloreversion reaction of BT. In addition, the slope of the time profiles decreases with a decrease in the temperature, indicating that the cycloreversion reaction quantum yield decreases with a decrease in the temperature. The linear relationship between ln(10A(t) − 1) − ln(10A(0) − 1) and t was confirmed for all of the time profiles in the temperature region of 253−343 K. From the slope of the linear relationship in Figure 2b at each temperature, the cycloreversion reaction quantum yield was obtained. In this estimation, the yield of 0.35 at 293 K was used.32 The temperature dependence of the cycloreversion reaction quantum yield thus obtained is plotted in Figure 2c, showing that the yield monotonously increases with increasing temperature; 0.26 at 253 K to 0.42 at 343 K. As reported for other diarylethene derivatives,3,35 the cycloreversion reaction quantum yield of BT(c) clearly shows temperature dependence. Temperature dependence of the cyclization reaction quantum yield was obtained by the absorption spectrum at PSS under the steady-state light irradiation in the UV region. Because both isomers have absorption bands in the UV region, the ratio between the closed- and open-ring isomers at PSS is represented by eq 2. ΦC ελ C = c C ΦO εoλ CO

(2)

Here, ΦC, ΦO, εCλ , and εOλ are, respectively, cyclization yield, cycloreversion yield, molar absorption coefficient of the closedring isomer, and that of the open-ring isomer at the excitation wavelength, λ. CC and CO are, respectively, the concentration of the closed-ring isomer and that of the open-ring isomer at PSS with the irradiation at λ. In this estimation, the cycloreversion reaction quantum yield at the excitation wavelength was assumed to be the same as that obtained for the 520 nm excitation. In the actual estimation, the isosbestic point at 316 nm was employed as the excitation wavelength (λ). Thus, we can obtain the ΦO value from the ΦC and the ratio between CC and CO because both molar absorption coefficients at the isosbestic point are the same. The temperature dependence of the cyclization reaction quantum yield thus obtained is shown in Figure 3a, indicating no remarkable dependence of the cyclization reaction in the temperature region examined here (253−343 K). As mentioned in the introductory section, the open-ring isomer of diarylethene derivatives has two conformers in the ground state;32 parallel (P) conformer and antiparallel (AP) conformer. The geometry of the AP conformer with C2v symmetry is favorable for the cyclization, while the P conformer cannot undergo the conrotatory cyclization reaction. Actually, it was experimentally confirmed from fs to μs laser spectroscopies that the preset derivative, BT(o) undergoes the cyclization reaction only in the AP conformer within 0.5 ps in the early stage after the excitation.27 Hence, the cyclization reaction quantum yield is dependent on the population of the AP conformer in the ground state. To obtain the information on the temperature dependence of the fraction of the AP

Figure 3. (a) Temperature quantum yield of BT in dependence of the fraction solution in the ground state. the eyes.

dependence of the cyclization reaction n-hexane solution. (b) Temperature of AP conformers of BT in n-hexane Dashed line of each figure is a guide for

conformer in the ground state, we applied NMR spectroscopy and analyzed by the standard method.36 As clearly shown in Figure 3b, the ratio between the AP and P conformers in the ground state is almost independent of the temperature. By summarizing the above results, it can be concluded that the cyclization reaction quantum yield is independent of the temperature in the region of 253−343 K. Temperature Dependence of the Dynamic Behaviors. Figure 4a shows time-resolved transient absorption spectra of the closed-ring isomer of BT, BT(c), in n-hexane solution at 343 K, excited with a femtosecond 510 nm laser pulse. A sharp positive absorption around 710 nm and a broad band around 550 nm appear within the response function of the apparatus, together with the negative signals around 480 nm. On the basis of the previous investigation at room temperature,30 these positive absorption bands can be ascribed to the Sn ← S1 transition of the closed-ring isomer and the negative absorption to the bleaching signal of BT(c) in the ground state. With an 4865



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In addition, the cyclization reaction takes place in the AP conformer with the time constant of 0.45 ps after the excitation in n-hexane solution, and no additional cycloreversion process occur in a longer time region.27 It should be noted that the portion of the AP conformer, ca. 0.65, is larger than the cyclization reaction quantum yield although the cyclization reaction occurs quite rapidly27 with the time constant of 0.45 ps. These results indicate that some portion of the AP conformer undergoes the cyclization but the rest of the excited AP conformer is deactivated into the ground state rapidly. Because the cyclization reaction quantum yield and the portion of the AP conformer were independent of the temperature, the branching ratio between the cyclization and the deactivation into the open-isomer in the ground state is also independent of the temperature. As mentioned in the introductory section, it has been reported from the theoretical investigations23a,33,34 that the cyclization reaction takes place along with the potential surface including the conical intersection (CI) where the fate of the excited molecule is determined. That is, some of the molecules can undergo the cyclization but others result in the deactivation into the ground state. The present experimental results of the cyclization reaction profiles might be interpreted along these theoretical predictions because the reaction quantum yield of the cyclization is not unity (for the excited AP conformers) in spite of the extremely short time constant of the production of the closedring isomer. In addition, temperature independence of the reaction quantum yield could support the mechanism with the branching at the CI point; the yield of the target process is in general expected to be temperature dependent because the apparent activation energy of each pathway is different from each other in the usual processes involving several competitive reactions. In contrast to the cyclization reaction, the cycloreversion reaction quantum yield increased with an increase in the temperature as was shown in Figure 2b, and the lifetime of the excited state of the closed-ring isomer shortened with increasing temperature. In order to elucidate reaction profiles of the cyclization process, we first employed the simple model that the two reactions, cycloreversion and nonradiative deactivation, occur independently in the excited state as shown in Scheme 2. Rate

Figure 4. (a) Time-resolved transient absorption spectra of BT(c) in n-hexane solution at 343 K, excited with a femtosecond 510 nm laser pulse with the energy of 1.0 μJ/pulse. (b) Temperature dependence of time profiles of transient absorbance of BT(c) in n-hexane solution, monitored at 700 nm and excited with a femtosecond 510 nm laser pulse with the intensity of 1.0 μJ/pulse. Temperature was varied from 253 to 343 K by every 10 K. Solid lines are curves calculated with a single-exponential function.

Scheme 2

increase in the delay time, the positive absorption around 710 nm decays to the baseline. However, the negative absorption remains at and after ca. 100 ps following the excitation, which is ascribed to the cycloreversion reaction. Similar time evolution of the transient absorption spectra was observed also in the dynamics in the temperature region of 253−343 K. Figure 4b shows the time profile of the transient absorbance monitored at 700 nm at various temperatures. The positive absorption (Sn ← S1 absorption of BT(c)) at each temperature decayed to the baseline in a few hundreds of picosecond time region. Solid lines in this figure are curves analyzed with a single-exponential function. With an increase in temperature from 253 K to 343 K, the decay time constant becomes short. Mechanism of the Cyclization Reaction. As was shown in previous sections, the cyclization reaction quantum yield of ca. 0.3 and the ratio between the AP and P conformers (0.65:0.35) were independent of the temperature in 253−343 K.

constants of both processes, kO for cycloreversion and kND for nonradiative deactivation, can be estimated by the cycloreversion reaction quantum yield, ΦO, and the lifetime of the excited state, τ, by using following equations: Φo =

τ= 4866

kO k O + kND

1 k O + kND

(3)

(4)



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obtained by the experimental results were 0.35 at 295 K and 0.42 at 343 K as was shown in Figure 2b, indicating that the nonradiative process leading to the ground state of the closedring isomer is in competition with the transition from the excited state to the CI position as shown in Scheme 3. This analysis also indicates that the deactivation process into the ground state is involved in the excited state of BT(c). On the basis of the second model, the rate constant for the transition to the CI, kCI, and that for the nonradiative decay directly leading to the ground state, kn, were obtained by the following equations. The cycloreversion reaction quantum yield, Φo, is represented by eq 5, where χ is the branching ratio toward the open-ring isomer formation at the CI. As was discussed in previous sections, χ in the present system is 0.55.

Rate constants estimated on the basis of the first model are shown in Figure 5, indicating that both rate constants are

Φo =

χk CI k CI + k n

(5)

The lifetime of the excited state, τ, is the summation of both rate constants of kCI and kn. τ= Figure 5. Temperature dependence of rate constants of the cycloreversion and nonradiative deactivation calculated on the basis of Scheme 2. Black and red circles are the rates for cycloreversion and nonradiative deactivation, respectively.

1 k CI + k n

(6)

Figure 6 shows the temperature dependence of the two rate constants obtained by using eqs 5 and 6. Although both rate

temperature dependent and that the apparent activation energy for the cycloreversion process is larger than that of the nonradiative process. In addition, it is obvious that the contribution from the rapid nonradiative deactivation is larger than that of the cycloreversion at every temperature examined in the region of 253−343 K. Although the analysis on the basis of the first model showed the rapid deactivation process, the role of the CI, which was predicted by the theoretical investigations,23a,33,34 has not yet been clarified. Hence, in the second analysis, we assumed that the common CI is involved for both cyclization and cycloreversion as shown in Scheme 3. The branching ratio between Scheme 3

Figure 6. Temperature dependence of rate constants of the cycloreversion and nonradiative deactivation via conical intersection calculated on the basis of Scheme 3. Black and red circles are the rates for cycloreversion and nonradiative deactivation, respectively.

constants increased with increasing temperature, the temperature dependence is much larger for kCI. The activation energies were 4.2 kcal/mol for kCI and 1.3 kcal/mol for kn on the basis of the Arrhenius equation. These results indicate that the deactivation into the ground state of BT(c) contributes less at the high temperature region. The value of kCI/(kCI + kn) at 343 K was 0.75, while this value at 253 K was 0.5. It should be remarked that both models require very rapid nonradiative deactivation pathways with the rate constant in the order of 1010 s−1, which is much larger than those for typical organic dyes in the excited state. Although the origin of this nonradiative deactivation process is not yet completely elucidated, some molecular motions along with the axis not directing toward the cycloreversion may contribute to this nonradiative decay

the closed-ring isomer and the open-ring isomer at the CI point can be determined by the cyclization reaction in the following manner. As stated in previous sections, the cyclization reaction quantum yield in the excited state of the open-ring isomer was 0.30 at all the temperatures examined. In addition, the ratio between AP and P conformers in BT(o) in the temperature region of 253−343 K was 0.65:0.35 from NMR data in nhexane-d14 as was shown in Figure 3b. From these results, the branching ratios at the CI could be estimated to be 0.55 for the open-ring isomer and 0.45 for the closed-ring isomer. In the case that all of the closed-ring isomers after the excitation can arrive at the CI position, the cycloreversion reaction quantum yield could be 0.55. However, the cycloreversion extents 4867



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process. Actually, the theoretical investigation predicted the presence of the molecular motion leading to the CI position that is different from the point for the photochromic reactions and is connected to the ground state.34 In addition, it is worth noting that the summation of both yields for the cyclization from the AP conformer and the cycloreversion increases with increasing reaction temperature; 0.72 at 253 K and 0.87 at 343 K. This temperature dependence suggests that both the cyclization and cycloreversion reactions take place via the common CI. By summarizing the above results and discussion, it is strongly suggested that the potential surface in S1 state may be treated as adiabatic and both reactions of the cyclization/ cycloreversion takes place via the common CI, especially at the high temperature region. A more detailed examination on potential surface in the S1 state by comparing with another diarylethene derivative with lower cycloreversion reaction quantum yield is now under investigation, results of which will be published soon.

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AUTHOR INFORMATION

Corresponding Author

*(M.I.) Phone/Fax: +81-3-3985-2397. E-mail: iriem@rikkyo. ac.jp. (H.M.) Phone: +81-(0)6-6850-6241. Fax:+81-(0)6-68506244. E-mail: [email protected]. Present Address ∥

Department of Materials Science and Biotechnology, Graduate School of Science and Engineering, Ehime University, Matsuyama, Ehime 790-8577, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by Grand-in-Aid for Research in Priority Areas “New Frontiers in Photochromism (No. 471)” from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan.

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REFERENCES

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