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J. Phys. Chem. C 2008, 112, 11150–11157
Dynamics of Cyclization, Cycloreversion, and Multiphoton-Gated Reaction of a Photochromic Diarylethene Derivative in Crystalline Phase Kensuke Tani,† Yukihide Ishibashi,† Hiroshi Miyasaka,*,† Seiya Kobatake,‡ and Masahiro Irie*,§ DiVision of Frontier Materials Science, Graduate School of Engineering Science and Center for Quantum Science and Technology under Extreme Conditions, Osaka UniVersity, Toyonaka, Osaka 560-8531, Japan, Department of Applied Chemistry, Graduate School of Engineering, Osaka City UniVersity, Sumiyoshi, Osaka 558-8585, Japan, and Department of Chemistry, Rikkyo UniVersity, Nishi-Ikebukuro 3-34-1, Toshima, Tokyo 171-8501 Japan ReceiVed: January 14, 2008; ReVised Manuscript ReceiVed: March 17, 2008
Cyclization and cycloreversion reactions of a photochromic diarylethene derivative, bis(2-methyl-5phenylthiophen-3-yl)perfluorocyclopentene, in crystalline phase were investigated by means of picosecond laser spectroscopy. Rapid cyclization within a few picoseconds, as comparable to that in solution phase, was confirmed for the colorization reaction in the crystalline phase. On the other hand, cycloreversion reaction took place with a time constant of 25 ps. In addition, nonlinear enhancement of the cycloreversion process was observed. The excitation intensity dependence of the reaction profiles and the correlation of the apparent reaction yield with the time interval of two pulses employed for the excitation clearly revealed that the successive two-photon absorption process via the S1 state, leading to higher excited states, opened the efficient cycloreversion process. The dynamics and mechanisms of the multiphoton-gated reaction in the crystalline phase were discussed by comparing the present results with those in solution phase. Introduction Photochromism is a photoinduced reversible isomerization in a chemical species between two forms. Quick property change via photoinduced chemical-bond reconstruction has been attracting much attention from viewpoints of fundamental chemical reaction processes and the application to optoelectronic devices such as rewritable optical memory and switches.1–15 Among various photochromic molecules, diarylethenes with heterocyclic aryl rings have been admitted to be a new type of thermally stable and fatigue-resistant photochromic compounds. Both isomers of diarylethene derivatives are different from each other not only in their absorption spectra, but also in various physical and chemical properties.4–13 In addition, photochromic reactions in the crystalline phase of diarylethene derivatives14 are accompanied with unique properties such as large cyclization reaction yield, dichroism under the polarized light, and reversible nanoscale surface morphology change. In relation to these properties, the application to the light-driven actuators has been recently reported for the photochromic reactions in the crystalline phase.15 Although a number of reaction dynamics by means of ultrafast spectroscopy16–25 have been reported for diarylethene derivatives, most investigations were restricted in solutions because the production of the thermally stable isomer prevented a large number of the accumulation for the precise measurement in the immobile crystalline phase.17b,25 In view of the unique properties in the crystalline phase, it is crucial to directly elucidate the reaction dynamics in the crystalline phase, and the comparison * Corresponding authors. (H.M.) Phone: +81-6-6850-6241. Fax: +816-6850-6244. E-mail:
[email protected]. (M.I.) Phone and Fax: +81-3-3985-2397. E-mail:
[email protected]. † Osaka University. ‡ Osaka City University. § Rikkyo University.
of the results with those in solution may provide fundamental information on the reaction in the solid crystals. Along this line, we applied picosecond transient absorption spectroscopy to the photochromic reactions of one of the diarylethene derivatives in the crystalline phase. The preparation of the specimen in the transparent KBr tablet allows a reliable measurement of transient absorption spectra with ca. 5-ps temporal resolution. In addition, from the viewpoint of the application, the photochromic systems for the actual utilization require several conditions such as (a) thermal stability, (b) low fatigue, (c) rapid response, (d) high sensitivity, and (e) nondestructive readout capability. Since the reaction in the excited state takes place in competition with various processes in a finite lifetime, the large rate constant of the photochromic reaction (the quick response) is of importance for an increase in the reaction yield (the high sensitivity) and a decrease in undesirable side reactions resulting in low durability (the low fatigue). On the other hand, the nondestructive readout capability is in conflict with the above properties fulfilling conditions b-d. Hence, the introduction of gated reactivity is required for the photochromic systems with nondestructive capability while reading out by the absorption of the light. On this gated reactivity, we recently reported laserinduced enhancement (more than 50 times) of cycloreversion reaction in photochromic diarylethene and fulgide derivatives.23–25 Excitation intensity and pulse duration effects of the reaction profiles revealed that higher excited states attained by multiphoton absorption could open the effective photochromic reaction channel. This multiphoton-gated reaction may provide a new approach to erasable memory media with nondestructive readout capability. Not only from the viewpoint of the application but also from the basic viewpoints of photochemistry, the selective excitation to a specific electronic state leading to the target reaction provides an essential approach for the control of the photochemical reactions.
10.1021/jp8003272 CCC: $40.75 2008 American Chemical Society Published on Web 07/02/2008
Picosecond Dynamics of Diarylethene in Crystals Although the previous results on the excitation intensity dependence and numerical analysis strongly suggested that successive two-photon absorption leading to the highly excited state was attributed to the mechanism for the enhancement of the cycloreversion reaction,23–25 the mode of the two-photon absorption, simultaneous or stepwise process, still needs to be clarified by more direct measurement. To explore the applicability of the multiphoton-gated reaction and elucidate the mechanisms of nonlinear cycloreversion reaction in the crystalline phase, we conducted the present investigation. The correlation of the apparent reaction yield with the time interval of two excitation laser pulses directly revealed that the successive twophoton absorption via the S1 state was responsible for the increase in the apparent cyclization reaction yield. In addition, the reaction yield in highly excited state attained by this successive two-photon absorption process was 0.4 from the detailed analysis of the excitation intensity dependence of the cycloreversion yield under the picosecond laser excitation. On the basis of these results, dynamics and mechanisms of nonlinear cycloreversion process will be discussed together with the onephoton reaction processes in the following sections. Experimental Section Bis(2-methyl-5-phenylthiophen-3-yl)perfluorocyclopentene (PC1) was synthesized and purified.14d The solid polycrystalline specimen was prepared as follows. KBr powder (Kishida Corporation, IR spectral grade) was shattered into fine particles in an agate mortar and dried under reduced pressure after heating at 100 °C for more than 24 h. After adding ca. 1-2 mg of the sample (the open-ring isomer of PC1) into the KBr powder, we equalized the grain size by a filter (200 mesh, 75 µm). About 400 mg of the mixed KBr powder was put on the KBr tablet holder (Shimadzu) and pre-evacuated using a vacuum pump for 5 min. Finally, we pressed this sample under 80 kN for 20 min by a hand press under vacuum and obtained a transparent KBr tablet with a thickness of 1.0 mm. The size of the crystal of PC1 in this specimen was ca. 1-5 µm, and these crystals were dispersed in the KBr tablet. The closed-ring isomer in these polycrystalline samples was prepared by steady-state UV-light irradiation of the solid sample. The conversion yield from open to closed form was estimated from the absorption spectrum of the closed isomer. In the present study, the sample contained 2-3% of the closed form. Hence, the closed form could be regarded as a guest molecule in the open-form crystal. On the other hand, the cyclization reaction was studied in the pure crystal of the open form. A picosecond laser photolysis system with a repetitive modelocked Nd3+:YAG laser was used for transient absorption spectral measurements.26 The second or third harmonic light with 15-ps fwhm and 0.5-1 mJ energy was focused into a spot with a diameter of ca. 1.5 mm and used for excitation. To obtain the effective size of the focal point of the excitation light, the excitation intensity dependence of PC1 in n-hexane23c was used as a reference in the present work. Picosecond white continuum generated by focusing a fundamental pulse into a 10-cm quartz cell containing D2O and H2O mixture (3:1) was used as a monitoring light. The sample position of the laser irradiation was replaced during the measurement under the repetition rate
