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J. Phys. Chem. C 2007, 111, 2730-2737
Laser Multiphoton-Gated Photochromic Reaction of a Fulgide Derivative Yukihide Ishibashi,† Masataka Murakami,† Hiroshi Miyasaka,*,† Seiya Kobatake,‡ Masahiro Irie,§ and Yasushi Yokoyama| 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, Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu UniVersity, Hakozaki 6-10-1, Higashi-ku, Fukuoka 812-8581, Japan, and Department of Materials Chemistry, Graduate School of Engineering, Yokohama National UniVersity, Tokiwadai, Hodogaya, Yokohama 240-8501, Japan ReceiVed: August 6, 2006; In Final Form: October 27, 2006
A cycloreversion (ring-opening) process of one of the photochromic fulgide derivatives, 2-[1-(2,5-dimethyl3-fulyl)-2-methylpropylidene]-3-isopropylidenesuccinic anhydride, was investigated by means of picosecond and femtosecond laser photolysis methods. The drastic enhancement of the reaction yield was observed by the picosecond laser irradiation. On the other hand, the cycloreversion reaction yield under femtosecond laser exposure was almost consistent with the steady-state light irradiation. The excitation intensity effect of the reaction profiles revealed that the successive multiphoton absorption process leading to higher excited states opened the efficient cycloreversion process. The multiphoton-gated reaction for photochromic compounds as diarylethene derivatives was confirmed also for a fulgide derivative. The similarities and differences of the reaction profiles in higher excited states between fulgides and diarylethenes were discussed from the viewpoints of selection rule of optical transition and its dependence on molecular structures.
Introduction Photochromism is a photoinduced reversible isomerization in a chemical species between two forms. The chemical bond reconstruction via photoexcitation induces not only the change of absorption spectra but also the change of various physical and chemical properties such as fluorescence spectra, refractive indices, oxidation/reduction potentials, chiral properties, and so on. The quick change of these properties induced by the photoexcitation has been attracting much attention1-15 from the viewpoints of the application to optoelectronic devices such as memories and switches. Also, from the basic viewpoint of the chemical reaction, the photochromic reaction can be regarded as an important chemical process for the investigation because the details of the reaction profiles occurring with the time origin at the photoexcitation can be precisely and directly elucidated by using time-resolved measurements, so that we could acquire information on the detailed reaction mechanisms that cannot be easily obtained for the usual thermal reaction processes. Among a number of photochromic molecules, diarylethene3 and fulgide4,5 derivatives undergo cyclization (ring-closing) and cycloreversion (ring-opening) reactions in 6π-electrocyclic systems in an ultrafast time scale,16-30 and both isomers in the photochromic reactions are thermally stable at room temperature. Although this thermal stability is an important condition in the application toward optical switches and memories, photochromic systems for the actual application require several other condi* Corresponding author. Phone: +81-6-6850-6241. Fax: +81-6-68506244. E-mail:
[email protected]. † Osaka University. ‡ Osaka City University. § Kyushu University. | Yokohama National University.
tions such as (a) low fatigue, (b) rapid response, (c) high sensitivity, and (e) nondestructive readout capability. Because the reaction in the excited state generally takes place in competition with various processes in a finite lifetime, the large rate constant of the photochromic reaction (the quick response) is of crucial 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 read-out capability is not compatible with the previous properties fulfilling conditions a-c. Hence, the introduction of a gated function is required for the photochromic systems with nondestructive capability while reading out by the absorption of the light. Recently, we reported multiphoton-induced enhancement of the cycloreversion reaction in photochromic diarylethene derivatives.26-29 The excitation intensity and pulse duration effects of the reaction profiles revealed that higher excited states attained by stepwise two-photon absorption could open the effective photochromic reaction channel. This multiphoton-gated reaction may provide a new approach to erasable memory media with a 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 seems to provide an important approach for the control of photochemical reactions. In relation to this, it is worth noting that the one-photon absorption directly pumped to a higher excited state did not lead to the efficient cycloreversion reaction in these diarylethene derivatives with efficient two-photon cycloreversion properties.28 These results indicate that not the energy of the excitation but the character of electronic states, depending on the mode of the excitation, takes an important role in the enhancement of the cycloreversion reaction.
10.1021/jp0650578 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/23/2007
Multiphoton-Gated Photochromic Reaction
J. Phys. Chem. C, Vol. 111, No. 6, 2007 2731
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The selection rule of the optical transition is generally sensitive to the molecular structure. Fulgide derivatives have a lower symmetry than those of diarylethenes, while the fulgide derivatives undergo cyclization and cycloreversion reactions in the 6π-electrocyclic systems as diarylethene derivatives. Hence, in the present paper, we have investigated the photochromic reaction profiles of fulgides under ultrafast laser pulsed excitation to explore the applicability of the multiphoton-gated reaction as well as to investigate the difference in the reactivity depending on the mode of the excitation and on the molecular symmetries. In the following, we will present the reaction dynamics depending on the pulse duration of the pump laser light sources and the excitation intensity effects of the photochromic reactions. By comparing these results with those previously reported for diarylethene derivatives, we will discuss a laser-induced cycloreversion reaction process from the viewpoints of mode of excitation and its dependence on molecular structures. Experimental Procedures A picosecond laser photolysis system with a repetitive modelocked Nd3+:YAG laser was used for transient absorption spectral measurements.31 The second harmonic (532 nm) with a 15 ps excitation pulse was focused into a spot with a diameter of ca. 1.5 mm. Picosecond white light continuum generated by focusing a fundamental pulse into a 10 cm quartz cell containing a D2O and H2O mixture (3:1) was employed as a monitoring light. A sample cell with 2 mm optical length was used for the 532 nm excitation. The sample solution was circulated during the measurement under the repetition rate of
