One- and multi-photon cycloreversion reaction dynamics of diarylethene derivative with asymmetrical structure, as revealed by ultrafast laser spectroscopy.

A cycloreversion reaction of a photochromic diarylethene derivative, 1-(2-methyl-3-benzothienyl)-2-(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene, with asymmetrical structure, in n-hexane solution, was investigated by means of picosecond and femtosecond laser spectroscopic methods. Femtosecond laser spectroscopy revealed that the excited state (S(1)) of the closed form with a lifetime of 1.3 ps undergoes the cycloreversion reaction in competition with the internal conversion and the apparent reaction yield was independent of the excitation intensity. On the other hand, picosecond laser excitation at 532 nm led to the drastic enhancement of the cycloreversion reaction yield by a successive two-photon absorption process via the S(1) state, leading to higher excited states with a large cycloreversion yield of 0.60 +/- 0.1. For the multiphoton-enhanced cycloreversion reaction, we reported that the efficient cycloreversion reaction of a diarylethene derivative with C(2v) symmetry could occur by stepwise, visible two-photon excitation but did not take place by UV one-photon absorption to a higher excited state. In the present asymmetrical system, both UV one-photon and visible two-photon absorption opened the efficient cycloreversion reaction. Similarities and differences of the reaction profiles in higher excited states between diarylethene derivatives with symmetrical and asymmetrical structures are discussed from the viewpoint of the selection rule of optical transition and its dependence on molecular structures.