Solvent Viscosity Effects on Photochromic Reactions of a Diarylethene Derivative As Revealed by Picosecond Laser Spectroscopy

Abstract

The solvent viscosity effect on photochromic reaction processes such as cyclization and cycloreversion of a diarylethene derivative, 2-bis(2,4,5-trimethyl-3-thienyl)maleic anhydride (TMTMA), was investigated by means of steady-state spectroscopy, picosecond transient absorption, and fluorescence measurements. The cyclization reaction yields from the excited open-ring isomer decreased with an increase in solvent viscosity. Picosecond transient absorption spectroscopy revealed that the cyclization reaction took place via two pathways; very rapid reaction immediately after the excitation in competition with the relaxation to the fluorescent state and the rather slow reaction from the relaxed fluorescent state with the time constant of several hundred picoseconds. The latter slow process was strongly dependent on the solvent viscosity, whereas the former rapid process was almost independent of the viscosity. This viscosity dependence of the reaction from the fluorescent state was attributed to the solvent molecular friction to the rotation of the thienyl moieties leading to the favorable geometry for the cyclization. For the cycloreversion reaction, much smaller solvent viscosity dependence was observed and reaction time constants less than 1−2 ps were estimated by transient absorption spectroscopy. By integrating these experimental results with the potential energy surfaces predicted from theoretical investigation, the mechanism of the photochromic reaction of TMTMA was discussed.