Abstract

To study the effects of the solvent on the dynamics of photoisomerization of polar molecules, the isomerization of p-dimethylamino-benzonitrile (DMABN) was studied in n-alcohol and n-nitrile solutions. DMABN undergoes both a structural change (90° twist about the amino–phenyl bond) and an electronic change (charge transfer from the dimethylamino portion to the benzonitrile moiety) in the excited state in polar solvents¹. In the twisted internal charge transfer (TICT) model, B^* refers to the planar form (twist angle of 0°), while A^* refers to the twisted (90°) charge transfer state. In alkanes, only the B^* fluorescence is observed, which indicates that the excited state potential surface connecting the planar and twisted forms is dependent on solvent polarity². Despite numerous kinetic studies³–⁶, the role that the solvent plays in determining the dynamics of TICT is still the subject of controversy. We wish to clarify the effects of solvent viscosity, polarity and hydrogen bonding on the rate of the B^* to A^* transition. To accomplish this, the rate constants were measured for DMABN in a wide range of neat, normal nitriles and alcohols at various temperatures.

© 1986 Optical Society of America