CIS-trans photoisomerization of 1-(9-anthryl)-2-phenylethylenes: effects of substitution and solvent polarity

Abstract

The trans and cis isomers of 1-(9-anthryl)-2-(4-R-phenyl)ethylene (RH, phenylsulphonyl and formyl; 1, 2 and 3 respectively) were studied by fluorescence techniques, laser flash photolysis and steady state irradiation. The quantum yields of fluorescence (φ f) and of cis-trans photoisomerization (φ c → t, φ t → c), the fluorescence decay kinetics and some properties of the triplet state were measured in various solvents at room temperature and below. After excitation and fast relaxation, the excited trans singlet state ( 1t *) of 1 is deactivated mainly by fluorescence, internal conversion at the trans side and intersystem crossing ( 1t * → 3t *). In addition, for 2 and 3 internal rotation about the CC double bond in the excited singlet state is proposed. The observed triplet state, characterized by its absorption spectrum and quenching as well as by energy transfer measurements, is assigned to the 3t * configuration. The cis triplet of 1 – 3 could only be observed in rigid media. By comparing the effects of solvent polarity and temperature on φ f and φ t → c with those on the triplet yield, a triplet mechanism for trans → cis photoisomerization of the three styrylanthracenes is excluded. The small value of φ t → c for 1 in virtually all solvents and its reduction for 2 and 3 in non-polar solvents at room temperature are partly due to internal conversion (not involving the perpendicular 1p * configuration) and partly due to the enhancement of φ f. The pathways for cis → trans photoisomerization are discussed.