Photoisomerization of 1,2-diarylethylenes: Photophysical properties of 5( E)-styryl-1,3-dimethyluracil

Abstract

The photophysical properties of 5( E)-styryl-1,3-dimethyluracil (5( E)-SDU) were investigated. The fluorescence quantum yield and the lifetime are low and short relative to those of the parent aromatic hydrocarbon; this is probably because of the contribution of the n,π * state to the fast radiationless decay. However, the fluorescence of 5( E)-SDU at room temperature is not significantly affected by the solvent polarity, the presence of salts or the pH. This observation indicates that the 1(n,π *) state lies above the lowest 1(π,π *) state and that the vibronic mixing between the two states is not extensive. The highly positive fluorescence and the fluorescence excitation polarization confirm that the lowest excited singlet state has the π,π * configuration. The strong temperature dependence of the fluorescence intensity may be due to the operation of the activated process of twisting in the first excited singlet state competing with flurescence, and the rotation barrier is estimated to be 3.0 kcal mol −1. The strong quenching of fluorescence intensity by ethyl iodide at room temperature may be indicative of inefficient intersystem crossing. Moreover, only very weak phosphorescence is detected at 77 K and its quantum yield is too low to estimate in either isopentane-ethyl ether (1:1, v/v) or ethanol, supporting the idea of inefficient intersystem crossing suggested by laser flash photolysis results. Therefore, the major radiationless process appears to be internal conversion including rotatory radiationless decay. From the relatively long phosphorescence lifetime and its highly negative polarization the lowest triplet state is considered to be the 3(π,π *) state.