Triplet and radical ion properties of (E)-1-styrylpyrene and (E)-3-styrylchrysene

Abstract

The photophysical and photochemical features of trans isomers of 1-styrylpyrene (StPy) and 3-styrylchrysene (StCh) have been studied in benzene and acetonitrile solution at room temperature, and in 2-methyltetrahydrofuran (MTHF) at low temperatures. The major deactivation process of the excited trans singlet state (¹trans*) is fluorescence, (fluorescence quantum yield, ϕ_F= 0.7–0.9), whereas singlet → triplet (S₁→ T₁) intersystem crossing is inefficient (as is trans→cis photoisomerization of StPy). Quenching of the ¹trans* state by diethylaniline (DEA) leads to an exciplex fluorescence in benzene. Fluorescence quenching by 4-bromodimethylaniline (via the external heavy-atom effect) enhances the formation of the trans triplet configuration (³trans*). In acetonitrile, the radical anion (trans˙^–) of the two 1,2-diarylethenes was observed by optical and conductometric detection in the presence of DEA. Photoionization with a low quantum yield (ϕ_PI= 0.007–0.015) was observed in neat acetonitrile. The properties of the radical cation (trans˙⁺) were studied after photoinduced electron transfer from ¹trans* to 1,4-dicyanobenzene (DCNB). The effect of temperature on the yield and decay kinetics of ³trans* was analysed for StCh in MTHF and a mechanism is proposed for the trans→cis photoisomerization via an upper excited triplet state.