The effect of solvent polarizability on the fluorescence of trans-1-(2-naphthyl)-2-phenylethene conformers. Conformer-specific fluorescence from the cis isomer

Abstract

A matrix of trans-1-(2-naphthyl)-1-phenylethene (t-NPE) fluorescence spectra obtained from benzene solutions at 20 °C by varying the excitation wavelength (λ_exc) and the oxygen concentration is resolved into pure conformer fluorescence spectra by use of principal component analysis with self-modeling based on optimum global Stern–Volmer constant criteria. The resulting fractional contributions of the two components to the fluorescence spectra are combined with observed fluorescence quantum yields as a function of λ_exc to obtain the conformer-specific quantum yields. These quantum yields and fluorescence lifetimes are used to determine conformer-specific radiative and radiationless rate constants. Comparison with results from an analogous study in methylcyclohexane reveals pronounced, differential enhancements of all these rate constants in benzene (Bz). Preliminary measurements of emission from a cis-1-(2-naphthyl)-1-phenylethene (c-NPE) solution in Bz at 20 °C, under static cell conditions, are dominated by sequential two-photon-induced t-NPE fluorescence due to excitation of t-NPE formed photochemically from the cis isomer. The spectra also reveal a weak structureless broad emission, which is assigned to ¹c-NPE* by comparison with the published low temperature fluorescence spectrum of c-NPE in a hydrocarbon glass. The conformational origin of this emission is addressed.