Absorption and emission anomalies in solutions of trans-azastilbenes and related compounds possibly caused by association

Abstract

Absorption and emission anomalies have been investigated for trans-styrylpyridines, dipyridylethylenes, and some other aza-analogues of stilbene-like molecules in aliphatic hydrocarbon solvents at low temperatures. Fluorescence spectra, quantum yields, and lifetimes have been measured at different excitation and emission wavelengths. On cooling, the structured fluorescence spectrum of the monomer is gradually replaced by a new broad red-shifted band which becomes prominent when irradiation is carried out at the red tail of the absorption spectrum. Its intensity decay is multi-exponential and the measured lifetimes depend on both λ_ex and λ_em, reflecting a continuum of new species of longer lifetime than the monomeric compounds. The extent of these anomalies depends on solvent, concentration, temperature, rate of cooling, and structure of the olefin. The emission spectra of the crystalline azastilbenes differ from the broadband emission which is tentatively assigned to association products with a more or less narrow size distribution.