Viscosity-induced emission anomalies in 1,2-diarylethylenes and in distyryl-benzenes and -naphthalenes

Abstract

trans-Stilbenes and a variety of compounds related to trans-1-styrylnaphthalene exhibit normal fluorescence behaviour in fluid solutions. At high viscosities (>10⁵ cP) of the medium, anomalous phenomena start to appear: the spectra change their shape and vary with the excitation wavelength, particularly with excitation in the red tail of the absorption spectrum. The extent of these anomalies increases with the viscosity and with the size of the aryl group. Another anomaly, observed in 1,2-di-(9- anthryl)ethylenes, is a viscosity-controlled. very pronounced enhancement of fluorescence yields. Our working hypothesis is based on the restriction, at high viscosities, of the rate of torsional relaxation, so that emission is mainly from molecules still having the geometry of the absorbing state. Irradiation at very low energies excites selectively those ground-state molecules having a geometry similar to that of the relaxed excited state. In the dianthrylethylenes, the rate of non-radiative decay processes S₁→S₀ associated with torsional relaxation is reduced at high viscosities, causing the observed strong enhancement of fluorescence yields, in addition to the variation of spectral shape with the temperature and the excitation wavelength.