Effect of temperature on the photophysical properties of styrylpyridines

Abstract

The effect of temperature on the photophysical parameters (fluorescence quantum yield and lifetime) of the three isomeric trans-styrylpyridines has been studied in 3-methylpentane and a mixture of ethyl ether, isopentane and ethanol in the proportions 5 : 5 : 2 (v/v). The Arrhenius parameters (A= 10¹²–10¹³ S^–1, ΔE= 1–3 kcal mol^–1) indicate that the activated twisting around the double bond, which is dominant at room temperature, takes place in the singlet manifold, as in stilbene. At low temperature (below 100–150 K, depending on nitrogen position) the S₁ [graphic omitted] T₁ process becomes important and, because of differences in the relative weight of intersystem crossing, different photophysical behaviour is exhibited by the three isomers. The observed dependence of the lifetimes on the excitation wavelength gives hints about the presence of conformational isomers.