Solvent and temperature dependence of the fluorescence quantum yields of substituted stilbenes

Abstract

The temperature and solvent dependence of the fluorescence quantum yield of trans‐4‐cyano‐4′‐methoxy stilbene (1) and trans‐4‐nitro‐4′‐methoxy stilbene (2) have been studied. The activation energy of the deactivation of excited 1 does not depend on the polarity of the solvent, whereas the activation energy for 2 increases with decreasing polarity. The results indicate that the deactivation mechanisms of the S₁ states are different in the two molecules. In the case of trans‐4‐cyano‐4′‐methoxy stilbene intersystem crossing is negligible and the temperature dependent process is attributed to a transition from the first excited singled state tr¹ to the singlet phantom state p¹ of the twisted form of the molecule over a small activation barrier E_a (E_a = 2.8 kcal/mol in toluene). Intersystem crossing, however, predominates in the case of trans‐4‐nitro‐4′‐methoxy stilbene. It is concluded that the deactivation rate has two components one of which is temperature independent and the other temperature dependent. The results are in line with consequences from studies on the mechanism of the photochemical cis‐trans‐isomerization of 1 and 2.