Mechanism of the photoisomerization of 1,3-diphenylallyl carbanion

The fluorescence of 1,3-diphenylallyl lithium (DPALi⁺) in 2-methyltetrahydrofuran (MTHF) is due to the presence of the loose ion pair having the trans,trans conformation: no fluorescence from the cis,trans conformer could be detected. Conformational photoisomerization accompanies fluorescence. The lifetime and the quantum yield of fluorescence increase with decreasing temperature and reach limiting values by about −100 °C. A mechanism in which the excited state is deactivated by fluorescence and non-radiative decay in competition with activated twisting about the allylic skeleton is proposed and shown to be compatible with the observed temperature dependences. Alternative mechanisms involving the corresponding allylic radical are discussed.