Abstract
Abstract The 1,3-diphenylallyl carbanion exists predominantly in the trans,trans conformation in equilibrium with a small proportion of the cis,trans isomer. Fluorescence was observed from the trans,trans conformer both in the form of the tight and the loose ion pairs but none was detected from the cis,trans conformer. Fluorescence lifetimes were determined using pulsed radiation from the Daresbury synchrotron; where the type of ion pair remains constant, lowering the temperature causes the fluorescence lifetime to increase from the shortest values measured (about 100 ps) to a limiting value of about 5 ns. Analysis shows that this behaviour is accommodated by a mechanism that involves deactivation of the excited state by fluorescence and non-radiative decay in competition with activated twisting about the allylic skeleton. The Arrhenius parameters have been determined for the title carbanion and a number of related species. Generally, the pre-exponential factors for tight ion pairs are considerably greater than for loose pairs. Alternative mechanisms involving the formation of the corresponding allylic radical by the loss of an electron are shown to be incompatible with the experimental results.
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