FLUORESCENCE OF ALPHA,OMEGA-DIPHENYLPOLYENYL CARBOCATIONS

The fluorescence lifetimes of the alpha,omega-diphenylpolyenyl carboca tions in CH2Cl2 solution increase with decreasing temperature toward l imiting values. This behavior is very similar to that of the correspon ding alpha,omega-diphenylpolyenyl carbanions, and the quantitative dif ferences are surprisingly small. By analogy with the anions, it is pro posed that deactivation of the excited state occurs by fluorescence, i nternal conversion, and by skeletal twisting of these conformationally labile ions. This last method of deactivation is an activated process and becomes negligible at low temperatures.