Theoretical studies on photodissociation of carbazole in high excited triplet states

A qualitative and semiquantitative explanation of “double flash” photochemical behaviour of carbazole is found by means of Molecular Orbital studies of ground and excited states of carbazole and carbazyl radical using the CNDO/S method. The need to excite to quite high energy triplet states in order to produce the homolytic breakdown of N-H bond is justified by the fact that the triplet states of carbazole correlating with ground and first excited states of carbazyl radical are, respectively, T₁₅ and T₁₂. These triplet states appear to be populated indirectly by means of the second flash that at first populates some other nearby triplet states (T₉ to T₁₁) from T₁ followed by an Internal Conversion (IC) from these to the T₁₅ and T₁₂ states. This procedure is favoured by the energy lowering of the T₁₅ and T₁₂ triplet states on enlarging the N-H bond and by the important contamination of these two states owed to spin-orbit coupling.