Energetics of photocyclization of polyphenylamines and assignment of the intermediate: a time-resolved photoacoustic calorimetric study

The photocyclization reactions of diphenylamine (DPA), N-methyldiphenylamine (MDPA) and triphenylamine (TPA) have been studied in methanol solvent through nanosecond/microsecond flash photolysis and time-resolved photoacoustic calorimetry (TRPAC). It is confirmed for all the amines that the long-lived dihydrocarbazole intermediate (<SUP>1</SUP>XDHC<SUP>0</SUP>) is formed through a cascade of processes. However, DPA behaves differently from the other two members of the series so far as the rates of the individual processes are concerned. The quantum yield of triplet formation and the quantum yield of the photocyclization of the amine triplet have been estimated for the DPA system. In conjugation with the experiments, theoretical calculations, employing AM1-SCI method, have been performed to propose a structure for the intermediate. For all the amines, the intermediate has been proposed to be of zwitterionic nature in the ground state. The enthalpies of formation of the intermediate (<SUP>1</SUP>XDHC<SUP>0</SUP>) from MDPA and TPA have been determined experimentally to be around 51 kcal mol<SUP>-1</SUP> and the values agree well with the calculated ones