EFFECT OF ADDED CHLOROMETHANES ON THE PHO TOCYCLIZATION OF DIPHENYLAMINES

Photocyclization of diphenylamine (DPA) to carbazole has been known to proceed via dihydrocarbazole (M) produced from the excited triplet st ate (T). The quantum yield (Phi(c),) for the formation of carbazole is markedly affected by the addition of chloromethanes (Q): Phi(c), repr esented as a function of [Q], Phi(c), (Q), shows a convex curvature; i .e. Q causes both acceleration and retardation of the photocyclization reaction. The present study was carried out in order to elucidate the role of Q in the photocyclization reaction of diphenylamines. Fluores cence of DPA is quenched markedly by the addition of Q. The rate const ant of fluorescence quenching (k(2)) increases with the increase in el ectron affinity of Q, indicating that the quenching of the lowest exci ted singlet state (S-1) is caused by the electron transfer from S-1-st ate of the amine to Q. However, the quenching of S-1 by Q cannot fully explain the variation of Phi(c), in the whole range of [Q] studied. C omputer simulation of Phi(c),(Q) revealed that dehydrogenation of M by Q accelerates the photocyclization, while the quenching of S-1 and T by Q retards the photocyclization. In this simulation process of Phi(c ),, we introduced acceleration and retardation facters, F-a and F-r, w hich depend on the nature of Q. The plot of both log F-a and log F-r v s K-q gave straight lines, suggesting that the electron-transfer proce sses play an important role in both acceleration and retardation in th e photocyclization as well as quenching of fluorescence. Detailed disc ussions on the chemical mechanism of acceleration and retardation were made. The differences between the N-substituted and N-unsubstituted a mine were also discussed.