CHARGE-TRANSFER-INDUCED PHOTOREDUCTION OF AZOALKANES BY AMINES

The unique DBH-type azoalkanes 1, which exhibit high intersystem cross ing quantum yields, have made possible the exploration of the bimolecu lar photoreduction of the n,pi triplet-excited azo chromophore. In th e laser-flash photolysis, amines were found to quench the triplet azoa lkane la with high rate constants (k(q) ca. 10(8) M-1 s(-1)). Steady-s tate photolysis of the azoalkanes la and Ib (phi(ISC) ca. 0.5) in the presence of primary, secondary, and tertiary aliphatic amines gave hig h chemical yields of the corresponding hydrazines 4a and 4b in competi tion with the unimolecular products, namely the housanes 2 and the azi ranes 3. In contrast, the azoalkane Ic undergoes appreciable photoredu ction only in neat amines, while the azoalkane Id (phi(ISC) ca. 0.10) is not reduced even under such conditions. Except for N,N-dimethylbenz ylamine, the amine oxidation products of the azoalkane photoreduction are analogous to those obtained from the reactions of amines with trip let benzophenone. In marked contrast, the absolute quantum yields of p hotoreduction for azoalkanes 1 are substantially lower (0.01-0.06) tha n for benzophenone (0.3-1.0). Efficient deactivation of the triplet-ex cited states by charge-transfer (k(q)(CT)), which competes with hydrog en atom abstraction (k(H)(CT)), is postulated to account for the low q uantum yields. The efficiencies of photoreduction follow the trend pri mary approximate to tertiary much greater than secondary amines observ ed with benzophenone, for which secondary amines also display the poor est efficiency. Electron transfer to triplet-excited azoalkanes, analo gous to benzophenone, is observed for amines with low oxidation potent ials. Thus, when triphenylamine (E-ox=0.85 V versus SCE) is used, the formation of its radical cation can be readily detected by laser-flash photolysis.