Hydrogen-Bonding-Induced Electron Transfer from Triplet N,N-Dialkyl-1-naphthylamines to Benzophenone via Triplet Exciplexes

Abstract

Laser flash photolysis studies at 355-nm on the photoreactions of the benzophenone (BP) and N,N-dialkyl-1-naphthylamine, DANA (N,N-dimethyl-1-naphthylamine, DMNA, and N,N-diethyl-1-naphthylamine, DENA) system have been carried out with and without H2O and methanol in acetonitrile (ACN) at 295 K. In the nanosecond time scale, triplet energy transfer from triplet BP (3BP*) to DANA occurs with the efficiency φTET (0.74 for DMNA and 0.61 for DENA) regardless of the presence of H2O and methanol. After the formation of triplet DANA (3DANA*), the triplet exciplex 3(DANA···BP)* with weak charge-transfer character is produced with the equilibrium constant K1 (10 M-1 for DMNA and 9 M-1 for DENA) between 3DANA* and BP. The mechanism for the formation of 3(DANA···BP)* is shown in Scheme 1. In the presence of H2O and methanol, it is found that the intraexciplex electron transfer takes place to give the BP anion (BP•-) and DANA cation (DANA•+) radicals in the hydrogen-bonded triplet exciplex 3(DANA···BP)*HB by H2O or methanol. The mechanism for the production of DANA•+ and BP•- is proposed in Scheme 2. The equilibrium constants K2 for the formation of 3(DANA···BP)*HB with H2O and methanol obtained are 0.55 and 0.45 M-1 for DMNA, 0.50 and 0.40 M-1 for DENA. The rate constants ket for the intraexciplex electron transfer induced by hydrogen bonding are determined to be 2.5 × 107 s-1 for DMNA and 1.4 × 107 s-1 for DENA. It was revealed that the driving force for intraexciplex electron transfer is the negatively enlarged reduction potential of BP in 3(DANA···BP)*HB due to the hydrogen bonding to the carbonyl group of BP in 3(DANA··· BP)*.