HYDROGEN-BONDING-INDUCED ELECTRON-TRANSFER FROM TRIPLET N,N-DIALKYL-1-NAPHTHYLAMINES TO BENZOPHENONE VIA TRIPLET EXCIPLEXES

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 h ave 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 ((BP)-B-3) to DANA occurs with the efficiency phi(TE T) (0.74 for DMNA and 0.61 for DENA) regardless of the presence of H2O and methanol. After the formation of triplet DANA ((3)DANA), me trip let exciplex (3)(DANA...BP) with weak charge-transfer character is pr oduced with the equilibrium constant K-1 (10 M(-1) for DMNA and 9 M(-1 ) for DENA) between (3)DANA and BP. The mechanism for the formation o f (3)(DANA...BP) is shown in Scheme 1. In the presence of H2O and met hanol, it is found that the intraexciplex electron transfer takes plac e to give the BP anion (BP.-) and DANA cation (DANA(.+)) radicals in t he hydrogen-bonded triplet exciplex (3)(DANA...BP)(HB) by H2O or meth anol. The mechanism for the production of DANA(.+) and BP.- is propose d in Scheme 2. The equilibrium constants K-2 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 k(et) for the intraexciplex electron transfer induced by hydrogen bonding are de termined to be 2.5 x 10(7) s(-1) for DMNA and 1.4 x 10(7) s(-1) for DE NA. It was revealed that the driving force for intraexciplex electron transfer is the negatively enlarged reduction potential of BP in (3)(D ANA...BP)(HB) due to the hydrogen bonding to the carbonyl group of BP in (3)(DANA...BP).