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, DANA (N, DMNA, and N, DENA) system have been carried out with and without H 2 O and methanol in acetonitrile (ACN) at 295 K. In the nanosecond time scale, triplet energy transfer from triplet BP ( 3 BP*) to DANA occurs with the efficiency φ TET (0.74 for DMNA and 0.61 for DENA) regardless of the presence of H 2 O and methanol. After the formation of triplet DANA ( 3 DANA*), the triplet exciplex 3 (DANA‚‚‚BP)* with weak charge-transfer character is produced 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 of 3 (DANA‚‚‚BP)* is shown in Scheme 1. In the presence of H 2 O 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 H 2 O or methanol. The mechanism for the production of DANA •+ and BP •is proposed in Scheme 2. The equilibrium constants K 2 for the formation of 3 (DANA‚‚‚BP)* HB with H 2 O 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 determined to be 2.5 × 10 7 s -1 for DMNA and 1.4 × 10 7 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)*. Experimental SectionBenzophenone (BP) was purified twice by vacuum sublimation. N,N-Dimethyl-1-naphthylamine (DMNA) from Tokyo Kasei was purified by distillation in vacuo. N,N-Diethyl-1naphthylamine (DENA) was synthesized as follows: N-Ethyl-1-naphthylamine was stirred with a small portion of H 2 SO 4 in X

Efficient Photochemical Oxidation of Anisole in Protic Solvents: Electron Transfer driven by Specific Solvent–Solute Interactions

“…Actually, free-radical formation in the system BP/dimethylsulfide proved to be insignificant in neat ACN but is substantially enhanced in ACN/ water mixtures and in pure water. [71] Besides that, both the mentioned ET-initiated reaction in BP/tyrosine dyads [31] and the ET in the naphthylamine/BP system [50] experience an expressed switch-on in protic solvents.…”

“…[50] Similar reasoning accounted for the solvent effects on the intramolecular quenching of BP/tyrosine dyads, where the net hydrogen atom transfer follows an ET-PT pathway in protic solvents. [31,32] The strong acceleration of the triplet quenching that is observed herein points to a similar impact of the specific solvation on the activation barriers of an electron-transfer step.…”

A Mechanistic Study of Photoinduced Electron Transfer from Triplet Erythrosin to Various Quinones Using Time Resolved Absorption and ESR-CIDEP Measurements

“…Negative curvatures in S-V plots can also be observed from excited state complexes between other triplet dyes and quinones [39,40].…”

Effects of protic and aprotic solvents on quenching mechanisms involving dimethyl-substituted donors and tetracyanoquinodimethane (TCNQ)

“…The meanings of the symbols in eqn. (7) and ( 8) are given elsewhere.2 From Table 2 it is apparent that the values obtained k q from both the SV relations for a particular DÈA system are very similar in either solvent, indicating that Ñuorescence quenching should be of a dynamic type and static quenching plays an insigniÐcant role in non-radiative deactivation of the excited singlets of the donor molecules in the presence of TCNQ. It seems that the very weak ground-state complex discussed above, might make little contribution to the Ñuorescence quenching mechanism.…”

“…This may be further substantiated by measuring the dynamic Ñuorescence quenching rate constants, by using simple SternÈVolmer (SV) relations of steady-state [eqn. (7)] and time-resolved [eqn. (8)] types.…”