PROTON-INDUCED ELECTRON-TRANSFER REACTION FROM TRIPLET METHOXYNAPHTHALENES TO BENZOPHENONE VIA TRIPLET EXCIPLEX

A laser flash photolysis study at 355 nm has been carried out on acetonitrile-water (4:1 v/v) mixtures of the methoxynaphthalene-benzophenone-H2SO4 system. It is found that the proton-assisted photoionization reaction of triplet 1- and 2-methoxynaphthalenes (ROMe) produced by triplet sensitization of benzophenone (BP) effectively occurs to produce the corresponding methoxynaphthalene cation radical (ROMe.+) and benzophenone ketyl radical (>COH). The triplet energy-transfer reaction from triplet benzophenone ((BP*)-B-3) to ROMe takes place as the primary event to produce triplet methoxynaphthalene (3ROMe*). The 1:1 triplet exciplex 3(ROMe...>CO)* having a weak charge-transfer structure is readily formed between 3ROMe* and BP with the equilibrium constants K1 (10.1 M-1 for 1ROMe; 4.0 M-1 for 2ROMe at 290 K). In the presence of protons, the triplet protonated exciplex 3(ROMe...>C+OH)* is also produced by protonation to 3(ROMe...>CO)*. Subsequently, the intraexciplex electron-transfer reaction in 3(ROMe...>C+OH)* takes place effectively, resulting in the formation of ROMe.+ and >COH. The presence of protons in the ROMe-BP system assists the photoionization of ROMe with the much lower energy (triplet energy of ROMe) than its ionization potential. The reaction mechanism is discussed in detail.