Triplet Energy Transfer and Triplet Exciplex Formation of Benzophenone

Abstract

Abstract Photochemical and photophysical aspects of (1) triplet energy transfer (TET) from triplet benzophenone (BP) to naphthalene derivatives (NpD) and (2) formation and decay processes of triplet exciplexes between 3NpD* and BP have been studied by 355-nm laser photolysis techniques in the liquid phase. As an initial event, TET occurs from 3BP* to NpD competing with H-atom transfer (HT), electron transfer (ET), and induced quenching (IQ) on the nanosecond time scale. The TET rate constant increases with an increase of solvent polarity while that of HT decreases, indicating that the 3(n, π*) state of BP is slightly mixed with 1(π, π*) in polar media. After formation of 3NpD* by TET from 3BP*, chemical reactions via triplet exciplexes between 3NpD* and BP having loose sandwich-like structures with weak charge transfer character take place in the microsecond time scale, depending on the substituent groups of NpD. (1) HT from triplet 2-naphthylammoniun ion and triplet naphthol to BP occurs efficiently to yield the 2-naphthylamine cation radical and naphthoxy radical plus the benzophenone ketyl radical, respectively. The more protic H-atom in NpD is the more reactive in HT. (2) The proton-induced ET from triplet methoxynaphthalene to BP takes place effectively via the protonated triplet exciplex to give the methoxynaphthalene cation and benzophenone ketyl radical. (3) Hydrogen bonding-induced ET from triplet N, N-dialkyl-1-naphthylamine to BP occurs to yield the N-dialkyl-1-naphthylamine cation radical and benzophenone anion radical in the presence of protic solvents.