Photochemistry and photoinitiation activity of radical polymerization of 2‐substituted anthraquinone derivatives. III. Nanosecond laser flash photolysis study

Abstract

A nanosecond laser flash photolysis study was undertaken on a selected range of 2-substituted anthraquinone derivatives and the data are discussed in relation to the photoactivities in industrial photopolymerization. All the compounds give rise to a triplet-triplet absorption that is quenched by ground-state molecular oxygen. The transient half-lives range from 0.64 to 11.3 μs of which the 2-(1,1 -dibromomethyl) anthraquinone exhibits the longest life-time. In a reductive solvent, 2-propanol, hydrogen atom abstraction takes place with the compounds haveing the lowest-lying triplet 3nπ* state, whereas those with a low-lying triplet3ππ* state show mixed kinetics. In the latter case, a disproportionation reaction involving the semianthraquinone radical may be taking place, competing with the direct hydrogen atom abstraction reaction. In addition, investigation of the halogenated derivatives has indicated the possibility of the corresponding halo radicals being formed. In the presence of a tertiary amine, triethylamine, all anthraquinone derivatives show the formation of stable species related to either the exciplex or the radical ion pair. The extent of exciplex formation is more effective with compounds possessing a lowest-lying triplet 3ππ* excited state than those with a triplet 3nπ* excited state. The results from the nanosecond laser flash photolysis study show the differences in behavior toward hydrogen atom abstraction and electron transfer processes that is dependent on the nature of the low-lying triplet state and the type of substituent present, i.e., electron-donating or electron-withdrawing. © 1996 John Wiley & Sons, Inc.