Photochemistry and photophysics of α-hydroxy ketones

Time-resolved laser flash spectroscopy with transient detection by time-resolved W-vis, IR, and EPR has been employed to investigate the photochemistry and photophysics of a series of derivatives of 2-hydroxy-2-methyl-1-phenyl propanone, a model photoinitiator for free radical polymerization. H, Cl, and F substituents at the para position promote the n,pi* nature of the lowest triplet state, and favor fast and efficient a-cleavage from T-1 upon irradiation. In contrast, dimethylamino and thioether substitution in the para position of the benzoyl moiety change the configuration of the lowest triplet states into a pi,pi* and are characterized by a lack of a-cleavage from T-1. Alkoxy substitution in the para position represents an intermediate for which or-cleavage occurs efficiently; but at a relatively slow rate. Alkylation of the 2-hydroxy group promotes fast alpha -cleavage upon irradiation (picosecond time scale), in contrast to the analogous ester derivative for which slow cleavage was observed (microsecond time scale). The conclusions are consistent with phosphorescence characterization of the triplet states and are supported by photopolymerization studies.