Photochemistry and photophysics of alpha-hydroxy ketones

Time-resolved laser flash spectroscopy with transient detection by time-res olved W-vis, IR, and EPR has been employed to investigate the photochemistr y and photophysics of a series of derivatives of 2-hydroxy-2-methyl-1-pheny l propanone, a model photoinitiator for free radical polymerization. H, Cl, and F substituents at the para position promote the n,pi* nature of the lo west triplet state, and favor fast and efficient a-cleavage from T-1 upon i rradiation. In contrast, dimethylamino and thioether substitution in the pa ra position of the benzoyl moiety change the configuration of the lowest tr iplet states into a pi,pi* and are characterized by a lack of a-cleavage fr om T-1. Alkoxy substitution in the para position represents an intermediate for which or-cleavage occurs efficiently; but at a relatively slow rate. A lkylation of the 2-hydroxy group promotes fast alpha -cleavage upon irradia tion (picosecond time scale), in contrast to the analogous ester derivative for which slow cleavage was observed (microsecond time scale). The conclus ions are consistent with phosphorescence characterization of the triplet st ates and are supported by photopolymerization studies.