Photochemistry and Photophysics of α-Hydroxy Ketones

Time-resolved laser flash spectroscopy with transient detection by time-resolved UV−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,π* nature of the lowest triplet state and favor fast and efficient α-cleavage from T1 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 π,π* and are characterized by a lack of α-cleavage from T1. Alkoxy substitution in the para position represents an intermediate for which α-cleavage occurs efficiently, but at a relatively slow rate. Alkylation of the 2-hydroxy group promotes fast α-cleavage upon irradiation (picosecond time scale), in contrast to the analogous ester derivative for which slow cleavage was observed (microsecond time ...