Photochemistry and Photophysics of α-Hydroxy Ketones

Abstract

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 scale). The conclusions are consistent with phosphorescence characterization of the triplet states and are supported by photopolymerization studies.

Synthetic Routes for the Preparation of Silver Nanoparticles

“…The photochemistry of α-hydroxy ketones has been well characterized and the homolytic rupture of the α-cleavage bond favored by alkylation of the 2-hydroxy group occurs in the picosecond time scale after excitation [62]. The case of I-2959 is remarkable since is commercially available, water-soluble, and produces ketyl radicals with a quantum yield of 0.29 in fast photocleavage from a triplet state with a lifetime of just 11 ns [62].…”

Chemically Induced Dynamic Electron Polarization Studies of a pH-Dependent Free Radical Cage Formed in a Photoinitiator Labeled Poly(methacrylic acid)

“…[11][12][13] When 1 is covalently bound to PMA, an analogous photochemical primary process is expected except that the resultant benzoyl radical must be attached to the PMA backbone, while the ketyl radical is free to diffuse away. In this study we have observed and investigated the transient polarized ESR signal of the ketyl radical as a function of pH.…”

Studying the polymerization initiation efficiency of acetophenone-type initiators via PLP-ESI-MS and femtosecond spectroscopy

“…Thereby, T 1 states revealing a 3 (np*) character are known to be more efficient concerning radical formation than 3 (pp*) states. 34 Since orbital analyses of the T 1 states of the three investigated initiator molecules differ insignicantly from each other (all photoinitiators show at least partly the 3 (np*) character), the observed differences in radical generation seem to have other origins. Regarding internal rotation effects [35][36][37] (rotation around the C(O)-C(OH) and C(O)-C(Me) 2 bond, respectively) the transient data do not allow for an analysis with respect to different conformers.…”