Thioxanthone hydroquinone-O,O'-diacetic acid: photoinitiator or photostabilizer?

A photoinitiator for free-radical polymerization based on a thioxanthone chromophore containing two acetic acid functions was synthesized and characterized. Photophysical studies such as fluorescence, phosphorescence, and laser flash photolysis in addition to photopolymerization of acrylates were performed to elucidate the radical generation mechanism involving intramolecular electron transfer from the triplet state followed by decarboxylation. We found that the position of the acetic acid substituent is critical for the photoreactivity. In most solvents and acrylic monomers, if the acetic acid functionality is at the 1-position, the singlet excited states are deactivated rapidly before electron transfer can occur, resulting in negligible photoreactivity. The excited-state deactivation probably involves intramolecular H-bonding deactivation. The intramolecular H-bonding is disrupted by solvents that support intermolecular H-bonding, such as DMF and DMSO, leading to efficient intramolecular photoreaction.