Thioxanthonation of 2-naphthalene-thioacetic acid as a visible-light photoinitiator: The investigation of photophysical and photochemical properties

Abstract

Thioxanthonation of naphthalene thioacetic acid (TXNPSCH2COOH) resulted a visible light photoinitiator with the high molar absorptivity (epsilon 405: 5680 L.mol- 1.cm- 1) in the visible spectral region and made this compound an attractive photoinitiator for free radical polymerization reactions. Low fluorescence quantum yield (phi F: 0.06 in 2-MeTHF) and comparatively low triplet energy (186 kJ/mol in 2-MeTHF) was in favorable for efficient intersystem crossing to the triplet state which is necessary for production of initiating radicals. Indeed, photopolymerization of methyl methacrylate along with TXNPSCH2COOH (1 x 10-3 M) and a tertiary amine namely MDEA yielded nearly 25% conversion of monomer to polymer. All spectrophotometric studies including laser flash photolysis which gives triplet lifetime of initiator which is very important information for initiation mechanism of photoinitiators, indeed a long enough triplet lifetime of TXNPSCH2COOH (tau T: 1000 ns) allows for efficient reactions with co-initiators to generate radicals.