Naphthoic Acid Derivatives as Photosensitizers for Short-wavelength α-Hydroxyacetophenone Photoinitiators

Abstract

1-Hydroxycyclohexyl phenyl ketone (Omnirad 184), a photoinitiator that provides thermal stability and efficient photocleavage, is unsuitable for polymerization using light near 400 nm, typically generated by an LED lamp because Omnirad 184 shows less absorption for long-wavelength UV light. However, Omnirad 184 acquired a photosensitizing ability (under light near 400 nm) when a 1,4-dioxy-2-naphthoic acid derivative was added to it, affording accelerated polymerization, and enhanced final conversion. In this study, 1,4-dioxy-2-naphthoic acid derivatives were synthesized and their photosensitizing abilities were evaluated using Fourier transform infrared spectroscopy (FTIR). The results showed that phenyl 1,4-dihydroxy-2-naphthoate (DHNA-Ph) is the best photosensitizer. Quenching studies confirmed the interaction between the excited states of some 1,4-dihydroxy-2-naphthoic acid derivatives and Omnirad 184, which promotes the excitation of the latter. The results show that the interaction may involve electron transfer from the excited singlet state of the naphthoic acid derivatives to the initiator Omnirad 184. Although Omnirad 184 is a good photoinitiator, it cannot be used with a near UV and Visible LED (405 nm). This study aims to overcome this problem using different photosensitizers.