Substituted Stilbene-based D-π-A and A-π-A type oxime esters as photoinitiators for LED photopolymerization

Abstract

Three novel oxime esters based on the π-conjugated trans-stilbene group were designed and synthesized as photoinitiators (PIs) for light-emitting diode (LED) photopolymerization. Three substituents (thioether, sulfoxide, and sulfone) with different electron pushing and withdrawing abilities were introduced into the chromophore to constitute D-π-A (OXE-S) and A-π-A (OXE-SO, OXE-SO2) type structures of PIs to rationalize their relevant effects on free radical photoinitiation ability. Structure-activity relationship, electronic properties, and photodissociation mechanism were investigated using steady-state photolysis, density functional theory (DFT) calculations, and real-time 1H NMR. The compounds show good absorption under exposure to LED irradiation wavelength (i.e., 365–405 nm). The polarity of solvent influences the photoreactivity of this series of oxime esters. The photolysis rates and free radical generation quantum yields for the three PIs increase from polar to nonpolar solvent. For D-π-A type OXE-S, the photolysis rate increases 70 times from acetonitrile to toluene. The three PIs can efficiently initiate the polymerization of acrylate monomer under exposure to LED irradiation at 365–405 nm. The excellent absorption and efficient photoinitiation properties of the PIs demonstrate their great potential in photocuring applications.