Effect of substituents on 1,3,5-triphenylpyrazoline as light-emitting diode-sensitive initiators in photopolymerization

Abstract

By introducing different substituents at the N1 position, a series of 1,3,5-triphenylpyrazolines were designed and synthesized as photoinitiators (PIs) for cationic and free radical photopolymerization processes. As evaluated through ultraviolet–visible (UV–vis) spectroscopy, fluorescence spectroscopy, theoretical calculations, electrochemistry, and electron spin resonance–spin trapping tests, all PIs show good photoinitiating properties under light-emitting diode (LED) irradiation at 365–425 nm. Their light absorption within the UV–vis range (λmax = 352–390 nm; εmax = 26 000–40 700 M−1 cm−1) is strongly influenced by the substituents on the N1 phenyl ring. Remarkably, the introduction of the carbonyl group redshifts the maximum absorption spectra from 375 nm to 390 nm, which could well match the emission of LEDs in the range of 365–425 nm. The pyrazolines, together with iodonium salt and N-methyldiethanolamine, can generate acid and free radicals under LED irradiation and conduct high-performance LED-induced cationic and free radical photopolymerization processes under 365–425 nm irradiation. Cationic polymerization is enhanced by the introduction of an electron-pushing group, and free radical polymerization is promoted by the introduction of a carbonyl group at the N1 position. These pyrazoline-based photoinitiating systems have promising applications in LED-induced photopolymerization.