Remote effect of substituents on the properties of phenyl thienyl thioether-based oxime esters as LED-sensitive photoinitiators

Visible light-sensitive photoinitiators (PIs) are essential for light-emitting diode (LED) photopolymerization as LEDs are becoming the predominant light sources. In this work, five phenyl thienyl thioether-based oxime esters with different electron-donating and -withdrawing substituents on the para -position of the phenyl ring were designed and synthesized as LED-sensitive PIs. These oxime esters exhibited a maximum absorption band matched with the LED emission spectra (e.g., 365–425 nm). The remote substituents clearly affected the absorption of the oxime esters, and the principle was investigated via DFT calculations. The substituents also enhanced the initiation activities with the increase in the quantum yields of photo-induced free radical generation. Besides, upon comparison of the quantum yields of fluorescence, photolysis, and free radicals, a fast intersystem crossing was proposed for the excited states of these oxime esters, and results revealed the photoreactions were mainly from the triplet states. Finally, photo-DSC was employed to evaluate the actual photopolymerization. Results suggested that they could all efficiently initiate the free radical polymerization of acrylates under LED irradiation, making them suitable PIs for LED polymerization. This study could provide some significance to the design of PIs with improved efficiency.