Thioxanthone-based amidine: An efficient nonionic photobase generator for thiol-based click polymerization under visible LED light

Abstract

With the fast development of the long-wavelength irradiation sources, such as light emitting diodes (LED), which possess advantages regarding irradiation safety and energy utilization efficiency, it is highly desired to develop efficient photobase generators (PBGs) sensitive to visible light. Currently, the reported efficient visible light PBGs mainly undergo photodecarboxylation to generate the super bases, while the released CO2 can potentially cause bubbles and cracks in the cured materials. Moreover, these ionic PBGs generally have poor solubility in organic media and limited thermal stability. In the present work, we have investigated a nonionic amidine PBG TX-DBN, which contains thioxanthone as a chromophore and a reduced form of 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) as a latent base species. TX-DBN, with a strong absorption above 380 nm, showed excellent solubility in most organic solvents and commercial monomers/oligomers. Under irradiation with an LED lamp at 405 nm, TX-DBN undergoes a CN bond cleavage followed by a fast hydrogen transfer, leading to the generation of the super base DBN without the generation of CO2. The as-released active species, namely super bases and active free radicals, can trigger thiol-epoxy and thiol-ene polymerization to fabricate hybrid network.