New photocleavable structures III: Photochemistry and photophysics of pyridinoyl and benzoyl-based photoinitiators

Pyridine ketones have recently been found to be suitable α-cleavage photoinitiators for radical polymerization with excellent water solubility. Nanosecond time-resolved laser flash photolysis and steady-state photolysis were employed to study and compare the triplet photophysics and the photochemistry of 2-hydroxy-2-methyl-1-pyridin-3-yl-propan-1-one (1b) and its commercial benzoyl analogue 2-hydroxy-2-methyl-1-phenylpropan-1-one (Darocur 1173®, 1a), and of some of their less photoreactive derivatives, in acetonitrile solution. The triplet state quantum yield, measured by energy transfer to naphthalene, was smaller for 1b (ΦT = 0.10) than for 1a (ΦT = 0.25), but the photodegradation quantum yields in the presence of the radical trap TEMPO were comparable (ΦD = 0.6). The discrepancy between the yields of triplet formation and photodegradation points to a possible role of higher excited triplet states in the process of photoinduced α-cleavage. The reactivities of the primary radicals with a series of reaction partners showed that the benzoyl and the nicotinoyl radical have comparable reactivities towards polymerizable double bonds, but that the nicotinoyl radicals reacts 17 times faster with TEMPO. The previously observed reduced oxygen sensitivity in photopolymerization using 1b compared to 1a could be traced to a smaller reactivity towards O2 of the nicotinoyl radical compared to the benzoyl radical. The photodegradation kinetics of 1a and 1b under steady-state conditions were comparable, but significant differences were observed in a photoproduct study.