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Acylgermanes: Photoinitiators and Sources for Ge-Centered Radicals. Insights into their Reactivity

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journal contribution
posted on 20.11.2013, 00:00 by Dmytro Neshchadin, Arnulf Rosspeintner, Markus Griesser, Bernhard Lang, Sandra Mosquera-Vazquez, Eric Vauthey, Vitaly Gorelik, Robert Liska, Christian Hametner, Beate Ganster, Robert Saf, Norbert Moszner, Georg Gescheidt
Acylgermanes have been shown to act as efficient photoinitiators. In this investigation we show how dibenzoyldiethylgermane 1 reacts upon photoexcitation. Our real-time investigation utilizes femto- and nanosecond transient absorption, time-resolved EPR (50 ns), photo-chemically induced dynamic nuclear polarization, DFT calculations, and GC-MS analysis. The benzoyldiethylgermyl radical G• is formed via the triplet state of parent 1. On the nanosecond time scale this radical can recombine or undergo hydrogen-transfer reactions. Radical G• reacts with butyl acrylate at a rate of 1.2 ± 0.1 × 108 and 3.2 ± 0.2 × 108 M–1 s–1, in toluene and acetonitrile, respectively. This is ∼1 order of magnitude faster than related phosphorus-based radicals. The initial germyl and benzoyl radicals undergo follow-up reactions leading to oligomers comprising Ge–O bonds. LC-NMR analysis of photocured mixtures containing 1 and the sterically hindered acrylate 3,3-dimethyl-2-methylenebutanoate reveals that the products formed in the course of a polymerization are consistent with the intermediates established at short time scales.