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S. Ligon, M. Griesser, D. Neshchadin, A. Rosspeintner, V. Gorelik, B. Ganster, R. Saf, G. Gescheidt, N. Moszner, R. Liska:
"Advances in long wavelength initiators for photocurable dental adhesives and restoratives";
Poster: RadTech Asia 2013, Shanghai; 20.05.2013 - 24.05.2013.

Photocurable dimethacrylate resins have become more and more common in the last thirty years in dental restoratives. While current photocurable dental resins are both versatile and fairly easy to use, a major shortcoming is the relatively low photoreactivity of most initiators at patient friendly higher wavelengths. Solutions for dealing with this problem have been introduced in the form of highly reactive acylgermane based photoinitiators. These photoinitiators are analogous to traditional aryl ketone photoinitiators with substitution at the carbonyl carbon by germanium, which red shifts absorbance and increases quantum efficiency. Real-time characterization of initial radicals by time-resolved EPR clearly establishes a dialkylphenylgermyl and a benzoyl radical as the decisive reactive species. The formation of primary follow-up products has been followed by photo-CIDNP (Chemically Induced Dynamic Nuclear Polarization) spectroscopy. An overall analysis of products formed under thermodynamic control was achieved by trapping experiments with TEMPO and GC MS analysis. The reactivity of the germyl radical compares to related phosphorus-based radicals and it is shown that the initial germyl and benzoyl radicals undergo subsequent reactions to give derivatives carrying Ge-O bonds. Initiating efficiency of the Germanium based photoinitiators is an order of magnitude beyond that of camphorquinone/amine systems, the benchmark for curing with high wavelength dental lamps. Thus application of a greatly reduced portion of photoinitiator allows faster and thicker curing with higher wavelength dental lamps.

The authors wish to acknowledge the Christian Doppler Research Society for funding of this work.

Dental materials, Photoinitiators, Germanium, CIDNP