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As bisacylphosphine oxides are highly efficient, cleavable photoinitiators for visible light curing, we were interested in preparing diacylgermane 2. The UV–vis absorption maximum of the n−π* transition was red-shifted up to 50 nm. Photo-DSC results of 2 showed similar reactivity to bisacylphosphine oxides under broadband irradiation experiments. Irradiation with a dental LED lamp gave significantly improved initiation behavior compared to bisacylphosphine oxides and camphorquinone, especially in aqueous acidic primer formulations. Application-oriented tests in dental adhesive or composite formulations showed sufficient storage stability, high photocuring activity, excellent photobleaching behavior, and mechanical properties of the materials.

In this contribution a convenient synthetic method to obtain tetraacylgermanes Ge[C(O)R] (R=mesityl (1 a), phenyl (1 b)), a previously unknown class of highly efficient Ge-based photoinitiators, is described. Tetraacylgermanes are easily accessible via a one-pot synthetic protocol in >85 % yield, as confirmed by NMR spectroscopy, mass spectrometry, and X-ray crystallography. The efficiency of 1 a,b as photoinitiators is demonstrated in photobleaching (UV/Vis), time-resolved EPR (CIDEP), and NMR/CIDNP investigations as well as by photo-DSC studies. Remarkably, the tetraacylgermanes exceed the performance of currently known long-wavelength visible-light photoinitiators for free-radical polymerization.