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The formation of a stable triacylgermenolate 2 as a decisive intermediate was achieved by using three pathways. The first two methods involve the reaction of KO t Bu or alternatively potassium with tetraacylgermane 1 yielding 2 via one electron transfer. The mechanism involves the formation of radical anions (shown by EPR). This reaction is highly efficient and selective. The third method is a classical salt metathesis reaction toward 2 in nearly quantitative yield. The formation of 2 was confirmed by NMR spectroscopy, UV–vis measurements, and X-ray crystallography. Germenolate 2 serves as a starting point for a wide variety of organo-germanium compounds. We demonstrate the potential of this intermediate by introducing new types of Ge-based photoinitiators 4b – 4f . The UV–vis absorption spectra of 4b – 4f show considerably increased band intensities due to the presence of eight or more chromophores. Moreover, compounds 4d – 4f show absorption tailing up to 525 nm. The performance of these photoinitiators is demonstrated by spectroscopy (time-resolved EPR, laser flash photolysis (LFP), photobleaching (UV–vis)) and photopolymerization experiments (photo-DSC measurements).

Several studies have demonstrated the superb photoinitiating properties of acylgermane compounds (the “bright side”), which are employed e.g. for curing dental filling materials. In this work, we want to explore the “dark side” of these compounds: i.e., reactivity not induced by light. Voltammetric measurements (direct-current polarography and cyclic voltammetry) show that the acylgermanes serve as electron acceptors, where the present CO groups represent the centers of reduction (similar to benzaldehyde and acetophenone). The half-wave potentials are influenced by the number of the acyl groups and the moiety at the para position. EPR spectra reveal (and thus confirm the electrochemical results) that the acylgermane radical anions are closely related to the radical anions of benzaldehyde or acetophenone derivatives: First, hindered rotation about the C–CO bond in the acyl substituent leads to inequivalent ring protons at both the ortho and meta positions. Second, the same hyperfine coupling constant (HFC) pattern is visible: |a(1H para )| > |a(1H ortho )| > |a(1H meta )|. In addition, EPR data and DFT calculations suggest that the spin density is mainly confined to one of the aryl ketone moieties. Yet, some spin density can also be found at the Ge atom, which manifests itself in 73Ge (abundance 7.8%, I = 9/2) hyperfine satellites in EPR spectra. Markedly, the same substituent effects (i.e., the para substituent) connect the redox or radical anion chemistry (the “dark side”) with the UV–vis absorption behavior of the parent acylgermane photoinitiators (the “bright side”).

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