The “Dark Side” of Germanium-Based PhotoinitiatorsConnecting Redox Properties and Optical Absorption

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”).