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(1Hpara)|
> |a(1Hortho)| > |a(1Hmeta)|. 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”).