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

Abstract

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”).

The Chemistry of Acylgermanes: Triacylgermenolates Represent Valuable Building Blocks for the Synthesis of a Variety of Germanium-Based Photoinitiators

“…The reduction potential of 1 (−1.57 V vs SCE in DMF) falls well in line with the reduction potentials observed for other tetraacylgermanes, for which radical anions could be detected by EPR spectroscopy . Therefore, we investigated a THF solution of 1 after exposure to metallic potassium by EPR spectroscopy and could obtain an EPR spectrum (SI, Figure S14).…”

“…Recently, we investigated the redox chemistry of acylgermanes and discovered their “dark side.” Upon reduction with metallic potassium under inert conditions, the corresponding acylgermane radical anions are formed (see Scheme 2 ), which were characterized by EPR spectroscopy in combination with DFT calculations. 59 While monoacylgermanes display similar reduction potentials as benzaldehyde derivatives, the di- and tetraacylgermanes can be reduced at less negative potentials due to the presence of more electron-withdrawing acyl groups. 59 …”

“… 59 While monoacylgermanes display similar reduction potentials as benzaldehyde derivatives, the di- and tetraacylgermanes can be reduced at less negative potentials due to the presence of more electron-withdrawing acyl groups. 59 …”

“…Upon reduction with metallic potassium under inert conditions, the corresponding acylgermane radical anions are formed (see Scheme 2), which were characterized by EPR spectroscopy in combination with DFT calculations. 59 While monoacylgermanes display similar reduction potentials as benzaldehyde derivatives, the di-and tetraacylgermanes can be reduced at less negative potentials due to the presence of more electron-withdrawing acyl groups. 59 On the basis of the accessibility of acylgermanium based radical anions and taking into account the seminal work of Murphy and co-workers, merging the chemistry of strong bases and electron transfer, we now explore whether the reduction of acylgermanes provides synthetically attractive reaction pathways.…”

Synthesis, LIFDI Mass Spectrometry and Reactivity of Triacyl‐Germenolates

“…Recently we investigated the redox chemistry of acylgermanes and discovered that upon reduction (i. e. alkali metals or KO t Bu) under inert conditions, the corresponding acylgermane radical anions are formed (Scheme 1), which were characterized by EPR spectroscopy in combination with DFT calculations [4] . While monoacylgermanes display similar reduction potentials as benzaldehyde derivatives, the di‐ and tetraacylgermanes can be reduced at less negative potentials due to the presence of more electron‐withdrawing acyl groups.…”