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”).

中文翻译：

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锗基光引发剂的“黑暗面” —连接氧化还原特性和光吸收

多项研究表明，酰基锗烷化合物（“亮面”）具有出色的光引发性能，例如可用于固化牙科填充材料。在这项工作中，我们想探索这些化合物的“黑暗面”：即，不是光诱导的反应性。伏安法测量（直流极谱法和循环伏安法）表明，酰基锗烷充当电子受体，其中当前的C═O基团代表还原中心（类似于苯甲醛和苯乙酮）。半波电位通过在酰基的数量和所述部分的影响的对位置。EPR光谱揭示（从而证实了电化学结果），酰基锗烷自由基阴离子与苯甲醛或苯乙酮衍生物的自由基阴离子密切相关：首先，在酰基取代基中绕C–CO键旋转受阻会导致两个位置的环质子不等价在邻位和间位。其次，可以看到相同的超精细耦合常数（HFC）模式：a（¹ H _para）| > | a（_正交¹ H ）| > | 一个（^1个ħ_元）|。另外，EPR数据和DFT计算表明自旋密度主要限于芳基酮部分之一。但是，在Ge原子上也可以找到一些自旋密度，这在EPR光谱中的^73个Ge（丰度7.8％，I = 9/2）超细卫星中表现出来。明显地，相同的取代基效应（即对位取代基）将氧化还原或自由基阴离子化学（“暗面”）与母体酰基锗烷光引发剂的紫外可见吸收行为（“亮面”）联系起来。