More than Expected: Overall Initiation Efficiencies of Mono‑, Bis‑, and Tetraacylgermane Radical Initiators

We introduce a quantitative comparison of the overall initiation efficiency for a library of eight mono-, bis-, and tetraacylgermane visible light photoinitiators with p-methoxy and fluorine substitution patterns. Specifically, cocktail experiments were carried out, fusing pulsed laser polymerization (PLP) of methyl methacrylate (MMA) with subsequent high-resolution electrospray mass spectrometry (ESI-MS) analysis. The overall initiation efficiency reflects all critical events leading to macromolecular growth, i.e., from light absorption and cleavage of the initiator to radical initiation. Importantly, we show that the obtained overall initiation efficiencies are self-consistent when going from lower to higher substituted systems, evidencing the validity of the derived overall initiation efficiencies within the error margins of the experiment. Remarkably, the comparison of mono-, bis-, and tetraacylgermanes reveals a nonstoichiometric increase of 40–90% in the overall initiation efficiency with increasing number of benzoyl moieties per initiator (size dependence), e.g., a maximum value of 7.6 ± 1.8 for tetrabenzoylgermane versus benzoyltrimethylgermane. In addition, the size dependence observed for acylgermanes scales with the nπ* extinction coefficients with the number of benzoyl moieties of the respective photoinitiator. Finally, with increasing system size of the acylgermanes, a more complex channel branchingas suggested from time-dependent density functional theory (TDDFT) calculationsresults in substitution-dependent intersystem crossing (ISC) and cleavage quantum yields.