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

The formation of
a stable triacylgermenolate 2 as
a decisive intermediate was achieved by using three pathways. The
first two methods involve the reaction of KOtBu or
alternatively potassium with tetraacylgermane 1 yielding 2 via one electron transfer. The mechanism involves the formation
of radical anions (shown by EPR). This reaction is highly efficient
and selective. The third method is a classical salt metathesis reaction
toward 2 in nearly quantitative yield. The formation
of 2 was confirmed by NMR spectroscopy, UV–vis
measurements, and X-ray crystallography. Germenolate 2 serves as a starting point for a wide variety of organo-germanium
compounds. We demonstrate the potential of this intermediate by introducing
new types of Ge-based photoinitiators 4b–4f. The UV–vis absorption spectra of 4b–4f show considerably increased band intensities
due to the presence of eight or more chromophores. Moreover, compounds 4d–4f show absorption tailing up to 525
nm. The performance of these photoinitiators is demonstrated by spectroscopy
(time-resolved EPR, laser flash photolysis (LFP), photobleaching (UV–vis))
and photopolymerization experiments (photo-DSC measurements).