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

Abstract

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 KO t Bu 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).

Synthesis of Mixed‐Functionalized Tetraacylgermanes

“…The bond lengths are slightly elongated compared with the average Ge−C bond (1.97 Å) [24] and the average C=O bond (1.19 Å) [25] . The obtained crystallographic data of the mixed derivatives are similar to those of the symmetrical compounds [13] …”

“…By adding equimolar amounts of KO t Bu and subsequently 3.1 equiv of mesitoyl fluoride, the stable germenolate intermediate 5 is formed. This reaction could be accomplished via different synthetic routes, [13] but the direct method with KO t Bu and mesitoyl fluoride used for this work generally gives higher yields. After the addition of KO t Bu, the reaction solution turns yellow, which indicates consumption of KO t Bu and the formation of the potassium germanide (Scheme 3).…”

“…Furthermore, the curing depth also strongly depends on the absorption behavior of the formed polymer: the more effective the discoloration of the PI/monomer reactive mixture, the greater the curing depth [11, 12, 14–18] . Hence, we investigated the photobleaching behavior of the mixed acylgermanes and compared it with already published data [13, 14] …”

“…The absorbance traces are normalized to the initial absorptions at the observation wavelengths (maxima of n/σ–π* transitions; Ivocerin: 412.0 nm; 1 d : 378.0 nm, 4 a : 397.0 nm, 4 b : 400.0 nm, 4 c : 400.0 nm, 6 a : 380.0 nm, 6 b : 379.0 nm, 6 c : 379.5 nm, 6 d : 378.0 nm, 6 e : 373.0 nm). Data for Ivocerin and 1 d were taken from the literature [13] …”

“…[11,12,[14][15][16][17][18] Hence, we investigated the photobleaching behavior of the mixed acylgermanes and compared it with already published data. [13,14] Degassed solutionso f4a-c and 6a-e (A 385 % 0.6 to ensure comparability) in toluene/methyl methacrylate (MMA) (1/1 v/v) were irradiated with two different low-power LEDs emitting at wavelengths of 385 and 470 nm (LED 385 and LED 470;f or a detailedd escriptiono ft he LEDs, see Experimental Section and Figure S23 in the Supporting Information), which are typical for dentall amps. [19] The normalized absorbance curvesa sa functiono ft ime are depictedi nF igure 3.…”