In this contribution a convenient synthetic method to obtain tetraacylgermanes Ge[C(O)R] (R=mesityl (1 a), phenyl (1 b)), a previously unknown class of highly efficient Ge-based photoinitiators, is described. Tetraacylgermanes are easily accessible via a one-pot synthetic protocol in >85 % yield, as confirmed by NMR spectroscopy, mass spectrometry, and X-ray crystallography. The efficiency of 1 a,b as photoinitiators is demonstrated in photobleaching (UV/Vis), time-resolved EPR (CIDEP), and NMR/CIDNP investigations as well as by photo-DSC studies. Remarkably, the tetraacylgermanes exceed the performance of currently known long-wavelength visible-light photoinitiators for free-radical polymerization.

We present a comparative study of the photoinitiating efficiency of selected acylgermanes, focusing on wavelength-dependent photobleaching, decomposition quantum yields and radical reactivity.

Acylgermanes have been subject of great interest recently because of their low toxicity and the applicability as sources for germanium-centered radicals for visible-light induced free radical polymerization processes. We report on a novel and versatile method for the synthesis of tetraacylgermanes allowing the preparation of various tetra-substituted acylgermanes 1a−m. The formation of these derivatives was confirmed by NMR spectroscopy, mass spectrometry, and X-ray crystallography. UV−vis absorption spectra of the prepared compounds reveal absorption in the visible region. This transition was assigned by TD-DFT calculations. It enabled a general screening of the influence of different substitution patterns on the absorption properties. The radical formation upon irradiation was confirmed by TR-EPR spectroscopy.

We have developed a simple method for determining the quantum yields of photo-induced reactions. Our setup features a fibre coupled UV-Vis spectrometer, LED irradiation sources, and a calibrated spectrophotometer for precise measurements of the LED photon flux. The initial slope in time-resolved absorbance profiles provides the quantum yield. We show the feasibility of our methodology for the kinetic analysis of photochemical reactions and quantum yield determination. The typical chemical actinometers, ferrioxalate and ortho-nitrobenzaldehyde, as well as riboflavin, a spiro-compound, phosphorus- and germanium-based photoinitiators for radical polymerizations and the frequently utilized photo-switch azobenzene serve as paradigms. The excellent agreement of our results with published data demonstrates the high potential of the proposed method as a convenient alternative to the time-consuming chemical actinometry.

A modelling toolbox for classifying and predicting photoinitiator efficiencies is presented, considering absorption properties, dissociation quantum yields, light intensities, irradiation wavelengths, kinetics of monomer addition and side reactions.

A series of new bis(acyl)phosphane oxide (BAPO) photoinitiators has been synthesized and tested with respect to their efficiency in the initiation step of radical photopolymerizations. The transient absorption spectra of the phosphanoyl radicals obtained upon laser‐flash photolysis reveal maxima at ca. 450–460 nm. Rate constants for the addition of these radicals to the double bonds of butyl acrylate, methyl methacrylate, 1‐vinyl‐2‐pyrrolidone, and styrene have been determined. All phosphanoyl radicals have been found to react the most rapidly with styrene and the most slowly with butyl acrylate. Low fluorescence quantum yields of 0.1–0.3 % reveal that the studied BAPOs undergo efficient intersystem crossing, followed by α‐cleavage. The heat profiles of selected photopolymerizations have been observed using a high‐resolution infrared camera. Thermal‐imaging experiments show substantial monomer‐dependent exothermicity. All BAPO derivatives can additionally act as electron acceptors, as indicated by cyclic voltammetry and EPR spectroscopy.

Acylgermanes provide an outstanding photoinduced reactivity at very useful absorption wavelengths. This encouraged multidisciplinary research groups to utilize them as highly effective and non‐toxic photoinitiators particularly for medical applications. In this Minireview, we present the most recent breakthroughs to synthesize acylgermanes. We also outline mechanistic aspects of photoinduced reactions of several acylgermane derivatives based on fundamental spectroscopic insights. These studies may aid future developments for tailor‐made photoinitiators.

Star-shaped polymers represent highly desired materials in nanotechnology and life sciences, including biomedical applications (e.g., diagnostic imaging, tissue engineering, and targeted drug delivery). Herein, we report a straightforward synthesis of wavelength-selective multifunctional photoinitiators (PIs) that contain a bisacylphosphane oxide (BAPO) group and an α-hydroxy ketone moiety within one molecule. By using three different wavelengths, these photoactive groups can be selectively addressed and activated, thereby allowing the synthesis of ABC-type miktoarm star polymers through a simple, highly selective, and robust free-radical polymerization method. The photochemistry of these new initiators and the feasibility of this concept were investigated in unprecedented detail by using various spectroscopic techniques.