Anna Eibel

Tetraacylgermanes: Highly Efficient Photoinitiators for Visible‐Light‐Induced Free‐Radical Polymerization

In this contribution a convenient synthetic method to obtain tetraacylgermanes Ge[C(O)R]4 (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.

From mono- to tetraacylgermanes: extending the scope of visible light photoinitiators

We have investigated the inititiaton efficiency of carefully selected germanium-based photointiators for radical polymerization. To establish a systematic relationship between structure and reactivity, we have developed a convenient synthetic protocol for the preparation of a trisacylgermane, closing the gap from mono- to tetraacylgermane photoinitiators. The studied acylgermanes display distinct, wavelength-dependent photobleaching upon irradiation up to 470 nm. In particular, tetraacylgermanes featuring ortho-alkyl substituents reveal red-shifted n–π* bands, in line with excellent photobleaching upon visible light irradiation. Quantum yields of decomposition (determined at 385 nm) have been found to be highest for bisacylgermanes. Germyl radicals produced upon triplet-state α-cleavage of the acylgermanes react remarkably fast with monomers. Addition rate constants to (meth)acrylates range from 0.4–4.5 × 108 M−1 s−1, depending on the substitution pattern. These values are clearly higher than those reported for related phosphorus-centered radicals derived from acylphosphane oxides. We have further established the nature of the products and side-products formed at initial stages of the polymerizations using chemically induced dynamic nuclear polarization (CIDNP) experiments.

Star‐shaped Polymers via Simple Wavelength‐Selective Free Radical Photopolymerization

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.

Tetraacylstannanes as Long‐Wavelength Visible‐Light Photoinitiators with Intriguing Low Toxicity

Abstract The first tetraacylstannanes Sn[(CO)R]4 (R=2,4,6‐trimethylphenyl (1 a) and 2,6‐dimethylphenyl (1 b)), a class of highly efficient Sn‐based photoinitiators, were synthesized. The formation of these derivatives was confirmed by NMR spectroscopy, mass spectrometry, and X‐ray crystallography. The UV/Vis absorption spectra of 1 a, b reveal a significant redshift of the longest wavelength absorption compared to the corresponding germanium compounds. In contrast to the known toxicity of organotin derivatives, the AMES test and cytotoxicity studies reveal intriguing low toxicity. The excellent performance of 1 as photoinitiators is demonstrated by photobleaching (UV/Vis) and NMR/CIDNP investigations, as well as photo‐DSC studies.

Recent Advances in Germanium‐Based Photoinitiator Chemistry

Abstract 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.

Toward Matching Optically and NMR Active Volumes for Optimizing the Observation of Photo-Induced Reactions by NMR

Abstract In the recent years photo-induced reactions are becoming increasingly popular in many fields of chemistry comprising biological conversions, material/environmental science and synthesis. NMR monitoring of such reactions has been shown being advantageous and several strategies of providing an efficient irradiation of the NMR sample have been developed and reported. Here we show that adjusting the optical properties of the investigated solution to the active volume detected by the NMR experiment is valuable. This is shown with the help of three examples comprising photo-isomerization, photo-induced polymerization and CIDNP-detected bond cleavage. Adjusting the photo-active volume to the NMR-detectable portion of the sample provides a substantially more realistic kinetic information, background suppression and reduction of thermal and diffusional effects.

Probing the first steps of photoinduced free radical polymerization at water–oil interfaces

Phosphanoyl radicals, produced by α-photocleavage of exclusively octane-soluble acylphosphane oxides, rapidly react with both water- and octane-soluble monomers in water-in-octane microemulsions. The presented mechanistic insights open the possibility for the tailored construction of photo-generated copolymers comprising polar and non-polar blocks at the same time.

Choosing the ideal photoinitiator for free radical photopolymerizations: predictions based on simulations using established data

We present a tool for predicting the initiation efficiency of various type 1 (α-cleavage) photoinitiators for free radical photopolymerization. The amount of initiated polymer chains is crucial for the successful outcome of radical polymerizations. This is based on a subtle combination of intrinsic photoinitiator characteristics, i.e. quantum yields for dissociation, the absorption spectrum of the initiator in terms of its absorption bands and the corresponding absorption coefficients (e), the rate constants of the primary radicals toward the monomer, and side reactions such as oxygen quenching. Equally important are the emission properties of the utilized light source (irradiation wavelengths, light intensity). We highlight that a balanced combination of all these factors is crucial for achieving optimal initiation performance.