Wavelength-dependent rearrangements of an α-dione chromophore: a chemical pearl in a bis(hypersilyl) oyster.

The symmetric bissilyl-dione 3 reveals two well-separated n → π* absorption bands at λmax = 637 nm (ε = 140 mol-1 dm3 cm-1) and 317 nm (ε = 2460 mol-1 dm3 cm-1). Whereas excitation of 3 at λ = 360/365 nm affords an isolable siloxyketene 4 in excellent yields, irradiation at λ = 590/630 nm leads to the stereo-selective and quantitative formation of the siloxyrane 5. These remarkable wavelength-dependent rearrangements are based on the electronic and steric properties provided by the hypersilyl groups. While the siloxyketene 4 is formed via a hitherto unknown 1,3-hypersilyl migration via the population of a second excited singlet state (S2, λmax = 317 nm, a rare case of anti-Kasha reactivity), the siloxyrane 5 emerges from the first excited triplet state (T1via S1λmax = 637 nm). These distinct reaction pathways can be traced back to specific energy differences between the S2, S1 and T1, an electronic consequence of the bissilyl substited α-dione (the "pearl"). The hypersilyl groups act as protective ''oyster shell", which are responsible for the clean formation of 4 and 5 basically omitting side products. We describe novel synthetic pathways to achieve hypersilyl substitution (3) and report an in-depth investigation of the photorearrangements of 3 using UV/vis, in situ IR, NMR spectroscopy and theoretical calculations.

Surface-Initiated Polymerizations Mediated by Novel Germanium-Based Photoinitiators.

Since surface-initiated photopolymerization techniques have gained increasing interest within the last decades, the coupling of photoinitiators to surfaces and particles has become an important research topic in material and surface sciences. In terms of surface modification and functionalization, covalently coupled photoinitiators and subsequent photopolymerizations are employed to provide a huge variety of surface properties, such as wettability, stimulus responsive features, antifouling behavior, protein binding, friction control, drug delivery, and many more. For this purpose, numerous type I and type II photoinitiators or other photosensitive moieties have been attached to different substrates so far. In our studies, a convenient and straightforward synthetic protocol to prepare a novel germanium-based photoinitiator (bromo-tris(2,4,6-trimethylbenzoyl)germane) in good yields was developed. The immobilization of this photoinitiator at the surface of silicon wafers and quartz plates was evidenced by X-ray photoelectron spectroscopy (XPS). Employing visible-light-triggered surface-initiated polymerization of different functional monomers, including acrylamide, perfluorodecyl acrylate, and fluorescein-o-acrylate, surfaces with various features such as hydrophilic/hydrophobic and fluorescent properties were prepared. This was also achieved in a spatially resolved manner. The polymer layers were characterized by contact angle measurements, UV-vis/fluorescence spectroscopy, spectroscopic ellipsometry, and XPS. The thicknesses of the surface grafted polymer layers ranged between 10 and 126 nm.

Synthesis and Characterization of New Counterion-Substituted Triacylgermenolates and Investigation of Selected Metal-Metal Exchange Reactions.

The synthetic process to obtain triacylgermenolates with alternated counterions by single-electron-transfer reactions or by a direct approach is described. The formation of these derivatives was confirmed by NMR spectroscopy and UV-vis spectroscopy. Moreover, metal-metal exchange reactions of potassium-substituted triacylgermenolate 2a with MgBr2, ZnCl2, and HgCl2 are presented. Additionally, 2a was reacted with nBu4NBr, which led to the formation of ammonia-substituted triacylgermenolate 8. Furthermore, we reacted 2a with HCl/Et2O to obtain triacylgermane 9. Subsequently, we investigated the reaction of 9 with tBu2Zn and tBu2Hg. NMR spectroscopy, single-crystal X-ray crystallography, and UV-vis spectroscopy are employed for analysis of structural properties.

Isolable Geminal Bisgermenolates: A New Synthon in Organometallic Chemistry.

We have synthesized the first isolable geminal bisenolates L2 K2 Ge[(CO)R]2 (R=2,4,6-trimethylphenyl (2 a,b), L=THF for (2 a) or [18]-crown-6 for (2 b)), a new synthon for the synthesis of organometallic reagents. The formation of these derivatives was confirmed by NMR spectroscopy and X-ray crystallographic analysis. The UV/Vis spectra of these anions show three distinct bands, which were assigned by DFT calculations. The efficiency of 2 a,b to serve as new building block in macromolecular chemistry is demonstrated by the reactions with two different types of electrophiles (acid chlorides and alkyl halides). In all cases the salt metathesis reaction gave rise to novel Ge-based photoinitiators in good yields.

Isolable Geminal Bisgermenolates: A New Synthon in Organometallic Chemistry.

We have synthesized the first isolable geminal bisenolates L2K2Ge[(CO)R]2 (R=2,4,6-trimethylphenyl (2 a,b), L=THF for (2 a) or [18]-crown-6 for (2 b)), a new synthon for the synthesis of organometallic reagents. The formation of these derivatives was confirmed by NMR spectroscopy and X-ray crystallographic analysis. The UV/Vis spectra of these anions show three distinct bands, which were assigned by DFT calculations. The efficiency of 2 a,b to serve as new building block in macromolecular chemistry is demonstrated by the reactions with two different types of electrophiles (acid chlorides and alkyl halides). In all cases the salt metathesis reaction gave rise to novel Ge-based photoinitiators in good yields.