An In-Depth Mechanistic Investigation of the Radical Initiation Behavior of Monoacylgermanes

Two Photon Induced Pulsed Laser Polymerization with Near Infrared Light

We introduce two-photon (2P) pulsed laser polymerization (PLP) at 800 nm, demonstrating its working principle even through biological tissue. We show that 2P PLP is reliable in determining propagation rate coefficients on the example of the free radical polymerization of methyl methacrylate (MMA) at frequencies ranging from 10 to 100 Hz.

Generation of (amino)(boryloxy)carbenes from carbamoylboranes and their coupling reaction with aldehydes

Carbamoylboranes were found to react with various aldehydes under heating conditions to give α-hydroxycarboxamides in good yields. A DFT study supports the mechanism, which involves thermally generated (amino)(boryloxy)carbene intermediates. To our knowledge, this is the first report on the generation of (amino)(boryloxy)carbene intermediates from carbamoylboranes and its application to carbon-carbon bond-forming reactions.

Acyl metalloids: conformity and deviation from carbonyl reactivity

Once considered as mere curiosities, acyl metalloids are now recognized for their utility in enabling chemical synthesis. This perspective considers the reactivity displayed by acylboron, -silicon, -germanium, and tellurium species. By highlighting the role of these species in various transformations, we demonstrate how differences between the comprising elements result in varied reaction outcomes. While acylboron compounds are primarily used in polar transformations, germanium and tellurium species have found utility as radical precursors. Applications of acylsilanes are comparatively more diverse, owing to the possibility to access both radical and polar chemistry.

N-Aryl glycines as versatile initiators for various polymerizations

N-(1-Pyrenyl)glycine can act as a versatile initiator for various polymerizations.

Directed Electron Transfer in Flavin Peptides with Oligoproline-Type Helical Conformation as Models for Flavin-Functional Proteins

To mimic the charge separation in functional proteins we studied flavin-modified peptides as models. They were synthesized as oligoprolines that typically form a polyproline type-II helix, because this secondary structure supports the electron transfer properties. We placed the flavin as photoexcitable chromophore and electron acceptor at the N-terminus. Tryptophans were placed as electron donors to direct the electron transfer over 0-3 intervening prolines. Spectroscopic studies revealed competitive photophysical pathways. The reference peptide without tryptophan shows dominant non-specific ET dynamics, leading to an ion pair formation, whereas peptides with tryptophans have weak non-specific ET and intensified directed electron transfer. By different excitation wavelengths, we can conclude that the corresponding ion pair state of flavin within the peptide environment has to be energetically located between the S1 and S4 states, whereas the directed electron transfer to tryptophan occurs directly from the S1 state. These photochemical results have fundamental significance for proteins with flavin as redoxactive cofactor.

The Exploration of Aroyltrimethylgermane as Potent Synthetic Origins and Their Preparation

The synthetic utilities of acylgermanes are surprisingly rarely explored compared with their analogues. In this communication, the survey of aroyltrimethylgermane as potent synthetic origins has been studied. A variety of novel chemical transformations have been realized, including using the acylgermane group as a directing group in Rh-catalyzed aromatic C-H alkenylation reaction and Ir-catalyzed aromatic C-H amidation reactions. Additionally, a general approach for acylgermanes preparation has been established as well. The catalytic system proceeds effectively in the presence of Pd(OAc)2/BINOL-based monophosphite (L11) and allows for the straightforward access to a wide range of functionalized acylgermanes in high yields.

Evidence for ultrafast formation of tribenzoylgermyl radicals originating from tetraacylgermane photoinitiators

We report strong evidence for the ultrafast formation of tribenzoylgermyl radicals originating from tetraacylgermane photoinitiators on a ps-time scale. In comparison to previous studies, the result can be related to a high initiation efficiency.

Chasing BODIPY: Enhancement of Luminescence in Homoleptic Bis(dipyrrinato) ZnII Complexes Utilizing Symmetric and Unsymmetrical Dipyrrins

Dipyrromethene metal complexes are fascinating molecules that have applications as light-harvesting systems, luminophores, and laser dyes. Recently, it has been shown that structurally rigid bis(dipyrrinato) zinc(II) complexes exhibit high fluorescence with comparable quantum yields to those of boron dipyrromethenes or BODIPYs. Herein, eight new bis(dipyrrinato) Zn^II complexes, obtained from symmetric and unsymmetrical functionalization of the dipyrromethene structure through a Knoevenagel reaction, are reported. It was possible not only to vary the maximum visible absorption from 490 to 630 nm, but also to enhance the emission quantum yield up to 66 %, which is extraordinarily high for homoleptic bis(dipyrrinato) zinc complexes. These results pave the way for designing highly luminescent bis(dipyrrinato) zinc complexes.

High Performance Quantification of Complex High Resolution Polymer Mass Spectra

Modern soft ionization mass spectrometry provides chemical information on various polymers with unparalleled resolution and sensitivity. However, the interpretation of the resulting highly complex mass spectra is hampered by the sheer amount of contributing macromolecular species. For example, state-of-the-art reversible deactivation radical polymerization techniques, which are generally considered to be highly controlled, can still generate tens or even hundreds of species in a narrow mass window. Moreover, the multitude of species typically leads to partially overlapping isotopic patterns, further complicating the data evaluation. Herein, a rapid and powerful three-step methodical approach is introduced that enables the successful identification and quantification of the contributing species. The approach is subsequently implemented in "pyMacroMS", a high performance algorithm that allows for ultrafast processing of high resolution polymer mass spectra with varying complexities. The power of our algorithm is demonstrated on the example of a photochemical atom transfer radical polymerization (photoATRP) of three monomers, ultimately leading to 908 identified species. pyMacroMS is available free of charge under a GNU General Public License v3.0.

Ultrafast stimulated emission of nitrophenolates in organic and aqueous solutions

Early-time dynamics of nitroaromatics and its coressponding bases can give valuable insights into photo-induced reactions relevant to atmospheric and environmental processes. In this work, femtosecond broadband absorption spectroscopy between 350 and 700 nm has been applied to explore the ultrafast dynamics of o-, p- and m-nitrophenol anions (NP-) in basic organic and aqueous solution. Excitation at 400 nm promotes these compounds into the first bright electronic singlet state, which is a charge-transfer state. A surprising finding for all nitrophenolates was a characteristic, spectrally broad stimulated emission (SE) from the electronically excited state into the ground state. The corresponding lifetime was on the order of a few hundred femtoseconds for o- and p-NP- while it was roughly ten times larger for m-NP-. In line with earlier observations, the SE is governed by an out-of-plane torsional motion of the nitro group, leading to a close energetic approach of the relevant electronically excited singlet and ground states. Subsequent dynamics can be assigned to excited state absorption and ground state relaxation due to energy dissipation of the vibrational modes to the solvent that occur for up to several tens of picoseconds. No longer-lasting transient absorption (TA) was found; instead, a complete recovery of the ground state bleaching was observed indicating that triplet state relaxation is either not significantly involved in this spectral part or shifted to other regions. In the aqueous system, time constants for all processes are much smaller than in organic solution, a fact that can be explained by the larger dipole moment of the solvent and the correspondingly stronger intermolecular coupling between NP- and the aqueous solvent.

A Priori Prediction of Mass Spectrometric Product Patterns of Photoinitiated Polymerizations

We introduce a method for the a priori prediction of mass spectra of complex poly(methyl methacrylate)s initiated by photoinitiators featuring multiple cleavage points. The method is based on permutation mathematics using multinomial coefficients to predict the probability of each poly(methyl methacrylate) species' isotopic pattern contribution to the overall mass spectrum. The method assumes a statistical behavior for the cleavage of the photoinitiator. The excellent agreement of the predicted mass spectrum based on multinomial coefficients with the experimental mass spectrum confirms a multipoint cleavage mechanism of the assessed photoinitiators. We exemplify our method for the prediction of mass spectra of poly(methyl methacrylate)s initiated by four tetraacylgermane derivates and one bisacylgermane, recorded after visible light pulsed-laser polymerization by high resolution Orbitrap electrospray ionization mass spectrometry (ESI-MS). The excellent agreement of our approach with experimental data suggests that a wide array of polymer mass spectra of polymers initiated by initiators capable of multiple cleavage events can be quantitatively predicted.

Installing lactone chain termini during photoinduced polymerization

We exploit the Thorpe–Ingold effect as a spontaneous end group transformation method during photo-induced polymerization of methacrylates using the functional (2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropio-phenone) species as radical photoinitiator.