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GRK 2112:  Molecular Biradicals: Structure, Properties and Reactivity

Subject Area Physical Chemistry
Term from 2015 to 2020
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 262511252
 
Final Report Year 2022

Final Report Abstract

Biradicals are a fascinating class of molecules. Their electronic structure is characterized by two unpaired electrons in degenerate or near-degenerate molecular orbital, therefore their properties differ drastically from those of closed shell molecules. Biradicals play an important role as reactive intermediates in complex reactions, combustion, interstellar and atmospheric chemistry, but also as test cases for theories of chemical bonding and computational models. Furthermore, they are of interest for new optoelectronic materials, due to their unusual electronic properties. It was the aim of the research training group (RTG) to identify new routes to synthesize biradicals, investigate them, understand their photophysics and steer their chemical properties. Boron-based biradicals and biradicaloids are a novel and fascinating class of compounds that were characterized for the first time in the RTG. They were prepared over a wide range of sizes, from simple diborene, HBBH to large biradicals with B-B units. Depending on the ligands, the boron atoms either form a double bond or a twisted biradicalic structure. The discovery that reactive boron compounds are able to fixate nitrogen, was a particular highlight. Considerable insight was obtained from the close cooperation with theory, which is continued beyond the end of the funding period. For example, a computational analysis showed that beside the expected steric factors also electronic effects are responsible for the ligand-depending properties. Several fundamental reactive boron compounds were specroscopically characterized in the gas phase, among them diborene itself, a prototype inorganic biradical, the related iminoborane, H-B=N-H but also fundamental species like BH2 and ammonia borane, NH3BH3, which is of interest as a hydrogen storage material. A second class of novel compounds are π-conjugated organic biradicals and biradicaloids that are based on established dye chromophores. A number of such chrompohores (perylenebisimide, diketopyrrolopyrrole and isoindigo compounds) with two 4-hydroxyaryl substituents were synthesized in the group of F. Würthner. After oxydation, remarkably air stable biradicaloid pigments were formed. Depending on the chromophore and the π-conjugated substituents, it was possible to steer the biradicaloid character und thus the electronic properties of the compounds. In another class of organic molecules, molecular triads, biradicalic electronic states were generated by photoexcitation. The photoinduced dynamics was investigated by time-resolved spectroscopy. The application of magnetic field lead to a Zeeman-splitting of the charge-separated triplet state, which permitted to investigate and finally understand the spin chemistry of the compounds. A further goal of the RTG was the investigation of the structure and dynamics of excited electronic states of biradicals, as well as the mechanism of their chemical reactions in the gas phase and in solution by combining experimental and computational approaches. To gather more information, state-of-the-art spectroscopic methods like fs-time-resolved spectroscopy or photoionisation using coincidence-methods were combined with computational approaches like on the fly surface hopping dynamics or high-level quantum chemical methods. This collective approach permitted detailed insight into the chemical dynamics of biradicals. One cooperation of several groups (Brixner, Mitric, Marder and Steffen) succeeded in identifying an ortho-Benzyne derivate as an intermediate in a photochemical hexadehydro-Diels–Alder reaction. In another exemplary cooperation, the experimental groups Brixner, Dyakonov/Sperlich and Fischer elucidated jointly with the theory group of B. Engels the reactions of diphenylpropargylene (C6H5-CCC-C6H5) in ethanol and dichloromethane. A highlight in gas phase spectroscopy was the characterisation of cyclobutadiene, C4H4 by threshold photoelectron spectroscopy. C4H4 is a fundamental model for biradicality and antiaromaticity and is subject to vibronic coupling in both the neutral and cationic form. Furthermore, we expanded our spectroscopic tool box. For example, time-resolved spectroelectrochemistry was established in the RTG, to study the dynamics of ionic biradicals in solution. To summarize, almost all designated projects were successfully carried out and several new individual projects were started, advancing ideas originally developed within the RTG.

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