New Triazaborine Chromophores: Their Synthesis via Oxazaborines and Electrochemical and DFT Study of Their Fundamental Properties

Eight new and stable triazaborine chromophores featuring various substituents as donor and acceptor moieties were prepared and investigated. An interpretation of the measured electrochemical data (CV, RDV, and dc polarography) and UV-vis spectra and quantum chemical calculations are presented. In the homologous series the first reduction proceeds as a one-electron reversible process localized at the -N=C-C=N- part of the central heterocycle being in conjugation with the attached carbonyl. The first oxidation of triazaborines proceeds as a two-electron irreversible process, most probably of the ECE type, localized at the negatively charged boron atom and surrounding unsaturated structures, including the substituted phenyl ring. For a better understanding of the relationship between the structure and redox properties, the LFER approach was applied for the first oxidation as well as reduction potential using the Hammett sigma (para) substituent constants. The energies of the longest-wavelength absorption bands taken from UV-vis spectra were compared with the experimentally found differences E-ox - E-red and with calculated HOMO-LUMO gaps, and no systematic influence of substitution was found. The calculated optimized structures and displacement of the frontier orbitals confirmed the interpretations presented above.