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 σ (para) substituent constants.
The energies of the longest-wavelength absorption bands taken from
UV–vis spectra were compared with the experimentally found
differences Eox – Ered 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.