DFT, spectroscopic, and photoproduct study of 2-aminoethyldiphenylborinate and tetraphenyldiboroxane

The photochemistry and photophysics of 2-aminoethyldiphenylborinate (2APB) and tetraphenyldiboroxane (TPhB) were investigated by ab initio DFT, electronic paramagnetic resonance, steady-state and time-resolved spectroscopy and ESI-MS product analysis. UV–Vis spectra of the organoboranes in solvents of different dielectric constants showed bands at 200, 240 and 270 nm which were attributed to π → π* and n → π* transitions and to the aggregate band, respectively. The exact position of the π → π* bands were influenced by the dielectric character of the solvent leading to the inversion of the π → π* and n → π* transition energies. For the 2APB molecule, the fluorescence quantum yield increases with increasing polarity, whereas the effect was opposite and less intense for TPhB. Dipole moments, energy levels of the singlet and triplet-excited states, HOMO–LUMO gaps, spin density populations calculated using the DFT method, were in good agreement with the experimental results for both organoboranes. The orbital composition and spin density population reveals a relevant contribution of the boron atom to form organoborane and phenyl radicals by photolysis, as detected by EPR and confirmed by high-resolution mass spectrometry experiments.