UV-Photoelectron Spectroscopic Studies on 2-Arylethynyl-1,3,2-benzodiazaboroles

Gas-phase He I photoelectron spectra of a series of 2-arylethynyl-1,3,2-benzodiazaboroles BEP-X [1 (X = H), 2 (X = Me), 3 (X = OMe), 4 (X = SMe), 5 (X = NMe2), 6 (X = Br)] have been recorded and assessed by density functional theory calculations. The first ionization energies of these benzodiazaboroles are in the order 5 (7.0 eV) < 3 (7.15 eV) < 2 (7.2 eV) < 1 = 4 = 6 (7.3 eV). As an important result of this study it was disclosed that in contrast to the remaining diazaboroles the frontier orbitals of the amino derivative 5 lack contributions from the diazaborole part of the molecule. In keeping with this, the dipole moment of the ground state (5.51 D) is only slightly changed, to 6.60 D, upon electronic excitation. This also rationalizes the low Stokes shift of 2600 cm−1 in comparison to the other diazaboroles in this study (5900−7300 cm−1).