Luminescence Properties of C-Diazaborolyl-ortho-Carboranes as Donor-Acceptor Systems

Seven derivatives of 1,2-dicarbadodecaborane (ortho-carborane, 1,2-C2B10H12) with a 1,3-diethyl- or 1,3-diphenyl-1,3,2-benzodiazaborolyl group on one cage carbon atom were synthesized and structurally characterized. Six of these compounds showed remarkable low-energy fluorescence emissions with large Stokes shifts of 1510020260 cm-1 and quantum yields (FF) of up to 65?% in the solid state. The low-energy fluorescence emission, which was assigned to a charge-transfer (CT) transition between the cage and the heterocyclic unit, depended on the orientation (torsion angle, ?) of the diazaborolyl group with respect to the cage C?C bond. In cyclohexane, two compounds exhibited very weak dual fluorescence emissions with Stokes shifts of 1566018090 cm-1 for the CT bands and 19605540 cm-1 for the high-energy bands, which were assigned to local transitions within the benzodiazaborole units (local excitation, LE), whereas four compounds showed only CT bands with FF values between 832?%. Two distinct excited singlet-state (S1) geometries, denoted S1(LE) and S1(CT), were observed computationally for the benzodiazaborolyl-ortho-carboranes, the population of which depended on their orientation (?). TD-DFT calculations on these excited state geometries were in accord with their CT and LE emissions. These C-diazaborolyl-ortho-carboranes were viewed as donoracceptor systems with the diazaborolyl group as the donor and the ortho-carboranyl group as the acceptor.