title: Luminescence properties of C-diazaborolyl-ortho-carboranes as donor-acceptor systems.
abstract: Seven derivatives of 1,2-dicarbadodecaborane (ortho-carborane, 1,2-C(2)B(10)H(12)) 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 15100-20260 cm(-1) and quantum yields (Φ(F)) 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, ψ) 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 15660-18090 cm(-1) for the CT bands and 1960-5540 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 Φ(F) values between 8-32%. Two distinct excited singlet-state (S(1)) geometries, denoted S(1)(LE) and S(1)(CT), were observed computationally for the benzodiazaborolyl-ortho-carboranes, the population of which depended on their orientation (ψ). 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 donor-acceptor systems with the diazaborolyl group as the donor and the ortho-carboranyl group as the acceptor.
authors.affiliation: Fakultät für Chemie der Universität Bielefeld, 33615 Bielefeld, Germany. lothar.weber@uni-bielefeld.de
authors.forename: Lothar
authors.initials: L
authors.lastname: Weber
authors.forename: Jan
authors.initials: J
authors.lastname: Kahlert
authors.forename: Regina
authors.initials: R
authors.lastname: Brockhinke
authors.forename: Lena
authors.initials: L
authors.lastname: Böhling
authors.forename: Andreas
authors.initials: A
authors.lastname: Brockhinke
authors.forename: Hans-Georg
authors.initials: HG
authors.lastname: Stammler
authors.forename: Beate
authors.initials: B
authors.lastname: Neumann
authors.forename: Rachel A
authors.initials: RA
authors.lastname: Harder
authors.forename: Mark A
authors.initials: MA
authors.lastname: Fox