Luminescence Properties of C‐Diazaborolyl‐ortho‐Carboranes as Donor–Acceptor Systems |
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Authors: | Prof?Dr Lothar Weber Jan Kahlert Dr Regina Brockhinke Lena Böhling Dr Andreas Brockhinke Dr Hans‐Georg Stammler Beate Neumann Rachel A Harder Dr Mark A Fox |
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Affiliation: | 1. Fakult?t für Chemie der Universit?t Bielefeld, 33615 Bielefeld (Germany);2. Department of Chemistry, Durham University, Durham DH1 3?LE (UK) |
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Abstract: | 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 15100–20260 cm?1 and quantum yields (ΦF) 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 ΦF values between 8–32 %. 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 (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. |
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Keywords: | carboranes charge transfer diazaboroles donor– acceptor systems luminescence |
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