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.

Carborane‐Induced Excimer Emission of Severely Twisted Bis‐o‐Carboranyl Chrysene

“…These ICT states are notable for their flexible carboranyl C−C bond, which can vibrate and relax non‐radiatively, or radiatively with a highly red‐shifted emission 24, 25, 26, 27, 28, 29, 30. Such systems often exhibit aggregation‐induced emission (AIE),13, 14, 15, 17, 18, 19, 20, 22, 31, 32 that is, in an aggregated state where molecular motion is restricted, non‐radiative relaxation pathways from the ICT state are inhibited, leading to a significant increase in emission efficiency.…”

Substituent Effects on the Fluorescence Properties of ortho‐Carbor­anes: Unusual Emission Behaviour in C‐(2′‐Pyridyl)‐ortho‐carboranes

“…[9,15,16,17] No thin-film emissions were observed at room temperatures for the parent carboranes 1-3. Solid-state emissions at ambient temperatures were reported elsewhere for 1 (powder form: 395 nm; [53] silica gel form: 356 nm [5] ), 2 (silica gel: 380 nm [5] ) and 3 (silica gel: 345 nm [5] ).…”

“…(Figure 3) is solvent-dependent with an emission maximum at 329 nm in cyclohexane and at 442 nm in dichloromethane. Based on reported photophysical data of other ortho-carboranes, [12,13,[15][16][17][18][19] the emission at 329 nm is from local transitions at one or both aryl groups, whereas the emission at 442 nm is a charge transfer involving the carborane cluster. Low-energy emissions of 17 are also observed in toluene (440 nm), chloroform (408 nm) and acetonitrile (426 nm).…”

“…Photoexcitation of such dyads induced a charge transfer from an organic scaffold to the carborane cluster, which either led to luminescence quenching or to charge-transfer (CT) emissions or both -depending on the solvents used and whether the materials were investigated as solids. [7][8][9][10][11][12][13][15][16][17][18][19][20][21][22][23][24][25][26][27][28] Dual fluorescence with CT and local transitions has been observed in some orthocarboranes. [12,13,[15][16][17][18][19][20]28] The ortho-carborane unit plays a role similar to meta-and para-carborane clusters in the photophysical process of a compound if there is a stronger electron acceptor than the cluster in these systems, [5,6,29,30] and if the ortho-carborane group is connected to a donor at one or more boron cluster atoms.…”

“…[33] Also other highly luminescent metal complexes have been reported with the pyridylcarborane unit present. [46] Given that there have been many articles on the observed luminescence of 1-monoalkyl-, 1-monoaryl-and 1,2-diaryl-ortho-carborane derivatives, [7][8][9][10][11][12][13][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] by contrast 1-monomethyl-ortho-carborane 4, 1-monophenyl-ortho-carborane 7 and 1,2-diphenyl-ortho-carborane 16 were quoted to be non-emissive. [20,30] …”