Carborane Photochemistry Triggered by Aryl Substitution: Carborane‐Based Dyads with Phenyl Carbazoles

Abstract

A bright combination: a new type of donor-acceptor dyad, carbazolylaryl-substituted ortho-carboranes, which are conveniently prepared from the corresponding acetylenes and decaborane pathways, showed unique excited-state behavior associated with electron transfer unlike the meta- and para-counterparts.

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

“…(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).…”

“…Such low-energy emissions have been reported in other diaryl-ortho-carboranes and arise from charge transfer involving both the ring and the carborane cluster. [9,10,13,15,16,18,20,[22][23][24][25][26] The solid-state emission of 1,2-diphenylortho-carborane 16 here is expected as many derivatives containing the diphenyl-ortho-carborane unit emit in the solid state. …”

“…[4][5][6][7][8][9][10][11][12][13] The meta-or para-carborane cluster merely plays roles as a spectator, a spacer or an inductively electron-withdrawing group in these molecules or polymers. There are, however, rare exceptions where the clusters are actively involved in the excited states of bis(benzodiazaborolyl)-meta-and -para-carboranes.…”

“…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] …”