Carborane Dyads for Photoinduced Electron Transfer: Photophysical Studies on Carbazole and Phenyl‐o‐carborane Molecular Assemblies

Abstract

o-Carborane-based donor-acceptor dyads comprising an o-carboranyl phenyl unit combined with N-carbazole (1) or 4-phenyl-N-carbazole (2) were prepared, and their dyad characters were confirmed by steady-state photochemistry and photodynamic experiments as well as electrochemical studies. The absorption and electrochemical properties of the dyads were essentially the sum of those of the carbazole and o-carboranyl phenyl units; this indicates negligible interaction between the carbazole and o-carborane units in the ground state. However, the emission spectra of 1 and 2 indicated that carbazole fluorescence was effectively quenched and a new charge-transfer (CT) emission was observed in solvents, varying from hexane to acetonitrile, which exhibited large Stoke shifts. The CT emission properties of o-carborane-based dyads were further analyzed by using Lippert-Mataga plots to show that unit charge separation occurred to form a charge-separated species in the excited state, namely, 1⋅2. This excited-state species was confirmed by nanosecond transient absorption spectra and spectroelectrochemical measurements; the photoexcitation of carbazole generated the CT state in which a radical cation and anion were formed at the carbazole and o-carborane units, respectively, within a few nanoseconds. DFT calculations corroborated the presence of this CT species and showed localized populations of the highest singly occupied molecular orbital on 2 in the reduced anionic state. As a result, molecular assemblies formed by linking the carbazole group with the o-carborane cage through a phenylene or multi-phenylene spacer revealed that the photoinduced electron-transfer process occurred intramolecularly.

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

“…(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).…”

“…[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] …”