Enhanced π-Conjugation and Emission via Icosahedral Carboranes: Synthetic and Spectroscopic Investigation

Abstract

The ability of ortho-, meta- and para-carboranes to enhance the emission intensity has been compared. For this purpose a series of carborane-appended 1,3,5-triphenylbenzene (TB) and 1,3,5- tris(biphenyl-4-yl)benzene (TBB) containing three ortho-, meta- and para-carborane clusters directly attached to the conjugated cores have been synthesized employing Suzuki, Heck, and trimerization reactions. The incorporation of the icosahedral carboranes was associated with a red shift in the UV absorption spectrum of up to 13 nm as well as enhancements of the emission intensities of up to 154%. The presence of ortho-carboranes showed the maximum red shift in the UV spectrum whereas the maximum enhancement of the emission intensity was observed in the presence of meta-carborane clusters. The order of π-conjugation extension is found to be ortho > meta ≈ para. A comparative thermal analysis indicated o-carborane-appended trimers to be the most thermally stable in the series. Proton NMR spectra of reported carborane-appended trimers indicated that ortho- and meta-carborane cages have benzenelike characteristics.

C,C′-Bis(benzodiazaborolyl)dicarba-closo-dodecaboranes: Synthesis, structures, photophysics and electrochemistry

“…4,7,[9][10][11][12][13][14][15][16][17][18][19] Due to similarities in their sizes, the para-carborane motif -CB 10 H 10 C-has been compared with the para-phenylene spacer 1,4-C 6 H 4 -in donor-acceptor molecules. There, however, the carborane behaved as an effective insulator, whereas the phenylene-moiety is well known as an electron-conducting conjugated -system.…”

“…3 These carboranes have only recently been considered as potential components for OLEDs in photophysical studies on C,C-diarylated dicarbadodecaboranes. [4][5][6][7][8][9][10][11][12][13] Selected photophysical data for some C,C-diarylcarboranes are summarized in Chart 1.…”

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

“…[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] …”