Through-space charge transfer and emission color tuning of di-o-carborane substituted benzene

1,4-Di-(1-Ar-o-carboran-2-yl)benzene (Ar = 3,5-bis(trifluoromethyl)phenyl (1), phenyl (2), 4-n-butylphenyl (3), 4-N,N-dimethylaniline (4)) compounds that are electronically modulated at the C1-position of o-carborane with the electron-withdrawing or -donating aryl groups were prepared and characterized. The X-ray crystal structures of 1, 3, and 4 reveal that the two aryl groups on the C1-carborane carbon atoms are oppositely positioned, featuring overall C2-symmetry, and the C1-C2 bond length of carborane increases with the increasing order of electron-donating effect of an aryl group. UV-vis absorption spectra exhibit small low-energy absorption bands at around 275-300 nm for 1-3 while 4 shows a broad absorption tail at 350-400 nm. Although 1-4 show virtually no emission in solution, an intense aggregation-induced emission over the region ranging from 400-700 nm is observed in the solid state. Importantly, the emission wavelengths of 1-4 exhibit an apparent red-shift upon changing the aryl substituent from the CF3 to the NMe2 group (from 1 to 4). TD-DFT calculations suggest that the low-energy electronic transition is attributed to the intramolecular "through-space" charge transfer between the appended aryl group (HOMO) and the central phenylene ring (LUMO), and the greater change in the HOMO level by the substituent than that in the LUMO is responsible for the emission color tuning.