Effects of Multi-Carborane Substitution on the Photophysical and Electron-Accepting Properties of o-Carboranylbenzene Compounds

Multiple o-carborane substituted compounds, mono-, 1,3-bis-, and 1,3,5-tris[2-(4-butylphenyl)-o-carboran-1-yl]benzene (1-3), were prepared and characterized by multinuclear NMR spectroscopy and elemental analysis. The solid-state structures of 2 and 3 were also confirmed by single-crystal X-ray diffraction. While the mono-carborane compound 1 was nonemissive in the solution state at 298 K, the photoluminescence (PL) spectra of 2 and 3 exhibited weak-to-moderate emission ((em) = 352 nm for 2 and 363 nm for 3 in THF). Compounds 2 and 3 showed intriguing dual emission bands ((em) = 361 and 537 nm for 2 and (em) = 387 and 520 nm for 3) at 77 K, and in film, of which the low-energy band was dominant in the solid state. TD-DFT calculations on the S-1 optimized structures suggested that the low-energy fluorescence of 2 and 3 was attributed to the (4-butylphenyl) *(phenylene-o-carborane) intramolecular charge-transfer transition. The low-energy electronic transition of 2 and 3 was apparently associated with aggregation-induced emission, and an enhanced emission intensity ((em) = ca. 570 nm for 2 and (em) = ca. 550 nm for 3) was observed upon increasing the water fraction (f(w)) in THF/water mixtures. Furthermore, the PL spectroscopic experiments of poly(3-hexylthiophene-2,5-diyl) (P3HT) polymer films doped with 3 revealed the excellent electron-accepting properties of 3.