Photochemistry of hybrid organic–inorganic triarylborane-o-carboranes

Abstract

Para- and meta-substituted o-carboranes with a dimesityl(phenyl)borane (dmpb) group, namely, p-1, p-2, m-1, and m-2, were prepared and their electronic natures were evaluated by steady-state photophysical methods. It was found that excited states were greatly influenced by the linking position of the dimesityl-borane group(s) at the phenyl unit of diphenyl-o-carboranes. While intramolecular charge-transfer (ICT) prevails in both para- and meta-regioisomers, para-isomers show much enhanced ICT character, as evidenced by observation of a new emission band at λmax ≈ 570 nm in dichloromethane (DCM) solution. Furthermore, the solvatochromic shifts, Δυf, from n-hexane to DCM were significantly different for para- and meta-isomers, which showed 8105, 8184, 1834, and 1895 cm−1 for p-1, p-2, m-1, and m-2, respectively. The difference in dipole moment between ground and excited states for para- (∼31.8 D) and meta-isomers (∼15.0 D) by using Mataga–Lippert plot confirmed that the ICT occurs from dimesityl-borane to o-carborane in para-isomers while the ICT occurs only from dimesityl-borane to the bridged phenyl group in meta-isomers. Excited state estimation by electrochemical and DFT studies corroborated well to the ICT character, and even with engagement of the strong electron withdrawing dimesityl-borane group o-carborane unit still acts as an electron accepter as found in the para-isomers, p-1 and p-2.