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

摘要

Six new C,C′-bis(benzodiazaborolyl)dicarba-closo-dodecaboranes, 1,A-R_2-1,A-C_2B_(10)H_(10), where R represents the group 2-(1,3-Et_2-1,3,2-N_2BC _6H_4) or 2-(1,3-Ph_2-1,3,2-N_2BC _6H_4) and A is 2, 7 or 12, were synthesized from o-, m-, and p-dicarbadodecaboranes (carboranes) by lithiation and subsequent treatment with the respective 2-bromo-1,3,2-benzodiazaboroles. UV-visible and fluorescence spectra of all carboranes display low energy charge transfer emissions. While such emissions with Stokes shifts between 17,330 and 21,290 cm~(-1) are typical for C,C′-bis(aryl)-ortho-carboranes, the observed low-energy emissions with Stokes shifts between 8320 and 15,170 cm~(-1) for the meta- and para-isomers are unusual as high-energy emissions are typical for meta- and para-dicarbadodecaboranes. Fluorescence quantum yields (F) for the novel 1,7- and 1,12-bis(benzodiazaborolyl)-carboranes depend on the substituents at the nitrogen atoms of the heterocycle. Thus, the para-carborane with N-ethyl substituents 1,12-(1′,3′-Et_2-1′, 3′,2′-N_2BC_6H_4)2-1,12- C_2B_(10)H_(10) has a F value of 41% in cyclohexane solution and only of 9% in the solid state, whereas the analogous 1,12-(1′,3′-Ph_2-1′,3′,2′-N _2BC_6H_4)2-1,12-C_2B _(10)H_(10) shows quantum yields of 3% in cyclohexane solution and 72% in the solid state. X-ray crystallographic, computational and cyclic voltammetry studies for these carboranes are also presented. This journal is