3-Vinylborane-functionalized oligothiophenes: isomer-dependent electronic structure and fluorescence enhancement upon anion binding

Abstract

The effect of pendant conjugated vinylborane moieties on the electronic structure of oligothiophenes and their anion binding behaviour was investigated. The vinylborane groups were introduced by hydroboration of alkynyl-functionalized 2,5-dibromothiophene with dimesitylborane (Mes₂BH). The reaction yielded a 1 : 1 mixture of the α- and β-borylated isomers, which were separated and individually isolated by reverse-phase column chromatography. Grignard metathesis polymerization (GRIM) of the dibromothiophenes yielded oligomeric materials (M_n = 1.5 to 1.7 kDa) capable of anion binding. DFT analyses on borane-functionalized terthiophene model compounds indicated that efficient extension of conjugation from the thiophene main chain to the vinylborane pendant group in the sterically less encumbered β-isomer greatly impacts on the photophysical properties. Anion titration experiments revealed the strong binding of F⁻ and CN⁻, which is accompanied by a fluorescence enhancement that is especially pronounced for the β-isomer.