A Poly(9-Borafluorene) Homopolymer: An Electron-Deficient Polyfluorene with Turn-On Fluorescence Sensing of NH3 Vapor

A substituted poly(9-borafluorene) (P9BF) homopolymer, a boron congener of polyfluorene, is prepared by Yamamoto coupling of a triisopropylphenyl substituted borafluorene (1). As predicted by prior density functional theory (DFT) studies, P9BF has a reduced optical bandgap (E-g,E-opt = 2.28 eV) and a significantly lowered LUMO level (-3.9 eV, estimated by cyclic voltammetry (CV)) compared to polyfluorene. In addition to binding fluoride in solution, films of P9BF exhibit a reversible, simultaneous turn-off/turn-on fluorescence response to NH3 vapor. A 9-borafluorene-vinylene copolymer (P9BFV) is synthesized via Stille coupling, demonstrating that 1 can readily be incorporated into copolymers. The extended conjugation of P9BFV due to the inclusion of the vinylene group results in a reduced optical bandgap (2.12 eV) and LUMO (-4.0 eV, estimated by CV) compared to the homopolymer P9BF.