Luminescent Main-Chain Organoborane Polymers: Highly Robust, Electron-Deficient Poly(oligothiophene borane)s via Stille Coupling Polymerization

A series of polymers (PBnT, n = 2-5) with boron atoms incorporated into the conjugated polythiophene main chain have been prepared via Pd-catalyzed coupling of stannylated thienylborane monomers. The polymers exhibit excellent long-term chemical stability to air and moisture and remarkable thermal stability with decomposition temperatures reaching over 300 °C. The high stability is achieved by placing very bulky pendant groups, 2,4,6-tri-tert-butylphenyl (Mes∗) and 2,4,6-tris(trifluoromethyl)phenyl (^FMes), on boron that prevent attack by nucleophiles. All these polymers display strong absorptions in the visible region and intense fluorescence in both solution and the solid state with quantum yields of up to 38% and fast radiative decay constants (k_r) of up to 3.3 × 10⁸ s^-1. Density functional theory (DFT) studies on diborylated oligothiophene model compounds suggest that the strong absorption of the polymers results from π-π∗ transitions on the oligothiophene borane main chain with significant charge transfer to boron. The unusually intense luminescence in the solid state is favored by the rigid planar skeleton and steric shielding of the bulky pendent groups. The emission color can be tuned from blue to deep orange by varying the length of the π-conjugated oligothiophene spacer between the boron atoms. Spectroelectrochemical studies on a dimeric model compound in THF solution reveal reversible two-step reductions to give highly colored species, while the corresponding polymeric material precipitates at higher potentials after undergoing an initial reversible reduction. The LUMO energy levels of the polymers can be effectively lowered by introduction of electron-withdrawing pendent groups on boron, affording a versatile approach for development of electron-deficient boron-containing polymers with controllable electronic structures and photophysical properties. The facile modular synthetic approach combined with the exceptional stability opens the door to broad adoption of electron-deficient organoboranes in conjugated materials design and development.