Synthesis and Electropolymerization of Seven-Membered BF 2 Chromophores: Triphenylamine-Functionalized BOPYRENPY with Electrochromic Properties

Two new seven-membered boron-difluoride chromophores, BOPYRENPY-TPA and BOPYRENPY-BTD-TPA, were synthesized as the first electroactive BOPYRENPY derivatives designed for electropolymerization. Both compounds incorporate terminal triphenylamine (TPA) donor groups, enabling film growth via oxidative coupling of TPA radical cations to form tetraphenylbenzidine (TPB). Incorporation of an electron-deficient benzothiadiazole (BTD) spacer in BOPYRENPY-BTD-TPA generates a donor–acceptor–donor (D–A–D) framework. Steady-state and time-resolved fluorescence measurements show that the BTD unit induces a blue-shifted emission (λ em = 560 nm) with higher fluorescence quantum yield and longer excited-state lifetime (Φ F = 0.4, 2.8 ns), whereas BOPYRENPY-TPA emits at 590 nm with biexponential decay. Cyclic voltammetry reveals multiple redox processes, and continuous anodic cycling on transparent indium tin oxide electrodes produces homogeneous polymeric films. Spectroelectrochemical studies show reversible formation of TPB radical cations and dications, accompanied by color changes from orange (neutral) to green and bluish-green (oxidized). The resulting films exhibit rapid switching (2–3 s), high optical contrast (Δ T ≈ 75% at 750 nm), and excellent stability. This work represents the first electrosynthesized polymer derived from a seven-membered boron fluoride (BF 2 ) ring system and establishes BOPYRENPY as a robust platform for BF 2 -based electrochromic materials.