Solution-Processable Near-Infrared-Absorbing Dye: Thiophene-Substituted N-Phenylphenothiazine Radical Cations for Stable Thin Films

We report a π-extended N-phenylphenothiazine dye bearing thiophene substituents, designed to address the practical compromise between long-wavelength near-infrared (NIR) absorption and the isolability of a stable radical cation state. The target compound was synthesized via Suzuki–Miyaura cross-coupling and exhibited good solubility in common organic solvents. Cyclic voltammetry in dichloromethane showed a reversible one-electron oxidation at E0 = 0.19 V vs. Fc/Fc+. Chemical oxidation afforded the corresponding radical cation, which showed an intense NIR absorption maximum at 910 nm. DFT calculations support thiophene-induced narrowing of the HOMO–SOMO gap and predict a pronounced bathochromic shift of the main absorption band. The radical cation was isolated as a stable PF6− salt and readily processed into spin-coated films, which retained strong NIR absorption and remained stable for months under ambient conditions.