A pyridine-modified phenothiazinium photosensitizer: Electrochemical and spectroscopic behaviour upon electron transfer

• A photosensitizer QMB + demonstrates excellent stability under irradiation at 575 nm • Merged CV peaks of QMB + were separated by fast voltammetry and H + donor presence • QMB + retains strong fluorescence even after exhibiting its functional activity • The reduction product emits nearly five times more than QMB + • QMB⁺ has a slightly smaller HOMO-LUMO gap, causing red-shifted absorption The novel phenothiazinium photosensitizer QMB + demonstrates unique spectroscopic properties upon reduction or reaction with an electron donor. The incorporation of a pyridine moiety into the phenothiazine framework significantly stabilizes the methylene blue structure due to electron delocalization over this region, as confirmed by density functional theory calculations. Importantly, the in-situ fluorescence – spectroelectrochemical measurements during reduction revealed a decrease in the original QMB + emission at 713 nm, accompanied by the appearance of a new emission band at 633 nm. This observation demonstrates that QMB + retains its ability to emit red light; during the catalytic cycle, only an 80 nm shift in emission maxima occurs. The emission intensity of the reduction product is almost five times higher than the original fluorescence intensity of QMB + . The reversible 1e - /1H + change is accompanied by the change of colour from blue QMB + to violet radical cation QMB +• . The subsequent one-electron reduction leads to the leuco form, which is not colourless due to the presence of chromophores of an additional pyridine ring. This behaviour was proved to be facilitated in the presence of an efficient proton donor, which affects the required reduction potential. The QMB + dye exhibits excellent photostability (at least 3000 s) upon irradiation with 575 nm light and can have higher lipophilicity due to its chemical structure in comparison with MB + . Also, both dyes were applied to polyacrylic fabrics, and their light-fastness was evaluated.