Long-Lived Charge Separation and Ionosolvatochromism of Methylene Blue

Methylene Blue (MB⁺) is a thiazine dye that has gained significant interest due to its diverse applications in health and technology. Here, two important findings regarding the formation of MB⁺-derived species are presented: photoinduced long-lived charge separation in organic solvents and the mechanism of the dye's conversion to a red form. Previous studies have relied on ultrafast techniquessuch as flash photolysis and transient adduct formationto detect MB⁺'s excited states and radical intermediates. We now report on the production of three MB⁺-derived species formed after the exchange of the counterion Cl^- to OH^- in aprotic solvents, DMSO and toluene: MB^2+•, MB^•, and methylene red (MR⁺). The long-lived free radicals generated by charge separation and stabilized by π*-π* interactions were detected by the steady-state UV-visible spectroscopy and direct continuous-wave electron paramagnetic resonance (CW-EPR) spectroscopy. MR⁺ is an ionosolvatochromic species formed by trimers and tetramers of [MB⁺]-[OH^-] in DMSO and toluene. Theoretical calculations supported the spectral assignments of radical cations and aggregates. These findings demonstrate a novel approach to stabilizing MB⁺ radical cations and aggregates, and to the ionosolvatochromism of MB⁺, thereby unlocking new avenues for its application in catalysis, synthetic chemistry, postlithium batteries, and biomedical technologies.