A PREVIOUSLY UNREPORTED INTENSE ABSORPTION BAND AND THE pK, OF PROTONATED TRIPLET METHYLENE BLUE

Abstract— Excitation by a Q-switched giant ruby laser (1.2 J output at 694 nm ?50 ns flash) of 2–10 µM solutions of methylene blue in water, 30% ethanol in water or 50 v/v% water-CH₃CN at pH values in the range 2.0–9.3 converted the dye essentially completely to its T₁ state. The absorption spectrum of T₁ dye was measured in different media at pH 2.0 and 8.2 by kinetic spectrophotometry. Previously reported T-T absorption in the violet in acidic and alkaline solutions and in the near infrared in alkaline solution was confirmed. Values found for these absorptions in the present work with 30% ethanol in water as solvent are λ_max - 370nm, ε_max, - 13,200 M^-1 cm^-1 at pH 2 and λ_max,?420nm, ε_max 9000 M^-1 cm^-1, λ_max, - 840 nm, ε_max - 20,000 m ^-1 cm^-1 at pH 8.2. Long-wavelength T-T absorption in acidic solution is reported here for the first time: λ_max, ? 680 nm, e_max? 19,000 M cm^-1 in 30% ethanol in water at pH 2. Observation of a pH-independent isobestic point ? 720 nm confirms that the long-wavelength absorptions are due to different protonated states of the same species, MB⁺(T₁) and MBH²⁺(T₁). The pK_a of MBH²⁺(T₁) in water was determined from the dependence on pH of absorption at 700 and 825 nm to be 7.1₄± 0.1 and from the kinetics of decay of triplet absorption to be 7.2. The specific rate of protonation of MB⁺(T₁) by H₂PO₄ in water at pH 4.4 was found to be 4.5 ± 0.4 times 10⁸M^-1s^-1.