PREVIOUSLY UNREPORTED INTENSE ABSORPTION-BAND AND THE PKA 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 λmax ‐ 370nm, εmax, ‐ 13,200 M‐1 cm‐1 at pH 2 and λmax,˜420nm, εmax 9000 M‐1 cm‐1, λmax, ‐ 840 nm, εmax ‐ 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: λmax, ˜ 680 nm, emax˜ 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 pKa 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. Copyright © 1979, Wiley Blackwell. All rights reserved