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A PREVIOUSLY UNREPORTED INTENSE ABSORPTION BAND AND THE pK, OF PROTONATED TRIPLET METHYLENE BLUE
Author(s) -
Ohno T.,
Osif T. L.,
Lichtin N. N.
Publication year - 1979
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1979.tb07178.x
Subject(s) - chemistry , protonation , absorption (acoustics) , analytical chemistry (journal) , methylene blue , absorption spectroscopy , solvent , absorption band , spectrophotometry , ethanol , nuclear chemistry , chromatography , materials science , optics , organic chemistry , composite material , catalysis , ion , physics , photocatalysis
— Excitation by a Q‐switched giant ruby laser (1.2 J output at 694 nm ˜50 ns flash) of 2–10 µ M solutions of methylene blue in water, 30% ethanol in water or 50 v/v% water‐CH 3 CN at pH values in the range 2.0–9.3 converted the dye essentially completely to its T 1 state. The absorption spectrum of T 1 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 + (T 1 ) and MBH 2+ (T 1 ). The pK a of MBH 2+ (T 1 ) in water was determined from the dependence on pH of absorption at 700 and 825 nm to be 7.1 4 ± 0.1 and from the kinetics of decay of triplet absorption to be 7.2. The specific rate of protonation of MB + (T 1 ) by H 2 PO 4 in water at pH 4.4 was found to be 4.5 ± 0.4 times 10 8 M ‐1 s ‐1 .