PICOSECOND AND 2-STEP LIF STUDIES OF THE EXCITED-STATE PROTON-TRANSFER IN 3-HYDROXYXANTHONE AND 7-HYDROXYFLAVONE METHANOL SOLUTIONS - REINVESTIGATION OF TAUTOMER AND ANION FORMATIONS

The methanol solutions of 3-hydroxyxanthone (3-HX) and 7-hydroxyflavone (7-HF) show both strong absorption (lambda 310-320 nm) and very weak bands (350-400 nm) at room temperature. The former and latter absorption bands are ascribed to normal and anion forms in equilibrium, respectively. The excitations of the normal forms (probably 1:1 and 1:2 H-bonded complexes) of both compounds exhibit a large Stokes-shifted anion and tautomer fluorescence at 430-530 nm. Picosecond fluorescence and nanosecond/picosecond two-step LIF studies demonstrate that the anion and tautomer are generated by the excited-state proton transfer (ESPT) from 1:1 and 1:2 complexes of 3-HX/CH3OH (7-HF/CH3OH), respectively. The rise times of tautomer and anion fluorescence were consistent with decay times of the respective normal forms. These picosecond fluorescence studies confirmed and developed the proposed mechanism of the ESPT and relaxation process in 3-HX reported previously, though two types of the tautomers of 7-HF proposed previously were partially corrected to the tautomer and anion generated by the ESPT in this paper.