Anomalous fluorescence from the azaxanthone ketyl radical in the excited state

The time-resolved absorption and fluorescence spectra of the azaxanthone (AX) ketyl radical (AXH.) in the excited state (AXH.(Dn) (n = 1 or 2)) were observed during the nanosecond-picosecond two-color two-laser flash photolysis. AXH. showed dual fluorescence peaks at 460 and 645 nm, which were assigned to the D2 --> D0 and D1 --> D0 transitions, respectively. It was found that the lifetime of the D2 --> D0 fluorescence (1.0 ns) was longer than that of the D1 --> D0 fluorescence (0.4 ns). The fluorescent quantum yields of the D1 --> D0 and D2 --> D0 fluorescence were estimated to be 0.0008 +/- 0.0002 and 0.05 +/- 0.02, respectively. These anomalous emitting properties can be attributed to the pyridine ring in AX. AXH. is a new example of a neutral radical which violates Kasha's rule.