Photophysics, photochemistry and kinetics of photochromic 2H-pyrans and chromenes

Nine naturally occurring and synthetic chromenes and 2H-pyrans (oxo and sulfo) derivatives have been examined using microsecond and nanosecond laser flash techniques. Triplet transients have only been observed for the coumarin-substituted chromenes (xanthyletin and seselin). Photochromism of the coumarin-substituted chromenes arose dominantly via an excited triplet state in about 800 ns. For the other chromenes and 2H-pyrans, the colored forms arose only from an excited singlet state and subnanosecond rise times were observed. Fluorescence lifetimes have been determined for some chromenes at room temperature as well as 77 K. For xanthyletin, a fluorescence lifetime of less than 30 ps at 295 K in hexane was observed.