SPIRONAPHTHOPYRAN PHOTOCHROMISM - PICOSECOND TIME-RESOLVED SPECTROSCOPY

Photochromism of a spironaphthopyran (1'-(methoxyethyl)-3',3',5'-trimethyl-spiro[2H-1-naphthopyran-2,2'-indoline]) is studied with picosecond transient absorption and picosecond time-resolved resonance Raman spectroscopies. A photochromic reaction occurring within 50 ps after ultraviolet (287 nm) irradiation produces merocyanine species which absorb at 574 nm in both polar and nonpolar solvents. For as long as 1.5 ns, neither absorption nor resonance Raman spectroscopy reveals any changes in the merocyanine isomer(s) initially formed. In polar solvents, a second transient absorption (400-460 nm) appears both before the merocyanine isomer(s) is formed and whenever the merocyanine isomer(s) is excited at 574 nm. No back-reaction from the merocyanine isomer(s) to the spiro form is observed when the sample is irradiated at either 574 or 425 nm with delays of 100 ps to 1 ns after 287-nm excitation. A reaction mechanism describing spiropyran photochromism is suggested from these results.