Picosecond Flash Photolysis of Photochromic Spiropyrans

The transient species involved in photocoloration and photobleaching of indoline6-nitrosubstituted spiropyran solutions at room temperature were studied bypicosecond flash photolysis. Transient absorption spectrum of spiropyran colorlessform consists of two bands peaked at 440 and 585 nm and is assigned to the photoinducedintermediate non-planar cis configuration with the breaked C—O bond of thepyran ring. This transient is found to have the buildup time shorter than 10 ps andinitial decay times of about 10 ns in benzene and 5 ns in ethanol. The colored formexhibits the transient absorption peaked at 460 nm with the lifetime of about 200 psin ethanol which is assigned toS n← S 1 absorption of colored form. The predominantdecay channel of the colored form S 1 state is shown to be internal conversion S 1 ⇝ S 0 with the quantum yield of about 0.70 ± 0.15.