Picosecond Flash Photolysis of Photochromic Spiropyrans

The transient species involved in photocoloration and photobleaching of indoline 6-nitrosubstituted spiropyran solutions at room temperature were studied by picosecond flash photolysis. Transient absorption spectrum of spiropyran colorless form consists of two bands peaked at 440 and 585 nm and is assigned to the photoinduced intermediate non-planar cis configuration with the breaked C—O bond of the pyran ring. This transient is found to have the buildup time shorter than 10 ps and initial decay times of about 10 ns in benzene and 5 ns in ethanol. The colored form exhibits the transient absorption peaked at 460 nm with the lifetime of about 200 ps in ethanol which is assigned to Sn ← S1 absorption of colored form. The predominant decay channel of the colored form S1 state is shown to be internal conversion S1 ⇝ S0 with the quantum yield of about 0.70 ± 0.15.