Ultrafast Studies of Photochromic Spiropyrans in Solution

The photochromic reaction dynamics of the spiropyran molecule 1’,3’,3’-trimethyl-6-hydroxyspiro[2H1 -benzopyran-2,2’-indoline] (HBPS) in solution have been studied with picosecond and femtosecond transient electronic absorption spectroscopy. Following excitation near 300 nm, the C-0 bond of the molecule breaks in less than 100 fs to form a metastable species. A small fraction of this metastable species re-forms the broken C-0 bond on the time scale of 200 fs. The major fraction of the metastable photoproduct vibrationally relaxes in a few picoseconds, and then undergoes isomerization to form a merocyanine product with a decay time constant of about 100 ps, depending on solvent viscosity. This isomerization decay is faster at shorter probe wavelengths and slower at longer wavelengths, indicating that this isomerization gives rise to a red-shifted absorption spectrum. The final merocyanine isomers are stable on the nanosecond time scale. All the results indicate that the initial steps of the photochromic reaction process of HBPS are extremely fast.