The ring-opening dynamics of the photochromic switch 1′,3′- dihydro-1′,3′,3′-trimethyl-6-nitrospiro[2H-1-benzopyran-2, 2′-(2H)-indole] in tetrachloroethene is studied with both femtosecond time-resolved ultraviolet (UV)/visible and UV/mid-infrared (IR) pump-probe spectroscopy. During the first picosecond we identify two new transient features in the UV/vis experiments, the first of which we assign to spiropyran S 1 → Sn absorption (lifetime ≤ 0.2 ps). The second feature (lifetime 0.5 ± 0.2 ps) we tentatively assign to the merocyanine T2 state. After 1 ps both probing methods show biexponential merocyanine formation kinetics, with average time constants of 17 ± 3 and 350 ± 20 ps. In the UV/IR experiments, the initial dynamics show more dispersion in formation times than in the UV/vis measurements, whereas the slower time constant is the same in both. A weak transient IR signal at ∼1360 cm-1 demonstrates that this biexponentiality is caused by a sequential isomerization between two merocyanine species. Lifetimes provide evidence that the merocyanine S1 state is not involved in the photochemical reaction.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry