title: Sequential merocyanine product isomerization following femtosecond UV excitation of a spiropyran.
abstract: 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 S1 --> S(n) absorption (lifetime < or = 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 approximately 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.
authors.affiliation: Department of Physical Chemistry, Uppsala University, P.O. Box 579, S-751 23 Uppsala, Sweden.
authors.forename: Ann-Kathrin
authors.initials: AK
authors.lastname: Holm
authors.forename: Omar F
authors.initials: OF
authors.lastname: Mohammed
authors.forename: Matteo
authors.initials: M
authors.lastname: Rini
authors.forename: Emad
authors.initials: E
authors.lastname: Mukhtar
authors.forename: Erik T J
authors.initials: ET
authors.lastname: Nibbering
authors.forename: Henk
authors.initials: H
authors.lastname: Fidder