Femtosecond laser studies of the photoisomerization reactions of cis-stilbene

The isomerization of stilbene (1,2-diphenylethylene) has become a model reaction for the study of photoinduced molecular rearrangement and as a probe of solvent effects associated with isomerization and molecular rotation. This work constitutes one of the first in-depth studies of the cis-stilbene photochemical reactions on the femtosecond and picosecond time scales. Amplified pulses from a Colliding Pulse Modelocked laser were used to generate femtosecond pulses at various wavelengths to study the stilbene system. Several types of transient absorption and anisotropy experiments were done to study the cis-stilbene to trans-stilbene, cis-stilbene to dihydrophenanthrene, and dihydrophenanthrene to cis-stilbene reactions. Transient absorption spectra of cis-stilbene are reported over the range 250-1100nm. The excited state absorptions disappear at rates that are weakly dependent on solvent friction. The cis-stilbene lifetimes are reported in several alkane solvents, and methanol, and ranged from ca. 0.45ps in methanol to ca. 1ps in hexane and ca. 1.4ps in hexadecane. A barrier crossing process for the cis-stilbene excited state isomerization is indicated when these results are compared to several theoretical models for the excited state potential energy surface. The trans-stilbene and dihydrophenanthrene product molecules are formed rapidly and vibrationally hot, the products cool on the tens-of-picoseconds time scale. The trans-stilbene product is formed with a time constant which is indistinguishable from the cis-stilbene decay time, and dihydrophenanthrene is formed within a few hundred femtoseconds after the disappearance of the cis-stilbene reactant. Experiments on the dihydrophenanthrene ring opening indicate that the reaction is ultrafast ($\u3c$500fs) leading towards dihydrophenanthrene and cis-stilbene ground state products. Anisotropy measurements of cis-stilbene in the region of the product absorptions demonstrate an unexpectedly high alignment between the reactant cis and product trans, and an unexpectedly low degree of alignment between reactant cis and the product dihydrophenanthrene. These results indicate a significant angular displacement of the ethylene bond during isomerization which suggests that the isomerization reactions do not occur on a simple one dimensional potential energy surface