The viscosity dependence and reaction coordinate for isomerization of cis-stilbene

Abstract

Femtosecond spectroscopic methods are used to study the dynamics following optical excitation of cis-stilbene molecules in hydrocarbon solvents. Transient absorption spectra of cis-stilbene over the range 320–1100 nm are reported. The anisotropies of these transients permit the assignment of the various excited electronic states in this region to A type in C2 symmetry. The excited state absorptions disappear at rates that are weakly dependent on solvent friction and comparisons with simple statistical mechanical theories and various potentials indicate that there is likely to be a barrier crossing process responsible for the observed decay times being in the range 0.7–1.4 ps. For observation times longer than ca. 100 fs an exponential decay of the cis population is observed and the transient spectrum does not appear to change in shape. A Kramers model fits the frictional dependence of the decay rates. Direct observation of what appears to be trans isomer ground states at 335 nm is reported following excitation of cis at 312 nm. The appearence time of this species is indistinguishable from the cis-disappearence time and any intermediate in the process cis→product (trans) is found to have a lifetime of less than 150 fs. This result implies that hot trans-stilbene molecules are produced in the isomerization. Previous studies probing at 312 nm may have detected cooling of these hot molecules. By means of polarized light excitation of cis and detection of the photoproduct it is found that the reactant and product of the isomerization are aligned as if the reaction coordinate were a twisting about the double bond by 180° while maintaining the orientation of the twofold axis in the laboratory frame. This high alignment, in view of obvious possible depolarizing influences in the solution, suggests a more intricate reaction coordinate involving the motion of the ethylene carbons.

Femtosecond fluorescence up-conversion spectroscopy of a rotation-restricted azobenzene after excitation to the S1state

“…30 However, the experimental data on cis-stilbene available at the time showed mono-exponential behaviour. [30][31][32] The reported exponential decay time (E1 ps) for cis-stilbene is significantly longer than that for trans-and cis-AB. Further, recent work showed that there is a shallow well on the excited cis-stilbene PES, supporting some low-frequency vibrations.…”

“…33,34 The actual isomerization coordinate for stilbene is assumed to be a mixture of several internal coordinates showing torsional motion around the ethylenic CC bond, including out-of-plane CH wag and other oscillatory motions on the multidimensional PES, which minimize solvent friction. 30,31 The term ''hulatwist'' motion has been coined to describe the combined effect. 33,34 However, the photoisomerization quantum yield of F ¼ 0 for the trans-stilbenophane shows that under the experimental conditions used neither rotation nor hula-twist nor inversion nor another type of motion appears to be a feasible reaction coordinate for that molecule.…”

Photoisomerization of Stilbene: A Spin-Flip Density Functional Theory Approach

“…However, there are some striking differences between the cis-to-trans and trans-to-cis isomerization processes. First, the isomerization proceeds much faster in the former (∼2 ps) 7,8,11,14,16,18,24,25,27,32 than in the latter (10À200 ps). 9,10,24,25,36 The experiments illustrate that there is a relatively large barrier (∼3 kcal/mol) on the trans-to-cis isomerization path, 3,4,6,31 while the cis-to-trans reaction involves a negligible barrier or a barrierless process.…”

Detailed study of the direct numerical observation of the Kramers turnover in the LiNC⇌LiCN isomerization rate

“…3,4 In the context of chemical reaction rate theory, [5][6][7][8][9][10][11] an elusive question has centered on the existence (or lack thereof) of chemical reactions which give rise to increasing rates in the low-friction regime and thereby a true Kramers turnover. Reports observing apparent increases in the rates with friction include the isomerization of transstilbene, 12,13 isomerization of cyclohexane, 14 internal rotation of N, N-dimethyltrichloroacetamide, 15 the isomerization of CH 3 -CN CN-CH 3 , 16 and the degradation of β-carotene. 17 Unfortunately, these have not been entirely conclusive because the solvent conditions allowed for unaccounted changes in temperature, pressure, or other factors, and not just the friction.…”