Structural Effects on the Isomerization Dynamics of trans-Stilbenes:  IVR, Microcanonical Reaction Rates, and the Nature of the Transition State

Abstract

Picosecond time-resolved fluorescence spectroscopy has been used for studying the intramolecular dynamics of trans-stilbene (TS) derivatives. Nonradiative decay rate constants (k nr ) of jet-cooled 4-methoxy-transstilbene (MS), 4,4′-dimethoxy-trans-stilbene (DMS), 2-phenylindene (PI), 4,4′-dihydroxy-trans-stilbene (DHS), and 1-(trans-β-styryl)cyclohexene (SCH) have been obtained from time-resolved fluorescence measurements with wide-band detection from photoselected vibrational states in the S 1 manifold. Time-and frequencyresolved fluorescence have been measured to characterize intramolecular vibrational energy redistribution (IVR) in the first three of these molecules. Statistical nonadiabatic RRKM theory is applied to model the energy dependence of k nr and extract the reaction barriers. Dispersed fluorescence is measured and analyzed in order to determine the substitution effect on vibrational frequencies most significant to the RRKM calculations. While k nr is almost invariant to para substitution of a methoxy group, with similar isomerization thresholds for MS and TS (E 0 MS ∼ E 0 TS ), a dramatic slowing of the rate is observed in both disubstituted derivatives, and reaction barriers of ∼1800 and 1700 cm -1 are derived for DMS and DHS, respectively. The altered conjugation of the system in SCH produces a large blue shift of the 0 0 0 and results in a lowering of the isomerization barrier (E 0 SCH < 740 cm -1 ). The twisting around the ethylenic double bond (C e -C e ) is prevented upon intramolecular bridging in PI, but another nonradiative channel, attributed to twisting around the C e -C φ single bond, is modeled by RRKM theory with E 0 PI ∼ 1770 cm -1 . From the electronic structure effects on both E 0 and the shift of the S 1 transition, we elucidate the nature of the transition state, as zwitterionic, and we correlate substitution effects with the HOMO/LUMO (frontier orbital) description. The results are relevant to studies in polar solutions and to the dimensionality of the reaction coordinate in the dynamics of barrier crossing.

Torsional potential energy surfaces and vibrational levels in trans Stilbene

“…The frequency of n 36 , which corresponds to the flap motion contaminated by the torsion around the central CyC bond, is calculated at ca. 60 cm 21 , in agreement with the assignments of CL [20] and SZZ [18], who revised the 33 cm 21 attribution by SMI [22]. For the n 37 , which is associated with the torsional vibration around the C -C single bonds, experimental findings of an anharmonic, low frequency vibration is supported by the theoretical estimates.…”

“…Its S 0 and S 1 potential energy surfaces, with special emphasis to the potential energy curves along the torsion around the ethylenic CyC bond, have been studied by many authors and by different theoretical approaches [1 -12]. Spectroscopically, several IR, Raman and UV studies in solution, solid matrices and supersonic beams have been performed on the vibrational frequencies to gain information on the mechanism and the dynamics of the photoisomerization process [7,9,10,[13][14][15][16][17][18][19][20][21][22].…”

Anisotropy of ensembles of free polyatomic photoisomers

“…The average rotation period (rotation about the axis of the average moment of inertia by 2π radians) for a free trans-stilbene molecule at 300 K is equal to ≈2π⋅2700 femtoseconds. At the same time, as established in experiments, the cis-trans isomerization process for stilbene occurs over a period of 300 femtoseconds [20]; the photoisomerization of retinal in the visual pigment rhodopsin occurs in 200 femtoseconds [21], while internal rotation in ethylene about the carbon bond during the photoreaction occurs in 30 femtoseconds [22].…”

“…In fact, as established in the example of photoisomerization of stilbene, intramolecular vibrational redistribution for comparable excesses of vibrational energy is about two orders of magnitude faster than the process of isomerization of the molecule [20]. So we can consider photoisomers as quasi-rigid molecular tops, rotating about each other and about the corresponding reaction coordinate.…”

Quantum-classical modeling of photoisomerization of polyatomic molecules

“…The IVR in trans-stilbene is fast, compared to the time scale of the photoisomerization, and was estimated to take about 20 ps. [32][33][34][35][36][37][38][39] This makes the transition S 1 → S 2 irreversible since the molecule very fast twists to the transition state with the energy, considerably lower than the minimum of the S 1 PES. Such a situation is similar to the nonadiabatic transitions in the "inverted region" of the free-energy-gap law where the transitions are thermally activated but irreversible.…”