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

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-trans-stilbene (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 frequency-resolved 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 E0 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