Excess energy dependence of adiabatic photoreactions in jet-cooled donor-acceptor stilbenes—A model compound study

Monte, Ch.; Hoffmann, K.; Siemoneit, Andreas; Staak, Michael; Zimmermann, P.; Rettig, Wolfgang; Lapouyade, R.

doi:10.1063/1.50160

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“…Finally, using the above picture, it is interesting to compare our model with other recent results on substituted stilbenes. A number of studies by Rettig and co-workers have focused on the dynamics of donor-acceptor substituted stilbenes and singleand double-bond bridged analogues, in solution 76,[79][80][81][82] and in supersonically cooled samples. 83 One of these molecules, 4-(dimethylamino)-4′-cyano-trans-stilbene (DCS), has also been studied under jet-cooled conditions by Daum et al 84 The donor-acceptor stilbenes have S 1 -S 0 origins which are strongly red-shifted (from 4000 to 6000 cm -1 ) and isomerization barriers lower than for TS.…”

Section: Discussionmentioning

confidence: 99%

“…73 These experimental and theoretical studies investigate the effect of electronic structure on the dynamics and should be applied also to other related systems. [81][82][83]88 The nature of the transition state elucidated here is important to studies in polar solutions and to the dynamics of barrier crossing. Further ab initio studies of the electronic states at different configurations should address the nature of the reaction coordinate.…”

Section: Discussionmentioning

confidence: 99%

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

et al. 1997

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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.

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