13020-78-5Relevant academic research and scientific papers
Photochemistry of Stilbene Adsorbed on Silica Gel and NaX Zeolite. A Diffuse Reflectance Laser Flash Photolysis Study
Lednev, I. K.,Mathivanan, N.,Johnston, L. J.
, p. 11444 - 11451 (2007/10/02)
Diffuse reflectance laser flash photolysis (266, 308, or 355 nm) of either cis- or trans-stilbene (St) adsorbed on silica gel or included in NaX zeolite leads to the formation of the trans-St radical cation with λmax at 475 nm at high laser powers.At low laser intensities trans-St also yields radical cation while cis-St photocyclizes to give dihydrophenanthrene with λmax at 450 nm.In contrast to the results for irradiation of stilbene alone on solid supports, irradiation of the cis-St/TNM charge transfer complex on silica or zeolite leads to a mixture of both trans- and cis-St.+ (λmax at 510 nm), demonstrating that the cis radical cation is stable with respect to isomerization on these two solids.This result, in combination with product studies which demonstrate that there is substantial cis-trans isomerization within a single laser pulse, leads to the conclusion that the formation of trans-St.+ following laser irradiation of cis-St occurs via cis-trans isomerization followed by photoionization of trans-St.Laser irradiation of St or pyrene on NaX zeolite results in strong transient signals in the 500-600 nm region due to trapped electrons, in addition to the signals due to radical cations.The effects of both water and oxygen on the trapped electron and radical cation have been examined.The trapped electron can be photobleached with a second 532 nm laser pulse.The bleaching does not lead either to trapping of the electron by ground state aromatic to give its radical anion or to recombination with the radical cation to regenerate the starting material.This suggests that irradiation leads to a redistribution of the electron to other zeolite sites.
Cis-stilbene isomerization: Temperature dependence and the role of mechanical friction
Todd, David C.,Fleming, Graham R.
, p. 269 - 279 (2007/10/02)
The fluorescence decay time of cis-stilbene has been measured in a variety of solvents over a large temperature range.An isoviscosity Arrhenius plot in n-alkanes yields an activation energy of 386+/-29 cm-1.We interpret this result as an upper limit for the cis-stilbene to trans-stilbene barrier in nonpolar solvents.Isoviscosity plots in small alcohols are nonlinear, indicating complicated behavior in this solvent type.The excited state decay times in n-alcohols and n-alkane solvents correlate well with each other when plotted as a function of chain length, n.We infer from this plot that macroscopic viscosity is a poor measure of the friction felt by the isomerizing species when changing solvent, and that the potential energy surface is not substantially altered between n-alkanes and n-alcohols with n 5.Decay times measured in 2-propanol at 490, 475, 453, and 440 nm emission differ by no more than 90 fs, indicating that there is little or no spectral evolution during the excited state lifetime.A short component in the fluorescence anisotropy decay of cis-stilbene and a decrease in the excited state lifetime due to deuterium substitution for the ethylenic hydrogens are observed in both polar and nonpolar solvents.Treating DHP formation as a thermally activated process which competes with the cis to trans isomerization, we determine a range of model dependent cis to trans activation energies in alkanes of -300 to +380 cm-1.
Femtosecond laser studies of the cis-stilbene photoisomerization reactions
Sension, Roseanne J.,Repinec, Stephen T.,Szarka, Arpad Z.,Hochstrasser, Robin M.
, p. 6291 - 6315 (2007/10/02)
Femtosecond laser studies have been performed on the photoisomerization reactions of cis-stilbene to obtain the most detailed understanding to date of a polyatomic isomerization reaction in a condensed phase environment.These experiments demonstrate that vibrationally hot product molecules are formed within a few hundred femtoseconds of the escape of the molecule from the cis* region of the potential energy surface.Although the cis to trans reaction may proceed via a twisted intermediate structure, this intermediate is not intercepted on the ca. 150 fs time scale.The frictional effects on the cis to trans reaction coordinate are found to be important and account for the anisotropy of the trans product molecules.Specific experiments presented in detail are the absorption spectrum of electronically excited cis molecules (cis*); the anisotropy decays for cis* showing motion along the reaction coordinate; the detection of the trans-stilbene product using transient fluorescence and transient absorption, confirming that the reaction generates hot product states and that the Franck-Condon modes are largely spectators in the reaction; the anisotropy (alignment) of trans product molecules illustrating the effect of friction coupling overall motion to the reaction coordinate; and a theoretical treatment of three-pulse anisotropy experiments.
