Ultrafast ring-opening reactions: a comparison of α-terpinene, α-phellandrene, and 7-dehydrocholesterol with 1,3-cyclohexadiene

Abstract

The excited and ground state dynamics of a series of 1,3-cyclohexadiene derivatives and their hexatriene photoproducts are studied using ultrafast broadband UV-visible transient absorption spectroscopy. The substitution pattern around the cyclohexadiene backbone alters the excited state potential energy surface in the Franck-Condon region as evidenced by changes in the excited state absorption and fluorescence properties of the systems. Unsubstituted 1,3-cyclohexadiene and alpha-phellandrene exhibit no excited state absorption while a strong excited state absorption in the visible spectral region is observed for both alpha-terpinene and Provitamin D3. Steric factors introduced by the ring substitutions determine the dominant rotational isomer at equilibrium for the hexatriene photoproducts. Coupling to the solvent during the conformational relaxation from the initial helical all-cis hexatriene is unique to each photoproduct, but the relaxation process occurs on a 6-10 ps timescale regardless of the size or substitution pattern on the triene.

Ultrafast polyene dynamics: the ring opening of 1,3-cyclohexadiene derivatives

“…These measurements agree with the older experiments in most aspects. 79 The probe continuum does not reach the 0-0 transition of the tZt-HT isomer (Fig. 4), but the spectra are consistent with earlier studies over the region covered.…”

“…6). 79 The studies by Marzec et al present the structural and thermodynamic properties of the open ring photoproducts, and use a conformation-sensitive analysis to show that the photochemistry of a-TP and a-PH consist exclusively of ringopening, with no signatures of a ''dewar'' form that can be found in other ring-opening reactions such as that of a-pyrone. 87,88…”

“…We have used broadband UV-visible transient absorption spectroscopy and integrated fluorescence spectroscopy to investigate the excited state dynamics of a-PH and a-TP. 79,86 To effectively probe a broad absorption range, we employed a white light continuum generated in CaF 2 with the 400 nm second harmonic of a 800 nm Ti:sapphire laser. This type of setup allows for simultaneous monitoring of the visible and UV spectral range with over 20 000 cm À1 of bandwidth (a spectral range of 270 to 600 nm).…”

Conformer-specific photochemistry imaged in real space and time

“…(10) The photochemical dynamics leading from the Franck-Condon region to the ring-opened photoproducts of αPH have been investigated by several ultrafast spectroscopic studies in both the gas and condensed phase, however largely without sensitivity to the structural dynamics, reactant conformers, or photoproduct isomers. (11)(12)(13)(14)(15) Here, we report on a combined megaelectronvolt ultrafast electron diffraction (UED) and ab initio multiple spawning (AIMS) (16)(17)(18) study of conformer-specific dynamics in the photochemical ringopening of gas-phase αPH with unambiguous conformersensitivity confirming but going far beyond the qualitative orbital-symmetry based WH rule predictions. In both our static diffraction measurements and quantum chemical calculations, we find the eq-αPH conformer to dominate in our gas phase sample (see supplementary note 1).…”

The Entropic Origin of Solvent Effects on the Single Bond cZt-tZt Isomerization Rate Constant of 1,3,5-cis-Hexatriene in Alkane and Alcohol Solvents: A Molecular Dynamics Study

“…70,71 More specifically, one can expect trend reversal when the range of conformational motion exceeds the size of the hydrogenbond cage in alcohol solvents, thereby leading to a higher enthalpic barrier. Such studies are underway and will be reported in a future publication.…”