Ultrafast non-adiabatic dynamics of methyl substituted ethylenes: The π3s Rydberg state

Abstract

Excited state unimolecular reactions of some polyenes exhibit localization of their dynamics at a single ethylenic double bond. Here we present studies of the fundamental photophysical processes in the ethylene unit itself. Combined femtosecond time-resolved photoelectron spectroscopy (TR-PES) and ab initio quantum chemical calculations was applied to the study of excited state dynamics in cis-butene, trans-butene, trimethylethylene, and tetramethylethylene, following initial excitation to their respective π 3s Rydberg states. The wavelength dependence of the π 3s Rydberg state dynamics of tetramethylethylene was investigated in more detail. The π 3s Rydberg to ππ * valence state decay rate varies greatly with substituent: the 1,2-di-and tri-methyl substituted ethylenes (cis-butene, trans-butene, and trimethylethylene) show an ultrafast decay (∼20 fs), whereas the fully methylated tetramethylethylene shows a decay rate of 2 to 4 orders of magnitude slower. These observations are rationalized in terms of topographical trends in the relevant potential energy surfaces, as found from ab initio calculations: (1) the barrier between the π 3s state and the ππ* state increases with increasing methylation, and (2) the π 3s/ππ* minimum energy conical intersection displaces monotonically away from the π 3s Franck-Condon region with increasing methylation. The use of systematic methylation in combination with TRPES and ab initio computation is emerging as an important tool in discerning the excited state dynamics of unsaturated hydrocarbons.

Hexamethylcyclopentadiene: time-resolved photoelectron spectroscopy and ab initio multiple spawning simulations

“…However, hints for such a shift can also be found in published TRPES data of ethylene, although analyzed differently there. 13 In butadiene, an induction period of about 50 fs was observed in simulations. 21 In this case, it can be connected to two vibrational degrees of freedom, the dihedral angle between the two conjugated double bonds and a double bond torsion angle.…”

“…This is especially important because a profound understanding of photoinduced dynamics almost always necessitates a detailed comparison of experiment with theory. Many model systems have been investigated by time-resolved experiments [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] or dynamical simulations. [20][21][22][23][24][25][26] We believe that fundamental questions could be answered more easily by joint studies connecting experimental approaches with such simulations.…”

Excited state dynamics in SO2. I. Bound state relaxation studied by time-resolved photoelectron-photoion coincidence spectroscopy

“…49 These fits incorporated the FWHM of the experimental cross-correlation, c, and time-zero, t 0 , determined from separate Gaussian fits to the temporal profiles of H 2 O + ion signals and/or bands in the covariant photoelectron spectra. These cross-correlation functions and the minimum number of exponential decay components to nontrivially fit the experimental data provided a basis for the 2D global fits.…”

The Dynamophore – Localization of Excited State Dynamics Studied by Time–Resolved Photoelectron Spectroscopy

“…Another example highlighting the influence of the surrounding can be seen when exciting ethylene to its 3s-Rydberg-state [2,4]. While increased substitution to up to three methyl groups only leads to a retardation of the dynamics due to a slower twisting motion, addition of the fourth methyl group pushes the Rydberg state energetically below the *-state and causes the dynamics to slow down by two-three orders of magnitude.…”

“…Various types of dynamophores are recurring in our investigations, the most important being the ethylene moiety. Ethylene exhibits a characteristic set of motions to return from the excited *-state to the electronic ground state: It twists about its double bond and either pyramidalizes at one of the two carbon atoms or forms a H-bridged structure which can lead to a [1,2]-hydrogen migration [2][3][4]. Other examples of dynamophores are the allyl-group (CH=CH-CH2) which performs an [1,3]-H shift or the cyclohexa- [1,3]-diene group exhibiting a ring opening reaction [5].…”