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

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 pi 3s Rydberg states. The wavelength dependence of the pi 3s Rydberg state dynamics of tetramethylethylene was investigated in more detail. The pi 3s Rydberg to pi pi* 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 (similar to 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 pi 3s state and the pi pi* state increases with increasing methylation, and (2) the pi 3s/pi pi* minimum energy conical intersection displaces monotonically away from the pi 3s Franck-Condon region with increasing methylation. The use of systematic methylation in combination with TR-PES and ab initio computation is emerging as an important tool in discerning the excited state dynamics of unsaturated hydrocarbons. [doi:10.1063/1.3652966]