Substituent Effects on Electrocyclic Reactions: Ultrafast Ring-Opening of α-Phellandrene Stimulated by Impulsively Excited Molecular Vibrations

The ring-opening reaction of 1,3-cyclohexadiene (CHD), involving a concerted electrocyclic process with simultaneous electronic and structural rearrangement, has been widely studied for its ability to modulate molecular functionality and confer specific biological activity. In this study, we investigate how substituents influence the ring-opening dynamics of (-)-α-phellandrene (αPH), a naturally occurring CHD derivative, following two-photon photoexcitation to Rydberg states using 3.1 eV pump pulses. The dynamics are probed by time-resolved photoelectron spectroscopy (TRPES) with extreme ultraviolet light (29.5 eV) in combination with quantum chemical calculations. Unlike CHD, which undergoes ballistic ring opening, αPH exhibits a stepwise reaction pathway in the ground state. This difference arises from the steric and mass effects of the isopropyl substituent, which elevate the energy of the first transition state and create a potential barrier. Ultrafast excitation to Rydberg states impulsively triggers coherent intramolecular vibrations, resulting in periodic isomerization between equatorial (eq) and axial (ax) conformers in the ground state of αPH at intervals of 436 fs. These oscillations facilitate the gradual accumulation of the ring-opened product. Intrinsic reaction coordinate (IRC) analyses further reveal that substitution on the CHD moiety substantially alters the potential energy surface, thereby modulating the reaction mechanism. These findings highlight the crucial role of structural substituents in governing ultrafast ring-opening dynamics in conjugated cyclic molecules.