Helical-photon-dressed states determining unidirectional π-electron rotations in aromatic ring molecules.

This article theoretically investigates how helical-photon-dressed states govern the unidirectional rotation of π-electrons in aromatic ring molecules under circularly polarized (CP) or elliptically polarized (EP) laser irradiation, using a three-electronic-state model within the semi-classical light–molecule interaction framework and rotating wave approximation (RWA). Analytical solutions for the wave functions and π-electron angular momenta of the three dressed states were derived, revealing that two states correspond to classical rotation (matching the laser's helicity) and one to non-classical rotation (opposite direction), with the sum of their angular momenta always zero—a principle termed the sum law. The study applies this theory to benzene (high symmetry) and toluene (low symmetry), showing that non-classical rotation is forbidden in benzene under CP lasers but allowed in toluene with EP lasers; it also extends the model to include nuclear vibrational effects in the adiabatic approximation, confirming the sum law's validity. These findings provide a fundamental basis for understanding laser-induced π-electron dynamics in aromatic molecules, with implications for ultrafast switching devices and magnetic field generation via ring currents.