Abstract
Herein we review a theoretical study of unidirectional π-electron rotation in aromatic ring molecules, which originates from two quasi-degenerate electronic excited states created coherently by a linearly polarized ultraviolet/visible laser with a properly designed photon polarization direction. Analytical expressions for coherent π-electron angular momentum, ring current and ring current-induced magnetic field are derived in the quantum chemical molecular orbital (MO) theory. The time evolution of the angular momentum and the ring current are expressed using the density matrix method under Markov approximation or by solving the time-dependent Schrödinger equation. In this review we present the results of the following quantum control scenarios after a fundamental theoretical description of coherent angular momentum, ring current and magnetic field: first, two-dimensional coherent π-electron dynamics in a non-planar (P)-2,2’-biphenol molecule; second, localization of the coherent π-electron ring current to a designated benzene ring in polycyclic aromatic hydrocarbons; third, unidirectional π-electron rotations in low-symmetry aromatic ring molecules based on the dynamic Stark shift of two relevant excited states that form a degenerate state using the non-resonant ultraviolet lasers. The magnetic fields induced by the coherent π-electron ring currents are also estimated, and the position dependence of the magnetic fluxes is demonstrated.
Highlights
In this review we present the results of the following quantum control scenarios after a fundamental theoretical description of coherent angular momentum, ring current and magnetic field: first, two-dimensional coherent π-electron dynamics in a non-planar (P)-2,2’-biphenol molecule; second, localization of the coherent π-electron ring current to a designated benzene ring in polycyclic aromatic hydrocarbons; third, unidirectional π-electron rotations in lowsymmetry aromatic ring molecules based on the dynamic Stark shift of two relevant excited states that form a degenerate state using the non-resonant ultraviolet lasers
This paper briefly provides an overview of the theoretical study of quantum laser control of coherent π-electron dynamics in low-symmetry aromatic ring molecules, which we have undertaken in recent several years [39,40,41,42,43, 68,69,70,71,72,73]
The essential principles to generate the π-electron angular momentum and ring current in a low symmetry aromatic ring molecule are first to create a superposition of two electronic excited states using two linearly polarized lasers, and second to select the clockwise or anticlockwise rotational component from the non-stationary time evolution of the coherent state using pump and dump lasers with properly designed polarization directions
Summary
Recent progress in laser science and technology has facilitated the coherent control of ultrafast charge migration dynamics in molecular systems [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21]. Quantum Control of π-Electron Dynamics polarization of the applied laser defines the rotational direction of the angular momentum [27]. In contrast in ring molecules with low-symmetry π-electrons cannot be rotated by using the circularly polarized lasers, due to the absence of electronic excited states degenerate, that would receive the photon angular momentum. The number of unidirectional rotations during the duration can be estimated from the energy difference between the quasi-degenerate excited and ground states This only applies to the ideal case in which any dephasing processes disturbing the electronic coherence are omitted. These molecules exhibit several localization patterns of coherent π-electron rotations. Each angular momentum pulse represents the unidirectional π-electrons rotation, which begins with acceleration and ends with deceleration
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