Laser-Control of Ultrafast π-Electron Ring Currents in Aromatic Molecules: Roles of Molecular Symmetry and Light Polarization

Abstract

Being motivated by the recent progress in attosecond laser technology, we theoretically explore the strategy of inducing ultrafast electron dynamics inherent to aromatic molecules, i.e., ring currents by means of polarized laser pulses. The main topic of discussion is how to control the direction of ring currents in an aromatic molecule of low symmetry, for which the design of an efficient control pulse cannot be achieved intuitively. We first consider a system with a single aromatic ring and show that coherent π-electron angular momentum, which oscillates with time, can be produced and controlled by a polarized laser pulse with its ellipticity and orientation properly chosen. Nonadiabatic couplings with molecular vibration gradually weaken the angular momentum, while the vibrational amplitude strongly depends on the polarization of incident light. This suggests the conversion of the polarization dependence of ring current into that of subsequent vibration, which may open a way to detect laser-driven ultrafast electron dynamics by vibrational spectroscopy. The laser-control scheme for the ring current is then extended to a molecule with two aromatic rings, which exhibits characteristic phenomena absent in that with a single ring. We demonstrate that two-dimensional switching of the direction of angular momentum is possible in such molecules. In addition, ring current can be localized at a specific ring by tailored lasers. The application of the present control method to polycyclic aromatic hydrocarbons will lead to the development of next-generation organic optical switching devices.

Dynamical Jahn-Teller effects on the generation of electronic ring currents by circularly polarized light

“…Recent experiments realized ring-currents in ring-shaped organic chromophores with strong magnetic fields and showed that the light absorption properties of the molecules were substantially modified [1]. From a theory perspective, it is well understood that circularly polarized light can also trigger ring-currents by resonantly exciting the electrons to a manifold of doubly-degenerate electronic states of (E) symmetry related to a symmetry axis of the molecule [4,[8][9][10][11][12]. It is inevitable by symmetry, though, that the (E × e) Jahn-Teller (JT) effect couples the (E) electronic states and (e) vibrational modes in all molecular point groups able to support ring currents [13][14][15][16], thus potentially mixing the two pristine ring-current circulation directions defined in the uncoupled limit [17,18].…”

Ring currents modulate optoelectronic properties of aromatic chromophores at 25 T

“…the application of a magnetic field on a macroscopic conducting wire ring gives rise to a directional current. Despite the prevalence of ring currents in chemical systems, a detailed understanding of what effect the applied magnetic field has on molecular electronic orbitals is still an active area of investigation (24)(25)(26)(27)(28). The effects of these induced currents in aromatic organic molecules are commonly seen in NMR spectroscopy, where the induced magnetic field arising from the circular current can either enhance or detract from the Zeeman splitting of 1 H nuclei, resulting in modified chemical shifts in the NMR spectrum as compared to the absence of the ring current (29,30).…”

Charge currents and induced magnetic fields in a bounded two-dimensional hydrogen atom

“…Also, in general, ring currents in 2D systems have an important application in optoelectronics, e.g. in design of novel optical switching devices [46,47]. The dependence of energy levels of the confined system of 2D hydrogen on the applied magnetic field and the radius of the bounding circle has been presented by us in an earlier study [48].…”