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REVIEW article

Front. Phys., 16 July 2021 | https://doi.org/10.3389/fphy.2021.675134

Quantum Control of Coherent π-Electron Dynamics in Aromatic Ring Molecules

www.frontiersin.orgHirobumi Mineo1,2*, www.frontiersin.orgNgoc-Loan Phan3 and www.frontiersin.orgYuichi Fujimura4*
  • 1Atomic Molecular and Optical Physics Research Group, Advanced Institute of Materials Science, Ton Duc Thang University, Ho Chi Minh City, Vietnam
  • 2Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Vietnam
  • 3Department of Physics, Ho Chi Minh City University of Education, Ho Chi Minh City, Vietnam
  • 4Department of Chemistry, Graduate School of Science, Tohoku University, Sendai, Japan

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.

1 Introduction

Recent progress in laser science and technology has facilitated the coherent control of ultrafast charge migration dynamics in molecular systems [121]. Controlled charge migration can generate unidirectional currents and current-induced magnetic fields, which can be used as a guiding principle for the next-generation ultrafast optoelectronic devices [22, 23]. Laser control of π-electron rotations in aromatic ring molecules is a typical example. Pioneering studies on the generation of coherent π-electron rotations in high-symmetry ring molecules such as Mg porphyrin having degenerate excited states have been reported by the Manz group [2426]. π-Electron rotations were induced by degenerate electronic excited states, which were coherently created by circularly polarized ultraviolet (UV) lasers. The generation mechanism of the photon angular momentum involves the photon transfer from the circularly polarized UV lasers to the π-electrons, thereby the left- or right-handed circular 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. Thus, it was commonly understood that coherent π-electron rotations could not be generated in low-symmetry aromatic ring molecules. However, Kanno et al. [2831] have invalidated this long-established understanding by demonstrating that π-electrons in oriented chiral aromatic ring molecules can be rotated by the coherent excitation of a pair of quasi-degenerate π-electronic excited states using a linearly polarized UV/vis laser pulse with a properly designed photon polarization direction [28, 29]; the polarization direction of the pulse determines the initial direction of the π-electron rotation, whether right- or left-handed one. The duration of the unidirectional rotation is inversely proportional to the energy difference between the two quasi-degenerate excited states, and inverse rotation begins after the duration because the coherent state is not a nonstationary state rather than an eigenstate. Pump and dump laser pulses with their properly designed polarization directions for these lasers are applied to eliminate the reverse rotation. 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. It is expected that unidirectional ring currents produce much stronger magnetic fields than traditional static magnetic fields [32, 33].

The concept of the conventional ring current has already been established previously and plays an important role in interpreting the magnetic properties and aromaticity of conjugated molecular systems [34, 35]. The molecules investigated based on the ring current are in the ground state, and the current densities for evaluation of the ring current are calculated using the first-order electronic wave function in the time-independent magnetic field and in a permanent magnetic dipole. Such a conventional ring current should be called an incoherent current.

In this article, we present the most recent results of our theoretical studies on the quantum control of coherent π˗electron rotations in low-symmetry aromatic ring molecules. Low-symmetry aromatic ring molecules are not rare, but rather common: high-symmetry aromatic molecules often become low-symmetry ones by substitution of a functional group, or under environment conditions. In the next section, the fundamental theory for the quantitative analysis of coherent π-electrons in a low-symmetry aromatic ring molecule is introduced. Here, the time-dependent coherent angular momentum, ring current, and ring current-induced magnetic field are analytically derived in a closed form in the quantum chemical MO theory. Time-evolutions of these quantities are derived by the density matrix method [3639]. The temporal behavior of coherence is determined by the off-diagonal element of density matrix. Then, the Markov approximation is ultilized for considering the dephasing effects on the conherent angular momenta and ring currents. The magnitudes of the electronic angular momenta and ring currents are expressed as the summation of the expectation values of the corresponding one-electron operators in the aromatic rings. The bond current between the nearest neighbor carbon atoms, Ci and Cj, is defined as an electric current flowing through a half plane perpendicular to the Ci Cj bond. The coherent bond current in an aromatic ring is defined as the average of all bond currents. The application of this theory to a nonplanar chiral aromatic molecule, (P)-2,2’-biphenol, is briefly described. (P)-2,2’-biphenol has four patterns of coherent π-electron rotations along with the two phenol rings because of its nonplanar geometrical structure [39, 40]. A sequence of the four rotational patterns can be controlled through a coherent excitation of two electronic states with two requirements: the symmetry of the two electronic states, and their relative phase. Quantum switching of coherent π-electron rotations is proposed [40]. In Section 3, the key points are summarized for the application of the quantum optimal control method for controlling coherent ring currents in polycyclic aromatic hydrocarbons (PAHs). These molecules exhibit several localization patterns of coherent π-electron rotations. Therefore, how to set up the target state for a desired localization pattern is crucial [41, 42]. However, we demonstrate that the target state can easily be set up using the Lagrange multiplier method. As examples, two types of current localizations for the simplest PAH, anthracene, are adopted [42]: current localization to the designated benzene ring, and the perimeter ring current. In Section 4, a convenient scenario involving unidirectional π-electron rotations in low-symmetry aromatic ring molecules is described [43]. The basic idea behind unidirectional electron rotations is to degenerate two nondegenerate excited states by utilizing dynamic Stark shifts. A degenerate state induced by dynamic Stark shifts is called a dynamic Stark-induced degenerate electronic state (DSIDES). Two linearly polarized continuous lasers with different frequencies and phases are used to form a DSIDES, where each laser is set to selectively interact with each electronic state through non-resonant excitation. As a result, the unidirectional π-electron rotation is driven by the lasers. This scenario was applied to toluene. The resulting angular momenta can be represented by a pulse train having an angular momentum. Each angular momentum pulse represents the unidirectional π-electrons rotation, which begins with acceleration and ends with deceleration.

2 Coherent π-electron Dynamics in Low-Symmetry Aromatic Ring Molecules

In this section we present the theoretical formalism for the coherent π-electrons in a low-symmetry aromatic ring molecule to derive analytical expressions for the coherent angular momentum, ring current and the current-induced magnetic field within the framework of the quantum chemical MO treatment [39, 40].

2.1 Equations of Motion for Coherent π-Electron Rotations Induced by Ultrashort UV/Vis Lasers

The expectation values of the coherent π−electron angular momentum and the ring current operators in an aromatic molecule are generally expressed as

O^(t)=nd3r1d3rnΨ(t)O^(r)Ψ(t).(1)

Here, O^(r) is a single-electron operator for the angular momentum or current. Ψ(t) is the wave function of the π−electrons in laser field F(t) at time t, n denotes the number of electrons, and ri express the ith electron coordinates. Since the optically-allowed electronic excited states of the conjugated aromatic rings are of our interest, the electronic wave function can be expanded by the two electronic configurations, i.e., the ground configuration Φ0, and the singly excited ones Φα as

Ψ(t)=c0(t)Φ0+αcα(t)Φα,(2)

where Φ0 is defined by a single Slater determinant as Φ0(r1,,rn)=ϕ1ϕaϕbϕn with ϕnϕn(rn). Here, ϕa and ϕb are the occupied orbitals, Φα is the electronic wave function for a single electron excited configuration α:aa, i.e., the single electron transition from the occupied molecular orbital (MO) a to the unoccupied MO a, when Φα(r1,,rn)=ϕ1ϕa'ϕbϕn.

The coherent and incoherent temporal behaviors of the electrons induced by a laser field F(t) can be obtained directly by solving the coupled electronic equations of motion expressed by the density matrix elements ραβ(t) under the initial population conditions ρ00(0)=1 and ραα(0)=0 for α0 and ραβ(0)=0 for αβ, that is, there is no electronic coherence at the initial time such that

dραβ(t)dt=iγ(Vαγ(t)ργβ(t)ραγ(t)Vγβ(t))(iωαβ+γαβ)ραβ(t).(3)

Here, ραβ(t)cα(t)cβ(t) is the density matrix element, and Vαγ(t) is the coupling matrix element between states α and γ via the molecule-laser interaction V^(t)=μF(t), where μ denotes the transition dipole moment operator, γαβ(=12(γα+γβ)+γαγ(d)) are the dephasing constants in the Markov approximation [38, 39] and ωβαis the frequency difference between the two electronic states α and β. Here, γα(γβ) is the non-radiative transition rate constant of the electronic state α(β), and γαβ(d) are the pure dephasing constants induced by the elastic interaction between the heat bath and molecule of interest.

Because we are interested in the coherent behaviors of dipole-allowed quasi-degenerate π-electronic excited states in the visible or UV region of an aromatic ring molecule, Eq. 1 can be rewritten in terms of singly excited configurations {Φκ} if κ0 as

O(t)=nd3r1d3rnTr(ρ(t)O(r)),(4)

where Oαβ(r)=Φα|O^(r)|Φβ.

In Eq. 4, the coherence between singly excited state configurations is considered, and the coherence between the ground state and the excited state is neglected because this coherence is much shorter compared to that between the singly excited state configurations. In this case, the coherence time is proportional to the inverse of the energy gap between the two electronic states.

The coherent π-electron angular momentum is formulated in the quantum chemical MO theory, and the π-orbital ϕk associated with the optical transition is expanded in terms of a linear combination of atomic orbitals χi as

ϕk=ick,iχi,(k=a,a,b,b),(5)

where χi denotes the atomic orbital, and ck,i indicates the molecular orbital coefficient.

Equation 4 can be expressed by using the Eq. 5 as

O^(t)=2nα<βImρβα(t)ij(δabcaicbj+δabcaicbj)d3rχiiO^(r)χj.(6)

Here, the temporal behavior of the expectation value O^(t) can be expressed using the off-diagonal density matrix element ρβα(t). The suffixes (a, a’) ((b, b’)) corresponds to the electronic configurations α(β), respectively.

2.2 Coherent π-Electron Angular Momentum

Consider the spatially fixed aromatic ring molecule with N aromatic rings. The electron angular momentum operator is defined as O^(r)=lZ=KO^K(r) with O^K(r)=lZ,K=i(xK/yKyK/xK)n,K. Here lZ,Kis the electronic angular momentum operator of the component perpendicular to ring K. Coordinates xK and yK are defined on the aromatic ring K, and n,K is the unit vector perpendicular to the aromatic ring. The expectation value of Kth angular momentum operator is given in terms of a 2pz carbon atomic orbitals (AOs) as

lZ,K(t)d3rKΨ(t)|lZ,K|Ψ(t)=2nen,Kα<βImρβα(t)ijK(δabcaicbj+δabcaicbj)×d3rχi(xyyx)χj,(7)

where ne is the total number of electrons in the system.

Note that in Eq. 7 the dependence of the electron angular momentum on the laser intensity is reflected in the imaginary part of the off-diagonal density matrices Imρβα(t).

2.3 Coherent π-Electron Ring Current

From Eq. 6, the perpendicular component of the time-dependent electric current flowing through a surface S (SeeFigure 1) is generally defined as

J(t)=2neα<βImρβα(t)ij(δabcaicbj+δabcaicbj)d3rχiiJ^(r)χj.(8)

FIGURE 1
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FIGURE 1. The interatomic bond current Jij(t) and bridge bond current JB,pq(t) are depicted. Here, i and j indicate the positions of two atomic sites in the bond Ci − Cj, and each carbon atom Cp, or Cq belongs to a different aromatic ring P or Q, which are in neighbors.

Here J^(r)=e2mei() is the current density operator. () is the nabla operating on the atomic orbital on the right-hand side (left-hand side).

Equation 8 can be expressed as

J(t)=i<jJij(t),(9)

where

Jij(t)=2neα<βImρβα(t)(δab(caicbjcbicaj)+δa'b'(caicbjcbicaj))Jij,(10a)

with

Jij=Sd2rχin,Sχj,(10b)

where n,S is a unit vector perpendicular to a surface S, which is given as n,S=rjri|rjri|, and the surface S is set at the center of the carbon bond Ci − Cj. The surface integrations in Eqs. 8, 10b are carried out over the half-plane S (seeFigure 1). Using Slater type AOs for {χi}, Jij in Eq. 10b can be expressed in analytical form [40].

We introduce the bridge bond currentJB,pq(t) which is a specific case of an interatomic bond current, and is defined as the current bridging two aromatic rings P and Q, passing from the nearest neighbor carbon atom at site p to q. Here, each carbon atom Cp, or Cq belongs to a different aromatic ring P or Q which are in the neighbors. (seeFigure 1).

The bridge bond current is given in terms of the interatomic current, Jpq(t), as

JB,pq(t)=Jpq(t)cosθd.(11)

Here, θd is the dihedral angle between the two rings P and Q [39].

We now define an averaged ring current along ring K, JK(t), which is defined by taking the average of all bond currents as

JK(t)1nK(i<j)KnKJij(t),(12)

where nK is the number of bonds in ring K.

2.4 Application to Nonplanar (P)-2,2’-biphenol

(P)-2,2’-biphenol is one of the typical nonplanar chiral aromatic ring molecules, which has two aromatic rings connected through the C-C bridge bond. For convenience, hereafter, we denote L (R) for the left- (right-) hand side phenol ring of (P)-2,2’-biphenol. Figure 2A exhibits the geometrical structure of (P)-2,2’-biphenol with the transition dipole moments vectors between the ground and excited states. (P)-2,2’-biphenol was assumed to be fixed on a surface by a non-conjugated chemical bond or was oriented in the space by the molecular orientation techniques by laser [4446]. The laboratory-fixed Y-axis was set parallel to the single chemical bond bridging two phenol molecules, and the rotational axis of the molecule belonging to point group C2 was set along the laboratory-fixed Z-axis which was parallel to the surface normal. The ground state geometry of (P)-2,2’-biphenol was optimized using the DFT-B3LYP level theory with the 6-31G+(d,p) basis set in the GAUSSIAN09 code [47]. The dihedral angle θd between the two phenol rings, was found to be 108.9 based on the density functional theory (DFT) calculations. The calculated geometrical structures are provided in Refs. [39, 40].

FIGURE 2
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FIGURE 2. (A) Geometrical structure of (P)-2,2’-biphenol belonging to point group C2 and the transition dipole moments between the ground state (g) and three excited states a, b1 and b2. (B) Three electronic excited states and the dipole allowed transitions used to create the coherent electronic states. A and B are the irreducible representations of point group C2. Dotted lines indicate the laser band widths that cover the superpositions of two electronic excited states (a b1), (a b2) and (b1 b2) respectively. (C) Linearly polarized unit vector of laser e+ (e) used to generate the in-phase superposition (a+b1) (out-of-phase superposition (a-b1)).

To create the coherent angular momenta and ring currents in (P)-2,2’-biphenol, we focused on the three dipole-allowed electronic excited states (a, b1, and b2) as shown in Figure 2B. The transition energies from the ground (g) to the a, b1, and b2 states, which were calculated under the optimized ground state geometry using the TD-DFT B3LYP level of theory [39, 40] were 6.67, 6.78, and 6.84 eV, respectively.

2.4.1 Generation of Two-state Electronic Coherence Using Linearly Polarized UV Pulses

For a creation of the coherent angular momentum and ring current using linearly polarized laser pulses, it is essential to prepare for an electronic coherence with a fixed relative phase between two electronic states, a and b1, i.e., in-phase (a + b1) or out-of-phase (a - b1). The principle behind the preparation of the electronic coherence (a + b1) ((ab1)) using a linearly polarized laser with polarization unit vector e+ (e-) is schematically demonstrated in Figure 2C [39, 40].

In the frame work of the three-excited state model, there are three types of two-electronic coherent states represented as b1±b2, a±b1 and a±b2, respectively. At the initial time each electronic coherence can be generated by applying a linearly polarized UV laser with a properly selected laser polarized direction. Because of the non-planar geometry of the molecule, the angular momentum is two-dimensional, and the two ring currents flow are on the two different planes, the direction of total angular momentum is dependent on the symmetry of the coherent state created by the laser: The Z-directional angular momentum (ring current) is generated for the created coherent state with the A-irreducible representation of the C2 point group, while the X-directional angular momentum (ring current) is generated for the coherent state with B.

It is remarkable that even though the third excited state is located between the two excited states in the three-excited states system, the coherent electronic state can still be created if the applied linearly polarized lasers satisfy the following conditions. For example, for the (a ± b2) electronic coherence, the conditions for the linearly polarized lasers with polarization vectors e± are given as

μgae±=±μgb2e±andμgb1e±=0,(13)

or equivalently,

e±=μgb1×(μgb2μga)/|μgb1×(μgb2μga)|.(14)

Thus, if the laser overlaps the electronic states a, b1, and b2, the (a + b2) or (ab2) electronic coherent state can be created selectively.

2.4.2 Coherent Angular Momentum Quantum Beats and Bond Currents

Figure 3 exhibits the temporal behaviors of the angular momenta calculated for three types of electronic coherences, (b1+b2), (a + b1) and (a + b2), each of which is created by a linearly polarized UV laser pulse with a properly selected polarization direction of laser. For the (b1+b2) electronic coherence, the total angular momentum parallel to the Z-axis, lZ, is created together with two ring components lL = lR, whereas for both the (a + b1) and (a + b2) electronic coherences, the total angular momenta parallel to the X-axis, lX, are generated with lL = lR, with a π-phase shift. Similarly, for out-of-phase electronic coherences, (b1b2), (ab1) and (ab2), the angular momentum can be given by the corresponding in-phase electronic coherence with a π-phase shift. The simple sinusoidal temporal behavior originates from the coherence of two electronic states with the oscillation period corresponding to a frequency difference between the two-electronic states. This is referred to as the angular momentum quantum beats, which are similar to the fluorescence quantum beats which originate from the vibronic coherence [48]. We note that π-electrons rotate a few times in a unidirectional manner within a half cycle of the oscillation. This unidirectional π-electron rotation can produce a unidirectional ring current and corresponding current-induced magnetic flux. In principle, this enables the design of ultrafast switching devices which consist of organic aromatic ring molecules.

FIGURE 3
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FIGURE 3. Temporal behaviors of the coherent angular momenta, total values lZ (lX), and components, lL (lR) of L (R) ring for three types of the electronic coherence and the directions of the bond currents at the initial time: upper panel for (b1 + b2) electronic coherence excited by laser pulse with amplitude of 0.19 TW/cm2; middle panel for (a + b1) electronic coherence excited by laser pulse with amplitude of 0.83 TW/cm2; lower panel for (a + b2) electronic coherence excited by the laser pulse with amplitude of 3.32 TW/cm2. The dephasing constants were set as γb1b2 = γab1= γab2 = 0.01 eV (∼1/50 fs −1). The arrows above the C – C bonds indicate the initial directions of the currents. Note that the bridge bond current J1,7 = 0 for the (b1 +b2) electronic coherence, while J1,70 for the (a +b1) and (a +b2) electronic coherences. Reprinted with permission from Ref. [39] Copyright (2013) American Institute of Physics.

The angular momenta lZ(t) and lX(t), shown in Figure 3, are obtained by the summation of the angular momenta created in the L and R aromatic rings lL(t) and lR(t) using the following relationship lZ(t)=2lL(t)eZsinθd2 with lL(t)eZ=lR(t)eZ, and lX(t)=2lL(t)eXcosθd2 (where lL(t)eX=lR(t)eX). Table 1 lists the angular momenta of (P)-2,2’-biphenol with the dihedral angle θd=108.9where the electronic coherence is maximum at time t=t, i.e., when the magnitude of the imaginary part of the density matrix element is maximized (Imραβ(t)=12) and dephasing effects are neglected. This results in lX(t)>0 or lX(t)<0 for the (a + b1) and (a + b2) electronic coherences, respectively, and in lZ(t)<0 for the (b1 + b2) electronic coherence.

TABLE 1
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TABLE 1. Angular momenta of the two phenol rings lLlL(t)nL and lRlR(t)nR, and the resulting angular momenta lX and lZ at the maximum coherence time t = t∗1).

The magnitudes of the bond current, Jij calculated at the maximum coherence time, are presented in detail in [40]. The magnitudes of the averaged ring current over the C-C bonds at the maximum coherence time, J¯ are on the order of tens of μA, i.e., J¯=161, 86.5 and 63.4 μA for (b1 + b2), (a + b1), and (a + b2) coherences, respectively.

Effects of dephasings on coherent π-electron angular momentum and ring currents were treated in the Markov approximation, and time-independent dephasing constants were used under the assumption of instantaneous interactions between the system and phonon baths. In a system such as condensed phases, the Markov approximation is broken down. Non-Markov response of coherent should be essential. Time evolution of coherent ring currents were calculated in a hierarchical master equation approach beyond the Markov approximation has been treated [49].

2.4.3 Ultrafast Quantum Switching of Angular Momentum

Consider the quantum control of π-electron rotations for two-dimensional angular momentum switching based on the results shown in Table 1. Here, two-dimensional quantum switching is defined as a sequential pulse of the electronic angular momentum with its constant sign (positive or negative) along the Z- or X-axis. Note that any quantum switching step should be completed before the reverse rotation of the π-electrons begins, because it may otherwise disturb the signal. Consider a sequential four-step control, which is expressed as lZ()lX(+)lZ(+)lX(). This indicates the switching of rotational patterns in the order CC → AC → AA → CA, where the symbol C (A) means clockwise (anti-clockwise) direction, and for example, CA rotation means clockwise and anti-clockwise rotations along the phenol rings L and R, respectively. Here, lX(+) (lZ()) means the π-electron angular momentum along the X- (Z-) axis with a positive (negative) sign, i.e., anti-clockwise (clockwise) rotation of π-electrons around the corresponding axis.

Figure 4A presents a 3D plot of the angular momentum as it switches based on the sequential four-step control scheme. From Figure 4A, we can see that the π-electron rotations are successfully manipulated by the pulses depicted in Figure 4B. That is, both the rotational axis parallel to the Z- or X-axis and the rotational directions around those axes, clockwise or anti-clockwise, are manipulated by the sequential four-step process. In Figure 4B, each switching step of control was performed using pump and dump pulses with specific polarization directions and phases. The laser pulse with an amplitude of 1.2 GV/m (= 1.9 × 1011 W/cm2) was used in the second and fourth steps. The dynamic Stark shifts between electronic states a, b1 and b2 were on the order of 0.01 eV [39], indicating that the Stark effects could be omitted in Figure 4A.

FIGURE 4
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FIGURE 4. (A) Sequential four-step quantum switching of π-electron rotations in (P)-2,2’-biphenol. (B) The sequential overlapped pump and dump laser pulses. Reprinted with permission from Ref. [40] Copyright (2012) American Chemical Society.

The pulses shown in Figure 4B have two features. The first feature is that the pump (dump) pulse for each step has polarizations, eαβ()(eαβ(+)) or eαβ(+) (eαβ()). Each pulse has an energy width that is large enough to coherently excite two quasi-degenerate electronic excited states, as shown in Figure 2. The second feature is that the pump and dump laser pulses partially overlap. In the creation of the CC rotation, for example, the electric field of the pump pulse was Eb1b2(+)(t)=eb1b2(+)Eb1b20sin2(πt/Tb1b2)sin(ωc,b1b2t), while the electric field of the dump pulse was Eb1b2()(t)=eb1b2()Eb1b20sin2(π(ttb1b2pd)/Tb1b2)sin(ωc,b1b2t+π/2). Here, Tb1b2 (= 60.9 fs) indicates the oscillation period between b1 and b2 states, Eb1b20 is the amplitude of the laser pulse, ωc,b1b2 is the central frequency between the two excited states b1 and b2; and tb1b2pd (the time interval between the pump and dump laser pulses) was set to Tb1b2/2 (Supporting information in Ref. [39]). The angle between the two polarization vectors, eb1b2(+) and eb1b2(), was 113.5°.

With respect to ultrafast quantum switching, it is crucial to create the overlap of the pump and dump pulses, where the resulting electric field rotates as an elliptically polarized electric field in the overlapped time domain, and the dump laser pulse reverse the rotation that occurs during this region. As a result, the angular momentum of the π-electrons is nullified.

2.4.4 Coherent Ring Current-Induced Magnetic Field

There have been interesting reports on the evaluation of the magnetic fields of atoms and oriented heteronuclear diatomic molecules, AlCl and BeO [32, 50, 51]. Strong and unidirectional magnetic fields are generated from the degenerate electronic states of these atoms and molecules excited by circularly polarized intense laser pulses. We estimated the magnetic fields (magnetic field flux density) generated by the coherent ring currents of (P)-2,2’-biphenol. The results may provide fundamental information for designing ultrafast switching devices controlled by current-induced magnetic fields as well as coherent ring currents [5254].

As an example, consider the magnetic field induced by the ring current for the (b1 + b2) electronic coherence. In Figure 5, BK(t,h) represents the current-induced magnetic field along the central axis perpendicular to ring K (L or R) as a function of the height h above the Z-axis at t=t under the maximum coherence condition (when Imρb1b2(t)=1/2). An expression for the magnetic field BK(t,h) was derived by taking into account the interatomic bond currents with the 2pz Slater AOs [40]. Note that BL(t,h)=BR(t,h) for the (b1 + b2) electronic coherence (Table 1). It is interesting to compare the behaviors of BK(t,h) with those calculated using

BKSRL(t,h)=μ0JK(t)2rsin3η,(15)

which was derived using a simple ring loop (SRL) model. Here, μ0=4π107[Wb/(Am)] is the magnetic constant, r (= rij) is the ring radius, and η (=sin1(r/r2+h2)) is defined as the angle between the Z-axis and a straight line depicted from the point on ring K that crosses the Z-axis. BK(t,0) at η=π/2 is the magnetic field measured at the center of ring K. The magnitude of BK(t,0) induced by JK(t) = 100 μA is 448 mT at the center of the ring K with r = 0.14 nm. It can be found from Figure 5 that the magnitudes of BKSRL(t,h) are overestimated near the aromatic ring plane 0h<1 Å, while the magnitudes are reasonable for h>1 Å, although slightly different results can be observed between the two magnetic fields for large values of h. At the center of the aromatic ring, BK(t,h=0)=0.66BKSRL(t,h=0). This can be understood from the result that the π-electron current density is densely distributed over the aromatic ring. A subtle difference between BKSRL(t,h)and BK(t,h) is observed for large h, although both magnetic fields BKSRL(t,h)=0 and BK(t,h)=0 approach zero. This deviation originates from the approximation that the aromatic ring is not a considered to be a perfect ring.

FIGURE 5
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FIGURE 5. Induced magnetic fields for the (b1 +b2) electronic coherence as a function of height h measured from the center of aromatic ring K at the maximum coherence time t=t. BKSRL(t,h) and BK(t,h) represent the induced magnetic field calculated by a simple ring loop model, and the field calculated by the expression that explicitly takes into account the coherent ring currents, respectively. The inset panel defines the coordinate system for height h and angle η. Rc denotes the center of the phenol ring.

It is necessary to check whether the value of BK(t,0) is larger than the one corresponding to the magnetic field BLaser induced by the applied laser field F when the induced magnetic fields are created during an ultrasfast laser pulse excitation at the early time regime. The magnitude of magnetic field BLaser can be estimated from a simple formula |BLaser | = |F|/c with c = 3.0 × 108 ms−1. The calculated magnitude of magnetic field BLaser with |F| = 1.0 GV/m is approximately 7.4BK(t,h), which is on the same order as the field induced by the ring current J = 100 μA. This implies that we need a careful examination to observe the current-induced magnetic flux, or to use the electro-magnetic device as a switching control tool.

Thus far, we have taken into account the ring current-induced magnetic fields of the low-symmetry aromatic ring molecule (P)-2,2’-biphenol, in which nondegenerate two electronic excited states are coherently excited by the linearly polarized UV lasers. Here we briefly discuss the ring current-induced magnetic field for the degenerate electronic excited states of an aromatic molecule induced by the intense circularly polarized UV laser pulse. Yuan and Bandrauk [2, 33] have numerically demonstrated that the circularly polarized ultrashort pulses are generated from the molecular high-order harmonic generation using the intense linearly and circularly polarized laser pulses. This indicates a possibility for the ultrashort circularly polarized UV laser pulses to create the ring current-induced magnetic fields in high symmetric aromatic ring molecules. We can estimate the magnitude of the ring current-induced magnetic field of benzene within the SRL model using Eq. 15. Here, J(t) is the electric ring current of the degenerated electronic state. The electronic spectrum of benzene is characterized by the dipole-allowed transition from the ground state to the third singlet electronic excited state (1E1u). For an equal population between the two states at t=t, BSRL(t,0)=874 mT is obtained using the maximum value of the coherent ring current J(t)=195 μA evaluated under the π-electron approximation [40, 55]. For comparison, we obtained BKSRL(t,0)=874 mT and BK(t,0)=579 mTwhen J(t)=195 μAfor (P)-2,2’-biphenol. Note that the same magnitudes of the induced magnetic field BKSRL(t,0) for (P)-2,2’-biphenol and BSRL(t,0)=874 mT for benzene were obtained in the SRL model because the two parameters in the SRL model, the radius of the ring r, and the ring current J(t) have the same values for both types of ring molecules. The same tendency in the magnitudes between BKSRL(t,0) and BK(t,0) is also observed for benzene.

3 Current Localization in Polycyclic Aromatic Hydrocarbons

In this section we consider a localization of coherent ring current in polyatomic aromatic hydrocarbons (PAH) [41, 42]. There exist various current localization patterns in PAH simply because of their geometrical structures consisting of many benzene units. Quantum optimal control method is a general and reliable one to choose a desired current localization pattern from the various patterns using control lasers. The quantum control method has successfully been applied to manipulation of molecules such as coherent control of chemical reactions [56, 57]. After a brief introduction of quantum optimal control method [41, 42, 58], we demonstrate how the target state is set up for the designated ring current using the Lagrange multiplier method. Here, the π-electron ring current is expressed in terms of the interatomic bond currents between two adjoining C–C atoms. At final, the target states are derived for the two types of current localization: the localized ring current, which indicates that the ring current is localized to the designated aromatic ring in linear PAH, and the perimeter ring current in linear PAH [41, 42].

3.1 Quantum Optimal Control Approach

The objective functional to be maximized, is defined as [5961].

J[F]=Ψ(T)|O^T|Ψ(T)α00Tdt(F(t))22Im[0Tdtξ(t)|it(H0μF(t))|Ψ(t)],(16)

where H0 is a Hamiltonian in absence of field, and Ψ(t) is the time-dependent wave function in the electric field F(t). Here, the target operator O^T is expressed as O^T=|ΨT><ΨT|, where ΨT is the target state wave function at the final time T, defined as ΨT=aca(T)Φa, and is equal to Ψ(T) under the optimal condition, O^T=|aca(T)Φabcb(T)Φb|. The penalty factor α0 is introduced to suppress the magnitude of the electric field F(t). ξ(t)is the time-dependent Lagrange multiplier. The third term in Eq. 16 is added to decouple the boundary conditions of the equations for Ψ(t) and ξ(t) as indicated in Eq. 17. Taking the variational condition δJ[F]=0, the following coupled equations are obtained,

itΨ(t)=(H0μF(t))Ψ(t),(17a)
itξ(t)=(H0μF(t))ξ(t),(17b)

where

F(t)=1α0Imξ(t)|μ|Ψ(t).(17c)

Here, Ψ(t) satisfies the initial condition Ψ(0)=Φ0, and ξ(T) satisfies the condition, ξ(T)=O^TΨ(T) at final time T. Note that by solving Eq. 17, we obtain the optimal solution Ψ(T), which is equal to ΨT.

3.2 Setup of Target Operators

Consider the ring current localization to a designated ring χ in a PAH, as shown in Figure 6. From Eqs. 10a, 12, the ring current on ring κ at the target time T is expressed as

Jκ(T)1nκα=1nβ=1n(i<j)κnκJij,αβ(T)=α=1nβ=1nJκ,αβIm(cα(T)cβ(T))=Jκ(c1,c2,cn),(1κm)(18a)

with cici(T), and

Jκ,αβ=2neeme(i<j)κnκ{(δab(caicbjcbicaj)+δab(caicbjcbicaj))Jij}.(18b)

FIGURE 6
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FIGURE 6. π-Electron ring currents Jl and bridge bond currents JBl (1lm, 1lm1) in a linear planar polycyclic aromatic hydrocarbon (PAH). Jχ refers to the π-electron ring current localized on aromatic ring χ.

Here, n is the number of electronic excited states. Hereafter, we write cα(T) as cα for simplicity.

The target state ΨT=αcαΦα can be determined by applying the Lagrange multiplier method to the ring currents at the target time T in Eq. 18a, which provides the coupled equations in terms of the configuration interaction coefficients with {Recα,Imcα}. The target operator is given as O^T=|ΨT><ΨT|.

The coupled equations for the ring current localization to aromatic ring χ can be expressed as

β=1nJχ,βαImcβ+κχmβ=1nλκJκ,βαImcβ+λχRecα=0,(1αn)(19a)
β=1nJχ,βαRecβκχmβ=1nλκJκ,βαRecβ+λχImcα=0,(1αn)(19b)
α=1n((Recα)2+(Imcα)2)1=0,(19c)

and

β=1nα=1nJκ,αβ(ImcαRecβRecαImcβ)=0 .(19d)

A brief derivation of Eq. 19 is summarized in Appendix A.

The coefficients {Recα,Imcα}with λl included are numerically determined by applying the Newton-Raphson method to the coupled equations in Eq. 19. The target operator for the localization is obtained as O^T=|αcαΦαβcβΦβ|.

Now consider the bridge bond currents JBl shown in Figure 6, and the perimeter ring current of a PAH with m aromatic rings, JP1ml=1mJl, which is defined as the average of the π-electron ring currents at each aromatic ring site. For the perimeter ring current, the bridge bond currents JBl'(1lm1) flowing among the nearest neighbor aromatic rings, should be zero at the target time. The target operators for the bridge bond currents, JBl'( 1lm1), and perimeter ring current, JP, can be derived from Eq. 19 by replacing Jκ,βα with JBl',βα and JP,βα, respectively, because JBl' and JP can be written in terms of {Recα,Imcα} as

JP=β=1nα=1nJP,αβ(ImcαRecβRecαImcβ),(20a)

and

JBl=β=1nα=1nJBl,αβ(ImcαRecβRecαImcβ),(20b)

respectively.

Similar to Eq. 19, the coupled equations for the perimeter ring current are derived in Appendix B.

3.3 Application of Current Localization Control to Anthracene

We applied the quantum optimal control (QOC) method described in the preceding section to anthracene, one of the smallest PAHs [42]. Anthracene (D2h) was assumed to be fixed on a surface (the XY plane), or oriented spatially by lasers The molecular geometry was optimized in the MP2/6-311+g(d,p) level theory using the GAUSSIAN09 code [47]. As demonstrated in Figure 7A, anthracene consists of three aromatic rings, which are called L-, M-, and R- rings respectively.

FIGURE 7
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FIGURE 7. (A) Symmetry-adapted ring currents (B1g) in anthracene, perimeter ring current, and middle ring current. (B) The electronic excited states adapted for laser control of π-electron ring in anthracene (D2h), and the non-zero transition dipole moments between the ground/excited-excited states. The solid (dashed) arrows represent the transition dipole moments which are parallel to the X (Y) axis.

Consider the two types of π-electron localized ring currents of anthracene: the perimeter current flowing along the outside chemical bonds of anthracene, and the middle ring current localized to the M-ring. Both types of localized ring currents belong to the irreducible representation B1g of the D2h point group, which are symmetry-adapted. The excited states contributing to the two ring currents need to belong to the B3u and B2u representations, because each corresponding electronic coherence created by the two excited states belongs to the B1g representation. The excited state for the localized ring current is S3 with B3u representation, and the other two excited states S5 and S6 with B2u representation as shown in Figure 7B. This defines a “symmetry-adapted ring current” [41, 42].

The excited state energies in Figure 7B were calculated to be E3=5.10 eV, E5=5.71 eV, and E6=5.78 eV. Here, S4 (B3u) with E4=5.13 eV was excluded because the oscillator strength was negligibly small (f = 0.0013) in our numerical simulation. The non-zero transition dipole moments relevant to the coherent π-electron ring current control were calculated using the time-dependent density functional theory (TDDFT) method as, μS0,S3=(3.98,0,0) , μS0,S5=(0,0.64,0), and μS0,S6=(0,0.44,0) (in a.u.) for the ground and excited states. The vapor absorption spectrum of anthracene shows the strongest absorption peak at 5.30 eV [62], which corresponds to E3=5.10 eV in the TDDFT calculation.

Figure 8 shows the QOC results for the generation of an anti-clockwise perimeter ring current in anthracene. The target state is expressed from the results obtained using the Lagrange multiplier method as

ΨAAA=0.717iΦS3+0.581ΦS5+0.385ΦS6.(21)

FIGURE 8
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FIGURE 8. Quantum optimal control simulations for generation of the perimeter ring current in anthracene: (A) temporal behavior of the three ring currents JL(t)=JR(t) (solid line), JM(t) (broken line), and those of the two bridge bond currents JB1(t)=JB2(t) (dotted line); (B) temporal behavior of the population in S0, and those in the three excited states: S3(B2u), S5(B2u), and S6(B2u); (C) optimized X- polarized laser pulse field FX(t) (left-hand side) and Y-polarized FY(t) (right-hand side); (D) Fourier transformed spectra of the two laser fields FX(ω) and FY(ω); (E) five components of the perimeter ring current at an arbitrary time. Reprinted with permission from Ref. [40] Copyright (2012) American Chemical Society.

Here, the subscripts specifying the target state, for example, ACA for ΨACA, indicates the anti-clockwise ring currents in two aromatic rings, L and R, and a clockwise ring current in aromatic ring M. It should be noted that the target state for a generation of any coherent ring current is expressed in terms of its complex form. The control target time was set to T = 60 fs. The matrix elements of π-electron ring currents JΧ,αβ (X = L, M, R) and the bond current JBi,αβ (i = 1, 2), for two excited states α, β are presented in Table 2. Here the π-electron ring current flowing in an anticlockwise direction is defined as positive, whereas the bond current flowing toward the Y direction is defined as positive (SeeFigure 6). Figure 8A shows the temporal evolutions of the coherent ring currents Jχ for the three aromatic rings of anthracene, χ=L, M, R, and those for B1 and B2. It can be observed from Figure 8A that the two bond currents JB1,αβ and JB2,αβ vanish at the target control time of T = 60 fs, and all ring currents in three aromatic rings exhibit positive. This indicates that the coherent π-electrons rotate clockwise along the outside (perimeter) of the aromatic ring. The laser-controlled π-electrons includes a ring current averaged over the three aromatic rings, Jp(JL+JM+JR)/3=89.0 μA, which is the perimeter ring current.

TABLE 2
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TABLE 2. Matrix elements of the coherent π-electron ring currents for the three aromatic rings of anthracene, Jx,αβ(X = L, M, R), and those of the coherent bond currents shared by the two adjacent aromatic rings, JBi,αβ (i = 1, 2) (seeFigure 8E).

Figure 8B shows the temporal behavior of the ground state S0, and three excited states S3 (B3u), S5 (B2u), and S6 (B2u) populations during the QOC process, which are induced by the two control lasers. Figure 8C shows the X and Y-components of the electric field F(t) generated by the control lasers, and Figure 8D shows the Fourier transformed spectra FX(ω) and FY(ω) in the ultraviolet (UV) frequency domain. By analyzing the temporal behavior of the electric field F(t) of the control lasers in Figure 8C and the Fourier transformed spectra of the control laser fields in Figure 8D, the mechanisms of the laser-controlled ring currents in anthracene can be clarified. It is evident that from the modulation in Figure 8C the two linearly (Y-) polarized lasers with a relative phase zero induce the electronic coherence between the two excited states S5 (B2u) and S6 (B2u), and that the linearly polarized laser pulse parallel to the X-axis creates the perimeter ring current on the molecular plane.

Thus far, we have considered the QOC procedure for generating an anti-clockwise perimeter current. The QOC procedure for generating a clockwise perimeter current can be carried out in the same manner as described above (SeeEq. 21) by considering the target state,

ΨCCC=ΨAAA=0.717iΦS3+0.581ΦS5+0.385ΦS6.(22)

We carry out the QOC procedure to generate the anticlockwise ring current localized to the middle ring of anthracene. The target state is expressed as

Ψ0A0=0.707ΦS30.440iΦS5+0.553iΦS6.(23)

The target state for the middle ring current with the clockwise flow is given by the relationship Ψ0C0=Ψ0A0. Figure 9 exhibits the QOC results, where Figure 9A displays the temporal evolutions of ring current localization control to the three aromatic rings, indicating that JM is a ring current with 64.4 μA at the target control time of 60 fs, but on the other hand the ring currents JL and JR of the other two aromatic rings vanish. This indicates that the ring current localized to the middle aromatic ring is created by the control laser pulses presented in Figures 9C,D. The temporal behaviors of the populations shown in Figure 9B are nearly the same as those of the perimeter ring currents as shown in Figure 8B. As presented in Figure 9C, the two linearly (Y-) polarized lasers induce the electronic coherence between the two excited states S5 and S6 with a definite relative phase π, in contrast to the results presented in Figure 8C. In the mechanism of generation between the perimeter and the middle ring currents there is no difference except the phase difference between the two excited states S5 and S6, because the temporal behavior in both Figure 8B and Figure 9B and the temporal behavior in the X-polarized laser pulse in both Figure 8C and Figure 9C are similar to each other.

FIGURE 9
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FIGURE 9. Quantum optimal control simulations for generation of the middle ring current in anthracene: (A) temporal behavior of the ring currents JL(t)=JR(t) (solid line) and JM(t) (broken line); (B) temporal behavior of the S0 population and those of the three excited state S3(B2u), S5(B2u) and S6(B2u) populations; (C) optimized X- and Y-polarized laser pulse fields FX(t) (left-hand side) and FY(t) (right-hand side); (D) Fourier transformed spectra of the laser pulse fields FX(ω) and FY(ω). Reprinted with permission from Ref. [42] Copyright (2017) American Institute of Physics.

Main difference between the temporal behaviors of the Y-polarized lasers in Figure 8C and Figure 9C can be explained briefly in the following. The two coherent states created by the Y-polarized lasers can be expressed as

F+(t)=FS5sin(ωS5t)+FS6sin(ωS6t),(24a)

and

F(t)=FS5sin(ωS5t)FS6sin(ωS6t).(24b)

For simplicity, the amplitudes of the two coherent states are assumed to have the same value, i.e., FS5=FS6FY. The above expressions can then be simplified to

F+(t)=FYsin[(ωS5+ωS6)t2]cos[(ωS5ωS6)t2],(25a)

and

F(t)=FYcos[(ωS5+ωS6)t2]sin[(ωS5ωS6)t2].(25b)

Here, the frequency difference |ωS5ωS6|/2 yields an oscillation beating period of 120 fs, which is nearly the same as that of the quantum beat frequency observed in Figure 8C and Figure 9C.

Having clarified the control mechanism of the coherent ring current generation, we can semi-quantitatively reproduce the above results in Figures 8, 9 by using an analytical method [42]. This indicates, in principle, that two types of ring current localizations, the perimeter ring current and the middle ring current in linear PAHs, can be generated in experiment using three coherent UV lasers without a sophisticated QOC device.

For QOC numerical simulations of the ring current localization control in anthracene, we have only considered the perimeter and the middle ring currents generations, which are symmetry-adapted, while we did not consider a ring current localization to the L- or R-ring of anthracene, which are created by a symmetry-broken procedure, as demonstrated for naphthalene in Ref. [41]. That is, other excited state(s) with gerade symmetry must be considered in addition to the excited states with ungerade symmetry as described in the previous subsection.

It is important to consider how to observe the ultrafast coherent ring currents in PAHs. We proposed a method to detect the direction of the atto-second coherent ring current by tracking the femtosecond molecular vibrational motions that can induce the nonadiabatic couplings [30]. Rodriguez and Mukamel [63] have proposed measuring the circular dichroism (CD) of the ring current using the pump-probe method. Recently, two methods have been proposed for the detection of the ultrafast coherent ring currents. One method, recently proposed by Yuan et al., Bandrauk’s group [64], is utilizing the atto-second detection method of the coherent electronic dynamics in molecules with the temporal and spatial resolutions using the circularly polarized ultrashort UV pump and X-ray probe laser pulses. The other method is to utilize the time-resolved scanning microscopy (STM) and the magnetic force microscopy (MFM) to detect the ring current-induced magnetic fields during the ultrashort time [6567].

It is also expected that to apply our methods to planar or non-planar extended molecular systems, such as graphene sheets. In principle, our method can be extended to these large systems, by considering the symmetry of the molecular system and the constraints on the π-electron ring currents.

Using laser control, it is also essential to maintain the created ring current at the target region (path) at least during one vibrational period of the PAH [68]. Otherwise, the created ring current would dissipate quickly because of the vibronic interactions and/or the nonadiabatic couplings between the two electronic excited states [30, 6971].

4 Dynamic Stark-Induced π-Electron Rotations in Low-Symmetry Aromatic Ring Molecules

In this section, we present a convenient method for inducing unidirectional π-electron rotations in aromatic ring molecules with low symmetry [43]. The basic idea behind the induction of unidirectional electron rotations is to degenerate two nondegenerate excited states by utilizing dynamic Stark shifts, as demonstrated in Figure 10. We refer to this as the dynamic Stark-induced degenerate electronic state (DSIDES) [43]. Two linearly polarized continuous lasers operating at different frequencies and phases are used to form DSIDES: Each laser is set to selectively interact with each electronic state through non-resonant excitation. The lower and higher excited states are shifted up and down with the same population, respectively, and the DSIDES is formed at the center between them. As a result, unidirectional π-electron rotation is driven by two lasers. In the laser control scenario, only one input parameter out of the four possible parameters (frequency and intensity for each laser), is required to induce the DSIDES formation.

FIGURE 10
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FIGURE 10. Dynamic Stark-induced degenerate state with the Rabi frequencies (Ω1 and Ω2) using two stationary linearly-polarized lasers with the frequencies, ωa and ωb, and detuning frequencies Δ1 andΔ2 respectively.

First, the DSIDES formation is described in a three-electronic state model under the fixed nuclei condition, and the time-dependent expectation value of the angular momentum operator of π-electrons is derived and analytically expressed. Next, to demonstrate the applicability of the control scenario, the results of the DSIDES treatment for toluene, which is a typical aromatic ring molecule of low symmetry belonging to Cs point group, are presented.

4.1 Formation of Dynamic Stark-Induced Degenerate Electronic State DSIDES and the Resulting Angular Momentum

Consider the coherent π-electron angular momentum in an aromatic ring molecule with low symmetry excited by non-resonant, stationary UV/visible lasers. The molecule of our interest is one oriented in a space or attached to a surface, as mentioned in the preceding sections. We adopt a three-electronic state model in the frozen nuclei approximation. The three electronic states including the ground state specified by the energy ε0(ω0), and two excited states specified by ε1(ω1) and ε2(ω2), as shown in Figure 10. Here, DSIDES can be formed by two stationary linearly polarized lasers with detuning frequencies Δ1=ωaω10<0 and Δ2=ωbω20>0. The frequency difference between the two excited states is expressed as ω21=ω2ω1. The dynamic Stark shifts are denoted by the Rabi frequencies Ω1 and Ω2, as presented in Figure 10. The wave function of the total system in the stationary lasers is defined through laser-molecular interactions as

Φ(t)=c0(t)exp(iω0t)ϕ0+c1(t)exp(iω1t)ϕ1+c2(t)exp(iω2t)ϕ2.(27)

Here, the normalization condition for the coefficients c0(t), c1(t), and c2(t) are as follows,

|c0(t)|2+|c1(t)|2+|c2(t)|2=1.(28)

The system Hamiltonian interacting with the electric fields is given as

H(t)=H0+V(t),(29)

where H0 satisfies H0ϕi=εiϕi for i =0, 1, and 2. In Eq. 29, the interaction Hamiltonian V(t) between the system and the two electric fields is written as

V(t)=μFacos(ωatζa)μFbcos(ωbtζb).(30)

Here μ=er is the electronic dipole moment operator, where r means the electron coordinate. Fα=Fαeα (α=a,b) is the electric field with amplitude Fα and photo-polarization vector eα, ζα is the initial phase, and ωα is the central frequency. In Eq. 30, the two electric fields denoted by a and b induce non-resonant transitions from the ground state to the two excited states. The selective transition conditions are set by the choice of the laser polarization vectors (ea and eb) satisfying μ02ea and μ01eb, respectively.

The time-dependent Schrödinger equation can be written as

iΦ(t)t=H(t)Φ(t).(31)

The coefficients must satisfy the coupled differential equation

it(c0(t)c1(t)c2(t))=H(t)(c0(t)c1(t)c2(t)),(32)

where the interaction Hamiltonian,

H(t)=(0V01a(t)V02b(t)V10a(t)00V20b(t)00),(33)

is applied with V01a(t)=<ϕ0|μFa|ϕ1>cos(ωatζa)exp(iω10t) and V02b(t)=<ϕ0|μFb|ϕ2>cos(ωbtζb)exp(iω20t).

Equation 32 can be rewritten in the rotating approximation (RWA) and solved under the following restriction conditions to obtain the analytical solutions for the time-dependent coefficients {ci(t)}. For this purpose, we introduce three conditions that can be set experimentally:

i.VV01a=V02b               for the transition magnitudes, (34a)
ii.ΔΔ2=Δ1>0       for detuning frequencies, (34b)

And

iii.Δ=Ωω212to induce the degeneracy condition, (34c)

where the transition magnitudes are V01aμ01Fa/(2), and V02bμ02Fb/(2).

In Eq. 34c, the two dressed states are assumed to have equal energies, i.e., ΩΩ1=Ω2(=Δ2+2V2), which is called the Rabi frequency [43]. The three conditions lead to a reduction of the input parameters of the two lasers, such that the two amplitudes (Fa and Fb), and two central frequencies (ωa and ωb) are reduced to one input parameter. We take FaF hereafter. Analytical expressions for time-dependent coefficients {ci(t)} are given in Appendix C.

The time-dependent angular momentum defined as an expectation value of an angular momentum operator L^Z=iφ=(l^Z, l^Z=iφ), can be expressed,

LZ(t)=<Φ(t)|L^Z|Φ(t)>=2Im(lZ,12)Im[c1(t)c2(t)exp(iω21t)].(35)

Here, lZ,12=<ϕ1|l^Z|ϕ2>=<ϕ2|l^Z|ϕ1>=lZ,21 and lZ,12=iImlZ,12.

4.2 Unidirectional π-Electron Rotations in Toluene

We calculate LZ(t) (derived in the preceding subsection) in a real molecule, toluene. The simplest three-electronic state model is applied for toluene because the quasi-degenerate states S3 (A") and S4 (A′) in toluene (Cs) correspond to the doubly degenerate state S3 (E1u) in benzene (D6h): Note that the S3 (E1u) state is a dipole-allowed excited state in benzene (D6h), whereas the other two lower excited states, S2(B1u) and S1(B2u), are dipole-forbidden. The geometry optimization of toluene was carried out with the MP2 level of theory [43]. The CS symmetry of toluene indicates that one of the hydrogen atoms belonging to the methyl group is perpendicular to an aromatic ring plane (Figure 11A).

FIGURE 11
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FIGURE 11. (A) Geometrical structure of toluene molecule (CS) in the ground state (S0) with the directions of the two electronic transition dipole moments; (B) Parameters for the three electronic states adopted to induce the unidirectional angular momentum: ωS4S3=0.10 eV as the energy difference between S4 and S3, μS3S0,X=5.27 D, and μS4S0,Y=5.67 D as the transition dipole moments, ES3=8.2 eV and ES4=8.3 eVas the vertical transition energies. The angular momentum matrix element between S3 and S4, Z,S3S4=0.672i, is perpendicular to the aromatic ring.

Figure 12A shows the calculated time-dependent angular momentum expectation values LZ(t) with respect to the relative phase ζ = −π/2 (ζ = +π/2) between the two lasers for the left (right) panel. These were calculated using Eq. 35 combined with Eq. 34. Here, the amplitude of the electric field Fa is adopted as the input parameter F (≡ Fa). Other parameters are shown in Table 3A, while Table 3B shows the time-dependent populations in the three electronic states.

FIGURE 12
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FIGURE 12. (A) Expectation values of the angular momentum operator LZ(t) for toluene, calculated with the laser input parameter values F = 1.0, 1.5 and 2.0 GV/m. The left (right) panel shows LZ(t) with respect to the relative phase between two lasers ζ = −π/2 (ζ = +π/2); (B) Temporal evolutions of the populations in S0, S3, and S4 for F = 1.0, 1.5 and 2.0 GV/m. Reprinted with permission from Ref. [43] Copyright (2016) Royal Society of Chemistry.

TABLE 3A
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TABLE 3A. Input parameter F (≡ Fa) and other parameters for a generation of unidirectional π-electron rotation in toluene.

TABLE 3B
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Table 3B. Calculated physical properties of the unidirectional π-electron rotation in toluene.

The time-dependent angular momenta plotted in Figure 12A are comprised of angular momentum pulse trains of the same shape for each value of F. Each angular momentum pulse corresponds to the unidirectional π-electron rotation, which begins with acceleration and ends with deceleration. The direction of the π-electron rotations is determined by the relative phase ζ between the two lasers: clockwise rotation for ζ=+π/2, and anti-clockwise rotation for ζ=−π/2. Here we discuss how the unidirectional π-electron rotations are created. We first note that LZ(t) can be rewritten under the two conditions, Eq. 34a and Eq. 34b, for Ω≈Δ as

LZ(t)4Im(lZ,12)V2Δ2Ω4(1cosΩt)sin((2Δ+ω21Ω)t+ζ).(36)

Equation 36 expresses no unidirectional π-electron rotations. By introducing the third condition, Eq. 34c, which provides for the formation of the doubly degenerate dressed states with ζ = ±π/2, Eq. 36 can be expressed as

LZ(t)4Im(lZ,12)V2Δ2Ω4(1cosΩt)sin(Ωt+ζ).(37)

This indicates a unidirectional π-electron rotation whose direction is determined by the relative phase ζ, that is, LZ(t) > 0 for ζ = −π/2 and LZ(t) < 0 for ζ = +π/2, as demonstrated in Figure 12. The dotted line shown in Figure 12A represents LZ(t) for F = 1.5 GV/m as calculated by Eq. 37, which well reproduces LZ(t) curve calculated using Eq. 35 without the approximation of Ω≈Δ approximation. Thus, the unidirectional π-electron rotation originates from the DSIDES formed by the two non-resonant lasers with a definite relative phase of ±π/2.

As demonstrated in Figure 12A, the oscillation periods of the angular momentum pulses become shorter (higher Rabi frequency Ω), and those amplitudes decrease as F increases. These two behaviors result from the third restriction condition for degeneracy (Eq. 35c) used in the derivation of LZ(t). This degeneracy condition should be satisfied for the two dressed states with equal energies to maintain their energies located at the center of the two excited states, even though the laser intensities of two lasers increase. This results in an increase in the detuning parameter Δ of the two lasers, that is, a decrease (increase) in ωa (ωb), as shown in Table 3A.

Figure 12A exhibits that the maximum magnitude of the angular momentum occurs in the vicinity of F = 1.5 GV/m. This can be explained by noting that the constant in Eq. 37, V2Δ2/Ω4, with Rabi frequency of Ω=Δ2+2V2, reaches its maximum at Δ2=2V2, which in turn provides the maximum angular momentum. Using the relationship Δ2=2V2for the dressed states with equal energies in Eq. 34c, the electric field F is estimated as F=2Δ/(μ10)X = 1.52 GV/m with Δ = 0.118 eV, which is close to the F = 1.5 GV/m value shown in Figure 12A.

Figure 12B shows the F-dependence of the |cS0(t)|2, |cS3(t)|2 and |cS4(t)|2 populations in toluene, which were calculated using Eqs. (B2). The relationship between the excited state populations, |cS3(t)|2=|cS4(t)|2, is maintained because the conditions represented by Eqs. 34a, 34b, were used for a derivation of LZ(t). Inversion of populations between the ground and excited states occurs for F = 1.5 GV/m and 2.0 GV/m, as demonstrated in Figure 12B. By comparing Figures 12A,B, it can be observed that major components of the unidirectional angular momentum pulse are created during when the population inversion occurs.

We now estimate the physical constants associated with π-electron rotations, which are listed in Table 3B: one cycle count of π-electron rotation Nr, and the magnitude of ring current J in one cycle of Rabi oscillation with periodicity T ≡ 2π/Ω. In particular for aromatic molecules the ring current is an important physical quantity because it is directly related to the magnetic field induced by the ring current. For simplicity, we consider the classical model of π-electron rotation, in which Nr is defined as NrT/τ. Here τ=2πmer/p=2πmer2/|L¯Z| is the rotational period of a π-electron having an angular momentum averaged over one cycle period of Rabi oscillation (L¯Z). me is the electron mass, and r indicates the radius of the aromatic ring. L¯ was calculated using Eq. 37 substituting for LZ(t) as

L¯Z=1T0TdtLZ(t)=2ImlZ,12Ω4V2Δ2sinζ.(38)

The ring current can be approximately expressed as JL¯ZJ0/ [3941, 43]. Here on a single aromatic ring, J0 = 200 μA was a typical value of the ring current estimated with a unit angular momentum ħ. Table 3B exhibits the calculated ring current J, cycle counts of π-electron rotations along the aromatic ring Nr and three values of F adopted in Figure 12, together with the parameters used in this model calculation. In section 3, we presented a pump-dump control procedure for the generation of sequential unidirectional ring currents in a 2,2’-biphenol molecule having non-degenerate excited states. The same order of magnitudes of the pump- and dump- pulse lasers were applied in the vicinity of ∼1 GV/m, considering that the number of aromatic rings differs between biphenol and toluene. This can be explained by the point that the magnitudes of the ring current J or the angular momentum L¯Z are proportional to that of the coherence between the two excited states, but not proportional to the number of aromatic rings. It is remarkable that in Table 3BJ is not proportional to Nr, it is actually proportional to Nr/T. This is originated from the different behaviors between J and Nr related to the F-dependence shown in Table 3B.

5 Summary and Perspective

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 [3943, 6873]. 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 (coherent) 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. Here, the direct product of the irreducible representations of each excited states in the molecular point group belongs to that of the molecular rotation, (Rx, Ry or Rz), and the relative phase between the two linearly polarized lasers are fixed in order to determine the initial direction of the angular momentum and ring current. In additon to the laser conditions, aromatic ring molecules also need to satisfy the follwing conditions to generate the π-electron angular momentum and ring current. First two electronic excited states are nearly degenerate, and second, those states can be excited from the ground states using UV lasers, i.e., transitions between the electronic excited states and ground states are optically allowed.

On the basis of the above principles, analytical expressions for coherent π-electron angular momentum, ring current, and ring current-induced magnetic field are derived in the quantum chemical MO theory. A coherent π-electron ring current is defined as the average of the bond currents. Temporal behaviors of these coherent quantities are calculated using the density matrix method within the Markov approximation. Quantum simulations of the coherent quantuities have been performed for (P)-2,2’-biphenol. Based on the simulation results a new quantum control method for two-dimensional ultrafast angular momentum switching was proposed. The essential point of the quantum control is to use a sequence of the overlapped pump and dump pulses with a selected relative phase and selected polarization directions between the two lasers. The results for the (P)-2,2’-biphenol indicate that this new control scheme can potentially be used for the design and realization of ultrafast multi-dimensional electronic switching devices, or field-effect transistor devices.

The familiar quantum optimal control procedure was applied to an optiaml control of the coherent π-electron ring currents in planar PAHs with D2h symmetry. For this purpose, we have to set up the target state for a desired ring current pattern. The final target wave functions are determined via the Lagrange multiplier method by solving the coupled equations of motion under the constraints that the ring currents must satisfy. The method was applied to anthracene. The creation of the perimeter current and the middle ring current in anthracene were successfully realized. The control mechanisms were elucidated by analyzing the time-dependent behaviors of the control laser puses and the populations of the relevant electronic states. Concerning the types of molecules used in the above laser control studies, polycyclic planar aromatic molecules like PAHs, coronene and benzoic acids dimers are applicable for a laser control of perimeter or localized ring currents.

Another laser control method for the π-electrons unidirectional rotation in low symmetric aromatic rings was presented. The basic idea of the control method is to degenerate two nondegenerate excited states by using dynamic Stark shifts. Doubly degenerate excited state was created by the Stark shift using the two linearly polarized non-resonant lasers with a definite relative phase of ±π2, which determines the angular momentum direction (clockwise or anti-clockwise π-electron rotation). Applying the RWA, the coherent π-electron angular momentum in a three-electronic state model was analytically expressed in a closed form, and the proposed control theory have been applied to toluene (Cs). The numerical simulations indicated that the resulting angular momentum comprised sequential angular momentum pulses with a positive or negative value depending on the relative phases between the two lasers.

Some theoretical studies on laser control of the coherent π-electron rotations that were not included in this review are noteworthy. Probing molecular chirality, right-handed or left-handed chiral molecules, is a central issue in natural science. E.x., Phenylalanine is an essential amino acid, and L-enantiomer is found in natural system. On the other hand, D-enantiomer is synthesized artificially, and racemate phenylalanine is used for dietary supplement. However, in conventional methods such as CD spectroscopy and optical rotatory dispersion (ORD) spectroscopy [74, 75], the signals are very week because these processes involve second-order evaluations. As a possible of laser control scheme of the unidirectional π-electron rotations in low symmetric aromatic ring molecules, we previously proposed an efficient enantiomer-probing scenario for chiral aromatic molecule [71, 72]. A pair of nondegenerate excited states becomes degenerate by applying the dynamic Stark shift in the presence of two non-resonant UV lasers, producing an enantiomer-specific angular momentum. In that study, phenylalanine was adopted as an example for the numerical simulation. The resulting enantiomer-specific magnetic fluxes were on the order of a few Tesla, with periods of several tens of femtoseconds.

The contribution of vibrational motion to coherent π-electron rotations is also an important research topic that was not discussed in this review. In our previous research, vibrational effects on the coherent π-electron rotations in (P)-2,2’-biphenol were theoretically studied in the adiabatic approximation [73]. It was found that the low-frequency torsion mode around the bridge causes modulations in the beat of the ring current [73]. The vibrational effects on dynamic Stark-induced π-electron rotations in aromatic ring molecules with low symmetry were also studied using the displaced harmonic oscillator (DHO) model in the adiabatic approximation [72]. A pair of the lowest vibronic state in the two electronic excited states was degenerated using the two linearly polarized UV lasers. The two potential displacements between the ground state and the two electronic excited states were used as the parameters. The shapes of the sequential angular momentum pulses were affected by the potential displacements.

From one perspective, theoretical treatments beyond the adiabatic approximation [28, 31, 7477] are expected to be incorporated such that other electronic excited states interact with the two relevant excited states through nonadiabatic couplings. It is crucial to clarify the effect of nonadiabatic couplings to the unidirectional π-electron rotations and determine how to maintain π-electron rotations by way of quantum optimal control techniques. In large PAHs, once π-electron rotation is realized at a localized ring site and once a site-selective coherent ring current is generated, the localized ring current is transferred from site to site using the laser pulses, i.e., transferred ring currents, which allow for an ultrafast switching function at the selected local site. Furthermore, the site-selective coherent ring current and transferred ring current can create the induced magnetic fields. These electromagnetic fields are expected to provide ultrafast probing of local sites in large molecular systems, biomolecules with chiral aromatic ring molecules, and PAHs. Further development of theoretical treatments involving photon-induced electronic coherence in molecular systems would be promising in the near future.

Author Contributions

The authors made a substantial, direct, and intellectual contribution to the work and approved it for publication. Original draft preparation, software, formal analysis, and funding acquisition, HM; Conceptualization, supervision, project administration, and validation, YF; Writing—original draft preparation, HM and YF; Investigation, writing—review and editing, HM, N-LP, and YF.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

HM and YF are grateful to Dr. M. Yamaki, and Professors Y. Teranishi, G-S. Kim, Y.J. Yan, and H. Nakamura for their contribution in earlier works. YF would like to thank Professor S. H. Lin for his critical comments and financial support.

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Appendix

A Brief derivation of target operators, calculated using the coupled equations for the ring current localization (Eq. 19).

In general, the Lagrange function LJ consists of “the function to be maximized or minimized” and “all related constraint conditions of the system,” For the localization of the π-electron ring current, Jχ in PAHs, LJ(c1,,cn,λ1,,λm) can be rewritten as

LJ(c1,,cn,λ1,,λm)=Jχ+λ1J1++λχ1Jχ1+λχ(α=1n|cα|21)+λχ+1Jχ+1++λmJm,(A1)

Where the first term Jχ is the target ring current to be maximized, while the other terms with Lagrange multiplier λκ, JκJκ(T), are irrelevant to the localization under the constraint conditions, such that J1==Jχ1=Jχ+1==Jm=0, and (α=1n|cα|21)=0, which indicates the number conservation of the π-electrons associated with the ring current.

The partial derivatives of Eq. 19 with respect to xα=(Recα,Imcα), λχ and λκ are then

LJxα=Jχxα+κχmλκJκxα+λχxα(β=1n|cβ|21)=0,(1αn),(A2a)
LJλχ=α=1n|cα|21=0,(A2b)

And

LJλκ=Jκ=0,(1κm,κχ),(A2c)

Respectively, where Jl(i.e.,Jχand Jκ) can be rewritten in terms of Recα and Imcα as

Jl=αnβnJl,αβ(ImcαRecβRecαImcβ)(1lm).(A3)

By substituting the partial derivatives of Jl in Eq. (A3), with respect to Recα and Imcα into Eq. (A2a), we obtain 2n+m coupled equations with 2n+m variables as

β=1nJχ,βαImcβ+κχmβ=1nλκJκ,βαImcβ+λχRecα=0,(1αn),(A4a)

And

β=1nJχ,βαRecβκχmβ=1nλκJκ,βαRecβ+λχImcα=0,(1αn).(A4b)

Equations (A2b), (A2c) can be expressed as

α=1n((Recα)2+(Imcα)2)1=0,(A4c)

And

β=1nα=1nJκ,αβ(ImcαRecβRecαImcβ)=0 .(A4d)

Equation (A4) contains the final equations, which are the same as those in Eq. 19.

B Brief derivation of target operators, calculated using the coupled equations for the perimeter ring current (Eq. 20).

Similar to the ring current localization, the Lagrange functional for the perimeter ring current is given as

LJP(c1,,cn,λ1,,λm)=JP+λ1JB,1++λm1JB,m1+λm(α=1n|cα|21),(B1)

Where JP is the perimeter ring current (Eq. 20a) to be maximized, and JB,l are the bridge bond currents (seeEq. 20b), which should be zero at the target time t = T, JB,l=0 where 1lm1. The last term indicates the normalization condition for the total target wave function α=1n|cα|21=0.

In the same manner as the ring current is localized to a designated ring, the coupled equations for the perimeter ring current are obtained by taking partial derivatives of the Lagrange functional LJP (Eq. (B1)) with respect to Recα, Imcα, and λl.

The coupled equations for the perimeter ring current can then be expressed as

β=1nJβαPImcβ+l=1m1β=1nλlJβαB,lImcβ+λmRecα=0,(1αn)(B2a)
β=1nJβαPRecβl=1m1β=1nλlJβαB,lRecβ+λmImcα=0,(1αn)(B2b)
α,β=1nJαβB,l(ImcαRecβRecαImcβ)=0,(1lm1)(B2c)

And

α=1n((Recα)2+(Imcα)2)1=0.(B2d)

C Calculated results of time-dependent coefficients {ci(t)} in Eq. 31.

Equation 32 can be rewritten in the rotating approximation (RWA) as

c˙0(t)=iV01aexp(iΔ1t)c1(t)iV02bexp(iΔ2t+iζ)c2(t),(C1a)
c˙1(t)=iV01aexp(iΔ1t)c0(t),(C1b)
c˙2(t)=iV02bexp(iΔ2tiζ)c0(t),(C1c)

Where ζζaζb is the laser relative phase between the two lasers. Note that the laser phases ζa and ζb are transferred to the electronic states.

The coupled differential equations in Eq. (B1) are analytically solved under the two conditions described by Eqs. 34a, 34b by setting the initial condition: c0(0)=1, c1(0)=c2(0)=0 [43] such that

c0(t)=Δ2+V2(eiΩt+eiΩt)Ω2,(C2a)
c1(t)=V2Ω2[2ΔeiΔt+(ΩΔ)ei(Ω+Δ)t(ΩΔ)ei(ΩΔ)t],(C2b)
c2(t)=eiζV2Ω2[2ΔeiΔt+(ΩΔ)ei(Ω+Δ)t(ΩΔ)ei(ΩΔ)t].(C2c)

Keywords: quantum control, electron dynamics, coherent ring current, ring current localization, angular momentum, quantum switching, Stark-induced degenerate electronic state

Citation: Mineo H, Phan N-L and Fujimura Y (2021) Quantum Control of Coherent π-Electron Dynamics in Aromatic Ring Molecules. Front. Phys. 9:675134. doi: 10.3389/fphy.2021.675134

Received: 02 March 2021; Accepted: 23 June 2021;
Published: 16 July 2021.

Edited by:

Philip Bucksbaum, Stanford University, United States

Reviewed by:

Sheng Meng, Institute of Physics (CAS), China
Luca Evangelisti, University of Bologna, Italy

Copyright © 2021 Mineo, Phan and Fujimura. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Hirobumi Mineo, mineo@tdtu.edu.vn; Yuichi Fujimura, fujimurayuichi@m.tohoku.ac.jp