1 Introduction
Electron dynamics induced by ultrafast short laser pulses in molecular systems have
been actively studied in recent years [1–8], because of its potential usefulness for
realization of organic electronic devices [9, 10]. In particular, rotational dynamics
of π-electron with mobile and polarizable characters in aromatic ring molecules is
interesting since large magnitudes of π-electron ring currents are in principle
generated by intense laser pulses. There are two types of electron ring currents
depending on whether its molecule is of highly symmetric like benzene or lower
symmetry like chiral aromatic molecules. The quantum simulations of π-electron
ring current were performed in highly symmetric planar molecules, Mg-porphyrin
[11–13], and benzene [14]. Here degenerated electronic states are excited by using
circularly polarized UV laser pulses. The rotational direction of the resultant πelectron ring current depends on whether the right or left circularly polarized laser
pulse is applied, and the current is unidirectional. On the other hand, linearly
polarized UV laser pulses can create a “coherent” π-electron ring current in a chiral
aromatic molecule. Kanno et al. carried out the π-electron rotation simulations for
2,5-dichloro[n](3,6) pyrazinophane [15–19], which is a planar chiral aromatic
molecule. The rotational direction of π-electron ring current is determined by
whether the superposition of quasi-degenerate states excited by linearly polarized
UV laser pulse is in-phase or out-phase. The simulation results indicate that the
chiral aromatic molecule is a candidate of ultrafast switching device.
Recently we have investigated coherent π-electron dynamics of a nonplanar
chiral aromatic molecule (P)-2,2′-biphenol [20–23]. This is a nonplanar chiral
aromatic molecule with axial chirality, which has two aromatic rings (called L and
R hereafter) linked through a single chemical bridge bond. We have classified four
possible rotational patterns of π-electron rotations [20, 21] as CC, AC, CA, and AA
where C(A) represents clockwise (anticlockwise) rotational direction. Because of
the nonplanar geometrical structure of (P)-2,2′-biphenol, the resultant π-electron
ring current and angular momenta are two dimensional. This indicates that nonplanar chiral aromatic molecules like (P)-2,2′-biphenol may serve as a twodimensional ultrafast switching device. For an effective quantum control of the
ultrafast sequential switching among the above four rotational patters, overlapped
pump and dump pulses with a properly selected relative phase and a laser polarization direction are necessary in addition to the time delay between two pulses.
In this review our attention is focused on the theoretical foundations of coherent
π-electron rotations in a nonplanar chiral aromatic molecule. In Sect. 2 we derive an
expression of π-electron ring current and angular momentum by solving the density
matrix method under Markov approximation. In Sect. 3 we show our numerical
results of π-electron ring current and angular momentum of (P)-2,2′-biphenol as an
example. Here, we provide an efficient method for a creation of coherent two
electronic excited states by using a properly selected linearly polarized UV pulse,
160
H. Mineo and Y. Fujimura
Electron dynamics induced by ultrafast short laser pulses in molecular systems have
been actively studied in recent years [1–8], because of its potential usefulness for
realization of organic electronic devices [9, 10]. In particular, rotational dynamics
of π-electron with mobile and polarizable characters in aromatic ring molecules is
interesting since large magnitudes of π-electron ring currents are in principle
generated by intense laser pulses. There are two types of electron ring currents
depending on whether its molecule is of highly symmetric like benzene or lower
symmetry like chiral aromatic molecules. The quantum simulations of π-electron
ring current were performed in highly symmetric planar molecules, Mg-porphyrin
[11–13], and benzene [14]. Here degenerated electronic states are excited by using
circularly polarized UV laser pulses. The rotational direction of the resultant πelectron ring current depends on whether the right or left circularly polarized laser
pulse is applied, and the current is unidirectional. On the other hand, linearly
polarized UV laser pulses can create a “coherent” π-electron ring current in a chiral
aromatic molecule. Kanno et al. carried out the π-electron rotation simulations for
2,5-dichloro[n](3,6) pyrazinophane [15–19], which is a planar chiral aromatic
molecule. The rotational direction of π-electron ring current is determined by
whether the superposition of quasi-degenerate states excited by linearly polarized
UV laser pulse is in-phase or out-phase. The simulation results indicate that the
chiral aromatic molecule is a candidate of ultrafast switching device.
Recently we have investigated coherent π-electron dynamics of a nonplanar
chiral aromatic molecule (P)-2,2′-biphenol [20–23]. This is a nonplanar chiral
aromatic molecule with axial chirality, which has two aromatic rings (called L and
R hereafter) linked through a single chemical bridge bond. We have classified four
possible rotational patterns of π-electron rotations [20, 21] as CC, AC, CA, and AA
where C(A) represents clockwise (anticlockwise) rotational direction. Because of
the nonplanar geometrical structure of (P)-2,2′-biphenol, the resultant π-electron
ring current and angular momenta are two dimensional. This indicates that nonplanar chiral aromatic molecules like (P)-2,2′-biphenol may serve as a twodimensional ultrafast switching device. For an effective quantum control of the
ultrafast sequential switching among the above four rotational patters, overlapped
pump and dump pulses with a properly selected relative phase and a laser polarization direction are necessary in addition to the time delay between two pulses.
In this review our attention is focused on the theoretical foundations of coherent
π-electron rotations in a nonplanar chiral aromatic molecule. In Sect. 2 we derive an
expression of π-electron ring current and angular momentum by solving the density
matrix method under Markov approximation. In Sect. 3 we show our numerical
results of π-electron ring current and angular momentum of (P)-2,2′-biphenol as an
example. Here, we provide an efficient method for a creation of coherent two
electronic excited states by using a properly selected linearly polarized UV pulse,
160
H. Mineo and Y. Fujimura