Masahiro Yamaki

Molecular motors driven by laser pulses: Role of molecular chirality and photon helicity

The results of a theoretical study on molecular motors driven by laser pulses are presented. The roles of molecular chirality and photon helicity in determination of their unidirectional rotation are clarified. An expression for an instantaneous angular momentum of motors driven by lasers in the density matrix formalism was derived. Assuming randomly oriented molecular motors, the initial distribution-averaged instantaneous angular momentum in the dipole approximation was obtained. Taking into account parity inversion symmetry of molecular motors in the averaged instantaneous angular momentum, it is shown that the directions of the averaged instantaneous angular momentum of (R)- and (S)-chiral molecular motors are opposite, but that the magnitudes are the same. This is independent of polarization of laser fields. That is, the chiral motors driven by a linearly polarized optical field creates a unidirectional motion in a molecular fixed frame. On the other hand, the direction of rotation in the laboratory fixed frame is decided by a circularly polarized laser regardless of its molecular chirality. A simple example of real chiral molecular motors is used to demonstrate the interplay of molecular chirality and photon helicity in determination of their unidirectional rotation. The internal rotation of the CHO group plays the role of the engine of the motor. The time evolution of the rotational wave packets of the molecular motors driven by linearly or circularly polarized laser pulses was numerically evaluated and the dynamical behaviors were analyzed. Effects of temperature on the instantaneous angular momentum of the molecular motors are presented as well.

Mechanism of unidirectional motions of chiral molecular motors driven by linearly polarized pulses

The mechanism of the unidirectional rotational motion of a chiral molecular motor driven by linearly polarized laser pulses was theoretically studied. A simple aldehyde molecule was adopted as a chiral molecular motor, in which a formyl group (–CHO) was the rotating part of the motor. Temporal evolutions of the instantaneous angular momentum averaged over an ensemble of randomly oriented motors were taken as a measure of the unidirectional motion. The contour plots of the averaged instantaneous angular momentum were obtained by using a quantum master equation approach that took into account relaxation effects and a classical trajectory approach. Two regimes are found in the contour plots. One is an intense laser field regime in which the laser–motor interaction energy exceeds the asymmetric potential barrier. In this regime, the motors are unidirectionally driven in the intuitive direction, i.e., the gentle slope of the potential. The other regime is a subthreshold laser intensity regime in which unintuit...

Quantum control of unidirectional rotations of a chiral molecular motor

A quantum control method is presented for designing electric fields of laser pulses to drive a chiral molecular motor in desired, rotational directions. Intuitive or counter-intuitive rotational motion of a chiral motor, 2-chloro-5-methyl-cyclopenta-2,4-dienecarbaldehyde, was controlled by electric fields of ps-laser pulses with mid-infrared central frequencies. The control mechanism is discussed by analyzing the time- and frequency-resolved spectrum of the electric fields of the laser pulses. Timing of laser pulses is the essential factor for controlling unidirectional motions.

Optical Control of Chiral Molecular Motors

Results of theoretical treatments of optical control of chiral molecular motors driven by linearly polarized laser pulses are presented. Both quantum and classical simulations of time-dependent quantum mechanical expectation values were performed to identify the rotational direction of real molecular motors. The rotational direction was found to be toward the gentle slope of the asymmetric potential energy surface of a chiral molecule of interest, which is the intuitive direction of rotation. The mechanism of its unidirectional motion is a non-resonant multi-photon forced-rotation induced by linearly polarized intense laser pulses. Relaxation effects of randomly oriented molecular motors were investigated using the Lindblad-type quantum master equation. Quantum control of unidirectional rotation of a chiral molecular motor is presented. Counter-intuitive rotation as well as intuitive rotation is generated by using a quantum control theory. Time- and frequency-resolved spectra of the electric field of the optimal laser were evaluated to analyze the origin of both intuitive and counter-intuitive rotations. A femtosecond pump-dump control method via an electronic excited state is shown to be one of the effective methods for avoiding effects of couplings between motors and solvents.

Quantum coherent control of pi-electron rotations in a nonplanar chiral molecule

The results of a theoretical investigation of coherent -electron dynamics for nonplanar (P)2,2’-biphenol induced by ultrashort linearly polarized UV pulses are presented. The coherent ring currents originate from an excitation of a pair of quasi-degenerate electronic states by an ultrashort linearly polarized UV laser pulse. The magnitudes of the electronic ring currents are expressed as the sum of expectation values of the corresponding operators in the two phenol rings (L and R rings). Here, L (R) denotes the phenol ring in the left (right)-hand side of (P)2,2’-biphenol. For electrons in (P)-2,2’-biphenol, there are four possible rotational patterns. The bond current of the bridge bond linking the L and R rings is zero for the symmetric coherent state, while it is nonzero for the antisymmetric coherent state.

Quantum Control of Laser-driven Chiral Molecular Motors

The design and control of functional molecular machines and devices is one of the fascinating and challenging research targets in molecular science (Feringa et al., 2000; Kinbara & Aida, 2005; Kay et al., 2007). They were originally inspired from biological machines such as ATP synthases (Boyer, 1993; Abrahams et al., 1994) and myosin and kinesin (Julicher et al., 1997). They now include various kinds of artificial molecular machines such as transmitters, shuttles, nanocars and logic gates (Balzani et al., 2008), which can be driven by external forces at the molecular level. Some of them are not simply sizeddown versions of macroscopic machines and are controlled at the quantum level (Roncaglia & Tsironis, 1998). Lasers are energy sources over a wide range of wave lengths from mid-infrared to ultraviolet, which make it possible to drive various sizes of molecular machines without any direct contact. Lasers are expected to play an important role as a source of external forces for controlling molecular machines because lasers have various controlling-parameters such as central frequencies, pulse shapes, photon polarizations and time differences between two pulses (Assion et al., 1998; Gouliemakis et al., 2004). Based on coherent control theory (Kosloff et al. 1989; Shi & Rabitz, 1990; Shapiro & Brumer, 2000), laser pulses can be designed to produce the maximum desired target with minimum laser energy (Assion et al., 1998; Rice & Zhao, 2000; Gordon & Fujimura, 2002; Bandrauk et al., 2002). Molecular machines can be controlled through coherent interactions between lasers and molecules at a quantum level (Hoki et al., 2003). The procedures are sometimes called “quantum ignition” for driving molecular motors (Fujimura et al., 2004). The time evolution is obtained by solving the time-dependent Schrodinger equation or the Liouville equation (Sugawara & Fujimura, 1994; Ohtsuki et al., 1999; Hoki et al., 2001). Application of coherent control theory enables extraction of key factors for driving molecular motors with a unidirectional motion, though we have to wait for further experimental progress to carry out coherent control experiments on artificial molecular machines. In this chapter, we present fundamental principles for unidirectional motions of chiral molecular motors driven by linearly polarized laser pulses having no photon helicity.

Quantum control of a chiral molecular motor driven by linearly polarized laser pulses

We present a theoretical study on quantum control of a chiral molecular motor driven by linearly polarized ultrashort laser pulses. Electric fields of laser pulses to drive the motor in desired directions are designed using a quantum control method.