Theoretical Design of an Aromatic Hydrocarbon Rotor Driven by a Circularly Polarized Electric Field † Masahiro Yamaki, ‡ Kunihito Hoki, ‡ Takato Teranishi, ‡ Wilfredo Credo Chung, ‡ Fabio Pichierri, § Hirohiko Kono, ‡ and Yuichi Fujimura* ,‡ Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Sendai 980-8578, Japan, COE Laboratory, Tohoku UniVersity, IMRAM, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ReceiVed: May 22, 2007; In Final Form: July 8, 2007 An aromatic hydrocarbon rotor without functional groups is theoretically designed. Such a molecular rotor is free from long-range electrostatic interactions. Induced dipole interactions are the rotor-driving forces under a nonresonant excitation condition. As an example, a molecular rotor with a condensed aromatic ring, a pentacene moiety mounted on a phenyl-acetylene axle that is driven by a circularly polarized electric field is considered. Results of simulations of the quantum dynamics of a rotor that take into account short-range rotor-bath interactions are presented by numerically solving the density matrix equations of the rotational motions. 1. Introduction In recent years, molecular rotors (motors) have been studied from both experimental and theoretical points of view. 1-15 Special attention has been given to optical electric-field-driven molecular rotors. 16-21 It has been shown that rotational motions can be controlled by optimizing variables of optical electric fields such as polarization direction, central frequency, pulse width, and so on. 21 The rotary arm of the rotor consists of a functional group such as NO 2 or CHO, whose dipole moment is of a magnitude that is sufficiently large for the rotor to be driven by electromagnetic fields of reasonably low intensities. On the other hand, such a molecular rotor is subjected to environmental perturbations, especially through long-range electrostatic interactions. This may cause a decrease in rotational power. Therefore, it is worth designing a molecular rotor without any functional group that is free from long-range electrostatic interactions. One of the candidates is a rotor made of hydro- carbon compounds without functional groups that consists of anisotropic, condensed aromatic rings that have a high polar- izability to be driven by electromagnetic fields. In this paper, we first present results of a theoretical design of an aromatic hydrocarbon rotor driven by a circularly polarized electromagnetic field. The rotor designed consists of a rotational arm, a shaft, and a base, which are made of pure hydrocarbons with a condensed aromatic ring. Similar aromatic hydrocarbons have actually been synthesized for molecular rotors. 22-23 We next present results of quantum dynamics simulation of the rotor by evaluating time-dependent expectation values of the rotational angular momentum operator. We clarify dephasing effects on the rotator dynamics by using the Redfield equation with secular approximation. Short-range collisions between the rotor and atoms or molecules in the environment are the main cause of rotational power loss of rotors. In the next section, we first describe the time-dependent interaction Hamiltonian in the Born-Oppenheimer approxima- tion within the semiclassical treatment of the rotor-radiation field interaction. We then show an outline of rotor dynamics in the density matrix formalism based on the so-called Redfield theory to take into account short-range interactions between a rotor and atoms or molecules in the heat bath. In Section 3, we present results of quantum dynamic simulations of a real molecular rotor, pentacene, that is surrounded by colliding substances under finite temperature conditions. 2. Theory (a) An Aromatic Hydrocarbon Rotor Driven by a Circu- larly Polarized Time-Dependent Electric Field. Consider an aromatic hydrocarbon rotor attached at a surface as shown in Figure 1. The internal, hindered-rotational coordinate is denoted by φ. We restrict ourselves to a one-dimensional rotor, neglect- † Part of the “Sheng Hsien Lin Festschrift”. * Corresponding author. E-mail: fujimurayuichi@mail.tains.tohoku.ac.jp. Telephone: +81 22 795 7715. Fax: +81 22 795 7715. ‡ Department of Chemistry, Graduate School of Science, Tohoku University. § COE Laboratory, Tohoku University. Figure 1. Molecular rotor that has pentacene as its engine attached to a surface through an acetylene unit and a benzene ring. Its rotation is induced by a circularly polarized electromagnetic field. 9374 J. Phys. Chem. A 2007, 111, 9374-9378 10.1021/jp073953t CCC: $37.00 © 2007 American Chemical Society Published on Web 08/24/2007