Molecular dynamics of a grid-mounted molecular dipolar rotor in a rotating electric field

Abstract

Classical molecular dynamics is applied to the rotation of a dipolar molecular rotor mounted on a square grid and driven by rotating electric field E() at T Ӎ 150 K. The rotor is a complex of Re with two substituted o-phenanthrolines, one positively and one negatively charged, attached to an axial position of Rh 2 4؉ in a [2]staffanedicarboxylate grid through 2-(3-cyanobicyclo[1.1.1]pent-1-yl)malonic dialdehyde. Four regimes are characterized by a, the average lag per turn: (i) synchronous (a < 1͞e) at E() ‫؍‬ ͦE()ͦ > Ec() [Ec() is the critical field strength], (ii) asynchronous (1͞e < a < 1) at Ec() > E() > Ebo() > kT͞, [Ebo() is the break-off field strength], (iii) random driven (a Ӎ 1) at E bo() > E() > kT͞, and (iv) random thermal (a Ӎ 1) at kT͞ > E(). A fifth regime, (v) strongly hindered, W > kT, E, (W is the rotational barrier), has not been examined. We find E bo()͞kVcm ؊1 Ӎ (kT͞)͞kVcm ؊1 ؉ 0.13(͞GHz) 1.9 and E c()͞kVcm ؊1 Ӎ (2.3kT͞)͞kVcm ؊1 ؉ 0.87(͞GHz) 1.6 . For > 40 GHz, the rotor behaves as a macroscopic body with a friction constant proportional to frequency, ͞eVps Ӎ 1.14 ͞THz, and for < 20 GHz, it exhibits a uniquely molecular behavior.M olecular construction kits promise access to giant molecules shaped like regular grids and scaffolds carrying active and͞or mobile groups (1-4). A sturdy artificial square grid polymer, albeit irregular and only ϳ150 nm across, has been reported (5, 6), and time appears right to use computer simulation of these ''designer solids'' to analyze the role of thermal motion in molecular machinery and to identify the best synthetic targets.One of the active elements proposed (4, 7) for incorporation in these molecular scaffolds is a dipolar rotor, capable of rotational motion in response to an outside driving force. Molecular rotors driven unidirectionally by light (8), or bidirectionally by heat (9-12) or chemically (13, 14) have been synthesized. Alternating electric field has been used to affect intramolecular motion (15), and intense laser field of rapidly rotating linear polarization has been used to drive the rotation of a chlorine molecule (16). Free hydroxyl groups on oxide surfaces behave as two-dimensional arrays of interacting rotors, as do phospholipids in certain membranes and dipolar molecules adsorbed on flat surfaces or intercalated in layered solids (17). Biological motors (18) are under study. The velocity at which a dissolved chiral molecule is propelled through a solution by circularly polarized microwaves has been theoretically estimated (19)(20)(21).Previously (22), we examined computationally the unidirectional motion of a grid-mounted propeller-shaped rotor driven by a stream of gas. This windmill rotor acted as a microphone, transducing linear motion of gas molecules into rotational motion of electric charges. A propeller-shaped rotor driven by a rotating electric field in the presence of a gas might produce a pressure differential, acting as a loudspeaker or a gas pump.The dielectric response of a dipolar rotor is intrinsically nonlinear and ther...

Symmetry and dynamics of molecular rotors in amphidynamic molecular crystals

“…2, a set of periodic rotary potentials suggests that rotators with a given rotational symmetry order (C n ) will have energy profiles with "n" minima and maxima, angular displacements of 360∕n°, and barriers that become smaller as the rotator approaches the shape of a cylinder when n → ∞. As the number of energy minima n increases, there is a greater spatial resolution that may be addressed with external stimuli when a suitable dipole is included in the structure (18,19). Increasing the symmetry of the rotators is also interesting because it allows the formation of gears with n teeth when the vertices of the rotator are extended in a radial manner.…”

An oscillating electric field with thermal noise increases the rotational diffusion and drives rotation in a dipole

“…Grimzewski et al [3] observed rotation of a single molecule within a supramolecular bearing. After the first examples of unidirectional rotating molecular motors based on simple organic molecules [4,5], several works reported on this subject [6][7][8][9][10][11][12][13][14]. Also great theoretical efforts have been devoted to understanding and proposing nanoscale rotors and stators as molecular motors capable of transforming a driven random rotation efficiently into a directed translational motion [15][16][17].…”

Quantum ignition of intramolecular rotation by means of IR + UV laser pulses

“…This approach may serve as alternative to chemical, electrochemical or photochemical stimuli of molecular rotors [4][5][6][7][8][9], which is an important challenge in molecular engineering [10][11][12][13].…”