Calculations of Light-Absorption Driven Molecular Rotors

We investigated the behavior of large molecular systems containing dipolar molecular rotors and motors. The computations, performed on the HPC computers, use the universal force field and Newton equations of motion implemented in our customized molecular dynamics program TINK, and commercial ab initio quantum chemical programs. The purpose of these calculations is to design new materials with controllable surface friction, materials with nonlinear response to electric fields, and other interesting properties for future devices. At 2005's conference we presented the principle of a fast unidirectional molecular rotor driven by absorption of light pulses. Now we outline results demonstrating the performance of an improved design of a synthetically accessible active motor molecule carrying a paddlewheel on a rotation axis. The structure has been optimized in a series of molecular dynamics simulations. It is designed in such a way that the absorption of a photon causes the separation of charges, and their mutual attraction then causes the rotor to make half a turn in the course of a few picoseconds. After about a dozen picoseconds after charge recombination the rotor completes a full turn. The structure has been designed in a way that favors unidirectional motion. The rates of charge separation and recombination can be controlled by a choice of donor and acceptor structures. With suitable choices, continuous irradiation or a train of light pulses would thus be expected to induce a fast unidirectional rotation of the rotor, which could be used to propel a thin layer of a fluid along a surface ("molecular pump") if the rotors were mounted in an organized array on a surface