| The operation of a molecular motor is studied theoretically. The motor consists of a dibenzofulvene rotor sterically geared into a chiral, linked trityl stator. The rotor is actuated by light absorption with subsequent isomerization about the exocyclic double bond of dibenzofulvene. Because this torsional motion is geared into a chiral potential, there exists the possibility for preferential clockwise or counter-clockwise rotation.;As an initial simple model system, the non-adiabatic transition between coupled harmonic oscillator states was first studied, with the analysis focusing on the results in phase space. For the motor itself, molecular mechanics and PM3 semi-empirical calculations were used to determine rotation barriers of the initial motor design and variations. The electronic structure of the ground and excited states of the dibenzofulvene rotor is studied by ab initio calculations. These are used with molecular mechanics potentials for the rotor-stator gearing to obtain a qualitatively reasonable potential surface for the full motor. These calculations show that the motor is locked at ambient temperatures in the absence of light absorption. The photoisomerization is studied by classical trajectories on the excited electronic state following photon absorption. Nose-Hoover chain dynamics is used to create thermal initial conditions for the Franck-Condon region on the ground state.;The results of these calculations are interpreted in terms of the preference for the unidirectional bias in rotary motion that would be required for a useful nanoscale motor. The structure features and experimental conditions required to optimize motor performance are discussed. |
