Synchronous Rotation Dynamics in a Molecular Motor

Direct measurements of molecular motion during chemical reactions are essential to understand how molecular machines perform work. In most systems, however, the reaction rate is dictated by the probability of reaching the transition geometry by thermal fluctuations, thereby masking the underlying molecular motions. Here, we study the dynamics of rotation around the central double bond of an artificial light-driven molecular motor by femtosecond transient absorption and fluorescence spectroscopy. We observe a first rotation step, assigned to a rotation of ∼28° which occurs synchronously across the motor ensemble and without an activation barrier, such that the measurements are representative of molecular dynamics. We can thus estimate the rotation speed and the relative importance of inertia, friction and strain, and propose a simplified nanomechanical model for the molecular motor. The results suggest a new framework to investigate work at the nanoscale and provide tools to analyze the mechanics of molecular machines, both synthetic or biological.