Light-Driven Reciprocating Host–Guest Molecular Machines

General operating principles have been established for light-driven molecular machines with different ground- and excited-state potential energies of a Brownian particle in a surrounding force field. The optomechanical coupling between the time dependences of the particle photoexcited-state population and its coordinate is described in terms of a theory developed for the laser-pulse controlled reciprocating Brownian photomotor which consists of the guest dye and host cavitand molecules. The time dependences of spectroscopic and mechanical characteristics have been calculated in the approximation of parabolic potential profiles of the ground and excited states. As shown, the dependence of the average velocity of reciprocating motion on the repetition period of ultrashort laser pulses is a nonmonotonic function, with its maximum specifying the optimal operating mode of the molecular machine. In the case when the inclusion of the dye molecule into the cavitand interior hinders the formation of the fluorescence-quenching twisted state and the deactivation rate constant of the guest molecule decreases with its coordinate, the fluorescence lifetime of the completely cavitand-included dye lengthens significantly, in agreement with the available experimental data. The presented analytical relations between mechanical and optical characteristics of the reciprocating molecular machines under study offer wide opportunities for controlling their motion as well as their observed fluorescence.