Mixed Quantum-Classical Simulations on Model Donor-Bridge-Acceptor Triads

Marcus theory predicts electron transfer rates between donor and acceptor systems. Since its inception in the 1950's, this theory has been widely applied to topics in many disciplines and has become an instrumental model in describing the efficiencies of photovoltaics. A key aspect of this theory requires knowledge of the nuclear reorganization of system and solvent alike and computation of energies associated with such nuclear motions is often difficult or impossible for systems with large number of degrees of freedom. We develop here a mixed quantum mechanical/molecular dynamics model to investigate charge transfer dynamics in a set of large organic Donor-Bridge-Acceptor triad molecules. Specifically, we are interested in the differences in electron and nuclear behavior relating to small changes in the molecular makeup of Carotenoid-Porphyrin-Fullerene triads. Our model approximates excitation energies on the order of 1.9 eV which agrees with absorption spectra for these triads and isolated porhyrins. Via electron population analysis, we monitor charge migration to the acceptor in time. Approximations of the charge transfer rates reveal ultrafast (picosecond scale) electron dynamics consistent with experimental literature. We then correlate nuclear dynamics with the charge transfer process using the Short-Time Fourier Transform technique. Broadly, the porphyrins undergo higher energy vibrations, whereas the fullerenes see low energy modes. Aryl side groups exhibit torsional motions relative to the porphyrin. Aryl linkers between bridge and acceptor are restricted from such motions and therefore express ring distortion modes. Finally, we find an amide linker mode that is directionally sensitive to electron motion. This work supports the notion of vibrationally coupled ultrafast charge transfer found in both experimental and theoretical studies and lays a foundational method for identifying key vibrational modes for parameterizing future theoretical models.