Coherent photoexcited dynamics and intermolecular conical intersections
Sergei Tretiak, Los Alamos National Laboratory
In this talk, I will overview some applications of semiempirical Non-adiabatic EXcited-state Molecular Dynamics (NEXMD) framework developed at several institutions and recently released to public. The NEXMD code is able to simulate tens of picoseconds photoinduced dynamics in large molecular systems with hundreds of atoms in size and dense manifold of interacting and crossing excited states. A variety of implemented non-adiabatic dynamics algorithms (including surface hopping, Ehrenfest, and ab initio multiple cloning) permits the flexibility of choosing the right approximation with respect to numerical cost. In particular, I will exemplify ultrafast coherent excitonic dynamics guided by intermolecular conical intersections (CoIns). Both simulations and time-resolved two-dimensional electronic spectroscopy track the coherent motion of a vibronic wave packet passing through CoIns within 40 fs, a process that governs the ultrafast energy transfer dynamics in molecular aggregates. Our results suggest that intermolecular CoIns may effectively steer energy pathways in functional nanostructures.
In the second example, we use non-adiabatic excited state dynamical simulations to compute X-ray Raman signals, which are able to sensitively monitor the coherence evolution in a rigid synthetic heterodimer. The observed coherences have vibronic nature that survives multiple conical intersection passages for several hundred femtoseconds at room temperature. These spectroscopic signals are possible to measure at XFEL facilities, paving the way for detailed coherence measurements in functional organic materials. Observed relationships between spatial extent/properties of excitonic wavefunctions and resulting electronic functionalities allow us to understand and potentially manipulate excited state dynamics and energy transfer pathways toward optoelectronic applications.
Relevant references
T. Nelson, A. White, J. Bjorgaard, A. Sifain, Y. Zhang, B. Nebgen, S. Fernandez-Alberti, D. Mozyrsky, A. Roitberg and S. Tretiak, “Non-adiabatic Excited-State Molecular Dynamics: Theory and Applications for Modeling Photophysics in Extended Molecular Materials”Chem. Rev. 120, 2215 (2020)
W. Malone, B. Nebgen, A. White, Y. Zhang, H. Song, J. Bjorgaard, A. Sifain, B. Rodriguez-Hernandez, S. Fernandez-Alberti, A. E. Roitberg, T. Nelson and S. Tretiak, “NEXMD Software Package for Non-adiabatic Excited State Molecular Dynamics Simulations” J. Chem. Theory Comput. 16, 5771 – 5783 (2020).
T. R. Nelson, D. Ondarse-Alvarez, N. Oldani, B. Rodriguez-Hernandez, L. Alfonso-Hernandez, J. F. Galindo, V. D. Kleiman, S. Fernandez-Alberti, A. E. Roitberg, S. Tretiak,“Coherent Exciton-Vibrational Dynamics and Energy Transfer in Conjugated Organics”Nature Comm., 9, 2316 (2018).