Prize Talk: Justin Jankunas Doctoral Dissertation Award (Finalists). Talk Title: Exciton-Vibration Dynamics Using Real-Time Path Integrals

Over the past two decades, understanding the quantum dynamics of molecular excited states has been a central theme in chemical physics research. Of particular relevance has been the question, "how do nuclear vibrational motions impact the dynamics of inter-molecular excitation energy transfer (EET)?", which has key bearings on our understanding of photosynthetic light harvesting, and on the design of new materials for photovoltaic applications. Numerical simulations of EET are often rendered intractable or severely approximate due to the exponential scaling of quantum mechanics with the combined Hilbert space size of electronic states and nuclear vibrations, and the need for an astronomical number of wave function-based calculations to incorporate thermal effects. During my PhD, I worked on the development of two numerically exact methods based on the Feynman path integral: the modular path integral (MPI) and the small matrix path integral (SMatPI) methods, that allow explicit inclusion of all nuclear normal modes at finite temperatures in EET simulations of large molecular aggregates. Using these methods, we performed the first simulation of the bacterial LH2 complex comprising all 24 excitonic states, and all nuclear normal modes (50 per chromophore) at room temperature, and discovered new aspects about its microscopic quantum dynamics. Particularly, we established that the remarkable (~90%) efficiency and (~1 ps) timescale of EET observed experimentally are enabled by the two-ring arrangement of chromophores, and quantum effects associated with nuclear vibrational motions. We also investigated EET in synthetic aggregates of perylene bisimides and metalloporphyrins, as well as several vibronic models. We illustrated how key features in the ultrafast dynamics of perylene bisimides are modulated by strongly coupled motions of the perylene core. Working with experimentalists, we also showed that small changes in the interplay of electronic and vibrational timescales can speed up exciton relaxation in porphyrin dimers by an order of magnitude, and introduce asymmetric temperature dependent features in the spectra of metalloporphyrin complexes. I will show key glimpses from these stories exhibiting the relevance of nuclear vibrations in quantitatively modulating inter-molecular EET dynamics.