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Research

Comparison between simulation (Markland) and 2D-IR experiments (Fayer) on self-assembled alkyl monolayers functionalized with photocatalysts on gold surface.

The NSF Center for Quantum Molecular Design (Center for First Principles Design of Quantum Processes) is focused on developing fully-predictive theoretical methods for atomistic quantum mechanical molecular dynamics for the rational theory-aided-design of molecules and materials. Strong ties to experimentalists ensure that the methods are well validated. Specific collaborative projects include: (1) tailoring transport and reactivity of proton defects in complicated environments such as metal organic frameworks (MOFs) and interfaces, (2) understanding and manipulating excitation energy transport in natural photosynthetic systems, and (3) elucidating and controlling the factors governing fluorescence vs. photoisomerization in fluorescent proteins. The central theoretical framework is ab initio molecular dynamics (AIMD), including localized and periodic basis sets, ground and excited-state simulations, and efficient dynamics techniques including non-adiabatic and other quantum mechanical effects for large spatial and temporal scales. Broader impacts from the research activities are significant since the enhanced theoretical methods allow for unprecedented insight and control in areas that are difficult to address experimentally. Broader impacts also result from the development of new theory tools, graduate student training at the interface of theory and experiment, early research opportunities for undergraduates from diverse backgrounds, and the use of visualization and haptic devices to convey the complexities of quantum processes to broad audiences. The Center model integrates theory and experiment and provides innovative training of graduate students and postdoctoral researchers. New software tools are also being developed and disseminated to a diverse audience of professional chemists and students.

Focus Area I: Tailoring Transport and Reactivity of Proton Defects

Control of transport and reactivity of proton defects is essential for elucidating and tailoring chemical systems ranging from enzyme active sites and proton channels in biology to molecular catalysts and proton and hydroxide exchange membranes for clean energy. In CQMD, we will undertake an integrated theoretical and experimental collaboration on the dynamics of proton transfer in hydrogen bond networks. The experiments (Fayer) will exploit recent advances in ultrafast non-linear IR spectroscopy, including 2D IR vibrational echo as well as time resolved fluorescence. These methods will provide detailed experimental observations on the dynamics of H-bond networks and proton migration in nanoconfined environments. These experiments will be closely coupled to ab initio molecular dynamics (AIMD) simulations leveraging recent advances in multiple time-scale approaches combined with methods to include nuclear quantum effects via path integral simulations to yield the unprecedented efficiency needed to treat these systems at the required sizes, time-scales, and accuracy (Markland, Tuckerman). We will incorporate recent advances in highly efficient GPU-accelerated electronic structure theory and AIMD (Martínez). These advanced quantum simulation methods will provide predictive capabilities and molecular level understanding of the experimental observables. The strong interactions between ultrafast experimental observations and simulations of these systems will not only shed light on the physics of the systems but also provide stringent molecular level insights and validation for the theoretical predictions

Focus Area II: Designing Excitation Energy Transport

Electronic excitation energy transport is critical to photosynthesis in living organisms and also for man-made light-harvesting systems such as photovoltaics and solar-to-fuel applications. Therefore, a detailed understanding of the mechanisms for this transport is sorely needed. Even more exciting is the possibility of controlling this transport through molecular/supramolecular architectures - a task which nature has already accomplished, but which we have yet to grasp. We will (1) develop a thorough understanding of how protonation state, conformational changes, and other factors in the ambient environment influence the photoabsorption, excitation energy transport, and excitation energy dissipation characteristics in light-harvesting systems and (2) develop a theoretical (Martínez)/experimental (Schlau-Cohen) workflow to enable directed modifications to tune these characteristics. In Phase I, we will focus on natural photosynthetic systems, including LHCII/LHCSR from green algae and OCP from cyanobacteria.

Focus Area III: Converting Photon Energy into Mechanical Motion and Reactivity

An extremely intriguing technological possibility is the use of light to generate directed mechanical motion or reactions on the atomic scale. Such photomechanical action is already known to be the driving mechanism for many naturally occurring systems. For instance, the operation of many photoactive proteins revolves around light-induced photoisomerization about a C=C double bond. Examples include retinal proteins such as rhodopsin (where mechanical motion influences protein-chromophore binding and initiates signaling) and bacteriorhodopsin (where mechanical motion leads to vectorial proton transport). The objectives of this Focus Area are to (1) develop a detailed understanding of how the composition of the surrounding environment influences the characteristics and operation of existing photomechanical/photochemical systems and (2) develop a theoretical (Martínez)/experimental (Boxer, Gaffney) workflow for the design of new photomechanical/photochemical systems with desired characteristics.