CAREER: Time-Resolved Multi-Pulse Spectroscopy of Solvated Aza-Aromatics

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Molecules that can absorb light and use the energy to drive chemical reactions are potentially interesting for new technologies in energy storage, water treatment, and industrial chemical production. However, understanding and controlling these processes is challenging because energy is generally expended quickly but stored slowly. For example, filming a speeding bullet (energy flow) poses different challenges than filming a flower growing (energy storage). In this project, funded by the Chemical Structure Dynamics and Mechanisms (CSDM-A) program of the Chemistry Division, Professor Cody W. Schlenker of the University of Washington (UW) is using advanced laser techniques to provide fundamental understanding of how light absorption drives chemical reactions, two processes with rates as different as the velocity of a speeding bullet and the growth of a flower. The scientific knowledge gleaned from this project has the potential to improve sunlight-to-fuel conversions, solar water decontamination, and possibly even new avenues for light-driven industrial chemical production. Professor Schlenker's Research-Integrated Science Education (RISE) Program focusses on helping to equip low-income and potential first-generation college students with the tools needed to apply to college, succeed in STEM majors, and enter the STEM workforce. The group integrates research and education to develop new peer-generated science tutorials presented by students from the most ethnically and culturally diverse public high-schools in Washington State (e.g. Chief Sealth High School in Southern West Seattle through a partnership with the UW's Math & Science Upward Bound program). One example product of the RISE Program: Step-by-step, peer-led-learning video science experiments are distributed freely on a RISE-dedicated YouTube channel. The research group also leverages existing UW resources, for example, through a partnership with the UW Clean Energy Institute. Organic photochemistry may lay the conceptual groundwork for new research in the field of solar water splitting. Cheap and scalable solar hydrogen could positively impact energy and food sustainability, since ammonia fertilizer production is resource intensive.The scientific objective of this project is to understand the role of inter-molecular excited states in the photochemistry of hydrogen-bonded molecular complexes. This research is focused particularly on understanding how nitrogen-containing molecules known as azaarenes photochemically react by removing hydrogen atoms from water and alcohols. To achieve this goal, the research team uses spectroscopic and electrochemical signals associated with transient, photogenerated, excited-states and free radicals. The population of these species are monitored as a function of the temporal delay between an initial ultrafast visible laser pulse (pump) and a subsequent infrared pulse (push) using transient absorption, photoluminescence, and electrical current detection. This approach is notable because it has the potential to extract ultrafast time information from the relative yields of much slower chemical reactions. Azaarenes play critical roles in photosynthetic assemblies, DNA photo-protection, photo-stabilization of industrial pigments, and they are common chromophores in renewable energy research focused on proton-coupled electron transfer (PCET). In these capacities, azaarenes can hydrogen-bond with hydroxyl groups. These hydrogen bonding interactions often alter the chromophore's photophysics and photochemical reactivity in seemingly unintuitive ways. The critical mechanistic links between the branching ratio among ultrafast photophysical pathways and the much slower subsequent photochemical reactions of azaarene molecules may constitute a step forward. Broader impacts of this work include potential societal benefits resulting from clarifying whether inter-molecular electronic excited-states that arise from hydrogen bonding interactions serve as a chemical gateway to photon-initiated reactions. Additionally, this project provides training opportunities for graduate and undergraduate students, integrating research concepts into public engagement activities for science outreach.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.