Cooperative dynamics in ligand binding reactions at the solution/solid interface

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Many large molecules have several chemically reactive sites. Oftentimes these sites behave independently, with the chemical reactions occurring at one site having no effect on the others. This is not always the case, however. In some instances, the binding at one site can change the chemical reactivity of another. Cooperation between reactive sites plays an important role in problems ranging from biology to molecular electronics. However, investigating cooperativity is challenging, since many experimental methods have trouble distinguishing the reactive sites. With support from the Macromolecular, Supramolecular, and Nanochemistry and the Chemical, Structure, Dynamics and Mechanisms A programs in the Division of Chemistry, Professors Ursula Mazur, Bhaskar Chilukuri, and K. W. Hipps at Washington State University are exploring cooperative effects in the binding of small molecules on nearby sites in large chemical complexes using a high resolution imaging method. The discoveries could aid in the design of new materials, as well as have broad implications for advancing technologies related to sensing, catalysis, and optoelectronics. The project's findings are incorporated into undergraduate courses at Washington State University, providing modern research examples to highlight concepts in chemistry, materials science, and engineering. The research labs involved in the project also provide training opportunities for graduate and undergraduate students, including a number of female and minority students.Working with their students, Professors Mazur, Chilukuri, and Hipps are synthesizing large, electron-rich molecules that consist of multiple fused porphyrin rings, and then depositing them on conducting substrates such as Au, graphite, and MoS2. The team then exposes the porphyrin multimers to small molecules (e.g. O2, CO, and NO) that react with the metal centers. By imaging the products with scanning tunneling microscopy (STM), they can directly observe of cooperative binding. Experimental observations are compared with molecular dynamics and density functional theory calculations to determine if communication occurs through the metal support or the intramolecular pi structure, as well as the extent to which intermolecular coupling occurs. Professors Mazur, Chilukuri, and Hipps are involved in undergraduate education and mentoring students from underrepresented groups.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.