Probes, Labels, Traps, and Reporters: Exploring the Modern Spin Chemistry Landscape

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In this project, funded by the Chemical Structure, Dynamics, and Mechanisms-B Program of the Chemistry Division, Professor Malcolm D. E. Forbes of the Department of Chemistry at Bowling Green State University is investigating the structure, changes (dynamics), and reactivity of free radicals. Free radicals are short-lived, very reactive species resulting from the fragmentation of stable molecules which have been exposed to heat or light. Free radicals are characterized by having unpaired electrons, in contrast to most stable molecules in their lowest energy state, which have all of their electrons paired. Using a specialized spectroscopic technique called time-resolved electron paramagnetic resonance, free radicals can be observed on short time scales in organized structures such as vesicles (biological cell mimics), micelles (oil droplets in water), bubbles, polymer matrices, and other supramolecular structures such as organic nanocrystals. The results provide a deeper understanding of entangled spin states, which have potential use in quantum computing, quantum sensing, and quantum information systems. The scientific impact of the work reaches the fields of medicine, quantum computing (control of highly entangled electron spin systems), coatings (thin film curing and drying), and photocatalysis and drug delivery. The Forbes' laboratory is active in public outreach to libraries and museums. They participate in a summer research program for high school students in Northwest Ohio.Spin wave function evolution and molecular motion are intimately connected in spectroscopic methods, especially time-resolved electron paramagnetic resonance (TREPR). Their dynamics can be analyzed through the use of theoretical models such as the nano-reactor model for spin-correlated radical pairs. A major thrust of the project is to place the radical-triplet pair mechanism (RTPM) of chemically induced electron spin polarization on a stronger theoretical footing, and in doing so make it more useful for understanding the nature of molecular motion in confined spaces on the nanoscale. The advantages in using RTPM spin polarization for experimental studies of molecular dynamics in both homogeneous and heterogeneous systems include the magnitude of the electron spin polarization observed in the nitroxide is directly dependent on the number of encounters it experiences with the photoexcited triplet state, providing quantitative translational diffusion information. The team also investigates the line shape of the nitroxide spin reporter (intensities, line widths, hyperfine couplings) gives information about polarity and viscosity, in particular rotational mobility. The excited triplet state is silent in the TREPR spectrum because of fast electron spin relaxation, induced by modulation of the dipolar interaction and this advantages is exploited by the team. Finally, neither the triplet precursor or the nitroxide are consumed during the experiment.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.