Spin, Exciton and Chemical Dynamics in Crystalline Solids

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With support from the Chemical Structure, Dynamic & Mechanism B Program of the NSF Chemistry Division, Professor Miguel A. Garcia-Garibay of the Department of Chemistry and Biochemistry at UCLA will investigate the use of light and crystals to explore a new platform for Quantum Information Science (QIS). Crystals make it possible to design chemical processes and reactions that cannot occur in liquids or in gases. They open the door for a deeper understanding of reaction mechanisms and for the exploration and design of solvent-free synthesis and applications in materials science. While molecules in crystals do not have the freedom of motion that is expected for chemical reactions to take place, reactions in crystals can be engineered in a reliable manner by taking advantage of a strategy used by nature based on biological structures that can capture and guide the energy contained in sunlight. Examples of such sunlight-induced, or “photobiological” processes include photosynthesis, DNA repair, and the biological compasses used by birds to guide their navigation across the planet. Similar to those systems, reactions in crystals rely on the capture of light to generate excited states that go on to form highly reactive intermediates, whose fate is controlled by the crystal’s rigidity, homogeneity, and order. Pulsed lasers aimed at selected crystalline ketones will be used to generate relatively long-lived magnetic species known as “triplet radical pairs”. These consist of two extremely close unpaired electrons formed by the cleavage of a single chemical bond, which causes their magnetic spins to be strongly entangled and be relatively impervious to external magnetic perturbations. These characteristics endow them with great potential for applications in Quantum Information Science (QIS). The second part of this project explores a relatively exotic type of chemical reaction where a single light particle, or “photon”, leads to many chemical events. These so-called quantum chain reactions are enabled by the strong energetic interactions between neighboring molecules, which lead to the transformation of photons into “excitons” and are helped by reactants that contain and unleash large amounts of energy. The third objective of this project is to provide a multi-disciplinary training ground for students who will contribute to the intellectual and human infrastructure needed to support our country’s academic and industrial enterprises. The PI will lead the annual UCLA Physical Sciences Fair, “Exploring your Universe,” which attracts ca. 7,000-10,000 visitors to campus, and group members will have a booth to share with the public the exciting things that can be done with crystals and light.The Garcia-Garibay group has established the structural and energetic requirements needed to engineer a number of photochemical reactions in the crystalline solid state. For the intellectual merit of this project, they will take advantage of nanocrystals (ca. 200 nm) suspended in water in order to detect the absorption spectra and decay kinetics of triplet excited states and triplet radical pairs to explore two photochemical processes that can only occur in crystalline solids: (1) the chemical and spin dynamics of highly entangled triplet radical pairs as a potential platform for quantum information science (QIS), and (2) the potential of quantum chain reactions with triplet chain carriers as a powerful signal amplification mechanism. With triplet excitons providing efficient energy delocalization mechanisms, these reactions are expected to reach chain lengths with as many as 106-109 reactions per photon absorbed, depending on the lifetime of the triplet excited states. For the first part, they take advantage of a series of 1,1-diphenyl-2-propanones and 1,1,1-triphenyl-2-propanones with a variety of substituents at C3 to study the kinetics of intersystem crossing of triplet radical pairs. While ketones with radical stabilizing substituents at C3 are known to lose CO to form triplet alkyl-alkyl radical pairs that go on to give products from a radical-radical combination reactions, ketones with high C2-C3 bond dissociation energies can only undergo an alpha-cleavage of the C1-C2 bond to form triplet acyl-alky radical pairs. These intermediates go back to the starting material after intersystem crossing to the corresponding singlet acyl-alkyl radical pairs. Much of what it is known about intersystem crossing in radical pairs comes from experiments where long inter-radical distances lead to weak exchange interactions with dominant hyperfine coupling (hfc) intersystem crossing mechanisms. By contrast, radical pairs in crystalline solids offer the opportunity to explore a new frontier: one where rigid radicals are highly entangled as a result of a very large singlet-triplet gap. Long triplet lifetimes are observed despite having the two electrons within van der Waals distance as the result of (a) inefficient hfc, (b) unfavorable geometries for spin-orbit coupling (SOC), and (c) spin-lattice relaxation slowed down by crystal rigidity. Long-lived entanglements makes these crystal-trapped radical pairs excellent qubit pair candidates for applications in quantum information science. For the exploration of a triplet exciton-enabled quantum chain reaction, this research will study solid state reactivity of crystalline sensitized Dewar benzenes. These are known to undergo an efficient triplet state adiabatic reaction where the triplet state Hückel benzene photoproduct is a competent energy donor that can carry out a quantum chain. Altogether, the work covered by this project is expected to improve basic understanding of reaction mechanisms and chemical reactivity in rigid materials and provide students the opportunity to receive a highly interdisciplinary training.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.