One of the most interesting and promising frontiers in the chemical sciences involves the use of crystalline solids exposed to light for the purpose of carrying out special chemical transformations. Reactions in crystals differ from processes in liquids and in gases because molecules in the crystalline state are held rigidly and orderly, such that they all have the same fate when a reaction occurs. With support from the Chemical Structure, Dynamics & Mechanisms B Program of the Chemistry Division, Professor Miguel A. Garcia-Garibay of the Department of Chemistry and Biochemistry at The University of California, Los Angeles, uses crystals to explore two types of unusual but promising chemical reactions. One reaction examines the effect of magnetic fields. Magnetic fields play a role in the navigation systems of migrating birds and have the potential to control some types of chemical systems. The second reaction, known as a quantum chain reaction, is process where a single light particle, or photon, leads to many chemical events (an amplification process). Quantum chain reactions in crystals are enabled by strong interactions between neighboring molecules. This interaction transforms photons into moveable "excitons", facilitated by chemical reactions that retain and unleash large amounts of energy. A third objective of this grant is to demonstrate the advantages of reactions in crystals for the conversion of simple chemicals into high value substances. This multifaceted project provides an excellent multi-disciplinary training ground for students in quantum information systems (QIS). It helps develop the intellectual and human infrastructure needed to support our country's academic and industrial enterprises related to quantum computing, quantum biology, and sensing. The connection between biological systems and molecules in crystals provides Professor Garcia-Garibay with an opportunity to develop a freshman-level course on the role of light in the origins of life. This course develops multimedia teaching and demonstration aids that are shared with the general public.
Over the last few years the Garcia-Garibay group has established the structural requirements needed to engineer reactions in crystalline solids. This project addresses three aspects of chemical dynamics that have not been accessible by studying reactions in liquid solutions or in the gas phase. First, by taking advantage of crystalline ketones which have radical stabilizing substituents at the two alpha-carbons, the researchers are able to generate triplet radical pairs under conditions not available previously. With strongly interacting spins, these radical pairs have a relatively large Singlet-Triplet energy gap. When combined with the use of nanocrystalline suspensions as a medium to carry out laser flash photolysis (pump-probe) experiments, this project provides a unique opportunity to measure the dynamics of intersystem crossing in the solid state. Structural modifications of the starting ketone measured by single crystal X-ray diffraction and solid state nuclear magnetic resonance provide opportunities to analyze the kinetics of the triplet excited states and sequential radical pair intermediates. The interplay of spin dynamics and chemical dynamics is probed by measuring the kinetics of intersystem crossing as a function of external magnetic fields and magnetic isotope effects. Other aspects of solid state photochemical reactivity that are encompassed by this project include the total synthesis of the natural product Chimonanthin using solid state photodecarbonylation as the key step. The triplet exciton-enabled quantum chain reaction of crystalline Dewar benzenes is also explored. This chain process is expected to give a large number of product molecules per photon absorbed. Altogether, the research activities in this project improve the fundamental understanding of reaction mechanisms and chemical reactivity in rigid materials and provide students with an 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.