Spin, Exciton, and Electron Transfer Chemistry And Dynamics In Crystalline Solids

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

With support from the Chemical Mechanism, Function, and Properties Program of the Chemistry Division, Professor Miguel A. Garcia-Garibay of the Department of Chemistry and Biochemistry at UCLA is taking advantage of crystals to explore the remarkable control of chemical reactivity that occurs when molecules are designed to break and make bonds upon the absorption of light. This project addresses the motion of light particles in crystals, also known as triplet excitons, which are transported by excited electrons moving from one molecule to another in a way that they have the same spin state, known as a triplet state. With the proper molecular design, triplet excitons can lead to the formation of reactive species where the two separated electrons are forced to remain at very close distances such that the evolution of their spin states may have potential applications in quantum information science. Triplet excitons can also enable a novel chemical amplification strategy where one photon leads to hundreds of chemical reactions by enabling a quantum chain reaction. In addition to exploring a frontier of chemical knowledge, this multifaceted project provides an excellent multi-disciplinary training ground for students who will join the intellectual and human infrastructure needed to support our country’s leadership in areas of critical scientific and technological development. Over the last few years the Garcia-Garibay group has established the structural and energetic requirements needed to engineer reactions in crystalline solids by taking advantage of excited states or reactive intermediates known to have pathways that break and make bonds in consecutive exothermic reactions. The overarching theme of studies described in this project is the use of unpaired electrons in triplet excited states and triplet reactive intermediates as a means to extend and control their chemical impact and/or the time needed for specific applications. First, they describe the use of non-reactive aromatic ketones to obtain information on how crystal packing may affect triplet exciton self-quenching dynamics. Next, they turn to reactive crystalline aryl-alky ketones to generate triplet radical pairs to analyze a largely unexplored frontier of spin and chemical dynamics that may have potential in quantum information science. This includes conditions where triplet radical pairs display unprecedented lifetimes as a result of large singlet-triplet energy gaps, large zero-field splitting, nearly colinear orbital geometries, and limited spin-lattice relaxation, all of which lead to very slow intersystem crossing rates. Then, to take advantage of triplet excitons, they propose the use of crystalline Dewar acenes with very long triplet lifetimes to extend the length of quantum chains resulting from adiabatic isomerization to their triplet valence-bond products. Notably, such quantum chains may result in hundreds, and potentially thousands, and millions of chemical reactions per absorbed photon. Finally, they will use knowledge earned by studying adiabatic reactions in crystals to explore the potential of chemiluminescent reactions using a series of 1,2-dioxetanes as a promising approach to detect magnetic polarization in crystals by the thermal generation of excited ketones within the probe of a solid-state NMR instrument. This may lead to the first observation of chemically induced dynamic nuclear polarization (CIDNP) in small molecule crystals. 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.