Molecular Information and Crystal Control in Solid State Photochemistry. Radical Pair Dynamics, Synthetic Applications and Triplet Quantum Chains

化学科学中最有趣和最有前途的前沿领域之一是利用暴露在光下的结晶固体进行特殊的化学转化。晶体中的反应不同于液体和气体中的过程，因为处于晶态的分子被严格而有序地持有，因此当发生反应时，它们都有相同的命运。在化学系化学结构、动力学和机理B项目的支持下，加州大学洛杉矶分校化学和生物化学系的Miguel A.Garcia-Garibay教授利用晶体探索了两种不寻常但有希望的化学反应。其中一种反应是考察磁场的影响。磁场在候鸟的导航系统中发挥作用，并有可能控制某些类型的化学系统。第二个反应称为量子连锁反应，是一个单一的轻粒子或光子导致许多化学事件的过程(放大过程)。晶体中的量子链反应是由相邻分子之间的强相互作用实现的。这种相互作用将光子转化为可移动的“激子”，通过化学反应保留和释放大量能量。这笔赠款的第三个目标是证明晶体中的反应在将简单的化学品转化为高价值物质方面的优势。这个多方面的项目为量子信息系统(QIS)的学生提供了一个极好的多学科训练场地。它有助于发展所需的智力和人力基础设施，以支持我国与量子计算、量子生物学和传感相关的学术和工业企业。生物系统和晶体中的分子之间的联系为加西亚-加里巴伊教授提供了一个机会，开发一门关于光在生命起源中的作用的大一课程。本课程开发多媒体教学和演示辅助工具，与公众共享。在过去的几年中，加西亚-加里巴伊小组已经确定了在晶体固体中设计反应所需的结构要求。这个项目涉及化学动力学的三个方面，这些方面是通过研究液体溶液或气相中的反应无法获得的。首先，通过利用在两个α-碳上具有稳定自由基取代基的结晶酮，研究人员能够在以前没有的条件下产生三重态自由基对。由于自旋相互作用强，这些自由基对具有相对较大的单态-三态能隙。当结合使用纳米晶体悬浮液作为介质进行激光闪光光解(泵浦-探测)实验时，该项目为测量固体中的系间交叉动力学提供了一个独特的机会。用单晶X-射线衍射法和固体核磁共振测定了起始酮的结构变化，为分析三重态激发态和顺序自由基对中间体的动力学提供了机会。通过测量作为外加磁场和磁同位素效应的函数的系间交叉动力学，探讨了自旋动力学和化学动力学的相互作用。该项目涵盖的固态光化学反应的其他方面包括使用固态光脱甲基化作为关键步骤来完全合成天然产物Chimonantin。还探讨了晶体杜瓦苯的三重态激子使能量子链反应。这一链式过程有望在每个吸收的光子中产生大量的产物分子。总而言之，该项目的研究活动提高了对刚性材料中反应机理和化学反应的基本了解，并为学生提供了接受高度跨学科培训的机会。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.