Novel Cation Radical and Exciplex Chemistry

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With this award, the Chemical Structure, Dynamics, and Mechanism B Program is supporting Professor Joseph P. Dinnocenzo at the University of Rochester to investigate the selective formation of highly reactive aryl radicals under mild conditions. The mechanistic insight gained from these studies can provide the knowledge necessary to rationally utilize the chemistry in synthesis and in the design of new materials. In addition, research on newly discovered cationic exciplexes and exciplexes that exhibit dual emissions will provide knowledge on these novel excited state intermediates. Research to better understand the fundamental photophysical properties these exciplexes intermediates will provide valuable knowledge for the design of efficient photoinduced electron transfer reactions and for potential use in sensing applications. Student co-workers will learn to carry out chemical syntheis and to conduct mechanistic studies in photochemistry. including a range of less traditional techniques (fluorometry, transient absorption spectroscopy, time-correlated single photon counting, electrochemistry, and modern computational methods). Thus, students trained with NSF support will be well prepared to tackle a variety of multidisciplinary problems in their professional, scientific careers.This project will also benefit undergraduate students through the NSF REU summer program at the University of Rochester with particular attention paid to recruiting students from groups traditionally underrepresented in science and engineering.Recent work in the Dinnocenzo laboratory has revealed that aryltrialkylstannane cation radicals undergo nucleophile-assisted fragmentation reactions in which less stable aryl radicals are often formed in preference to more stable alkyl radicals - an seemingly contrathermodynamic result. This selectivity is unprecedented in ion radical chemistry. Proposed research will test the generality of these remarkable results by exploring the fragmentation reactions of aryltrialkyl-germane and -silane cation radicals through the combined use of synthesis, steady-state photooxidation experiments, and nanosecond transient absorption spectroscopy. In addition, two types of novel, charge-transfer exciplex intermediates recently discovered in the Dinnocenzo laboratory will be investigated. One type of exciplex is formed by reaction of cationic, excited state acceptors with neutral donors, which leads to the formation of emissive cationic exciplexes that are without precedent. Research objectives include determining the electronic nature of the exciplexes (e.g. electronic coupling matrix elements) through theoretical fitting of the radiative lifetimes of the exciplexes (determined by time-correlated single photon counting) and their fluorescence spectra, exploring the generality of cationic exciplexes, and measuring some of their photophysical properties. A related project will investigate examples uncovered in the PI's lab where two exciplex emissions are observed from a single acceptor and donor. Preliminary results indicate dramatically different solvent reorganization energies for the two types of exciplexes. Proposed research will determine the electronic properties of the exciplexes and explore their generality.