Stable organic radicals are rare and limited to specific structural families that have important applications as molecular magnets, probes in biological systems, polarizing agents and radical initiators for polymerization. Self-assembly offers a potential strategy for stabilizing organic radicals outside the known structural classes. Herein, we examine the effect of assembly on the photophysics of benzophenone, a well-known triplet sensitizer. Typically, upon excitation, benzophenone undergoes rapid intersystem crossing to its triplet state, which can abstract nearby hydrogens to afford reactive ketyl radicals that recombine, creating covalent C-C bonds. Benzophenone containing <i>bis</i>-urea macrocycles and urea-tethered benzophenone linear analogues were synthesized and their crystal structures determined. These crystalline materials control the distinct microenvironments around the benzophenone carbonyl group. Upon UV-irradiation, they display remarkably persistent triplet radical pairs. In contrast, UV-irradiation of these materials in solution shows no radical formation. We investigate the factors that lead to emergent organic radicals by comparing the microenvironment around the benzophenone carbonyl, the type of proximal hydrogens, their estimated bond dissociation energies, and the rigid supramolecular network. These endogenous photogenerated radicals can employed as polarizing agents for magic angle spinning (MAS) dynamic nuclear polarization (DNP) NMR enhancement. DNP enhancement is observed over a broad field range &gt;120 mT and the materials are easily recovered and reused, highlighting the advantages of using these materials over traditional polarizing agents. Finally, we explore the use of these materials for photooxidations in the solid state and as suspended catalysts in solution.

Enhancing the stability of photogenerated benzophenone triplet radical pairs through supramolecular assembly

2019 Spring National Meeting

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