Probing photochemically-induced dynamic transitions by magic-angle-spinning NMR combined with in-situ irradiation

Abstract

Light is often the stimulus of choice for dynamically changing structure and function in materials and other contexts, including not only many smart materials and photovoltaics, but also biological and biochemical processes. Proper understanding of the associated mechanisms requires insights into the underlying dynamic molecular and (photo)chemical transformations. Nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for investigating these types of processes, providing unique chemical, structural, and dynamic information at the atomic level. Yet, while liquid-state NMR is increasingly regularly coupled with in-situ irradiation, it is limited to soluble samples. Here we report on the application of in-situ illumination to magic angle spinning (MAS) solid-state NMR (ssNMR). An in-situ irradiation setup adapted to a customized MAS probe enabled sample illumination with high efficiency. We describe its technical features and illustrate its capabilities through representative case studies. These include photoresponsive azobenzene derivatives in solution and sequestered in hydrogels, a chemical actinometer in solution and UV-polymerization of vitrimer-like elastomers. We show how different ssNMR polarization transfer techniques can enable the study of dynamic transitions that accompany light-triggered (dis)assembly and cross-linking processes. We report on the efficient use of not only sapphire but also thin-wall zirconia rotors, in an approach compatible with the widely used Bruker-type MAS ssNMR instrumentation. We envision that these methods will enable diverse new research directions on photoresponsive materials, but also studies on photopharmacology, optogenetics, photocatalysis, and other light-driven systems.