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

Light is often the stimulus of choice for dynamically changing function in materials, from controlling phase transitions and dynamic chemistry to in-situ polymerizations, crosslinking, and 3D printing. The high temporal and spatial control over irradiation underpins the function of photovoltaics, smart materials, biological systems, and optogenetics research. Proper understanding of the associated mechanisms requires insights into the dynamic molecular and (photo)chemical transformations. NMR spectroscopy is a powerful tool for investigating such processes, providing unique information at the atomic level, especially if light can be delivered directly during the NMR experiment. While liquid-state NMR is increasingly regularly coupled with in-situ irradiation, it is limited to soluble sample states. 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 with relevance to different segments of photochemistry and materials science. 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 and many other types of samples, where liquid state NMR is not applicable due to their viscous or solid nature.