Visualizing molecular diffusion direction and processes in the solid state via dichromatic fluorescent cocrystalization transformation

Solid-state molecular motion (SSMM) plays a critical role in enriching material properties and functions, yet visualizing its detailed dynamics including direction, progression, and velocity to thoroughly elucidate its mechanism remains a great challenge. Herein, an intermolecular charge transfer (ICT)-mediated fluorescence strategy is developed to visualize and regulate SSMM in binary cocrystal systems, utilizing 6-methoxy-2-acetylnaphthalene (MA)/1,2,4,5-tetracyanobenzene (TCNB) as modeling platform. This approach leverages the reversible transformation between MA/TCNB = 1/1 (MT1, yellow emission) and MA/TCNB = 1/2 (MT2, green emission) crystalline phases, enabling real-time visualization of detailed SSMM information via localized dichromatic fluorescence signatures. It reveals an unexpected significant disparity in diffusion rates between electron donor MA and electron acceptor TCNB during the diffusion process at their interface, even exhibiting unidirectional diffusion from MA to TCNB within a restricted time. Applications demonstrated include pretreatment-free impurity analysis of MA (a key impurity in naproxen) down to 0.1% leveraging the fluorescence shift, and real-time monitoring of transesterification reactions based on the differing interaction capabilities of various naproxen esterification derivatives with TCNB. This study not only provides fundamental insights into SSMM and cocrystallization mechanisms but also demonstrates the potential of charge transfer cocrystallization for molecular sensing and dynamic reaction monitoring.