Solid-State NMR Unveils Analyte-Induced Structural Change in Luminescent MOF for Chemical Sensing

Understanding analyte-induced structural changes in luminescent metal–organic frameworks (L-MOFs) is critical for the rational design of guest-responsive materials in sensing applications. Here, we employ multinuclear solid-state NMR (SSNMR) spectroscopy─including 13C, 31P, 63Cu, and 31P–63Cu correlation experiments─to elucidate the mechanism underlying photoluminescence (PL) enhancement in the Cu4I4-based L-MOF, [Cu4I4 (Py3P)2]n, upon exposure to volatile organic compounds (VOCs). Although powder X-ray diffraction (PXRD) detects no apparent geometric changes, 31P–63Cu J-resolved correlation spectroscopy reveals a shift in the relative orientation between the 31P chemical shift anisotropy (CSA) and 63Cu electric field gradient (EFG) tensors, indicating a guest-induced reorganization of the local coordination environment. Additional 1H–13C local-field measurements show no change in ligand dynamics, supporting the conclusion that the PL enhancement originates from a change in coordination environment rather than suppression of ligand dynamics. This work illustrates that the measurements of orientation-dependent SSNMR parameters can probe subtle, otherwise undetectable structural changes that govern the optoelectronic response of guest-responsive materials. The approach is broadly applicable for probing structural changes in MOFs responding to external stimuli and will help in the rational design of stimuli-responsive materials.