Identification of Phase Changes in a Model 2D MOF through Polarization-Dependent Raman Spectroscopy

Metal-organic frameworks (MOFs) are molecular lattices with highly tunable structures and correspondingly diverse properties. However, distinguishing MOFs that contain the same metal nodes and ligands but different connectivities is nontrivial, especially when the MOF samples approach the monolayer limit. Here we show that polarization-dependent Raman spectroscopy can be used to distinguish between two closely related structural motifs of a model MOF comprised of Mo₂(isonicotinate)₄ clusters. We find that certain Raman modes active under cross-polarization become more intense in the partially under-coordinated (nominally 1D) phase of the normally 2D MOF, reflecting the character of connectivity in the MOFs rather than the formal symmetry of their unit cells. This polarization-dependent change in intensity is supported by density functional theory (DFT) calculations of vibrational modes in the two phases. We anticipate polarization-resolved Raman spectroscopy will be a useful tool for identification of MOF structures, particularly for samples for which conventional structure determination is difficult or untenable.