Minimal π-Electronic Systems-Photoantenna Unlocking Dual Emission in Zeolite-like MOFs with Lanthanides

Abstract π-Conjugated organic photoantenna ligands, such as bipyridine, are widely used to sensitize the luminescence of photoluminescent lanthanide ions (e.g., Eu3+ and Tb3+), whereas the photoantenna potential of minimal π-electronic systems remains largely unexplored. Here we report lanthanide metal-organic frameworks (MOFs, Ln = Eu, Gd and Tb) constructed from isonicotinic acid (NA), one of the smallest π-electronic system ligands. Single crystal X-ray diffraction analysis reveals tetranuclear Ln4O4 clusters with two distinct NA coordination modes and forms the zeolite-like MOFs. Photophysical studies show that NA functions as an effective photoantenna, giving rise to excitation-wavelength-dependent ligand-centered emission in GdNA and efficient energy transfer to lanthanide ions in EuNA and TbNA. In particular, EuNA exhibits excitation-dependent coexistence of ligand-centered and lanthanide-centered emissions, arising from coordination-mode-dependent energy relaxation pathways. These findings demonstrate that minimal π-electronic system ligands can provide versatile luminescence control in lanthanide MOFs.