Enhancing Dynamic Spectral Diffusion in Metal-Organic Frameworks through Defect Engineering.

The crystal packing of organic chromophores has a profound impact on their photophysical properties. Molecular crystal engineering is generally incapable of producing precisely spaced arrays of molecules for use in photovoltaics, light-emitting diodes, and sensors. A promising alternative strategy is the incorporation of chromophores into crystalline metal-organic frameworks (MOFs), leading to matrix coordination-induced emission (MCIE) upon confinement. However, it remains unclear how the precise arrangement of chromophores and defects dictates photophysical properties in these systems, limiting the rational design of well-defined photoluminescent materials. Herein, we report new, robust Zr-based MOFs constructed from the linker tetrakis(4-carboxyphenyl)ethylene (TCPE4-) that exhibit an unexpected structural transition in combination with a prominent shift from green to blue photoluminescence (PL) as a function of the amount of acid modulator (benzoic, formic, or acetic acid) used during synthesis. Time-resolved PL (TRPL) measurements provide full spectral information and reveal that the observed hypsochromic shift arises due to a higher concentration of linker substitution defects at higher modulator concentrations, leading to broader excitation transfer-induced spectral diffusion. Spectral diffusion of this type has not been reported in a MOF to date, and its observation provides structural information that is otherwise unobtainable using traditional crystallographic techniques. Our findings suggest that defects have a profound impact on the photophysical properties of MOFs and that their presence can be readily tuned to modify energy transfer processes within these materials.

Kinetic Trapping of Photoluminescent Frameworks During High-Concentration Synthesis of Non-Emissive Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) are porous, crystalline materials constructed from organic linkers and inorganic nodes with potential utility in gas separations, drug delivery, sensing, and catalysis. Small variations in MOF synthesis conditions can lead to a range of accessible frameworks with divergent chemical or photophysical properties. New methods to controllably access phases with tailored properties would broaden the scope of MOFs that can be reliably prepared for specific applications. Herein, we demonstrate that simply increasing the reaction concentration during the solvothermal synthesis of M2(dobdc) (M = Mg, Mn, Ni; dobdc4- = 2,5-dioxido-1,4-benzenedicarboxylate) MOFs unexpectedly leads to trapping of a new framework termed CORN-MOF-1 (CORN = Cornell University) instead. In-depth spectroscopic, crystallographic, and computational studies support that CORN-MOF-1 has a similar structure to M2(dobdc) but with partially protonated linkers and charge-balancing or coordinated formate groups in the pores. The resultant variation in linker spacings causes CORN-MOF-1 (Mg) to be strongly photoluminescent in the solid state, whereas H4dobdc and Mg2(dobdc) are weakly emissive due to excimer formation. In-depth photophysical studies suggest that CORN-MOF-1 (Mg) is the first MOF based on the H2dobdc2- linker that likely does not emit via an excited state intramolecular proton transfer (ESIPT) pathway. In addition, CORN-MOF-1 variants can be converted into high-quality samples of the thermodynamic M2(dobdc) phases by heating in N,N-dimethylformamide (DMF). Overall, our findings support that high-concentration synthesis provides a straightforward method to identify new MOFs with properties distinct from known materials and to produce highly porous samples of MOFs, paving the way for the discovery and gram-scale synthesis of framework materials.