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

Abstract

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; dobdc^4- = 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 H₂dobdc^2- 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.

Figure 1.

Standard synthesis of M₂(dobdc) under dilute solvothermal conditions (concentration of linker shown). R = alkyl, H.

Figure 2.

a) Synthesis of Mg₂(dobdc) and/or CORN-MOF-1 (Mg) at varying linker concentration. b) PXRD patterns at increasing reaction concentrations. Transition from Mg₂(dobdc) (dashed gray lines) to CORN-MOF-1 (Mg) (dashed pink lines) is indicated. The pattern corresponding to the previously reported SCXRD structure of Zn₂(dobdc) is included for reference (black). c) SEM image of CORN-MOF-1 (Mg) synthesized at 0.15 M with stirring. d) SEM image of Mg₂(dobdc) synthesized at 0.01 M without stirring.

Figure 3.

Pawley refinement of the PXRD pattern (λ = 1.5406 Å) of CORN-MOF-1 (Mg). The shown fit corresponds to the $R\overline{3}$ space group with a = 27.612(2) Å and c = 7.2853(3) Å. R_wp = 3.85%. The black ticks indicate calculated Bragg peak positions.

Figure 4.

a) ATR-IR spectra of Mg₂(dobdc) and CORN-MOF-1 (Mg). b) 77 K N₂ adsorption (closed circles) and desorption (open circles) isotherms of activated CORN-MOF-1 (Mg). Inset: DFT-calculated pore size distribution of CORN-MOF-1 (Mg), assuming a cylindrical pore shape with a metal oxide surface. c) ¹H and d) CP ¹³C MAS SSNMR spectra of Mg₂(dobdc) and CORN-MOF-1 (Mg). All SSNMR data were collected at a field strength of 9.4 T and a MAS rate of 25 kHz.

Figure 5.

a) Visual comparison of H₄dobdc (left), CORN-MOF-1 (Mg) (center), and Mg₂(dobdc) (right) upon irradiation with UV light (230 nm), confirming the uniquely strong solid-state photoluminescence of CORN-MOF-1 (Mg). b) Relative PL intensities of CORN-MOF-1 (Mg), Mg₂(dobdc), and H₄dobdc powder.

Figure 6.

a) Normalized, solvent-dependent PL spectra for H₄dobdc (0.1 mM). Normalized, solvent-dependent PL spectra for suspensions (1 mg/mL) of b) Mg₂(dobdc) and c) CORN-MOF-1 (Mg). d) Spectral slices of the H₄dobdc powder time-resolved PL capturing prompt emission (0 ns) and delayed excimer emission (5 ns) as a function of temperature. e) Equivalent spectral slices of the Mg₂(dobdc) time-resolved PL capturing prompt (0 ns) and delayed excimer (3 ns) emission. Temperature-dependent spectra for H₄dobdc and Mg₂(dobdc) are normalized to the prompt PL signal to demonstrate how the excimer PL intensity increases at lower temperature due to the suppressed nonradiative decay. Time zeroes were set at the point of maximum PL intensity, and gate widths of 1 ns were used. f) Normalized, temperature-dependent steady state PL of CORN-MOF-1 (Mg). THF = tetrahydrofuran, iPrOH = isopropanol, rt = room temperature (295 K).

Figure 7.

a) PL decay curves for 0.1 mM H₄dobdc in THF (dark green triangles), CORN-MOF-1 (Mg) powder (blue circles), H₄dobdc powder (light green triangles), and Mg₂(dobdc) powder (red squares). Decay curves were extracted by integrating over the entire PL spectrum. b) Spectral evolution of H₄dobdc powder within the first 3 ns. c) Spectral evolution of Mg₂(dobdc) within the first 1.5 ns. The excimer formation is near the 1 ns resolution of the ICCD. d) Spectral evolution of CORN-MOF-1 (Mg) within the first 25 ns.

Figure 8.

a) Synthesis of Mg, Mn and Ni analogues of CORN-MOF-1, and M₂(dobdc) converted from the corresponding CORN-MOF-1 analogues. b) PXRD patterns of CORN-MOF-1 (Mg, Mn and Ni) and the converted M₂(dobdc) (M = Mg, Mn and Ni) analogues. SEM images of the conversion of CORN-MOF-1 (Mg) hexagonal plates to Mg₂(dobdc) rods by heating in DMF at 150 °C after c) 0 h, d) 24 h, e) 72 h, f) 120 h. R = alkyl, H.