Acid modulation is among the most widely employed methods for preparing metal–organic frameworks (MOFs) that are both stable and highly crystalline, yet there exist few guiding principles for selecting the optimal modulator for a given system. Using the Zr-based MOFs UiO-66 and UiO-68-Me2 (UiO = Universitetet i Oslo) as representative materials, here we present for the first time an in-depth structure–activity study of acid modulators and identify key principles of modulation for the synthesis of highly crystalline Zr-MOFs. By applying whole pattern fitting of powder X-ray diffraction patterns as a technique for evaluating modulator efficacy, complemented by scanning electron microscopy, 1H NMR, and thermogravimetric analysis (TGA), we demonstrate that the key to effective modulation is competition between the linker and modulator for coordination to the Zr secondary building units (SBUs). Specifically, we illustrate that a close match in pK a and structure between the linker and modulator favors larger and more well-defined crystallites, particularly with sterically unhindered aromatic acid modulators. Based on our findings, we demonstrate that 5-membered heteroaromatic carboxylic acids are among the most efficient acid modulators identified to date for the synthesis of several representative Zr-MOFs with fcu net topologies. In addition, we find that coordination modulation is superior to exogenous acid modulation at higher modulator concentrations. Finally, we compare 1H NMR and TGA as data-driven methods for quantifying linker deficiencies in modulated MOF syntheses. The guiding principles established herein have critical implications for the scalable and controllable synthesis of highly crystalline and stable MOFs relevant to chemical separations, gas storage, and catalysis.

Conjugated microporous polymers (CMPs) are porous organic materials that display (semi)conducting behavior due to their highly π-conjugated structures. As such, they are promising next-generation materials for applications requiring both conductivity and porosity, such as supercapacitive energy storage and electrochemical sensing. However, most CMPs and related porous aromatic frameworks (PAFs) are currently prepared using expensive transition metal-based catalysts under solvothermal conditions, significantly increasing their manufacturing costs. Herein, we demonstrate that the ionothermal cyclotrimerization of methyl ketones via the aldol reaction represents a new strategy for the solvent-free synthesis of CMPs and PAFs. Specifically, we show that 1,3,5-triacetylbenzene and tetrakis(4-acetylphenyl)methane can be polymerized in molten zinc chloride to produce highly conjugated and microporous materials, as confirmed by 77 K N 2 adsorption measurements in conjunction with UV−vis, Raman, and solid-state NMR spectroscopies. The CMP prepared from 1,3,5-triacetylbenzene demonstrates higher charge storage capacities (up to 172 F/g) than a commercially available supercapacitor carbon, reflecting the promise of cyclotrimerized CMPs for electrical energy storage applications.

A centimeter-sized, laser-induced phase-separated (LIPS) solution droplet, which was formed by tightly focusing a continuous-wave near-infrared laser beam at the glass/solution interface of a millimeter-thick layer of glycine in D2O with a supersaturation ratio, S, of 1.36 was irradiated with a single unfocused nanosecond near-infrared laser pulse in order to study the effect of non-photochemical laser-induced nucleation (NPLIN) on the droplet, as well as to help characterize the behavior of the LIPS droplet. Additionally, a control NPLIN experiment was conducted on an S = 1.50 supersaturated solution of glycine/D2O in the same cell to better understand the differences between NPLIN in a LIPS droplet and an ordinary supersaturated solution. These experiments revealed that NPLIN could nucleate crystals within a LIPS droplet, although the growth of these crystals was inhibited during the first 5 min of the droplet’s relaxation. For the first 40 min of its relaxation, the LIPS droplet was observed to be more labile to spontaneous nucleation than the control S = 1.50 solution, although the growth of spontaneously nucleated crystals was also inhibited during the first 5 min of the droplet’s relaxation. This suggests that although the macroscopic phase boundary between the LIPS droplet and the surrounding solution disappeared after approximately 5 min, the full microscopic relaxation of the LIPS droplet took at least 40 min. The resulting crystals were analyzed using powder X-ray diffraction, and 100% of crystals formed within the LIPS droplet induced by NPLIN with linearly polarized light and by spontaneous nucleation were α-glycine, while crystals formed outside of the LIPS droplet were mixtures of α- and γ-glycine. The results suggest that the LIPS droplet and the surrounding solution are not equilibrium phases of aqueous glycine, but phases in which optical gradient forces have induced a partitioning of large and small solute clusters.

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 (TCPE 4− ) 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.

Metal–organic frameworks enable the delivery of hydrogen sulfide (H2S), an endogenous gasotransmitter with potential therapeutic value for treating disorders such as ischemia-reperfusion injury.