Exercising fine control over the synthesis of metal–organic frameworks (MOFs) is key to ensuring reproducibility of physical properties such as crystallinity, particle size, morphology, porosity, defectivity, and surface chemistry. The principle of modulated self-assembly – incorporation of modulator molecules into synthetic mixtures – has emerged as the primary means to this end. This perspective article will detail the development of modulated synthesis, focusing primarily on coordination modulation, from a technique initially intended to cap the growth of MOF crystals to one that is now used regularly to enhance crystallinity, control particle size, induce defectivity and select specific phases. The various mechanistic driving forces will be discussed, as well as the influence of modulation on physical properties and how this can facilitate potential applications. Modulation is also increasingly being used to exert kinetic control over self-assembly; examples of phase selection and the development of new protocols to induce this will be provided. Finally, the application of modulated self-assembly to alternative materials will be discussed, and future perspectives on the area given.

https://pubs.rsc.org/en/content/articlepdf/2020/SC/D0SC01356K

Modulated self-assembly of metal–organic frameworks

Applications of reticular diversity in metal–organic frameworks: An ever-evolving state of the art

Synthesis, modifications and applications of MILs Metal-organic frameworks for environmental remediation: The cutting-edge review

Dynamic nuclear polarization utilizing photoexcited triplet electrons (triplet-DNP) has great potential for room-temperature hyperpolarization of nuclear spins. However, the polarization transfer to molecules of interest remains a challenge due to the fast spin relaxation and weak interaction with target molecules at room temperature in conventional host materials. Here, we demonstrate the first example of DNP of guest molecules in a porous material at around room temperature by utilizing the induced-fit-type structural transformation of a crystalline yet flexible metal-organic framework (MOF). In contrast to the usual hosts, 1 H spin-lattice relaxation time becomes longer by accommodating a pharmaceutical model target 5-fluorouracil as the flexible MOF changes its structure upon guest accommodation to maximize the host-guest interactions. Combined with triplet-DNP and cross-polarization (CP), this system realizes an enhanced 19 F NMR signal of guest target molecules. 

Triplet Dynamic Nuclear Polarization of Guest Molecules through Induced Fit in a Flexible Metal–Organic Framework**

Dynamic nuclear polarization utilizing photoexcited triplet electrons (triplet-DNP) has great potential for room-temperature hyperpolarization of nuclear spins. However, the polarization transfer to molecules of interest remains a challenge due to the fast spin relaxation and weak interaction with target molecules at room temperature in conventional host materials. Here, we demonstrate the first example of DNP of guest molecules in a porous material at around room temperature by utilizing the induced-fit-type structural transformation of a crystalline yet flexible metal–organic framework (MOF). In contrast to the usual hosts, 1H spin-lattice relaxation time becomes longer by accommodating a pharmaceutical model target 5-fluorouracil as the flexible MOF changes its structure upon guest accommodation to maximize the host–guest interactions. Combined with triplet-DNP and cross-polarization (CP), this system realizes an enhanced 19F NMR signal of guest target molecules. 

https://rss.onlinelibrary.wiley.com/doi/am-pdf/10.1002/ange.202115792

A modulated synthesis approach based on the chelating properties of oxalic acid (H2 C2 O4 ) is presented as a robust and versatile method to achieve highly crystalline Al-based metal-organic frameworks. A comparative study on this method and the already established modulation by hydrofluoric acid was conducted using MIL-53 as test system. The superior performance of oxalic acid modulation in terms of crystallinity and absence of undesired impurities is explained by assessing the coordination modes of the two modulators and the structural features of the product. The validity of our approach was confirmed for a diverse set of Al-MOFs, namely X-MIL-53 (X=OH, CH3 O, Br, NO2 ), CAU-10, MIL-69, and Al(OH)ndc (ndc=1,4-naphtalenedicarboxylate), highlighting the potential benefits of extending the use of this modulator to other coordination materials.

Maria del Carmen Martin

Papers

Overcoming Crystallinity Limitations of Aluminium Metal-Organic Frameworks by Oxalic Acid Modulated Synthesis.