Alankriti Bajpai

Single Crystal-to-Single Crystal Site-Selective Postsynthetic Metal Exchange in a Zn–MOF Based on Semi-Rigid Tricarboxylic Acid and Access to Bimetallic MOFs

The metal ions in a neutral Zn-MOF constructed from tritopic triacid H3 L with inherent concave features, rigid core, and peripheral flexibility are found to exist in two distinct SBUs, that is, 0D and 1D. This has allowed site-selective postsynthetic metal exchange (PSME) to be investigated and reactivities of the metal ions in two different environments in coordination polymers to be contrasted for the first time. Site-selective transmetalation of Zn ions in the discrete environment is shown to occur in a single crystal-to-single crystal (SCSC) fashion, with metal ions such as Fe(3+) , Ru(3+) , Cu(2+) , Co(2+) , etc., whereas those that are part of 1D SBU sustain structural integrity, leading to novel bimetallic MOFs, which are inaccessible by conventional approaches. To the best of our knowledge, site-selective postsynthetic exchange of an intraframework metal ion in a MOF that contains metal ions in discrete as well as polymeric SBUs is heretofore unprecedented.

A fluorescent paramagnetic Mn metal–organic framework based on semi-rigid pyrene tetra­carboxylic acid: sensing of solvent polarity and explosive nitroaromatics

Fluorescence quenching by paramagnetic metal ions is attenuated in an Mn metal–organic framework (Mn-MOF) based on a pyrene linker. Based on solvent-dependent emission, the Mn-MOF is shown to serve as a solvent polarity probe. Further, sensing of nitroaromatics is demonstrated with a detection limit of ∼125 p.p.m. for TNT.

Flue-gas and direct-air capture of CO2 by porous metal-organic materials.

Sequestration of CO2, either from gas mixtures or directly from air (direct air capture), is a technological goal important to large-scale industrial processes such as gas purification and the mitigation of carbon emissions. Previously, we investigated five porous materials, three porous metal–organic materials (MOMs), a benchmark inorganic material, Zeolite 13X and a chemisorbent, TEPA-SBA-15, for their ability to adsorb CO2 directly from air and from simulated flue-gas. In this contribution, a further 10 physisorbent materials that exhibit strong interactions with CO2 have been evaluated by temperature-programmed desorption for their potential utility in carbon capture applications: four hybrid ultramicroporous materials, SIFSIX-3-Cu, DICRO-3-Ni-i, SIFSIX-2-Cu-i and MOOFOUR-1-Ni; five microporous MOMs, DMOF-1, ZIF-8, MIL-101, UiO-66 and UiO-66-NH2; an ultramicroporous MOM, Ni-4-PyC. The performance of these MOMs was found to be negatively impacted by moisture. Overall, we demonstrate that the incorporation of strong electrostatics from inorganic moieties combined with ultramicropores offers improved CO2 capture performance from even moist gas mixtures but not enough to compete with chemisorbents. This article is part of the themed issue ‘Coordination polymers and metal–organic frameworks: materials by design’.

Highly Selective Separation of C2H2 from CO2 by a New Dichromate-Based Hybrid Ultramicroporous Material

A new hybrid ultramicroporous material, [Ni(1,4-di(pyridine-2-yl)benzene)2(Cr2O7)]n (DICRO-4-Ni-i), has been prepared and structurally characterized. Pure gas sorption isotherms and molecular modeling of sorbate-sorbent interactions imply strong selectivity for C2H2 over CO2 (SAC). Dynamic gas breakthrough coupled with temperature-programmed desorption experiments were conducted on DICRO-4-Ni-i and two other porous materials reported to exhibit high SAC, TIFSIX-2-Cu-i and MIL-100(Fe), using a C2H2/CO2/He (10:5:85) gas mixture. Whereas CO2/C2H2 coadsorption by MIL-100(Fe) mitigated the purity of trapped C2H2, negligible coadsorption and high SAC were observed for DICRO-4-Ni-i and TIFSIX-2-Cu-i.

Towards an understanding of the propensity for crystalline hydrate formation by molecular compounds

The propensity for crystalline hydrate formation by molecular compounds that are devoid of strong hydrogen-bond donors has been analyzed and rationalized through a Cambridge Structural Database (CSD) survey, systematic hydrate screening experiments and computational studies.

Self-assembly of sterically-rigidified 3-connecting benzenetribenzoic acid into (6,3) and (3,3) nets and stabilization of water channel in the crystal lattice

A rigid 3-connecting triacid MeBTB was designed and synthesized in the quest of guest inclusion in the pores of honeycomb network structures generated based on the acid dimer-mediated self-assembly. Crystallization of MeBTB is indeed found to lead to a (6,3) net that is 3-fold interpenetrated. Charge-assisted hydrogen bond-mediated self-assembly in the presence of KX/dibenzo-18-crown-6 is likewise found to lead to a (3,3) honeycomb net, which is also 3-fold interpenetrated. When contrasted with the results of self-assembly of sterically-unhindered 1,3,5-benzenetribenzoic acid 2 and those of analogous tribenzoic acid based on mesitylene, that is MTB, the sterics built into the structure of MeBTB allow engineering of ordered assemblies with reduced interpenetration and higher solvent-accessible volumes. A limited, yet meaningful number of structures demonstrates the fact that the rigid building blocks, while responding to the expectations based on aggregation via acid dimer synthon, are most likely to present rich diversity in terms of synthon adoptions and bring up surprises in the self-assembly through inclusion of adventitious water. Crystallization of MeBTB in MeOH–DME led to a disappearing solvate form in which the helically organized acids are found to sustain a water channel.

Readily accessible shape-memory effect in a porous interpenetrated coordination network

An interpenetrated flexible metal-organic material exhibits only the second example of a shape-memory effect in a porous material. Shape-memory effects are quite well-studied in general, but there is only one reported example in the context of porous materials. We report the second example of a porous coordination network that exhibits a sorbate-induced shape-memory effect and the first in which multiple sorbates, N2, CO2 and CO promote this effect. The material, a new threefold interpenetrated pcu network, [Zn2(4,4′-biphenyldicarboxylate)2(1,4-bis(4-pyridyl)benzene)]n (X-pcu-3-Zn-3i), exhibits three distinct phases: the as-synthesized α phase; a denser-activated β phase; and a shape-memory γ phase, which is intermediate in density between the α and β phases. The γ phase is kinetically stable over multiple adsorption/desorption cycles and only reverts to the β phase when heated at >400 K under vacuum. The α phase can be regenerated by soaking the γ phase in N,N′-dimethylformamide. Single-crystal x-ray crystallography studies of all three phases provide insight into the shape-memory phenomenon by revealing the nature of interactions between interpenetrated networks. The β and γ phases were further investigated by in situ coincidence powder x-ray diffraction, and their sorption isotherms were replicated by density functional theory calculations. Analysis of the structural information concerning the three phases of X-pcu-3-Zn-3i enabled us to understand structure-function relationships and propose crystal engineering principles for the design of more examples of shape-memory porous materials.

New Insights into an Old Problem. Fluorescence Quenching of Sterically-Graded Pyrenes by Tertiary Aliphatic Amines

Although the quenching of singlet-excited states of aromatic molecules by amines has been studied for several decades, important aspects of the mechanism(s) remain nebulous. To address some of the unknowns, steric, and electronic factors associated with the quenching of the singlet-excited states of three electronically related aromatic molecules, pyrene, 1,3,6,8-tetraphenylpyrene (TPPy), and 1,3,6,8-tetrakis(4-methoxy-2,6-dimethylphenyl)pyrene (PyOMe), by a wide range of tertiary aliphatic amines have been assessed quantitatively. Correlations among the steric and electronic properties of the amines and the pyrenes (e.g., sizes, shapes, conformational labilities, excitation energies, and oxidation or reduction potentials) have been used in conjunction with the steady-state and dynamic fluorescence quenching data and DFT calculations on the ground and excited state complexes to make quantitative assessments of the steric and electronic factors controlling the quenching processes. PyOMe is a rather rigid bowl-like molecule that, in its electronic ground state, does not make stable complexes with amines in solution. TPPy has a shallower bowl-like shape that is much more flexible. Experiments conducted with a crystalline ground-state complex of an amine and PyOMe demonstrate (as assumed in many other studies but not shown conclusively heretofore) that the geometry needed for quenching the excited singlet state of PyOMe must place the lone-pair of electrons of the amines over the π-system of the pyrenyl group. Furthermore, there is a significant dependence on the shape and size of the amine on its ability to quench PyOMe, but not on the less conformationally constrained TPPy. The conclusions obtained from these studies are clearly applicable to a wide variety of other systems in which fluorescence from an aromatic moiety is being quenched, and they provide insights into how weak host-guest pairs interact.

Tri- and tetraarylanthracenes with novel λ, χ and ψ topologies as blue-emissive and fluorescent host materials in organic light-emitting diodes (OLEDs)

Three unique λ-, χ- and ψ-shaped tri- and tetaarylanthracenes, i.e., ANT1–3, have been designed and synthesized by Suzuki coupling as a key reaction for application as emissive and host materials in OLEDs. The regioisomeric anthracenes ANT2 and ANT3 display intriguingly similar photophysical, electrochemical and thermal properties, while their lower analog, i.e., ANT1, exhibits different properties. All the three arylanthracenes show respectable thermal stabilities with high Tds in the range of 372–427 °C and moderate Tgs in the range of 90–114 °C. Their fluorescence quantum yields are in the range of 45–67%. Their utility in OLEDs has been demonstrated by fabrication of doped- and non-doped devices. In particular, ANT1 is shown to emit deep-blue light with moderate efficiencies and CIE coordinates that are very close to those demanded by the NTSC (National Television System Committee). It has also been shown to serve as a fluorescent host for sky-blue and green dopants, with the luminance being higher for the latter.

Influence of Cations on the Fluorescence Quenching of an Ionic, Sterically Congested Pyrenyl Moiety by Iodide in Water

Quenching of the excited singlet states of a water-soluble, sterically congested tetraarylpyrene, 1,3,6,8-tetrakis(2,6-dimethyl-4-(α-carboxy)methoxyphenyl)pyrene (Py4C), by a series of iodide salts has been investigated by steady-state and time-resolved fluorescence measurements. Access to the pyrenyl group of Py4C is restricted sterically as a result of the four flanking (2,6-dimethylphenoxy)acetic acid groups and the energy costs associated with their rotation. Deprotonation of the carboxylic acid groups of Py4C permits examination of ion-ion electrostatic interactions on the rates of quenching by iodide salts in which different steric and electrostatic factors are introduced by varying the cationic portions. At the same concentrations and with the same cations, chloride anions are ineffective quenchers. The quenching rate constants of Py4C by iodide are found to correlate linearly with the ionic radii of the cations and their enthalpies of hydration. These correlations are discussed in terms of the Hofmeister series. Furthermore, the results indicate that the cations that flank Py4C decrease the quenching efficiency of iodide through polarization and shielding effects (i.e., lowering the effective charge), which isolate to varying degrees the π-system. The effects of the different cations on quenching the fluorescence of a simpler and sterically unencumbered pyrenyl derivative, 1-pyrenylbutyric acid (PyBu), by iodide are much smaller. Overall, the results with Py4C indicate that the fluorescence quenching efficiency by iodide is influenced by direct interactions with the cations associated with the carboxylate groups of Py4C and not the solvation of water molecules. This observation is germane to a topic of current debate: Are the effects of the cations more closely related to bulk water properties or to direct ion-ion interactions? The conclusions obtained from these studies are applicable clearly to a wide variety of other systems in which ion pairing influences cooperative or inhibitory interactions.