Non-centrosymmetric Metal-organic Frameworks Research Articles

Structure-Property Relationship of Piezoelectric Properties in Zeolitic Imidazolate Frameworks: A Computational Study.

Metal-organic frameworks (MOFs) are a class of nanoporous crystalline materials with very high structural tunability. They possess a very low dielectric permittivity εr due to their porosity and hence are favorable for piezoelectric energy harvesting. Even though they have huge potential as piezoelectric materials, a detailed analysis and structure-property relationship of the piezoelectric properties in MOFs are lacking so far. This work focuses on a class of cubic non-centrosymmetric MOFs, namely, zeolitic imidazolate frameworks (ZIFs) to rationalize how the variation of different building blocks of the structure, that is, metal node and linker substituents affect the piezoelectric constants. The piezoelectric tensor for the ZIFs is computed from ab initio theoretical methods. From the calculations, we analyze the different contributions to the final piezoelectric constant d14, namely, the clamped ion (e140) and the internal strain (e14int) contributions and the mechanical properties. For the studied ZIFs, even though e14 (e140 + e14int) is similar for all ZIFs, the resultant piezoelectric coefficient d14 calculated from piezoelectric constant e14 and elastic compliance constant s44 varies significantly among the different structures. It is the largest for CdIF-1 (Cd2+ and -CH3 linker substituent). This is mainly due to the higher elasticity or flexibility of the framework. Interestingly, the magnitude of d14 for CdIF-1 is higher than II-VI inorganic piezoelectrics and of a similar magnitude as the quintessential piezoelectric polymer polyvinylidene fluoride.

The Power of Heterometalation through Lithium for Helix Chain-Based Noncentrosymmetric Metal–Organic Frameworks with Tunable Second-Harmonic Generation Effects

We demonstrate herein a general strategy for developing acentric or chiral nonlinear optical MOFs from centrosymmetric ligands by shifting the asymmetrical roles to the metal centers through heterometalation. With 1,3,5-benzenetricarboxylic acid (H3BTC), a very common polyfunctional centric ligand, the Li+/Zn2+, Li+/Co2+, or Li+/Cd2+ heterometallic combinations successfully led to five new noncentrosymmetric metal–organic frameworks, namely, {[Zn(HBTC)(H2O)]}n (SNNU-71), {Li4[Cd2(HBTC)(BTC)2]}n (SNNU-72), {CoLi(BTC)(DMA)2}n (SNNU-73), {CdLi2(BTC)4/3(H2O)3}n (SNNU-74), and {Li2[Zn3Li5(BTC)4(MTAZ)(H2O)4]}n (MTAZ = 5-methyl tetrazole, SNNU-75). All these MOFs are acentric or chiral on the base of helical chain building blocks and four colorless members of them show remarkable and tunable SHG effects. Remarkably, more than 1000 metal-BTC frameworks are reported to date and most of them are centrosymmetric due to the C3-symmery of BTC ligands. Clearly, our heterometalation approach greatly increases the possib...

Fructose calcium chloride MOFs for NLO applications

Non-centrosymmetric Metal Organic Frameworks (MOF) based on coordination of carbohydrates on alkaline metal ion represent an ideal category of compounds to plan functional materials with interesting second-order non-linear optics properties. Two MOFs obtained from fructose and calcium chloride, [Ca(C6H12O6)(H2O)2]Cl2 (1) and [Ca(C6H12O6)2(H2O)2]Cl2·H2O (2), have been synthetized in ethanol and by solid-solid interactions and have been characterized with X-ray diffraction and IR and RAMAN spectroscopy. The two compounds have also been studied with DFT theoretical calculations to investigate their relative stability and foresee their optical properties. Finally, the second harmonic generation (SHG) properties have been evaluated by SHG microscopy. It has been demonstrated that the coordination of fructose on the calcium ion causes an improvement of the SHG intensity with respect to the fructose itself and that there is a strictly correlation between the calculated dipole moment and the first static hyperpolarizability values and intensity of the SHG signal. Furthermore, the remarkable difference in the SHG intensity of the two compounds studied, suggests that this physical property can be "controlled" not only by tuning the composition of the system, but also by the disposition of the MOF's building units in the crystal. The metal-carbohydrate based MOFs analyzed in this work have a favorable combination of thermal and chemical stability, transparency, and second-order optical non-linearity and are thus potential candidates for applications in electro-optic devices.

Solvent-directed synthesis of chiral and non-centrosymmetric metal–organic frameworks based on pyridine-3,5-dicarboxylate

Three new metal–organic frameworks Mn(3,5-PDC)(H2O)•(glycol) (JUC-58), M(3,5-PDC)(H2O)(glycol)•(H2O) (JUC-59-M, M=Co, Ni), have been synthesized by the assembly of tri-connected ligand pyridine-3,5-dicarboxylic acid (3,5-PDC) and different metals under the solvothermal condition in DMF and glycol. JUC-58 is an unusual example of 3-D chiral MOF with helical channels, while JUC-59-Co and JUC-59-Ni are isostructural and have non-centrosymmetric structures with a very rare 3-connected non-interpenetrated lig-b net. It is worth noting that glycol could play an important role in the formation and stabilization of these networks.

Rational Synthesis of Noncentrosymmetric Metal–Organic Frameworks for Second-Order Nonlinear Optics

The term nonlinear optics (NLO) was coined to describe the nonlinear relationship between dielectric polarization P and electric field E in optical media. NLO is a cornerstone of the emerging field of photonics, in which photons instead of electrons are used for signal transmission and processing. The vision of photonic signal transmission, processing, and storage has attracted a great deal of attention from both the engineering and the scientific communities because of its great impact in many of the existing and future information technologies. The first step toward realization of these revolutionary technologies is to develop tools to manipulate photons. For example, it is desirable to develop materials with the ability to alter the frequency of light, to amplify light signal, and to modulate light intensity or phase factors. NLO phenomena can be the key to achieving these important functions. One of the most common NLO behaviors is second-harmonic generation (SHG), in which a NLO material mediates the “adding-up” of two photons to form a new one with twice the frequency. The SHGphenomenonwas first demonstrated by Franken et al. in 1961. In their pioneering work, a laser beam with a wavelength of 694.2 nm was irradiated through a quartz crystal and an output ultraviolet radiation with a wavelength of 347.1 nm (double frequency) was detected. After this discovery, numerous nonlinear optical phenomena have been studied and a number of NLO-active materials have been developed. Second-harmonic generation can be quantitatively described by the second-order nonlinear optical susceptibility χ, a third-rank tensor with 27 components. The tensor elements are related to each other tomeet the requirements of both inherent and structural symmetries, which greatly reduces the number of independent components of the susceptibility tensor. Only crystals in noncentrosymmetric crystal classes can have nonvanishing χ. Moreover, for material crystallizing in the noncentrosymmetric 422, 622, and 432 crystal classes, the second-order NLO response might also vanish due to structural symmetry as well as Kleinman’s symmetry. Many inorganic compounds crystallize in noncentrosymmetric space groups and have been found to be SHG active. Some important examples are potassium dihydrogen phosphate (KDP = KH2PO4), lithium niobate (LiNbO3), and barium sodium niobate (Ba2NaNb5O15). 7 New inorganic compounds have been explored for NLO applications including but not limited to metal borates 12 and metal oxides. Recent structural studies on the inorganic systems have led to a better understanding of crystal growth/packing, paving the way for potentially manipulating their crystallization tendency to form noncentrosymmetric structures. Since the 1970s molecular NLO materials, including organic, organometallic, and inorganic complexes, have been of increasing interest to synthetic chemists. 19 The existing library of organic compounds was first screened, and the urea crystal has become a SHG standard because of its high SHG efficiency and usual availability. In a microscopic view, the second-order NLO susceptibility χ is related to the first hyperpolarizability β of a molecule. According to the classical two-level model, β is enhanced by a large transition moment and a large dipole moment difference between the ground and the charge transfer excited state. A donor acceptor type of molecule often possesses both a large transition moment and a large excited state dipole moment. As a result, most of the organic SHG chromophors belong to this category. However, most of the molecules with large β values also possess a large dipole moment, which induces formation of centrosymmetric assemblies of molecules due to dipole dipole interactions. One of the methods to avoid the centrosymmetric alignment of molecular dipoles is to trap them inside the channels of asymmetric porous host structures. 28 Other methods include formation of poled polymers in which the required asymmetry is imposed by the external electric field 35 and the Langmuir Blodgett (LB)