Noncentrosymmetric MOFs

Keywords Diamondoid net Metal-organic framework Noncentrosymmetric MOFs Nonlinear optics Octupolar symmetry Push-pull effect Second harmonic generation... 

Some other naturally chiral carboxylate ligands were also selected to construct chiral MOFs. For example, assemblies of Zn " and Cd " with o-camphoric acid in combination with some semi-rigid N-donor auxiliary ligands, such as 1,3- and l,4-bis(imidazol-l-ylmethyl)benzene and 4,4 -dipyridylsulfide, result in the formation of chiral MOFs [M(l,3-bimb)(D-ca)] (M = Zn, Cd), [Zn(l,4-bimb)(D-ca)] and a noncentrosymmetric MOF [Cd2(dpys)(D-ca)2(H20)2] H2O , in which the ID Zn (Cd )/D-ca chains are linked by N-donor ligands to form either a 2D network or a ladder-like structure [21]. Preliminary experimental results indicate that they exhibit SHG intensities ca. 0.3, 0.1, 0.3, and 0.8 times, respectively, as large as that of urea. In fact, many MOFs with nonaromatic chiral ligands are... 

The systematical design of noncentrosymmetric MOFs based on ID chains is more difficult compared to those with high-dimensional stmctural motifs because the ID chain systems lack control in two other directions. Except when using enantiopure chiral ligands, most of the noncentrosymmetric linear chains (ID MOFs) are synthesized accidentally and normally have a weak SHG activity. However, some 2D and 3D MOFs constructed by helical chain motifs can display good SHG responses. 

Imidazole derivatives have been used with metal ions to form noncentrosymmetric MOFs as they have strong tendency to induce helical structures of axial chirality. The im-N M donor-acceptor systems in these MOFs are expected to enhance the hyperpolarizabiUty p and thus the second-order NLO susceptibility. By introducing polar substituents such as - CN into the imidazole ring, the p value... 

In this chapter we briefly review the recent progresses in the design and synthesis of metal-organic frameworks as a new class of materials with potential for NLO application. The noncentrosymmetric MOFs, discussed herein are synthesized by using either chiral ligands or unsymmetrical ligands to assemble with suitable metal ions under hydro(solvo)thermal conditions. Additionally, incorporation of alkali or alkaline earth ions into Cd carboxylate frameworks is also an effective way to produce noncentrosymmetric MOFs. 

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