Octupolar Cyclotriphosphazene-Cored Self-Standing Covalent Organic Framework Membranes as Nonlinear Optical Materials: Impact of Linkage Types and Material Forms.

Conjugated and processable self-standing vinylene-linked covalent organic framework membranes (COFMs) are highly demanding for photonics and optoelectronics. In this work, we have fabricated the first cyclotriphosphazene (CTP) cored vinylene-linked self-standing COFM (CTP-PDAN). For comparison purposes, we have successfully fabricated the imine-linked congener (CTP-PDA). Leveraging the inherent nonlinear optical (NLO) response of the CTP core, both membranes were directly mounted to evaluate NLO parameters using the open-aperture (OA) Z-scan technique. Direct measurement of NLO responses on membranes is advantageous and free from solvent and scattering effects, making it a more practical approach compared to the conventional dispersion mode. The OA Z-scan transmission yields a reverse saturable absorption signature exhibiting a higher NLO absorption coefficient (ß) of 58.37 cm/GW for CTP-PDAN, compared to that of the imine-linked CTP-PDA COFM (ß = 8.5 cm/GW). These results can be correlated to the efficient conjugation through the vinylene linkage in CTP-PDAN compared to the imine linked congener.

Terahertz Conductivity of Free-Standing 3D Covalent Organic Framework Membranes Fabricated via Triple-Layer-Dual Interfacial Approach.

Processable covalent organic framework membranes (COFM) are emerging as potential semiconducting materials for device applications. Nevertheless, the fabrication of crystalline and free-standing 3D COFMs is challenging. In this work, a unique time and solvent-efficient triple-layer-dual interfacial (TLDI) approach for the simultaneous synthesis of two 3D COFMs from a single system is developed. Besides, for the first time, the optical conductivity of these free-standing 3D COFMs is analyzed using terahertz (THz) spectroscopy in transmission mode. Interestingly, these membranes show excellent transmittance at THz frequencies with very high intrinsic THz conductivities. The evaluated scattering time and plasma frequency of the free carriers of the COFMs are highly promising for future applications in optoelectronic devices in THz frequencies.

Carbon-Carbon Linked Organic Frameworks: An Explicit Summary and Analysis.

Organic frameworks with carbon-carbon (CC) linkage are an important class of materials owing to their outstanding chemical stability and extended π-electron delocalization resulting in unique optoelectronic properties. In the first part of this review article, the design principles for the bottom-up synthesis of 2D and 3D sp/sp2 CC linked organic frameworks are summarized. Representative reaction methodologies, such as Knoevenagel condensation, Aldol condensation, Horner-Wadsworth-Emmons reaction, Wittig reaction, and coupling reactions (Ullmann, Suzuki, Heck, Yamamoto, etc.) are included. This is discussed in the context of their reaction mechanism, reaction dynamics, and whether and why resulting in an amorphous or crystalline product. This is followed by a discussion of different state-of-the art bottom-up synthesis methodologies, like solvothermal, interfacial, and solid-state synthesis. In the second part, the structure-property relationships in CC linked organic frameworks with representative examples of organocatalysis, photo(electro)catalysis, energy storage and conversion, magnetism, and molecular storage and separation are analyzed. The importance of linkage type, building blocks, topology, and crystallinity of the framework material in connection with the structure-property relationship is highlighted. Finally, brief concluding remarks are presented based on the key development of bottom-up synthetic methods and provide perspectives for future development in this field.

Synthesis, characterization and magnetism of metal-organic compounds: role of the positions of the coordinating groups of a meso-flexible ligand in placing anisotropy to exhibit spin-canting behaviour.

In continuation of our recent investigation on flexible ligands, three new metal-organic coordination framework containing compounds, formulated as {Co(2)(L2)2(px3ampy)(2)}(n)·npx3ampy (1), {Co(L2)(px3ampy)(0.5)}(n) (2) and {Co(2)(L3) (px3ampy)(2)(H(2)O)}(n)·3nH(2)O (3) have been synthesized using three structurally different meso-flexible polycarboxylate ligands, H(2)L1(4,4'-methylenebis-(oxy)dibenzoic acid), H(2)L2 (3,3'-methylenebis-(oxy)dibenzoic acid) and H(4)L3 (5,5'-methylene-bis(oxy)diisophthalic acid) bearing the flexible spacer in the middle of the skeleton, along with a long flexible pyridyl ligand px3ampy (1,4-bis(3-pyridylaminomethyl)benzene). Compounds 1-3 have been characterized by single crystal X-ray diffraction analysis, IR spectroscopy and thermogravimetric (TG) studies including elemental analysis. The crystal structure determinations reveal that compound 1 has a 1D ladder-like structure and compounds 2and 3 are characterized by 3D frameworks. Compound 2 possesses a tiling of a snz net and compound 3 has a 3D interpenetrated motif with a (4,4) connecting 2-nodal net. The variable temperature magnetic susceptibility measurements demonstrate the dominating antiferromagnetic nature of all three title compounds; interestingly, however, compounds 2 and 3 exhibit ferromagnetic interactions due to the uncompensated magnetic moment of the system at low temperatures. Compound 2 illustrates the occurrence of spin canted antiferromagnetic ordering at T(c) ≈ 25 K with a coercive field (H(c)) of 900 Oe at 10 K due to the inclusion of magnetic anisotropy caused by the twisting of the concerned ligand skeleton. Compound shows comparatively weak ferromagnetic ordering with T(c) ≈ 9.5 K, for which weak magnetic anisotropy is present because of two different coordination environments (octahedral and tetrahedral) between two cobalt centers. A structure-function relationship has been described based on the position of the coordinating groups with respect to the flexible center of the ligand skeleton as well as the coordination angle between the ligand and the metal ion.