Building Conjugated Donor–Acceptor Cross-Links into Metal–Organic Frameworks for Photo- and Electroactivity

Abstract

We convert a coordination network into a covalent solid, while maintaining the crystallinity and greatly enhancing the framework rigidity and redox-active and photochemical properties. Specifically, intensely light-absorbing push–pull functions are postsynthetically installed by reacting the electrophilic TCNE (tetracyanoethylene) guests and the electron-rich alkyne side arms on a microporous Zr–organic framework, generating black microporous crystallites with a band gap smaller than 1.0 eV. The reaction proceeds in the known [2 + 2] cycloaddition–retroelectrocyclization mechanism and extensively establishes conjugated (polyene) bridges across the linker molecules. The donor (4-methoxyphenyl) and acceptor (dicyanovinyl) couples of the polyene bridges also act as an efficient fluorescent quencher and can be selectively installed in a thin outer layer of the host crystallite to form a core–shell assembly for turn-on fluorescent sensing of small amine molecules in water solutions.

Design of Photothermal Covalent Organic Frameworks by Radical Immobilization

“…33 We employed a classic type of radical generator, para- cycloaddition−retroelectrocyclization (CA−RE) reaction of tetracyanoethylene (TCNE) with the ethynyl groups suspended on the TPB-BPTA-COF (Figure 1, SI). 34 GT-COF-3 was thus transformed to radical COF by the structural resonance of carbonitrile-equipped buta-1,3-dienes and their radicals. Notably, the synthesis of TPB-BPTA-COF and GT-COF-3 was high-throughput with good yields: one batch of synthesis could obtain more than 1 g of COFs, compatible with large-scale production.…”

Linker Deficiency, Aromatic Ring Fusion, and Electrocatalysis in a Porous Ni8-Pyrazolate Network

“…The 12-connected Ni 8 O 6 node can be considered topologically equivalent to the Zr 6 O 4 (OH) 4 12+ unit (e.g., as in the UiO series), which features a Zr 6 octahedron and an O 2– or OH – species on its 8 faces. One difference, however, is noted: while Ni 8 -pyrazolate nets reported so far uniformly feature full linker occupancy (i.e., one Ni 8 O 6 node per six ditopic linkers to give the 12 connections), the Zr 6 O 4 (OH) 4 unit was often found to fall short of the full 12 connections, ,− e.g., with the missing linkers filled in by monotopic formate/acetate or OH – /H 2 O ligands. Another difference lies in the softer Ni­(II)-pyrazolate pair (relative to Zr 4+ -carboxylate), which imparts more covalency and stability to the bonding, making the resultant framework less prone to deformation and degradation.…”

Uniting Form and Function, Stability and Reactivity in Open Framework Materials

“…21 In Zr-Dc4mo, the electron-donating methoxy groups render the pendant alkyne units reactive towards TCNE (tetracyanoethylene) guests in the [2+2] cycloaddition-retroelectrocyclization (CA-RE) mechanism and extensively establish conjugated (polyene) bridges across the linker molecules (Figure 7). 26 The crosslinking CA-RE reactions convert the coordination network into a covalent solid, while maintaining the crystallinity and greatly enhancing the framework rigidity and redox-active and photochemical properties. For example, it transformed from an insulator into a semiconductor (with a conductivity of 10 ¹5 S/m as per two-probe measurement) upon taking up Cu(I) ions from a saturated acetonitrile solution of Cu(CH 3 CN) 4 BF 4 .…”