Saturated Linkers in Two-Dimensional Covalent Organic Frameworks Boost Their Luminescence

Abstract

The development of highly luminescent two-dimensional covalent organic frameworks (COFs) for sensing applications remains challenging. To suppress commonly observed photoluminescence quenching of COFs, we propose a strategy involving interrupting the intralayer conjugation and interlayer interactions using cyclohexane as the linker unit. By variation of the building block structures, imine-bonded COFs with various topologies and porosities are obtained. Experimental and theoretical analyses of these COFs disclose high crystallinity and large interlayer distances, demonstrating enhanced emission with record-high photoluminescence quantum yields of up to 57% in the solid state. The resulting cyclohexane-linked COF also exhibits excellent sensing performance for the trace recognition of Fe3+ ions, explosive and toxic picric acid, and phenyl glyoxylic acid as metabolites. These findings inspire a facile and general strategy to develop highly emissive imine-bonded COFs for detecting various molecules.

Linkages Make a Difference in the Photoluminescence of Covalent Organic Frameworks

“…We speculate that the ACQ effect is the main source of poor fluorescence for imCOF. This conclusion is consistent with a recent paper, which demonstrates that the fluorescence of imine‐linked COFs can be enhanced by increasing the interlayer distance to inhibit interlayer interaction [41] . For viCOF, a red‐shift of emission peak is observed in MeOH (579 nm) as compared with that in solid‐state (569 nm) (Figure 3h).…”

“…This conclusion is consistent with a recent paper, which demonstrates that the fluorescence of iminelinked COFs can be enhanced by increasing the interlayer distance to inhibit interlayer interaction. [41] For viCOF, a red-shift of emission peak is observed in MeOH (579 nm) as compared with that in solid-state (569 nm) (Figure 3h). The red-shift phenomenon of emission peak for dispersed viCOF is completely opposite to the blue-shift of emission peak for dispersed imCOF, and more importantly, the increase in fluorescence intensity is not as drastic as that of imCOF, indicating that the ACQ effect is not the main factor in viCOF.…”

A Multifunctional Covalent Organic Framework Nanozyme for Promoting Ferroptotic Radiotherapy against Esophageal Cancer

“…The peaks at 157 and 99 ppm unambiguously identified the presence of the carbon atoms in the CN and C–I functional groups, whereas the peak at 72 ppm was associated with the Fc groups . In addition, a moderately intense band at 1616 cm –1 , attributable to the CN stretching vibration, in the attenuated total reflection–Fourier transform infrared (ATR–FTIR) spectrum was observed, supporting our conclusion that imine-linked COFs has been successfully synthesized (Figure E) . Two bands, at 3362 and 1689 cm –1 , corresponding respectively to amino and aldehyde group vibrations, unambiguously confirmed the presence of binding defects at the edges of the COF particles, which functioned as the active sites for Fc modification .…”

Hexagonal 2D covalent organic frameworks from nonpolar and symmetric electron-accepting substituents for electron transport layers in near-infrared PeLEDs

“…21 In particular, 2D COFs with hcb topology usually have good electrical conductivity compared to other topologies due to their graphene-like planar structure, in which carbon atoms are arranged in hexagonal shape to form highly conjugated π orbitals. 22–24 What new properties would these materials with hcb topology have? To our knowledge, attempts in this area have never been reported systematically.…”