Luminescent Covalent Organic Frameworks Research Articles

Stable Hydrazone-Linked Covalent Organic Frameworks Containing O,N,O'-Chelating Sites for Fe(III) Detection in Water.

Two stable crystalline hydrazone-linked covalent organic frameworks (COFs) (Bth-Dha and Bth-Dma) containing functional O,N,O'-chelating sites have been designed and successfully synthesized by the Schiff-base condensation reactions between benzene-1,3,5-tricarbohydrazide (Bth) and 2,5-dihydroxyterephthalaldehyde (Dha) or 2,5-dimethoxyterephthal-aldehyde (Dma), respectively. Bth-Dma exhibits strong fluorescence in the solid state and in an aqueous dispersion, while no fluorescence can be observed for Bth-Dha. Interestingly, the as-synthesized Bth-Dma can be used as a turn-off fluorescence sensor for the Fe(III) ion in aqueous solution with outstanding selectivity and sensitivity. The recognition process can be attributed to the coordination interaction between Fe(III) ion and the O,N,O'-chelating sites in the pore wall of Bth-Dma COF, as verified by X-ray photoelectron spectroscopy and 1H NMR spectroscopy. To the best of our knowledge, this is the first report on the rational design of luminescent COF with predesigned O,N,O'-chelating sites as a fluorescence sensor for highly selective and sensitive metal ion detection. This work may pave the way for designing luminescent COF sensors with functional binding sites for detecting specific metal ions.

Luminescent covalent organic framework as a recyclable turn-off fluorescent sensor for cations and anions in aqueous solution

Covalent organic frameworks (COFs) have shown great potential for use in ion sensing; however, applications of existing COFs are limited to sensing either cations or anions.

Mechanochromic luminescent covalent organic frameworks for highly selective hydroxyl radical detection.

The mechanochromic luminescence phenomenon was first reported in the low-cost mechanochemical (MC) synthesis process of a covalent organic framework (COF-TpMA (MC)) prepared using triformylphloroglucinol (Tp) and melamine (MA) as precursors, which shows excellent performance as a hydroxyl radical (˙OH) detector in living systems. This work not only shows a new kind of mechanochromic luminescent material but also exhibits a close relationship between the π-π stacking and luminescence properties of COF materials.

A mechanism of the luminescent covalent organic framework for the detection of NH3

The comparison of the two systems shows that the addition of NH3 can affect the luminescence behavior of Ph-An-COF.

Dual luminescent covalent organic frameworks for nitro-explosive detection

The assembly of two electron-rich fluorescent monomers produces a new dual luminescent covalent organic framework (DL-COF), which exhibits nitro explosives detecting ability with high sensitivity (Ksv∼ 106M−1,Kq∼ 1015M−1s−1) and high selectivity (at ppb level).

Impact of electronically excited state hydrogen bonding on luminescent covalent organic framework: a TD-DFT investigation

ABSTRACTThe investigation of intermolecular hydrogen bonding between the luminescent polypyrene covalent organic framework and formaldehyde (PPy-COF-HCHO) was carried out with the density functional theory and time-dependent density functional theory. The strengthening of the photoexcited hydrogen bond C = O---H–C was verified via geometric structures, electronic transition energies, binding energies, UV-Vis and infrared spectra comparison in both ground state and excited state of the PPy-COF's truncated representative fragment. From the frontier molecular orbitals examination, natural population analysis, and plotted electron density difference map demonstrated that the strengthened hydrogen bond facilitated the rearrangement of electron density between H-donor and H-acceptor moieties which should account for charge transfer and ultimate fluorescence quenching. Interestingly, the energy gap between excited state and triplet state of the hydrogen-bonded complex showed the possibility of the intersystem crossing. The MOMAP programme further confirmed the quenching process because there was a lower fluorescent rate constant for the donor–acceptor PPy-COF-HCHO complex compared to free PPy-COF fragment. Results above significantly highlighted the high sensitivity of the PPy-COF towards organic analyte, i.e. the formaldehyde and can be employed as a sensor.

Luminescent Covalent Organic Frameworks Containing a Homogeneous and Heterogeneous Distribution of Dehydrobenzoannulene Vertex Units.

Finding new ways to construct crystalline multiple-component covalent organic frameworks (COFs) has become an important focus. Herein we report the synthesis of three novel COFs containing a homogeneous and heterogeneous distribution of π-conjugated dehydrobenzoannulene (DBA) vertex units. The COFs were synthesized by reacting different ratios of C3-symmetric DBA catechol monomers with C2-symmetric pyrene-2,7-diboronic acid (PDBA) to yield three COFs, Py-DBA-COF 1, Py-DBA-COF 2, and Py-MV-DBA-COF. All three materials are highly crystalline and display unique luminescent properties in the solid state.

Highly Emissive Covalent Organic Frameworks.

Highly luminescent covalent organic frameworks (COFs) are rarely achieved because of the aggregation-caused quenching (ACQ) of π-π stacked layers. Here, we report a general strategy to design highly emissive COFs by introducing an aggregation-induced emission (AIE) mechanism. The integration of AIE-active units into the polygon vertices yields crystalline porous COFs with periodic π-stacked columnar AIE arrays. These columnar AIE π-arrays dominate the luminescence of the COFs, achieve exceptional quantum yield via a synergistic structural locking effect of intralayer covalent bonding and interlayer noncovalent π-π interactions and serve as a highly sensitive sensor to report ammonia down to sub ppm level. Our strategy breaks through the ACQ-based mechanistic limitations of COFs and opens a way to explore highly emissive COF materials.

A robust and luminescent covalent organic framework as a highly sensitive and selective sensor for the detection of Cu2+ions

A hydrogen bond assisted azine-linked covalent organic framework, COF-JLU3, was synthesized under solvothermal conditions. Combining excellent crystallinity, porosity, stability and luminescence, it can be the first COF as a fluorescent sensor for toxic metal ions, exhibiting high sensitivity and selectivity to Cu(2+).

A new triazine functionalized luminescent covalent organic framework for nitroaromatic sensing and CO2 storage

A hexagonal COF has been designed via Schiff-base condensation reaction between 1,3,5-tris-(4-aminophenyl)triazine and 1,3,5-triformylphloroglucinol. It showed excellent sensing behavior towards nitroaromatic compounds through fluorescence quenching and excellent CO2 uptake.

Excited-state properties of a blue luminescent covalent organic framework by the time-dependent density-functional tight-binding method

Excited-state properties of a blue luminescent two-dimensional (2D) layered covalent organic framework (COF) have been studied by the time-dependent density-functional tight-binding (TD-DFTB) method and compared with available experimental data. For single layer fragment, signatures in absorption and emission spectra are revealed, which show the typical π–π∗ transitions and intramolecular charge transfer (ICT) character. The influence of interlayer stacking has also been considered. Our calculations on the interlayer spacing of the COF suggest that adjacent layers are considerably more stable if their stacking arrangement is horizontally offset by ≈1.4Å compared with the eclipsed AA stacking structure. Excite-states calculations of high-symmetry eclipsed AA stacking and low-symmetry offset stacking are also considered. The low-symmetry offset stacking shows higher luminescence intensity but lower carrier transport ability. These calculations will be helpful for understanding interlayer excitation and carrier mobility, and the balance of the two processes is essential to the designing of the COF-based optoelectronic devices.