Excellent Crystallinity and Stability Covalent–Organic Frameworks with High Emission and Anions Sensing

Abstract

Covalent-organic frameworks (COFs) are a new class of porous crystalline frameworks with high 𝝅-conjugation and periodical skeletons. The highly ordered 𝝅-conjugation structures in some COFs allow exciton migration and energy transfer over the frameworks, which leads to good fluorescence probing ability. In this work, two COFs (TFHPB-TAPB-COF and TFHPB-TTA-COF) are successfully condensed via the Schiff base condensation reaction. The intramolecular hydrogen bonds between imine bonds and hydroxyl groups form the excited-state intramolecular proton transfer (ESIPT) strategy. Owing to intramolecular hydrogen bonds in the skeleton, the two COFs show high crystallinity, remarkable stability, and excellent luminescence. The COFs represent a good sensitivity and selectivity to fluoride anions via fluorescence turn-off. Other halogen anions (chloride, bromide, and iodine) and acid anions (nitrate and hydrogen carbonate) remain inactive. These results imply that only fluoride anion is capable of opening the hydrogen bond interaction and hence break the ESIPT strategy. The detection limit toward fluoride anion is down to nanomoles level, ranking the best performances among fluoride anion sensors systems.

Dual emission of ESIPT-capable 2-(2-hydroxyphenyl)-4-(1H-pyrazol-1-yl)pyrimidines: interplay of fluorescence and phosphorescence

“…For example, these fluorophores are widely used in analytical chemistry, and the design of ESIPT-capable sensors for analytes of different nature (cations, anions, molecules) has become one of the highly grown interdisciplinary areas in recent years. [75][76][77][78][79] Due to spatial separation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) ESIPT-fluorophores can have narrow singlet-triplet energy gaps facilitating thermally activated delayed fluorescence (TADF). [80][81][82][83] Some ESIPTcapable organic compounds are known to exhibit phosphorescence 84,85 and even room temperature phosphorescence (RTP).…”

Lanthanide Functionalized Covalent Organic Frameworks Hybrid Materials for Luminescence Responsive Chemical Sensing

“…and "turn-on-off-on-off", respectively, both of them originate from the ESIPT strategy in the skeleton of COFs containing hydroxyl groups (proton donor) and imine bonds (proton acceptor) for that their hydrogen bond interaction is replaced by deprotonation from the hydroxyl unit and F À . [52,53] (b) PO 4 3À . The imine-linked conjugated COF (IC-COF) from 1,5-diaminonaphthalene and 2,4,6-tris(4formyl phenoxy)-1,3,5-triazine shows the luminescence quenching to PO 4 3À , which is also for the proton transfer from the terminal NH 2 groups of the IC-COF to PO 4 3À and the formation of the imine bond.…”

“…[49,50] (2) Anions. [42,[52][53][54] Different from metal ions, the luminescent COFs for sensing on anions have no apparent character, depending on the special type of anions. and "turn-on-off-on-off", respectively, both of them originate from the ESIPT strategy in the skeleton of COFs containing hydroxyl groups (proton donor) and imine bonds (proton acceptor) for that their hydrogen bond interaction is replaced by deprotonation from the hydroxyl unit and F À .…”

“…Two reported systems include TFHPB‐TAPB‐COF and TFHPB‐TTA‐COF via condensation of 1,3,5‐tris(4‐formyl‐3‐hydroxyphenyl)‐benzene (TFHPB) with 1,3,5‐tris(4‐aminophenyl) benzene (TAPB) or 4,4′,4′′‐(1,3,5‐triazine‐2,4,6‐triyl)trianiline (TTA), and ACOF of hydrazine hydrate and 5′‐(4‐formyl‐3‐hydroxyphenyl)‐3,3′′‐dihydroxy‐[1,1′ : 3′,1′′‐terphenyl]‐4,4′‐dicarbaldehyde. Its luminescence response modes involve “turn‐off” and “turn‐on‐off‐on‐off”, respectively, both of them originate from the ESIPT strategy in the skeleton of COFs containing hydroxyl groups (proton donor) and imine bonds (proton acceptor) for that their hydrogen bond interaction is replaced by deprotonation from the hydroxyl unit and F − [52,53] . (b) PO 4 3− .…”

“…Hg 2+ also performs the similar luminescence quenching sensing behavior to COFs probe for the similar responsive mechanism, which mentions two COFs systems, the thioether‐functionalized COF‐LZU8 and TFPPy‐CHYD by condensation of flexible carbo hydrazide (CHYD) and 1,3,6,8‐tetrakis(4‐formylphenyl)‐pyrene (TFPPy) [49,50] Anions [42,52–54] . Different from metal ions, the luminescent COFs for sensing on anions have no apparent character, depending on the special type of anions.…”