[3+3] Imine and beta-ketoenamine tethered fluorescent covalent-organic frameworks for CO2 uptake and nitroaromatic sensing

Imine and beta-ketoenamine based covalent-organic frameworks (COFs) are nitrogen rich organic porous materials which offer enhanced affinity for carbon dioxide. In this article, synthesis, gaseous uptake and chemo-sensing properties of four new 2-D COFs with [3+3] structural motifs have been reported. The COFs have been synthesized from readily available C-3-symmetric aldehyde and amine building units via Schiff base condensation. Prior to the synthesis, reactivity and structural integrity of the building blocks were appropriated by a fully characterized model Schiff base (TAPB-Benz) obtained from the condensation of 1,3,5-tris(4'-aminophenyl)benzene (TAPB) and benzaldehyde. Reaction of 1,3,5-tris(4'-aminophenyl) benzene (TAPB) and 1,3,5-tris(4'-amino-3',5'-isopropylphenyl)benzene (iPrTAPB) with 1,3,5-tris(4'-formylphenyl)benzene (TFPB) and 1,3,5-triformylphluroglucinol (TFP) in dry dioxane and acetic acid (cat.) resulted in the formation of crystalline 2-D frameworks, TAPB-TFPB, iPrTAPB-TFPB, TAPB-TFP and iPrTAPB-TFP. The COFs feature permanent porosity with high surface area and carbon dioxide uptake. Among these, iPrTAPB-TFP revealed the highest surface area of 756 m(2) g(-1) (Brunauer-Emmett-Teller) and 1515 m(2) g(-1) (Langmuir) and carbon dioxide uptake of 105 mg g(-1) (273 K, 1 atm). Notably with 180 mg g(-1) (273 K, 1 atm), TAPB-TFP shows the highest CO2 uptake capacity among all the COFs which is also comparable to previously reported high CO2 uptake capacity COFs. Furthermore, due to the inherent fluorescent capability of triphenylbenzene, the COFs are endowed with fluorescence and fluorescence chemo-sensing ability for polynitroaromatic analytes