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

Abstract

Imine and β-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 C₃-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 m² g⁻¹ (Brunauer–Emmett–Teller) and 1515 m² 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.