Structural motifs of oxacalix[4]arene for molecular recognition of nitroaromatic explosives: Experimental and computational investigations of host-guest complexes

Two new structural motifs based on oxacalix[4]arene appended N-(3-bromopropyl) phthalimide moieties, FON3PPh and PON3PPh was prepared, and its fluorogenic behaviors towards nitroaromatic explosives were investigated. The sensors exhibit preferential binding towards 4-nitro toluene (4-NT) and 2,6-dinitro toluene (2,6-DNT) based on the conformational orientation of the oxacalix[4]arene-host cavity. Spectrofluorimetric and 1H NMR titration studies support drastic changes inferred due to the complexation of the nitroaromatic explosives with FON3PPh and PON3PPh. The linear concentration range of FON3PPh for 4-NT is 5 μM to 4.5 mM with the detection limit of 2.4 μM and for PON3PPh, the linear concentration range 2,6-DNT is 0.5 μM to 5 mM with the detection limit of 0.1 μM. The binding ability of FON3PPh and PON3PPh for 4-NT and 2,6-DNT was determined as 1.15 × 104 M−1 and 3.02 × 104 M−1, respectively. Further, Density Functional Theory (DFT) based conformational modeling suggested that the spatial orientation of the guests in conformationally different hosts (PON3PPh and FON3PPh) lead to a varied selectivity pattern. The calculations also reveal that the complexation is thermodynamically favorable and selective. This work provides methodology to understand selective recognition of explosives based on structurally altered motifs of supramolecular architectures. The cytotoxic activity of FON3PPh and PON3PPh was also evaluated against human cancer cell lines in vitro.