Highly Selective and Sensitive Detection of Picric Acid Explosive by a Bisporphyrin Cleft: Synergistic Effects of Encapsulation, Efficient Electron Transfer, and Hydrogen Bonding

Abstract

A flexible diethylpyrrole-bridged bisporphyrin (H4DEP) is employed to encapsulate highly explosive picric acid (PA) in its cleft, and information about the chemical environment in the interior of the host–guest assembly is obtained. The synthesis, X-ray structure, and photophysical properties of the encapsulated host–guest complex (H4DEP·PA) are reported here. The host–guest assembly is also retained in solution with a binding constant (K) of (2.63 ± 0.2) × 105 M–1. Such an efficient complexation is due to the perfect match of the host–guest size, H bonding, and strong π–π interactions between the host and guest, which collectively make the binding of PA rapid and highly selective with a detection limit of 2.4 ppm. The host H4DEP can also discriminate the highly explosive PA from other nitroaromatic compounds. The extremely low HOMO–LUMO gap (HOMO = highest occupied molecular orbital, LUMO = lowest unoccupied molecular orbital) in the host–guest complex leads to an efficient and highly facile electron transfer from the host to the guest, which results in substantial quenching of the emission intensity of H4DEP. Such an efficient electron transfer is only possible if the donor and acceptor moieties are close to each other, as this lowers the reorganization energy. All of the experimental evidence is further substantiated by DFT calculations.