Imidazole-appended 9,10-anthracenedicarboxamide probe for sensing nitrophenols and selective determination of 2,4,6-trinitrophenol in an EtOH-water medium

A novel imidazole-appended anthracenedicarboxamide "switch-off" probe, AIM-D, was designed and synthesized for nitrophenol sensing. AIM-D selectively detected 2,4,6-trinitrophenol (TNP) relative to other nitrophenol derivatives through a ratiometric fluorescence response in 1:1 EtOH-H₂O and exhibited a large Stern-Volmer quenching constant (K_sv = 2.69 × 10⁷ M^-1) and a binding ratio of 1:2. Mono- and dinitrophenols also quenched anthracene emission to a moderate (∼37%) or great (∼80%) extent depending on the position(s) of the nitro group(s) on the aromatic ring. The limit of detection of TNP with AIM-D was 1 ppb. The selective ratiometric discrimination of TNP over other nitrophenols was attributed mainly to protonation-induced electron transfer aided synergistic coulombic, multiple hydrogen bonding and π_an-π_TNP interactions in a symmetrical manner. Monomer quenching resulted from resonance energy transfer and a new enhancement at 520 nm occurred as a result of the effective inhibition of intramolecular charge transfer through intermolecular sequential opposite charge flow between AIM-D and TNP. However, mono- and dinitrophenol sensing attributed to the nitro group oriented resonance energy transfer from the latter to the former without inducing protonation at imidazole resulted in only monomer band quenching. An AIM-D coated paper strip was applied to the detection of nitrophenols in sea and river water samples and showed a limit of detection of 20 nM. The interaction of AIM-D with nitrophenols was examined by UV-visible, fluorescence, and ¹H NMR spectrometry and supported by DFT calculations.