Aggregation-induced emission (AIE)-active N-arylated ferrocenyl pyrazole chromophores: second-order nonlinear optical and turn on/off fluorescence for the detection of picric acid in mixed aqueous media

Abstract

A series of new donor–acceptor (D–A)-type N-arylated ferrocenyl pyrazole chromophores 1–4 have been synthesized and spectroscopically characterized. The molecules 2–4 crystalize in the centrosymmetric space group, but show a weak second harmonic generation (SHG) effect in bulk due to the presence of non-covalent interactions reducing the anti-parallel alignments. The absorption properties were examined in a wide range of solvents from polar to non-polar, shows a positive solvatochromism, because of the larger dipole moment in the excited state. All the derivatives exhibit weak emission in the solution state (CH₃CN: H₂O), owing to the quenching nature of ferrocene, but strong emission was observed in their aggregated state because of the restriction of intramolecular rotation, and this phenomenon is known as aggregation-induced emission (AIE). The AIE-state is further utilized to detect picric acid in a mixed aqueous medium by a fluorescence turn on/off method. It was carried out in a water fraction of f_w = compounds 1 and 2 at 80%, and compounds 3 and 4 at 90%, which shows highly selective and sensitive fluorescence quenching response towards picric acid in a mixed aqueous solution among several nitroaromatic compounds. The density functional theory revealed that the fluorescence quenching mechanism is a photo-induced electron transfer process between N-arylated ferrocenyl pyrazole and picric acid. The low quenching constant (K_sv = 2.54 × 10⁷ M⁻¹) and detection limit (25.4 nm) of picric acid was observed in compound 1, due to the presence of π-electron-rich delocalization over the aromatic ring, which interact with picric acid via non-covalent interactions that make effective charge transfer from N-arylated ferrocenyl pyrazole to picric acid.