High-fidelity detection of picric acid following proton prompted fluorescence quenching by a novel acridine yellow G-based binuclear ZnII-metallacycle

Abstract

Synthesis of a highly fluorescent binuclear Zn^II-metallacycle (1) based on a novel Schiff base (H₂L) viz. 2,7-dimethylacridine-3,6-diyl-bis(azaneylylidene)-bis(methaneylylidene)-bis(3-diethylaminophenol) is described and characterized through various spectral (FT-IR, ¹H-NMR and HRMS) methods followed by structural authentication of 1 by single crystal X-ray diffraction analysis. The Zn^II-metallacycle 1 is strategically incorporated with four –N(C₂H₅)₂ groups, which enables it to exhibit excellent selectivity towards picric acid (PA) among other nitro aromatic compounds (NACs) in the form of fluorescence quenching (71%). The vital role of –N(C₂H₅)₂ is substantiated by performing UV-vis, fluorescence, and ¹H-NMR spectral and DFT studies on 1 with all NACs and comparing it with a model complex 2, which lacks the –N(C₂H₅)₂ group. Studies revealed that the quenching involved a static and dynamic mechanism with a quenching constant (K_SV) of 3.44 × 10⁴ M and LOD of 7.62 × 10⁻⁸ M. The static pathway involved the formation of a H-bonded ground state ion pair complex of 1 and PA [1-(C₂H₅)₂N–H⁺⋯O⁻–picrate] and the dynamic quenching is attributed to energy transfer from the excited state of 1 to the ground state of PA facilitating polar–polar resonance between them. Conclusively, 1 has been demonstrated as a significant probe, which can be employed for naked eye detection of PA in solid and liquid phases.