Deciphering the excited state behaviour of 2-pyridin-2-yl-quinoline based systems with fluorescence quenching response for the selective detection of cu(II) ions

Novel fluorescence probes, PyQ1-PyQ4, based on 2-pyridyl quinoline moiety were developed and thoroughly investigated for solvent dependent fluorescence modulation and metal sensing ability. The estimated photophysical properties reveal significant solvatochromic effect on fluorescence Stokes shift ranging within 2318–7480 cm −1 in different solvents. Likewise, fluorescence yields and lifetime show the variation of about 40 and 14 times, respectively indicating strong solvent dependent fluorescence behaviour. The PyQ derivatives exhibit a distinct fluorescence “turn-off” behaviour in the presence of Cu(II) ions, highlighting their potential for selective Cu(II) detection among other metal cations. The results demonstrated that all the four probes exhibited almost ∼98 % fluorescence intensity decrease upon binding through quinoline nitrogen atoms to copper ion in 1:1 stoichiometry. Theoretical calculation results using density functional theory (DFT) and natural bond orbital (NBO) analysis predict significant ligand to metal excited state charge redistribution in presence Cu(II) ions, leading to the chelation enhanced quenching (CHEQ) of fluorescence. The limit of detection (LOD) and limit of quantification (LOQ) were found to be 1.0–2.4 μM in different cases. Additionally, PyQ1 behaves as a ratiometric fluorescent probe for the detection of Zn(II) and Hg(II) with the corresponding LODs of 4.2 μM and 11.0 μM, respectively. The results underscore the potential application of the developed PyQ derivatives as solvatochromic probe with significant modulated fluorescence response in presence of certain metal ions. • 2-pyridin-2-yl-quinoline systems were developed with diverse structural features. • Fluorescence response and photophysical properties are solvent dependent. • Efficient ligand to metal charge transfer occurs in presence of Cu (II) ions. • Almost 98 % quenching of fluorescence results a “turn-off” fluorescence sensor. • Lower values LOD and LOQ confirm the sensitive detection of Cu (II) ions in solution.