Fluorescent turn on detection of zinc(II) ion using a hydrazone derived from 4-tert-butyl-2,6-diformylphenol and 3-hydrazineylquinoxalin-2(1H)-one

A polydentate chelating ligand (LH) was synthesized from 4-tertbutyl-2,6-diformylphenol and 3-hydrazineylquinoxalin-2(1H)-one and characterized by mass spectrometry, IR and NMR spectroscopy. LH is weakly fluorescent in EtOH - HEPES buffer (5 mM, pH = 7.4, 8:2, v/v) but exhibited a turn on response with Zn2+ ion having a workable pH range of 6-12 and thus is compatible with biological pH. This sensor is selective for Zn2+ ion, over various metal ions and anions as evident from competition experiments. The probable mechanism of sensing by LH is restriction of ESIPT, PET and C = N isomerization processes and on -set of CHEF process upon coordination to Zn(II) ion. Reversibility was established using EDTA test and applicability in various real samples were evaluated. The calculated limit of detection for Zn2+ ion was 95 nM and Job plot as well as 1H NMR titration suggested a 2:1 binding ratio between Zn2+ ion and LH. From the reaction (in ethanol) of ZnCl2 with LH, in 2:1 ratio, crystalline solids of composition [Zn2(L)Cl3]& sdot;2H2O 2.5DMF was isolated and its molecular structure was established by single crystal XRD studies. DFT/TDDFT calculation supported the experimental electronic absorption spectrum.