Aza-phenol Based Macrocyclic Probes Design for “CHEF-on” Multi Analytes Sensor: Crystal Structure Elucidation and Application in Biological Cell Imaging

Metal bound macrocyclic compounds found in biological
systems inspired
us to design and synthesize two Robson-type macrocyclic Schiff-base
chemosensors, H2L1 (H2L1=1,11-dimethyl-6,16-dithia-3,9,13,19-tetraaza-1,11(1,3)-dibenzenacycloicosaphane-2,9,12,19-tetraene-1,11-diol)
and H2L2 (H2L2=1,11-dimethyl-6,16-dioxa-3,9,13,19-tetraaza-1,11(1,3)-dibenzenacycloicosaphane-2,9,12,19-tetraene-1,11-diol).
Both the chemosensors have been characterized with different spectroscopic
techniques. They act as multianalyte sensor and exhibit “turn-on”
fluorescence toward different metal ions in 1X PBS (Phosphate Buffered
Saline) solution. In presence of Zn2+, Al3+,
Cr3+ and Fe3+ ions, H2L1 exhibits ∼6-fold enhancement
of emission intensity, while H2L2 shows ∼6-fold enhancement of emission intensity
in the presence of Zn2+, Al3+ and Cr3+ ions. The interaction between the different metal ion and chemosensor
have been examined by absorption, emission, and 1H NMR
spectroscopy as well as by ESI-MS+ analysis. We have successfully
isolated and solved the crystal structure of the complex [Zn(H2L1)(NO3)]NO3 (1) by X-ray crystallography. The crystal structure
of 1 shows 1:1 metal:ligand stoichiometry and helps to
understand the observed PET-Off-CHEF-On sensing mechanism. LOD values
of H2L1 and H2L2 toward metal ions
are found to be ∼10–8 and ∼10–7 M, respectively. Large Stokes shifts of the probes
against analytes (∼100 nm) make them a suitable candidate for
biological cell imaging studies. Robson type phenol based macrocyclic
fluorescence sensors are very scarce in the literature. Therefore,
the tuning of structural parameters as the number and nature of donor
atoms, their relative locations and presence of rigid aromatic groups
can lead to the design of new chemosensors, which can accommodate
different charged/neutral guest(s) inside its cavity. The study of
the spectroscopic properties of this type of macrocyclic ligands and
their complexes might open a new avenue of chemosensors.