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)-di-benzenacycloicosaphane-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)-di-benzenacycloicosaphane-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 similar to 6-fold enhancement of emission intensity , while H2L2 shows similar to 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 successfu l l y 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 similar to 10(-8) and similar to 10(-7) M, respectively. Large Stokes shifts of the probes against analytes (similar to 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 parameter s 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 ca v i t y . The study of the spectroscopic properties of this type of macrocyclic ligands and their complexes might open a new avenue of chemosensors.