SYNTHESIS, STRUCTURAL STUDIES AND SENSING PROPERTIES OF FLUORESCENT MOLECULES AND METAL ORGANIC FRAMEWORKS.

Abstract

Fluorescent materials have gained the attention of the researchers over the past years to be used as turn on/turn off sensors in detection of various analytes. Apart from conventional fluorescent molecules various new hybrid materials have been explored that can be potentially used as sensors. One such material is luminescent metal organic frameworks (MOFs) which are generally formed by using either fluorescent organic linkers or lanthanide/d10 metal ions. For the exploration of possible fluorophores in fluorescent molecules as sensors, symmetric functionalized anthracene unit has not been investigated much. Therefore, anthracene with symmetric functionalization by the receptor units can be treated as possible chemosensors for sensing metal ions, hazardous compounds and antibiotics. For the sake of convenience, the work presented in the thesis has been divided into the following chapters. The first chapter of the thesis embodies general introduction and data survey of the literature regarding fluorescent chemosensors and its type. The chapter also includes the different signaling mechanism that exists for the chemosensors based on photo physical processes, followed by brief introduction of the aim behind the present work. The second chapter of the thesis deals with a multi responsive and selective fluorescent chemo sensor with anthracene as a fluorophore and phosphonic acid group as receptor (anthracene-9,10-diylbis(methylene))diphosphonic acid (ANDP) for detecting Fe3+, benzaldehyde and few antibiotics. The sensor was characterised by single crystal X-ray diffraction and 1H-NMR. ANDP could selectively and sensitively detect Fe3+ and benzaldehyde in DMSO-water with detection limits of 5.09 μM and 39.8 μM respectively. 1H-NMR and FT-IR proved the interaction of Fe3+ with the sensor and the oxygen of phosphonic acid group is responsible for the interaction in both the sensing of iron ion and benzaldehyde. Mainly nitro group based antibiotics (NFZ, MDZ, FZD) and TCL sensitively quenched the fluorescence of the sensor with the detection limit of 2.07 μM, 1.25 μM, 1.19 μM and 0.66 μM. The quenching mechanism was further discussed and it was found that the photo induced electron transfer (PET) was responsible for the quenching of fluorescence intensity of ANDP. The HOMO and LUMO energies were estimated by DFT calculations. The third chapter of the thesis presents the solvothermal reaction of cobalt nitrate/zinc nitrate with 2,7-di(4H-1,2,4-triazol-4-yl)benzo[lmn]-[3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone (L1) ligand and 1,3,5-tricarboxylic acid (TCA)/5-sulphoisophthalic acid (5-SIA) resulted in the formation of Co(II)/Zn(II) organic frameworks. The synthesised MOFs were structurally characterised by single crystal X-ray diffraction studies including IR and NMR techniques. The approximate particle size of the MOFs was calculated by Debye-Scherrer formula using the powder X-ray diffraction (PXRD) technique, which came out to be 40 nm for Co-MOF and 62 nm for Zn-MOF. The particle size encouraged us to investigate these as imaging probes for human colon cancer cell line (HCT-15). Cytotoxicity assay studies showed that the MOFs are non-toxic to the cells and show good cellular internalisation. The emission properties of MOFs were further utilised for the sensing of 2,6-dichloro-4-nitroaniline (DCN) and 2,4,6-trinitophenol (TNP) as sensitive and selective detection of pesticide DCN and explosive TNP. Co/Zn-MOFs showed good quenching response to the analytes with low limit of detection (LOD). The quenching efficiency of Zn-MOF is more than that of Co-MOF which may be attributed to the interaction of co-ligands with the analyte. The quenching mechanism was further encountered and it was found out that electron and energy transfer is responsible for the quenching of emission of the MOFs. The LUMO and HOMO energy levels were evaluated by density function theory (DFT) calculations. The fourth chapter of the thesis deals with synthesis of a new fluorescent anthracene based probe (ANSN) with 2-mercaptopyridine as receptor at 9 and 10 positions. ANSN selectively and sensitively detects the presence of toxic Hg2+ ion and furazolidone (FZD) antibiotic in water via turn off mechanism with detection limit of 0.80 μM and 0.12 μM, respectively. The sensor was characterised by 1H-NMR, 13C-NMR and single crystal X-ray diffraction techniques. The stoichiometry and the interaction of Hg2+ with ANSN was examined by job plot and FT-IR spectroscopy stating that the complex exist in 1:2 stoichiometric ratios. Moreover, the charge transfers and the stability of the complex was inferred by DFT calculations. Theoretical studies demonstrated that the quenching of fluorescence of ANSN by FZD was explained by photoinduced electron transfer (PET). The probe revealed good selectivity for both the quenchers by performing anti-interference experiments. Therefore, this study concludes that ANSN can be easily and widely used as a promising sensor for sensitive and selective detection of FZD and Hg2+ ion in water samples. The fifth chapter of the thesis contains the experimental details, materials and reagents, instrumentation information and different types of spectroscopic measurements. Synthetic procedures for different type of ligands and metal organic frameworks have also been added in this chapter that has been reported in chapter 2-4.