Synthesis and photophysical properties of Benzimidazole / Benzothiazole based chemosensors

Abstract

Heterocyclic rings like benzimidazole/benzothiazole act as a pharmacore in the structure of many clinically useful chemotherapeutic agents and is critical precursor for generating new derivatives with a wide spectrum of biological activity. Benzimidazole/Benzothiazole rings were introduced in the structure of various chemosensors based on following considerations: (i) it changes the spatial effects within one molecule; (ii) realize the real-time detection (quickly induce the fluorescent response); (iii) biocompatible nature; (iv) therapeutic nature (v) range of possible derivatives. Chemosensors are abiotic compounds which bind with analyte through noncovalent interactions like van der wall forces, hydrogen bonding and then convert chemical information in to analytically useful signals. An optical chemosensor consist of two units i.e. binding unit and chromophore or fluorophore moeity as a signalling subunit. Fluorescent chemosensors act through diverse mechanisms like photoinduced electron transfer (PET), internal charge transfer (ICT), molecular rigidification, excimer & exciplex formation, electronic energy transfer (EET) or Förster resonance energy transfer (FRET). In last few years, formulation of chemosensors detecting cations, anions or neutral molecules has been of great interest to many scientists, including chemists, biologists, clinical biochemists and environmentalists. Numerous chemosensors based on benzimidazole/benzothiazole ring, working in organic medium or semi aqueous medium are reported. For in vivo sensing of analyte and sake of environment, chemosensors are required whose operation can be assessed in partially or fully aqueous medium. However, it is not easy to solublise benzimidazol/benzothiazole based aromatic compounds in water due to their hydrophobic nature. Moreover, sensing of anions in water has always been a challenging field of supramolecular chemistry. Sensors for anions are less available in aqueous medium because of the competitive interaction felt by analyte from water molecules for H-bonding sites. This trouble was overcome through sensing anions in aqueous medium employing cation or counter anion displacement assay. Exclusive optical features of nano particles have made them a preferred choice for formulation of chemosensors. Nano particles themselves or nano particles coated with organic receptors have proved to be excellent examples of chemosensors with high sensitivity and selectivity. Orientation of binding sites of organic receptors acting as chemosensor changes upon coating on nano particles, thus, their selectivity was increased to a great extent. Coating of organic receptor on nano particles alters their detection limit of analyte due to change in the number of exposed binding sites. Thus, overall sensing properties of a receptor are greatly enhanced upon coating on nano particles. Advantages of ZnO nano particles such as less toxicity, easy synthesis, range of possible nanostructures, high stability against oxidants and photo-oxidants and transparency to visible light has made it a best choice to be used. Synthesis of improved benzimidazole/benzothiazole based chemosensors for various analytes with increased water solubility, selectivity and detection limit is of immense benefit for in vivo or in vitro detection of analytes. We have synthesized range of benzimidazole/benzothiazole based chemosensors for diverse analytes i.e. metal ions like Mg(II), Fe(III), Cr(III), Cu(II), Zn(II), Ag(I), Hg(II); anions like F- , Cl- , Br- , PO4 3- ; and neutral molecules like oxalic acid, L-cysteine and 3-mercaptopropanoic acid. Deficiency as well as excess of these analytes hinders smooth functioning of many bioprocesses; therefore, monitoring their concentration in various samples is an essential requirement. We have tried to perform recognition in increased content of water through following various strategies. We have synthesized Schiff bases through single step reaction of 2-(2-aminophenyl) benzimidazole/benzothiazole with variety of aldehydes to have different binding environment and cavity of appropriate size for different metal ions. Receptor synthesized through treatment of 2-(2-aminophenyl)benzimidazole with 2-hydroxy-5- nitrobenzaldehyde has acted as a selective sensor for simultaneous detection of Mg(II) and Cr(III) in semi aqueous medium using two different techniques. Other receptor with pyridine-2-carboxyldehyde act as a sensor for Cr(III) ion and then its metal complex with Cr(III) act as a differential sensor for two anions i.e. Fand HSO4 - in semi aqueous medium following two different mechanisms of cation displacement and counter anion displacement assay. Receptors with pyrrole-2-carboxyldehyde, salicyldehyde, 4- (diethylamino)-2-hydroxybenzaldehyde and an azo dye act as a selective chemosensor for Cu(II) and then their metal complex with Cu(II) as a sensor for phosphate, L-cysteine hydrochloride, 3-mercaptopropanoic acid and oxalic acid in semi aqueous medium through following cation displacement assay. Receptor synthesized through reaction of 2-(2-aminophenyl)benzothiazole with 2-hydroxynaphthaldehyde act as a selective sensor for Fe(III) through fluorescence emission spectroscopy and for HSO4 - in semi aqueous medium through UV-Vis absorption spectroscopy. To increase the content of water in the sensing medium, benzothiazole based amidine containing nitro group is processed in to Organic nano particles. This acts as a sensor for Ag(I) and its complex with Ag(I) act as a sensor for Clin complete aqueous medium. Formation of ONPs had allowed desirable sensing of anion in pure aqueous medium. To meet the aim of recognition of analyte in complete aqueous medium, we processed the benzimidazole/benzothiazole based Bigneilli compounds into organic nano particles (ONPs) through precipitation method. Receptors formed through coupling benzimidazole moiety with pyrene moiety inhibits aggregation induced enhanced emission (AIEE) on formation of ONPs. They act as multifunctional sensor for Zn(II) and Fe(III) in two different mediums i.e. organic medium (Acetonitrile) and aqueous medium (ONPs). Chromogenic Bigneilli compound formed through coupling benzimidazole ring with 2- hydroxy-5-nitrobenzaldehyde act as a selective sensor for Hg(II) in aqueous medium (ONPs); where reversibility of sensor is proved titrating it with iodide. Crystal of Cu(II) complex of Bigneilli compound coupling benzimidazole and pyridine moiety was developed. This gives us information about the binding sites involved in binding with metal ion. Subsequently, oleylamine was used to form long chain hydrocarbon with different polar end groups like imine, amine, urea and thiourea. Surfactant triton-X-100 was used to solublise these receptors in complete aqueous medium. Receptor formed through treating oleylamine and salicyldehyde comprising imine linkage was found to be a selective sensor for Zn(II) ion with detection limit in nano molar range. Other receptor formed by treating oleylamine with 1-isothiocyanatonaphthalene comprising thiourea linkage acts as a selective sensor for Hg(II). Benzimidazole/Benzothiazole based compound with long side chain are checked for metal binding in complete aqueous medium following identical approach. Benzimidazole based receptor sense Hg(II) and then its complex with Hg(II) as a selective sensor for Brthrough following cation displacement mechanism. Challenging task of sensing cations and anions was achieved by using surfactant for dissolving organic receptor in aqueous medium. Finally, benzimidazole/benzothiazole based receptors with amine linkage were synthesized by treating 2-aminobenzimidazole/benzothiazole with different aldehydes followed by reduction. Few of these receptors were found to act as a sensor for Cr(III) and Zn(II) though with interference from some of other tested metal ions. These simple receptors were then decorated on ZnO nano particles. Recognition studies of ZnO coated receptors immensely increase the selectivity of sensor along with detection limit.