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Abstract : This thesis reports on the use of the element boron in organic chemistry. Its role in catalysis, as well as its broad utility when in the form of a boronic acid functional group is demonstrated. Boric acid and boronic acids have applications in numerous kinds of chemical reactions as catalysts. Boric acid is demonstrated in this work to catalyse the esterification of a- hydroxycarboxylic acid starting materials, including carbohydrates, typically in excellent yield. A series of reactions were conducted to demonstrate the utility and limitations of this technique. Included in this work is the synthesis of the carbohydrate, KDO. Furthermore, a series of esters were generated using salicylic acid as a starting material, one of which was subjected to x-ray crystallographic studies. Also in this thesis a novel type of boronic acid catalysed amide forming reaction is described. The reaction is shown to proceed rapidly under mild reaction conditions with little purification required to give a pure product. Structural identification of the amide products is discussed and hypothesised molecular configurations are presented. Fluorescence sensors are described as a practical application of boron {u2013} polyol interactions. Supporting theories are outlined and published work is summarised, compared and contrasted. The carbohydrates sialic acid and KDO are identified as molecular targets for boronic acid based fluorescence sensors. The benefits of multiple binding sites and optimised molecular geometry are clearly shown in the results of fluorescence assays. Sensor molecules reported in this thesis demonstrated selective binding to the carbohydrates, sialic acid and KDO.
Doctoral Thesis / Dissertation from the year 2016 in the subject Chemistry - General, grade: 9.0, , language: English, abstract: This thesis deals with Fluorochemosensors. Fluorescence spectroscopy and ultraviolet techniques have been applied to various analytical, bio-analytical, environmental, medical and forensic investigations. Several analytical methods that are offered for recognition of target concerned such that flame photometry, AAS, HPLC, mass spectrometry, ion sensitive electrode, microprobe analysis, neutron activation analysis, have been developed. But these methods are expensive and time uncontrollable process that involves complicated instrumentation and do not allow permanent monitoring. When compared to absorption techniques, flourimetric method is more sensitive and selective and rapidly performed. In nature, any compound analysed by using a suitable analytical technique which basically depends on the nature and properties of the target compound. If the target compound exhibit phenomenon called as Luminescence where the emission of electromagnetic radiation of longer wavelength to that of absorbed radiation can be seen are analysed by using the modern spectroscopic technique called as "flourimetry" . Hence, significant hard works are life form complete to develop selective fluorescent sensor for recognition of targeted species. To blind date different fluorescent molecular sensors with different excitation and emission wavelengths comprise be employed such like coumarin, 1,8-naphthamide, pyrene, xanthenes, cynine, squaraine, boron dipyrromethene difuoride, nitrobenzofurazan etc. In outline, cinnamaldehyde - rhodamine based signalling systems were designed and synthesized for the selective recognition of Fe+3 ions. The cinnamaldehyde molecule was used as a recognition moiety and rhodamine-B was used as a signalling moiety. The excellent fluorescent response to Fe+3 in ACN solution can be detected even by the naked eye, which provides a facile method for the visual detection of Fe+3. Complexation of the Fe+3 ions opens the spirolactum ring of rhodamine moieties to produce specific color change as well as fluorescence development.
Chemistry has become an important tool in solving problems in biology and medicine. Chemists are engaged in developing new devices at the nanoscale level to aid in clinical diagnosis, real time monitoring of analytes, and analyzing neural activity, etc. Our research is aimed at designing and synthesizing fluorescent sensors to detect various divalent metal ions under physiological conditions. We designed and synthesized a chemosensor based on the principle of photoinduced electron transfer as a signal transduction mechanism in order to detect the presence of various divalent metal cations, particularly zinc. We report the synthesis, characterization and its spectrophotometric titrations with various metal ions. This sensor signals the presence of metal ions by a fluorescence signal and offers a significant advantage as it exhibits "off-on" behavior in the presence and absence of metal ions. The primary sensor was found to be very sensitive and selective toward zinc ions. We also aimed at optimizing the performance of this chemosensor by taking advantage of electron donating/withdrawing abilities of different functional groups by using computational methods. We devised a new scheme to synthesize it, unfortunately the modified sensor was not realized for unknown reason. We are currently involved in devising an alternate strategy to synthesize and develop new sensors for neutral molecules.
In this dissertation, we established a new approach assisted by computational chemistry to design fluorescent sensors. The approach is applicable to predict the behavior of a fluorophore-bridge-receptor sensor based on photoinduced electron transfer (PET). Our first designed rhodamine based pH sensor exhibits strong fluorescence under acidic conditions and very weak fluorescence under basic conditions, just as the computations predicted.
Over the past few decades, there has been significant interest in developing fluorescent probes, because they are useful tools for biological studies. As effective analytical techniques, fluorescent probes utilize distinct advantages offered by fluorescence detection in terms of sensitivity, selectivity, and fast response time. When fluorescent probes interact selectively with target molecules, ions or biological specimens, they can generate large optical responses. Since most ions or molecules, such as Zn2+, Ca2+, or pyrophosphate ion (PPi), are non-fluorescent, chemosensors having analyte binding-triggered fluorescence are appealing in many fields, like analytical chemistry, clinical biochemistry, medicine, and environmental science.This dissertation is devoted to the design, synthesis, and characterization of novel fluorescent sensors for Zn2+ and its associated applications. Chapter II of this dissertation presents several novel terpyridine-based fluorescent sensors with different substituents affecting the electronic and steric nature of the terpyridine (tpy) fluorophore. Sensors are designed to establish the correlation between sensor structure and its photophysical properties. Low temperature fluorescence is used to evaluate the essential role of intramolecular charge transfer (ICT) in zinc binding-induced fluorescence changes. The tpy molecular fragment has a relatively large [pi]-conjugated system which enables the potential [pi-pi] interaction between two tpy platforms and affects the fluorescence of tpy ligands. Chapter III introduces a dimeric tpy ligand containing two tpy fragments connected via a meta-phenylene unit. The detailed spectroscopic study shows that this ligand displays an attractive fluorescence turn-on, in sharp contrast to mono(tpy) ligand that shows fluorescence quenching upon binding Zn2+. The result suggests the existence of delicate structural influences on fluorescence of tpy derivatives.Chapter IV is devoted to 2-(2'-hydroxyphenyl)-1,3-benzoxazole (HBO) and 2-(2'-hydroxyphenyl)-1,3-benzothiazole (HBT) derivatives featured with a structural potential of excited-state intramolecular proton transfer (ESIPT). The study reveals additional information on the binding of HBO or HBT to metal cations, which aids the sensor design for Zn2+ and PPi detection. The molecular design aims to realize ESIPT process control upon complexation with an analyte. Chapter V is devoted to the synthesis of bis(HBO) derivatives which bind Zn2+ selectively and emit near-infrared (NIR) fluorescence as a consequence of metal ion binding-induced ESIPT turn-on. Preliminary cell stain experiment was conducted and indicated the potential biological applications.