Small Organic Molecule Based Gelators and Their Sensing Applications

Mohar, Mrittika (2020) Small Organic Molecule Based Gelators and Their Sensing Applications. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Mrittika Mohar (16RS056)) 16RS056.pdf - Submitted Version Restricted to Repository staff only Download (14MB)

Abstract

The thesis entitled as “Small organic molecule based gelators and their sensing applications” is about the synthesis, characterization of various gelators based on conventional or non-conventional materials. These gels have been applied as chemosensors for vaious guest species. This thesis is divided into six chapters. Chapter 1 provides the introduction of rest of the chapters. It mainly deals with the supramolecular chemistry and gels. Various aspects of gelation like gelator design, role of solvent in gelation, stimuli for gelation, characterization of gel, etc. have been highlighted. Need of chemosensors in today’s world is very much crucial and nowadays gel-based chemosensing are gaining huge popularity because of increasing pollution and contamination of pollutant in food, water, etc. Some of the reported literature of gel-based sensing have been discussed for understanding the topic clearly. Chapter 2 entitled “Phenylalanine based low molecular weight gelator for the removal of metal ions and dyes from wastewater” describes the synthesis of a two component gel from non-conventional materials which is associated with remediation of several environment pollution. In this present work, methyl ester of phenylalanine (Phe-OMe), a non-gelator has been successfully applied to produce two-component charge transfer induced organogel and metallohydrogel using picryl chloride and Cu(II) respectively. These gels and their gelation processes were utilized to remove various toxic metal ions and ionic dyes (especially zwitterionic dye) from wastewater. Phe-OMe can drop down these metal concentrations below the permissible limit of WHO within ten minutes. The organogel showed instant response i.e. gel to sol towards volatile base or acid. This property makes the gel act as sensors of harmful acidic and basic vapors like HCl, ammonia, etc. in the laboratories as well as in different industries. Chapter 3 entitled “Molecular engineering of a non-gelator tyrosine containing tripeptide to derive organogel” deals with the synthesis of a series of organogel derived from substituted tyrosine derivatives via applying the concept of molecular engineering. Phenylalanine and tyrosine are structurally very similar, only phenylalanine forms gel in water as well as organic solvents. From that point of view the challenge was taken to transform tyrosine into a gelator. So the concept of molecular engineering was applied. It was observed that a single nitro group is sufficient enough to transform a nongelator into a gelator. Not only a Boc-protected amino acid but also tripeptide was also found to form stable organogel. Theoretical studies were also performed to understand the interactions involved during the gelation process. The gels derived from 1, 2, and 4 were found to be responsive towards strong base like hydroxide and this base responsive gel to sol transformation also helped us to get information regarding gel model. Chapter 4 titled as “Cascade sensing of iodide and fluoride by tryptophan derived low molecular weight gelator” describes the design of an optical cascade sensor of iodide and fluoride ions based on simple tryptophan derivative containing urea functional group. The tetrahydrofuran solution of the sensor becomes yellow in the presence of iodide ion and this binary yellow solution shows bright green emission under 366 nm UV irradiation in the presence of fluoride ion. The sensor is one of the very few examples of cascade sensors. The sensor was utilized to detect iodide present in water which can be helpful for monitoring of iodide in real samples. A stable organogel was obtained from the same compound in 1,2-dichlorobenzene which showed acid vapor responsive gel to sol transformation. This can be utilized for sensing acid vapors present in the air. Along with that, the gel shows good selectivity for adsorbing zwitterionic dye compared to cationic, anionic or neutral dye. Chapter 5 titled as “A Metallogel Based on a Zwitterionic Spirocyclic Meisenheimer Complex: Sensing of Fluoride Ions in Water and Moisture Content in Organic Solvents” reports the synthesis of a stimuli-responsive metallogel. Picric acid and N,N' -dicyclohexylcarbodiimide derived zwitterionic spirocyclic Meisenheimer complex was mixed with Fe³⁺ ion in 2:1 ratio in chloroform to produce the gel. Interestingly, the Meisenheimer complex shows bright yellow emission under 366 nm UV light. But the Fe³⁺ based metallogel does not show any emission under UV irradiation. However, in the presence of fluoride contaminated water, the metallogel surface shows bright yellow emission while checking inside a UV chamber. Thus the non-fluorescent metallogel was utilized as a sensor of aqueous fluoride ion. The sensor is able to detect aqueous fluoride down to 156 ppb concentration. The dried metallogel solution acted as a sensor of moisture. The fluorescence intensity of the dried metallogel solution was found to increase with an increase in the percentage of water. In this way, the xerogel was utilized as a sensor of undesired water content present in different organic solvents which can detect moisture content with a minimum detection limit of 0.04% for dichloromethane. Chapter 6 titled as “2,4,7-Triaminofluorenone as a Multi-Analyte Colorimetric Sensor of Fluoride, Acetone Vapor, and Other Harmful Compounds ” describes the synthesis of 2,4,7-Triaminofluorenone, a fluorenone derived multi-analyte colorimetric chemosensor via an extremely cost-effective synthetic pathway. The sensor molecule can detect various analytes such as fluoride ion, acetone vapor, and picric acid. The violet solution of the sensor molecule becomes deep blue in the presence of fluoride ion. It can detect fluoride down to 120 ppb level. The compound in the solid state shows vapochromism only in the presence of acetone. The grey colored solid coating of the sensor molecule becomes blue in the presence of acetone vapor. This property was utilized to sense acetone vapor. Color Grab, an android based app, was utilized to detect acetone in real time down to 10 ppm level. Apart from that, the same sensor molecule was also tested for sensing picric acid, volatile acid and base like HCl and NH₃.

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