The present study describes the synthesis of a chromophoric system 2-(5-(4-dimethylamino-benzylidin)-4-oxo-2-thioxo-thiazolidin-3-yl)acetic acid and its incorporation into starch through esterification of the hydroxyl group by the free carboxyl function of the chromophoric system by DCC coupling. The products were characterized by UV-visible, fluorescence, FT-IR, and NMR spectroscopic methods. The newly developed system was subjected to photoresponsive studies such as light absorption, light stabilization and fluorescence emission. The free chromophoric system and the coupled product were also subjected to thermal analysis. The results show that modification enhances the light absorption and light fastening properties of the chromophoric system. Thermal stability of the polymeric system greatly enhances on attaching the chromophoric system. In view of these results the newly developed system is proposed as a nature friendly, green, and photoactive product which could find application in dyes, inks, paints, and so forth.
Starch is an inexpensive commodity that has been used for food and many nonfood purposes for centuries. Starch, one of the main polysaccharides in the world, has been paid much attention because of its biodegradability and abundance as a renewable resource. It has been widely used as a raw material for biodegradable plastics. The chemical modification of starch is of interest because the modification would not change the fundamental skeleton of starch, would keep the original physicochemical and biochemical properties, and finally would bring new or improved properties. Chemically, starch is composed of two different molecules, amylose and amylopectin. In amylose, the glucose molecules are linked in a “linear” fashion; however, the tetrahedral chemistry of carbon (and the bond angles that result from this chemistry) gives amylose an overall spiral shape. Amylopectin, on the other hand, has a linear arrangement of glucose molecules which includes, at regular intervals, a different kind of linkage between two adjacent glucoses. This different linkage results in the formation of a branched structure and an overall treelike shape for this molecule. Starch consists of amylose and amylopectin, which are polymers of glucose, linear and branched, respectively. Each glucose unit potentially has three reactive hydroxyl groups that are the basis of all derivatizations. Native starches in its derivative forms open a wide scope in pharmaceutical-, food-, and nonfood based industries. Starch modification is generally achieved through derivatization such as esterification, etherification, cross-linking, acid hydrolysis, enzymatic hydrolysis heat treatment, and grafting of starch [
In the present study, functional modification of starch has been achieved by DCC coupling. The free hydroxyl group of the biopolymeric core material was esterified with free carboxylic group of the chromophoric system. Thus we could develop a thermo stable and photo responsive system on starch. Modified starches have a very promising industrial prospects on application basis, due to its more biocompatible and environmental friendly nature as compared to synthetic polymers.
Starch was purchased from Merck (Germany). Dimethylaminopyridine (DMAP) and dicyclohexylcarbodiimide (DCC) are commercial samples and are used as purchased from E. Merck India Ltd.
2-(5-(4-dimethylamino-benzylidin)-4-oxo-2-thioxo-thiazolidin-3-yl)acetic acid (2.25 g), starch (1 g), DMAP (200 mg), and DCC (1 g) were separately dissolved in DMF and mixed together. The mixture was stirred at room temperature for 2 hours and at 80°C for 6 hours. The byproduct dicyclohexyl urea (DCU) was removed by warming-cooling-filtration process and the solvents were removed on a vacuum rotary evaporator and dried. It was purified by column chromatography using silica column and 10 : 2 chloroform-methanol solvent system and dried in vacuum [
Spectroscopic analyses such as UV-visible, FT-IR, and 1HNMR have been done in order to visualize the chemical changes in the structure of starch. The green, nature friendly starch functionalized with 2-(5-(4-dimethylamino-benzylidin)-4-oxo-2-thioxo-thiazolidin-3-yl)-acetic acid is a novel photoresponsive system and reported in the present paper for the first time.
Synthesis of 2-(5-(4-dimethylamino-benzylidin)-4-oxo-2-thioxo-thiazolidin-3-yl) acetic acid.
IR spectrum of 2-(5-(4-dimethylamino-benzylidin)-4-oxo-2-thioxo-thiazolidin-3-yl)acetic acid was recorded in the solid state as KBr discs in the operating frequency range 4000–400 cm−1. 3440 cm−1(broad):
IR spectrum of 2-(5-(4-dimethylamino-benzylidin)-4-oxo-2-thioxo-thiazolidin-3-yl)acetic acid.
The proton NMR spectrum of the product was recorded in chloroform using a 500 MHz 1H NMR spectrophotometer. 1H NMR: 10.32 ppm (1H,s: –COOH), 7.42 ppm (2H,d:aromatic proton a), 6.74 ppm (2H,d:aromatic proton b), 4.81 ppm (1H,s:Hc), 3.65 ppm (2H,s:Hd), 3.09 ppm (6H,s:N(CH3)2) (Figure
H1 NMR spectrum of 2-(5-(4-dimethylamino-benzylidin)-4-oxo-2-thioxo-thiazolidin-3-yl)acetic acid.
The end hydroxyl functionalities of starch were esterified with the free carboxyl group of 2-(5-(4-dimethylamino-benzylidin)-4-oxo-2-thioxo-thiazolidin-3-yl)acetic acid through DCC coupling using DMAP as the catalyst (Scheme
Functional modification of starch with 2-(5-(4-dimethylamino-benzylidin)-4-oxo-2-thioxo-thiazolidin-3-yl) acetic acid.
IR spectrum was recorded in the operating frequency range 4000–400 cm−1. IR(KBr): 3326 cm−1(broad):
IR spectrum of starch modified with 2-(5-(4-dimethylamino-benzylidin)-4-oxo-2-thioxo-thiazolidin-3-yl)acetic acid.
The proton NMR spectrum of the product was recorded in chloroform using a 500 MHz 1H NMR spectrophotometer. 1H NMR: 7. 75 ppm (2H,d:Ha), 6.75 ppm (2H,d:Hb), 5.0 ppm (1H,S: OH group unreacted), 4.90 ppm (1H,s:Hc), 3.55 ppm (2H,s:Hd), 3.09 ppm (6H,S:N(CH3)2), 1.37–2.17 ppm (m: aliphatic protons of starch). The signal corresponding to COOH proton observed in the proton NMR spectrum of the dye was absent here (Figure
1H NMR spectrum of starch modified with 2-(5-(4-dimethylamino-benzylidin)-4-oxo-2-thioxo-thiazolidin-3-yl)acetic acid.
The UV-visible spectra of the chromophoric system 2-[5-4-dimethylamino-benzylidine-4-oxo-2-thioxo-thiozolidine-3-yl]acetic acid and starch modified with2-[5-4-dimethylamino-benzylidine-4-oxo-2-thioxo-thiozolidine-3-yl]acetic acid were recorded in chloroform. UV-visible spectra showed an intense band at 474 nm for the chromophoric system and a band centred at 503 nm for the coupled product. The
UV-visible spectra of (a) 2-[5-4-dimethylamino-benzylidine-4-oxo-2-thioxo-thiozolidine-3-yl]acetic acid and (b)starch modified with 2-[5-4-dimethylamino-benzylidine-4-oxo-2-thioxo-thiozolidine-3-yl]acetic acid.
The low molecular chromophoric system 2-[5-4-dimethylamino-benzylidine-4-oxo-2-thioxo-thiozolidine-3-yl]acetic acid and its polymer-bound analogue were subjected to light fastening studies. Solutions of the two systems were prepared in chloroform with same molar concentrations. These solutions were subjected to irradiation under visible radiant energy. The changes in the UV-visible absorption spectra as a function of time of irradiation were noted. A sudden, but gradual, decrease in intensity was noted on irradiating the low molecular chromophoric system under visible radiant energy. The intensity of absorption was 2.75 at zero time and this was decreased to 1.8 on prolonged irradiation for 5 hours. The starch modified with 2-[5-4-dimethylamino-benzylidine-4-oxo-2-thioxo-thiozolidine-3-yl]acetic acid showed high stability even after prolonged irradiation. The intensity of absorption was recorded as 3.2 at zero time. This remained nearly the same on irradiation for 5 hours. This shows the efficient light fastening property of the polymer anchored chromophoric system. The irradiation results are shown in Figure
Irradiation studies of (a) 2-[5-4-dimethylamino-benzylidine-4-oxo-2-thioxo-thiozolidine-3-yl]acetic acid and (b) starch modified with 2-[5-4-dimethylamino-benzylidine-4-oxo-2-thioxo-thiozolidine-3-yl]acetic acid.
The fluorescence emission spectra of 2-[5-4-dimethylamino-benzylidine-4-oxo-2-thioxo-thiozolidine-3-yl]acetic acid and starch coupled with 2-[5-4-dimethylamino-benzylidine-4-oxo-2-thioxo-thiozolidine-3-yl]acetic acid were recorded in chloroform. The excitation wavelength was 400 nm. The emission maximum for the free chromophoric system was observed at 554 nm and the coupled product was observed at 567 nm. Compared to the pure chromophoric system, the coupled product shows appreciable increase in emission maximum. The intensity of fluorescence emission was also greatly enhanced on attaching to the polymer. The fluorescence spectra of modified as well as pure chromophoric system are shown in Figure
Fluorescence spectra of (a) 2-[5-4-dimethylamino-benzylidine-4-oxo-2-thioxo-thiozolidine-3-yl]acetic acid and (b) starch modified with 2-[5-4-dimethylamino-benz ylidine-4-oxo-2-thioxo-thiozolidine-3-yl]acetic acid.
In the case of the TG curve of starch, a sudden degradation with 90% weight loss occurs at 350°C. The first thermal event occurs at a weight loss of 15%; this is due to the dehydroxylation taking place in the core system (see Table
Thermal analysis data (TG) of starch.
Mass % | Start temp. | End temp. | Weight loss % |
---|---|---|---|
80–100 | 104.53 | 261.75 | 10 |
60–20 | 277.74 | 350.70 | 80 |
TG-DTA curve of starch.
TG of chromophoric system shows four mass loss steps. In the first thermal event between 137–203°C, 36 mass % loss was observed (see Table
Thermal analysis data (TG) of 2-(5-(4-dimethylamino-benzylidin)-4-oxo-2-thioxo-thiazolidin-3-yl)acetic acid.
Mass % | Start temp. | End temp. | Weight loss % |
---|---|---|---|
60–100 | 137.19 | 203.54 | 36.04 |
40–60 | 206.28 | 245.61 | 17.086 |
20–40 | 294.18 | 375.58 | 18.04 |
TG-DTA curve of 2-(5-(4-dimethylamino-benzylidin)-4-oxo-2-thioxo thiazolidin-3-yl)acetic acid.
It was noted that TG of modified starch shows two mass loss steps as shown in the curve. In the first thermal event 117–387°C, 65.03% mass loss is observed (see Table
Thermal analysis data (TG) of starch functionalised 2-(5-(4-dimethylamino-benzylidin)-4-oxo-2-thioxo-thiazolidin-3-yl)acetic acid.
Mass % | Start temp | End temp | Weight loss % |
---|---|---|---|
100–40 | 117.87 | 387.58 | 65.03 |
40–20 | 467.42 | 644.86 | 26.37 |
TG-DTA curve of starch modified with 2-(5-(4-dimethylamino-benzylidin)-4-oxo-2-thioxo-thiazolidin-3-yl)acetic acid.
The thermogravimetric analysis of modified polymer, chromophoric system, and core material were compared. The analysis shows that the degradation temperature was increased in the case of starch modified with 2-(5-(4-dimethylamino-benzylidin)-4-oxo-2-thioxo-thiazolidin-3-yl)acetic acid compared to the core or the low molecular chromophoric system. The overall thermal stability of polymer was also enhanced upon functional modification with the chromophoric system.
In the present study the environment friendly natural polymeric system such as starch was used as the core material for developing nature friendly, “green” photoactive system based on polymeric systems. Modified starches have very promising industrial prospects on application basis, due to its more biocompatible and environment friendly nature as compared to synthetic polymers. The paper reports the development of photoactive, thermostable starch moiety. Starch was modified with chromophoric system, namely, 2-(5-(4-dimethylamino-benzylidin)-4-oxo-2-thioxo-thiazolidin-3-yl)acetic acid by DCC coupling between the carboxyl function of chromophoric system and hydroxyl functionalities of starch. The modified products were characterised and compared by spectroscopic techniques. The modified starch shows remarkable red shift and increase in intensity of absorption and emission. Light irradiation studies of the 2-(5-(4-dimethylamino-benzylidin)-4-oxo-2-thioxo-thiazolidin-3-yl)acetic acid and starch modified with 2-(5-(4-dimethylamino-benzylidin)-4-oxo-2-thioxo-thiazolidin-3-yl)acetic acid were compared. The modified system shows excellent light fastening properties. The fluorescence emission maximum of starch coupled with 2-[5-4-dimethylamino-benzylidine-4-oxo-2-thioxo-thiozolidine-3-yl]acetic acid was observed at 567 nm. TGA studies were conducted to determine the effect of esterification on the thermal stability of starch. The results indicated that modified starch has increased thermostability compared to the unmodified starches.
The authors thank the Department of Science and Technology (Ministry of Science and Technology), the Government of India, New Delhi, and University Grant Commission, New Delhi, for financial support by awarding major and minor research projects.