PVA@RHB@OG ORGANOGEL PHOTOCATALYST FOR PHOTOCATALYTIC C-H ACTIVATION FOR C-N/C-S BOND FORMATION

Abstract

Although using solar energy for pharmaceutical manufacture presents hurdles, the potential benefits in terms of sustainability, cost-effectiveness, and technical innovation make it an exciting path for the scientific community to investigate and develop further. We present a highly efficient solar light active supramolecular network organogels as a photocatalyst. The as-prepared PVA@RhB and PVA@RhB@OG organogels as a photocatalyst catalytic materials were used to create C-N and C-S bonds for drug intermediates and value-added products because of their high bond energy, low polarity, and strong inertia C-H bond activation. The current research work sets a new standard for the environmentally friendly, highly selective C-H activation for the production of C-N/C-S bonds under solar light.

Specification

DESC:FIELD OF THE INVENTION
The present invention relates to PVA@RhB@OG Organogel Photocatalyst for Photocatalytic C-H Activation for C-N/C-S Bond Formation.
BACKGROUND OF THE INVENTION
References which are cited in the present disclosure are not necessarily prior art and therefore their citation does not constitute an admission that such references are prior art in any jurisdiction. All publications, patents and patent applications herein are incorporated by reference to the same extent as if each individual or patent application was specifically and individually indicated to be incorporated by reference.
Luminous energy is receiving more attention these days since it is a cost-effective and "green" renewable energy source. Therefore, it is necessary to comprehend how to catch and use light energy[1]. Photocatalysis is gaining interests for its ability to utilize solar light for catalytic processes.[2]The photocatalysis method holds promise for addressing several environmental challenges and develop sustainable technologies for various industrial processes.Among various applications of photocatalysis, C-H bond activation via photocatalysis has garnered great attention in organic synthesis because it offers a greener and better alternative to traditional methods that mostly require harsh working conditions and expensive transition metal catalyst.[3] In this process, a photocatalyst absorbs light photons that triggers generation of reactive species like radicals or excited states. These species subsequently reacts with organic substrates, causing the activation of C-H bonds.[3, 4] By controlling the conditions of the reaction and selecting the appropriate photocatalyst, the precise selectivity and modification of particular C-H bonds within intricate organic molecules can be attained. C-H bond activation via photocatalysis has risen as a powerful technique for the synthesis of pharmaceuticals, agrochemicals, and fine chemicals due to its mild reaction conditions and lowered waste generation.[5, 6] The prime focus in photocatalysis are centered on enhancing the effectiveness and specificity of photocatalysts by causing various modifications and tailoring of photocatalyst material.[7, 8]
Hydrogels are three-dimensionally crosslinked polymeric networks with a variety of properties, including low slip friction, shock absorption, and swelling and deswelling.[9, 10] Because of these properties, hydrogels have found significant uses in a wide range of industries, including drug delivery,[11, 12] tissue engineering,[13, 14] effluent disposal,[15, 16] and biomolecular filters. [17, 18] Hydrogels can only be used in fields where load bearing is required because their mechanical qualities are severely diminished in a state of high water absorption[19]. High mechanical property hydrogels have progressively drawn more attention in recent years. Hydrogels with exceptional strength and durability have numerous applications, such as contact lenses, artificial muscles, artificial organs, and cartilage replacement[20]. Although natural hydrogels with low mechanical and chemical stability, including chitosan and gelatin gels, have good biocompatibility, their uses are limited.
As a biocompatible and water-soluble synthetic polymer, polyvinyl alcohol (PVA) has several -OH functional groups that can be cross-linked to create hydrogels[21]. Because of its distinct structure, PVA's mechanical qualities can be enhanced even more while maintaining high biocompatibility. As a result, PVA hydrogels with high durability and appropiate strength are suitable candidates because they have outstanding functionalization and basic properties and have a wide range of possible applications[22].
Strengthening and toughening often include chemical or physical cross-linking[23]. Chemical cross-linking can be achieved by several methods, including, radical polymerization, energy radiation, chemical interactions between enzyme crosslinking and functional groups [24]. Physical cross-linking relies on intermolecular interactions, such as hydrophobic, ionic/electrostatic, and hydrogen bonding, which are often achieved by freeze-thaw cycles, annealing, and other methods[12]. Despite extensive study on toughening and strengthening PVA hydrogels, still they lack versatile and ubiquitous methods for modifying their mechanical properties. Herein, we describe an easy method to fabricate PVA@RhB and PVA@RhB@OG organogel photocatalysts with high strength and toughness via dual chemical and physical cross-linking as illustrated in Scheme 1.
Here, we synthesised a novel organogel polymer with favourable physical and fluorescent characteristics. By esterification at 120 °C, the carboxyl group (-COOH) and hydroxy group (-OH) of polyvinyl alcohol (PVA) can be crosslinked to create a stable organogel polymer in polar aprotic solvent dimethyl sulfoxide (DMSO). Next, we added rhodamine B (RhB) and Orange G (OG) from the acceptor to create effective artificial light-harvesting devices with a high antenna effect.[25-27] Therefore, successfully synthesized novel PVA@RhB and PVA@RhB@OG organogels photocatalysts show high efficiency in photocatalytic performance for solar light initiated aerobic oxidative cyclization of thiobenzamide 1a to 3,5-diphenyl 1,2,4-thiadiazole 2a C-N and C-S bond formation via C-H bond activation with excellent 97% yield
Several patents issued for photocatalysts but none of these are related to the present invention. Patent US10449530B2 relates to photocatalysts for reduction of carbon dioxide and water are provided that can be tuned to produce certain reaction products, including hydrogen, alcohol, aldehyde, and/or hydrocarbon products. These photocatalysts can form artificial photosystems and can be incorporated into devices that reduce carbon dioxide and water for production of various fuels. Doped wide-bandgap semiconductor nanotubes are provided along with synthesis methods. A variety of optical, electronic and magnetic dopants (substitutional and interstitial, energetically shallow and deep) are incorporated into hollow nanotubes, ranging from a few dopants to heavily-doped semiconductors. The resulting wide-bandgap nanotubes, with desired electronic (p- or n-doped), optical (ultraviolet bandgap to infrared absorption in co-doped nanotubes), and magnetic (from paramagnetic to ferromagnetic) properties, can be used in photovoltaics, display technologies, photocatalysis, and spintronic applications.
Another patent US9205420B2 relates to nanostructures and compositions comprising nanostructures, methods of making and using the nanostructures, and related systems. In some embodiments, a nanostructure comprises a first region and a second region, wherein a first photocatalytic reaction (e.g., an oxidation reaction) can be carried out at the first region and a second photocatalytic reaction (e.g., a reduction reaction) can be carried out at the second region. In some cases, the first photocatalytic reaction is the formation of oxygen gas from water and the second photocatalytic reaction is the formation of hydrogen gas from water. In some embodiments, a nanostructure comprises at least one semiconductor material, and, in some cases, at least one catalytic material and/or at least one photosensitizing agent.
Another patent US7541509B2 discloses a photocatalyst nanocomposite which can be used to destroying biological agents includes a carbon nanotube core, and a photocatalyst coating layer covalently or ionically bound to a surface of the nanotube core. The coating layer has a nanoscale thickness. A method of forming photocatalytic nanocomposites includes the steps of providing a plurality of dispersed carbon nanotubes, chemically oxidizing the nanotubes under conditions to produce surface functionalized nanotubes to provide C and O including groups thereon which form ionic or covalent bonds to metal oxides, and processing a metal oxide photocatalyst sol-gel precursor in the presence of the nanotubes, wherein a nanoscale metal oxide photocatalyst layer becomes covalently or ionically bound to the nanotubes.
Another patent CN107349937B discloses a preparation method of a graphene-based bimetallic sulfide nano composite photocatalyst, which is characterized in that a series of graphene-based bimetallic sulfide composite photocatalysts are synthesized by a simple and mild one-step hydrothermal method, and the morphology size and the oriented growth on the surface of graphene of bimetallic sulfide are regulated and controlled by controlling the hydrothermal reaction temperature, the reaction time, the addition amount of graphene and the content of a metal salt compound in a composite system; in addition, the preparation method provided by the invention is simple in process operation, low in raw material price and suitable for large-scale industrial production.
Another patent CN102921416B relates to a silver doped grapheme-zinc oxide nano composite photocatalytic material and a method for preparing the same and belongs to the technical field of nano composite materials and photocatalysis. Graphite oxide is subjected to ultrasonic dispersion to obtain a dispersion solution of graphene oxide; precursors of silver ions and zinc ions are added in the dispersion solution of the graphene oxide, and the mixture is placed into a reaction kettle to be subjected to hydro-thermal treatment to be prepared into the silver doped grapheme-zinc oxide nano composite photocatalytic material in situ after the pH is regulated to an alkaline condition. Photocatalytic degradation experiments show that the silver doped grapheme-zinc oxide nano composite photocatalytic material prepared through the method has good adsorption and visible light photocatalytic degradation effects on rhodamine B and is an ideal nano composite photocatalytic material.
OBJECTS OF THE INVENTION
Main object of the present invention is PVA@RhB@OG organogel photocatalyst for photocatalytic C-H activation for C-N/C-S bond formation.
Another object of the present invention is to achieve selective activation of C-H bonds under mild conditions without harsh chemicals.
Another object of the present invention is to use solar light as the primary energy source for photocatalysis, reducing reliance on artificial light or heat.
Another object of the present invention is to increase reaction efficiency and speed in C-H activation and subsequent C-N/C-S bond formation using the supramolecular network organogel.
Another object of the present invention is to develop a reusable organogel matrix that aligns with green chemistry principles, minimizing waste and environmental impact.
SUMMARY OF THE INVENTION
This summary is provided to introduce a selection of concepts, in a simplified format, that are further described in the detailed description of the invention.
This summary is neither intended to identify key or essential inventive concepts of the invention and nor is it intended for determining the scope of the invention.
To further clarify advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof, which is illustrated in the appended drawings.
For the preparation of PVA@RhB organogel photocatalyst, 200 mg of RhB was dissolved into 100 mL DMSO solution, and the reaction mixture was stirred for 30 minutes at 120oC. After that, 1.5 gm of polyvinyl alcohol was dropped into the reaction solution and refluxed at 120oC for 5 hours. The temperature of the solution was cooled down to 50oC and ethylene glycol was added to it, the solution was then vigorously stirred for 30 minutes. The resultant mixture was deep freezed at -20 oC to obtain the PVA@RhB organogel photocatalyst.
To synthesize PVA@RhB@OG organogel photocatalyst, the solution of 100mg of OG dye and 1.0 ml of ethylene glycol were added to the previously synthesized PVA@RhB organogel photocatalyst, and was stirred for 30 minutes at 50oC. The solution was deep freeze at -20 oC, PVA@RhB@OG organogel photocatalyst was obtained.
Herein enclosed a PVA@RhB@OG organogel photocatalyst comprising of
Polyvinyl alcohol (PVA), rhodamine B (RhB), ethylene glycol, dimethyl sulfoxide (DMSO), Orange G (OG), thiobenzamide, dimethylformamide (DMF).
A method for the preparation of PVA@RhB@OG Organogel Photocatalyst for Photocatalytic C-H Activation for C-N/C-S Bond Formation comprising the steps of
Dissolving 200 mg of RhB into 100 mL DMSO solution, and the reaction mixture was stirred for 30 minutes at 120oC;
Dropping 1.5 gm of polyvinyl alcohol into the reaction solution and refluxed at 120oC for 5 hours after that;
Cooling the temperature of the solution to 50oC and ethylene glycol was added to it;
Stirring the solution vigrously for 30 minutes;
freezing the resultant mixture at -20 oC to obtain the PVA@RhB organogel photocatlyst;
adding the solution of 100mg of OG dye and 1.0 ml of ethylene glycol to the previously synthesized PVA@RhB organogel photocatalyst, and stirred for 30 minutes at 50oC;
obtaining the PVA@RhB@OG organogel photocatalyst after the solution was deep freezed at -20 oC;
adding 2 mol% PVA@RhB@OG organogel photocatalyst, 100 mg of thiobenzamide to 3mL of DMF solution in a 25mL glass vial;
stirring for 2-3hours while being under solar light irradiation in an aerobic atmosphere at ambient temperature;
washing the reaction mixture with 5mL H2O followed by 15mL ethyl acetate after completion of the reaction;
filtering the PVA@RhB@OG organogel photocatalyst by phase separation method;
filtering the joined organic phases over MgSO4 to trap moisture and filtrate evaporated under reduced pressure; and
obtaining a pure product of 3,5-diphenyl 1,2,4-thiadiazole.
The Photocatalytic aerobic oxidative cyclization of thiobenzamide 1a into 3,5-diphenyl 1,2,4-thiadiazole 2b via C-N/C-S bond formation via C-H bond activation.
The obtained crude product was then purified by silica gel column chromatography using a gradient solution of hexane ethyl acetate to (95%) yield a pure product.
BRIEF DESCRIPTION OF THE DRAWINGS
The illustrated embodiments of the subject matter will be understood by reference to the drawings, wherein like parts are designated by like numerals throughout. The following description is intended only by way of example, and simply illustrates certain selected embodiments of devices, systems, and methods that are consistent with the subject matter as claimed herein, wherein:
Figure 1. Schematic representation of structure and formation intermolecular of PVA@RhB and PVA@RhB@OG organogel photocatalyst.
Figure 2. Photocatalytic aerobic oxidative cyclization of thiobenzamide 1a into 3,5-diphenyl 1,2,4-thiadiazole 2a in presence of PVA@RhB@OG organogel photocatalyst.
Figure 3. (a) U-V visible spectra of RhB, OG, PVA@RhB, and PVA@RhB@OG organogel photocatalyst along with (b) Tauc plot of PVA@RhB@OG organogel photocatalyst to calculate band gap energy.
Figure 4. (a) FTIR spectra of RhB, OG, PVA@RhB, and PVA@RhB@OG photocatalyst and (b) X-ray diffraction pattern of RhB PVA@RhB, OG, and PVA@RhB@OG photocatalyst.
Figure 5. Cyclic voltammogram of PVA@RhB and PVA@RhB@OG organogel photocatalys.
Figure 6. (a) Tafel plot of RhB, PVA@RhB, OG, and PVA@RhB@OG organogel photocatalyst and (b) Nyquist plots of RhB, PVA@RhB, OG and PVA@RhB@OG organogel photocatalyst measured by EIS spectra.
Figure 7. SEM images of (a) PVA@RhB organogel photocatalyst and (b) PVA@RhB@OG organogel photocatalyst.
Figure 8. EDX spectrum of (a) PVA@RhB organogel and (b) PVA@RhB@OG organogel.
Figure 9. Schematic illustration of photocatalytic aerobic oxidative cyclization of thiobenzamide 1a into 3,5-diphenyl 1,2,4-thiadiazole 2a in presence of PVA@RhB@OG organogel photocatalyst.
BRIEF DESCRIPTION OF THE TABLES
Table 1. Optimization table for photostatic efficiency.
Table 2. Optimization table for experimental reaction conditions.
DETAILED DESCRIPTION OF THE INVENTION
The detailed description of various exemplary embodiments of the disclosure is described herein with reference to the accompanying drawings. It should be noted that the embodiments are described herein in such details as to clearly communicate the disclosure. However, the amount of details provided herein is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the present disclosure as defined by the appended claims.
It is also to be understood that various arrangements may be devised that, although not explicitly described or shown herein, embody the principles of the present disclosure. Moreover, all statements herein reciting principles, aspects, and embodiments of the present disclosure, as well as specific examples, are intended to encompass equivalents thereof.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms "a"," "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises," "comprising," "includes" and/or "including," when used herein, specify the presence of stated features, integers, steps, operations, elements and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof.
It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may, in fact, be executed concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved.
In addition, the descriptions of "first", "second", "third", and the like in the present invention are used for the purpose of description only, and are not to be construed as indicating or implying their relative importance or implicitly indicating the number of technical features indicated. Thus, features defining "first" and "second" may include at least one of the features, either explicitly or implicitly.
Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, e.g., those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
In some embodiments of the present invention, For the preparation of PVA@RhB organogel photocatalyst, 200 mg of RhB was dissolved into 100 mL DMSO solution, and the reaction mixture was stirred for 30 minutes at 120oC. After that, 1.5 gm of polyvinyl alcohol was dropped into the reaction solution and refluxed at 120oC for 5 hours.
In some embodiments of the present invention, temperature of the solution was cooled down to 50oC and ethylene glycol was added to it, the solution was then vigrously stirred for 30 minutes. The resultant mixture was deep freezed at -20 oC to obtain the PVA@RhB organogel photocatlyst.
In some embodiments of the present invention, to synthesise PVA@RhB@OG organogel photocatalyst, the solution of 100mg of OG dye and 1.0 ml of ethylene glycol were added to the previously synthesized PVA@RhB organogel photocatalyst, and was stirred for 30 minutes at 50oC. The solution was deep freezed at -20 oC, PVA@RhB@OG organogel photocatalyst was obtained.
In some embodiments of the present invention, photocatalytic aerobic oxidative cyclization of thiobenzamide 1a into 3,5-diphenyl 1,2,4-thiadiazole 2b via C-N/C-S bond formation via C-H bond activation is shown in fig 1. In a 25mL glass vial, 2 mol% PVA@RhB@OG organogel photocatalyst, 100 mg of thiobenzamide were added to 3mL of DMF solution, stirred for 2-3hours while being under solar light irradiation in an aerobic atmosphere at ambient temperature.
In some embodiments of the present invention, the reaction progress was monitored by thin layer chromatography (TLC) method. After completion of the reaction, the reaction mixture was washed with 5mL H2O followed by 15mL ethyl acetate and then PVA@RhB@OG organogel photocatalyst was filtered out by phase separation method. The joined organic phases was filtered over MgSO4 to trap moisture and filtrate evaporated under reduced pressure.
In some embodiments of the present invention, the obtained crude product was then purified by silica gel column chromatography using a gradient solution of hexane ethyl acetate to (95%) yield a pure product.
Herein enclosed a PVA@RhB@OG organogel photocatalyst comprising of
Polyvinyl alcohol (PVA), rhodamine B (RhB), ethylene glycol, dimethyl sulfoxide (DMSO), Orange G (OG), thiobenzamide, dimethylformamide (DMF).
A method for the preparation of PVA@RhB@OG Organogel Photocatalyst for Photocatalytic C-H Activation for C-N/C-S Bond Formation comprising the steps of
Dissolving 200 mg of RhB into 100 mL DMSO solution, and the reaction mixture was stirred for 30 minutes at 120oC;
Dropping 1.5 gm of polyvinyl alcohol into the reaction solution and refluxed at 120oC for 5 hours after that;
Cooling the temperature of the solution to 50oC and ethylene glycol was added to it;
Stirring the solution vigrously for 30 minutes;
freezing the resultant mixture at -20 oC to obtain the PVA@RhB organogel photocatlyst;
adding the solution of 100mg of OG dye and 1.0 ml of ethylene glycol to the previously synthesized PVA@RhB organogel photocatalyst, and stirred for 30 minutes at 50oC;
obtaining the PVA@RhB@OG organogel photocatalyst after the solution was deep freezed at -20 oC;
adding 2 mol% PVA@RhB@OG organogel photocatalyst, 100 mg of thiobenzamide to 3mL of DMF solution in a 25mL glass vial;
stirring for 2-3hours while being under solar light irradiation in an aerobic atmosphere at ambient temperature;
washing the reaction mixture with 5mL H2O followed by 15mL ethyl acetate after completion of the reaction;
filtering the PVA@RhB@OG organogel photocatalyst by phase separation method;
filtering the joined organic phases over MgSO4 to trap moisture and filtrate evaporated under reduced pressure; and
obtaining a pure product of 3,5-diphenyl 1,2,4-thiadiazole.
The Photocatalytic aerobic oxidative cyclization of thiobenzamide 1a into 3,5-diphenyl 1,2,4-thiadiazole 2b via C-N/C-S bond formation via C-H bond activation.
The obtained crude product was then purified by silica gel column chromatography using a gradient solution of hexane ethyl acetate to (95%) yield a pure product.
EXAMPLE 1
EXPERIMENTAL SECTION
Materials and Methods
Polyvinyl alcohol (PVA), rhodamine B (RhB), ethylene glycol, dimethyl sulfoxide (DMSO), Orange G (OG), thiobenzamide, dimethylformamide (DMF), and ethyl acetate were purchased from sigma Aldrich and TCI. Deionised water was used throughout the experiment.
Instruments and measurements
UV-Visible absorbance spectra were recorded on a Shimadzu UV-1800 spectrometer. Fourier transform infrared spectroscopy (FTIR) spectra were recorded on a Nicole 6700 (made by Thermo Scientific, USA) spectrometer. X-ray diffraction (XRD) of synthesized organogel was recorded on Bruker, AXS D8 Advance X-ray diffractometer and Cu Ka radiation. Scanning electron microscope (SEM) images were recorded on a FET Phillips instrument [Model No. 200k VLAB6, (FEL TECNAI G2-20S- Twin)] operated at (?=0.15406 nm) 200 kvEDX spectra were recorded on an Axis Nova spectrometer (KRATOS). Cyclic voltammetry, electrochemical impedance spectra and Tafel plot were recorded on CHl608E, 220V instrument.
Synthesis of PVA@RhB organogel photocatalyst
For the prepartion of PVA@RhB organogel photocatalyst, 200 mg of RhB was dissolved into 100 mL DMSO solution, and the reaction mixture was stirred for 30 minutes at 120oC. After that, 1.5 gm of polyvinyl alcohol was dropped into the reaction solution and refluxed at 120oC for 5 hours. The temperature of the solution was cooled down to 50oC and ethylene glycol was added to it, the solution was then vigorously stirred for 30 minutes. The resultant mixture was deep freeze at -20 oC to obtain the PVA@RhB organogel photocatalyst. (Scheme 1)
Synthesis of PVA@RhB@OG organogel photocatlyst
To synthesise PVA@RhB@OG organogel photocatalyst, the solution of 100mg of OG dye and 1.0 ml of ethylene glycol were added to the previously synthesized PVA@RhB organogel photocatalyst, and was stirred for 30 minutes at 50oC. The solution was deep freezed at -20 oC, PVA@RhB@OG organogel photocatalyst was obtained. (Scheme 1).
Aerobic oxidative cyclization of thiobenzamide into 3,5-diphenyl 1,2,4-thiadiazole
Photocatalytic aerobic oxidative cyclization of thiobenzamide 1a into 3,5-diphenyl 1,2,4-thiadiazole 2b via C-N/C-S bond formation via C-H bond activation is shown in fig 1. In a 25mL glass vial, 2 mol% PVA@RhB@OG organogel photocatalyst, 100 mg of thiobenzamide were added to 3mL of DMF solution, stirred for 2-3hours while being under solar light irradiation in an aerobic atmosphere at ambient temperature. The reaction progress was monitored by thin layer chromatography (TLC) method. [28] After completion of the reaction, the reaction mixture was washed with 5mL H2O followed by 15mL ethyl acetate and then PVA@RhB@OG organogel photocatalyst was filtered out by phase separation method. The joined organic phases was filtered over MgSO4 to trap moisture and filtrate evaporated under reduced pressure. The obtained crude product was then purified by silica gel column chromatography using a gradient solution of hexane ethyl acetate to (95%) yield a pure product. The optimization experiments were carried out following the same procedure with required changes in some variables as outlined in Table 1-2. [29-31] The crude product was identified by using 1H-NMR. [1H-NMR (CDCl3) 7.83-7.86 (m, 2H), 7.48-7.51 (m, 2H), 7.37-7.47 (m, 6H)]
EXAMPLE 2
Result and discussion
U-V Visible absorption spectral studies
UV-visible spectroscopy is used to illustrate the absorbance peak of RhB, OG, PVA@RhB and PVA@RhB@OG organogel photocatalyst (fig. 2a). The absorption peak of RhB dye was observed nearly at 553nm.[32] The RhB dye coupled through ester linkage with PVA to form PVA@RhB organogel photocatalyst. Therefore, due to coupling the absorbance of PVA@RhB photocatalyst increased. Additionally, OG shows absorbance at 450-500nm which when further coupled with PVA@RhB resulted in the formation of PVA@RhB@OG organogel photocatalyst, which showed the red shift at 500-550nm i.e. enhancement of absorbance capacity. This showed the better photocatalytic activity for better charge excitation capability of the PVA@RhB@OG organogel photocatalyst.[33] As shown inset of figure 2b, the optical band gap calculated Eg = 2.07 eV by using Tauc plot method. As per literature, the electronic properties of semiconductor material are based on the optical band gap. The optical band gap 2.07ev indicated the semiconductor nature of photocatalyst. The lower band gap and high absorption capacity of PVA@RhB@OG organogel photocatalyst as compared to RhB, PVA@RhB organogel and OG resulted in the high photocatalytic activity of semiconductor nature PVA@RhB@OG organogel photocatalyst.
FTIR and XRD analysis
Fourier transform infrared (FTIR) spectroscopy were investigated to study the vibrational frrequency occurred via stretching of the characteristic bonds.[34] Fig.3a, illustrated the FTIR spectra of RhB, PVA@RhB organogel, OG and PVA@RhB@OG organogel photocatalyst. As observed in fig. 3a, the RhB dye have the characteristic peaks at 1587, 1686 and 1644cm-1, which occurred due to stretching vibrations of aromatic hydrocarbons vibrations (?C=C),[35] carboxylic group (?C=O) [36] and aromatic ring (?C-H), respectively. [37] However, FTIR spectra of PVA@RhB organogel photocatalyst show left shifted C=O peak at 1720 cm-1 [38] which indicated the ester linkage between PVA and RhB. Therefore, we can confirm the presence of ester linkage in the PVA@RhB photocatalyst via FTIR spectroscopy. Furthermore, the spectra of PVA@RhB@OG organogel photocatalyst show slightly shifted peaks as compare to PVA@RhB photocatalyst due to OG functionalization.
X-ray diffraction analysis is a prominently used method for investigating the crystal structure and physical change upon cross linking of the compound.[39] In fig. 3b, the XRD pattern of the RhB with scan range between 5-45o shows well defined sharp and intense multiple peaks 2? at 19.72 o,14.56 o, 19.3 o, 22.3o, 23.3o. Which reveal the highlly crystalline behavior of starting material RhB.[40] However, the XRD spectrum of PVA@RhB organogel photocatalyst exhibited right-shifted broad and weak peaks 2? at 20.1o and 22.5o with decrease in crystallinity, as seen by the absence of multiple peaks, show amorphous nature. The XRD spectra of OG show multiple sharp and intense peaks which indicate the crystalline behavior of OG. We observed that, the intensity of the diffraction peak PVA@RhB@OG photocatalyst was increased and slightly shifted due to the OG surface functionalization on the PVA@RhB organogel photocatalyst. The freeze-dried PVA@RhB and PVA@RhB@OG organogel photocatalyst was synthesized at various cross-linked densities maintained the amorphous behavior.
Electrochemical Studie with cyclic voltammetry
Cyclic voltametry experiment were performed to illustrate an electrochemical performance of PVA@RhB and PVA@RhB@OG organogel photocatalyst by using glassy carbon (working) electrode, calomel (reference) electrode and platinum (counter) electrodes in 0.1M tetrabutylammonium hexafluorophosphate (TBAFP) electrolyte to determine position of HOMO-LUMO in band gap.[41] As shown in Fig. 4, oxidation and reduction onset potential for PVA@RhB organogel photocatalyst were 1.11 V and -1.16 V, respectively. According to these results, the HOMO-LUMO band gap values calculated 2.27 eV. Furthermore, the oxidation and reduction potential of PVA@RhB@OG organogel photocatalyst were 1.25 V and -0.86 V, with resulted HOMO-LUMO band gap energy 2.11 eV. Because of smaller band gap 2.11 eV in PVA@RhB@OG photocatalyst provided better charge excitation capability for better photocatalytic activity than PVA@RhB organogel photocatalyst. The smaller bandgap of PVA@RhB@OG organogel photocatalyst enhances the solar light absorption, and OG provides additional reaction sites for the oxidation reactions, leading to aerobic oxidative cyclization.
Kinetic studies via Tafel plot
Tafel graph is used to estimate the interrelation between the overpotential and the logarithm of the current density in an electrochemical reaction for RhB, PVA@RhB, OG and PVA@RhB@OG organogel photocatalyst demonstrated in fig 5a. The slope of the graph is inversely proportional to rate of reaction which means if the slope of the graph is more, then rate of reaction will be less and vice -versa. From the graph, the slope of RhB was more and when it is coupled with PVA to form PVA@RhB organogel photocatalyst, forms an ester linkage between the components and therefore due to coupling, its slope becomes less. In the same way, the slope of the OG is also more which if further coupled with PVA@RhB organogel photocatalyst leading to the formation of PVA@RhB@OG organogel photocatalyst whose slope is less which means lower the value of Tafel slope for the PVA@RhB@OG organogel photocatalyst shows the faster charge transfer and due to this performs better photocatalytic activity. The high durability of the PVA@RhB@OG photocatalyst and faster charge transfer kinetics provide better charge excitation capability and makes them a better photocatalyst.[42]
Electrochemical impedance spectroscopy (EIS) analysis
The electrical charge resistance of compounds effectively characterized by electrochemical impedance spectroscopy (EIS), and the measured impedances were analyzed using a Nyquist plot.[43] Fig. 5b illustrated, the Nyquist plot for RhB, OG, PVA@RhB and PVA@RhB@OG organogel photocatalyst, which was recorded in 0.01 M H2SO4 electrolytic solution. The charge transfer resistance is often expressed by the semi-circular arc diameter and the x-intercept on the real axis. A smaller arc diameter and x- intercept proximity to the origin point suggest that photo-induced electron/hole pairs on the electrolyte have better charge separation and transfer efficiency. The RhB dye have large semicircles arc than PVA@RhB organogel photocatalyst, which had small arc as demonstrated in Figure 5b. Similarly, OG had large arc radius whereas PVA@RhB@OG organogel photocatalyst small arc radius semicircles. From graph, the small semicircle arc radius of PVA@RhB@OG organogel photocatalyst in the high frequency region correspond to the substantial large charge transfer resistance while the RhB, OG, and PVA@RhB organogel photocatalyst large ones in the low frequency region corresponds to diffusive resistance with low transferability.[44]
Morphology studies by SEM (scanning electron microscopy)
The scanning electron microscopy (SEM) was performed to investigate morphology. The fracture surface of the PVA@RhB and PVA@RhB@OG photocatalyst was observed to be an unevenly interconnected network structure in fig. 6a-b. According to published research, the SEM picture of RhB [45] reveals uniformly dispersed cauliflower like morphology. The SEM image of PVA show orderly arranged round structure. While SEM image of PVA@RhB organogel photocatalyst (fig. 6a) exhibited highly rough and densely porous surface maintained with spherical shape. Moreover, SEM image of PVA@RhB@OG organogel photocatalyst (fig.6b) exhibit a spherical white foam like structure on the porous round structured surface (from RhB and PVA) maintained with spherical shape which confirms the addition of OG with PVA@RhB organogel photocatalyst to form PVA@RhB@OG organogel photocatalyst.
Energy dispersive X -ray spectroscopy (EDX)
The elemental composition of material were determined by energy - dispersive X -ray spectroscopy (EDX) analysis.[46] The EDX of PVA@RhB organogel photocatalyst (fig.7a) demonstrated the presence of elements like C,O,N, and Cl. Moreover, the EDX spectra of PVA@RhB@OG photocatalyst (fig. 7b) shown the presence C, O, N, Na, S, and Cl. The appearance of Na and S (from OG) clearly indicated the functionalization of OG into PVA@RhB to form PVA@RhB@OG organogel photocatalyst.
Optimization of reaction parameters
In optimization Table 1, highest yield 97% was observed with irradiation of solar light in the presence of PVA@RhB@OG organogel photocatalyst and aerobic condition (entry 7) but in the absence of light (dark) no reaction takes place. Similarly, at inert atmosphere, the reaction was not taking place. So, these controlled experiments revealed that, without irradiation of light, air and photocatalyst, the product was formed just in trace.[47]
Table 1. Optimization table for photocatystic efficiency.

aReaction conditions: Substrate 1a (1.0 mmol), DMF (3mL), solar light, photocatalyst (2 mol%), the reaction was performed at ambient temperature in air atmosphere. Percentage (%) yield of the product 2a. bThe reaction was carried out without air. cThe reaction was performed without photocatalyst. dThe reaction was performed in dark. eThe reaction was performed at inert condition.
Further, the reaction conditions were optimized with respect to different solvents in presence of light, air, and PVA@RhB@OG organogel photocatalyst. In Table 2, observed that in all solvents (DMSO, MeCN, MeOH, EtOH, THF and DMF) the yield was greater than 50%.[28] therefore, the reaction does not vary based on the reaction environment. When reaction was performed in DMF, it gave product with 97% yield in 2 hours with respect to reaction conditions. (Table 2, entry 6)
Feasible mechanistic pathway for aerobic oxidative cyclization for C-N and C-S bond formation to form 3,5-diphenyl 1,2,4-thiadiazole from thiobenzamide
On the basis of our survey and literature review, a feasible mechanism for the formation of 3,5-diphenyl 1,2,4-thiadiazole from thiobenzamide is recommended in scheme 2.[48-51] The photo redox catalyst, PVA@RhB@OG organogel photocatalyst is photoexcited under the illumination of light emitted from a 5W Blue LED light . On irradiation the photocatalyst reaches to its more stable triplet from less stable singlet state followed by single electron transfer (SET).[52] Sulfur radical cation c developed from thiolic form a' by involving single electron transfer from PVA@RhB@OG*. The sulfur radical d formed after a H+ loss. The cyclodesulfurization of d with other sulfur radical gives e and again on proton loss from sulfur gives f. Now the sulfur radical f reacts with O2• - radical give peroxysulfenate g. The O2• - radical is produced in the photocatalytic cycle of PVA@RhB@OG organogel photocatalyst. The intramolecular cyclization reaction of g gives the desired product 3,5-diphenyl 1,2,4-thiadiazole 2a with the liberation of SO22- (good leaving group). [53]
Conclusion
In summary, we successfully fabricated PVA@RhB@OG organogel photocatalysts by a facile cross linking of PVA and RhB followed by OG functionalization. The obtained PVA@RhB@OG organogels photocatalyst showed much enhanced solar light photocatalytic activities. The photocatalytic activity of PVA@RhB@OG organogel photocatalyst was determined by various characterizations like UV, FTIR, CV, Tafel, EIS. The PVA@RhB@OG organogel photocatalyst possess remarkable photocatalytic activity towards solar light driven aerobic oxidative cyclization reactions. i.e., it is a extremely particular photocatalyst that sustainably promotes C-N and C-S bond formation through C-H bond activation which have various medicinal application. This approach is a top - notch substitute to the existing 3,5-diphenyl 1,2,4-thiadiazole synthesis with the advantages of maximum productivity, long- term viability, and eco- conscious reagents i.e., light, and air. Furthermore, the reaction is carried out in a simple one pot operation under mild conditions and endure many functional groups. So, we reported the PVA@RhB@OG photocatalyst involving aerobic oxidative cyclization of thioamide into 3,5-diphenyl 1,2,4-thiadiazoles with excellent yield (97%) and the superiority was proved by optimization tables. This research work offers a novel concept and foundation for the creation of artificial light-harvesting materials based on multifunctional fluorescent organogels made with PVA or other cross-linking agents.
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,CLAIMS:1. A PVA@RhB@OG organogel photocatalyst comprising of
Polyvinyl alcohol (PVA), rhodamine B (RhB), ethylene glycol, dimethyl sulfoxide (DMSO), Orange G (OG), thiobenzamide, dimethylformamide (DMF).
2. A method for PVA@RhB@OG organogel photocatalyst as claimed in claim 1, wherein the said method for the preparation of PVA@RhB@OG Organogel Photocatalyst for Photocatalytic C-H Activation for C-N/C-S Bond Formation comprising the steps of
a. Dissolving 200 mg of RhB into 100 mL DMSO solution, and the reaction mixture was stirred for 30 minutes at 120oC;
b. Dropping 1.5 gm of polyvinyl alcohol into the reaction solution and refluxed at 120oC for 5 hours after that;
c. Cooling the temperature of the solution to 50oC and ethylene glycol was added to it;
d. Stirring the solution vigrously for 30 minutes;
e. freezing the resultant mixture at -20 oC to obtain the PVA@RhB organogel photocatlyst;
f. adding the solution of 100mg of OG dye and 1.0 ml of ethylene glycol to the previously synthesized PVA@RhB organogel photocatalyst, and stirred for 30 minutes at 50oC;
g. obtaining the PVA@RhB@OG organogel photocatalyst after the solution was deep freezed at -20 oC;
h. adding 2 mol% PVA@RhB@OG organogel photocatalyst, 100 mg of thiobenzamide to 3mL of DMF solution in a 25mL glass vial;
i. stirring for 2-3hours while being under solar light irradiation in an aerobic atmosphere at ambient temperature;
j. washing the reaction mixture with 5mL H2O followed by 15mL ethyl acetate after completion of the reaction;
k. filtering the PVA@RhB@OG organogel photocatalyst by phase separation method;
l. filtering the joined organic phases over MgSO4 to trap moisture and filtrate evaporated under reduced pressure; and
m. obtaining a pure product of 3,5-diphenyl 1,2,4-thiadiazole.
3. The method as claimed in claim 1, wherein the Photocatalytic aerobic oxidative cyclization of thiobenzamide 1a into 3,5-diphenyl 1,2,4-thiadiazole 2b via C-N/C-S bond formation via C-H bond activation.
4. The method as claimed in claim 1, wherein the obtained crude product was then purified by silica gel column chromatography using a gradient solution of hexane ethyl acetate to (95%) yield a pure product.

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