A series of fluorescence-containing indazole-fused ring systems were made with the support of g-C3N4/CuO as a catalyst via non-conventional (microwave) method. We have synthesized g-C3N4/CuO nanocomposites by mechanochemical process; further, its morphology and composition were studied using various instrumental techniques like Fourier transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM). Also, we have synthesized pyrimido[1, 2-b] indazole-4-yl methanol motifs without any solvent in single-step methodology utilizing microwave irradiation. The pyrimido[1, 2-b] indazole-4-yl methanol motifs were optimized using response surface methodology (RSM). This preparation was effortlessly accessible, and the overview of the substrates was authorized. The pyrimidoindazole core structures exhibit the most remarkable photo physical properties. Most of the pyrimidoindazole scaffold appears in solvatochromism and excited with blue–green fluorescence shift while using ethyl acetate as solvent. This result indicates that synthesized pyrimidoindazole core motifs have prodigious potential as fluorophores which will help us to study several applications.
The carbon nitride materials are said to be more stable (g-C3N4) at ambient conditions [1,2,3,4]. It is made up of earth abundant carbon and hydrogen elements with two-dimensional networks. Conjugated metal-free polymer of g-C3N4 is having a layer of planer structure which is similar to graphite [5, 6]. They are having tremendous features like non-metal, environmentally friendly, tunable electronic structure, good thermal and chemical stability. With utilization of these properties, g-C3N4 is employed as an effective catalyst in various reactions [7, 8]. For example, it is observed in the reduction of oxygen in fuel cells as an electro catalyst , in Knoevenagel condensation as a heterogeneous catalyst, activation and conversion of carbon dioxide [12, 13], the process of degradation of various pollutants as photo catalyst, and generating hydrogen from water [9, 10].
Various procedures have been taken up for increasing the rate and efficiency of the catalyst g-C3N4, which includes chemical doping, physical compounding, and morphology control [14,15,16,17,18,19]. The physical modification procedures of g-C3N4 incorporated with various semiconductors such as titanium dioxide, zinc oxide, etc., are well reported [20,21,22,23,24,25]. The combinations of the procedures were also noted. Therefore, it is worth to find novel nanocomposites by incorporating g-C3N4 with semiconductors, and afterwards, utilizing a sequence of chemical and photographic reactions to enhance their catalytic activity.
A process involving three or more reagents combined in a single vessel with suitable conditions (incorporation of majority atoms of starting materials) is called as multicomponent reactions (MCRs) [26, 27]. Due to this, a number of reaction steps have been eliminated, separation of intermediate has been avoided, and any sort of purification has been neglected. When compared to other methodology, MCRs have proved to be powerful and more efficient; subsequently, it takes lesser time and effort to prepare organic motif [26, 27]. This is the reason why we tend to use a single chemical transformation. These products have indeed received special interest in pharma industry as they have a simple preparation of number of libraries of compounds with possible biological activities .
The ring junction heterocyclic system is one of the emerging fields in organic synthesis, which allows modifications in post reactions [29, 30]. Fused ring junction motifs are essential core structures in alkaloids and show versatile advantages in the biological field [31,32,33,34]. Pyrimidoindazoles and fused tricyclic indazoles have great bio-medicinal properties . In most of the synthetic challenges with high biological properties pyrimido[1,2-b]indazole scaffolds receives a highest role in action which includes viral infections, proliferation disorders, auto-immune disorders etc., . Drugs named Crestor, Gleevec, and Aggrenox are having pyrimidine system in it (Fig. 1) . To overcome the above reason, the evolution of these complex molecules has emerged with high requirements to serve the globe. However, pyrimido[1,2-b]indazole core analogs, direct synthesis was not explored; hence, it receives great attention.
In literature, authors reported that the preparation of pyrimido[1,2-b]indazole-3-carbonitrile motifs (Scheme 1 reaction 1) via multicomponent assembly process . Also, the development of pyrimido[1,2-b]indazoles was stated from substituted anilines and indazole over a superficial strategy . Shinde et al. discovered a methodology to prepare pyrimidine-fused indazole through oxidation, Michael addition, cyclization and Knoevenagel condensation (Scheme 1, reaction 2) .
Metal salts like silver, copper, gold and palladium were utilized for metal-catalyzed reactions to result in fused heterocycles [40,41,42,43]. Specifically, copper has been considered as one of the attractive metals that has a long history in organic chemistry. Mostly, for several industrial processes, CuO has been utilized as a catalyst for various organic transformations. Compared with other metal oxides such as Fe2O3, TiO2, and ZnO, only few reports have described the synthetic strategies adopted for CuO nanostructures along with the introduction of their related applications. The authors reported synthesis of 2-alkenyl imidazo[1,2-a]pyridines using oxidative aerobic cyclization [44, 45]. In this manuscript, we have developed a novel approach to synthesize pyrimidoindazole derivatives in a straightforward, one-pot manner, and further, the synthesized analogs were subjected to investigate fluorescence studies.
Synthesis of catalyst
We prepared graphitic carbon nitride (g-C3N4) at atmospheric air conditions by direct heating of melamine. The detailed protocol was discussed in detail here. Fix around 500 °C in muffle furnace and place 5 g of melamine in it for 2 h. After 120 min, sample was taken out of furnace and kept for air dry at room temperature; once dried, further it was grained using mortar and pestle. Meanwhile, CuO NPs were prepared by the method which we already reported . Then, various quantities of g-C3N4 samples were mixed with synthesized CuO NPs without any solvent and stirred continuously until navy-blue color appears. Then, the solid mixture was placed in furnace for 2 h at 550 °C. Then, the reaction mixture was allowed to place in RT and further, the resultant samples were named according to their concentrations as follows CuO, 0.05 g g-C3N4/CuO, 0.1 g g-C3N4/CuO, 0.15 g g-C3N4/CuO, 0.25 g g-C3N4/CuO, 0.35 g g-C3N4/CuO, 0.5 g g-C3N4/CuO, respectively. Moreover, numerous experiments have been conducted to compare the results (Fig. 2).
Available chemicals and solvents were procured from commercial resources. Rigaku Ultima-IV advanced X-ray diffractometer was used for X-ray diffraction (XRD) to detail the crystalline phases of the material; sample spectra from Fourier transform infrared (FT-IR) were reported using KBr pressed pellets on a NICOLET IS10 spectrometer. A Transmission electron microscopy (TEM, Philips Tecnai 12) was used to identify the structure and morphology of the as-synthesized samples. Melting points are identified using open capillary tubes by Elchem Microprocessor-based DT apparatus. Sineo-UWave-1000 MW-UV-Us reactor has been utilized for the experiment. The 1H and 13C NMR were verified by a Bruker Avance 400 MHz spectrometer with the help of DMSO-d6 solvent, Mass spectroscopy (MS) was analyzed using high-resolution mass spectroscopy (HR-MS). UV–vis spectra were measured using Shimadzu Ultraviolet (UV)–visible spectrometer. UV irradiation was done using an 8-lamp photo reactor by Heber. Luminescence property was recorded in the Hitachi F-7000 FL spectrometer. The column chromatography was eluted in 4:6 ratios present in the ethyl acetate (EA) and pet ether (PE) solvent system.
Microwave-assisted synthetic procedure for the preparation of Pyrimidoindazole compounds
1 mM of 1H-indazol-3-amine, 1 mM of aldehyde, 1 mM of propargyl alcohol, 0.5 g C3N4/CuO (10%), and PTSA (10%) were taken in the reaction chamber and then it was kept under MW conditions (100 W, 70 °C) to get the final product. Thin-layer chromatography (TLC) was used to detect the reaction framework and the mixture had been further isolated with ethyl acetate and water. The collected organic residue was dried out using Na2SO4. Further, the dissolvable was vaporized and the mixture was refined by column chromatography.
RSM (Response Surface Methodology) and BBD were applied for the investigation of the interaction between the independent variables for reaction yield. From this current study, independent variables were R1, R2, and R3 and the response is reaction yield. Each independent variable has been allowed to vary into three levels: lower level (− 1), central level (0) and upper level (+ 1), shown in Table 1. The Minitab 14 software was employed to study and analyze the relationship between dependent and independent variables. The experimental data have been further studied and investigated using a second-order polynomial equation with a high value of correlation coefficients (R2) in the form of the following equation:
Here, Y means the predicted response, β0 means constant of the quadratic equation, βi means the linear coefficient, βii means the squared coefficient and βij means cross-product coefficient.
Results and discussion
The crystal structures of CuO, pure g-C3N4 and 0.5 g g-C3N4/CuO nanocomposites were deliberated by X-ray diffraction data. As shown in Fig. 3, the diffraction peaks of pure CuO NPs at 32.35º, 35.26º, 39.35º, 48.97º, 53.15º, 58.90º, 61.78º, 66.35º, 68.88º, 72.13º corresponded to the (110), (111), (111), (202), (020), (202), (113), (004), (220) and (311) planes of CuO (JCPDS No. 81-0792). The results are further found to confirm the monoclinic CuO formation which matches the JCPDS No. 81-0792 for the pure CuO nanoparticles . As shown in Fig. 3, the diffraction peaks at 13.2° and 27.4° corresponded to the planes (1 0 0) and (0 0 2) found to be compatible with the typical interplanar staking peaks of the inter-layer structural packing . The diffraction peaks of both the g-C3N4 and CuO could be seen for 0.5 g g-C3N4/CuO nanocomposites, which confirm the presence of g-C3N4 and CuO in the g-C3N4/CuO nanocomposites.
FT-IR spectra depict the formation of 0.5 g g-C3N4/CuO and CuO nanocomposites (Fig. 4); it is to be concluded that, for clear g-C3N4/CuO, the broad absorption band was noted to be at 3300–3100 cm−1. It corresponds to NH2 and –NH stretching vibration. Also, it indicates interaction of intermolecular H2 bonding [48,49,50]. The various peaks marked at 1629 cm−1 1546 cm−1, 1452 cm−1, 1313 cm−1and 1230 cm−1 were found to be linked with the aromatic C–N stretching [51,52,53,54]. Sharp peak in 804 cm−1 helps to identify s-triazine rings out of plane bending [9, 55]. However to obtain pure CuO, the peak of absorption was found to be at 3296 cm−1 which can be attributed to symmetric and asymmetric stretching of NH2 groups in ethanolamine. C–N stretching and NH group in plane bending were found at 1480 cm−1. At a peak of 1101 cm−1 , C–O stretching vibration was found to be located [56, 57]. The peak of s-triazine ring modes in g-C3N4 is clearly weakened as a result of dispose in Fig. 4, which may be due to the interaction of the H2 bond among g-C3N4 and CuO which makes g-C3N4 nanosheets as very think and small in size.
TEM and EDX
TEM and EDX results of the 0.5 g g-C3N4/CuO nanocomposite have been depicted in Fig. 5a–d. From Fig. 5a, thin sheet-like structure of g-C3N4 nanosheets has been noted; on the other hand, wrinkle and overlap of CuO [52, 58,59,60] are found to be darker. The size in terms of the diameter of the CuO nanoparticles is found to be 10 nm (Fig. 5b). Figure 5 clearly depicts the images of g-C3N4/CuO nanocomposites. It was found to have an increased particle size of CuO in the nanocomposite (30–50 nm approximately). The observations showed that the g-C3N4 surface contains CuO nanoparticles which were agglomerated. Similarly, Fig. 5d displays EDX of g-C3N4/CuO nanocomposites, Cu, C and N which can enable us to reload the Cu nanoparticles onto the g-C3N4 surface. Additionally, EDS, which is shown in Table, determined the actual content of Cu atomicity percentage.
Microwave-assisted synthesis of pyrimidoindazole compounds
After the successful preparation and fabrication of 0.5 g g-C3N4/CuO, its catalytic applicability was explored in the three-component A3 coupling reaction of aldehydes, amines, and alkynes. To investigate the potentiality of the nanocomposites in A3 coupling reaction, multiple time reaction has been carried out. Figure 3 reveals that a minor quantity of CuO is in XRD peaks not capable of A3 coupling reaction. However, in the absence of CuO or even in the presence of g-C3N4, this reaction does not occur. Optimum reaction condition was proved only in the presence of g-C3N4/CuO. It is clearly demonstrated that 0.5 g g-C3N4/CuO is required to attain maximum conversion. The reaction rate was varied with respect to the catalyst, i.e., g-C3N4 < CuO < 0.5 g g-C3N4/CuO. We observed a significant increase in the reaction rate while utilizing 0.5 g g-C3N4/CuO in A3 coupling reaction. As per the literature methods were compared with the present work in Table S1.
For the fusion of pyrimido [1, 2-b] indazole analogs (Fig. 6), we utilized multicomponent protocols (three reactions) as follows: substituted 1H-indazol-3-amine 1, substituted aldehyde 2 [1–12] and substituted propargyl alcohol 3 with 0.5 g g-C3N4/CuO as a catalyst and PTSA (p-Toluene sulfonic acid) were clearly explained in Scheme 2. In the non-conventional method, initially, the reaction was refluxed at 80 ºC without catalyst, and no product is found. Later, we carried out the reaction in the presence of CuBr, CuCl, CuO, CuI, Cu(OAc)2, and CuSO4 as catalyst. Gratifyingly, the generation of Pyrimido[1,2-b]indazole was observed in less quantity (Table 2, entry 1–14). The best result was obtained using the 0.5 g g-C3N4/CuO catalyst (92%, entry 12) in the presence of PTSA at 30 min. To minimize the reaction time, we move towards modulating the ratio of catalyst; MW condition time was reduced by 5 min, providing a similar isolated product (95% entry 13) under solvent-free condition. Thus, the finest yield was attained by carrying out the reaction in MW (microwave method) in the presence of 0.5 g g-C3N4/CuO (10%) in PTSA at 100 W which is tabulated in (Table 2, entry 13).
Later optimizing the reaction states, we substituted Pyrimido [1, 2-b] indozol-4-yl) methanol with various groups, –Me, –Cl, –Br, and –OCH3 which results in better yields of compounds 4 (a–u) as shown in Table 3. Here, the scope of the substitution aldehydes is also investigated in Table 4. Even though, Br-, Cl-, OCH3-substituted aldehydes readily react with an amine to give excellent yields due to electron-releasing nature. A little variety in the reactiveness of ortho-, meta-, and para-substituted benzaldehydes (3a–k) was unmistakably evident from the yields of the resultant products 4 (a–u). It is eminent that 4-Cl and 4-Br which have more electronegativity reacted with amines selectively to give higher yield. Unsuccessfully, benzaldehydes holding strong electron-withdrawing groups, for example, CN, NO2 neglected to respond with an amine under improved conditions, providing drop quantities of the resultant products. Moreover, we highly noted that the occurrence of electron rich amines showed more efficient transformation over the electron-deficient substitutedamines. For the situation, the presence of electron-contributing groups like, –Me, –Cl, –Br, and –OCH3 in aldehydes on amines exhibited satisfactory reactivity, to give corresponding products. We substituted primary alkynes that show good yield compared to secondary alkynes. Synthesized molecules were described by 1H NMR, 13C NMR and HR-MS analyses (Table S2).
A possible mechanism for synthesizing pyrimidoindazole core motifs using the g-C3N4/CuO catalyst is as follows: the first step involved reaction between –NH2 and –CHO which gives imine formation. The imine I was reacted with g-C3N4/CuO supported acetylene to get an intermediate II. Further, the intermediate II changes into another isomeric form III. The intermediate III undergoes six endo dig cyclization with the formation of IV. Next progress is demetallization and then auto-oxidation which will provide 4a (Scheme 3). Recycling the catalyst effectively is very useful for practical purposes.
The recyclability of 0.5 g g-C3N4/CuO is of vital importance from both economic and the synthetic aspects. To investigate this problem, the recyclability of the catalyst must be checked to prepare pyrimidoindazole core motifs. The reusability of the catalyst was, therefore, explored by separating the 0.5 g g-C3N4/CuO nano-composites from the reaction mixture with implicit filtration, washed with ethanol and dried completely using a vacuum oven at a temperature of 80 °C over a time period of 3 h and reusing it during subsequent cycles. The catalyst recovered after the reaction can be reused at least 5 times with minute loss in its catalyst activity (Fig. 7). To affirm the catalyst recyclability, the obtained catalyst was studied with the aid of XRD technique. The XRD pattern of the reused catalyst indicates the presence of both peaks in the catalyst structure, which reaffirms that g-C3N4/CuO is recyclable (Fig. 8).
Further, the synthesized compounds were examined for fluorescence spectroscopy emission spectra, and it was analyzed in Ethyl acetate (10−5 M).
According to the fluorescence activity, the compounds quantum yield (\(\phi\)) was calculated .
where \(\phi\)R denotes reference quantum yield; Is and IR denote sample integral area and the reference integral area; ODs and ODR represents sample and reference excited absorbance; ηs and ηR denote the sample solvent and the reference solvent refractive index.
Here, the standard is taken for the fluorescence activity known as tryptophan  (Table 4). Whereas, synthesized compounds were prepared in ethyl acetate for analysis. To find the solvatochromism effect; all the synthesized molecules were dissolved in different solvents and recorded the UV-visible spectroscopy. The solvents were selected, based on their polarity [DMSO (dimethyl sulfoxide), ACN (acetonitrile), DMF (dimethylformamide), methanol, ethanol, acetone, DCM (dichloromethane), THF (tetrahydrofuran), EA (ethyl acetate), chloroform, i-propanol] nature in (Fig. S1) and analyzed in UV–visible spectroscopy.
The emission spectra of all pyrimidoindazole motifs in various polar and non-polar solvents are shown in Fig. S2. The absorption and emission maxima of all synthesized compounds are summarized in Table 5. Pyrimidoindazole core motifs showed good fluorescence activity in different solvents. Compounds like 4p and 4q exhibited intense fluorescence property in a non-polar solvent. With regard to the influence of the fused ring system, all compounds exhibited a hypsochromic shift. The absorption and emission spectra of the above two (4p, 4q) compounds appeared in the same range. Moreover, the fluorescence quantum yield of the compound 4p was found to be higher than all other compounds. All synthesized core motifs exhibit good fluorescence properties.
Applications of RSM in fitting models
Pyrimido[1, 2-b]indazole preparation using microwave is evaluated via RSM methodology. Table 6 shows the RSM experiment results for the synthesis of pyrimido [1, 2-b] indazole-fused motifs. The fitting of the BBD experimental data to the quadratic multiple regression model is shown by the following equation,
Here, R1 denotes catalyst concentration (mol%), R2 denotes time (mins), and R3 denotes power (Watt). The current model ANOVA (analysis of variance) is shown in Table 7. The computed F value of the yield (80.87) along with a p value less than 0.05 implies that the model is more significant at the 95% confidence level. Also, from the ANOVA analysis, no lack of fit (F value = 11.66) represents the pure error. The R2 value is 97.92% which indicates that the present model is having a more significant and better quality of fits. The experimental and theoretical yield values were correlated in Fig. 9.
Effect of parameters
Figure 10a presents the response surface plot showing the effect of catalyst amount (R1), time (R2) and their interactions on pyrimido [1, 2-b] indazole preparation at 30 min. It was observed that the percentage of yield increased with increasing catalyst level up to 15 mol%. The increase in the amount of catalyst increases the product yield. Figure 10b displays the effect of variable catalyst dose and power (Watt) on the product yield. With increasing the microwave power and catalyst amount, the product yield becomes higher. Again, it can be seen that by increasing the catalyst up to an optimum amount, the product yield increased. Figure 10c shows the response surface plot as a function of time vs microwave power. The maximum yield of the product was obtained in 30-min time and 100 W of power. By increasing the time from 15 to 30 min at catalyst concentration 5–10 mol%, the percentage of yield increased. From the results, the optimized condition for the reaction was 10 mol% catalyst, 30 min time, and 100 W power. Also, in Fig. 11a–c, the 3D surface plots represents the interaction between all three independent variables.
Theoretical DFT calculations for pyrimido[1, 2-b] indazole-fused motifs
There are various methods to find excitation energy; one of the simplest methods involves the difference between HOMO and LUMO. Both HOMO–LUMO orbital are called as a frontier molecular orbital (FMOs). Electronic and optical properties are decided by these orbitals. The HOMO indicates the ability to donate an e- (electron); also LUMO indicates the ability to accept an e- pair. The HOMO and LUMO for the energy bandgap for all synthesized compounds are represented in Table 8. The energy range between HOMO and LUMO orbitals is − 5.54 and − 5.81 eV, − 1.76 and − 2.36 eV, respectively. Specifically, compounds 4b, 4c, 4d, in general, had the smallest ΔE (bandgap 3.37 eV, 3.353 eV), indicating fast electron transfer and radical transmission between HOMO and LUMO (Fig. 12). This might be one of the reasons why compounds demonstrate good fluorescence-emitting behavior. This result shows a close relationship with the resonance effect-based electro-density of compounds 4b, 4c, 4d, which is shown in Fig. 12. Based on these observations, we can expect the HOMO–LUMO distance to be considered as an important parameter in the choice of fluorescence compounds among all synthetic derivatives evaluated.
Graphitic carbon nitride/copper oxide composite was successfully prepared by varying the quantity of g-C3N4. CuO nanoparticles were successfully embedded on to the surface of g-C3N4. g-C3N4, and CuO possesses a synergetic effect between them which results in high catalytic activity. Finally, we have reported the g-C3N4/CuO catalyst-mediated synthesis of pyrimido [1,2-b] indazole-fused motifs through the microwave method which reduces time with cheap investing materials. The product yield was optimized using RSM methodology. These results imply that the experimental yield was good, when compared with the theoretical yield. The synthesized analogs by one-pot multicomponent synthesis resulted in luminescence property. Further, the stability of synthesized compounds was identified as a property of solvatochromatism which also results in the excellent solvent effect.
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We thank the organization of the Vellore Institute of Technology for giving all the facilities to succeed in this work. Devi Priya thank CSIR-SRF (09/844(0052)/2018 EMR-I) for providing grant. Also, we thank Prof. G. Madhumitha for Microwave support under her DST Grant (No. SB/FT/CS–113/2013).
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Priya, D.D., Khan, M.M.R. & Roopan, S.M. Fabricating a g-C3N4/CuO heterostructure with improved catalytic activity on the multicomponent synthesis of pyrimidoindazoles. J Nanostruct Chem (2020). https://doi.org/10.1007/s40097-020-00350-0
- Multicomponent reaction
- BBD model
- Microwave methods