Introduction

Chalcones, both naturally occurring and synthetic analogs, have a broad range of biological activities. The chalcone nucleus is one of the most common and well-known intermediates, found in a wide range of flavonoids and medicinal agents. This component is used to biosynthesize naturally occurring secondary metabolites including flavonoids and isoflavonoids [1-5]. Naturally occurring and synthetic chalcone compounds have proven to have potential biological activity and harmless profiles. Chalcones are the versatile materials for the development of various heterocyclic frameworks such as pyrazoline [6], oxazoline [7], thiazine [8], oxazine [9], pyrimidine [10], etc. Chalcones, widely recognized as 1,3-diaryl-2-propen-1-ones, are open-chain flavonoids that have a three-carbon, α, β-unsaturated carbonyl system appending the two aromatic rings [11]. Furthermore, Chalcones containing indanone moiety are likely to be the most pondered molecules in therapeutic science in view of their broad scope of pharmacologically exercises [12]. 1-indanone and its structural analogs have been thought to play a significant role in the field of medicinal chemistry [13]. The presence of active methylene hydrogens contiguous to the carbonyl gathering of indanone makes it significant in various organic transformations. Lately, indanone derivatives with anticancer [14], antibacterial [15], antiviral [16], antifungal [17], antimalarial [18], anti-inflammatory [19] exercises have been accounted for. Indole is a potent supporting molecule that aids in the broadening of the main pharmacophore by transforming its functional groups [20]. It's worth noting that the indole moiety is an essential pharmacophoric fragment for drug design and production, according to the literature. Indole is a notable heterocyclic compound containing a pyrrole ring with a benzene ring fused to α, β position. Indole bearing compounds possess various types of biological activities such as antibacterial [21], antifungal [22], antiviral [23], antimalarial [24], and anti-HIV [25]. Some of the indole compounds are quite effective antioxidants, protecting both lipids and proteins from peroxidation and influence the antioxidant efficacy in biological systems [26, 27]. Similarly, chalcone (1,3-diaryl-2-propen-1-one) moiety frames the focal center for an assortment of biologically active compounds with various medicinal properties. From the perspective of molecular design, the combination of two pharmacophores into a single molecule has been found to enhance the biological properties of the resultant hybrid molecule. The remarkable bioactivity of indole-based chalcones, as well as their unusual structural variety, makes them appealing targets for drug development. Green chemistry principles have been found to be utilized for the synthesis of various organic compounds [28-32].

Over the past few years, theoretical calculations based on DFT have been effectively used to assess various structural aspects of synthetically and pharmacologically important organic motifs [33, 34]. The DFT/B3LYP method using various basis sets has been found to be extremely useful in studying the structural, chemical, and spectroscopic properties of molecules [35-38]. The Quantum chemical method allows the prediction of different properties based on knowledge of a few quantum superpositions and the use of standard programming for electronic structure calculations [39-50]. DFT has received a lot of attention in the last two decades because it is less computationally expensive. B3LYP stands for "Becke, 3-parameter, Lee-Yang-Parr". The density functional theory based on theoretical quantum calculations has been effectively used to explore the structural and chemical properties of organic molecules [51-56] The importance of spectroscopic and quantum calculations in predicting various quantum chemical parameters and thermodynamic aspects has been discovered [57-59]. Taking into consideration of entire mentioned properties and the future province of the chalcone derivatives, we have designed (E)-2-((1H-indol-3-yl)methylene)-2,3-dihydro-1H-inden-1-one molecule and explored for the investigation of their structural, chemical, electronic, thermodynamic, and quantum chemical parameters.

Materials and Methods

General remarks

All chemicals and solvents (Make- Sigma Aldrich and SD Fine) were purchased and used as received. The ¹H NMR and ¹³C NMR experiments were carried out on sophisticated multinuclear FT-NMR Spectrometer (500 MHz) model Advance-II (Bruker). The compound was dissolved in CDCl₃ and the chemical shifts were reported in ppm using TMS as an internal standard reference. The reactions were monitored by using thin-layer chromatography on Merck Aluminium TLC plate, silica gel coated with fluorescent indicator F254. All of the glassware was cleaned and dried in the oven before use.

Experimental procedure for the synthesis of (E)-2-((1H-indol-3-yl) methylene)-2,3-dihydro-1H-inden-1-one (IMDHI)

The compound IMDHI was synthesized using the Claisen-Schmidt condensation reaction. In a typical synthesis scheme, 2,3-dihydro-1H-inden-1-one (1, 10 mmol) and 1H-indole-3-carbaldehyde(2, 10 mmol) were mixed in 10 mL ethyl alcohol. 5 mL 30 percent NaOH was added to this. Then, the alkaline mixture was stirred at room temperature for 5-10 minutes until the formation of the product. The reaction was smothered by pouring onto crushed ice after it was completed (as determined by TLC). It was then acidified with dilute HCl, and the crude product obtained was filtered, dried, and recrystallized with hot ethanol to get crystals of the pure product (3). The reaction is presented in Scheme 1.

Spectral data

(E)-2-((1H-indol-3-yl)methylene)-2,3-dihydro-1H-inden-1-one: Dark Yellow colour; Yield; m.p.: 158-160 ℃; ¹H NMR (500 MHz, CDCl₃) δ 3.90 (s, 2H), 7.33 – 7.30 (m, 2H), 7.68 (s, 2H), 7.84 (s, 1H), 7.92 (m, 2H), 7.97 (m, 2H), 8.12 (m, 1H), 8.75 (s, 1H); ¹³C NMR (126 MHz, CDCl₃) δ 35.73, 111.47, 119.52, 121.42, 123.57, 123.73, 123.80, 124.18, 126.04, 126.24, 126.41, 126.53, 127.41, 132.57, 134.79, 137.19, 153.84, 208.21.

Computational details

The Gaussian 03 package was used to perform all calculations [60]. DFT with the B3LYP (Becke three-parameter Lee–Yang–Parr) exchange-correlation functional was used for all computational calculations. The molecular structure of IMDHI was optimized by employing the 6-31G(d,p) basis set. On fully DFT optimized geometry, the geometrical parameters, frontier molecular orbitals, global reactivity parameters, and MESP surfaces were all predicted using the same basis set. Ionization potential, electron affinity, electronegativity, chemical hardness and softness, global electrophilicity, and chemical potential were also calculated using HOMO and LUMO energy values.

Antimicrobial screening

The antimicrobial activities of the IMDHI molecule were determined using the Agar diffusion assay (Disc diffusion process, Disc size 6 mm) [61, 62]. Compound concentration was determined by making a stock solution [1000 microgram per mL] of each compound in distilled water. The concentration of 100 micrograms per disc was used in the assay. Microbiological media used for bacteria was Nutrient agar (Hi-media); composition (gL^-1): sodium chloride, 5.0; beef extract 10.0; peptone 10.0 (pH 7.2). Microbiological media for fungi was potato dextrose agar (all ingredients of Hi media); composition (gL^-1): potatoes infusion, 200; dextrose, 20; Agar, 15; final pH (at 25°C) 5.6±0.2. For antibacterial evaluation, Chloramphenicol was used as a standard and, Amphotericin-B for antifungal screening. Antibacterial screening was observed against Escherichia coli [2109] Staphylococcus aureus(NCIM 2079) Proteus vulgaris (NCIM 2172)andBacillussubtilis [2063] while the antifungal screening was observed against Aspergillus niger (NCIM 545) and Candida albicans (NCIM 3471). [NCIM stands for National Collection of Industrial Microorganisms, and NCL is National Chemical Laboratory, Pune, India].

Results and Discussion

Chemistry

The title compound IMDHI was synthesized by the well-known Claisen-Schmidt reaction at room temperature. Solution of 30 percent NaOH was used to accomplish the transformation. This synthetic strategy is dependent on high product yields. The synthesized compound was examined using ¹H NMR and ¹³C NMR spectroscopic methods. The ¹H NMR and ¹³C NMR examination of the title compound have shown a great connection with their structures. In ¹H NMR spectra ofthe IMDHI molecule, the peak at 3.90 ppm (m, 2H) affirms the presence of saturated -CH₂ group of indanone counterpart in the synthesized compound. In ¹³C NMR spectra, the same –CH₂ group can be confirmed by observing a peak at 35.73 ppm. The aromatic protons were observed in the 7.30 to 8.75 ppm range in ¹H NMR. The presence of carbonyl function in the synthesized compound can be confirmed by observing a peak at 208.21 ppm in ¹³C NMR.

Computational study

Molecular structure, bond length, bond angle study

The optimized molecular structures of the IMDHI molecule at B3LYP/6-31G(d,p) basis set is presented in Figure 1. The DFT study uncovered that the IMDHI molecule had C1 point group symmetry, demonstrating that the molecule was overall asymmetric. IMDHI molecule with dipole moment 5.9583 Debye indicated that molecule had appreciable polarity. The total energy of the IMDHI molecule was −1029.21 (Table 3).The bond length and bond angle data of the title molecule are given in Table 1 and Table 2. The olefinic bond length in the IMDHI molecule was 1.3521 Å. The C20-C21 bond is the longest aromatic double bond and C17-C18 is the shortest aromatic double. The carbonyl bond (C9-O13) length is nearly 1.2252 Å. All-inclusive C3-C10 bond is the longest bond while N28-H29 shortest bond. All other bond length values are showing good agreement with the structures of the title molecule. The C4-C9-C14 bond angle is 106.1648°. Likewise, other bond angle data are rightly matching with the various bond angles of the IMDHI molecule.

HOMO-LUMO and reactivity descriptors

The HOMO-LUMO orbitals of the molecule under investigation were obtained by using the TD-DFT method at CAM-B3LYP basis set. The HOMO-LUMO orbitals are called frontier molecular orbitals (FMOs) put together. The FMOs provides lots of information about the molecules' stability and reactivity. The energy gap between FMOs is very crucial to comprehend numerous significant electronic aspects of the molecules. The HOMO-LUMO of the title molecule with energy difference is given in Figure 2. The electronic parameters are given in Table 3. The IMDHI molecule is found to possess 6.59 eV electron ionization potential and 2.39 eV electron affinity value. Using Koopman's theorem, various global reactivity parameters such as chemical hardness (ɳ), chemical softness (σ), global electrophilicity (ω), electronegativity (χ) and chemical potential (μ) were calculated from HOMO-LUMO energy values [63]. The chemical hardness and softness values were found for the IMDHI molecule 2.10 eV and 0.48 eV^-1, respectively.

Mulliken atomic charges and MESP analysis

Mulliken charges emerge from Mulliken population analysis [64]. It gives a way to calculate partial atomic charges from computational simulations. The Mulliken charges are charges dependent on the charge density. The Mulliken atomic charges of the IMDHI molecule are given in Table 5. The investigation on Mulliken atomic charges revealed that all hydrogen atoms of the IMDHI molecule are having a positive character. In view of Mulliken population analysis, the most negative and positive carbon atoms in the IMDHI molecule are C10 (-0.290093) and C9 (0.339950), respectively. Figure 3 portrays the MESP of the IMDHI molecule. MESP correlates the total charge distribution with dipole moment, electronegativity, and partial charges, and site of chemical reactivity of a molecule. It gives a visual strategy to understand the relative polarity of a molecule and serves as a useful quantity to explain the hydrogen bonding, reactivity, and structure-activity relationship of molecules. It is the potential energy of a proton at a particular location near a molecule. Different values of the electrostatic potential at the surface of a molecule appear with different colors. In general, the attractive potential appears in red color regions and those of repulsive potential appear in blue.

Antimicrobial Study

The antimicrobial examination of the synthesized IMDHI molecule was performed against two Gram-negative (E. coli&P.Vulgaris) and two Gram-positive (S. aureus&B. subtilis) bacteria whereas antifungal examination was performed against A.Niger and C.albicans fungal species. The posteffects of the antimicrobial evaluation are presented in Table 6. The outcomes imply that the IMDHI molecule is a very good antimicrobial agent against some of the tested strains. The IMDHI molecule has shown strong activity against E. coli, P.Vulgaris,and, B. subtilis bacterial strains but has not shown antibacterial action against the S. aureusand antifungal action against A.Niger and C.albicans. The antimicrobial data presented here could give a great stage to extra advancement of the antimicrobial agent. The minimum inhibitory concentration (MIC) was studied using the previously reported method [65].The MIC values of the synthesized compound were evaluated and the results are depicted in Table 7, which stresses the differential of sensitivity pathogens toward the title compound.

Conclusion

In the current progression of investigation work, we have analyzed the geometrical parameters, frontier molecular orbitals, global reactivity parameters, MESP surfaces of the title molecule, and found that these computed outcomes are in worthy conformity with the experimental data. The synthesized chalcone was characterized by instrumental methods like PMR and CMR. IMDHI molecule with dipole moment 5.9583 Debye showed that molecule had appreciable polarity. The MESP plot showed the negative potential location for oxygen atom and the positive potential locations for hydrogen atoms. Our present study shows that IMDHI molecule pharmacological significant may be due to the combination of two pharmacophores; indole and indanone into a single molecule; yet, further intensive investigations are expected to affirm this prediction toward this path. The current quantum chemical examination may add a perception of properties of title the molecule and may similarly help its usage for additional created applications.

Acknowledgments

The Authors acknowledge the central instrumentation facility (CIF), Savitribai Phule Pune University, Pune for the spectral analysis. The authors thank Mr. Mahesh Patil for the antimicrobial study. The authors are very much thankful to Dr. B. S. Jagdale, Principal of Arts, Science and Commerce College, Manmad, for permission and providing necessary research facilities. The authors are thankful to Prof. Dr. A. B. Sawant for the Gaussian study. Dr.Aapoorva P. Hiray Coordinator, M. G. Vidyamandir institute is gratefully acknowledged for the Gaussian package.

Funding

We have not gotten any sort of funding for the exploration work.

Conflict of Interest

The authors declared that they do not have any conflict of interest regarding this research article.

Orcid:

Rahul A. Shinde: https://orcid.org/0000-0003-1067-9654

Vishnu A. Adole: https://orcid.org/0000-0001-7691-7884

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