Photophysical Approach of Biological Active Benzofuran Derivatized Pyrrole with Green Synthesized Silver NPs Using C. Roseus Leaves: Computational and Spectroscopic Study

The electronic absorption and fluorescence emission spectra of N-(2,5-dimethyl-pyrrol-1-yl)-2(5-methoxybenzofuran-3-yl)acetamide (DPMA) molecule were recorded in various solvents at room temperature with the aim of estimation of ground state (μg) and excited states (μe) dipole moments using Lippert’s, Bakshiev’s and Kawski-Chamma-Viallete’s equations. The results were signified that, the excited state dipole moment is greater than the ground state dipole moment, which indicates the excited state dipole moment is more polar than the ground state dipole moment. Ecofriendly green synthesis of silver nanoparticles (Ag NPs) were synthesized using catharanthus roseus (C.R) leaf extract was done. These synthesized Ag NPs were used as

The highly fluorescent materials with extended π-conjugation continue to attract much interest because of their applications as sensors and biosensors, electroluminescent materials, lasers and other optoelectronic devices [30][31][32][33][34].
Catharanthus roseus (C.R) is originated at the Indian Ocean Island of Madagascar country in East Africa. This plant does not require too much of water and nutrition, which is globally found in tropical areas especially in Southern Asia. It has immense ayurvedic medicinal values in its roots, leaves, stem and flowers. Whereas antioxidant property found in roots [35], antidiabetic [36], antimicrobial [37] and antiplasmodial [38] activities found in leaves.
This paper reports estimation of ground state and excited state dipole moments of DPMA molecule using solvatochromic shift method, analysis of microscopic solvent polarity ( N T E ) and fluorescence quenching and fluorescence lifetime of biologically active DPMA molecule using Ag NPs by catharanthus roseus (C.R) leaf extract. Herein we are also report the green synthesis of Ag NPs and used as fluorescence quenchers.

Spectroscopic method
Absorption and fluorescence spectra were recorded on a UV-Visible spectrophotometer (Hitachi U-3310, Japan) and fluorescence spectrophotometer (Hitachi F-7000, Japan), respectively at room temperature.

Computational method
The complete computational studies are performed using Gaussian 09w software along with DFT method and B3LYP/6-311++G (d, p) basis set [40]. Gauss View 5.0 software [41] is used to visualization of the molecule. Highest occupied molecular orbital (HOMO) represent electron donating ability of the molecule and lowest unoccupied molecular orbital (LUMO) represents electron accepting ability of the molecule. MEP map gives the valuable information about the shape, size and charge region present in the molecule and also provides the net electrostatic effect caused by the total charge distribution.

Solvatochromic shift method
By taking into the account of simplest quantum-mechanical second order perturbation theory of absorption maxima ( a v ) and emission maxima ( f v ) (in wavenumbers), band shifts of spherical solute molecule in different solvents of varying dielectric constant ( ) and refractive index ( n ) [42].

Constant
(1) ) , ( n F  will get the S1 Lippert's polarity function is given by Here S1, S2 and S3 are the slope values using the Lippert's (5), Bakshiev's (6) and Kawski-Chamma-Viallete's (7) equations. Where g  and e  are the ground state and excited state dipole moments, h is the Planck's constant, is the velocity of light in vacuum, is the Onsager cavity radius of the solute molecule and the value was calculated by atomic increment method by Edward [45] and also determined by molinspiration application,

Fluorescence quenching
Fluorescence quenching is the phenomena of decrease in the fluorescence intensity of a sample. This includes exited-state reactions, molecular re-arrangements, energy transfer, ground state complex formation and collisional quenching [46]. Fluorescence quenching of organic molecules in solvents carried out by using different solvents i.e. aniline [47] and carbon tetrachloride [48]. In recent years chemically and green synthesized nanoparticles were also used as quenchers i.e. TiO2 [49], Ag [50], ZnS [51], ZnO [52][53] NPs.

Stern -Volmer Equation
For steady state

Preparation of C.roseus leaves extract solution
Fresh leaves of C. roseus (C.R) were collected from Karnatak University, Dharwad. The C.R leaf extract solution was prepared by using 15 g of C.R leaves, rinsed and washed with deionized water. The same amount of C.R leaves cut into small pieces, the chopped C.R leaves were mixed with 100 mL of distilled water and heated for 15 minute. The cooled leaf broth solution was filtered through Whatman filter paper and stored in ice cubes.

Green synthesis of Ag Nanoparticles
Green synthesis of Ag NPs has been carried out based on the literature with some modification [54]. The 10 mL of C.R leaves extract solution was added drop-wise to 0.01M of AgNO3, which was prepared in 10 mL of distilled water. This mixture was stirred well with the help of magnetic stirrer for 5 minutes, color changes from pale yellow to dark brown indicating the formation of Ag NPs as shown in Fig 2. This mixture was added in 50 mL of Teflon made autoclave which was heated to 160 o C for 6 hours, and then cooled to room temperature. After collecting the mixture from autoclave, separation was done by the centrifuging at the rate of 3000 rpm for 15 minute, the Ag NPs were obtained.

ET (30) solvent polarity and normalized solvent polarity of solvents
Here ET (30) and E T N were solvent polarity and normalized solvent polarity.ET (30) i.e.
calculated solvent polarity values were obtained based on the maximum absorption of the DPMA molecule dissolved in various solvents using equation (10).
The solvent polarity ET (30) and normalized solvent polarity ( ) values are given in Table 3 ET (30) E T N values have been defined based on the equation (11) using water and tetramethylsilane (TMS) as most polar and least polar, respectively. Therefore E T N scale ranges from 0.00 for TMS to 1.00 for water [55][56].

Results and Discussion
The DPMA was dissolved in various solvents i.e.1,4-dioxane, acetonitrile, chloroform butan-2-ol, DMF, DMSO, ethanol, ethyl acetate, methanol, THF, and toluene. The Fig 3 and Fig 4 represents the maximum absorption and maximum emission of wavelengths, respectively, in various solvents. These wavelengths were converted into wavenumbers (cm -1 ), the difference between maximum absorption and maximum emission is known as Stoke shift as shown in Table 1.
DPMA molecule interacts with various solvents, considering their refractive index ( n ) and dielectric constant (  ) which was used in polarity functions Bakshiev's and Kawski-Chamma-Viallete's equations by solvatochromic shift method is shown in Table 2. The graphs Vs.(

Optimization process
The optimization process gives the minimum energy confirmation of the structure. The title molecule is completely optimized by DFT method with B3LYP along with 6-311++G (d, p) basis level. The ground state optimized structure along with dipole moment is shown in Fig 11 and Gauss view optimized structure is presented in Fig 12.

Molecular Electrostatic Potential
Molecular electrostatic potential (MEP) provides the necessary information regarding molecular size, shape, importantly positive, negative and neutral electrostatic potential areas are expressed in terms of color coding technique. Different values of the electrostatic potential are represented by various colors, red represents the most electrostatic negative potential, blue represents the region of most positive electrostatic potential and green represents the region of zero potential. The potential increases in the order of red < orange < yellow < green < cyan < blue [60,61]. Herein the color code lies between -0.02304 a.u (dark red) to 0.0240 a.u (dark blue) as shown in Fig 13. In the present molecule, highest electronegative region present around oxygen atom in the benzofuran ring. Second highest electronegative region present around the oxygen atom lies between the CH3 group and benzofuran ring. The highest electropositive region present around the hydrogen atoms present in CH2 group. Second highest electropositive region present around hydrogen atom attached to nitrogen atom.

Frontier molecular orbital studies (FMO)
Energies and distributions of the FMO are very important indicators of the reactivity.
Highest occupied molecular orbital (HOMO) represent electron donating ability of the molecule and Lowest unoccupied molecular orbital (LUMO) represents electron accepting ability of the molecule [62].The kinetic stability of the molecule is indicated by HOMO-LUMO energy gap [63] and energy gap is obtained by energy difference between HOMO and LUMO. The energies of LUMO and HOMO and the related energy gaps are computed using optimized structure of the molecule. Fig.14 shows the two dimensional structure of the energy gaps between HOMO and LUMO. The energies of HOMO and LUMO are -5.59 and -1.03 eV respectively. The energy gap between HOMO-LUMO is 4.53 eV. The small energy gap obtained represents more polarizability and so this chalcone predicted promising NLO properties. The ionization potential and chemical hardness and other properties are estimated using Koopman's theorem [64].
Generally large energy gap indicates that, molecule is hard and stable, whereas the small energy gap indicate that molecule is soft and reactive. The energy gap of the DPMA is 4.53 eV. Hence the molecule is soft and reactive.

TEM-EDX analysis of Ag nanoparticles
Morphological nature of the bio-synthesized Ag NPs is studied with Transmission electron microscopy (TEM). Here we observed the monodispersed spherical nature of the Ag NPs. Due to some bioactive molecule, there is agglomeration between some particles but all are in similar size in the range of 20 nm -40 nm. The average size of synthesized Ag NPs is around 28 nm. This regularity in particles size is due to the phytoconstituent residual present in the C.R leaves, this act as capping agent for the growth of the nanoparticle. The Fig 15 (a)   Bakshiev's (6) and KCV (7).
Change in dipole moment calculated using Equation (10).