Estimation of ground and excited state dipole moments of newly synthesized coumarin molecule by Solvatochromic shift method and Gaussian software

Received: 10/Apr/2019, Accepted: 19/Apr/2019, Online: 30/Apr/2019 Abstract: Absorption and fluorescence spectra of newly synthesized coumarin molecule namely 4-((3-methoxyphenoxy)methyl)-7-hydroxy-2H-chromen-2-one [3-MPHC] have been recorded at room temperature (300 K) in solvents of different polarities. The effects of solvents on the spectral properties are discussed. Using Lippert‟s, Bakshiev‟s and Kawski-Chamma-Viallet‟s equations the ground (μg) and excited state (μe) dipole moments were estimated. Variation of Stokes shift with the solvent dielectric constant along with refractive index was made use of to estimate dipole moments. Density Functional Theory (DFT) was studied using Gaussian software 16. It was observed that excited state dipole moment was higher than the ground state dipole moment. Further we have analyzed microscopic solvent polarity parameter.


I. INTRODUCTION
Estimation of the ground and excited state dipole moments of newly synthesized coumarin molecule is important, because the values of dipole moments provide information about the change in electronic redistribution upon excitation.Coumarin is the most important naturally occurring oxygen containing heterocyclic compound.From few decades coumarin and its derivatives are well recognized for their biological traits such as anticoagulant, antibacterial, anti-microbial, analgesic antipyretic, anti-neoplasm, anti-metastatic and fungicidal activity [1][2][3].Coumarin compounds are also used as food preservatives, additives and cologne in cosmetics.Therefore, determination of the coumarin content in different pharmaceutical and commercial products has become an interesting field of research.Coumarin molecules are very good media for efficient optical brighteners, laser molecules and broad-band dye lasers in blue-green domain of radiation.These compounds are used as rodenticides, fluorescent indicators.Using different methods many researchers worked on the ground and excited state dipole moments by experimentally and theoretically for coumarins, fluorescent molecules, organic dyes and laser dyes [4][5][6][7][8][9].We observed there is a blue shift in Iron doped ZnO nanoparticles at different molarities by Wet Chemical Method [20].However, there are no reports available in the literature on the estimation of ground and excited state dipole moment values of the coumarin molecule 4-((3-methoxyphenoxy)methyl)-7-hydroxy-2H-chromen-2-one .This prompted us to carry out the present work to estimate and compare the µ g and µ e of 3-MPHC.

II. MATERIALS AND METHODS
The coumarin derivative 3-MPHC was synthesized as per the procedure mentioned in the reference [10].The molecular structure of 3-MPHC is as shown in Fig. 1.Sample solutions were prepared by adding required amount of solute into different solvents.The solvents used were, Ethyl acetate (EA), Acetone, Dimethyl sulfoxide (DMSO), Acetonitrile (AN), 1-Butanol, Ethanol, Methanol, Toluene, Tetrahydrofuran (THF) and Dichloromethane (DCM) all were of spectroscopic grade.The solvents were used without any further purification.The required solutions were prepared at a fixed solute concentration (10 μ M/L).The concentration of solute was kept sufficiently low in order to minimize the effect of self absorption.Absorption spectra were recorded on a PG Inst.Ltd., model T-90+, UV-Vis.absorption spectrophotometer and fluorescence spectra on a Fluorolog-3 spectrofluorometer (Horiba-Jobin-Yvon).Linear fit was done by using Origin pro 8 software.

III. EXPERIMENTAL ESTIMATION OF GROUND AND EXCITED STATE DIPOLE MOMENTS
The independent equations used for the estimation of ground state and excited state dipole moments of the 3-MPHC molecule are as follows, Lippert"s equation [11] ) , ( Bakshiev"s equation [12] ) , ( Kawski-Chamma-Viallet"s equation [13,14] ) , ( 2 where a  and f  are absorption and fluorescence maxima wavelength in cm -1 , respectively.The other symbols "' and "n' are dielectric constant and refractive index respectively. The expressions for F 1 (ε, n) [Lippert"s polarity function], F 2 (ε, n) [Bakshiev"s polarity function] and F 3 (ε, n) [Kawski-Chamma-Viallet"s polarity function] are given as From equations ( 4), ( 5), ( 7) and ( 8) it follows that   linear graphs with slopes m 1 , m 2 and m 3 respectively and are given below, Where μ g and μ e are the ground and excited state dipole moments of the coumarin molecule, respectively.The symbols 'h' and 'c' are Planck constant and velocity of light in vacuum, respectively and 'a' is the Onsager"s cavity radius of the coumarin molecule.The value of 'a' is evaluated using atomic increment method by Edward [15].In general case, the dipole moments μ g and μ e are not parallel but they form certain angle [16].By assuming the ground state and excited state dipole moments are parallel, the following equations are obtained on the basis of equations ( 8) and ( 9).

Difference in Dipole Moment Using Molecular Microscopic Solvent Polarity Parameter
Understanding the polarization dependence or hydrogen bonding effect on spectral characteristics, it may be worthwhile to use molecular microscopic solvent polarity N T E function proposed by Reichardt [17].The theoretical basis for the correlation of the Stokes shift with N T E was proposed by Reichardt and developed by Ravi et al. [18].Accordingly equation ( 13) is obtained and (12) and E T (solvent) = 28,591/λ max corresponds to the peak wavelength in the red region of the intramolecular charge transfer absorption of the pyridinium-N-phenolatebetaine dye.Using this method, change in dipole moment is calculated from the plots of Stokes shift versus N T E .According to the following equation: Where Δ μ = (µ e -µ g ) and "a' are the change in dipole moment and Onsager cavity radius of the coumarin molecule of interest respectively.In the expression, the Onsager radii of the both Betaine dye (a D ) and coumarin derivative (a) enter as a ratio, as the values of a D and Δμ D are known (6.2 Å and 9 D, respectively [17]).

Effect of Solvents on Absorption and Fluorescence Emission Spectra
Absorption spectra and emission spectra of 3-MPHC molecule in different solvents are shown in Fig. 2 and Fig. 3 respectively.It is observed that the absorption wavelength found nearly in the range 275-350 nm and emission wavelength is nearly in the range 375-425 nm.Absorption maxima, emission maxima, Stokes shift and arithmetic mean Stokes shift wave numbers in cm -1 of 3-MPHC are given in table 1.The band shift occurred in the emission spectra and there is increase in the Stokes shift values with increasing solvent polarity which indicates that there is an increase in the dipole moment on excitation.Polarity functions values like Lippert"s

Estimation of ground and excited state dipole moments
The graphs obtained for Lippert"s polarity function versus Stokes shift (Fig. 4), Bakshiev"s polarity function versus Stokes shifts (Fig. 5) and Kawski-Chamma-Viallet"s polarity functions versus arithmetic mean Stokes shifts (Fig. 6) are found to be linear.From these graphs we obtained slopes to estimate the ground and excited state dipole moments using the relevant formulae.We also analyzed the change in dipole moment using microscopic solvent polarity parameter ( N T E ).The variation of Stokes shift with microscopic solvent polarity parameter is shown in Fig. 7. Onsager cavity radius, statistical correlation values, dipole moments in ground and excited states and change in dipole moments of 3-MPHC are given in table 3.  f) Change in dipole moment calculated using equations (10) and (11).
g) Change in dipole moment calculated using N T E equation (12).

Theoretical studies using Gaussian 16 software
From the theoretical calculations using Gaussian-16 program for the ground state optimized geometries, the estimated ground state dipole moment and Time dependent DFT studies, excited state dipole moments of the 3-MPHC molecule is tabulated in table 4. The optimized molecular geometry of the corresponding molecule is shown in Fig. 8; the arrow mark indicates the direction of dipole moment.The HOMO, LUMO and total density matrix of the molecule 3-MPHC are as shown in Fig. 9, Fig. 10 and Fig. 11 respectively.From these figures, it is apparent that the HOMO of 3-MPHC molecule mainly localized on the electron donating aliphatic chain, whereas the LUMO level shifted to peripheral electron accepting moiety results to an obvious spatial separation of frontier orbital"s.The polarity of a molecule depends on its electron distribution over the molecule.The absorption of a suitable electromagnetic radiation will cause transition of an electron from HOMO to LUMO orbital.Consequently, the movement of an electron from the low energy orbital to the higher energy orbital results in a change of dipole moment with respect to the ground state dipole moment [19].V. CONCLUSION We studied the effect of solvents on photophysical parameters for 3-MPHC molecule by experimentally and theoretically.We observed that 3-MPHC molecule undergo positive solvatochromism with increase in solvent polarity, indicates that the involvement of π → π * transition.The dipole moment of 3-MPHC molecule is more in the first excited singlet state than in the ground state.This indicates the existence of a more relaxed excited state, due to intramolecular charge transfer.We compared the experimental values of dipole moments with theoretical values; it is found that the ground state dipole moment is same in both the case.Experimentally the solvent effect is more on 3-MPHC molecule rather than the theoretical value.From microscopic solvent polarity function we estimated the difference in dipole moment and are compared.This is the first report on extensive study of photophysical characteristics of biologically active 3-MPHC molecule which helps to understand the usage of this molecule in various fluorescence applications as well as in the field of medicine.

Table 1 .
Absorption maxima, fluorescent maxima wave numbers in cm -1 , Stokes shift and arithmetic Stokes shift data

Table 2 .
Polarity functions and microscopic solvent polarity parameter for respective solvents

Table 3 .
Radius, statistical correlation values and dipole moments of 3-MPHC in the ground and excited states (in Debye)