Effects of solvent polarity on the absorption and fluorescence spectra of 3-cyano-7-hydroxy-4-methylcoumarin: Determination of the dipole moments.

The Absorption and fluorescence emission spectra of 3-cyano-7-hydroxy-4-methylcoumarin (3C7H4M) were studied in solvents of different dielectric constant ε and refractive index n . Experimental ground and excited state dipole moments were established by means of solvatochromic shift method. Both the ground state and excited state dipole moments were established. Results revealed that the excited state dipole moments of 3C7H4M were higher than those of the ground state. Further it is evident from these results that, the changes in the dipole moments on electronic excitation are small.


Introduction
Classical studies on photochemical properties of molecules require knowledge of spectral features.Fluorophoreshavedistinctive electronic and photonic properties such as, relatively shorter lifetime, high quantum yield and broad spectral band width which can be useful in a number of technological applications (Raikar et al., 2006).
Coumarin dyes are well known photosensitizing agents (Zhang, Zhang, & Xia, 2008).They can be used in photobiology because of their photodynamic actions.They are known to show interesting photochemical behavior, particularly dimerization in polar and non-polar solvents (Perez-Rodriguez, Aguilera, & Figueroa, 2003).
The widespread occurrence of coumarin derivatives in nature (Kirsch, Abdelwahab, & Chaimbault, 2016) and their variety of applications have made their study very interesting.Dipole moments of short-lived speciesare of significant interest as they provide bothelectronic and geometrical structure information of the dye (Sharma, Jain, & Rastogi, 2007).The information on dipole moments of electronically excited species is helpful in the designing nonlinear optical materials and understanding their photochemical transformations (Patil, Melavanki, Kapatkar, Ayachit, & Saravanan, 2011).Fluorescence experimental data on excited states are handy in the parameterization of semi-empirical quantum chemical procedures for these states (Xie, Ho, Truelove, Corry, & Stewart, 2010).
Determination of the ground and excited state dipole moments of dye molecules is important, because the values so obtained provide information about the change in electronic distribution upon excitation (Biradar, Siddlingeshwar, & Hanagodimath, 2008)The most popular technique for the determination of excited state dipole moments is based on the Lippert-Mataga equationLippert(1955), Mataga, Kaifu& Koizumi, 1956), in which absorption and fluorescence shiftsdescribed by dielectric constant ε and refractive index n are followed using the solvent polarity.
In this paper we report determination of dipole moments using different solvent parameters, refractive index, dielectric constant ε, spectral parameters like stokes shift.We have calculated the ground and excited state dipole moments of 3C7H4M dye by solvent perturbation method based on absorption and fluorescence shift in solvents with varied polarities.
Change in dipole moment will be obtained from the slope of the graph of Ṽ a -Ṽ f versus the solvent polarity,

Experiment
3C7H4Mwas purchased from Aldrich Chemical Co. and was used without further purification.The molecular structure of 3C7H4M is as given in Fig. 1.All the solvents used viz., methanol, ethanol, ethyl acetate, THF, and n-hexane, were of spectroscopic grade.Electronic absorption spectra were recorded on a Merc UV-11 Model UV-Vis spectrophotometer.Fluorescence spectra were taken by using a Biobase-BKF93 Model F2000 fluorospectrometer at room temperature.Refractive index was measured using ATAGO model pocket refractometer PAL-1.
Staining solutions of 3-cyano-7-hydroxy-4-methylcoumarin in water and methanol were prepared by dissolving 0.1 mg of the dye in 100 mL of distilled water (1 ppm).Iodine was prepared as per standard laboratory procedure.Each mixture was stirred for dye solubilization and filtered before use.

Results and discussion
Absorption and fluorescence emission spectra of 3-cyano-7-hydroxy-4-methylcoumarin (3C7H4M) and was recorded in solvents of different solvent parameters of dielectric constant ε and refractive index η.Figs.2-and 3 show the absorption and fluorescence spectra of 3C7H4M respectively in solvents with various polarity indices as shown in table 1.The absorption and emission spectra of 3C7H4M show maximum values around 340nm and 400nm respectively for THF, 350nm and 400nm respectively for ethyl acetate, 400nm and 444nm for ethanol respectively, 410nm and 440nm for methanol and 440nm and 444for water.
However, the hexane solution did not show any absorbance and emission.This resulted in stokes shift ranging from 04-60nm on changing the solvent from water to THF and the emission spectra.The absorption and emission maxima, dielectric constant and refractive index of 3C7H4M in different solvents are given in Table 1.ε =Relative permittivity of the solvent.f(ε, η)=Solvent polarity calculated using eqn (3).f(ε, η)+2g( η)=Solvent polarity calculated using eqns (3) and ( 4).
The charge transfer band shows a shift of about 04-60 nm in the absorption spectra on changing the solvent from THF to water and the emission spectra show smaller shift as compared with the absorption spectra.Thehighly pronounced absorption shift between water and other solvents implies that the ground state energy distribution is affected to a greater extent possibly due to the polar nature of 3C7H4M.
The values of the Stokes shifts are also indicative of the charge transfer transition.On changing the solvent from a low polar, like THF to high polar like ethyl acetate shows a difference in Stokes shift of about 9nm again indicative of a charge transfer transition.The large magnitude of the Stokes shift indicates that the excited state geometry could be different from that of the ground state.The general observation is that there is an increase in the Stokes shift values with increasing solvent polarity which shows that there is an increase in the dipole moment on excitation.
Figs. 4 and 5 show the spectral shifts (in cm -1 ) ν˜a-ν˜f and ν˜a + ν˜f of 3C7H4M in polarity functions f (ε, n) andf(ε, n) + 2g (n).A linear progression was done data was fit to a straight line.
Theslopes m 1 and m of the fitted lines shown in Figs.4and 5.Where M is the molecular weight of the dye; ρ is the density of the dye; N is the Avogadro's number.We have approximated the density of this compound to be1 g/mL.By using Eqns.
(3)and ( 4), we get μg = 3.08 D and μe = 3.9 D and the changein the dipole moments (Δμ= μ gμ e )is 0.82 D. From Eqns. ( 8) and ( 9), the dipole moments μ g and μ e depends not only on m 1 and m 2 but also on radius of the solute.The linear dependence of spectralshifts on polarity function (Fig. 4 and 5) exhibits a good correlation.These experimental values were obtained in solution phase, where the solvent (matrix) is expected to introduce strong perturbation.Further it is evident from these results that, the changes in the dipole moments on electronic excitation are small.

Conclusion
Both excitation and emission wavelengths of 3C7H4M in different solvents varying in polarity index has been determined.The Onsager radius has been calculated and the surface area of the molecule has been established, assuming the molecule as spherical and density is 1 g/mL.The determinations of dipole moments of C7H4M in the ground and excited states have shown that C7H4M is more polar in the excited state than the ground state.

Table 1 .
The calculated values of the solvent polarity parameters