Introduction

Coumarin derivatives and analogues are well-known for their photochemical and photophysical properties as well as for their interesting second-order nonlinearities [1,2,3,4,5,6]. In addition, the coumarin unit is known to undergo a reversible photo-induced cyclodimerization by irradiation at λ > 300 nm that leads to stable cyclobutane based dimers, whereas the reverse photocleavage reaction occurs at shorter wavelengths (λ < 280 nm) [7,8,9]. Coumarin-derived Cu (II)-selective fluorescent sensor and studied the fluorescence quenching mechanism by femtosecond time-resolved fluorescence (TRF) spectroscopy and quantum calculations [10]. Coumarin derivatives studied to their widespread industrial use as dye lasers [11]. In recent years, there has also been a drive to synthesize coumarin-based organic dyes for use in high-efficiency dye-sensitized solar cells (DSCs) [12, 13]. Coumarin and its derivatives have attracted significant interest in a wide range of pharmaceutical research areas such as anti-inflammatory, hepatoprotective, antiviral, anticarcinogenic and anticoagulant activities [14,15,16,17] and they have various applications in food constituents, stabilizers and clinical use [18].

Knowledge of the excited state charge distribution and the dipole moments of the molecules are important in designing the nonlinear optical materials [19, 20] and understanding the photochemical processes. Methods available for the determination of dipole moments can be classified as either external or internal. External methods includes electric dichroism [21], Stark splitting of rotational levels [22] and microwave conductivity [23] are consider to be more accurate in the determining the excited state dipole moment of the simple organic molecules, however the quite popular internal method for the determination of the singlet excited state dipole moment is based on the analysis of absorption and fluorescence maxima. The procedure is evolved by Lippert’s [24], Bakshiev’s [25] and Kawski-Chamma-Viallet’s equations [26] followed by the variation of Stokes shift with solvent polarity function.

In several investigations, they have reported regarding the estimation of ground and excited state dipole moments using various organic compounds [27,28,29,30,31,32,33,34,35]. However there is no report available in literature for the determination the ground and excited state dipole moments of these three 4-(2,6-Dibromo-4-methyl-phenoxymethyl)-benzo[h]chromen-2-one(DMB), 4-(6,7-Dimethoxy-3,4-dihydro-isoquinoline-1-ylmethyl)-6-methyl-chromen-2-one(DIM) and 4-((p-tolyloxy)methyl)-6-methoxy-2H-chromen-2-one (TMC) coumarin molecules. In this paper, we have estimated and compared the ground state and the excited state dipole moments of these three coumarin molecules by various methods.

Materials and Methods

The novel coumarin derivatives namely; 4-((2,6-dibromo-4-methylphenoxy)methyl)-2H-benzo[h]chromen-2-one (DMB), 4-((3,4-dihydro-6,7-dimethoxyisoquinolin-1-yl)methyl)-6-methyl-2H-chromen-2-one (DIM) and 4-((p-tolyloxy)methyl)-6-methoxy-2H-chromen-2-one (TMC) were synthesised according to references [36, 37]. The molecular structures of these three molecules are shown in Fig. 1. Solvents used in the present study were trichloroethylene, benzene, toluene, tetrahydrofuran, dioxin, acetone, acetonitrile, methanol and dimethyl sulfoxide [DMSO] were purchased from S-D Fine Chemicals Ltd. India and used without any further purification.

Fig. 1
figure 1

The molecular structure of coumarin molecules (DMB, DIM and TMC)

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Several experimental techniques were used to characterize the coumarin molecules at room temperatures like Hitachi model U-3310 UV–visible spectrophotometer was used to record the electronic absorption spectra and Hitachi model F-7000 fluorescence spectrophotometer was used to record fluorescence spectra. The LCR Data Bridge was used to measure the dielectric constant and Abbe’s refractometer was used to measure the refractive index of the coumarin molecules. All these measurements were carried out by keeping dye concentration very low about 1 × 10− 5 M in order to avoid the self-absorption.

The dielectric constant of the solution ε xy can be calculated using the equation

$${\varepsilon _{xy}}=\frac{{{C_S} - {C_L}}}{{{C_A} - {C_L}}}$$
(1)

where

C S :

Capacitance of the solution.

C A :

Capacitance of the air.

C L :

Capacitance due to leads.

Solvatochromic Shift Method

The ground state and excited state dipole moments for all three molecules are calculated using three independent equations proposed by Lippert’s, Bakshiev’s and Kawaski-Chamma-Viallte’s.

  1. I.

    Lippert’s equation [24] is given by

$$\overline {{\upsilon _a}} - \overline {{\upsilon _f}} =m{\text{ }}\Phi (\varepsilon ,n)+{\text{Constant}}$$
(2)
  1. II.

    Bakshiev’s equation [25]

$$\overline {{\upsilon _a}} - \overline {{\upsilon _f}} ={m_1}{\Phi _1}(\varepsilon ,n)+{\text{Constant}}$$
(3)
  1. III.

    Kawski-Chamma-Viallete’s equation [26]

$$\frac{{\overline {{\upsilon _a}} +\overline {{\upsilon _f}} }}{2}= - {m_2}{\Phi _2}(\varepsilon ,n)+{\text{Constant}}$$
(4)

where \(\Phi (\varepsilon ,n)=\left[ {\frac{{\varepsilon - 1}}{{2\varepsilon +1}} - \frac{{{n^2} - 1}}{{2{n^2}+1}}} \right]\) is the Lippert’s polarity function.

\({\Phi _1}(\varepsilon ,n)=\frac{{2{n^2}+1}}{{{n^2}+2}}\left[ {\frac{{\varepsilon - 1}}{{\varepsilon +2}} - \frac{{{n^2} - 1}}{{{n^2}+2}}} \right]\) is the Bakshiev’s polarity function.

\({\Phi _2}(\varepsilon ,n)=\left[ {\frac{{2{n^2}+1}}{{2\left( {{n^2}+2} \right)}}\left( {\left[ {\frac{{\varepsilon - 1}}{{\varepsilon +2}} - \frac{{{n^2} - 1}}{{{n^2}+2}}} \right]} \right)+\frac{{3\left( {{n^4} - 1} \right)}}{{2{{({n^2}+2)}^2}}}} \right]\) is the Kawski-Chamma-Viallete’s polarity function.

In all above equations \(\overline {{\upsilon _a}}\) and \(\overline {{\upsilon _f}}\) are absorption and the fluorescence maxima wavenumbers (in cm− 1) respectively. n and ε are the refractive index and the dielectric constant of solvent respectively. From Eqs. (2), (3) and (4) graphs of \(\left( {\overline {{\upsilon _a}} - \overline {{\upsilon _f}} } \right)\) versus ɸ (ε, n), \(\left( {\overline {{\upsilon _a}} - \overline {{\upsilon _f}} } \right)\) versus ɸ 1 (ε, n) and \(\left( {\frac{{\overline {{\upsilon _a}} +\overline {{\upsilon _f}} }}{2}} \right)\) versus ɸ 2 (ε, n) gives the linear graphs with slopes m, m 1 and m 2 respectively and are given by

$$m=\left[ {{{\frac{{2\left( {{\mu _e} - {\mu _g}} \right)}}{{hc{a^3}}}}^2}} \right]$$
(5)
$${m_1}=\left[ {{{\frac{{2\left( {{\mu _e} - {\mu _g}} \right)}}{{hc{a^3}}}}^2}} \right]$$
(6)
$${m_2}=\left[ {{{\frac{{2\left( {\mu _{e}^{2} - \mu _{g}^{2}} \right)}}{{hc{a^3}}}}^{}}} \right]$$
(7)

where

\({\mu _g}\) :

Ground state dipole moment.

\({\mu _e}\) :

Excited state dipole moment.

h :

Planck’s constant.

c :

Velocity of the light in vacuum.

a :

Onsager cavity radius of the solute molecule which can be calculated by adding an atomic volume of the constituting the molecules as suggested by Edward [38].

From Eqs. (6) and (7) the value of \({\mu _g}\) and \({\mu _e}\) are given by

$${\mu _g}=\frac{{{m_2} - {m_1}}}{2}{\left[ {\frac{{hc{a^3}}}{{2{m_1}}}} \right]^{\frac{1}{2}}}$$
(8)
$${\mu _e}=\frac{{{m_2}+{m_1}}}{2}{\left[ {\frac{{hc{a^3}}}{{2{m_1}}}} \right]^{\frac{1}{2}}}$$
(9)

and

$$\frac{{{\mu _e}}}{{{\mu _g}}}=\left[ {\frac{{{m_2}+{m_1}}}{{{m_2} - {m_1}}}} \right]{\text{for }}{m_2}>{m_1}$$
(10)

Guggenheim Method

The ground state dipole moment of all three solute molecules was calculated by Guggenheim method [39] which involves the refractive index and dielectric constant of the solute molecules and is given by

$$\mu _{g}^{2}=\left[ {\frac{{27kT}}{{4\pi N\left( {{\varepsilon _x}+2} \right)\left( {n_{x}^{2}+2} \right)}}} \right]\Delta$$
(11)

Where \(\Delta =\left[ {{{\left( {\frac{{{\varepsilon _{xy}} - {\varepsilon _x}}}{C}} \right)}_{C \to 0}} - {{\left( {\frac{{n_{{xy}}^{2} - n_{x}^{2}}}{C}} \right)}_{C \to 0}}} \right]\).

Here symbols k, T, N, ε, n and C are Boltzmann’s constant, the temperature in Kelvin, Avogadro’s number, dielectric constant, refractive index and concentration respectively. The suffixes x and xy refer to the solvent and solute respectively. ∆ is extrapolated intercepts of the plots \(\left( {\frac{{{\varepsilon _{xy}} - {\varepsilon _x}}}{C}} \right)\) versus C and \(\left( {\frac{{n_{{xy}}^{2} - n_{x}^{2}}}{C}} \right)\) versus C with respect to infinite dilution (C→0).

Results and Discussion

The spectral shift data of absorption and emission spectra along with the Stokes shift of all three coumarin derivatives namely; 4-(2,6-Dibromo-4-methyl-phenoxymethyl)-benzo[h]chromen-2-one (DMB), 4-((3,4-dihydro-6,7-dimethoxyisoquinolin-1-yl)methyl)-6-methyl-2H-chromen-2-one (DIM) and 4-((p-tolyloxy)methyl)-6-methoxy-2H-chromen-2-one (TMC) are shown in Tables 1, 2, and 3 respectively. From these spectral shift data, one can identify the spectral transition namely; n→ π*, π→ π* etc [40, 41]. From Tables 1, 2, and 3 it can be notified that on the decrease in the solvent polarity which decreases the Stokes shift value of the solute molecules confirming that the π→π* transition in spectral levels. The shifting of the band occurs due to differences in the stabilization of ground and the excited state thus it causes a change in energy gap between these electronic states. And Table 4 summarizes the calculated values of the solvent polarity functions Φ (ε, n), Φ 1 (ε, n), Φ 2 (ε, n) along with the solvent parameters namely the dielectric constant (ε) and refractive index (n).

Table 1 Spectral shift data for DMB molecule in different solvents
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Table 2 Spectral shift data for DIM molecule in different solvents
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Table 3 Spectral shift data for TMC molecule in different solvents
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Table 4 Solvent parameters and calculated polarity functions
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Figures 2, 3, and 4 shows the linear graphs of \(\left( {\overline {{\upsilon _a}} - \overline {{\upsilon _f}} } \right)\) versus ɸ (ε, n), \(\left( {\overline {{\upsilon _a}} - \overline {{\upsilon _f}} } \right)\) versus ɸ 1 (ε, n) and \(\left( {\frac{{\overline {{\upsilon _a}} +\overline {{\upsilon _f}} }}{2}} \right)\) versus ɸ 2 (ε, n) for DMB, DIM and TMC molecules which gives the slopes m, m 1 and m 2 respectively. The slope, intercept, the number of data points and the statistical correlation values are shown in Table 5 and the good correlation value is obtained in all these cases, which clear that polarity functions exhibit quite good correlation with the spectral shifts for the selected number of data points. Generally, the deviation from the linearity may be due to specific solute–solvent interactions.

Fig. 2
figure 2

Lippert’s plots for all three molecules in different solvents

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Fig. 3
figure 3

Bakshiev’s plots for all three molecules in different solvents

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Fig. 4
figure 4

KCV plots for all three molecules in different solvents

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Table 5 Statistical treatment of the correlations for the solvent spectral shift of DMB, DIM and TMC molecules
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The ground state dipole moment value of all three solute molecules [DMB, DIM and TMC] obtained from the Guggenheim method were found to be 2.71, 2.33 and 2.47 D respectively and are shown in Table 6. The excited state dipole moment values were calculated using Eq. (9), Lippert Eq. (5), Bakshiev’s Eq. (6) and Kawski-Chamma-Viallet’s Eq. (7) are shown in Table 6. In all three solute molecules, the value of \({\mu _e}\) calculated from the Lippert’s equation is larger, this may be due to the solute–solvent interaction evolved in the Lippert’s equation [24]. The values of the ground and the excited state dipole moments of these three coumarin derivatives are different; this may be due to the structural difference between these three molecules. The change in dipole moment (∆μ) from Eqs. (8) and (9) are also shown in Table 6. There is a large magnitude of the Stokes shift in the system studied, which may indicate that the excited state charge distribution is different from the ground state. In general, there will be an increase in Stokes shift on increase in the solvent polarity, which shows that there is an increase in dipole moment on the excitation.

Table 6 The Onsager radius, the ground and excited state dipole moments of DMB, DIM and TMC molecules
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Conclusion

In summary, we have studied photophysical properties for novel coumarin derivatives namely; 4-(2,6-Dibromo-4-methyl-phenoxymethyl)-benzo[h]chromen-2-one(DMB), 4-((3,4-dihydro-6,7-dimethoxyisoquinolin-1-yl)methyl)-6-methyl-2H-chromen-2-one (DIM) and 4-((p-tolyloxy)methyl)-6-methoxy-2H-chromen-2-one (TMC). Absorption and fluorescence spectra of above said molecules were recorded in different polar and nonpolar solvents. From these spectral shift data, we observed that on the increase in solvent polarity the Stokes shift value also increases confirming spectral transition may be due to being π→π* and we have calculated and compared the dipole moments of these three coumarin molecules in the ground and excited states by solvatochromic shift method and Guggenheim method. It is notified that all three Coumarin molecules possess higher dipole moment in the excited state than in the ground state. It is inferred that the excited state for above said molecules is more polar than the ground state. Further, an Eq. (10) can be used to estimate the value of excited state dipole moment by pre-knowledge of the value of ground state dipole moment, without a necessity of knowing the Onsager cavity radius of the solute molecule. The present investigations may shine in the design of nonlinear optical materials.