Introduction

The scientific concept of magic bullet coined by German Nobel Laureate, Paul Ehrlich is the need of the hour when the deadly diseases like cancer has taken over the world [1]. The challenge is to create “smart bomb” which can destroy the tumor cells and prevent the risk of collateral damage [2]. Hydrazone derivatives are used in such treatments where the drug is coupled to antibodies using acid-labile hydrazone bonds [3]. Literature studies reveal that Schiff bases have a wide range of biological properties. Among these some of the fluoro-based Schiff base have been found out to be cytotoxic concerning the human embryonic cell line (HEL) [4]. Several Schiff base metal complexes are used for second order non-linear optics (NLO). These effects can only be induced by molecules and materials which lacks a centre of symmetry (in order to allow a restoring force which is asymmetric), usually with dipolar D-π-A structure where π is a conjugated system (bridged) [5].

When a molecule absorbs a photon, the electrons of HOMO (Highest Occupied Molecular Orbital) get promoted to LUMO (Lowest Unoccupied Molecular Orbital) which is followed by an intramolecular charge transfer. The excited molecule has a greater dipole moment because of highly polar state which can be accounted from the conformational change that happens upon excitation in comparison to the ground state [6, 7]. The fluorophores are very helpful in order to analyze the microenvironment properties that surround the molecule as they undergo Donor-Acceptor (D-A) molecular bond rotations and the D-A nucleus is responsible for significant charge redistribution upon photoexcitation (Fig. 1a) [7,8,9,10]. In order to study this fundamental physical-chemical process in bulk solution, the determination of ground state dipole moment and excited state dipole moment is of utmost importance as it further helps in the determination of nucleophilic and electrophilic attack sites in some of the photochemical reactions [11]. The knowledge of the excited state dipole moment is useful in determining how a photochemical transformation progresses or to elucidate the nature of the excited state [12]. There are quite a few methods which are available for the determination of electric dipole in different electronic states of a solute molecule in any solvent [13,14,15]. The most prominent existing method is the one based on change in intensities and the band obtained in the electronic spectra for solvents having different polarities which is called as Solvatochromic method [16, 17].

Fig. 1
figure 1

(a) The interaction between Donor and Acceptor (D-A) groups and (b) the presence of D-A groups in the chemical structure of FNEB

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Solvatochromism can be described using innumerable methods such as the characteristics of the solvent and its spectral parameters [18]. Theories described are based on the specific electrostatic solute-solvent interactions explained by Onsager, where solvent is described as a dielectric continuum hosting the solvent molecules into an cavity which is of Onsager type (Onsager’s reaction-field theory) [19]. The theory explains that the solute’s electric dipole moment polarizes the solvent so that an electric field is induced; hence the reaction field experienced by the solute is directly proportional to electronic state dipole moment of the compound [20].

In the current study, we report the synthesis of FNEB which is a novel fluoro-based Schiff base containing an electron donor and acceptor group (Fig. 1b). The azomethine (-C=N-) derivative is obtained by the condensation of an active carbonyl containing compound with a primary amine. The excitation and emission spectra of FNEB were recorded in fifteen solvents in order of increasing polarities. The dipole moments of ground state and excited state were computed using the theoretical relations in Solvatochromic approaches. Catalan and Kamlet-Abboud-Taft parameters were determined from multiple linear regression analysis (MLR) in order to study the interaction between the solute and solvent. The dipole moments were also calculated computationally using TD-DFT method.

Experimental and Computational Studies

Materials and Methods

All the chemicals used in the present research work were obtained from Sigma-Aldrich-Merck Limited. FNEB was purified by recrystallization using ethyl alcohol. Thin layer chromatography (TLC) was performed to confirm purity of the synthesized compounds using silica gel coated aluminium plates (Merck). Solvatochromic shift studies were done using spectroscopic graded solvents like methanol, ethanol, butanol, isopropanol, acetonitrile, dimethyl sulphoxide, N, N-dimethylformamide, acetone, dichloromethane, tetrahydrofuran, ethyl acetate, diethyl ether, 1,4-dioxane, n-Hexane and toluene purchased from S-D Fine Chemicals Limited, India. 1H and 13C NMR were recorded on 400 MHz Bruker NMR using DMSO-d6 (Dimethyl sulphoxide-d6) as the solvent and Me4Si (tetramethylsilane) as the internal standard. Infra Red (IR) spectrum was recorded using Bruker-Alpha-T ATR/FTIR spectrometer. The emission and excitation spectrum were recorded for low concentration (10−5 M) of analyte using Shimadzu (UV/Visible 1800) and Shimadzu Spectrofluorometer (RF5301PC). The solvatochromic results were analyzed using OriginPro Software. The relative permittivity (dielectric constant) of solvent and compound dissolved in the solvent were recorded using Cromtech LCR Meter (Model JC2812B) and refractive indices of compound dissolved in suitable solvent, air and solvent were recorded using Abbe’s Refractometer. CHI608 Electrochemical analyzer was used to determine HOMO and LUMO energies experimentally. Theoretical computational study were done using Gaussian 09 W software program where TD-DFT mode was used. TD-DFT computational analyses were used to study the property of surface (nucleophilic or electrophilic sites) in FNEB. These studies were done using Gaussian 09 W software which was also used to optimize the geometry of FNEB in both the electronic states (Fig. 2). The theoretical Light Harvesting efficiency (LHE) was computed using B3LYP/6–31++G(d) basic set. The chemical stability of FNEB was determined using computational and cyclic voltammetry by the use of obtained energy gap between the frontier orbitals.

Fig. 2
figure 2

Optimized geometry of FNEB using TD-DFT in (a) Ground state and (b) Excited state

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Theory

The estimation of dipole moments of the electronically singlet excited state (μex) and ground state (μgr) for any organic compound can be determined using both experimentally and theoretically. Guggenheim method is used for determination of experimental ground state dipole moment (μgr). DFT was also used for determination of the dipole moments in both the states.

Experimental Calculation for Ground State Dipole Moment

The determination of μgr using E. A Guggenheim method is shown in Eq. (1)

$$ {\upmu}_{\mathrm{gr}}=\sqrt{\left[\frac{27\mathrm{KT}}{4\uppi \mathrm{N}\left({\upvarepsilon}_1+2\right)\left({\mathrm{n}}_1^2+2\right)}\times \frac{\varDelta }{\mathrm{C}}\right]} $$
(1)

Where C is concentration of solute in a given solvent, \( \varDelta =\left({\upvarepsilon}_{12}\hbox{--} {\mathrm{n}}_{12}^2\right)-\left({\upvarepsilon}_1\hbox{--} {\mathrm{n}}_1^2\right) \), here ε12 and n12 are relative permittivity and refractive index of the solution containing solute, ε1 and n1 are relative permittivity and refractive index of the pure solvent.

Theoretical Calculations for Ground State Dipole Moment

Bilot and Kawski [21, 22] derived expressions for effect of Solvatochromism as a function of both relative permittivity and solvent’s refractive index which is shown as

$$ {\overline{\upupsilon}}_{\mathrm{Ab}\overline{\mathrm{s}}}{\overline{\upupsilon}}_{\mathrm{Flu}}={\mathrm{m}}^{(1)}\mathrm{f}\left(\mathrm{n},\upvarepsilon \right)+\mathrm{constant} $$
(2)
$$ {\overline{\upupsilon}}_{\mathrm{Abs}}+{\overline{\upupsilon}}_{\mathrm{Flu}}=-{\mathrm{m}}^{(2)}\upvarphi \left(\mathrm{n},\upvarepsilon \right)+\mathrm{constant} $$
(3)

Where \( {\overline{\upupsilon}}_{\mathrm{Abs}} \) and \( {\overline{\upupsilon}}_{\mathrm{Flu}} \) are wave number (cm−1) for absorption and fluorescence respectively and φ(n,ε) is given as the sum of solvent polarity functions, i.e., f(n,ε) + 2 g(n).

The functions f(n,ε) and g(n) are given using the following expressions

$$ \mathrm{f}\left(\mathrm{n},\upvarepsilon \right)=\frac{\upvarepsilon -1}{\upvarepsilon +2}-\frac{{\mathrm{n}}^2-1}{{\mathrm{n}}^2+2}\frac{2{\mathrm{n}}^2+1}{{\mathrm{n}}^2+2} $$
(4)
$$ \mathrm{g}\left(\mathrm{n}\right)=\frac{3}{2}\frac{\left[{\mathrm{n}}^4-1\right]}{{\left[{\mathrm{n}}^2+2\right]}^2} $$
(5)

The variables m(1), m(2) are slopes of linear graphs for both Eqs. (2) and (3) of Bilot-Kawski and their values can be given as follows

$$ \mathrm{m}(1)=\frac{\kern0.5em 2{\left({\upmu}_{\mathrm{ex}}-{\upmu}_{\mathrm{gr}}\right)}^2}{{\mathrm{hca}}^3} $$
(6)
$$ \mathrm{m}(2)=\frac{2\ \left({\upmu_{\mathrm{ex}}}^2-{\upmu_{\mathrm{gr}}}^2\right)}{{\mathrm{hca}}^3} $$
(7)

where μgr and μex are the electronic ground and excited state dipole moments of the molecule, “h” is the Planck’s constant, ‘c’ is the velocity of light and ‘a’ is the Onsager radius. The Onsager cavity can be calculated using Edward’s atomic increment method by the use of van der Waal’s volume [23]. The Onsager radius is calculated using Eq. (8).

$$ \mathrm{a}=\sqrt[3]{\left[\frac{3\mathrm{M}}{4{\uppi \updelta \mathrm{N}}_{\mathrm{A}}}\right]} $$
(8)

Where the density of the solute is denoted by δ, M is molecular weight and NA is the Avagadro’s number.

Considering that the symmetry of the molecule remains unchanged in both ground and excited states, the dipole moment for both the states are considered as parallel [22]. Based on Eqs. (6) and (7), the following expressions were derived.

$$ {\upmu}_{\mathrm{gr}}=\frac{{\mathrm{m}}^{(2)}-{\mathrm{m}}^{(1)}}{2}{\left[\frac{\left({\mathrm{hca}}^3\right)}{\mid 2{\mathrm{m}}^{(1)}\mid}\right]}^{1/2} $$
(9)
$$ {\mu}_{ex}=\frac{m^{(2)}+{m}^{(1)}}{2}{\left[\frac{\left( hc{a}^3\right)}{\mid 2{m}^{(1)}\mid}\right]}^{1/2} $$
(10)
$$ {\mu}_{ex}={\mu}_{gr}\left[\frac{m^{(1)}+{m}^{(2)}}{m^{(2)}-{m}^{(1)}}\right] $$
(11)

If the dipole moments μgr and μex are not parallel, then the angle (φ) made is determined using Eq. (12)

$$ \cos\ \upvarphi =\frac{1}{2{\mu}_{ex}{\mu}_{gr}}\left\{\left({\mu_{ex}}^2+{\mu_{gr}}^2\right)-\frac{m^{(1)}}{m^{(2)}}\left({\mu_{ex}}^2-{\mu_{gr}}^2\right)\right\} $$
(12)

Therefore,

$$ \upvarphi ={\cos}^{-1}\left[\frac{1}{2{\upmu}_{\mathrm{ex}}{\upmu}_{\mathrm{gr}}}\left\{\left({\upmu_{\mathrm{ex}}}^2+{\upmu_{\mathrm{gr}}}^2\right)-\frac{{\mathrm{m}}^{(1)}}{{\mathrm{m}}^{(2)}}\left({\upmu_{\mathrm{ex}}}^2-{\upmu_{\mathrm{gr}}}^2\right)\right\}\right] $$

Experimental Calculations for Dipole Moment in the Excited State

The experimental excited state dipole moment is determined by three separate equations which were suggested by Lippert-Mataga {Eq. (13)} [24, 25], Baksheiv {Eq. (14)} [26] and Kawski-Chamma-Viallet {Eq. (15)} [27, 28] are given below.

$$ {\overline{\upupsilon}}_{\mathrm{Abs}}-{\overline{\upupsilon}}_{\mathrm{Flu}}={\mathrm{m}}_1{\mathrm{P}}_1+\mathrm{constant} $$
(13)
$$ {\overline{\upupsilon}}_{\mathrm{Abs}}+{\overline{\upupsilon}}_{\mathrm{Flu}}={\mathrm{m}}_2{\mathrm{P}}_2+\mathrm{constant} $$
(14)
$$ \frac{\left({\overline{\upupsilon}}_{\mathrm{Abs}}+{\overline{\upupsilon}}_{\mathrm{Flu}}\right)}{2}={\mathrm{m}}_3{\mathrm{P}}_3+\mathrm{constant} $$
(15)

Where P1, P2, P3 represents the solvent polarity functions {Eqs. (16), (17) and (18)}, m1, m2 and m3 represent the slopes of the linear relationship {Eqs. (19), (20) and (21)} which can be given as

$$ {\mathrm{P}}_1=\left\langle \frac{\upvarepsilon -1}{2\upvarepsilon +1} - \frac{{\mathrm{n}}^2-1}{2{\mathrm{n}}^2+1}\right\rangle $$
(16)
$$ {\mathrm{P}}_2=\mathrm{f}\left(\mathrm{n},\upvarepsilon \right) $$
(17)
$$ {\mathrm{P}}_3=\left[\left(\frac{{\mathrm{P}}_2}{2}\right)+\left(\frac{3\left({\mathrm{n}}^4-1\right)}{2{\left({\mathrm{n}}^2+2\right)}^2}\right)\right] $$
(18)

The refractive index of the solvent is denoted by ‘n’ and the relative permittivity by ‘ε’. The linear plots were plotted using Eqs. (13), (14) and (15) and corresponding slopes m1, m2 and m3 were obtained which is obtained by the following equations.

$$ {\mathrm{m}}_1=\frac{\kern0.5em 2{\left({\upmu}_{\mathrm{ex}-}{\upmu}_{\mathrm{gr}}\right)}^2}{{\mathrm{hca}}^3} $$
(19)
$$ {\mathrm{m}}_2=\frac{\kern0.5em 2{\left({\upmu}_{\mathrm{ex}-}{\upmu}_{\mathrm{gr}}\right)}^2}{{\mathrm{hca}}^3} $$
(20)
$$ {\mathrm{m}}_3=\frac{2\ \left({\upmu_{\mathrm{ex}}}^2-{\upmu_{\mathrm{gr}}}^2\right)}{{\mathrm{hca}}^3} $$
(21)

Based on microscopic polarity parameter \( {\mathrm{E}}_{\mathrm{T}}^{\mathrm{N}} \), Reichardt proposed a much improved technique when compared to the regularly used bulk solvent polarity functions [29], which was used to determine μexusing Eq. (22) and for determining dipole moment change (δμ) from ground to excited state.

$$ {\overline{\upupsilon}}_{\mathrm{Abs}}-{\overline{\upupsilon}}_{\mathrm{Flu}}={\mathrm{m}}_4{\mathrm{E}}_{\mathrm{T}}^{\mathrm{N}}+\mathrm{constant} $$
(22)
$$ {\mathrm{m}}_4=11307.6\left[{\left(\frac{\updelta \upmu}{{\updelta \upmu}_{\mathrm{B}}}\right)}^2{\left(\frac{{\mathrm{a}}_{\mathrm{B}}}{\mathrm{a}}\right)}^3\right] $$
(23)

Where δμB is the change in dipole moment of betaine dye and ‘aB’, ‘a’ are Onsager radius of betaine and solute molecule respectively. By the use of Eq. (23), the dipole moment for the molecule in the excited state is calculated from Eq. (24)

$$ {\upmu}_{\mathrm{ex}}=\sqrt{\left[\frac{{\mathrm{m}}_481}{11307.6\kern0.5em {\left(\raisebox{1ex}{$6.2$}\!\left/ \!\raisebox{-1ex}{$\mathrm{a}$}\right.\right)}^3}\right]}+{\upmu}_{\mathrm{gr}} $$
(24)

The electrostatic solute-solvent interactions which are non-specific in nature as described by Onsager is explained using multiple linear regression (MLR) analysis based on linear salvation energy relationship (LSER) [30]. This concept of MLR was proposed by Katritzky and Kamlet-Abboud-Taft [31], which was used for correlation of excitation, emission wavelength, Stoke’s shift with hydrogen bond acceptor (β) ability, hydrogen bond donor (α) and the solventsdipolarity/polarizability (π*) [32] and can be expressed as

$$ \mathrm{z}={\mathrm{z}}_0+\mathrm{a}\upalpha +\mathrm{b}\upbeta +\mathrm{c}\uppi \ast $$
(25)
$$ \mathrm{z}={\mathrm{z}}_0+\mathrm{a}\left(\frac{\upvarepsilon -1}{2\upvarepsilon +1}\right)\kern0.6em +\mathrm{b}\left(\frac{{\mathrm{n}}^2-1}{2{\mathrm{n}}^2+1}\right)\kern0.36em +{\mathrm{cE}}_{\mathrm{T}} $$
(26)

Where z is the Stoke’s shift (\( {\overline{\upupsilon}}_{\mathrm{Abs}}-{\overline{\upupsilon}}_{\mathrm{Flu}} \)) and z0 indicates the physiochemical properties of the shift which does not depend on solvent (in gaseous phase) [(\( {\overline{\upupsilon}}_{\mathrm{Abs}}-{\overline{\upupsilon}}_{\mathrm{Flu}} \))0]. ε is the dielectric constant/ relative permittivity, n is refractive index and ET (30) scale is the microscopic solvent polarity parameter for a given solvent. The polarizability of two different interrelated solvents in the solvatochromic shift approach is distinguished by using Catalan method.

Generalized Treatment of Solvent Effect Using Catalan Method

Catalan method was used in MLR to estimate the non-specific solvent effects by splitting the traditional single-parameter into polarizability (SP) term and dipolarity term (SdP) along with solvent acidity (SA) and solvent basicity (SB) [33]. The expression for particular approach is given as below

$$ \mathrm{z}={\mathrm{z}}_0+\mathrm{aSA}+\mathrm{bSB}+\mathrm{cSP}+\mathrm{dSdP} $$
(27)

Amongst the four coefficients a, b, c and d, d is not prone to any change in its equivalent parameter for the spectroscopic parameter.

Determination of the Quantum Yield (Φ)

Quantum yield of a molecule can be determined using single point method. The integrated fluorescence intensity (I) and optical density (OD) of the compound is compared to optical density (ODR) and intensity (IR) of reference (anthracene in ethyl alcohol) at room temperature. Φ is given as Eq. (28)

$$ \Phi ={\Phi}_{\mathrm{R}}\frac{\mathrm{I}}{{\mathrm{I}}_{\mathrm{R}}}\frac{{\mathrm{OD}}_{\mathrm{R}}}{\mathrm{OD}}\frac{{\mathrm{n}}^2}{{\mathrm{n}}_{\mathrm{R}}^2} $$
(28)

Where, ΦR is quantum yield of reference (standard value of 0.27). The constants n and nR are the refractive indices of solvent and reference respectively and ratio of squares of n and nR is assumed to be 1.

Determination of Frontier orbital’s (HOMO and LUMO) Energies

Computational Studies

The difference in the energies of frontier orbitals, i.e., highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) gives the band gap (EHOMO – ELUMO) for the considered molecule was found out by TD-DFT studies using Gaussian 09 software. TD-DFT/PCM (B3LYP/6-31G (d,p)) mode was used. The geometry of molecule in both ground state and excited state were optimized. Mulliken charges for each atoms were also generated. Finally, molecular electrostatic potential plot was done to evaluate the electrophilic and nucleophilic sites.

Cyclic Voltammetry Method

Typical cyclic voltammograms for the given compound and the internal standard ferrocene were obtained in an organic solvent using lithium perchlorate in acetonitrile as the supporting electrolyte. The electrode system consisted of aqueous silver/silver chloride electrode as reference electrode, Platinum (Pt) electrode as working electrode and platinum counter electrode at ambient temperature is used. HOMO-LUMO was determined using oxidation potential (Eox) values. The energies for HOMO and LUMO were calculated using Eqs. (29) and (30)

$$ {\mathrm{E}}_{\mathrm{HOMO}}=-\left[\left({\mathrm{E}}_{\mathrm{ox}}\hbox{--} {\mathrm{E}}_{\mathrm{ferrocene}}\right)+4.8\right]\ \mathrm{eV} $$
(29)
$$ {\mathrm{E}}_{\mathrm{LUMO}}=-\left[\left({\mathrm{E}}_{\mathrm{red}}\hbox{--} {\mathrm{E}}_{\mathrm{ferrocene}}\right)+4.8\right]\ \mathrm{eV} $$
(30)
$$ {\mathrm{E}}_{\mathrm{G}}=-\left[\left({\mathrm{E}}_{\mathrm{HOMO}}\hbox{--} {\mathrm{E}}_{\mathrm{LUMO}}\right)\right]\ \mathrm{eV} $$
(31)

Where, EHOMO and ELUMO represent the energies of HOMO and LUMO respectively. Eferrocene is reduction potential for internal standard ferrocene. Ered is reduction potential of the molecule in consideration. The standard value for reduction potential for ferrocene-ferrocenium ion is 4.8 and the electrochemical gap between frontier orbitals energies is given by EG as shown in Eq. (31).

Global Reactivity Parameters

DFT measures are used in the calculations of global reactivity parameters such as electronegativity (χ), hardness (η) and softness (σ) [34]. Chemical potential (ρ) is known as first order derivative of the electron systems total electronic energy (E) with respect to total electrons (N) at an external potential due to nuclei V(r) [35] or can defined as the negative value of electronegativity as described by Iczkowski and Margrave [36] as shown in Eq. (32).

$$ -\uprho =\upchi =-\left(\frac{\mathrm{\partial E}}{\mathrm{\partial N}}\right)\mathrm{V}\left(\mathrm{r}\right) $$
(32)

Hardness (η) is defined as the second order derivative of E with N at V(r) that gives an idea about the molecules reactivity and stability by Eq. (33) [37].

$$ \upeta =-\left(\frac{\partial^2\mathrm{E}}{\partial {\mathrm{N}}^2}\right)\mathrm{V}\left(\mathrm{r}\right)=-\left(\frac{\mathrm{\partial E}}{\mathrm{\partial N}}\right)\mathrm{V}\left(\mathrm{r}\right) $$
(33)

The chemical hardness is calculated by a finite difference approximation to Eq. (33) which is given in Eq. (34). Chemical hardness and electronegativity of the molecule is also given by Koopmans’ theorem [37] and is calculated by the Eqs. (34) and (35).

$$ \upeta =\frac{\mathrm{IP}-\mathrm{EA}}{2}=\hbox{--} \left[{\mathrm{E}}_{\mathrm{LUMO}}\hbox{--} {\mathrm{E}}_{\mathrm{HOMO}}\right] $$
(34)
$$ \upchi =\frac{\mathrm{E}\mathrm{A}+\mathrm{IP}}{2}=\hbox{--} \left[{\mathrm{E}}_{\mathrm{LUMO}}+{\mathrm{E}}_{\mathrm{HOMO}}\right] $$
(35)

Where IP is the ionization potential (eV) and EA is the electron affinity (eV).

Another global chemical reactivity parameter electrophilicity (ω) was introduced by Parr et al. [38] which is described as Eq. (36) [39].

$$ \upomega =\frac{\uprho^2}{2\upeta} $$
(36)

Electrophilicity index was defined as a measure of electrophilic power for a considered molecule. The global softness is a reciprocal of hardness [40] as given in Eq. (37).

$$ \upsigma =\frac{1}{\upeta} $$
(37)

Total energy change of the molecule is defined as ΔET =  − η/4.

All the above global reactivity parameters were computed by the use of TD-DFT/PCM method.

A molecule or the atoms are said to be electron acceptor, when they possess positive electron affinity and this cause them to undergo charge transfer reactions and the vice versa, i.e., electron donating power measures the IP (energy required for removing an electron from HOMO) of the donor molecule. The difference between EA and IP accounts for the overall energy balance (DE) which accounts for the energy gained/lost in an electron donor/acceptor transfer. The energy balance can be given by the difference of EA and IP. Electronegativity is a chemical property that defines the ability of an atom or a functional group to attract the electron density towards itself. The concept of η tells that a larger EG accounts for a molecule to be hard and smaller EG for it to be a soft molecule. This accounts for the fact that the molecule with least EG is more reactive.

Molecular Electrostatic Potential (MESP)

MESP is used in studying the relation between the structure and activity by the means of electrostatic property [41].

MESP surface diagram can be used to understand the molecules reactive sites wherein the negative regions represent electrophilic sites and the positive regions represent nucleophilic centers.

The identification of key features necessary for the “recognition” of one molecule by another is done by the use of MESP. It is a useful tool in studying both electrophilic and nucleophilic processes [42,43,44,45]. At the point r, MESP V(r) is given by Eq. (38).

$$ \mathrm{V}\left(\mathrm{r}\right)={\mathrm{Z}}_{\mathrm{A}}/\mid {\mathrm{R}}_{\mathrm{A}}\hbox{--} \mathrm{r}\mid $$
(38)

Where, V(r) is the electron density at r, ZA is the charge on the nucleus located at RA. The first term is due to the nuclear charge and the second is accounted to the electronic distribution.

Synthesis of (E)-2-Fluoro-N′-(1-(4-Nitophenyl)Ethylidene)Benzohydrazide (FNEB)

An equal amount of 2-fluorobenzohydrazide (1 mmol, 0.154 g), 4-nitroacetophenone (1 mmol, 0.165 g) was taken in a round bottomed flask and 2–3 drops of glacial acetic acid was taken along with ethanol as the solvent. The solution was refluxed for 5 h at 60–70 °C as shown in Scheme 1. The completion of reaction was observed using TLC (with 70% ethyl acetate and 30% n-hexane as solvent system). The crude yellow solid product thus obtained was purified using filtration, washing with cold ethyl alcohol and finally recrystallized using ethyl alcohol.

Scheme 1
scheme 1

Synthetic scheme of FNEB

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Characterization of (E)-2-Fluoro-N′-(1-(4-Nitophenyl)Ethylidene)Benzohydrazide (FNEB)

Pale yellow crystalline solid; Yield: 79%; mp: 219-220 °C; IR (KBr,cm−1): 3081.16 (Ar C – Hstr), 1664.66 (C=O str), 1613.45 (C=N str), 1346.73 (C-N str), 1110.24 (N-N str), 1526.68 and 1346.73 (Ar-NO2); 1HNMR (d6-DMSO), δ, ppm):8.30–7.40 (m, 8H, Ar-H), 7.20 (s, 1H, N-H), 2.50 (s, 3H, CH3); 13CNMR (d6-DMSO), δ, ppm):162.2, 160, 151, 144, 127.6, 123.5, 112, 17.

Results and Discussion

Effects of Solvents on Excitation and Emission Spectra of FNEB

The excitation absorbance (Fig. 3) and emission fluorescence (Fig. 4) spectra were recorded in fifteen solvents ranging from non-polar to polar aprotic and then finally to polar protic solvents at room temperature. The concentration of FNEB was maintained at 10−5 M to record its electronic spectra. The \( {\overline{\upupsilon}}_{\mathrm{Abs}} \), \( {\overline{\upupsilon}}_{\mathrm{Abs}}-{\overline{\upupsilon}}_{\mathrm{Flu}} \) in cm−1, the difference of the two respective wave numbers gave the Stokes shift (Δυ) and their sum (Συ) along with the average of their sum (Συ/2) for the synthesized compound FNEB were tabulated in Table 1. The polarity functions [P1(ε,n), P2(ε,n) and P3(ε,n)] along with Reichardt’s parameter \( \left({\mathrm{E}}_{\mathrm{T}}^{\mathrm{N}}\right) \) for the fifteen solvents are given in Table 2.

Fig. 3
figure 3

Excitation spectrum of FNEB in different organic solvents

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Fig. 4
figure 4

Emission spectrum of FNEB in different organic solvents

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Table 1 Solvatochromic studies of FNEB in different solvents
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Table 2 Solvent polarity functions along with the Reichardt’s parameter for solvents
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It is clear from Table 1 that on increase of the solvent polarity, a bathocromic shift takes place with maximum absorption wavelength ranging from 3 nm to 51.5 nm relative to n-hexane due to π-π* transition which occurs because of charge transfer of conjugated entity of FNEB. Bathocromic shift ranging from 8 nm to 165 nm relative to n-hexane were observed in maximum fluorescence wavelength, which is attributed to high influence of solvent polarity in the excited state of FNEB compared to its ground state. The Stoke’s shift was observed to increase from 12,037.04 cm−1 to 13,434.56 cm−1 with increased solvent polarity from n-hexane and toluene respectively. The above data conclude that FNEB is influenced by solvent polarity functions [P1(ε,n), P2(ε,n), P3(ε,n),Reichardt’s parameter \( \left({\mathrm{E}}_{\mathrm{T}}^{\mathrm{N}}\right) \)], hydrogen bond acceptor (β) ability, hydrogen bond donor (α) and the solvents dipolarity/polarizability (π*). The excited state of FNEB is stabilized more in polar solvents leading to a higher value of excited state dipole moment compared to that of the ground state dipole moment. Thus, the absorption spectra are least susceptible to the solvent polarity as compared to the emission spectra of FNEB. This was proposed after observing weak bathochromic shift in the absorption spectra.

Experimental and Theoretical Calculations of Dipole Moments for FNEB

The experimental dipole moment for the ground state was determined by Guggenheim Debye method using Eq. (1) and was found out to be 2.58 D. Density functional Theory (DFT) based on B3LYP/6-31G(d) were used to estimate ab-initio dipole moments of both ground state and the excited state which was found out to be μgr = 2.10 D and μex = 7.33 D. The spectral parameters were correlated in the solvatochromic studies of FNEB with the use of different solvent polarity approaches. The linear graphs for Bilot-Kawski, Lippert-Mataga, Bakhshiev, Kawski-Chamma-Viallet and Reichardt methods were plotted as shown in Fig. 5, where (\( {\overline{\upupsilon}}_{\mathrm{Abs}}-{\overline{\upupsilon}}_{\mathrm{Flu}} \)) and (\( {\overline{\upupsilon}}_{\mathrm{Abs}}+{\overline{\upupsilon}}_{\mathrm{Flu}} \)) were plotted against f(ε,n) and φ(ε,n) respectively, Stoke’s shift, its mean {\( {\overline{\upupsilon}}_{\mathrm{Abs}}-{\overline{\upupsilon}}_{\mathrm{Flu}} \)} and {(\( {\overline{\upupsilon}}_{\mathrm{Abs}}+{\overline{\upupsilon}}_{\mathrm{Flu}} \))/2} for FNEB (Table 1) were plotted against solvent polarity functions (Table 2).

Fig. 5
figure 5

Linear plots of solvent polarity functions f(ε,n), φ(ε,n), P1 (ε,n), P2(ε,n), P3(ε,n) and \( {\mathrm{E}}_{\mathrm{T}}^{\mathrm{N}} \) against (\( {\overline{\upupsilon}}_{\mathrm{Abs}}-{\overline{\upupsilon}}_{\mathrm{Flu}} \)), (\( {\overline{\upupsilon}}_{\mathrm{Abs}}+{\overline{\upupsilon}}_{\mathrm{Flu}} \),(\( {\overline{\upupsilon}}_{\mathrm{Abs}}+{\overline{\upupsilon}}_{\mathrm{Flu}} \))/2

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Table 3 summarizes the obtained linear equation from all the correlation approaches and from the slopes and intercepts obtained from Fig. 5, the dipole moments of FNEB in both ground and excited states were calculated. The Correlation coefficient values which are high (generally greater than 0.90) suggests a good linear relationship for the above correlation methods. The dipole moment in ground state and excited state were calculated using quantum-mechanical treatment of Bilot-Kawski using Eqs. (9) and (10). Incorporating Eq. (19) to (21) and (24), the other correlation methods were used to calculate excited state from respective slopes m1, m2, m3, m4, as given in Table 4. Onsager cavity radius (a) was determined using Eq. (8). By the use of Eq. (28), quantum yield (φ) of FNEB was found out to be 0.23.

Table 3 Linear plots for various correlation methods for FNEB
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Table 4 Dipole moments (μgr and μex) for FNEB
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From Table 4, it was found out that μex of FNEB is greater than μgr. The calculated values for the excited state dipole moment (μex) using Bilot-Kawski(2), Baksheiv and Reichardt approaches are unanimous. The higher values for other approaches in the determination of μex can be attributed for the polarizability effect due to the solute. The phenomenon of charge transfer (CT) can account for the difference in dipole moments and nature of the excited state. Using Eqs. (9) and (10), ground state dipole moment (μgr) and excited state dipole moment (μex) of FNEB were calculated (Bilot-Kawski approach) considering both the dipole moments to be parallel [16]. The value of μgr obtained from Gaussian software as well as Bilot-Kawski(1) are unanimous. The dipole moments for both the states as given by Bilot-Kawski are μgr = 1.30 D and μex = 1.56 D suggesting that the excited state of FNEB has charge separation and thus becomes more polar to the ground state which can be attributed to stronger solute-solvent interaction in case of electronically excited state. Using Eq. (11), μex/μgr (ratio of dipole moments) was found to be 0.83.

Multiple Linear Regression (MLR) Analysis

The solute-solvent interaction and the electronic transition mechanism are studied using multiple linear regression analysis (MLR). It is known that these interactions interfere with the shape, intensity and position of the absorption spectra. The influences of solvents in the absorption and fluorescence spectra were determined by the use of both Kamlet-Abboud-Taft and Catalan parameters. These parameters were correlated to determine solvatochromic properties of FNEB.

Kamlet-Taft parameters α, β and π*are determined using Eq. (25). Catalan parameters include polarizability (SP) term and dipolarity term (SdP) along with solvent acidity (SA) and solvent basicity (SB) as shown in Eq. (27). The values of these parameters are given in Table 5.

Table 5 Kamlet-Abboud-Taft and Catalan parameters values for MLR analysis
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The values of \( {\overline{\upupsilon}}_{\mathrm{Abs}} \), \( {\overline{\upupsilon}}_{\mathrm{Flu}} \), Δυ were correlated with the parameters using MLR. The electronic transition mechanisms between both the electronic states were analyzed, which is on the basis of solute-solvent interactions.

The regression analysis with correlation coefficient for FNEB using Kamlet-Abboud-Taft parameters is given in Eq. (39).

$$ {\displaystyle \begin{array}{l}{\overline{\upupsilon}}_{\mathrm{A}}=36766.76-466.47\upalpha \hbox{--} 1802.85\ \upbeta \hbox{--} 4240.38\ {\uppi}^{\ast };{\mathrm{R}}^2=0.96\\ {}{\overline{\upupsilon}}_{\mathrm{F}}=24649.98\hbox{--} 674.91\upalpha \hbox{--} 2059.02\ \upbeta \hbox{--} 4900.73\ {\uppi}^{\ast };{\mathrm{R}}^2=0.95\\ {}\varDelta \overline{\upupsilon}=12116.69+208.61\upalpha \hbox{--} 255.51\ \upbeta +660.96\ {\uppi}^{\ast };{\mathrm{R}}^2=0.88\end{array}}\Big\} $$
(39)

The regression analysis with correlation coefficients for FNEB using Catalan parameters is given in Eq. (40).

$$ {\displaystyle \begin{array}{l}{\overline{\upupsilon}}_{\mathrm{A}}=35814.25+281.78\mathrm{SA}\hbox{--} 313.00S\mathrm{B}+2072.28\mathrm{SP}-5584.12\mathrm{SdP};{\mathrm{R}}^2=0.90\\ {}{\overline{\upupsilon}}_{\mathrm{F}}=24359-295.16\mathrm{SA}+9.64\mathrm{SB}+609.47\mathrm{SP}\hbox{--} 6174.23\mathrm{SdP};{\mathrm{R}}^2=0.87\\ {}\varDelta \overline{\upupsilon}=12681.91+101.39\ \mathrm{SA}\hbox{--} 117.62\ \mathrm{SB}-1026.84\ \mathrm{SP}+930.15\ \mathrm{SdP};{\mathrm{R}}^2=0.82\end{array}}\Big\} $$
(40)

From Eq. (39), it can be observed that a slightly higher π* coefficient is obtained than the other coefficients of α and β. Therefore, using MLR analysis, it can be observed that the solvent dipolarity/polarizability (π*) has a vital influence on the interactions between the solute and the solvent. This implies that π* controls the excitation as well as the emission spectra as π* is more powerful than β and α. On the other hand, the relatively higher values of β cannot be neglected as the value is greater than α. The fairly higher values for π* and β indicate that both the absorption and fluorescence spectra show bathochromic shift. The comparatively greater π* and β in the emission spectral wavenumber \( {\overline{\upupsilon}}_{\mathrm{Flu}} \) than the excitation spectral wavenumber \( {\overline{\upupsilon}}_{\mathrm{Abs}} \) of FNEB indicates a more polarized state in excited state than the ground state. This leads to a higher μex value. MLR using Catalan parameter, dipolarity term (SdP) from Eq. (40) indicates a red shift. In comparison to the excitation dipolarizability, the emission dipolarizability influences the interactions between the solute and solvent suggesting a higher value of dipole moment of FNEB in its excited state. Using MLR analysis, it can be shown that \( {\overline{\upupsilon}}_{\mathrm{Abs}} \) and \( {\overline{\upupsilon}}_{\mathrm{Flu}} \) have relatively good correlation coefficients. The above results conclude solvents play a major role in the intramolecular charge transfer which is due to the polarizability/dipolarizability parameter of a particular solvent.

Determination of Frontier orbital’s Energies

The frontier orbitals namely Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) will give insights into the direction of charge transfer (CT). The energies of frontier orbitals were determined in ethanol. The energies give necessary details concerning CT and explains electron donor/acceptor ability where HOMO is responsible for the electron donor ability whereas LUMO for the electron acceptor ability. The difference in the energies (HOMO-LUMO energy gap) of both gives detail about the properties of the molecule’s chemical stability.

Determination of Frontier orbital’s Energies Using Computational Method

The red and green colored lobes correspond to positive phase and negative phase. It was observed that HOMOs for both ground state and excited state of FNEB are localized on azomethine linkage and carbonyl group of ortho-fluorobenzoyl moiety whereas LUMOs for both the states of FNEB are localized over the entire molecule except for 2-fluorobenzene nucleus (Fig. 6). Using the values of HOMO and LUMO energies for both ground and excited states, the global reactivity parameters were found out using Eqs. (32), (34), (35), (36) and (37) and have been tabulated in Table 6.

Fig. 6
figure 6

(a) The ground state HOMO (b) ground state LUMO (c) excited state HOMO and (d) excited state LUMO, for FNEB were computed by TD-DFT studies

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Table 6 Global reactivity parameters of FNEB
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The suitable position for the electrophilic and nucleophilic attack can be determined from the MESP plot (Fig. 7).

Fig. 7
figure 7

Molecular electrostatic potential (MESP) plot for FNEB (a) Ground state and (b) Excited state

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MESP can also be used to study the influence of solvents by means of hydrogen bonding. The scale for MESP symbolizes different potentials on the surface of FNEB molecule, which is electrostatic in nature. The red color represents negative phase (electrophilic site) whereas blue color represents positive phase (nucleophilic site). The carbonyl group and nitro group are susceptible to electrophilic attack as they are in the red/negative phase. The blue/positive phases are enclosed by the methyl group, benzene ring containing fluoro, imine group and all the hydrogens.

Determination of Frontier Orbital Energies Using Cyclic Voltammetry

HOMO and LUMO energies were also determined using cyclic voltammetry using the Eqs. (29) and (30) and the energy gap (EG) was calculated. The cyclic voltammograms of FNEB and the internal standard ferrocene were recorded in acetonitrile where lithium perchlorate was used as the supporting electrolyte and shown in Fig. 8. HOMO-LUMO energies were determined using the reduction and oxidation potential values.

Fig. 8
figure 8

Cyclic voltammograms of (a) Internal standard ferrocene and (b) FNEB

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HOMO (EHOMO) and LUMO (ELUMO) energies determined in acetonitrile were found to be 0.18 and 0.17 a.u. respectively. Global reactivity parameters were calculated using the HOMO and LUMO energies from the redox potential values determined using cyclic voltammetry (CV) and given in Table 6.

The use of Gaussian 09 gave insights into the charge distribution in FNEB from atomic charges of each atom in the optimized geometry (Fig. 2). The study was done in ethyl alcohol. In case of ground state, O atoms (8, 13, 14), all nitrogen apart from (N-7) and C (2,3,4,5,6,15,18,19,20,21) atoms have a significant negative charge and act as donor atoms. The C atoms (1, 9, 12, 17) and N (7) atoms carry significant positive charge, thereby acting as acceptor atoms. The charge distribution direction in solvent medium directs from C (12) to N (11) atom having magnitude +0.487 to −0.540 eV respectively in case of ground state whereas the direction of the charge distribution in case of the excited state is from C (9) to N (11) atom having magnitude +0.236 to −0.541 eV respectively. FNEB is more polar giving rise to difference in dipole moments of both the states as there is charge separation. The positive charge in all the hydrogen atoms indicate electron donating character of hydrogen compared to rest of the atoms. The above studies confirm the conclusions obtained from MLR analysis.

Light Harvesting Studies

Light Harvesting Efficiency (LHE) of a molecule is a very important physical property, which indicates the use of compounds in conventional Dye Sensitized Solar Cells (DSSC’s) for converting light energy to electrical energy. The LHE is mathematically related to the oscillating strength through the equation LHE = 1–10-A, where A is the oscillating strength corresponding to the UV excitation valuates obtained from the TD-DFT calculations. The results of TD-DFT calculations using B3LYP/6–31++G(d) in ethanol as solvent using i.e. fPCM model is provided in Table 7. There are three major transitions corresponding to wavelength 707.54 and 425.43 nm with oscillator strength 0.2195 and 0.1851 respectively. The calculated LHE is 0.52 and 0.34, which suggests that the in the first transition, 52.08% of energy is can be transferred to the conducting material in DSSC by photosensitization, while in second, only 34.70% of light energy can only be transferred.

Table 7 Light Harvesting studies of FNEB
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Conclusion

The influence in excitation and emission spectra by solvents of different polarities are computed using solvent polarity correlation methods. The optimized geometries of FNEB in ground and excited states were achieved using TD-DFT. The dipole moments in excited state was found to be higher than that in ground state based on quantum-mechanical treatment of Bilot-Kawski approach. Density functional Theory (DFT) based ab-initio estimation of dipole moments also indicated the same. All the correlation methods indicated that the excited state of FNEB has charge separation and thus more polar when compared to the ground state accounting for stronger solute-solvent interaction in case of electronically excited state. Multiple Linear Regression analysis using both Kamlet-Abboud-Taft and Catalan parameters suggested that the polarity term (π*) of solvents is responsible in controlling the excitation and emission spectra. The frontier orbitals energies were calculated computationally using TD-DFT calculations and cyclic voltammetry. The Light harvesting efficiency which was determined to gain insights into the photo-voltaic property of the molecule confirms the energy transfer of about 52.08% to conducting material in DSSC by photosensitization in the first transition whereas it decreased in the second transition.

The energies of the frontier orbitals were used to calculate the global reactivity parameters which give information about the surface of FNEB. MESP plots explain that the molecule has nucleophilic/electrophilic sites. Based on the results obtained, FNEB can be considered as a potentially suitable candidate for designing materials for luminescence and fluorescent probe applications.