The solvatochromism and electronic structure of

: The (E)-2-(2-hydroxystyryl)quinolin-8-ol (abbreviated as HSQ) molecule was synthesized and characterized. The ESIPT, solvatochromism properties, electronic structure, and ground and excited electric dipole moments of this molecule were measured using absorption and fluorescence spectra recorded in 13 different solvents. Its electronic structure via electronic transitions was investigated to find the quantitative values of solvatochromism properties by LSER calculations. The ESIPT mechanism was clarified; ground and excited dipole moments were determined using solvatochromic shift methods. The DFT (B3LYP)/6-311 ++ G(d,p) method and basis set with potential energy surface (PES) calculations of proton transfer were used to explain the ESIPT mechanism. NBO analysis, NLO properties, and behavior under an electric field were also determined.


Introduction
Quinoline derivatives have been used in many different applications such as optoelectronic materials and optical switches in nonlinear optics due to their unique electronic structure [1,2]. In addition, these compounds have some biological activities such as anticancer potential [1,3] and are used in the treatment of malaria, topical pathogenic parasitic Leishmania infection, antihepatitis B, etc. They have usage in textiles as well [4][5][6].
In particular, 8-hydroxyquinoline derivatives have attracted considerable interest due to their intramolecular proton transfer process properties. The physical and chemical effects of this challenging case in electronic structure are still being researched. Moreover, 8-hydroxyquinoline and its derivatives play an important role as chelating ligands for the preparation of various photo-and electroluminescent metal complexes [7,8] and fluorescent chemosensors for metal cations [9][10][11][12][13][14][15][16]. In addition, they exhibit strong pharmacological activities [17]. Naik et al., Filip et al., and Mehata et al. have investigated the electronic structure of 8-hydroxy quinoline and some of its derivatives [18][19][20].
Introduction of substituents, e.g., styryl groups, into the hydroxyquinoline ligand system enhances the thermal stability of metal complexes as compared to unsubstituted quinoline complexes and improves their solubility in organic solvents [21,22]. 2-Styrylquinoline derivatives are used as precursors for the synthesis of compounds exhibiting various biological activities, including antitumor [23], antifungal [24], antiinflammatory, and antiallergic [25]. In addition, they can be used as models for the design of molecular logic devices and supramolecular systems [26,27]. The traditional procedure for the synthesis of 2-styrylquinolines is based on condensation of quinaldine with aromatic aldehydes in acetic anhydride [28,29]. However, this method involves prolonged heating of the reaction mixture, while the yields of target compounds are reduced as a result of formation of products. In recent years, Ca(OTf) 2 catalyzed sp 3 C-H functionalization of azaarenes has been reported [30].
In the present study, we synthesized (E)-2-(2-hydroxystyryl)quinolin-8-ol (HSQ). Then we investigated its electronic structure, solvatochromism, electric dipole moment in the ground and excited state, and behavior under an electric field (EF). Knowledge of the interaction between solvent and solute and between solute and solute during electronic transitions and electronic structures of molecules provide important information on how electrons behave actively in photophysical and photochemical processes. Additionally, solvatochromism gives information involving the organic electronic material properties of molecules. It is worth noting that wavelength shifts (bathochromic and hypsochromic effect) occurring dependent on the solvent medium of electronic transitions can be explained by fitting to a quantitative model (polarity, dipolarity function, hydrogen bonding, or other physical parameters). The linear solvation energy relationship (LSER) was applied to analyze the solvent-solute interactions of the molecule. In order to achieve that, multilinear regression analysis (MLRA) was performed using Kamlet-Taft and Catalan parameters, the Marcus optical dielectric function, and Reichardt-Dimroth solvent parameters [31,32]. Excited state and ground state dipole moments of the investigated compounds were calculated using Bakhshiev, Lippert-Mataga, Bilot-Kawski, and Reichardt solvatochromic shift methods [32,33].

General
All experiments were carried out in predried glassware in an inert atmosphere of argon. 1 H NMR and 13 C NMR spectra were recorded in CDCl 3 on an Agilent NMR spectrometer (400 MHz). 1 H (400 MHz) and 13 C NMR (100 MHz) were recorded in CDCl 3 and the chemical shifts are expressed in ppm relative to CDCl 3 (δ 7.26 and 77.0 for 1 H and 13 C NMR, respectively) as the internal standard. Flash column chromatography was performed using thick-walled glass columns and silica gel (60-mesh; Merck). The reactions were monitored by thin-layer chromatography (TLC) using Merck 0.2-mm silica gel 60 F254 analytical aluminum plates, visualized by UV light. All extracts were dried over anhydrous magnesium sulfate and solutions were concentrated under reduced pressure using a rotary evaporator.
Both absorbance and fluorescence spectra were recorded in cyclohexane, benzene, toluene, o-xylene, diethyl ether, chloroform, 1-butanol, acetone, ethanol, methanol, acetonitrile, ethylene glycol, and water. In these measurements, ultraviolet-visible (UV-Vis) absorption spectra and steady-state fluorescence spectra were recorded by PerkinElmer Lambda-35 UV-Vis spectrophotometer and PerkinElmer LS-55 Model fluorescence spectrophotometer. All the measurements were obtained at room temperature. The solutions were prepared as about 1.12 µ M. Spectroscopic grade solvents and organic compounds were purchased from Sigma-Aldrich.

Quantitative solvent-solute interactions
LSER calculations of the title molecule were performed using Kamlet-Taft and Catalan parameters. These equations are shown below: In these equations, C 0 and C 5 are the statistical quantities corresponding to the values of these properties in the gas phase or inert solvents [47]. C 1 -C 4 are coefficients derived from Kamlet-Taft solvatochromism, which provides information on solvent-solute interactions during electronic transitions. C 1 (polarizability or dispersion/polarization) and C 2 (polarity or orientation/induction) show specific interactions, while C 3 (hydrogen bonding acceptor) and C 4 (hydrogen bonding donor) show nonspecific interactions [48][49][50].
In Catalan solvatochromism, C 6 (polarizability of solvent) and C 7 (dipolarity of solvent) solvatochromic coefficients indicate the global interactions, while C 8 (solvent of acidity) and C 9 (solvent of basicity) describe the nonglobal interactions, which occur during electronic transitions between solvent and solute [51].

Quantum chemical calculations
The three-dimensional ground state (S 0 ) geometries of all compounds were optimized in the gas phase without any symmetry restrictions using DFT [52] implemented hybrid functional B3LYP with the Gaussian 09W [53] software package. B3LYP is composed of Becke's three parameter exchange functional (B3) [54] and the nonlocal correlation functional by Lee, Yang, and Parr (LYP) [55]. In order to find the molecular structures with minimum energy, conformational analysis was performed with the B3LYP/6-311++G(d,p) method and basis set. Afterwards, H-bond transfer of O28-H21 with N10-H21 was performed by applying the TD-B3LYP/6-311 ++G(d,p) method and basis set. The vibrational analyses were carried out using the same basis set employed in the corresponding geometry optimizations. The frequency analysis did not yield any imaginary frequencies, indicating that the optimized structure of the molecule corresponds to at least a local minimum on the potential energy surface. The normal mode analysis was performed for 3N -6 vibrational degrees of freedom, with N being the number of atoms in the molecule.

Results and discussion
Through the cleavage of two C(sp 3 ) -H bonds of alkylazaarenes, synthesis of alkenyl product by green Lewis acid catalyst was considered. The HSQ was synthesized by the reaction of 2-methyl quinoline-8-ol and salicylaldehyde by the catalysis of Ca(OTf) 2 (Scheme).   Table 1 and Figure 2 show the absorbance and fluorescence spectra data of the HSQ molecule in various solvent media. We observed that the absorption spectrum of this molecule has four absorption bands in 13 different solvents. These electronic bands are observed at 248-256 nm (5.00-4.84 eV), 260-285 nm (4.77-4.35 eV), 293-311 nm (4.23-3.99 eV), and 344-361 nm (3.60-3.43 eV), respectively. The first electronic absorbance transition peak observed in the absorbance spectrum can be referred to as π -π * electronic transition, which was observed in cyclohexane, diethyl ether, methanol, acetonitrile, ethylene glycol, and water. Except for cyclohexane and diethyl ether, all of these solvents are polar protic, which may have an effect on electronic transitions. However, in cyclohexane, only intramolecular electronic transitions can take place. The second electronic band can be attributed to ππ * electronic absorption in the quinoline ring, while the first electronic band is attributed to ππ * electronic absorption transition in the phenol ring. The third electronic band occurred by ππ * electronic absorption transition due to conjugation between the quinoline and phenol rings.

Electronic transitions
The last band refers to nπ * electronic absorption transition, which is expected to be born out from the delocalization of n electrons as a result of the interaction of HSQ with solvents. The assignment of the absorption bands is made according to the transition energy (in eV) and the electron conjugation centers of the molecule. Naik and Math reported that the 8-hydroxyquinoline molecule has an excited state intramolecular proton transfer (ESIPT) mechanism [18]. Filip et al. investigated the electronic structure of the 8-hydroxyquinoline molecule in some solvents and thus an ESIPT structure has been explained for this molecule [19]. Electronic structure and dipole moments estimated by using polar and aprotic solvents the 2-,6-,7-, 8-hydroxyquinolines were researched by Mehata et al. [20]. The observed electronic transition wavelength values for both absorption and emission spectra in these works were smaller than those in the present work, because the phenoxy substituent gives rise to longer wavelengths. Greater wavelength values brought about a fine structure in the spectra of HSQ.  In the first three absorbance bands, there occurred an irregular hypsochromic shift in which the bands shifted towards smaller wavelengths with increasing polarity. During these electronic transitions, the molecule decreased the difference between ground and excited state energies dependent on the polarity of the solvent.
As the polarity increases, the wavelength of the last absorbance band shifts towards a higher value, which indicates a bathochromic shift. This electronic transition might be a result of excited state intramolecular proton transfer. The energy of this transition is increased as the polarity increases. As can be seen from Table   1, Stokes shifts vary in the range of 78-101 nm (5206-6597 cm −1 ) . This means that the studied molecule proves excited state intramolecular proton transfer. The fine structure observed in the fluorescence spectra supports an ESIPT mechanism too. The values of fluorescence bands are shown in Table 1. Three fluorescence bands were observed for the present molecule. The wavelength of the excitation was 360 nm. The first fluorescence band was at 408-418 nm (3.04-2.97 eV). The second fluorescence band was observed in all solvents at 428-454 nm  Stokes shifts between the electronic absorbance and fluorescence bands studied have a bathochromic effect, indicating solvatochromism. Figure 3 shows the open and closed forms of the present structure ( Figure   3a), geometry optimized structures of open and closed forms (Figure 3b), and excited state intramolecular proton transfer mechanism of the molecule (Figure 3c) studied by B3LYP/6-311++G (d,p) level of theory. As seen from Figure 3a, the distance between 21 H with 10 N was 2.1048 Å, which means that this proton can form hydrogen bonding with the nitrogen atom in the excited state.

Solvatochromism
Solvatochromism is the reversible change in the electronic spectroscopic properties (absorption, emission) of a chemical structure caused by solvents. In other words, the term solvatochromism is used to describe significant changes in the position (and sometimes intensity) of the UV-Vis absorption band accompanying the change in the polarity of the medium. Molecules of electronic transitions depending on the environment can be used in organic electronics [46,47].
The linear solvation energy relationship allows us to statistically identify the effects that contribute to solvatochromism, quantitatively and qualitatively defining the interactions between solvent and solute. The studied molecule was subjected to LSER for both absorbance and fluorescence using dielectric function and refractive index Kamlet-Taft parameters. In addition, LSER was applied using solvent polarizability, solvent dipolarity, solvent of acidity, and solvent of basicity done with Catalan parameters. The resulting statistical parameters for correcting the multiple linear regression equations and the proposed models are given below. The values of parameters used in the solvatochromism and electric dipole moment calculations are tabulated in Table 2.

Electric dipole moments
The ground and excited state dipole moments were estimated using solvatochromic shift methods. The ground state dipole moment was calculated using the Bilot-Kawski method, while the excited state dipole moment of HSQ was calculated using the Bilot-Kawski, Lippert-Mataga, Bakshiev, modified Bilot-Kawski, and Reichardt methods. Figure 4 depicts the correlation plots derived from the Bilot-Kawski, Lippert-Mataga, Bakshiev, modified Bilot-Kawski, and Reichardt methods. These calculations were used to obtain Stokes shifts and the sum of wavenumbers of the fluorescence and absorbance spectra. In addition, these calculations were used to find dielectric and refraction index functions and Onsager cavity radius. Moreover, the statistical parameters found in the experimental dipole moment calculations are listed in  [20]. The ground state dipole moment of HSQ with hydroxystyryl substituent decreases, while its excited state dipole moment increases. This case is attributed to the breaking of conjugation between the quinoline ring and hydroxystyryl substituent. A decrease in the excited state charge distribution compared to the ground state of charge distribution was observed. There is a considerable difference between the ground and the excited state dipole moments. Thus, in the excited state, large changes in electronic construction occurred.

ESIPT reaction
Proton   group toward the N atom is depicted in Figure 3c. In addition, we saw the proton transfer in the PES video.
In the ground state, O20-H21 length is 0.97438 Å, while H21…N10 bond length is 2.09721 Å. In the excited state, O20-H21 bond length in the ESIPT mechanism is calculated to be 2.35779 Å, whereas H21…N10 bond length is found to be 0.99721 Å. The energy difference between these two states in ground state is 0.6684 eV, while in excited state it is 0.5910 eV. It can be concluded from Figure 5 that the third absorbance, the second photoluminescence, and the fourth absorbance of the molecule occur in the closed form of the molecule, while the second fluorescence occurs by the electronic transition of the ESIPT mechanism.
In cyclohexane, we have two electronic absorption transition and two fluorescence bonds. The ESIPT mechanism was observed at the fourth electronic absorbance transition. According to Figure 5, the O-H bond is broken in the course of the second electronic transition. Therefore, H migrates from H-O to the N 10 atom. Figure 3c shows the ESIPT mechanism of HSQ [56].

Treatment under an Electric Field
HOMO and LUMO energies and the HOMO-LUMO gap ( ∆ε) of HSQ calculated by DFT-B3LYP/6-311 ++G(d,p) level of theory were -5.89 eV, -2.40 eV, and 3.49 eV, respectively. A large ∆ε value in the ground state indicates that HSQ is an insulator. The ground state dipole moment of the present compound was calculated as 3.3684 Debye by the same method. In order to tune the ∆ε and dipole moment, an external EF was applied in 3D (xyz-direction). The plots of ∆ε variation and dipole moment change versus applied external EF are depicted in Figures 6 and 7, respectively. As seen from Figure 6, ∆ε stays almost stable up to 0.04 V/Å EF. From 0.05 V/Å to 0.08 V/Å, ∆ε decreases drastically from 3.0330 eV to 0.8702 eV. At the higher applied EF, the decrease in ∆ε continues and the smallest ∆ε value is obtained at 0.17 V/Å as 0.3951 eV. It is important to note that at 0.07 V/Å EF and beyond HSQ gains an organic semiconductor character. LUMO energy starts to decrease under 0.05 V/Å EF, whereas the response of HOMO energies versus EF starts at 0.08 V/Å. The external EF was more effective on LUMO energy than on HOMO energy.
Dipole moment of the title compound was calculated to be 3.3684 Debye in the absence of EF by DFT-B3LYP/6-311++G(d,p) level of theory. In the range of 0.00-0.06 V/Å EF, dipole moment was 3.368-3.6786

Nonlinear optical (NLO) properties
NLO effects emerge from the interactions of electromagnetic fields in various media to generate new fields changed in phase, frequency, amplitude, or other diffusion characteristics from the incident fields [57]. NLO materials have been widely investigated by researchers recently due to their importance in applicability as the key functions of frequency shifting, optical logic, optical switching, optical modulation, and optical memory for the upcoming technologies in areas of telecommunications, signaling and optical interconnections [58][59][60][61].
A Taylor series expansion of the total dipole moment, µ tot (Eq. (7)), induced by the field represents the NLO response of an isolated molecule in an EF E i (ω) : where α , µ 0 , and β ijk are linear polarizability, the permanent dipole moment, and the first hyperpolarizability tensor, respectively. The isotropic (or average) linear polarizability can be calculated by Eq. (9) [62]: First hyperpolarizability is a third degree tensor that is represented by a 3 × 3 × 3 matrix. The 27 elements of the 3D matrix can be reduced to 10 components due to the Kleinman symmetry [62] such that β xyy = β yxy = β yyx = β yyz = β yzy = β zyy ; ...). The output file of a computation in Gaussian 09 provides the 10 values of this matrix as β xxx , β xxy , β xyy , β yyy , β xxz , β xyz , β yyz , β xzz , β yzz , and β zzz , respectively. The components of the first hyperpolarizability can be calculated using the following equation (Eq. (9)) [63]: The magnitude of β tot from Gaussian program output can be calculated by Eq. (10).
The calculations of the total molecular dipole moment ( µ tot ), linear polarizability ( α tot ) , and hyperpolarizability ( β tot ) from the Gaussian output were explained in a previous work [64], and DFT has been widely used as an effective method to compute the properties of NLO materials [65]. The electronic dipole moment µ tot , polarizability α tot , and the hyperpolarizability β tot data of all compounds were calculated at the B3LYP/6-311++G(d,p) level of theory using the Gaussian 09 package; the results are given in Table 5. The dipole moment of HSQ was calculated to be 3.37 Debye. Two hydroxyl groups attached to parent quinoline result in an increase in charge separation; thus, the observation of high dipole moment is not surprising.
The 3D-MEP surface counter map was obtained for B3LYP/6-311++G(d,p) optimized geometries to predict reactive sites for electrophilic and nucleophilic processes for the compounds and dipole moment observation. The electrostatic potential surface of HSQ is shown in Figure 8. Electrophilic reactivity regions (negative charge) are shown in red and yellow, while blue indicates nucleophilic reactivity [66]. For the title compound, the negative charge is mostly localized on the electron withdrawing hydroxy groups, as expected. The charge separation is very well observed for HSQ, which may be the reason for the high magnitude dipole moment. The average polarizability ( α tot ) data together with its components are listed in Table 5. The value of the calculated polarizability is equal to 38.0 Å 3 for HSQ. The hyperpolarizability value β tot (19.2 × 10 −30 cm 5 /esu) for the compound was much greater than that of urea (0.77 × 10 −30 cm 5 /esu) [67], which is one of the typical compounds used in research into the NLO properties of molecular systems. Therefore, it was used frequently as a threshold value for comparative studies [67]. The obtained results show that the title compound is a good candidate for NLO materials. The hyperpolarizability value for hydroxyquinoline itself was computed to be 3.5; thus, an increase in the conjugation path upon substitution of the hydroxystyryl link resulted in an increase in first hyperpolarizability. Comparison of the interfrontier energy gap data with the hyperpolarizability values is a good indicator of nonlinear properties. ∆ E for urea was calculated to be 8.2 eV at the same level of DFT method, whereas the ∆ E value for the present system was 3.49 eV.

Conclusions
In the present study, the electronic structure, ESIPT mechanism, experimental dipole moment, and solvatochromism properties of HSQ were investigated by both spectroscopic and computational methods. HSQ was found to be a solvatochromic material. Global electronic absorption and emission transitions indicated a bathochromic effect. Charge distribution in ground and excited states was almost the same, due to µ e being only 0.0144 times bigger than µ g . The equation µ e = (m B−K(2) + m B−K(1) ) /(m B−K(2) -m B−K (1) ) was used to obtain the charge distribution data [35][36][37][38][39][40]. It was observed that application of an external EF gave rise to a lower value of ∆ε and thus gave an organic semiconductor character of HSQ. The lowest HOMO-LUMO band gap was obtained at 0.17 V/Å as 0.3951 eV.