Solvent Effects and Metal Ion Recognition in Several Azulenyl-Vinyl-Oxazolones

Abstract

The spectral properties of several azulene-oxazolone derivatives containing a phenyloxazolone moiety linked to a substituted azulene ring via a C=C double bond were studied in different solvents of varying polarity. The solvatochromism and the ability of azulene-oxazolone derivatives to recognize heavy metal ions were investigated. In order to estimate the contribution of the non-specific and specific solute–solvent interactions, multiple linear regression analysis using Kamlet–Taft, Catalan and Laurence parameters was applied. These azulene derivatives demonstrate positive solvatochromism. The methyl and isopropyl substituents at the seven-membered azulene ring determine the highest red shifts of the absorption maxima of these azulenyl-vinyl-oxazolones. According to Catalan and Laurence models, the solvent polarizability is a more significant parameter in describing the solvatochromic properties of the azulene-oxazolone derivatives. The azulene-oxazolone compounds under study showed a good response to heavy metal cations (Cd²⁺, Hg²⁺, Cu²⁺ and Pb²⁺).

1. Introduction

Due to their unusual physicochemical properties (photophysical, electrochemical and photoelectrochemical properties), azulene derivatives present a broad range of applications in the research field of functional materials, including optical materials, electrochromic materials, conducting polymers, molecular switches, organic solar cells, molecular devices, and so on [1,2,3,4,5]. Furthermore, certain azulene derivatives exhibit antimicrobial, antifungal, anti-inflammatory and anticancer activity, and are utilized in photodynamic therapy [2,6,7].

The structure of azulene, a bicyclic nonalternant aromatic hydrocarbon (Figure 1a), consists of an electron-deficient seven-membered ring (tropylium cation) fused with an electron rich five-membered ring (cyclopentadienyl anion). Its polarity is reflected by the presence of a relatively large dipole moment (1.08 D) (Figure 1b). Even though both skeleton isomers, azulene and naphthalene, have the same composition (C₈H₁₀) and degree of unsaturation (five double bonds and two fused rings), azulene has distinct absorption bands and electronic transitions compared to its naphthalene isomer (Figure 2a). By varying the nature of the electronic substituents grafted on the azulene moiety in the positions where the electron density is the highest, the wavelength of the light absorbed can be finely tuned [8] and significant changes of the optical and photophysical properties have been observed [9,10,11].

Azulene exhibits a weak absorption band in the visible range which causes the blue color of this compound [9]. Due to a small energy gap between the ground state (S₀) and the first excited state (S₁), this weak absorption corresponds to a transition from the ground state (S₀) to the first excited state (S₁) (S₀ → S₁ transition), making azulene become the smallest isolable organic compound [12,13].

The literature data show that while electron-withdrawing groups (EWG) present at the C-1 and C-3 positions increase the S₀–S₁ gap by stabilizing the HOMO and LUMO+1 orbitals, having no effect on the LUMO orbital, electron-donating groups (EDG) at these positions destabilize the HOMO and LUMO+1 orbitals, reducing the S₀–S₁ gap [14]. Additionally, azulene presents a strong absorption band related to a transition from S₀ to the second excited state (S₂). In addition to these absorption bands, a third strong band showing an S₀ → S₃ transition was observed for azulene in the ultraviolet range [1,15,16,17]. In this way, the absorption bands of azulenes can be connected with four distinct spectral regions: λ > 500 nm (S₀ → S₁ transition), 350 nm < λ < 500 nm (S₀ → S₂ transition), λ = 250–350 nm (S₀ → S₃ transition), and λ > 250 nm (S₀ → S₄ transition) [16,17].

Azulene is one of the few chromophores that present a strong emission from the S₂ excited singlet state to S₀ at room temperature, distinctly from the great majority of aromatic compounds which present the S₁ → S₀ emission. This anomalous situation can be due to the low-lying first excited state S₁ and a large energy gap between S₁ and S₂ levels which contribute to the decrease of the radiationless transition rate between S₂ and S₁ [1,17,18]. Additionally, the anomalous emission from the S₂ state is determined by the fact that the energy gap between S₂ and S₁ states is over 10,000 cm⁻¹ (cf. 14,000 cm⁻¹ in azulene), while a mixture of S₂ and S₁ emissions occurs if the S₂ → S₁ energy gap has values between 9000 and 10,000 cm⁻¹ [19]. If the gap is below 9000 cm⁻¹, the internal conversion rate between S₁ and S₂ is higher than the relaxation rate from S₂ and the emission takes place from S₁ [1,3,16]. The nature of azulene fluorescence is highly dependent on substitution at the azulene moiety, which affects whether the emission is dominant from the S₂ or S₁ singlet excited state, except for azulene aldehydes whose emission from the S₃ and S₂ states depends on the nature of the hydrogen bonds present in the system [20,21].

Due to the ability of solvents to modify the physicochemical properties of molecules, the investigation of the solvent influence on the spectral characteristics of azulene derivatives can provide information about the inter- and intramolecular interactions with the solvent and the excited state energy. The solvent influence is associated with a change in polarizability, polarity, dielectric constant and viscosity of the surrounding microenvironment. The dependence of the electronic absorption spectra, mainly the position and intensity of the absorption bands, on the solvent polarity can be described by the interaction of the chromophoric groups with solvent through non-specific (dipolarity/polarizability) and specific (H-bonding, acceptor–donor) interactions [22,23,24,25]. The solvent effects were studied using different solvatochromic parameters and polarity scales, such as the empirical solvent polarity parameter E_T [26], the Catalan solvent scale, the Kamlet–Taft scale or the Laurence solvent scale, which were applied to investigate the nature of solute–solvent interactions for a given compound [24,25,26,27,28,29,30,31,32].

In this paper, a detailed investigation of the effects of solvents and substituents on the electronic absorption spectra of some azulenyl-vinyl-oxazolones (Figure 3) was performed. Each investigated compound can act as a ligand for heavy metal (HM) ions. Table S1 lists the codes and names of the investigated compounds. They contain the phenyloxazolone moiety attached to the substituted azulene by a C=C double bond. The azulene moiety is either substituted with electron-donating alkyl groups (4,6,8-Me₃, 5-iPr-3,8-Me₂) or unsubstituted, whereas the phenyloxazolone moiety is either substituted with the electron-withdrawing nitro group or unsubstituted. This polarized structure ensures the increased degree of asymmetry of these azulene compounds by increasing the dipole moment as compared to unsubstituted 4-(azulen-1-ylmethylene)-2-phenyloxazol-5(4H)-one.

The spectral response of the samples under study was discussed for solvents of different polarities. The contribution of the non-specific and specific solute–solvent interactions for these azulene derivatives was analyzed using the Kamlet–Taft, Catalan and Laurence solvent scales.

2. Materials and Methods

The spectral studies were performed in 1,4-dioxane, tetrachloromethane (CCl₄), toluene, chloroform (CLF), ethyl acetate (EtAc), dichloromethane (DCM), dichloroethane (DCE), acetone, methanol, acetonitrile (ACN), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO). All the solvents were of spectroscopic grade and were commercially acquired from Sigma-Aldrich. The azulene derivatives (Table S1) were prepared following a previously reported synthetic method based on the condensation of azulene-1-carbaldehydes with hippuric acids by Erlenmeyer–Plöchl synthesis [33] (Figure 3). Copper sulfate pentahydrate, cadmium sulfate hydrate, lead(II) acetate trihydrate and mercury(II) chloride (Sigma-Aldrich) were utilized as received. A SPECORD 210 Plus spectrometer (Analytik Jena, Germany) was used to record the electronic absorption spectra in solution. Quartz cells with an optical path length of 10 mm were used for measurements.

The spectral properties of azulene derivatives were investigated by titration experiments monitoring the intensity of the absorption bands at longer wavelengths upon the addition Cd²⁺, Cu²⁺, Pb²⁺ and Hg²⁺ cations. Stock solutions of each metal cation were freshly prepared before each measurement at a concentration of 10⁻³ mol/L in acetonitrile. To perform the absorption titration experiments, a solution (2.5 mL) of azulene derivative in acetonitrile was introduced into a quartz cuvette and then increasing volumes of each metal ion (1–1500 μL) were added. After mixing, the electronic absorption spectra were recorded.

3. Results and Discussion

To investigate the solvent effect on the electronic absorption spectra of azulene derivatives, the measurements were performed in solvents of varying polarities. The UV–Vis absorption spectra of the studied azulenes consist of distinct absorption ranges located around 400–600 nm (S₀ → S₂ transition), 300–400 nm (S₀ → S₃ transition) and 300–220 nm (S₀ → S₄ transition). The transition S₀ → S₁ appearing at wavelengths over 600 nm has a very low intensity, and it is not evident in the spectra (Figure 4 and Figure S1). The absorption bands in the range 300–600 nm can be attributed to a π–π* transition being localized on the whole system.

When the solvent polarity changes from dioxane (ε = 2.22) to dimethyl sulfoxide (DMSO) (ε = 47.24), the maximum of the absorption band at 405 nm for O1 shifts to longer wavelengths of about 492 nm (Table 1). The introduction of the electron donating methyl groups in the 4,6,8 positions and the methyl groups in the 3,8 positions and an isopropyl group in 5 position, leads to a greater red shift, namely 758 cm⁻¹ and 887 cm⁻¹, for O2 and O3, respectively, compared to O1. The substitution of the nitro electron-withdrawing group at the para position of the phenyl ring of oxazolone induces a lower red shift for compounds O5 (588 cm⁻¹) and O6 (429 cm⁻¹) due to the higher asymmetry degree in these compounds determined by the nitro group. In this way, by changing the position of the substituents on the azulene ring, the azulene compounds that present colors covering the visible range to a great extent (Figure 4 and Figure S1) can be obtained, giving the possibility of tuning the color of azulene derivatives. The absorption bands corresponding to the S₀ → S₃ transition are of lower intensity and are poorly defined (Figure 4). Although, most azulene compounds exhibit strong fluorescence, the azulene derivatives under study do not show fluorescence.

To gain an understanding of the solvatochromic behavior of the azulene derivatives, the Lippert–Mataga and Bakhshiev solvent polarity functions were utilized in a simple approximation, according to the relations (1) and (2) [24,26,34,35].

ν_max = ν₀ + 2(μ_e−μ_g)²/hca³F_LM(n,ε) + const(1)

ν_max = ν₀ + 2(μ_e−μ_g)²/hca³F_B(n,ε) + const(2)

In (1) and (2), ν_max denotes the absorption maximum wavenumber; ν₀ is the spectral position of the absorption maximum wavenumber in the gas phase; n is the refractive index; ε is the dielectric constant of the solvent; μ_g and μ_e represent the ground and excited state dipole moments, respectively; h is the Planck constant; c is the light velocity in vacuum; a is the Onsager cavity radius; and F_LM(n,ε) and F_B(n,ε) are the solvent polarity functions given by expressions (3) and (4), respectively.

F_LM(n,ε) = (ε − 1)/(2ε + 1)−(n² − 1)/(2n² + 1)(3)

F_B(n,ε) = {(ε − 1)/(ε + 2)−(n² − 1)/(n² + 2)}(2n² + 1)/(n² + 2)(4)

The plots F_LM(n,ε) and F_B(n,ε) as a function of ν_max do not show a linear trend, but present a high dispersion of the data (Figures S2 and S3), which denotes that the specific interactions must be taken into consideration for understanding the solvent effects.

In recent years, many multiparameter solvent polarity scales have been developed to quantitatively describe solvent–solute interactions, resulting in a good linear correlation between spectral data and solvent polarity parameters. Linear solvation energy relationship analysis was applied using the Kamlet–Taft [29], Catalan [30] and Laurence [31] models in order to explore the solvent influence on the electron absorption spectra of the investigated azulene derivatives. The linear solvation energy relationships that describe the above-mentioned empirical solvent scales can be formulated through Equations (5)–(7), respectively [28,29,30,31,32,36].

(5)${\mathsf{\nu }}_{\max }={\mathsf{\nu }}_{\max ,0}+{\mathrm{a}}_{\mathsf{\alpha }}\mathsf{\alpha }+{\mathrm{b}}_{\mathsf{\beta }}\mathsf{\beta }+{\mathrm{c}}_{\mathsf{\pi }}\mathsf{\pi }$

In (5), ν_max represents the wavenumber in the absorption maximum; ν_max,0 is the value of the wavenumber in the gas phase; π* is a measure of the dipolarity/polarizability of the solvent; α and β denote the solvent hydrogen bond donor acceptor (HBD) and solvent hydrogen bond acceptor basicity (HBA), respectively; and a_α, b_β and c_π are regression coefficients.

(6)${\mathsf{\nu }}_{\max }={\mathsf{\nu }}_{\max ,0}+{\mathrm{a}}_{\mathrm{SA}}\mathrm{SA}+{\mathrm{b}}_{\mathrm{SB}}\mathrm{SB}+{\mathrm{c}}_{\mathrm{SP}}\mathrm{SP}+{\mathrm{d}}_{\mathrm{SdP}}\mathrm{SdP}$

In (6), SA, SB, SP and SdP are parameters that refer to acidity, basicity, polarizability and dipolarity of the solvents, respectively, and a_SA, b_SB, c_SP and d_SdP are regression coefficients.

(7)${\mathsf{\nu }}_{\max }={\mathsf{\nu }}_{\max ,0}+{\mathrm{a}}_{\mathrm{DI}}\mathrm{DI}+{\mathrm{b}}_{\mathrm{ES}}\mathrm{ES}+{\mathrm{c}}_{\mathsf{\alpha }1}{\mathsf{\alpha }}_1+{\mathrm{d}}_{\mathsf{\beta }1}{\mathsf{\beta }}_1$

In (7), DI describes the dispersion and induction interactions, ES denotes the electrostatic interactions between permanent dipoles of the solute and the solvent, α₁ and β₁ are HBD and HBA abilities of the solvents, and a_DI, b_ES, c_α1 and d_β1 represent the corresponding regression coefficients.

The Kamlet–Taft (α, β and π*), Catalan (SA, SB, SP and SdP) and Laurence (D1, ES, α₁ and β₁) solvatochromic parameters used in multilinear regression analysis are given in Table 2 [24,29,30,37]. The solvent-independent correlation coefficients from Equations (5)–(7) are estimated using multiple linear regression analysis, and they reflect the relative contributions of each solvent factor on the spectral shift in ν_max.

The results of the multiple regression analysis using Equations (5)–(7) are illustrated in Table 3. The multilinear analysis of the absorption data according to the Kamlet–Taft solvatochromic model shows a poor correlation, evidenced by the low values of R² for all azulene derivatives (Table 3). This weak correlation can be due to the fact that in the Kamlet–Taft equation, the non-specific effects are included in a single parameter π*. In the Catalan model, these effects are split, and a very good fit was obtained (R² = 0.94 for O1). It is clear from Table 3 that the Catalan model gives better correlations than the Laurence model, as seen from the slightly higher values of R². The percentage contributions of Catalan and Laurence solvent parameters on the absorption spectral shifts of the azulene derivatives are given in Table 4. The multilinear fit of ν_max using SP, SdP, SA and SB parameters (Equation (6)) shows that the solvent polarizability parameter (SP) dominates the solvatochromism with a lower contribution of solvent dipolarity and acidity. The solvent polarizability has a major effect on the absorption energy of all studied azulenes with percentage contributions of 73.89%, 62.48%, 71.65%, 67.62% and 65.67%, respectively (Table 4). In this way, the specific interactions (α + β) have around 20% contributions to the solvent–solute interactions. According to the Catalan scale analysis, a good correlation was obtained for O2 (R² = 0.92) with the largest impact of solvent polarizability (71.65%), but compared to O5 and O6, the correlation is less good (R² = 0.86 and 0.74, respectively), with a higher contribution (30.28%) of the specific interactions for O6. This shows that the introduction of the nitro group in the p-position of the phenyl ring of the oxazolone determines a reduction of the non-specific interactions at the position of absorption maxima.

The multiple linear regressions in Figure 5, Figures S4 and S5 reveal a good correlation between the experimental and calculated data using Equation (6). In the fitting relations to the absorption spectral data (Table 3), the coefficients a, c and d have a negative sign for all azulene derivatives, which suggests a red shift of the absorption maxima with the increase of solvent polarizability and dipolarity. At the same time, the negative coefficients associated with SP, SdP and SA confirm the stabilization of the excited state compared to the ground state [38].

From the multilinear correlations using the Laurence solvent scale (Equation (7)), similar results were obtained. The fitting equations relating to the absorption data reveal that the non-specific interactions, given by the dispersion and induction contributions (DI ~75%) and the electrostatic interactions (ES ~15%), are the most important factors responsible for the red shift of the absorption band, while the specific interactions have a low influence for all compounds (Table 4).

Given the outstanding binding characteristics of azulene compounds with metal ions, they can be used for heavy metal ion detection [39,40]. The metal ion recognition behavior of the azulene derivatives was evaluated upon their titration by successive increments of metal ion, and the changes in the electronic absorption spectra were monitored. The evolution of the UV–Vis spectra of O2 and O5 derivatives with the four metal ions at different concentrations is depicted in Figure 6, Figure 7, Figures S6 and S7. The addition of each cation caused the progressive decrease in intensity of the absorption bands of azulene derivatives (Figure 6, Figure 7, Figures S6 and S7), with the exception of the absorption band in the 250–270 nm range where, during titration, an increase in absorbance was noticed due to the absorption of metal salts [41] (Figure 7b inset). A good linear relationship (R² = 0.99) was obtained for the plot of absorbance of the longer wavelength band versus the metal ion concentration for O2 during titration with Cd²⁺ ions over the range of 2.71 × 10⁻⁵ to 5.35 × 10⁻⁴ M Cd²⁺ content (Figure 8). The decrease in intensity of the absorption band at longer wavelengths with adding increasing contents of metal ions was seen to be 35.5% for Pb²⁺, 32.6% for Cu²⁺, 30.4% for Cd²⁺ and 32.7% for Hg²⁺, taking into account the ratio A_f/A_o, where A_o is the absorbance before the addition of metal ion and A_f is the absorbance corresponding to the constant value after the addition of metal ion.

4. Conclusions

The aim of this work was to investigate the spectral characteristics of a series of structurally different azulene-oxazolone derivatives in various solvents and their spectroscopic absorption behavior in the presence of Cd²⁺, Hg²⁺, Cu²⁺ and Pb²⁺ ions. Each investigated compound contains a phenyloxazolone moiety linked to a substituted azulene ring through a C=C double bond. The azulene moiety substituted with electron-donating alkyl groups and the phenyloxazolone moiety substituted with an electron-withdrawing nitro group provides an increased degree of asymmetry of these azulene compounds compared to unsubstituted 4-(azulen-1-ylmethylene)-2-phenyloxazol-5(4H)-one. The solvent-dependent characteristics of some azulene-oxazolone derivatives were analyzed using Kamlet–Taft, Catalan and Laurence solvent scales. Catalan and Laurence multiparametric solvatochromic analysis showed that the solvent polarizability parameters (SP + SdP, α₁ + β₁) have a greater impact (over 80%) on the spectral shifts of the absorption maxima of the investigated azulenyl-vinyl-oxazolones. However, the specific interactions are quite important in the solvatochromic behavior. The solvatochromic response also depends on the position of the substituents from the azulenic ring. These results might be useful for the design of new solvatochromic compounds and to understand the nature of the solvent–solute interactions given by this polarized structure. In the presence of Cd²⁺, Hg²⁺, Cu²⁺ and Pb²⁺ metal ions, the main absorption band intensity decreases by about 30% and exhibits a linear dependence on the concentration of metal ions, showing a good recognition ability of these ions at 10⁻⁴ M.

Author Contributions

Conceptualization, E.-M.U. and A.A.; methodology, E.-M.U.; validation, E.-M.U. and A.A.; formal analysis, M.H., O.-T.M. and M.C.; investigation, M.H. and O.-T.M.; resources, E.-M.U.; data curation, A.A. and M.C.; writing—original draft preparation, A.A. and M.H.; writing—review and editing, E.-M.U. and A.A.; visualization, A.A.; supervision, E.-M.U.; project administration, E.-M.U. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Tables

Figure 1. (a) Alternant nature of naphthalene and nonalternant nature of azulene; (b) azulene and its polarized resonance structure.

Figure 2. (a) Unique fluorescence observed for naphthalene and azulene. (b) Probability of locating the electron (squares of coefficients of each wave−function) in the HOMO, LUMO, and LUMO+1 of azulene (left) and naphthalene (right).

Figure 3. Chemical structure of investigated azulenyl-vinyl-oxazolones: R1 = H; 4,6,8-Me3; 5-iPr-3,8-Me2; R2 = H; NO2.

Figure 4. UV–Vis absorption spectra in different solvents: (a) O3, (b) O6.

Figure 5. Linear relationship between experimental and calculated absorption maxima for (a) O3 using Equation (6) and (b) O3 using Equation (7).

Figure 6. Changes in the UV–Vis absorption spectra in acetonitrile solution upon the addition of increasing amounts of Cd2+ (a) for O2 and (b) for O5.

Figure 7. Changes in the UV–Vis absorption spectra in acetonitrile solution upon the addition of increasing amounts of (a) Cu2+ for O2 and (b) Pb2+ for O5. The inset shows the enlarged view of the 220–270 nm range.

Figure 8. Plot of the absorption change of O2 as a function of Cd2+ content.

Table 1

Spectroscopic data of the investigated azulenyl-vinyl-oxazolones in different solvents (ν_max, cm⁻¹).

Solvent	Azulene Derivative
	O1	O2	O3	O5	O6
1,4-Dioxane	21,500	20,960	20,080	20,080	20,080
CCl4	21,459	20,790	20,000	20,000	20,000
Toluene	21,321	20,703	19,920	19,920	19,920
CLF	21,141	20,449	19,455	19,455	19,455
EtAc	21,551	20,876	20,161	20,161	20,161
DCM	21,321	20,533	19,493	19,493	19,493
DCE	21,186	20,576	19,841	19,841	19,841
Acetone	21,459	21,321	19,723	19,723	19,723
Methanol	21,321	20,491	19,569	19,569	19,569
ACN	21,459	20,876	19,723	19,723	19,723
DMF	21,186	20,366	19,455	19,455	19,455
DMSO	21,008	20,202	19,193	19,193	19,193

Table 2

Empirical Kamlet–Taft (KAT) [29], Catalan [30,31] and Laurence [32] solvent parameters used in the multiple regression analysis.

Method	KAT			Catalan				Laurence
Solvent	α	β	π	SA	SB	SP	SdP	DI	ES	α1	β1
1,4-Dioxane	0.0	0.37	0.55	0.000	0.444	0.737	0.312	0.77	0.36	0.00	0.44
CCl4	0.0	0.0	0.28	0.000	0.044	0.768	0.000	0.82	0.10	0.00	0.00
Toluene	0.0	0.11	0.54	0.000	0.128	0.782	0.284	0.86	0.20	0.00	0.15
CLF	0.44	0.0	0.58	0.047	0.071	0.783	0.614	0.80	0.40	0.20	0.00
EtAc	0.0	0.45	0.54	0.000	0.525	0.674	0.535	0.71	0.51	0.00	0.52
DCM	0.30	0.0	0.82	0.040	0.178	0.761	0.769	0.78	0.60	0.10	0.00
DCE	0.0	0.0	0.807	0.030	0.126	0.771	0.742	0.80	0.74	0.00	0.00
Acetone	0.08	0.48	0.71	0.000	0.475	0.651	0.907	0.69	0.78	0.04	0.49
Methanol	0.93	0.62	0.60	0.605	0.545	0.608	0.904	0.64	0.84	1.00	0.54
ACN	0.19	0.31	0.75	0.044	0.286	0.645	0.974	0.67	0.84	0.23	0.37
DMF	0.0	0.69	0.87	0.031	0.613	0.759	0.977	0.78	0.87	0.00	0.69
DMSO	0.0	0.76	1.0	0.072	0.647	0.830	1.000	0.84	1.00	0.00	0.71

Table 3

Coefficients obtained by multivariate linear regression analysis of the investigated azulene derivatives using Kamlet–Taft (KAT), Catalan and Laurence solvent parameters.

Fitting Method	Ligand	y0 × 103	Correlation Coefficients				R2
KAT			aα	bβ	cπ*
	O1	21.70 ± 1.84	−86.73 ± 1.75	56.72 ± 2.17	−583.31 ± 3.10		0.435
	O2	20.60 ± 2.29	−379.22 ± 2.18	200.96 ± 2.70	−1368.62 ± 3.86		0.743
	O3	21.18 ± 2.52	−228.32 ± 2.40	−12.44 ± 2.98	−790.21 ± 4.25		0.498
	O5	18.86 ± 2.50	376.51 ± 2.31	156.34 ± 2.49	212.37 ± 3.83		0.401
	O6	18.09 ± 2.77	110.16 ± 2.63	429.86 ± 3.27	−233.94 ± 4.67		0.252
Catalan			aSA	bSB	cSP	dSdP
	O1	23.39 ± 2.55	−502.30 ± 1.32	85.15 ± 1.00	−2535.06 ± 3.33	−308.2 ± 68.26	0.943
	O2	22.70 ± 2.7	−822.99 ± 1.42	400.93 ± 1.07	−3447.95 ± 3.58	−846.20 ± 0.73	0.980
	O3	23.48 ± 4.16	−883.34 ± 2.16	44.75 ± 1.63	−3435.86 ± 5.44	−431.24 ± 1.11	0.928
	O5	21.35 ± 5.05	−428.37 ± 2.62	415.66 ± 1.98	−2774.90 ± 6.60	−484.54 ± 1.35	0.861
	O6	22.14 ± 13.70	−1377.65 ± 7.1	1141.29 ± 5.38	−5461.76 ± 17.89	−336.09 ± 3.66	0.743
Laurence			aDI	bES	cα1	dβ1
	O1	23.75 ± 4.14	−2774.5 ± 5.04	−507.2 ± 1.25	−305.15 ± 1.14	103.52 ± 1.30	0.900
	O3	23.03 ± 5.14	−3535.5 ± 6.25	−1322.9 ± 1.5	−423.99 ± 1.42	588.61 ± 1.61	0.955
	O2	23.92 ± 6.83	−3701.7 ± 830	−754.3 ± 2.06	−498.44 ± 1.89	137.89 ± 2.14	0.871
	O5	21.61 ± 7.58	−2869.8 ± 9.21	−746.93 ± 2.29	−222.79 ± 2.09	480.85 ± 2.37	0.792
	O6	20.28 ± 8.71	−2796.9 ± 10.58	−279.95 ± 2.63	−175.33 ± 2.40	447.12 ± 2.73	0.742

Table 4

Percentage contributions of the Catalan and Laurence solvent parameters to the solvatochromism of the investigated azulenes.

Fitting Method	Ligand	Relative Contribution of Correlation Parameters				Spec.*a (%)	Non-Spec.*b (%)
Catalan		SA (%)	SB (%)	SP (%)	SdP (%)
	O1	14.64	2.48	73.89	8.98	17.12	82.87
	O2	14.91	7.26	62.48	15.33	22.18	77.81
	O3	18.42	0.93	71.65	8.99	19.35	80.64
	O5	10.43	10.12	67.62	11.80	20.56	79.43
	O6	16.56	13.72	65.67	4.04	30.28	69.71
Laurence		DI (%)	ES (%)	α1 (%)	β1 (%)
	O1	75.18	13.74	8.26	2.80	11.07	88.92
	O2	60.21	22.53	7.22	10.02	17.24	82.75
	O3	72.69	14.81	9.78	2.70	12.49	87.50
	O5	66.42	17.28	5.15	11.12	16.28	83.71
	O6	75.60	7.56	4.73	12.08	16.82	83.17

^*a specific interaction; ^*b non-specific interaction.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/sym15020327/s1, Basic properties for ligands and characterization by elemental analysis, UV–Vis, ¹H NMR, ¹³C NMR, IR, MS; Lippert–Mataga equation; Bakhshiev equation; Figure S1: UV–Vis absorption spectra of azulene derivative in solvents of different polarities for (a) O3 and (b) O6; Figure S2: Lippert–Mataga solvent polarity function (FLM) plots for (a) O1 and (b) O6; Figure S3: Bakhshiev solvent polarity function plots for (a) O1 and (b) O6; Figure S4: Linear correlation plots using Equation (6) for (a) O2 and (b) O5; Figure S5: Linear correlation plots using Equation (7) for (a) O1 and (b) O2; Figure S6: Changes in the UV–Vis absorption spectra in acetonitrile solution upon the addition of increasing amounts of (a) Pb²⁺ for O2 and (b) Cu²⁺ for O5; Figure S7: Changes in the UV–Vis absorption spectra in acetonitrile solution upon the addition of increasing amounts of Hg²⁺ (a) for O2 and (b) for O5.

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