Electrochemistry of Rhodanine Derivatives as Model for New Colorimetric and Electrochemical Azulene Sensors for the Detection of Heavy Metal Ions

Abstract

Rhodanine (R) is a heterocycle having complexing properties for heavy metal (HM) ions. Considering the similar electron-donating character of diethylaminobenzene and azulene, electrochemical characterization of (Z)-5-(azulen-1-ylmethylene)-2-thioxo-thiazolidin-4-one (R1) and 5-(4 diethylamino-benzylidene)-2-thioxo-thiazolidin-4-one (R2) was performed to establish their common features. Chemically modified electrodes based on R1 and R2 were compared for HM recognition. Evidence for the formation of films was provided by scanning and controlled potential electrolysis, and HM recognition experiments were performed using their films. Parallel studies for analysis of HMs by complexation in solution were performed by UV-Vis. The analogy between R1 and R2 created the premise for easier selection of compounds for certain applications. The performance of the chemically modified electrodes was evaluated as detection limits for HMs. The azulene monomer (R1) proved to be the best candidate for Pb(II) detection, being about eight times more sensitive than R2. However, in solution, R2 proved to be a good choice for optical measurements, having a higher absorption coefficient. These results support the two ligands having different behaviors in homogeneous and heterogeneous systems.

1. Introduction

2-Sulfanilidene-1,3-thiazolidin-4-one (rhodanine, R) is a 5-membered ring heterocyclic organic ring with sulfur and nitrogen in the positions 1 and 3 (Figure 1). Some rhodanine-substituted derivatives are used as highly sensitive reagents for the determination of metal ions in analytical chemistry [1,2,3]. These derivatives are encountered in drug production and applied in the treatment of cancer and diabetes [4,5], as antimicrobial compounds [4,5,6], or as pan-assay interference compounds (PAINS), which give false positive results for biological samples [6,7,8]. Another category of applications of these compounds is their use in preparing polymer layers to protect various surfaces against corrosion [9]. Their efficiency as organic corrosion inhibitors is due to heteroatoms with high electron density, such as nitrogen, sulphur, or multiple bonds, acting as adsorption centres [10]. Rhodanines generate protective polymeric films against corrosion by physical and chemical adsorption, thereby reducing corrosive attack on mild steel [11,12] or copper [9,13,14]. Last but not least, due to their conductive properties, these films are used in the achievement of photovoltaic cells [15], with refs. [15,16,17] demonstrating improved solar energy capture efficiency.

The polymerization of R and its derivatives has been reported [9,10,11,12,13,14] and several practical applications are mentioned. Polyrhodanine films were obtained by electrochemical oxidation of rhodanine solutions in water [18,19] or organic media [20]. The polyrhodanine films synthesized so far have been shown to have good properties, such as high thermal stability and adequate solubility in common organic solvents, indicating their potential use as protective films for metallic or insulating surfaces and other materials [18].

This work is focused on the comparison of (Z)-5-(azulen-1-ylmethylene)-2-thioxo-thiazolidin-4-one (R1) and diethylaminobenzylidenerhodanine (R2). Compound R1 contains an azulen-1-yl moiety (Figure 1). Azulene is a non-alternating bicyclic aromatic hydrocarbon with an electron-deficient seven-carbon ring (tropylium cation ion configuration) fused to an electron-rich five-carbon ring (cyclopentadienyl anion configuration), which assigns a relatively large dipole moment (1.08 D) to this molecule. The electrochemical, optical, and photophysical properties of differently substituted azulene derivatives can vary significantly depending on the position and nature of the substituent [21,22,23,24]. The structure of compound R2 contains a phenyl substituted in the para position with a diethylamino group and has a polarity similar [25] to that of azulene (dipole moment 1.61 D for dimethylaminobenzene).

Azulene derivatives of R, such as R1, proved to be systems with high affinity for cations, with the push–pull structure being favourable for the construction of modified electrodes with complexing properties. Previous research on R1 [26] highlighted the electrochemical processes of this compound at anodic and cathodic potentials and the easy formation of films with complexing properties. Starting from R1, chemically modified electrodes (CMEs) were prepared [26]. They were tested for the analysis of HMs with good results [26]. This confirmed the literature data showing that azulene derivatives of R, known for their complexing properties towards HMs [26], can be used in the determination of metals, similar to the use of p-dimethylaminophenylene rhodanine for the accurate analysis of Ni, Cu, Zn, and Fe ions [27] or triarylamine rhodanine derivatives used for the detection of Ag(I) and Hg(II) ions by colorimetric methods [28,29].

Azulenic compounds require special conditions during synthesis to protect the azulenic moiety, which is easily oxidizable. The analogy between the azulene derivative of rhodanine and the aromatic p-dialkylaminobenzylidene rhodanine may provide a basis for preliminary testing of these more accessible compounds. Thus, the selection of compounds of interest with interesting properties for a specific application will be facilitated. This approach can reduce the costs associated with the discovery of new compounds (expensive reagents, working time under special conditions), which is generally the case for azulene derivatives.

Comparison of the electrochemical study of these compounds (Figure 1) revealed the similarity between the electrochemical processes of oxidation and reduction, which confirmed the similarity of the chemical properties of the two compounds [24] and was obvious considering the similar electron-donating character of the structures of R1 and R2. Consequently, their affinities for cations were expected to be similar. To evidence the similarity between the two ligands, some previous results referring to R1 [30] were used and a thorough study of R2 was carried out. The possibility of comparing these push–pull structures to build modified electrodes with complexation properties for HMs was also investigated for R2 and compared with that of R1. Parallel studies of HM complexation in solution by R1 and R2 were performed by UV-Vis.

2. Materials and Methods

(Z)-5-(Azulen-1-yl)methylene)-2-thioxothiazolidin-4-one (R1) was synthesized according to the literature [31]. Diethylaminobenzylidenerhodanine (R2) was purchased from LOBA CHEMIE (98%). All other reagents for voltammetric experiments were analytical grade. Acetonitrile, used as the solvent, was electronic grade 99.999% (Sigma Aldrich, St. Louis, MI, USA) and tetra-n-butylammonium perchlorate (Flukapuriss) was electrochemical grade 99%. These reagents were used as the solvent and supporting electrolyte, without further purification. Mercury(II) acetate (Fluka, Munich, Germany, ≥98%), copper(II) acetate monohydrate (Fluka, Munich, Germany, ≥98%), lead(II) nitrate (Fluka, Munich, Germany, ≥99.5%), and cadmium nitrate tetrahydrate (Fluka, Munich, Germany, ≥98%) were used for the recognition experiments.

The electrochemical experiments for ligand characterization and preparation of modified electrodes were performed by cyclic voltammetry (CV), differential pulse voltammetry (DPV), and rotating disk electrode voltammetry (RDE). The experiments were performed in a three-compartment cell coupled to a PGSTAT 12 AUTOLAB potentiostat. The working electrode (WE) was a glassy carbon (GC) disc with a diameter of 3 mm. GS was thoroughly prepared for each determination by polishing its surface with diamond paste (0.25 μm) and then rinsed with acetonitrile with acetonitrile. The reference electrode (RE) was Ag/10 mmol∙L⁻¹ AgNO₃ in 0.1 mol∙L⁻¹ tetra-n-butylammonium perchlorate (TBAP) in acetonitrile (CH₃CN) and a platinum wire was used as the auxiliary electrode (AE). For the HM recognition experiments, a transfer cell with a Pt wire was used as the AE and Ag/Ag Cl, KCl 3 M was used as the RE.

CV curves were generally recorded at a scan rate of 0.1 V∙s⁻¹. The DPV curves were recorded at 0.01 V∙s⁻¹ with a pulse height of 0.025 V and a step time of 0.2 s. The RDE curves were recorded at 0.01 V∙s⁻¹. After the experiment, the potentials were related to the potential of the ferrocene/ferrocenium redox couple (Fc/Fc⁺) for the purpose of comparisons. All electrochemical characterization experiments were performed under an argon atmosphere. The experiments were performed at room temperature (25 °C).

The chemically modified electrodes (CMEs) were prepared from millimolar ligand solutions in 0.1 M TBAP/CH₃CN by scanning or controlled potential electrolysis (CPE). Then, each modified electrode was washed with acetonitrile and placed in 0.1 M acetate buffer solution (pH = 4.5) where it was equilibrated and overoxidized following a procedure similar to that previously described [30,31]. The CMEs were tested as HM sensors in aqueous medium using the transfer cell that had the CME as the WE. Each CME was immersed for 10 (15) minutes under stirring in synthetic HM ion solutions. For recognition experiments using chemically modified electrodes or UV-Vis, the metal ion solutions were prepared from 10⁻³ M stock solutions of mercury(II) acetate, copper(II) acetate monohydrate, lead(II) nitrate, and cadmium nitrate tetrahydrate. Afterwards, the electrode was removed and rinsed with deionized water, then immersed in a cell containing 0.1 M acetate buffer solution (pH = 4.5). Here, it was kept for 3 min at −1.0 V, and then the DPV curve was recorded between −1.0 and +0.6 V.

The UV-Vis spectrometry study was performed on a JASCO V-670 in 1 cm optical path quartz cuvettes in freshly dried acetonitrile. Aliquots of heavy metal solutions were added to the ligand solution and UV-Vis spectra were recorded between 800 and 200 nm one minute after each addition.

3. Results

3.1. Electrochemical Characterization of the Ligands

The electrochemical behavior of the used ligands was studied in millimolar solutions in supporting electrolyte (0.1 M TBAP, CH₃CN) by electrochemical methods. Ligand solutions were obtained by successively diluting the initial solution, of higher concentration, with the appropriate amount of supporting electrolyte solution. The curves for reduction and oxidation processes were recorded starting from the equilibrium potential. For each electrochemical method used to characterize the ligands, voltammograms were recorded for the supporting electrolyte (with zero ligand concentration); their curves were shown with dashed lines. The peaks of the cathodic and anodic processes were denoted in the order in which they appeared in the DPV voltammograms. To be able to compare the values of the cathodic and anodic currents more easily, the currents for the cathodic processes were represented in absolute values.

3.1.1. Characterization of the Ligands by Cyclic Voltammetry

Characterization by cyclic voltammetry (CV) was performed in solutions of different concentrations (Figure 2), at different scan rates (Figure 3), and on different potential domains (Figure 4). For R1 ligand, 5 peaks corresponding to 5 reduction processes on the cathodic domain and 2 peaks corresponding to 2 oxidation processes on the anodic domain were identified [30]. For ligand R2, 3 peaks corresponding to 3 reduction processes on the cathodic domain and 5 peaks corresponding to 5 oxidation processes on the anodic domain were identified. Figure S1 shows the maximum values of the peak currents as a function of the ligand concentration in the solution, and in Table S1 the equations of the correlation lines are given. Figure 3b shows the variation of the peak currents for processes a1 and c1, respectively, with the square root of the scan rate. The equations of the correlation lines between the two quantities are given in

Table 1. Linear dependence of first anodic (a1) and cathodic (c1) peak currents (in A) on the square root of the scan rate (in V∙s⁻¹).

Ligand	Process	Equation	Adj. R-Square	Reference
R1	a1	$ipa1=-3.80\times {10}^{-6}+44.67\times {10}^{-6}\times \surd v$	0.998	[32]
R2	a1	$ipa1=-7.56\times {10}^{-7}+51.6\times {10}^{-6}\times \surd v$	0.996	this paper
	c1	$ipc1=-3.01\times {10}^{-7}-82.7\times {10}^{-6}\times \surd v$	0.999	this paper

. The peak potentials are given in

Table 2. Anodic and cathodic process assignments for R2 peak potentials (V vs. Fc/Fc⁺) from CV and DPV curves and half-wave potential (E_1/2) values from RDE.

Process	Method			Process Assignment
	CV	DPV	RDE (E1/2)
a1	0.526 qr	0.436	sh	Radical cation formation from amino group
a2	0.840 i	0.753	“peak” at 0.90	Oligomer oxidation
a3	i	1.191	-
a4	qr	1.518	-
a5	2.147 i	1.800	-
c1	−1.759 qr	−1.711	−1.70	Reduction of C=C double bond
c2	i	−2.217	−2.20	-
c3	−3.151 qr	−3.011	−3.10 **	Reduction of C=S bond

qr—quasi-reversible process, i—irreversible process; sh—shoulder; ** revealed only for low R2 concentration (0.5 mM).

.

3.1.2. Characterization of Ligands by Differential Pulse Voltammetry

Characterization by DPV was performed in solutions of different concentrations (Figure 5). Five peaks on the cathodic domain and 2 peaks on the anodic domain corresponding to the oxidation/reduction processes of R1 ligand were noticed [30]. For R2 ligand, 3 peaks on the cathodic domain and 5 peaks on the anodic domain were identified. Figure S2 shows the maximum values of the DPV peak currents depending on the ligand concentration in solution, and Table S2 shows the equations of the correlation lines. To correlate the processes highlighted in CV and DPV, the voltammograms for ligand R2 were represented in parallel in Figure S3.

3.1.3. Characterization of Ligands by Rotating Disk Electrode Voltammetry

In Figure 6, the voltammograms obtained by RDE at [R1] = 0.25 mM and for [R2] = 2 mM, at different rotation rates, are presented in parallel. The voltammograms obtained at a rotation rate of 1000 rpm for different concentrations of R1 and R2 are presented in Figure 7. To correlate the processes that were highlighted in RDE and DPV, the voltammograms obtained for R2 ligand are represented in parallel in Figure S4. Figure S5 shows in detail for [R2] = 1 mM the appearance of an anodic process evident at approximately 2 V, which decreased with increasing electrode rotation rate. Table 2 shows the values for the anodic and cathodic processes for R2 at a concentration of 2 mM by the previously listed electrochemical methods.

3.2. Preparation and Electrochemical Characterization of Modified Electrodes Based on the Aromatic Rhodanine

The electrochemical immobilization of R2 ligand on the glassy carbon electrode was carried out in millimolar solutions of the ligand by scanning or CPE at potentials close to those of the anodic processes, as shown in previous works [20,26,32,33,34,35]. After obtaining the chemically modified electrodes based on R2 (R2-CME), each electrode was rinsed with acetonitrile and transferred to ferrocene (Fc) solution in 0.1 M TBAP/CH₃CN. The CV curves of Fc obtained on the CMEs in different conditions were compared with those obtained on the unmodified electrode, and the differences were analyzed.

3.2.1. Formation of Modified Electrodes Based on the Aromatic Rhodanine by Scanning

Figure 8 shows the evolution of the CV curves during successive scans in the domain of processes a1–a5. The comparison of the CV curves recorded in transfer solution of Fc on the obtained CMEs and uncoated GC (Figure 6) suggested the formation of thin films. Figure 9 shows the evolution of potentials and peak currents on the obtained modified electrodes.

3.2.2. Formation of Modified Electrodes Based on the Aromatic Rhodanine by Controlled Potential Electrolysis

The chemical immobilization of the ligand by CPE was performed from solutions of R2 in 0.1 M TBAP/CH₃CN with a concentration of 2 mM. The corresponding chronoamperograms are given in Figure S6. When the modified electrodes were obtained by CPE, their transfer in Fc solution led to the CV curves shown in Figure 10A. Figure 10B,C show the evolution of peak potentials and currents for Fc on the obtained CMEs.

3.3. Heavy Metal Recognition Experiments Using Modified Electrodes Based on the Aromatic Rhodanine

In order to compare the electrodes modified with the two ligands, the recognition of HM ions on electrodes modified with R2 was realized. R2-CMEs were obtained by CPE from millimolar solutions of R2 in 0.1 M TBAP/CH₃CN. Figure 11 shows the stripping curves obtained by DPV recorded on R2-CMEs prepared from R2 and immersed in mixed solutions with different concentrations (for 10 min accumulation times). Figure 12A shows the dependence of the area of the stripping peak currents for each metallic ion that was present in the accumulation solutions. The DPV peak current dependence on Pb concentration is shown in Figure 12B. Figure 13 shows in parallel the DPV stripping curves recorded on modified electrodes based on R1 and R2 to highlight the difference between the two ligands.

3.4. UV-Vis Study of Pb Complexation in Solution

UV-Vis spectra for different concentrations of R1 or R2 were recorded to find the extinction coefficients and main wavelength absorption bands for R1 and R2, respectively. The variation of these spectra in the presence of the investigated HM ions was noticed. The dependence of absorbance on monomer concentration was recorded to find the stoichiometric ratio of the formed complexes. Figure S7 shows the UV-Vis spectra obtained for different concentrations of R1 and its inset gives the dependence of absorbance on R1 concentration for different wavelengths given in

Table 3. Linear dependence of absorbance peaks for R1.

Maximum Wavelength, nm 1	Equation	Adj. R-Square
λ1 = 457	$A_{\lambda 1}=0.01446+0.02962\times c$	0.998
λ2 = 435	$A_{\lambda 2}=0.03176+0.02947\times c$	0.988
λ4 = 289	$A_{\lambda 4}=0.02533+0.01688\times c$	0.969
λ5 = 231	$A_{\lambda 5}=0.0188+0.01642\times c$	0.964

¹ for λ₃, maximum values could not be read for all calibration determinations.

. Figure 14 shows the UV-Vis spectra for different [Pb] /[R1] ratios and the variation of the absorbance for the peak at 457 nm vs. [Pb]/[R1] ratio (I1 inset) and vs. [Pb]/([Pb] + [R1]) molar fraction (I2 inset). Figure S8 gives the UV-Vis spectra obtained for different concentrations of R2 for different wavelengths and the linear dependence of UV-Vis peak absorbance on concentration in the inset, which is given in

Table 4. Linear dependence of absorbance peaks for R2.

Maximum Wavelength, nm	Equation	Adj. R-Square
λ1 = 474	$A_{\lambda 1}=-0.02161+0.06887\times c$	0.992
λ2 = 321	$A_{\lambda 2}=0.00573+0.01793\times c$	0.994
λ3 = 295	$A_{\lambda 3}=0.00953+0.02491\times c$	0.993
λ4 = 251	$A_{\lambda 4}=0.00083+0.02139\times c$	0.993

. Figure 15 gives the UV-Vis spectra for different [Pb]/[R2] ratios and the variation of the absorbance for the peak at 474 nm vs. [Pb]/[R2] ratio (I1 inset) and vs. [Pb]/([Pb] + [R2]) molar fraction (I2 inset). The ratios r/m for (Pb)_m(R1)_r and (Pb)_m(R2)_r, according to the Mollard method for absorbance (A), are given in Tables S3 and S4, respectively.

4. Discussion

4.1. Comparison between Electrochemical Behavior of the Two Ligands

In the anodic domain, where the relevant peaks are a1 and a2 (Figure 2), the electrochemical behavior evaluated by CV for the ligands R1 and R2 in millimolar solutions in the supporting electrolyte was similar. For both ligands, linear dependence of the peak current on the ligand concentration was obtained, certifying the assignment of the peaks to some processes undergone by the ligand (for R1 according to [32] and for R2 according to Figure S1 and Table S1).

The dependencies of the peak currents a1 and c1 on the scan rate for both ligands were compared. For R2, Figure 3a showed the variation of the CV curves for different scan rates in the domain of processes a1 and c1, while Figure 3b showed the linear dependencies of the peak currents a1 and c1 on the radical of the scan rate, which confirmed a diffusive controlled charge transfer. The slope of the dependence of the anodic peak current a1 (Table 1) for both ligands was similar.

Figure 4 illustrated the character of the R2 peaks, summarized in Table 2, which was evidenced by CV on different scan domains and scan rates. According to Table 2, the first CV oxidation potential peak a1 for R2 (0.526 V) was at about the same positive potential as the azulene derivative R1 (0.579 V) [32]. The second oxidation peak for R2, a2, had a similar position with respect to the azulene derivative R1 (0.84 V vs. 1.143 V [32], respectively). This proved that the amino group in R2 enabled easy oxidation of the overall molecule compared to a pure aromatic compound such as phenylene-2thioxo-thitiazolidin-4-one. For cathodic processes, the CV c1 peak potential was −1.759 V for R2 and −1.202 V for R1 [32]. As expected, it was more difficult to reduce R2, which has an amino group.

The DVP peaks a1 and a2 (Figure 5) were situated in good accordance with the peaks registered by CV: 0.436 V and 0.753 V vs. 0.541 V and 1.022 V, respectively (Table 2). Linear dependence of the DPV peak current on concentration was found for both ligands: for R1 (according to [32]) and R2 (as shown in Figure S2). Figure S3 showed, with a dashed line, a good correlation between the potential peaks revealed by CV and DPV for R2.

The comparative RDE study (Figure 6 and Figure 7) showed a peak form for the anodic processes followed by a decrease down to the background values of RDE at potentials in the range of the a1 and a2 peaks. This confirmed the formation of insulating films for both monomers. In the cathodic domain of scanning, the two monomers had regular curves but the reduction potentials were different, being more negative for R2. Their half-wave potentials (E_1/2) taken from the RDE curves were in good agreement with the CV and DPV peak potentials (Table 2). Figure S4, containing a detailed image of the processes for different rotation rates and R2 concentrations, showed the correspondence between the processes evidenced by DPV and RDE. An evident asymmetry between the anodic and cathodic processes could be noticed, especially for the RDE curves. The anodic currents were much smaller than the cathodic ones due to film formation. This asymmetry was less evident in the DPV curves. With the increased rotation rate in RDE, the signal for the a1 peak increased, while the peak at 1.8 V, denoted as a5 in DPV, decreased (Figure S5). This evolution can be explained by the formation of insulating films at potentials higher than 0.8 V.

4.2. Comparison between the Formation of Chemically Modified Electrodes

This work found the common features of the CMEs in the preparation of R1-CMEs and R2-CMEs and their electrochemical characterization. This showed the formation of an insulating film either by scanning or CPE for both ligands. For R2, Figure 8A–H illustrated the decrease in the oxidation currents in successive cycles during preparation by scanning, which was more pronounced when the anodic limit of the scan increased. The transfer of modified electrodes in Fc solution led to modified CV curves, as shown in Figure 8A1–H1. Their shapes depended on the anodic limit of the scan during the preparation, being flatter for anodic limit values higher than 1 V. The analysis of the Fc signal on these curves, shown in Figure 9A, indicated that the anodic/cathodic potential peak (Epa/Epc) of Fc was constant until 1 V and then it increased in absolute value, whereas the formal potential (Ef) was constant. The difference between Epa and Epc (∆Ep) was constant until 1 V and then it increased (Figure 9B). The Fc anodic/cathodic peak current ipa /ipc was constant until 1 V and then it decreased in absolute value (Figure 9C). All of these facts confirmed the formation of insulating films when the scanning potential in the preparation was above 1 V.

When the CMEs were prepared by CPE starting from R2 solutions (as shown in Figure S6), the Fc probe on these CMEs (Figure 10A) had a slight variation with CPE potential. The detailed analysis of the Fc signal on these curves indicated that the anodic/cathodic potential peak (Epa/Epc) and the difference between Epa and Epc (∆Ep) of Fc were almoust constant until 1.8 V, and then they slightly increased in absolute value, whereas the formal potential (Ef) was practically constant (Figure 10B). The Fc anodic/cathodic peak current ipa /ipc slightly increased in absolute value until 1.8V and then it decreased (Figure 10C). All of these findings supported the formation of thin insulating films. The difference noticed between the two modes of preparation in the case of R2 can be attributed to the slow formation of films by scanning compared to CPE.

For R1, the two preparation modes did not generate such differences [32]. This can be correlated with the well-known polymerization capacity of the azulene moiety in comparison with the aromatic moiety.

4.3. Comparison between Heavy Metal Recognition Properties Using Modified Electrodes

The stripping curves recorded on R1-CMEs and R2-CMEs for different concentrations of mixed metals (

Table 5. Potential values (V) for DPV peaks of investigated metals using CMEs based on R1 and R2 in accumulation solutions containing mixtures of HMs with concentrations of 10⁻⁴ M for each ion.

Modified Electrode	Cd	Pb	Cu	Hg
R1-CME	−0.759	−0.533	−0.094	0.235
R2-CME	−0.757	−0.533	−0.089	0.235

) showed peaks for each of the metal ions in the accumulation solution at close potentials (Figure 11). Their intensities varied depending on the concentration. The shape of the obtained peak intensity vs. concentration curves (Figure 12A) was of the type of a saturation curve with different limiting currents. Among the metals tested, Pb showed the most intense signals and a relatively wide linear range between 10⁻⁸ and 10⁻⁵ M (Figure 12B), as in the case of R1-CMEs [32].

The stripping curves recorded on R1-CMEs and R2-CMEs (Figure 13) highlighted the difference between the two ligands in building sensitive CMEs for HMs. Both types of CME achieved better analysis of Pb(II) among the cations investigated, but their selectivity for the involved metal ions was different. R1-CMEs were found to be significantly better than R2-CMEs, as the Pb stripping area was about eight times larger. This meant that the azulene ligand R1 was a better ligand than R2 in the CME-based analysis of Pb.

The values provided in Table 5 showed that the stripping potentials for the cations were practically identical, which proved the similarity of the two rhodanines. This observation differed from other previous experimental conclusions because we obtained different values for the stripping potential of cations when we performed studies with electrodes modified with differently substituted azulenes [32]. The potential difference may be due to the formation of films with different structures due to the specific steric hindrance of each substitution.

4.4. Comparison between Heavy Metal Complexation of the Two Ligands in Solution by UV-Vis

The UV-Vis spectral investigations in micromolar solutions of each ligand revealed specific absorption wavelengths. These allowed the estimation of their extinction coefficients and the obtaining of calibration curves. The results for R1 and R2 (Figures S7 and S8, respectively) allowed the identification of the maximum wavelengths presented in Table 3 and Table 4, respectively, and the calculation of extinction coefficients for R1 (2.95∙10⁹ M⁻¹∙cm⁻¹ for the visible range and 1.66∙10⁹ M⁻¹∙cm ⁻¹ for the UV range) and for R2 (6.89∙10⁹ M⁻¹∙cm⁻¹ for the visible range and 2.3∙10⁹ M⁻¹ cm⁻¹ for the UV range).

The absorption bands at longer wavelengths for the R1 and R2 derivatives can be associated with a localized π → π* transition throughout the molecular system. The visible absorption band of R2 showed a red shift compared to R1 due to a higher degree of conjugation in R2. The absorption bands in the range 220–350 nm for R1 probably belong to the azulene ring, and the absorption bands around 320, 295, and 250 nm in R2, respectively, can be attributed to π → π* transitions linked to the dimethylaminobenzylidene unit.

Since Pb was found to be well identified using the CMEs, UV-Vis analysis of R1 or R2 solutions was performed for Pb. The variation of absorbance in the presence of increasing amounts of Pb showed the possibility of using this technique as an analytical tool based on the homogeneous complexation of Pb by the ligands.

The variation of the UV-Vis spectra for different amounts of metals (Figure 14) allowed the estimation of the ratio between Pb and R1 in the complex in solution and the establishment of its stoichiometry Pb:R1 = 3:4, a ratio calculated by two methods (inserts I1 and I2 in Figure 14).

Similarly, the variation of the UV-Vis spectra for different amounts of Pb (Figure 15) allowed the estimation of the ratio between Pb and R2 in the complex and the establishment of a similar stoichiometry Pb:R2 = 3:4 (the ratio calculated by two methods according to inserts I1 and I2 from Figure 15).

Although the comparison between R1 and R2 regarding Pb complexation by UV-Vis showed similar stoichiometric ratios in complexes for both structures, the higher extinction coefficient for R2 than R1 indicated that R2 was more efficient in solution for Pb complexation.

The UV-Vis study of these rhodanines provided important information on the role of the rhodanine moiety for optical applications, as has been reported for other rhodanine dyes [36].

5. Conclusions

As azulenic compounds require special conditions during synthesis, the analogy between azulenic derivatives of rhodanine and the aromatic p-dialkylaminobenzylidene rhodanine created the premise for easier selection of compounds for certain applications.

Comparison of the electrochemical and optical studies of the azulenic derivative of rhodanine and the aromatic p-dialkylaminobenzylidene rhodanine revealed the similarity between the two compounds, which was expected considering their similar electron-donating character. From electrochemical determinations using modified electrodes based on these ligands, the azulene compound was proven to be the best one, in terms of film formation and HM recognition, being about eight times more sensitive than R2. The electrodes modified with both rhodanines detected all the metal (Cd, Pb, Cu, Hg) ions in the accumulation solutions, but with the highest intensity for Pb. However, in solution, R2 was proven to be a good choice for the optical measurement of Pb concentrations, having a higher absorption coefficient. The two ligands had different behaviors in homogeneous or heterogeneous systems.