Synthesis,characterization,theoretical investigations and fluorescent sensing behavior of oligomeric azine-based Fe 3+ Chemosensors

Abstract

Three azine oligomeric esters were synthesized, characterized by IR, UV, ¹H, ¹³C{¹H} and GPC technique, and applied to chemosensor application. The sensitivity response of the oligomers towards the metal ion was evaluated for a metal ion series. The results have shown selective and sensitive “turn off” fluorescence response towards Fe³⁺ ion in DMF/H₂O (1:1, pH: 7.4, fluorophore: 5 μM) solution. The binding stoichiometry and binding constant of the fluorophores were calculated using the Stern–Volmer equation and Benesi–Hildebrand plots, respectively. The quenching of fluorophores on the addition of Fe³⁺ ion indicates the capability of fluorophore towards quantitative analysis of Fe³⁺. The dimer of oligomers was theoretically studied using DFT, B3LYP/6-311G level basic set to support and explain the quenching mechanism of LMCT, PET process and to explain the DC, AC electrical studies results. The electrical conductivity measurements of solid-state, I₂ doped and undoped oligomers were carried out and the conductivity gradually increases with increase in iodine vapor contact time of oligomers. The electrical conductivity was related with band gap and charge density values of imine nitrogen obtained by Huckel calculations. The dielectric measurements at different temperatures and frequencies were made by two probe method. Among the oligomers, EBHAP has recorded a high dielectric constant at the low applied frequency of 50 Hz at 373 K due to loosely attached π bonds resulting good polarization.

Keywords

Solution polymerization azine oligomers chemosensors electrical study DFT

Introduction

Nowadays, the development of highly sensitive and selective fluorescence chemosensors for the detection of various metal ions has gained significant relevance in environmental, medical, and biological applications because agriculture and industrial hasty growth causes excessive atmospheric pollution by introducing heavy metals and transition metals to the environment.^1,2 Moreover, the most abundant and biologically important transition metal is iron. The analysis of iron is much important for understanding the metabolic processes of biochemical cycling.^3,4 Because the high intake or deficit of metal ions in the human body might lead to a grave human health condition. Especially, the intake of excess iron causes several diseases such as cancer, anemia, kidney failure, Huntington’s, Alzheimer’s, Parkinson’s, and liver damage disease.^5–10 Many methods are reported for detecting the iron in a system such as electrochemistry, atomic absorption spectroscopy, and inductively coupled plasma mass spectrometry.^11–13 These methods are having some limitations to sense the particular metal ion in the system due to the interference of other metal ions. So, the widely accepted fluorescence technique has been adopted and is highly efficient, reliable, simple, low cost, instantaneous response, and capability of specific recognition of particular ions.¹⁴

Mostly, the conjugated compounds are used as a chemosensor for selectively detecting a particular ion in the system because of its ability to encounter the electron density changes in their backbone from the external structure contributions. The azine moiety (=N-N=) within the conjugated compounds shows well ligand nature towards the metal cations.^15,16 The azine moiety is similar to –C=N– and act as an efficient chemosensor for the detection of metal ions with a great fluorescent response.¹⁷ Among the organic compound, azin containing oligomers with conjugated double bond shows good response towards the metal ion in the chemosensor study and also have several advantages in terms of amplification, electronic communication and biological applications.^18–20

Here, we reported the synthesis of azine containing ester oligomers with different degree of substituents and their application in chemosensor study for detecting the Fe³⁺ ion and electrical properties with special attention on dielectric property. The results of chemosensor studies are indicating the sensitive and selective quenching response of oligomers towards the Fe³⁺ ion over the presence of other metal ions in the solution. The fluorescence mechanism of chemosensor study, conductivity and dielectric properties of oligomers are explained with the help of absorption and DFT structural parameter analysis.

Experimental

Materials

Hydrazine, glyoxal, 4-hydroxy-3-methoxybenzaldehyde, phenyl phosphonic dichloride, phenyl dichlorophosphate, and adipoyl dichloride were purchased from Merck Chemical Co. and used without further purification. Dimethylsulfoxide, tetrahydrofuran, dimethylformamide, ethanol, methanol, acetone, acetonitrile, ethyl acetate, heptane, toluene, n-hexane, and chloroform were obtained from Merck, India and used after proper distillations.²¹

Synthesis of oligomers

The monomer (EBHMM) 4,4-(ethane-1,2-diylidene bis(hydrazine-2,1-diylidene))bis (methanylylidene) bis(methanylylidene)) bis(2-methoxyphenol) was prepared by the procedure already reported.²² The oligomer was synthesized through solution polymerization by taking EBHMM, triethylamine, and phenyl phosphonic dichloride in the ratio of 1:2:1. In the chloroform solution of EBHMM in the ice bath, triethylamine was added drop-wise followed by the addition of phenyl phosphonic dichloride in chloroform and stirred at 0°C (Scheme 1). After 1 h, the reaction mixture was warmed to room temperature and refluxed for 2 h. The oligomer was precipitated by adding methanol into the reaction mixture. The oligomer was filtered, washed with methanol and dried in a hot oven at 80°C for 24 h.

Scheme 1.

Synthesis of oligomers EBHMP, EBHOP, and EBHAP.

The other oligomers, EBHOP and EBHAP were also synthesized by adopting the same procedure using phenyl phosphoric dichloride and adipoyl dichloride, respectively. The oligomers were obtained with a good yield.

(EBHMP:82%, EBHOP: 78%, EBHAP: 84%). EBHMP: ₁ HNMR (CDCl ₃ -d1): 11.81 (s, -OH), 8.62 (s, Hd), 8.53 (s, He), 7.83–6.92 (Ar-H), 3.79 (-OCH ₃ ). ¹³ CNMR (CDCl ₃ -d1):178.25 (C7), 174.47 (C1), 161.14 (C8), 151.42(C2), 133.15(C9)132.26(C11), 131.72(C10), 128.42 (C12), 128.26 (C4), 127.92 (C5), 122.04 (C6), 110.52 (C3), 55.89 (-OCH ₃ ). ³¹ PNMR (CDCl ₃ -d1): −0.92, −5.74, −11.94.

EBHOP: ¹ HNMR (CDCl ₃ -d1): 11.90 (s, -OH), 8.60(s, Hd), 8.58 (s, He), 7.57–7.01 (Ar-H), 3.84 (-OCH ₃ ). ¹³ CNMR (CDCl ₃ -d1): 161.10 (C7), 151.04 (C1), 144.96 (C8), 139.73 (C2), 132.21 (C9), 129.71 (C11), 125.57 (C4), 123.62 (C5), 122.79 (C6), 121.44 (C12), 120.45 (C10), 110.73 (C3), 56.02 (-OCH ₃ ). ³¹ PNMR (CDCl ₃ -d1): −11.66, −11.82, −17.69, −18.04.

EBHAP: ¹ HNMR (CDCl ₃ -d1): 11.90 (s, -OH), 8.61 (s, Hd), 8.59 (s, He), 7.59–6.99(Ar-H), 3.87 (-OCH ₃ ), 2.70 (d, Hf), 1.95 (t, Hg). ¹³ CNMR (CDCl ₃ -d1): 190.05 (C9), 160.32 (C7), 148.07 (C1), 146.32 (C8), 125.54 (C2), 123.49 (C4), 122.02 (C6), 113.68 (C5), 107.63 (C3), 55.03 (-OCH ₃ ), 28.67 (C10), 23.25 (C11).

Characterization techniques

FT-IR spectra of monomer and oligomers were recorded in the region of 4000–400 cm⁻¹ in KBr pellet using Perkin Elmer 8000 FT-IR spectrophotometer. Bruker AV400 MHZ spectrometer was used to record ¹H, ¹³C{¹H} and ³¹P NMR spectra using CHCl₃-d1 as solvent. The molecular weight of the oligomers was determined with gel permeation chromatography using polystyrene standard and eluted in DMF at a flow rate of 1.0 mL/min at 25°C on a Waters HPLC model (GPC, water 600 HPLC) fitted with water 2414 refractive index detector and Styragel HR5E 4E 2/0.5.

Optical properties

UV visible spectra of the fluorophores in dimethylformamide were recorded using Shimadzu double beam UV-240 on the spectrophotometer. Fluorescence measurements were implemented with Jasco FP-8200 spectrofluorimeter equipped with quartz cuvettes of 1 cm path length. Before starting the experiments, the stoke solution of oligomers and the nitrate salts solutions of Ag⁺, Bi³⁺, Pb²⁺, Al³⁺, Ba²⁺, Cr³⁺, Mg²⁺, Ca²⁺, Na⁺ Ce³⁺, Zn²⁺, Zr²⁺, Co²⁺, Ni²⁺, Mn²⁺, Cd²⁺, Hg²⁺, and Li⁺ and chloride salts solutions of Cu²⁺, La³⁺, K⁺, and Fe³⁺, Fe²⁺ were prepared using DMF/H₂O (1:1) at pH = 7.4. Binding studies of 5 μM concentration of fluorophore with these metal ions were studied in DMF/H₂O (1:1, pH = 7.4).

Theoretical study

The theoretical study of a dimer of oligomers was studied using DFT, B3LYP/6-311G level of basis set in the Gaussian 09 package to analyze the structural parameter and for supporting the quenching mechanism and dielectric properties.

Electrical study

The pellets for AC and DC electrical study were prepared using LMTP 1–25 ton pellet press. The compound was placed at 15 ton pressure for 1–2 min in the LMTP 1–25 manual hydraulic press tool to produce the strong pellet for electrical study. The dielectric characteristics of oligomers were investigated using Hioki 3532–50 LCR meter at different temperatures in the 50 HZ to 5 MHz frequency range. DC conductivity study of oligomers was carried out in the range of 1V–10V using Keithley electrometer (6517 B). The conductivity of iodine doped oligomers were examined at different time intervals by exposing the oligomers pellet with I₂ vapor in a desiccator at room temperature.

Results and discussion

Solubility and molecular weight

0.1 mg of the sample was used to analyze the solubility in 1 mL of series of solvents such as DMSO, THF, DMF, methanol, ethanol, CHCl₃, CH₂Cl₂, ethyl acetate, n-hexane, acetonitrile, and acetone at room temperature. Synthesized oligomers are soluble in DMSO, THF, DMF, CHCl₃, and CH₂Cl₂ and insoluble in ethanol, methanol including other non-polar solvents. The solubility of compounds is shown in Table 1. Compared with monomer EBHMM, oligomers exhibit less solubility due to the involvement of polar phenolic –OH increasing the molecular weight of the oligomerisation process which causes lowering the solubility of oligomers.

Table 1.

Solubility table of oligomers.

Compound	DMSO	DMF	THF	CHCl₃	CH₂Cl₂	AC.N	MeOH	EtOH	n-hexane	Acetone
EBHMP	+	+	+	+−	+	−	−	−	−	−
EBHOP	+	+	+	+	+−	−	−	−	−	−
EBHAP	+	+	+	+	+	−	−	−	−	−

Soluble: +, Partially soluble: ±, Insoluble: −.

Despite that, the oligomers are reasonably soluble in some polar solvents due to the presence of the terminal polar –OH group.²³ The weight average molecular weight (Mw), number average molecular weight (Mn) and polydispersity index (PDI) of EBHMP, EBHOP, and EBHAP were calculated from the polystyrene standard calibration curve. The molecular weight of the oligomers is given in SF.1, SF.2, and SF.3. The values of Mn, Mw and PDI are found to be 757, 926 g mol⁻¹, and 1.22 for EBHMP, 1316, 1409 g mol⁻¹, and 1.07 for EBHOP, 1068, 1172 g mol⁻¹, and 1.09 for EBHAP, respectively. The low PDI values of oligomers are indicating the less number of branches with structural homogeneity in the oligomer chain.²⁴

Spectral characterizations

The FT-IR spectra of monomer and oligomers are shown in SF.4 and their spectral data are given in Table 2. The –C=C– and aromatic –C-H stretching vibrations of monomer and oligomers bands are appearing in the range of 1513–1491 cm⁻¹ and 2930–2945 cm⁻¹, respectively. The band at 1591 cm⁻¹ of monomer and 1591–1583 cm⁻¹ of oligomers are attributed to the –C=N– band. The sharp band appeared at around 2503–2481 cm⁻¹ is due to the presence of the end phosphoric group of oligomers EBHMP and EBHOP. The band at 3487 cm⁻¹ is attributed to the –OH stretching vibrations of the monomer and the bands in the range of 3386–3448 cm⁻¹ are due to the terminal –OH of oligomers. The bands that appeared at 1185–1161 cm⁻¹, 1266–1280 cm⁻¹, and 1632 cm⁻¹ are representing P-O-C, P=O, and –C=O stretching vibrations of EBHMP, EBHOP, and EBHAP, respectively.^25–28

Table 2.

IR spectral data of oligomers.

Compounds	Wavenumber (cm⁻¹)
Compounds	-OH	-C=N-	-C=C-	Ar-H	P-O-C	P=O	O=P-OH	-C=O
EBHMP	3386	1591	1513	2935	1161	1280	2481	—
EBHOP	3402	1599	1491	2945	1185	1266	2503	—
EBHAP	3448	1583	1506	2937	—	—	—	1632

¹H NMR spectral data of oligomers are given in the experimental part and their images are shown in SF.5, SF.6, and SF.7. The chemical shift values of azomethine protons of oligomers are appearing between 8.93–8.47 ppm. The small deviation of azomethine protons towards the downfield is due to the deshielding effect of the benzene ring.²⁹ The methoxy and terminal –OH protons resonate at 3.87–3.79 and 11.90–11.81 ppm, respectively. The aromatic and aliphatic oligomeric ester protons resonate in the region of 7.83–6.99 and 2.70–1.95 ppm, indicating the incorporation of phenyl and adipoyl groups in the main chain of EBHMP, EBHMOP, and EBHAP oligomers, respectively.³⁰

¹³C{¹H} NMR spectra of oligomers are clearly shown in SF.8, SF.9, and SF.10. The methoxy carbon chemical shift of oligomers is appearing at around 56.02–55.03 ppm. The appearance of excess carbon shifts at their respective aromatic regions than the monomer gives evidence of the incorporation of the phenyl group in the main chain of EBHMP and EBHOP oligomers. Similarly, the appearance of ester signal at 190.05 and aliphatic carbon signals at around 28.67–23.25 ppm gives evidence to the EBHAP oligomer structure.

³¹P NMR spectra of oligomers are given in SF.11 and SF.12. The spectra of EBHMP and EBHOP shows sharp chemical shifts at −0.92, −5.74, −11.94 and −11.66, −11.82, −17.69, −18.04 ppm, respectively. The values strongly indicating the presence of the phosphoric group in the main chain and end group of oligomers.³¹

Thermogravimetric analysis

The thermal stability of oligomers was studied by TG-DTA analysis in a nitrogen atmosphere with a heating rate of 10°C/min. The thermogram of oligomers are shown in SF.13 and they record a multistep degradation. The results are indicating that the oligomers EBHMP, EBHOP, and EBHAP are stable up to 225, 297, and 250°C, respectively, and thereafter they start degrading. The initial weight loss of oligomers around 30°C–100°C occurred due to the occulted water molecules in the oligomer structure. The second step degradation at near 200°C of EBHMP and EBHAP and 250°C for EBHOP due to the degradation of methoxy group in the oligomer structure. After 300 C there are multi exothermic peaks appeared due to the degradation of ester linkage of oligomers.³² The temperatures corresponding to 5% and 50% weight loss and char residue remaining at 600°C are given in Table 3. The char yield (CR) can be applied as a conclusive factor for estimating the limiting oxygen index (LOI) of the oligomer based on the van Krevelen and Hoftyzer equation,³³

LOI = 17.5 + 0.4 CR

Table 3.

TG-DTA data of oligomers.

Compounds	^aT_on	^bT_max	5% weight loss	50% weight loss	% of residual at 600 (°C)	LOI
EBHMP	120	225	160	284	21.29	25.56
EBHOP	149	297	113	418	43.11	34.74
EBHAP	133	250	142	275	21.64	26.15

The LOI values of oligomers EBHMP, EBHOP, and EBHAP are 25.56, 34.74, and 26.15, respectively. The LOI values of oligomers are less than 45, show that they have reasonable self-extinguishing property. Based on LOI values, the EBHOP oligomer has high self-extinguishing and low flammability than the other oligomers.³⁴

Optical properties

The UV-Visible spectra of monomer and oligomers were recorded in DMF solution and the results are shown in SF.14. The absorption bands are shown by monomer and oligomers at 377 and 330, 337 and 345 nm for EBHMP, EBHOP, and EBHAP, respectively are due to n-π* transition of imine moiety. The absorption band at around 264 nm of monomer and oligomers are due to the $π―>π*$ electronic transition of aromatic ring. The incorporation of ester linkage in the backbone of the compound has affected the planarity of the oligomer and causes a hypsochromic shift from 377 nm of monomer wavelength. The bandgap values of monomer and oligomers were calculated using the equation Eg = 1242/λ onset and are 3.3 eV for monomer (EBHMM) and 3.76, 3.68, and 3.60 eV for EBHMP, EBHOP, and EBHAP oligomers, respectively. The presence of ester linkage in the main chain of oligomers shows the lack of conjugation and high steric hindrance and imparts a higher bandgap and low λmax value than the monomer.³⁵ The excitation and emission fluorescence of oligomers is depicted in SF.15.

In the fluorescence study, the excitation wavelength of oligomers was fixed as the respective absorption band values and the emission of oligomers are appearing at 415, 416, and 417 nm for EBHMP, EBHOP, and EBHAP, respectively. The oligomers show high fluorescence intensity values than the monomer because of its non-planar and high rigid structural nature.³⁶

The stoke shift values were calculated from the excitation and emission λmax of fluorophores ³⁷ and are listed in Table 4. The stoke shift is generated by the relaxation of electrons or the structural geometry of the molecule. The calculated stoke shift (Δλ_ST) values of oligomers EBHMP, EBHOP, and EBHAP are 85, 79, and 72, respectively. The oligomers have higher stoke shift values than the monomer due to the extension of the double bond in the oligomer chain.³⁸ The stoke shift value is most important for the optical and fluorescence study because, the large stock shift value can distinguish emission fluorescence photon from the excitation photon. The large stock shift values are reducing the background signal and is useful to construct the excellent fluorescence sensor.³⁹

Table 4.

Fluorescence spectral data of oligomers.

Compound	λEx^a	λEm^b	λmax (Ex)^c	λmax (Em)^d	ΔλS_T^e
EBHMP	324	409	330	415	85
EBHOP	326	406	337	416	79
EBHAP	329	408	345	417	72

^aExcitation wavelength for emission.

^bEmission wavelength for excitation.

^cMaximum excitation wavelength.

^dMaximum emission wavelength.

^eStokes shift.

Evaluation of selectivity

Selectively, sensing capability of oligomers was investigated with 100 equivalent of different metal ions in the buffer solution of DMF/H₂O (1:1, pH: 7.4, fluorophore: 5 μM). The excitation wavelength of emission spectra was fixed at 10 nm apart from that of absorption wavelength. The free fluorophore was investigated before the experiment and then the same concentration of different metal ions Ag⁺, Bi³⁺ Pb²⁺, Al³⁺, Ba²⁺, Cr³⁺, Mg²⁺, Ca²⁺, Na⁺ Ce³⁺, Zn²⁺, Zr²⁺, Co²⁺, Ni²⁺, Mn²⁺, Cd²⁺, Hg²⁺, Li⁺, Cu²⁺, La³⁺, K⁺, Fe³⁺, and Fe²⁺ were used to evaluate the selectivity of imine moiety of the compounds in DMF/H₂O (1:1, fluorophore = 5 μM, pH = 7.4) solution. In the study, the fluorophores are selectively sensing the Fe³⁺. It shows an effective quenching “On-Off” behavior of fluorophores with Fe³⁺ion through the energy or electron transfer processes. Such quenching behavior is due to the unfilled d subshell and paramagnetic property of Fe³⁺ ion.⁴⁰ All the fluorophores have shown well fluorescence quenching ability with Fe³⁺ ion in the selectivity study and the results are shown in Figure 1.

Figure 1.

Selective response of oligomers (100 equv. 1:1, fluorophore = 5 μM, pH = 7.4) with the different metal ions in DMF-H₂O solution.

Competitive analysis with other metals

In the competitive complex analysis, the fluorophore was studied to evaluate the sensitivity and selectivity towards 100 equivalent of Fe³⁺ ion over the presence of other metal ions in 100 equivalent individually. The addition of different metal ions with fluorophore-Fe³⁺ solution shows no significant changes in their quenching intensity. The results indicate the fluorophores can potentially detect the Fe³⁺ without having any interference with other competing metal ions (Cu²⁺, K⁺, La³⁺, Fe²⁺, Fe³⁺, Ba²⁺, Ag⁺, Al³⁺, Bi²⁺ Pb²⁺, Cd²⁺, Hg²⁺, Cr³⁺, Ce³⁺, Li⁺ Zn²⁺, Zr²⁺, Mn²⁺, Mg²⁺, Co²⁺, Ni²⁺, Ca²⁺, and Na⁺). The efficiency results are shown in Figure 2.

Figure 2.

Fluorescence response of oligomers with Fe³⁺ over the presence of other metal ions.

Quenching response towards Fe³⁺ ion concentration

The quenching responses of the fluorophore with different concentration of Fe³⁺ ion was investigated separately in DMF/H₂O solution. Increase in concentration of Fe³⁺ increases the quenching process and the its progress is arrested at around 65 equivalent. The fluorophore quenching is caused by electron or energy transfer and the paramagnetic nature of Fe³⁺ metal ion.⁴¹ Oligomers have shown the spot response and get saturated with less equivalent of Fe³⁺ ion which indicates that the oligomers are more selective and sensitive towards Fe³⁺ than that of monomer EBHMM. The results are shown in Figure 3. While studying the fluorophore with Fe³⁺ ion in the absorption spectrum shows a hypsochromic shift from their original absorption band. The results illustrate the ground state interaction between the fluorophore and metal ion. The blue shift is caused by the strong interaction between imine and Fe³⁺ ions in the solution through the ligand to metal charge transfer process (LMCT) ⁴² in the ground state. The process is confirmed by the comparison of the absorption spectrum of the fluorophore with and without quencher are shown in SF.16.

Figure 3.

Fluorescence quenching response of oligomers with different concentration of Fe³⁺ metal ions (0–100 equiv).

Stern–Volmer plot and limit of detection

The binding stoichiometry of fluorophore with Fe⁺ was investigated with the help of the Stern–Volmer equation given below,

I_{0} / I = 1 + Ksv [Q],

Here, I₀ and I are the fluorescence intensities of fluorophore in absence and presence of the quencher. The Stern–Volmer constant is Ksv and [Q] is the concentration of the quencher.⁴³

The linear line of the Stern–Volmer plot between I₀/I and [Q] indicating the dynamic and dominant diffusion process taken in the fluorophore solution upon the addition of Fe³⁺ ion.⁴⁴ The Stern-Volmer plot of oligomers is depicted in Figure 4. The correlation coefficient value of the Stern-Volmer linear plot was calculated R² = 0.98951, 0.98936, and 0.94734 for EBHMP, EBHOP, and EBHAP, respectively. This Stern–Volmer plot value R² of oligomers are higher than the monomer EBHMM due to the presence of more efficient binding sites in their chain indicating the high capability of qualitative determination of Fe³⁺ ion.

Figure 4.

Stern–Volmer plots of oligomers- Fe³⁺ complex.

The limit of detection (LOD) was calculated using the formula,

LOD = 3.3 σ / k

Where σ is the y-intercept of the regression line and k is the slope of the calibration graph.⁴⁵ The value of LOD obtained for EBHMP, EBHOP, and EBHAP are 2.9 x 10⁻⁸ M, 2.8 x 10⁻⁸ M, and 7.2 x 10⁻⁸ M, respectively.

Benesi–Hildebrand plot

Benesi–Hildebrand plot was used to determine the binding constant (Ka) of fluorophore using the linear fitting curve of Benesi–Hildebrand plot and found as 1.0060 M⁻¹, 1.00537 M⁻¹, and 1.0035 M⁻¹ for EBHMP, EBHOP, and EBHAP, respectively. The binding constant value of oligomers indicates a well binding ability of fluorophore towards Fe³⁺ ion in the sensor study more than that of monomer (1.0034 M⁻¹). The results are shown in Figure 5.

Figure 5.

Benesi–Hildebrand plots of oligomers- Fe³⁺ complex.

Time effect

The spot sensing response capability of fluorophore towards Fe³⁺ ion was studied in DMF/H₂O (quencher = 100 equiv. 1:1, fluorophore = 5 μM, pH = 7.4) solution. The addition of Fe³⁺ ion into the fluorophore solution causes quenching and attained saturation in 4 min with EBHMP and EBHOP, 6 min with EBHAP. This instantaneous quenching in the intensity represents the spot response of fluorophores towards Fe³⁺ ion. Therefore, it can be used for sensing the Fe³⁺ ion in real-time analysis.⁴⁶ The quenching of fluorophore with Fe³⁺ ion increases with increase in time, due to an increase in the number of bindings between fluorophore and quencher. The results of the effect of fluorescence quenching with time on fluorophore-Fe³⁺ are shown in SF.17.

Sensor efficiency in the different pH

The fluorophore with and without Fe³⁺ ion was analyzed at different pH ranges in DMF/H₂O solution (quencher = 100 equiv. 1:1, fluorophore = 5 μM) to get the response towards the detection of Fe³⁺ ion. 0.1 M NaOH and 0.1 M HCl were used to adjust the pH of the fluorophore and FeCl₃ solution from 1 to 13. Except for the neutral medium, the fluorophore shows the electron transition from benzene ring to imine and imine to benzene ring due to the formation of phenoxide ion and imine cation at low and high Ph, respectively.⁴⁷

When the pH of the solution increased, the equilibrium between phenol and phenoxide ion is shifted towards the formation of phenoxide ion and it causes the reduction of the energy gap between the HOMO and LUMO by increasing the conjugation of π electrons. Therefore, less energy is required for the electron transition and the intensity of fluorescence emission is gradually increasing.⁴⁸ The quenching and enhancing of fluorescence at the different pH is indicating the ideal pH for the detection of Fe³⁺ ion. The effect of pH on fluorophore with Fe³⁺ ion was examined and shown in SF.18.

Reversibility of the chemosensors

In the chemosensor, the reversible capability of the fluorophore with metal ions is an essential requirement in the detection of metal ions several times. Find the efficient reversibility of fluorophore with Fe³⁺ was investigated with ethylene diamine tetraacetic acid (EDTA) in DMF/H₂O solution (quencher = 100 equiv. 1:1, fluorophore = 5 μM, pH = 7.4). The intensity of quenched fluorophore by Fe³⁺ was gradually nullified with the addition of EDTA (10–100 equiv.) into the solution. The fluorophore reversibility on the addition of EDTA is shown in Figure 6. The results indicate that the fluorophore can be used for several cycles to detect the Fe³⁺ ion.

Figure 6.

The reversible capability of oligomers-Fe³⁺ with EDTA.

Computational method

The structural characterization of fluorophore and quencher was carried out using density functional theory (DFT). The dimer structure of oligomers was optimized using the B3LYP/6-311G level basic set and used for the analysis of the structural parameters. The optimized structures of oligomers dimer are shown in SF.19.

Frontier molecular orbital analysis

The energy gap value of dimers is analyzed using Frontier molecular orbital analysis. The chemical reactivity, optical polarizability and chemical hardness-softness of the molecules are characterized with the help of Frontier orbital band gap value.⁴⁹

The smaller energy gap value of a molecule indicates good stability with low excitation energy and low chemical hardness. The calculated energy gap values of EBHMP (E_HOMO = −5.7340 eV & E_LUMO = −2.54609 eV), EBHOP (E_HOMO = −5.7164 eV & E_LUMO = −2.6582 eV) and EBHAP (E_HOMO = −5.7706 eV & E_LUMO = −2.6953 eV) are 3.1879 eV, 3.0581 eV and 3.0752 Ev, respectively. Usually, the soft molecules are more polarizable than the hard molecule. The hardness value of molecules can be determined by the formula,⁵⁰

η = \frac{1}{2} (I - A)

Where E_HOMO and E_LUMO are the energies of HOMO and LUMO molecular orbitals. The η value (in a. u) of dimers EBHMP, EBHOP and EBHAP are −1.5939, −1.5290, and −1.5376, respectively. The hardness value of the oligomer’s dimer is lower than the monomer −1.62495. These values are indicating low chemical hardness ⁵¹ and less resistance to the charge transfer process.

The hydrated Fe³⁺ ion in the solution is highly feasible to form Fe(OH)₃ at pH above 7. So, the theoretical study was carried out for the quencher Fe(OH)₃ and found E_HOMO = −6.8395 eV and E_LUMO = −6.6583 eV with a bandgap value of 0.1812 eV. The results clearly illustrated that the metal ions can easily make a bond with azine molecules.

Mechanism of sensing

The study of the ‘turn off’ fluorescence mechanism of fluorophores with 100 equiv. of Fe³⁺ was investigated using UV absorption spectroscopy and DFT study of both quencher and fluorophores. The blue shift of fluorophore-Fe³⁺ from their original absorption band of the fluorophore from 10 to 50 nm indicate that the fluorophore-Fe³⁺complexation is taken place in the ground state through ligand (fluorophore) to metal charge transfer (LMCT) process.⁵²

Furthermore, the Frontier molecular orbital analysis gives the HOMO-LUMO energy value of fluorophores and quencher. The molecular orbital diagram gap of the dimer of oligomers and quencher are shown in SF.20 and SF.21 and the data are given in Table 5.

Table 5.

HOMO, LUMO and band gap value of oligomer’s dimer and quencher.

Compound	I = -Homo	A = -Lumo	Band Gap(eV)	MO.Number
EBHMP	−5.7340	−2.5460	3.1879	217 (O), 218(U)
EBHOP	−5.7164	−2.6582	3.0581	221 (O), 222(U)
EBHAP	−5.7706	−2.6953	3.0752	215 (O), 216(U)
Fe(OH)₃	−6.8395	−6.6582	0.1812	27(O),28(U)

In general, the quencher will act as an electron acceptor when its LUMO value is lower than the fluorophore. So, Fe (OH)₃ received electrons from the higher LUMO of fluorophore through stalling the photoinduced electron transfer (PET) process^53–55 in the fluorophore causes ‘turn off’ fluorescence in the sensor study. The “turn off” fluorescence mechanism of the fluorophore with Fe³⁺ ion is shown in Figure 7.

Figure 7.

“Turn off” fluorescence mechanism of oligomers with Fe³⁺.

Direct current electrical study

Conductivity

The Conductivity study of oligomers with and without doping iodine was measured in the different time from 0 to 80 h. The conductivity of the oligomers is gradually increasing with an increase in the doping time. The I-V Characteristics of oligomers with and without I₂ doping at different time intervals are shown in SF.22.

The extent of iodine doping and conductivity of oligomers also depends on the electron density of imine nitrogen present in the molecule and is shown in SF.23. The charge density of dimer imine nitrogen are in the order EBHAP (−0.2967) > EBHOP (−0.2960) > EBHMP (−0.2954) and is the same as the order of conductivity of the respective oligomers. The conductivity of oligomer was calculated using the formula ⁵⁶ σ = [(I X L)/(V X A)]. Where I is the current, V is the voltage, L is the thickness of the pellet and A is the cross-sectional area of the pellet. A graph is plotted between time and solid-state conductivity values measured at air atmosphere. The contour map with the electron density of the atoms shows in the SF. 24. When the electron pulling iodine coordinate with the electron-emitting nitrogen causes the positive radical (polaron) in the azomethine moiety of the oligomer. This is due to the formation of charge transfer complex by the dopant iodine with imine nitrogen and aromatic π e⁻ system of oligomers upon the removal of HOMO electrons of oligomers and create the positive polarons with the formation of I₃^– counter ion.^57,58 The mode of coordination of iodine with oligomeric nitrogen is given in Scheme 2.

Scheme 2.

Possibility of I₂ coordination with oligomers during doping.

This polaron has expedited the electron flow in the oligomer chain and increases the electrical conductivity. After the 86 h of iodine doping time the measured conductivity values of oligomers are in the order, EBHAP > EBHOP > EBHMP. Among all oligomers, EBHAP has higher electrical conductivity (10⁻⁵ Scm⁻¹) than the other two oligomers (10⁻⁸ Scm⁻¹). The oligomer EBHAP has a lack of electron distribution between the repeating unit through the ester unit but the flexible structure of the oligomer causes the π-π stacking interaction between them causes high electrical conductivity than the other oligomers.⁵⁹

The FT-IR and UV spectra of iodine doped oligomers are shown in SF. 25 and SF.26. The bonds between the atoms in the oligomers are modified by the iodine doping due to the formation of polarons in the structure like –C=N– to –C-N⁺–.⁶⁰ FTIR spectra of iodine-doped oligomers show weak aromatic C-H stretching vibration between 2750 and 2500 cm⁻¹ due to the interaction of iodine with the electron cloud of the benzene ring. Similarly, the appearance of broad stretching vibration between 1600 and 1450 cm⁻¹ is due to the interaction of iodine with –C=C– and –C=N– groups.⁶¹ This indicates the incorporation of an iodine atom in the backbone of the oligomers chain.⁶²

In the UV-visible spectrum, the absorption bands are redshift and appeared at 352, 349, 359 nm. The bandgap value is calculated and found 3.52, 3.56, and 3.45 nm for EBHMP, EBHOP, and EBHAP, respectively. The iodine doping with oligomers reduced band gap value and increases the electrical conductivity. DC conductivity studies confirm that the oligomers can be used as optoelectronic, semiconductive materials in electronic and photovoltaic applications.

Alternating current electrical study

Dielectric property

The temperature and frequency-dependent dielectric loss and dielectric constant of oligomers were measured. The dielectric loss and dielectric constant were plotted with logarithmic frequency and the graphs are shown in SF.27. The dielectric constant of the oligomer was calculated using the formula ⁶³ ɛ_r = Cd/ɛ₀A where C is the capacitance and d is the pellet thickness, A is the cross-sectional area of the pellet and ɛ₀ is the free space permittivity of the pellet.

The dielectric constant of the oligomers is decreasing with an increase in frequency and attained a constant value after a certain point. In lower frequency, oligomers show a high dielectric constant as it is having sufficient time to align with the field before it changes the direction.

But at a higher frequency, oligomers are not having sufficient time to align before the field direction changes and causes a low dielectric constant value. When the temperature is increased at a lower frequency, oligomers exhibit a high dielectric constant value due to minimized intramolecular force between oligomers and increases the thermal agitation of the oligomer chain. The segmental motion of the oligomer chain is frozen at low temperature.

When the temperature is increased at a constant frequency, the dielectric constant value of the oligomers are gradually increased. The analyzed results of oligomers are shown in SF. 28. Among the oligomers, EBHAP exhibit a high dielectric constant value and EBHMP shows a very low dielectric constant value at the constant frequency. EBHAP oligomer has more loosely attached π bonds than other oligomers resulting easy polarization and the dielectric constant value is high. So, EBHAP can be used to make a passive component like resistors and capacitors.⁶⁴ Since the dielectric loss of oligomers is high at low frequency and low at higher frequency regions, they are suitable for electro-optical device applications.⁶⁵

Impedance spectroscopy

The real part Z^′ and imaginary part Z^″ of oligomers impedance are shown in SF. 29. The presence of dipolar and orientational polarization in the oligomeric material, the impedance value of oligomers is higher at lower temperatures, indicating the relaxation process in the oligomer system. The decrease of Z^′ with increase in frequency is attributed to increase in electron mobility between the repeating unit of the oligomer. It can also be seen that the Z^′ remains nearly constant at higher frequencies, implying that the conductivity remains nearly constant at higher frequencies and is ascribed to the release of space charges, which promotes electron mobility.⁶⁶ The imaginary part $(Z^{″})$ and real part $(Z^{'})$ exhibits the same behavior in the impedance study. However, it increases initially with frequency and reaches a maximum at around 3x10² Hz, afterward decreases with increas in frequency and saturates to some constant value and the similar behavior is followed for the subsequent higher temperatures. It shows that there is a decrease in impedance due to increase in the mobility of the polarons.⁶⁷

Conclusion

A new series azine based ester oligomers were designed and synthesized for high-quality chemosensors application to evaluate the Fe³⁺ in the environment as well as in the biological system. The characteristic study of IR, ¹H NMR, ¹³C {¹H} NMR, and ³¹P NMR was used to confirm the structure of oligomers. The TG-DTA study showed that the oligomers have good self-extinguishing behavior. Photophysical properties of the fluorophores were investigated by using UV-Vis and fluorescence spectroscopy. The sensor studies are indicating that the designed fluorophores are exhibiting a highly selective and sensitive “turn off” response towards Fe³⁺ ion. The Stern–Volmer and Benesi–Hildebrand plots exhibit the high stoichiometry and binding constant of the fluorophore with Fe³⁺ resulted from the sensing ability towards Fe³⁺ ion in the real-time analysis than the monomer. Furthermore, the reversible capability of the fluorophore with EDTA shows it can be used for many cycles to sense Fe³⁺ ions. The fluorophore-Fe³⁺ quenching mechanism follows LMCT and PET processes. The enhanced electrical conductivity of iodine doped oligomers is explained with the help of HOMO-LUMO band gap values and absorption spectrum band gap values. I-V characteristic curves of oligomers exhibit that the current linearly increases with an increase in voltage and as well as iodine doping time. Among the iodine doped oligomers, EBHAP iodine doped oligomer has shown higher electrical conductivity of 10⁻⁵ Scm⁻¹ due to higher charge density on imine nitrogen than the other two oligomers. The high dielectric constant value of EBHAP at 373 K is due to easily polarized, loosely attached π bonds in the chain. At room temperature, the real part $(Z^{'})$ and imaginary part $(Z^{″})$ of impedance have shown higher peaks observed in these plots corresponds to a relaxation process. Oligomer EBHAP has a greater impedance value and is highly accomplished to make passive components like resistors and capacitors.

Supplemental Material

sj-pdf-1-hip-10.1177_09540083211055675 – Supplemental Material for Synthesis, characterization, theoretical investigations and fluorescent sensing behavior of oligomeric azine-based Fe³⁺Chemosensors

Supplemental Material, sj-pdf-1-hip-10.1177_09540083211055675 for Improved Synthesis, characterization, theoretical investigations and fluorescent sensing behavior of oligomeric azine-based Fe³⁺Chemosensors by Subramani Manigandan, Athianna Muthusamy, Raju Nandhakumar, Charles Immanuel David and Siddeswaran Anand in High Performance Polymers

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Athianna Muthusamy

Supplemental Material

Supplemental material for this article is available online.

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Supplementary Material

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Synthesis,characterization,theoretical investigations and fluorescent sensing behavior of oligomeric azine-based Fe 3+ Chemosensors

Abstract

Keywords

Introduction

Experimental

Materials

Synthesis of oligomers

Characterization techniques

Optical properties

Theoretical study

Electrical study

Results and discussion

Solubility and molecular weight

Spectral characterizations

Thermogravimetric analysis

Optical properties

Evaluation of selectivity

Competitive analysis with other metals

Quenching response towards Fe3+ ion concentration

Stern–Volmer plot and limit of detection

Benesi–Hildebrand plot

Time effect

Sensor efficiency in the different pH

Reversibility of the chemosensors

Computational method

Frontier molecular orbital analysis

Mechanism of sensing

Direct current electrical study

Conductivity

Alternating current electrical study

Dielectric property

Impedance spectroscopy

Conclusion

Supplemental Material

sj-pdf-1-hip-10.1177_09540083211055675 – Supplemental Material for Synthesis, characterization, theoretical investigations and fluorescent sensing behavior of oligomeric azine-based Fe3+Chemosensors

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

Supplemental Material

References

Supplementary Material

Quenching response towards Fe³⁺ ion concentration

sj-pdf-1-hip-10.1177_09540083211055675 – Supplemental Material for Synthesis, characterization, theoretical investigations and fluorescent sensing behavior of oligomeric azine-based Fe³⁺Chemosensors