Optical Anion Receptors with Urea/Thiourea Subunits on a TentaGel Support.

Two simple chemosensors with urea (L1) and thiourea (L2) groups were synthesized and studied by different spectroscopic techniques. Both receptors can sense acetate (Ac-), dihydrogen phosphate (H2PO4 -), and fluoride (F-) anions, accompanied by changes in UV-vis and 1H NMR spectra, and an optical response is observed as a color change of the solutions due to deprotonation and hydrogen-bonding processes. Also, L1 and L2 were supported on TentaGel resins (R1 and R2), and their fluoride-sensing properties in DMSO and water solutions were studied. Interestingly, R2 can sense fluoride ions in sample solutions of 100% water.

Introduction

In the past few years, work has been carried out to discover and improve analytical methods for a trustful sensing and detection of different species.[1] The detection, differentiation, and visualization of compounds as gases, liquids, and ions are important challenges for the design of optically selective chemosensors.[2] Real-time monitoring of the concentration of anions in aqueous solution and the qualitative determination in a wide range of concentrations are important in environmental and health issues as well as scientific and industrial applications.[3,4]

Among the techniques used for anion detection are electrochemical analysis, ion-selective electrode, and NMR.[5] However, these techniques present disadvantages as they use expensive instruments and require time-consuming and careful manipulation of well-trained technicians. Because of these disadvantages, colorimetric and fluorescent molecules are attractive for the design and study of new simple chemosensors capable of detecting species as anions as they could show great sensibility with detection limits of subparts per million.[6,7]

There are a big number of optical sensors that have been developed for anion detection. The used methods for the sensors depend on strong receptor–anion interactions, like acid–base interactions, electrostatic interactions, and hydrogen bonds, among others.[8−10]

Anion colorimetric sensors in which hydrogen bonding is involved have been designed and studied for many years.[11] Among these sensors are the ones using aromatic rings along with urea and thiourea groups as their NH units act as hydrogen-bond donors to bind to anions and cause a change in the chromogenic properties of the receptor. This can be translated as a color change of solution that can be perceived by the naked eye when anions are present.[12−14]

Some of the challenges to overcome for this kind of chemosensors are solubility and competition with the solvent: the most commonly used are polar aprotic solvents, like dimethyl sulfoxide (DMSO), which make the study of real samples difficult in aqueous solution and biological samples.[13]

One of the strategies used to solve these problems is to support anion receptors in different materials like Tentagel resins.[15] These copolymers, which consist of a polystyrene (PS) and polyethylene glycol (PEG), can be swelled in almost all solvents due to PEG so that the hydrophilic and hydrophobic properties are given to the resin.[16,17]

Herein, we describe the development of two simple urea and thiourea anion receptors (L1 and L2, respectively) containing a phenol and nitrophenyl rings as chromogenic groups. We investigated their anion-sensing properties toward acetate, phosphate, and fluoride anions in DMSO solution through different spectroscopic techniques. TentaGel HL-Br resin was chosen as a solid support, to which the receptors were linked by the reaction of their phenol group with the functional Br group of the resin. The functionalized resins were studied in aqueous and DMSO solutions for proving the fluoride-sensing capacity.

Results and Discussion

The receptors were synthesized in a single step, by the reaction of p-aminophenol with p-nitrophenyl isocyanate (L1) or p-nitrophenyl thiocyanate (L2) (Figure ). The receptors were obtained in quantitative yields, and their purity was verified by different techniques such as melting joint, FT-IR, 1H NMR, 13C NMR, UV–vis, and mass spectrometry. The data are presented in the Experimental Section.

Figure 1

Receptors L1 (left) and L2 (right) and their 1H NMR signal assignments.

Reactivity of Receptors toward Anions

Both receptors L1 and L2 were dissolved in DMSO during UV–vis experiments. The UV–vis spectrum of the free receptor L1 exhibited two π → π* absorption bands with maxima at 300 and 361 nm (for aromatic ring with NO2). These transitions were observed at 274 and 372 nm, respectively, for L2. The anion-binding ability of L1 and L2 toward the anions Cl–, H2PO4–, AcO–, HSO4–, NO3–, and F– was investigated through colorimetric analysis in DMSO.

Figure shows the UV–vis spectra of L1 and L2 receptors in the presence of the different anions studied, along with the photographs of the solutions. In the case of L1, a change in color was observed when F– was added to the receptor solution: the UV band showed a bathochromic shift (361–382 nm) and hypochromic effect, and a new band appeared at 499 nm, resulting in a change in color of the solution from pale yellow to red. When AcO– and H2PO4– anions were added to L1 solution, a bathochromic shift occurred (361–382 nm), and the hyperchromic effect was observed only in the case of AcO–.

Figure 2

Absorbance and color changes of L1 (A) and L2 (B) (5 × 10–5 M) treated with an excess of different guest anions (F–, AcO–, Cl–, and H2PO4–).

For the other studied receptor L2, the presence of Cl–, NO3–, and HSO4– ions did not cause any change in the spectrum (Figure ). The presence of F–, AcO–, and H2PO4– ions caused a bathochromic shift in addition to the appearance of a new band at 497 and 477 nm, respectively, accompanying a color change from pale yellow to purple in the case of F– and to bright yellow in the case of AcO– and H2PO4–. In order to study these changes in detail, UV–vis and 1H NMR titrations, as well as Fourier transform infrared (FT-IR) and theoretical computer studies, were carried out.

UV–Vis Titrations of L1 and L2 toward Anions in DMSO

Titrations of solutions of L1 and L2 were carried out by adding tetrabutylammonium (TBA) salts of F–, AcO–, and H2PO4– in DMSO. Figure B shows that the titration of L1 (5 × 10–5 M) with F– (0.15 M) exhibited a red shift of the absorption band (361–382 nm) as the anion concentration was increased. In addition, a new absorption band appeared at 490 nm due to charge transfer caused by the deprotonation of the NH groups of the receptor L1; this behavior is responsible for the color change of solution.

Figure 3

UV–vis titration in DMSO of (A) L1 (5 × 10–5 M) with TBA acetate (0.075 M). (B) L1 (5 × 10–5 M) with TBA fluoride (0.15 M). (C) L2 (5 × 10–5 M) with TBA acetate (0.018 M). (D) L2 (5 × 10–5 M) with TBA fluoride (0.018 M).

Titration of L1 (5 × 10–5 M) with AcO– (Figure A) and H2PO4– (Figure S4 in Supporting Information) showed a similar red shift from 361 to 380 and 375 nm, respectively, with increasing the anion concentration in the solution. However, no new absorption band appeared at a longer wavelength; this fact suggests that the type of interaction of L1 with AcO– and H2PO4– is hydrogen bonding. L1 showed higher sensibility toward AcO– (Figure A) and H2PO4 because of their geometry[18] as the titrations were performed at a lower concentration of these anions (0.075 M).

Titrations of L2 (5 × 10–5 M) with F– (Figure D), AcO– (0.018 M) (Figure C), and H2PO4– (Figure S5) showed a bathochromic shift (372 to 396, 390, and 375 nm, respectively) and a decrease in the absorbance band. Likewise, a new charge-transfer absorption band appeared at 497 with F– and 477 nm with AcO– and H2PO4– anions, indicating that the protons in the thiourea NH groups are more acidic than in the urea NH groups (L1).[19−21] Titrations of L1 and L2 with F–, and L2 with AcO– show a sigmoidal curve, differing from that predicted for ordinary 1:1-complexation. This observation suggests a positive cooperative binding process due to the presence of more than one binding site in the receptors L1 and L2. From the absorption titration plot, the binding schemes were interpreted by using an interactive data fitting procedure on the basis of eq , in which a compound L is in equilibrium between two states denoted by La and Lb

When a compound (or a receptor) L takes two states La and Lb that have different interactions with a guest ion at the same time in the same conditions (like conformational isomerism, tautomerism, and protonation equilibrium), the equilibrium is controlled by a single constant.

The equilibrium depends on the environmental conditions such as electrolyte and solvent effects. When the equilibrium is displaced by an anion effect, the stimulation is most reasonably expressed by the logistic function, and the concentrations of the two species are formulated as follows

Here, [L]t is the total concentration, [S] is the concentration of simulant (such as F– or AcO–), Ceq is the concentration at equivalent point or [S] at [La] = [Lb], and w is the width of response. The resulting spectral change is given by a weighted average of the intrinsic spectral intensities Ia of La and Ib of Lb at [L]t.

The curve fitting is performed for variables w and Ceq with the Excel spreadsheet. The values of Kism are reported in Table .

Table 1

Formation Constant (K/M–1) Determinate in Solution (DMSO) by UV–vis; the Asterisked Values are Kism

	K
receptor	F	Ac–	H2PO4–
L1	3.59 × 103 ± 3.00%*	3.66 × 103 ± 0.39%	2.54 × 103 ± 0.34%
L2	2.26 × 104 ± 1.06%*	2.79 × 104 ± 1.08%*	3.20 × 103 ± 0.29%

The titrations of L1 with AcO– and H2PO4– and L2 with H2PO4– present a hyperbolic curve in all the cases. The binding constants were calculated with eq based on the formation of 1:1 complex. Table reports the values of K obtained.where Aobs is the observed absorbance, AH is the absorbance of the free ligand, A∞ is the maximum absorbance change induced by the presence of the given guest, [G]T is the total concentration of the guest, [H]T is the total concentration of the ligand, and K is the binding constant.

The binding constants of the systems studied are summarized in Table , which were about 103 and 104. The higher affinities were found with L2; this can be attributed to the lower acidity of the NH hydrogens of thiourea. In both systems, the highest constants were presented by AcO–. However, the response of L1 to F– is selective for color change.

FT-IR Experiments

Besides obtaining FT-IR spectra of both receptors for characterization, the spectra were compared with those of the receptor bound to guest anions F–, AcO–, and H2PO4–.

Notable changes in the spectra were observed when the guest anions interacted with receptor L1 (Figure ). In all the three cases, a wide band at 3432 cm–1 appeared, attributed to formation of hydrogen bonds or deprotonation of the receptor. Also, the C=O band of urea shifted from 1658 to 1710 cm–1.

Figure 4

FT-IR spectra of L1, L1-F–, L1-AcO–, and L1-H2PO4–.

When L2 reacts with guest anions, also a new wide band appears at 3432 and 3460 cm–1, respectively, which represents the interaction between the anions and the receptor. On the other hand, there are three signals of C=S stretching in molecules with thiourea groups. In L2, these signals are at 1107, 1332, and 1597 cm–1. The last signal shifts to 1670 cm–1 when AcO– is present (Figure S6 of Supporting Information).

1H NMR Titrations

1H NMR titrations were performed to clarify the interaction mechanism between the receptors and the guest anions.

Addition of 1 equiv of F– to L1 (Figure ) and L2 (Figure S8) solutions resulted in the disappearance of the OH and NH proton peaks. This spectral change indicates the occurrence of the deprotonation process in the receptors. With increasing F– concentration, peaks corresponding to the phenolic ring protons shifted both upfield and downfield. This can be explained by two coexisting phenomena. First, by deprotonation of the hydroxy group, electron density on the phenolic group increases shielding of its protons and results in the upfield shift. Second, the interaction of the urea/thiourea group with the fluoride ion increases the aromatic ring polarity through space, resulting in the downfield shift.[22] A new triplet is shown at ∼16 ppm that can be associated to the formation of bifluoride ion (HF2–).[23,24]

Figure 5

1H NMR titration of L1 (10 mM) by adding known quantities of TBA fluoride (1 M) in DMSO-d6.

In contrast, addition of AcO– to L1 (Figure ) caused a downfield displacement of NH proton peaks until equilibrium was reached. Also, the OH proton peak moved downfield but disappeared upon addition of 2 equiv of AcO–, indicating deprotonation of this functional group. This behavior was observed also with H2PO4– (Figure S7). Other reported molecules with urea and thiourea groups have similar behavior that indicates the formation of parallel hydrogen bonds between the receptor and the anion.[25,26] Peaks of the aromatic ring protons did not suffer great changes, differing from protons of NH, but presented chemical shift due to the interaction.

Figure 6

1H NMR titration of L1 (10 mM) adding known quantities of TBA acetate (1 M) in DMSO-d6.

Even under the condition that AcO– forms hydrogen bonds with L1, in the case of L2, the addition of 1 equiv. AcO– resulted in the deprotonation of the phenol group and of the NH groups (Figures S9 and S10) as L2 is more acidic than L1 due to the thiourea group.[27]

Computational Study

In order to understand the results obtained, theoretical studies were carried out using the density functional theory with Gaussian 09 software package; OPBE exchange–correlation function was employed along with the 6-311+G(2d,p) basis set. All chemical structures were optimized in DMSO solvent by using a polarizable continuum model (PCM). Transition wavelengths, oscillator strengths, and main contribution were summarized in Table S1, and the calculated absorption spectra are in reasonable agreement with the experimental results, in special for L2. Figure presents the HOMO–LUMO and other important molecular orbitals portrayed for both receptors. The HOMOs in both receptors are mainly distributed on the aromatic ring of the phenol group side and covering their OH and NH (only one) groups, whereas the LUMOs in both receptors mainly cover the nitro group and the o- and p-positions in the aromatic ring with respect to the nitro group, although the HOMO–LUMO energy gaps are approximately equal in both receptors (5.6 eV for L1 and 5.8 eV for L2) and can be related to the first excited states as the first approximation. The results indicate that the main electronic transitions are: HOMO – 1 → LUMO and HOMO – 2 → LUMO, and HOMO → LUMO only for L1, all in singlet excited states. The HOMO – 1 and HOMO – 2 are located near to the LUMO region in both receptors, and the oscillator strength reflects a major contribution of these molecular orbitals to the allowed transitions. Although the absorption wavelengths calculated are displaced from experimental values for L1, the calculated absorption strengths are in reasonable agreement with the experimental results. In Figures S11 and S12, UV–vis spectra calculated for L1 and L2 are presented.

Figure 7

Main molecular orbitals in electronic transitions. For L1: (a) LUMO (−1.7 eV), (b) HOMO (−7.3 eV), (c) HOMO – 1 (−8.3 eV); for L2: (d) LUMO (−1.8 eV), (e) HOMO (−7.6 eV), (f) HOMO – 1 (−7.9 eV), and (g) HOMO – 2 (−8.0 eV). All molecular orbitals are portrayed with 0.04 au (electrons/Bohr[3]) isosurface values of electron density.

In order to understand the binding mechanism of L1 and L2 to guest anions, we explored at the OPBE/6-311+G(2d,p) level the hydrogen bond distances and stretching vibrational frequencies for the interaction of the receptors with the anions, F–, AcO–, H2PO4–, and Cl– in DMSO solvent with PCM. The molecular structures, bond distances, and vibrational frequencies of the NH groups of each receptor were determined. The binding mechanisms between receptors and anions are presented in Figures S13 and S14, and the hydrogen bond distances between the anions and the NH groups are also reported in Table . As we can see from Figures S13 and S14, F– approaches to a hydrogen atom at 1.497 Å for L1 and at 1.501 Å for L2. The distance of L1 is shorter than the other 1.710 Å. The NH group responsible for the shorter distance is bonded to the nitrophenyl group, and hence, this hydrogen atom is the most acidic. In addition, the NH bond distance of this group increases upon hydrogen bonding, and thereby the symmetrical and asymmetrical stretching frequencies move to lower frequencies, Δνsym = −542.5 cm–1 and Δνasym = −1108 cm–1 (see Table ). The same behavior is found for interaction between L2 and F–; in this case, the shifts of the stretching frequencies are Δνsym = −711.7 cm–1 and Δνasym = −1080 cm–1. These shifts of stretching frequencies reflect much weakened NH bonds, particularly the NH bond of the most acidic hydrogen atom which is mainly involved in the normal mode of asymmetrical stretching vibration. It is noteworthy that bond distances of NH groups increase when the receptor interacts with the anion unlike when this is alone. Since the hydrogen bonds are shorter than 2 Å, they can be classified as the category of very strong, which is enough for F– to take a proton of the NH group. In contrast to the preceding results, when Cl– approaches to both receptors, the stretching vibrational frequencies are shifted by Δνsym = −331 cm–1 and Δνasym = −396.5 cm–1 for L1 and Δνsym = −442.1 cm–1 and Δνasym = −377.8 cm–1 for L2. This result indicates that the hydrogen bonds between Cl– and both receptors are moderately strong, in agreement with distances larger than 2 Å, as shown in Table ; the NH bond distance slightly increases when the receptor interacts with the anion unlike when this is alone. For interaction between AcO– and both receptors, the stretching vibrational frequencies are shifted by Δνsym = −659.8 cm–1 and Δνasym = −740.9 cm–1 for L1 and Δνsym = −833.4 cm–1 and Δνasym = −896.8 cm–1 for L2. It should be noted that both shifts in L2 are larger than in L1, and the values are closer to Δνasym shifts found for interaction of F– with L1 and L2, in consistency with the shorter hydrogen bond distances of both oxygen atoms in AcO– that interacts with L2 compared to L1. Also, these hydrogen bonds can be classified as the category of strong, in particular for the interaction between AcO– and L2, and the hydrogen bond distances are close to those with F–. This behavior is similar to that of H2PO4–. According to all these results, it is feasible to support that F– can take a proton of the NH group (i.e., the most acidic proton) of both receptors. Similarly, AcO– and H2PO4– ions, when they interact with L2, can take the two protons of the receptor in the parallel form (Figures S13 and S14).

Table 2

Bond Distances of the NH Groups of the L1 and L2 Receptors, Hydrogen Bond Distances with Each Anion, and Stretching Vibrational Frequencies of the NH Groups of L1and L2 Receptors

receptor	anion	d (anion···HN)/Å	d (H–N)/Å	ν (H–N)/cm–1
L1	F–	1.497 (H26)	1.076 (H26–N11)	2487.7 (asym. stretching)
		1.710 (H25)	1.040 (H25–N7)	3073.9 (sym. stretching)
	Cl–	2.234 (H26)	1.021 (H26–N7)	3199.2 (asym. stretching)
		2.288 (H25)	1.027 (H25–N12)	3285.4 (sym. stretching)
	AcO–	1.736 (O21–H30)	1.050 (H30–N11)	2854.8 (asym. stretching)
		1.766 (O24–H29)	1.045 (H29–N7)	2956.6 (sym. stretching)
	H2PO4–	1.925 (O22–H27)	1.028 (H27–N7)	2854.8 (asym. stretching)
		1.899 (O21–H28)	1.031 (H28–N11)	2956.6 (sym. stretching)
L2	F–	1.515 (H26)	1.073 (H26–N7)	3205.4 (asym. stretching)
		1.610 (H25)	1.054 (H25–N12)	3258.9 (sym. stretching)
	Cl–	2.250 (H26)	1.033 (H26–N11)	3125.6 (asym. stretching)
		2.190 (H25)	1.035 (H25–N7)	3188.3 (sym. stretching)
	AcO–	1.679 (O21–H30)	1.060 (H30–N11)	2670.8 (asym. stretching)
		1.671 (O24–H29)	1.060 (H29–N12)	2732.7 (sym. stretching)
	H2PO4–	1.831 (O22–H27)	1.037 (H27–N7)	3068.6 (asym. stretching)
		1.822 (O21–H28)	1.038 (H28–N11)	3106.0 (sym. stretching)

Proposed Binding Mechanism

Using the data obtained by the different spectroscopic techniques, we propose the binding mechanisms of the receptors toward acetate, phosphate, and fluoride (Scheme ).

Scheme 1

Proposed Binding Mechanisms of (A) L1-F, (B) L1-AcO–, (C) L1-H2PO4–, (D) L2-F–, (E) L2-AcO–, and (F) L2-H2PO4–

As previously mentioned, receptor L1 binds to acetate and phosphate ions via hydrogen bonds. First, an approaching of the guest to the receptor occurs to cause a deprotonation of the phenol groups (Scheme B,C). After deprotonation, the urea group of L1 forms two parallel hydrogen bonds to two oxygen atoms of the acetate anion due to geometric complementarity.[18]

It is possible that L1 binds to fluoride with the same mechanism as L2 binds with acetate, phosphate, and fluoride anions (Scheme A,D,–F). Also, a deprotonation of the phenol group of the receptors takes place when guest anions approach to the receptor. Then, the anions bring about deprotonation of both NH groups of the receptors. This deprotonation depends on the acidity of the receptors due to electron-withdrawing properties of the functional groups.[18]

Synthesis and Colorimetric Detection of the Functionalized TentaGel Resins

In order to ensure practical applicability of the receptors L1 and L2, they were supported on the TentaGel resin and evaluated if the sensor response observed in solution is maintained. The synthesis of two functionalized resins was achieved: one resin with receptor L1 (R1) and the other with receptor L2 (R2). There was a color change of the functionalized resins with respect to the unfunctionalized TentaGel resin from light yellow to bright yellow (Figure S15); this indicates that the receptors react with the TentaGel resin free of functional groups.[15] The characterization of the resins was carried out by FT-IR and colorimetric tests.

The detection test for the anions, which showed colorimetric changes in solution, was carried out in DMSO and water. One milligram of resin (R1 or R2) was placed in 100 μL of solvent to allow the material to swell, and subsequently, 10 μL of the salt solution in water was added. Unfortunately, R1 did not show any response by adding the anion solutions in the concentration range evaluated. In contrast, resin R2 showed a color change response when the anion solutions of AcO–, H2PO4–, or F– were added. The intensity of the color increased by increasing the anion concentration from bright yellow to red (Figure S16). Interestingly, resin R2 showed a color change response by being swelled only with water (Figure ). The same color change was observed in solution, but it was necessary to increase the concentration by ten times with respect to the use in solution. The sensory response is considered outstanding since in solution, it was not possible to evaluate in a 100% aqueous medium in which the receptor was not soluble. One of the advantages of supporting to resins is that the receptors can be evaluated in other solvents, otherwise, it is not possible to do it in solution. The decrease in receptor response can be attributed to the differences in availability of the groups due to the surface effect such as the proximity to neighboring urea group or thiourea group to the polyethylene groups of PEG chain of the resins, leading to intramolecular hydrogen bonding between the urea NH protons and oxygen of the PEG.[12] In the case of R2, as the N–H are more acidic, it allows a better response. The effect of pH was also evaluated, and a color change response was found in solutions with a pH greater than 9.

Figure 8

Sensor resin with receptor L2 (R2), and its qualitative detection of TBA fluoride in water. (A) No fluoride, (B) 0.2 mM, (C) 0.4 mM, (D) 0.6 mM, (E) 0.8 mM, (F) 1 mM, (G) 2.5 mM, and (H) 5 mM.

Conclusions

Two simple chemosensors with urea/thiourea subunits, L1 and L2, to detect AcO–, H2PO4–, and F– were synthesized and studied.

The data obtained by FT-IR, UV–vis, and 1H NMR spectra were important for proposing the interaction mechanisms of both receptors. L1 binds to AcO– and H2PO4– by forming hydrogen bonds with the urea NH groups. However, F– deprotonates the phenol and NH groups of receptor L1. Likewise, AcO–, H2PO4–, and F– deprotonate receptor L2. Color changes in the receptor solution were observed more markedly with F–.

Both receptors were attached in TentaGel resins, and their sensing properties were qualitatively studied. No response was observed for R1, but R2 showed very high sensor response both in DMSO and water, mainly toward F–, with a change in color from bright yellow to dark red. This event is comparable to that observed for the receptors in solution.

These results may be helpful to continue studying different strategies to create new and better anion sensors supported on solid materials for overcoming the water solubility and competition challenges present in this kind of chemosensors.

Experimental Section

General

Absorption spectra were measured at room temperature on a PerkinElmer LAMBDA 20 UV–Vis spectrometer. NMR spectra were recorded at 25 °C on a Bruker AVANCE 400 spectrometer in DMSO-d6. FT-IR spectra were obtained on a PerkinElmer FT-IR spectrometer model Frontier. Mass spectra of electrospray ionization (ESI/MS) were obtained on 6130 Quadrupole LC/MS of Agilent Technologies. All reagents were obtained from commercial suppliers and used without further purification.

Synthesis of 1-(4-Hydroxyphenyl)-3-(4-nitrophenyl) Urea (L1)

Synthesis of L1 was carried out by adding p-aminophenol and p-nitrophenylisocyanate with a 1:1 stoichiometry into a reaction flask using dry dichloromethane (DCM) as the solvent. The mixture was left in agitation for 24 h at room temperature. Afterward, the product was obtained by precipitation and it was washed using acetone. The product was dried in a vacuum oven for 5 h. 90% yield, mp of 226–227 °C, ESI/MS: 317.8.

1H NMR (400 MHz, DMSO-d6): 6.71 (d, J = 8.32 Hz, 2H, ArHb), 7.24 (d, J = 8.32 Hz, 2H, ArHc), 7.67 (d, J = 8.56 Hz, 2H, ArHf), 8.18 (d, J = 8.56 Hz, 2H, ArHg), 8.64 (s, 1H, ArOHa), 9.20 (s, 1H, NHd), 9.37 (s, 1H, NHe).

13C NMR (400 MHz, DMSO-d6): 115.72, 117.68, 121.37, 125.58, 130.77, 141.17, 147.19, 152.61, 153.59.

FT-IR (KBr) cm–1: 841, 1227, 1330, 1460, 1560, 1615, 1658, 3300, 3371.

Synthesis of 1-(4-Hydroxyphenyl)-3-(4-nitrophenyl)thiourea (L2)

L2 was synthesized by adding p-aminophenol and p-nitrophenylisothiocyanate with a 1:1 stoichiometry into a reaction flask using dry DCM as the solvent. The mixture was left in agitation for 24 h at room temperature. The precipitated product was washed using ACN and dried in a vacuum oven for 5 h. 90% yield, mp of 146–147 °C, ESI/MS: 307.8.

1H NMR (400 MHz, DMSO-d6): 6.75 (d, J = 8.12 Hz, 2H, ArHb), 7.21 (d, J = 8.12 Hz, 2H, ArHc), 7.82 (d, J = 8.72 Hz, 2H, ArHf), 8.19 (d, J = 8.72 Hz, 2H, ArHg), 9.48 (s, 1H, ArOHa), 10.07 (s, 1H, NHd), 10.22 (s, 1H, NHe).

13C NMR (400 MHz, DMSO-d6) 115.63, 121.90, 124.75, 126.57, 130.50, 142.56, 147.00, 155.61, 179.87.

FT-IR (KBr) cm–1: 850, 1260, 1330, 1560, 1609, 3300, 3272.

Synthesis of Sensor Resins

The sensor resins were synthesized by adding TentaGel HL-Br resin (loading: 0.48 mmol/g) and L1/L2 with a 1:1.1 stoichiometry into a reaction flask. The reaction was performed in basic media (triethylamine) using DMSO as the solvent. Afterward, the mixture was left in agitation for 24 h at room temperature. After agitation, the resin was filtered and washed using ACN to get rid of the non-reacted receptor. The sensor resin was dried in a vacuum oven for 5 h.

Resin with L1 FT-IR cm–1: 2869, 1456, 1343, 1298, 1237, 1090, 947, 841, 756, 698, 1716, 1699, 1555, 1541.

Resin with L2 FT-IR cm–1: 2863, 1597, 1557, 1349, 1287, 1240, 843, 752, 695.

UV–Vis Experiments

The solutions of the receptors L1 and L2 and the guest anions were prepared in DMSO due to receptor solubility. Volume of the receptor solutions used in the UV–vis experiments was 3 mL. Absorption spectra were recorded by adding different amounts of anion solution of a known concentration into the receptor solution.

FT-IR spectra of the receptors within a KBr pellet were obtained. Likewise, FT-IR spectra of the guest anions were obtained. FT-IR spectra of receptors attached to TentaGel resins were obtained through ATR.

1H NMR Experiments

The solution of the receptors (10 mM in DMSO-d6) was titrated by adding known quantities of solution of TBA salts of fluoride (F–), monobasic phosphate (H2PO4–), and acetate (AcO–) (1 M). The chemical shift changes of the receptors were monitored.

Computational Studies

All structure calculations were performed with the software package Gaussian 09 (G09).[28] For this work, the OPBE exchange–correlation functional[29] was employed along with the 6-311+G(2d,p) basis set. The chemical structures were optimized in DMSO solvent using a PCM.[30] Frequency analysis confirmed that all optimized structures correspond to the global minima in the potential energy surface. Furthermore, time-dependent DFT calculations as implemented in G09[31,32] with CAM-B3LYP[33,34] were carried out for evaluating the first four and five singlet and triplet excited states in both L1 and L2 receptors; this was performed taking the optimized molecular geometries and using the same basis set and DMSO solvent.