Fluoride Anion Recognition by a Multifunctional Urea Derivative: An Experimental and Theoretical Study

In this work we demonstrate the ability of a multifaceted N,N′-disubstituted urea to selectively recognize fluoride anion (F−) among other halides. This additional function is now added to its already reported organocatalytic and organogelator properties. The signaling mechanism relies on the formation of a charge-transfer (CT) complex between the urea-based sensor and F¯ in the ground state with a high association constant as demonstrated by absorption and fluorescence spectroscopy. The nature of the hydrogen bonding interaction between the sensor and F¯ was established by 1H-NMR studies and theoretical calculations. Moreover, the recovery of the sensor was achieved by addition of methanol.


Introduction
Anion sensing and, as consequence, the design, synthesis and development of new sensors constitute nowadays a highly active research field [1][2][3][4][5]. It is well established that anion-sensor coordination takes place usually by hydrogen bonding and/or electrostatic interactions. Among others, sensors with an optical response (i.e., fluorescence) have been found to be the most suitable and attractive tools for anion recognition mainly due to their high sensitivity at low analyte concentration [6]. Within this context, different signaling mechanisms such as photoinduced electron transfer (PET) [7][8][9][10][11], excimer/exciplex formation [12,13], intramolecular charge transfer (ICT) [14,15], and excited-state proton transfer [16,17] have been previously reported in the literature.
After Wilcox [18] and Hamilton [19] discovered the interaction between urea-derivatives and phosphonates, sulphates or carboxylates forming stable 1:1 complexes, the urea moiety has played an essential role as one of the most suitable binding sites in the vast field of anion receptor chemistry. In general, recognition and detection of other anions such as fluoride (F¯) have attracted considerable interest because of its established role in dental care [20], treatment of osteoporosis [21] and its association with chemical weapons (e.g., nerve gases such as sarin, soman and GF chemical warfare The absorption spectrum of sensor 1 in the absence of halide anions was first measured in different solvents and showed two bands that corresponded to the two characteristic electronic transitions, ππ* at higher energy and nπ* transition at lower energy. Table 1 summarizes the photophysical data obtained for compound 1 in different solvents (for a comparison with various urea analogues, see Figures S2-S9 and Tables S1-S3). A red-shifted emission of 21 nm as well as an increase in the Stokes shift values when increasing solvent polarity (DMSO vs. CHCl3) was observed. On the other hand, a protic solvent provided practically the same results than a polar aprotic solvent (MeOH vs. MeCN, respectively). These results suggested an enhancement of the overall dipole moment on excitation albeit almost independent of the solvent.

Materials
All commercially available solvents and reagents for synthesis and analysis (p.a. grade) were used as received. Urea derivatives were synthesized and characterized as previously described and showed identical spectroscopic data to those reported [44]. Briefly, urea 1 was easily synthesized by an equimolar reaction of commercial (1S,2R)-1-amino-2,3-dihydro-1H-inden-2-ol and 3,5-bis(trifluoromethyl)phenyl isocyanate in methylene chloride at room temperature [44]. The organogel was prepared as following: urea 1 (8 mg, 0.02 mmol) was weighed and placed into a screw cap vial (4.5 cm lengthˆ1.2 cm diameter) and 1 mL of CHCl 3 was added. The closed vial was gently heated with a heat gun until the solid compound was completely dissolved. The obtained isotropic solution was cooled down spontaneously to room temperature affording the gel formation. No flow of the material was observed upon turning the vial upside-down at room temperature.

Absorption and Fluorescence Spectroscopy
Absorption spectra were recorded using a Cary 50 Bio UV-visible spectrophotometer (Varian, Palo Alto, CA, USA). Fluorescence and excitation spectra were carried out using a Fluoromax-4 spectrofluorometer (Horiba, Kyoto, Japan). The excitation and emission slit widths were 5 nm. The samples were placed into quartz cells of 1 cm path length. All measurements were performed at room temperature and compound concentrations were fixed as indicated.

Theoretical Calculations
Chemical structures were optimized with the GAUSSIAN09 program package [46] and the density functional theory (DFT) method. B3LYP functional [47] together with the standard basis set 6-31G(d) [48] and the CPCM-SCRF method [49,50] were used to model the solvent (acetonitrile). NMR shifts were computed on the optimized structures using the GIAO method [51].

NMR Spectroscopy
NMR spectra were recorded at 400 MHz on an AVANCE-II instrument (Bruker, Billerica, MA, USA).

Electron Microscopy
Electron microscopy images were obtained with a Merlin field emission scanning electron microscope (FESEM, resolution = 0.8 mm resolution, Carl Zeiss, Jena, Germany) equipped with a digital camera and operating at 10 kV (accelerating voltage) and 10 mA (emission current). The sample was prepared by freeze-drying the corresponding organogel. Prior to imaging, a 5 nm sized Pt film was sputtered (40 mA, 30 s) on the sample placed on carbon tape.

Results and Discussion
As mentioned above, urea-based compound 1 can not only function as a good organocatalyst for Friedel-Crafts alkylations [42,43], but also as a molecular building block for the bottom-up preparation of multiresponsive supramolecular gels in several organic solvents at concentrations ranging from 3 to 50 g¨L´1 ( Figure 1) [44]. During our preliminary studies we observed that a gel made from 1 in chloroform (c = 8 g¨L´1) collapsed in the presence of fluoride anions ( Figure 1). Similar behavior has been already reported with other gels, in which the addition of halide anions causes the disruption of intermolecular hydrogen bonding leading to either gel-to-sol transitions and/or colorimetric gel-to-gel transitions [52][53][54][55][56][57][58]. Furthermore, changes in the absorption spectra of 1 in the presence of halides in chloroform solution clearly appeared ( Figure S1), confirming the host-guest binding in all cases. At this point, we decided to focus our attention on the complete photophysical characterization of the potential molecular interaction between 1 and halide anions, as well as on the anion sensing signaling mechanism and selectivity of the process.
The absorption spectrum of sensor 1 in the absence of halide anions was first measured in different solvents and showed two bands that corresponded to the two characteristic electronic transitions, ππ* at higher energy and nπ* transition at lower energy. Table 1 summarizes the photophysical data obtained for compound 1 in different solvents (for a comparison with various urea analogues, see Figures S2-S9 and Tables S1-S3). A red-shifted emission of 21 nm as well as an increase in the Stokes shift values when increasing solvent polarity (DMSO vs. CHCl 3 ) was observed. On the other hand, a protic solvent provided practically the same results than a polar aprotic solvent (MeOH vs. MeCN, respectively). These results suggested an enhancement of the overall dipole moment on excitation albeit almost independent of the solvent. Having established the photophysical data of compound 1 in different solvents, we tested its ability to recognize halide anions in a suitable solvent. In this sense, DMSO possesses an absorption band in the range of 200-280 nm, which would interfere with the excitation in the fluorescence experiments (250 nm). MeOH was discarded because it is usually used for checking the reversibility of the recognition process. Thus, we decided to do the next studies in MeCN because it is a relatively inert solvent without any absorption band in the region between 210 and 600 nm and it is commonly used in photochemical/photophysical investigations. On the other hand, we used tetrabutylammonium (TBA) halides for our studies because they have good solubility in MeCN and TBA is one of the most common counterions used in this field [59][60][61]. Other counterions such as K + and Na + were not suitable for these studies because they can also be coordinated with the urea moiety [61].
The UV-vis absorption spectra of 1 in MeCN in the absence of F¯showed two bands centered at 249 nm and 290 nm. Upon titration with F¯, the ground state was affected; a bathochromic shift due to anion recognition on the two absorption maxima together with the appearance of an isosbestic point at 252 nm, clearly pointed out to the formation of new species ( Figure 2). The reverse was true for other halide anions (i.e., Cl¯, Br¯, I¯) as slight-to-negligible changes in the UV-vis spectra in MeCN were observed upon addition of these species ( Figure S10).
Sensors 2016, 162016, 16,658 Having established the photophysical data of compound 1 in different solvents, we tested its ability to recognize halide anions in a suitable solvent. In this sense, DMSO possesses an absorption band in the range of 200-280 nm, which would interfere with the excitation in the fluorescence experiments (250 nm). MeOH was discarded because it is usually used for checking the reversibility of the recognition process. Thus, we decided to do the next studies in MeCN because it is a relatively inert solvent without any absorption band in the region between 210 and 600 nm and it is commonly used in photochemical/photophysical investigations. On the other hand, we used tetrabutylammonium (TBA) halides for our studies because they have good solubility in MeCN and TBA is one of the most common counterions used in this field [59][60][61]. Other counterions such as K + and Na + were not suitable for these studies because they can also be coordinated with the urea moiety [61].
The UV-vis absorption spectra of 1 in MeCN in the absence of F¯ showed two bands centered at 249 nm and 290 nm. Upon titration with F¯, the ground state was affected; a bathochromic shift due to anion recognition on the two absorption maxima together with the appearance of an isosbestic point at 252 nm, clearly pointed out to the formation of new species ( Figure 2). The reverse was true for other halide anions (i.e., Cl¯, Br¯, I¯) as slight-to-negligible changes in the UV-vis spectra in MeCN were observed upon addition of these species ( Figure S10).
The corresponding absorbance/concentration plot is shown in Figure 3. From Equation (1), AbsCT and εCT represent the absorbance due to the CT band at 300 nm at different concentrations of F¯ and the molar absorption coefficient of the CT complex, respectively. A value of 3721 M −1 · cm −1 ± 340 M −1 · cm −1 for the εCT was obtained from the intercept in acetonitrile (log εCT = 3.6). Consequently, the corresponding KCT (slope) was found to be 4826 M −1 ± 111 M −1 . This moderate-high value of KCT The absorbance intensity of the ππ* transition decreased, whereas nπ* transition band remarkably enhanced. These changes confirmed the formation of a new species, named charge-transfer (CT) complex with new photophysical properties after the anion recognition. To examine the formation of the CT complex, difference spectra (i.e., Abs [1+F¯] -Abs 1 ) were obtained. The results showed the emerging of a new band at ca. 310 nm (Figure 2, inset), which was attributed to the CT complex absorption maximum. The formation constant of a CT complex (K CT ) was estimated spectro-photometrically by the Benesi-Hildebrand procedure (Equation (1)) [62]: The corresponding absorbance/concentration plot is shown in Figure 3. From Equation (1), Abs CT and ε CT represent the absorbance due to the CT band at 300 nm at different concentrations of F¯and the molar absorption coefficient of the CT complex, respectively. A value of 3721 M´1¨cm´1˘340 M´1¨cm´1 for the ε CT was obtained from the intercept in acetonitrile (log ε CT = 3.6). Consequently, the corresponding K CT (slope) was found to be 4826 M´1˘111 M´1. This moderate-high value of K CT indicates a strong intermolecular interaction between 1 and F¯in the ground state. This behavior was not observed in the presence of other halides such as Cl¯, Br¯and I( vide infra).  Figure S11 and Figure S12), which demonstrated the preference of sensor 1 for F¯. Furthermore, the detection limit of fluoride anion was established at around 0.003 mg·L −1 (see ESI, Figure S13); this fluoride concentration is categorized as low-fluoride water (up to 0.5 mg·L −1 ) [63]. Other related ureas, albeit lacking gelation ability, were found to be also suitable for sensing selectively fluoride anion among other halides ( Figure S14). These results suggest that, at least in this case, the self-assembly tendency of the sensor does neither hinder nor favor its anion sensing properties. In addition, sensor 1 showed high affinity for other anions such acetate and phosphate, while the reverse was true for hydrosulfate ( Figure S15).
A static fluorescence quenching of sensor 1 seemed to occur since the Stern-Volmer plot presented a non-linear behavior at high amounts of fluoride ( Figure S16). For comparison, a strong non-nucleophilic base such as 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU) was also used. In this case, fluorescence quenching followed a clearly fitted-linearity, in contrast than the one found for fluoride ( Figure S16). In addition, 1 H-NMR experiments showed that urea protons completely disappeared in the presence of DBU ( Figure S17). Therefore, the different effects obtained for the emission and 1 H-NMR of 1 in the presence of DBU and fluoride, support the formation of a CT complex between sensor 1 and fluoride.  Figure S11 and Figure S12), which demonstrated the preference of sensor 1 for F¯. Furthermore, the detection limit of fluoride anion was established at around 0.003 mg¨L´1 (see ESI, Figure S13); this fluoride concentration is categorized as low-fluoride water (up to 0.5 mg¨L´1) [63]. Other related ureas, albeit lacking gelation ability, were found to be also suitable for sensing selectively fluoride anion among other halides ( Figure S14). These results suggest that, at least in this case, the self-assembly tendency of the sensor does neither hinder nor favor its anion sensing properties. In addition, sensor 1 showed high affinity for other anions such acetate and phosphate, while the reverse was true for hydrosulfate ( Figure S15).
A static fluorescence quenching of sensor 1 seemed to occur since the Stern-Volmer plot presented a non-linear behavior at high amounts of fluoride ( Figure S16). For comparison, a strong non-nucleophilic base such as 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU) was also used. In this case, fluorescence quenching followed a clearly fitted-linearity, in contrast than the one found for fluoride ( Figure S16). In addition, 1 H-NMR experiments showed that urea protons completely disappeared in the presence of DBU ( Figure S17). Therefore, the different effects obtained for the emission and 1 H-NMR of 1 in the presence of DBU and fluoride, support the formation of a CT complex between sensor 1 and fluoride. non-nucleophilic base such as 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU) was also used. In this case, fluorescence quenching followed a clearly fitted-linearity, in contrast than the one found for fluoride ( Figure S16). In addition, 1 H-NMR experiments showed that urea protons completely disappeared in the presence of DBU ( Figure S17). Therefore, the different effects obtained for the emission and 1 H-NMR of 1 in the presence of DBU and fluoride, support the formation of a CT complex between sensor 1 and fluoride.   In order to establish the formation of [1---F]¯complex, steady-state fluorescence measurements were performed with a solution of 1 and F¯in acetonitrile ( Figure 5). Upon selective CT complex excitation at 320 nm (sensor 1 does not absorb in this region), its emission was detected, with a maximum at ca. 468 nm. In good agreement with the UV-absorption measurement, the corresponding excitation spectrum showed a maximum at 308 nm.
Sensors 2016, 162016, 16,658 In order to establish the formation of [1---F]¯ complex, steady-state fluorescence measurements were performed with a solution of 1 and F¯ in acetonitrile ( Figure 5). Upon selective CT complex excitation at 320 nm (sensor 1 does not absorb in this region), its emission was detected, with a maximum at ca. 468 nm. In good agreement with the UV-absorption measurement, the corresponding excitation spectrum showed a maximum at 308 nm.  Figure S18). In the presence of increasing equivalents of F¯, the urea resonances were gradually shifted to downfield by ca. 2 ppm and ca. 4 ppm, respectively, reflecting H-bond formation between receptor and anion. In addition, some effects on aromatic substituents (for instance, polarization-induced of the C-H bonds via a through-space effect) obtaining downfield shift due to deshielding effect by partial positive charge formed onto the proton [32], could be induced by this H-bonding interaction. In fact, this electrostatic effect was also detected in aromatic protons H 2 and H 3 showing a weak downfield shift upon addition of F¯ equivalents ( Figure S18), in good correlation with literature data [30,60]. Moreover, deprotonation was not involved in the signaling mechanism, as reflected by the incomplete disappearance of the Ha signal, even at high anion concentration. This suggests that the N-H amide bonding length increased during the anion recognition, which was confirmed by the multiplicity of H 1 , which changed from a double doublet in the absence of F¯ to a doublet upon addition of one equivalent of F¯ ( Figure S18). Furthermore, the effect of protic solvent on the fluorescence of [1---F]¯ complex also confirmed the H-bonding nature of this interaction. The addition of methanol to a mixture of 1 and F¯ in acetonitrile  Figure S18). In the presence of increasing equivalents of F¯, the urea resonances were gradually shifted to downfield by ca. 2 ppm and ca. 4 ppm, respectively, reflecting H-bond formation between receptor and anion. In addition, some effects on aromatic substituents (for instance, polarization-induced of the C-H bonds via a through-space effect) obtaining downfield shift due to deshielding effect by partial positive charge formed onto the proton [32], could be induced by this H-bonding interaction. In fact, this electrostatic effect was also detected in aromatic protons H 2 and H 3 showing a weak downfield shift upon addition of F¯equivalents ( Figure S18), in good correlation with literature data [30,60]. Moreover, deprotonation was not involved in the signaling mechanism, as reflected by the incomplete disappearance of the H a signal, even at high anion concentration. This suggests that the N-H amide bonding length increased during the anion recognition, which was confirmed by the multiplicity of H 1 , which changed from a double doublet in the absence of F¯to a doublet upon addition of one equivalent of F¯( Figure S18). Furthermore, the effect of protic solvent on the fluorescence of [1---F]¯complex also confirmed the H-bonding nature of this interaction. The addition of methanol to a mixture of 1 and F¯in acetonitrile led to the recovery of the emission band (Figure 6), which clearly supported the reversibility of the process by the interaction between the urea moiety and the protic solvent (see Figure S17 for additional 1

H-NMR experiments).
Sensors 2016, 162016, 16, 658 [60,64] for complexation of analogous compounds with F¯ ions. Similar results were obtained for the analogous ureas (Figures S19-S21). Moreover, both N-Ha and N-Hb GIAO-NMR shifts (δ) were also calculated in the absence/presence of fluoride anion. Upon addition of F¯, a strong shift to downfield was found for both hydrogen atoms, especially for Hb. The computed signal for Ha appeared at 9 ppm, while the computed signal for Hb appeared at 13 ppm. These data are in good agreement with those observed experimentally. Thus, the qualitative picture of the molecular recognition is well represented by the computational data. Overall, these computational results are in concordance with experimental observations where the formation of a complex between sensor 1 and F¯ in the ground state prevails over a possible deprotonation of the urea moiety.

Conclusions
In summary, urea 1 was found to selectively recognize fluoride anion among other halides as demonstrated by means of absorption as well as fluorescence spectroscopic data. The signaling mechanism relies on the formation of a CT complex between sensor 1 and F¯ in the ground state ( Figure 8). In this sense, upon recognition of the anion, new absorption and fluorescence bands of The foregoing experimental results pointed out that the signaling mechanism involved a CT complex formation through H-bonding interaction between 1 and F¯. To further support this conclusion, computational calculations at the B3LYP/6-31G(d) level of theory using the CPCM method (acetonitrile as solvent) were carried out. Optimization of the geometries in the absence and presence of F¯was performed (Figure 7) and the corresponding N-H bond distances were calculated. The values for N-H a and N-H b bond lengths without anion were found to be 1.017 Å and 1.010 Å, respectively. After fluoride binding complex optimization, the values of these bond distances were 1.043 Å for N-H a bond and 1.081 Å for N-H b bond. Although, bond-elongation was observed in both cases, it seemed not to be sufficient for hydrogen abstraction by F¯. The H---F distances were 1.686 Å (H a ---F) and 1.467 Å (H b ---F), which were close to the experimental values reported in the literature [60,64] for complexation of analogous compounds with F¯ions. Similar results were obtained for the analogous ureas ( Figures S19-S21).
Moreover, both N-H a and N-H b GIAO-NMR shifts (δ) were also calculated in the absence/presence of fluoride anion. Upon addition of F¯, a strong shift to downfield was found for both hydrogen atoms, especially for H b . The computed signal for H a appeared at 9 ppm, while the computed signal for H b appeared at 13 ppm. These data are in good agreement with those observed experimentally. Thus, the qualitative picture of the molecular recognition is well represented by the computational data. Overall, these computational results are in concordance with experimental observations where the formation of a complex between sensor 1 and F¯in the ground state prevails over a possible deprotonation of the urea moiety. both hydrogen atoms, especially for Hb. The computed signal for Ha appeared at 9 ppm, while the computed signal for Hb appeared at 13 ppm. These data are in good agreement with those observed experimentally. Thus, the qualitative picture of the molecular recognition is well represented by the computational data. Overall, these computational results are in concordance with experimental observations where the formation of a complex between sensor 1 and F¯ in the ground state prevails over a possible deprotonation of the urea moiety.

Conclusions
In summary, urea 1 was found to selectively recognize fluoride anion among other halides as demonstrated by means of absorption as well as fluorescence spectroscopic data. The signaling mechanism relies on the formation of a CT complex between sensor 1 and F¯ in the ground state ( Figure 8). In this sense, upon recognition of the anion, new absorption and fluorescence bands of this CT complex at longer wavelengths were clearly detected together with a high association constant. The nature of the H-bonding interaction between 1 and fluoride was unambiguously proven by 1 H-NMR studies and theoretical calculations. Finally, the recovery of the sensor was achieved using protic solvents such as methanol.

Conclusions
In summary, urea 1 was found to selectively recognize fluoride anion among other halides as demonstrated by means of absorption as well as fluorescence spectroscopic data. The signaling mechanism relies on the formation of a CT complex between sensor 1 and F¯in the ground state ( Figure 8). In this sense, upon recognition of the anion, new absorption and fluorescence bands of this CT complex at longer wavelengths were clearly detected together with a high association constant. The nature of the H-bonding interaction between 1 and fluoride was unambiguously proven by 1 H-NMR studies and theoretical calculations. Finally, the recovery of the sensor was achieved using protic solvents such as methanol.  Supplementary Materials: The following are available online at www.mdpi.com/link: Figure S1: Absorption spectra of 1 (0.04 mM) in chloroform in the presence of increasing amounts of the corresponding halide. Figure S2: Absorption spectra of 1 (0.01 mM) in chloroform, acetonitrile, methanol and dimethylsulfoxide at room temperature under aerobic conditions. Figure S3: Normalized excitation and emission spectra of 1 (0.01 mM) in chloroform, acetonitrile, methanol and dimethylsulfoxide at room temperature under aerobic conditions. Figure S4: Absorption spectra of 2 (0.01 mM) in dichloromethane, acetonitrile, methanol and dimethylsulfoxide at room temperature under aerobic conditions. Figure S5: Normalized excitation and emission spectra of 2 (0.01 mM) in dichloromethane, acetonitrile, methanol and dimethylsulfoxide at room temperature under aerobic conditions. Figure S6: Absorption spectra of 3 (0.01 mM) in dichloromethane, acetonitrile, methanol and dimethylsulfoxide at room temperature under aerobic conditions. Figure S7: Normalized excitation and emission spectra of 3 (0.01 mM) in dichloromethane, acetonitrile, methanol and dimethylsulfoxide at room temperature under aerobic conditions. Figure S8: Absorption spectra of 3 (0.01 mM) in dichloromethane, acetonitrile, methanol and dimethylsulfoxide at room temperature under aerobic conditions. Figure S9: Normalized excitation and emission spectra of 3 (0.01 mM) in dichloromethane, acetonitrile, methanol and dimethylsulfoxide at room temperature under aerobic conditions. Table S1: Photophysical data of sensor 2 Supplementary Materials: The following are available online at www.mdpi.com/link: Figure S1: Absorption spectra of 1 (0.04 mM) in chloroform in the presence of increasing amounts of the corresponding halide. Figure S2: Absorption spectra of 1 (0.01 mM) in chloroform, acetonitrile, methanol and dimethylsulfoxide at room temperature under aerobic conditions. Figure S3: Normalized excitation and emission spectra of 1 (0.01 mM) in chloroform, acetonitrile, methanol and dimethylsulfoxide at room temperature under aerobic conditions. Figure  S4: Absorption spectra of 2 (0.01 mM) in dichloromethane, acetonitrile, methanol and dimethylsulfoxide at room temperature under aerobic conditions. Figure S5: Normalized excitation and emission spectra of 2 (0.01 mM) in dichloromethane, acetonitrile, methanol and dimethylsulfoxide at room temperature under aerobic conditions. Figure S6: Absorption spectra of 3 (0.01 mM) in dichloromethane, acetonitrile, methanol and dimethylsulfoxide at  Figure S7: Normalized excitation and emission spectra of 3 (0.01 mM) in dichloromethane, acetonitrile, methanol and dimethylsulfoxide at room temperature under aerobic conditions. Figure S8: Absorption spectra of 3 (0.01 mM) in dichloromethane, acetonitrile, methanol and dimethylsulfoxide at room temperature under aerobic conditions. Figure S9: Normalized excitation and emission spectra of 3 (0.01 mM) in dichloromethane, acetonitrile, methanol and dimethylsulfoxide at room temperature under aerobic conditions. Table S1: Photophysical data of sensor 2 (c = 10´5 M). Table S2: Photophysical data of sensor 3 (c = 10´5 M). Table S3: Photophysical data of sensor 4 (c = 10´5 M). Figure S10: Absorption spectra of 1 (c = 0.04 mM) in the absence and with increasing amounts of Cl¯, Br¯and I¯(c = 0, 0.03 mMÑ1 mM) in acetonitrile at room temperature. Figure S11: Emission spectra of 1 (0.04 mM) in the presence of increasing amounts of chloride (λ exc = 252 nm), bromide (λ exc = 252 nm) and iodide (λ exc = 300 nm) in acetonitrile. Figure S12: Emission spectra of 1 (c = 0.04 mM, λ exc = 252 nm) in the presence of increasing amounts of a mixture of halide anions (c = 0, 0.004 mMÑ0.48 mM) in acetonitrile. Figure S13: Emission spectra of 1 (c = 0.04 mM, λ exc = 252 nm) in the presence of increasing amounts of fluoride (c = 0, 0.004 mM Ñ 0.02 mM) in acetonitrile. Figure S14: Changes in the maximum emission of sensors 2-4 upon titration with different halides in acetonitrile at room temperature. Figure S15: Changes in the emission of 1 (c = 0.04 mM, λ exc = 252 nm) at 332 nm upon titration with different anions in acetonitrile. Figure  S16

Conflicts of Interest:
The authors declare no conflict of interest.

Abbreviations
The following abbreviations are used in this manuscript: