4-( Pyren-1-ylimino ) methylphenol and its Silylated Derivative as Chromogenic Chemosensors Highly Selective for Fluoride or Cyanide

Two novel compounds, 4-(pyren-1-ylimino)methylphenol and 4-[(triisopropylsilyl)oxy] phenylmethylenepyren-1-amine) were synthesized. Solutions of 4-(pyren-1-ylimino)methylphenol in dimethyl sulfoxide (DMSO) are colorless, but on addition of cyanide and fluoride they become colored due to the deprotonation of the chemosensor. The system is highly selective toward cyanide with the addition of water. This compound can be solubilized in water with addition of cetyltrimethylammonium bromide above its critical micellar concentration. The pKa of the compound in water was determined as 10.49 ± 0.02 and this value is lowered in micellar medium to 7.49 ± 0.02, which means that only cyanide is sufficiently basic to achieve the deprotonation of the compound. Solutions of 4-[(triisopropylsilyl)oxy]phenylmethylenepyren-1-amine) in DMSO are colorless, but they are colored in the presence of cyanide and fluoride and only cyanide could be detected in DMSO-water mixture. The nucleophilic attack of the anions on the silicon center of the compound releases colored 4-(pyren-1-ylimino)methylphenolate, enabling the detection of nucleophilic analytes.


Introduction
9][20][21][22][23][24] This is due to the fact that CN -is a chemical species present in several processes, being fundamental in various industrial activities, such as metallurgy, mining and the fabrication of polymers. 25,26This anion is obtained by means of hydrolysis from certain fruit seeds 27 and roots 28 and is present in some neurotoxic warfare agents. 29][32] The definition of a particular system for the optical detection of an analyte, such as F -and CN -, as an optical chemosensor or as a chemodosimeter, is dependent on whether the process is reversible or irreversible, respectively. 33The main feature which these strategies have in common is the combination of a receptor site (for the recognition of the analyte) and a signaling unit (responsible for the detection of the recognized analyte). 1,9he simplest optical chemosensor that can be designed involves the use of an acid-base strategy. 8,14,34,35In this case, a suitable indicator is used, which is colorless in an organic solvent in its protonated form.The deprotonation of the compound, on the addition of a sufficiently basic anion, colors the solution and indicates the presence of the analyte.7][38][39] A system which is more selective toward the anion, for instance, CN -, can be achieved through synthetic modification of the molecular structure of the chemosensor, by the addition of small amounts of water, 40 using biphasic media, 22 or anchoring the chemosensor in a polymeric matrix. 10,41he chemodosimeter approach represents another interesting strategy for the detection of anionic species. 13,333][44][45][46][47][48][49][50][51][52][53][54] In the design of these

Instrumentation
Melting points were obtained on a Kofler hot stage and are uncorrected.The nuclear magnetic resonance (NMR) spectra were recorded on a 400 MHz spectrometer with DMSO-d 6 and deuterated propanone (C 3 D 6 O).Chemical shifts were recorded in ppm with the solvent resonance as the internal standard.Data are reported as follows: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet and m = multiplet), integration and coupling constants (Hz).Infrared (IR) spectra of the solid compounds were obtained with KBr pellets and the oil compound was used neat.Highresolution mass spectra were obtained with an electrospray ionization quadrupole time-of-flight mass spectrometer (HR ESI-MS QTOF).UV-Vis experiments were carried out on a HP 8452A spectrophotometer equipped with a thermostated bath and all measurements were performed at 25 °C, employing a 1 cm quartz cuvette.The maximum wavelengths (λ max ) of the UV-Vis spectra were calculated from the first derivative of the absorption spectrum.

Synthesis of the compounds 1-Nitropyrene (4)
Pyrene (2.53 g, 12.5 mmol) and acetic anhydride (6.50 mL, 68.8 mmol) were mixed in a 250 mL three-neck round-bottomed flask, with 50 mL of dried ethyl acetate and copper(II) nitrate (4.53 g, 18.8 mmol), under magnetic stirring and argon atmosphere.The mixture was stirred for 24 h at 55 °C and a yellow precipitate was formed.After this reaction time, the flask was cooled at room temperature and the inorganic materials were filtered off.The product obtained from the filtration was recrystallized from an ethanol/ethyl acetate mixture (1:1; v/v) and the pale yellow crystals collected were dried in an oven at 40 °C for 6 h, yielding 2.75 g (89% yield); mp obtained: 150-151 °C (mp lit.: 60  1-Nitropyrene (1.00 g, 4.0 mmol) and tin(II) chloride dihydrate (4.35 g, 19.2 mmol) were mixed with 60 mL of ethyl acetate in a 250 cm 3 three-neck round-bottomed flask, under magnetic stirring and argon atmosphere.The suspension mixture was heated to reflux for 6 h.Subsequently, the solution was cooled at room temperature, a sodium carbonate aqueous solution (20%; m/v) was added to obtain pH ca.8.0, and stirring was continued for 1 h.The organic phase was then extracted three times with ethyl acetate and dried with magnesium sulfate.The solid was filtered-off and the solvent was evaporated, resulting in 0.80 g (90% yield) of a light green product; mp obtained: 115-116 °C (mp lit.: 61   1-Aminopyrene (0.200 g, 0.92 mmol) and 4-hydroxybenzaldehyde (0.169 g, 1.38 mmol) were dissolved in 5 mL of dry ethanol in a 10 mL beaker and acetic acid (one drop) was added.The mixture was stirred at room temperature for 4 h.A precipitate was formed after the stirring time, the product was recrystallized and the crystals collected were washed twice, using an ethanol/ acetone mixture (1:1; v/v) as the solvent, giving 0.150 g (51% yield) of a green product, with mp 229-232 °C.IR (KBr) n max / cm − 4-Hydroxybenzaldehyde (0.427 g, 3.5 mmol) was dissolved in DMF (9 mL) in a 100 mL round-bottomed flask in an argon atmosphere.Imidazole (0.483 g, 7 mmol) and TIPS-Cl (1.0 g, 5.2 mmol) were then added and the mixture was stirred at room temperature.After 12 h, 100 mL of water was added to the reaction mixture and the organic fraction was extracted with n-hexane (1:1, v/v).The combined organic layers were washed with brine, dried over MgSO 4 and the solid was filtered off.The solvent was removed by rotary evaporation and the residue was purified by flash column chromatography (3:1 n-hexane/EtOAc), yielding 0.747 g (76% yield) of the product as a colorless oil.IR (neat oil) n max / cm −1 2867-2947 (C-H aliphatic), 1701 (C=O), 1509-1599 (C=C); 1

Visual detection and UV-Vis experiments
The following procedure was used for compounds 1a and 2. A 1.0 × 10 -2 mol L -1 stock solution of each compound was prepared in anhydrous acetone and stored in glass flasks closed with rubber stoppers to avoid the evaporation of the solvent.A volume of 10 µL was collected from this solution and placed in 5 mL volumetric flasks.After evaporation of the acetone, the solid was dissolved in dry DMSO, resulting in a dye solution of 2.0 × 10 -5 mol L -1 .This solution was distributed to several vials and subsequently the tetra-n-butylammonium salts were added, resulting in anion concentrations of 6.0 × 10 -4 and 3.0 × 10 -3 mol L -1 for 1a and 2 solutions.Similar experiments were carried out with DMSO-water mixtures to evaluate the influence of the medium on the performance of systems 1a and 2. The experiments involving compounds 1a and 2 in water were performed using 2.0 × 10 -5 mol L -1 of the compounds in the absence and in the presence of CTABr (3.0 × 10 -3 mol L -1 ).UV-Vis spectra were obtained for all solutions prepared in the absence and in the presence of the anions.

Titrations
Titration experiments were performed in DMSO with the preparation of solutions of 1a and 2 as previously described.These solutions were used to prepare the stock anion solutions, which were closed with rubber stoppers.The titrations were carried out by adding small amounts (2-50 µL) of the salt solution (with a microsyringe) to closed quartz cuvettes containing the solution of 1a or 2.
Titration experiments were also performed in DMSO/ water systems, using the minimum water content, which allowed selective detection of the anion.
Finally, the titration of 1a (2.0 × 10 -5 mol L -1 ) with CN - was carried out in a CTABr/water system.The absorbance values at 406 nm were recorded and used to construct the titration curve.pK a value for 1a in aqueous solution A solution of 1a was prepared at a concentration of 2.0 × 10 -2 mol L -1 in DMSO, and stored in a volumetric flask closed with a rubber stopper to avoid water absorption by the solvent.An aliquot of the solution, sufficient to give a concentration of the compound of 2.0 × 10 -5 mol L -1 , was collected with a microsyringe and placed in two volumetric flasks.Water at pH 5.01 (adjusted with 0.1 mol L -1 HCl) was added to one flask and the other was completed with water at pH 13.9 (adjusted with 0.1 mol L -1 KOH).The UV-Vis spectrum of the dye solution of pH 5.01 was recorded at 25 °C.The pH of the solution was measured and the UV-Vis spectrum was obtained after each addition of a small amount of dye solution at pH 13.9, until the pH was above 12.

NMR experiments
For the 1 H NMR experiments, 6.0 mg of 1a or 2 were placed in an NMR tube and 0.6 mL of DMSO-d 6 was added.The concentrations of 1a and 2 were 3.7 × 10 -2 and 2.5 × 10 -2 mol L -1 , respectively.Aliquots of a 0.6 mol L -1 tetra-n-butylammonium fluoride stock solution were added to the NMR tubes containing 1a and 2 and the 1 H NMR spectra were obtained after 5 min at room temperature.

Calculations
The equilibrium constants were calculated through the fitting of the least-squares regression curves using the ORIGIN 6.1 program.

Results and Discussion
Compounds 1a and 2 were synthesized according to the route shown in Scheme 1. Firstly, pyrene (3) was nitrated using copper(II) nitrate, acetic acid and acetic anhydride to form 1-nitropyrene (4) in 89% yield.Compound 4 was reduced under reflux with tin(II) chloride in ethyl acetate to form compound 5 with 90% yield.The condensation of compound 5 with 4-hydroxybenzaldehyde ( 6) in ethanol at room temperature provided compound 1a in 51% yield.The silylated intermediate 7 was obtained in 76% yield by reacting compound 6 with TIPS-Cl in DMF, using imidazole as a base.The condensation of the aldehyde 7 with amine 5 using the same conditions for the preparation of imine 1a led to the formation of compound 2 in 60% yield.The novel compounds 1a and 2 were fully characterized, exhibiting purity adequate for use in the further studies.

Acid-base strategy using compound 1a
Figure 2A shows the solutions of 1a in DMSO and DMSO-water mixtures and in the absence and presence Vol. 26, No. 12, 2015 of several anions.Solutions of 1a are colorless but on addition of hydroxide they become orange in color due to the deprotonation of the chemosensor.Of the several anions tested, only CN -, F -, and to a lesser extent, CH 3 COO -, were able to deprotonate the solutions of 1a.
Only F -and CN -are detected with the addition of 1% of water (v/v), while with the addition of 10% of water, the system becomes highly selective toward CN -.3][64][65][66][67] This is explained considering that the hydration energies of F -(-465 kJ mol −1 ), CH 3 COO -(−365 kJ mol -1 ), and H 2 PO 4 -(−465 kJ mol -1 ) are high in comparison with that of CN - (−295 kJ mol −1 ). 68CN -is less hydrated with the addition of water, being a more basic species in comparison with the other anions and thus more effective in the abstraction of the proton of 1a.Scheme 2 details the reaction of the deprotonation of colorless 1a by the basic anions F -and CN -generating the colored species 1b in solution.
Figure 3A shows the UV-Vis spectra for 1a in DMSO in the absence and presence of the anions studied.This compound has a band with a maximum at 384.0 nm (e max = 3.28 × 10 4 L mol -1 cm -1 ) and the addition of hydroxide causes the disappearance of this band simultaneously with the appearance of another band with a maximum at  4-(Pyren-1-ylimino)methylphenol and its Silylated Derivative as Chromogenic Chemosensors Highly Selective 468.0 nm (e max = 4.78 × 10 4 L mol -1 cm -1 ).Data show that on the addition of F -the UV-Vis spectrum is the same as that obtained with the addition of hydroxide, indicating that F -is very efficient as a base to deprotonate the dye.CN -is also able to deprotonate 1a, although with less efficiency than F -, while acetate has a small influence on the spectrum of the chemosensor.On the addition of 1% (v/v) of water (Figure 3B), acetate is not able to act as a base, while in the medium containing 4% (v/v) of water, CN -is more efficient as a base than F -(Figure 3C).In the 9:1 (v/v) DMSO-water mixture (Figure 3D), only CN -caused the appearance of the band in the visible region, this band being hypsochromically shifted to 436.0 nm, since the dye generated exhibited solvatochromism, i.e., its visible band changes position when the polarity of the medium is altered. 69ompound 1a was titrated with the anionic species able to change the color of the solutions, that is, F -in DMSO and CN -in a DMSO-water (9:1; v/v) mixture.The absorbance values for the band of 1b at 468.0 nm in DMSO and at 436.0 nm with the addition of 10% of water were plotted as a function of the anion concentration added.The experimental data were fitted with the use of In this equation, Abs is the absorbance value after each addition of an anion, Abs 0 is the initial absorbance without an anion added, Abs 11 is the maximum absorbance value obtained with the addition of an anion considering a 1:1 1a:anion stoichiometry, C A-is the anion concentration for each addition, and K 11 is the equilibrium constant.The results are given in Table 1 and show very good fits for all systems studied.
Figure 4a shows the UV-Vis spectra for the titration of 1a in DMSO with increasing amounts of F -.With the addition of the anion, the band with a λ max value of 384.0 nm related to compound 1a shows a reduction in the absorbance, with the simultaneous appearance of the band with λ max = 468.0nm, due to the appearance of 1b.An isosbestic point occurs at 410.0 nm suggesting the presence of two species (1a and 1b) in equilibrium.The corresponding titration curve obtained with experimental data for the absorbance at 468.0 nm is shown in Figure 4b.The data were fitted using eqaution 1, giving the value of K 11 = (4.98 ± 0.19) × 10 3 L mol -1 (Table 1).Titrations of 1a using CN -as the anion in DMSO and the 9:1 (v/v) DMSO-water mixture were performed (see Supporting Information (SI) section).The pattern observed for the experiment in DMSO was similar to those described for the titrations with F -.The K 11 value obtained from the titration of 1a with CN -in DMSO, of (1.56 ± 0.13) × 10 3 L mol -1 , is lower than that obtained with F -as anion, corroborating the fact that, in DMSO, F -is a stronger base than CN -. 72 In addition, the value of K 11 = (1.29 ± 0.03) × 10 3 L mol -1 obtained for the titration of 1a with CN -in the DMSO-water (9:1; v/v) mixture shows that water molecules have a minor influence on the action of CN -as a base, probably because DMSO interacts strongly with water through hydrogen bonding, making the anion species relatively free to interact with 1a.
Figure 5 shows the 1 H NMR spectra for compound 1a in DMSO-d 6 in the absence and in the presence of increasing amounts of F -. Upon addition of 1.0 equivalent of the anion, the singlet at d 10.22 ppm (H a ) of the hydroxyl group disappears, indicating the abstraction of the proton.Simultaneously, with the addition of more anion, the singlet at d 8. 77 ppm (H c ), corresponding to the proton of the imine group, and the doublet at d 6.99 ppm of the aromatic hydrogens at the ortho position in the phenol group (H b ) were upfield shifted, generating the spectrum of 1b (Figure 5E).
An attempt was made to use 1a for the detection of CN -in water, but the solubility of the compound in this solvent is very low.Since 1a has a lipophilic character, its solubility in water should, in principle, be improved with the use of a surfactant agent. 73Thus, CTABr was used in water above its critical micellar concentration (cmc = 9.0 × 10 -4 mol L -1 ), 74 and it was observed that the solubility of 1a was considerably improved.Preliminary tests showed that of the various anions tested only CN -  caused an alteration in the aspect of the system containing 1a, from colorless to yellow.A UV-Vis study demonstrated that on the addition of CN -the band of 1a in the UV region, with a maximum at 384.0 nm, was replaced by another band in the visible region, with λ max = 406.0nm (see SI).The pK a of 1a in water was determined as being 10.49 ± 0.02, while in micellar medium the pK a was lowered to 7.49 ± 0.02.Since the pK a values of HCN and HF are 9.21 and 3.18, respectively 72 their conjugate bases are not sufficiently strong to deprotonate 1a in water, but in micellar medium only CN -is sufficiently basic to deprotonate 1a.
Figure 6a shows the influence of the addition of increasing amounts of CN -on the UV-Vis spectrum of 1a in water containing CTABr above its critical micellar concentration.Significant changes were observed in the original spectrum on the addition of the anion, with the appearance of a band with λ max = 406.0nm. Figure 6b shows the corresponding titration curve, the shape of which is typical of a 1:1 1a:anion stoichiometry.The use of equation 1 to fit the experimental data provided the result K 11 = (9.38 ± 0.28) × 10 4 L mol -1 , a value larger than that obtained in DMSO or in the DMSO-water mixture.The detection and quantification limits were determined as 4.65 × 10 -7 mol L -1 and 1.55 × 10 -6 mol L -1 , respectively.
A chemodosimeter approach based on compound 2 Figure 7 shows the solutions of 2 in DMSO and DMSO-water mixtures in the absence and presence of several anions.Solutions of compound 2 in DMSO are colorless but they become colored (orange) in the presence of CN -and F -.In relation to the other anions, a very pale yellow color was observed for the solutions containing H 2 PO 4 -and CH 3 COO -.On the addition of 4% of water (v/v), only the effect of CN -and F -could be observed and only CN -could be detected in the 9:1 (v/v) DMSO-water mixture.
Figure 8 shows the UV-Vis spectra for solutions of 2 in the absence and presence of the anions.For compound 2 in DMSO there is a band with maximum at 384 nm (e max = 3.28 × 10 4 L mol -1 cm -1 ), which disappears with the addition of CN -and F -giving place to another band with λ max = 468 nm (e max = 4.79 × 10 4 L mol -1 cm -1 ).It is important to observe that the latter band corresponds to that observed for the product of the reaction of 1a with CN -and F -in DMSO.Thus, it suggests that the product of the reaction of 2 under these experimental conditions is species 1b.This is also corroborated by the fact that the product of the reaction of 2 with F -and CN -in DMSO with 10% (v/v) of water has a band with λ max = 436.0nm, at the same position as that verified in the experiment carried out with compound 1a.As observed for compound 1a, with the addition of water the system became highly selective for CN -in comparison with the species which provided positive results in DMSO, indicating that water in smaller amounts efficiently hydrates H 2 PO 4 -and CH 3 COO -, and at 10% (v/v) it hydrates F -, hindering the action of anions as nucleophilic species.Scheme 3 summarizes the chemodosimeter 12 ppm (H d ).The final spectrum coincides with the 1 H NMR spectrum of 1a in the presence of F -in excess (Figure 5E), revealing that the same species, 1b, was formed in the two cases.However, the spectra obtained using small amounts of the anion did not show any alteration in the position of the signals, which suggests that F -has a very weak interaction with the compound prior to the reaction with the silicon center.9][80] Alternatively, the interaction of two equivalents of F -with the silicon atom was reported in other papers. 53,76,81Mass spectrometric studies of 2 in the presence of an excess of F -and CN -showed that the anions caused the Si-O bond breaking, with the formation of 1b (see SI), without evidences for the occurrence of other intermediates.Therefore, the results suggest that there is a very weak interaction between F -and the hydrogen atom of the imine group in the compound before the action of the anion as a nucleophile.A comparison of the curves for the titration of 1a and 2 with F -suggests that the same interaction does not occur for compound 1a, where the anion simply acts as a base abstracting the phenolic proton.The UV-Vis spectra for the titration of compound 2 in DMSO with the addition of CN -(Figure 11a) show a  approach based on compound 2. The nucleophilic attack of the anionic species on the silicon center, through a nucleophilic substitution at silicon (S N 2@Si), 53,[75][76][77] releases colored 1b species as the leaving group, which makes this process effective for the detection of strongly nucleophilic analytes, such as F -and CN -in anhydrous DMSO.On the addition of water, the system becomes highly selective toward CN -, which remains an efficient nucleophile under these experimental conditions.
Figure 9a shows the UV-Vis spectra corresponding to the titration of compound 2 in DMSO with increasing amounts of F -. On the addition of F -a reduction in the intensity of the band related to 2 at 384 nm occurs with the simultaneous appearance of the band for 1b at λ max = 468 nm.The titration curve in Figure 9b was obtained using the absorbance values at 468 nm as a function of c(F -).
The sigmoidal profile of the curve and the absence of an isosbestic point on the UV-Vis spectra suggest that some 4-(Pyren-1-ylimino)methylphenol and its Silylated Derivative as Chromogenic Chemosensors Highly Selective reduction in the intensity of the band related to 2 at 384 nm and the simultaneous appearance of the band for 1b at λ max = 468 nm.An isosbestic point occurred at 410 nm, which suggests that one species (2) is gives rise to the product (1b) with no intermediate species.The titration curve in Figure 11b was obtained using the absorbance values at 468 nm as a function of c(F -).The curve does not show a sigmoidal shape, which suggests that there is no interaction of the anion with the compound prior to the nucleophilic attack.This corroborates the explanation given for the behavior of 2 in the presence of F -, since CN -does not form hydrogen bonds as effectively as F -. Equation 1 was used to fit the experimental data, providing further evidence for a 1:1 2:anion stoichiometry, with K 11 = (4.03± 0.22) × 10 3 L mol -1 (standard deviation (S.D.) = 0.055).The titration of 2 with CN -in DMSO with 10% (v/v) of water at 25 °C revealed a similar pattern (see SI), except for the fact that for the band corresponding to the appearance of 1b the λ max is 436 nm, as observed in the experiments with 1a and CN -in the same aqueous DMSO mixture.The detection and quantification limits were 3.59 × 10 -5 mol L -1 and 1.20 × 10 -4 mol L -1 , respectively.

Conclusions
Compounds 1a and 2 were synthesized and studied in two different strategies for the optical detection of F -and CN -.Compound 1a is a phenol and it was used as an anionic chromogenic chemosensor in DMSO through an acid-base strategy.With the addition of small amounts of water, the system could be used only for the visual and quantitative detection of CN -.Although 1a is not soluble in water at the concentration required for optical chemosensors, the use of a cationic surfactant (CTABr) was found to be a very efficient approach to improve the solubility of the compound and to lower its pK a value, enabling the use of the system for the highly selective optical detection of CN -.In addition, the detection and quantification limits of the method (4.65 × 10 -7 mol L -1 and 1.55 × 10 -6 mol L -1 , respectively) are close to the maximum level of c(CN -) in potable water allowed by the World Health Organization, which is 1.7 mmol L −1 . 82nother strategy was studied using the basic framework of 1a to construct the chemodosimeter 2. This chemodosimeter approach was studied in DMSO, which is based on the nucleophilic attack of the anionic species on the silicon center, through a nucleophilic substitution at silicon (S N 2@Si).Dye 1b is generated in this process, which signals the presence of F -and CN -.With the addition of 10% (v/v) of water, the system is able to detect only CN -.Since water hydrates F -, the action of this anion as a nucleophilic species is hindered.
In conclusion, the system described herein for anionic detection is versatile, allowing its use in organic and aqueous media.In addition, simple synthetic modifications can be envisioned, for instance in the conjugated bridge and the use of substituents at the phenol and pyrene moieties in the molecules, which should allow access to new fluorogenic systems and more efficient techniques for anion sensing.

Scheme 1 .
Scheme 1. Synthetic route for the preparation of compounds 1a and 2.

Figure 4 .
Figure 4. (a) Influence of the addition of increasing amounts of F -on the UV-Vis spectra of 1a (2 × 10 -5 mol L -1 ) in DMSO at 25 °C.The final concentration of F -was 1.1 × 10 -3 mol L -1 .(b) Curve showing the variation in the absorbance at 468 nm of 1a with the addition of increasing amounts of F -.

Figure 6 .
Figure 6.(a) Influence of the addition of increasing amounts of CN -on the UV-Vis spectra of 1a (2 × 10 -5 mol L -1 ) in CTABr/water at 25 °C.The final concentration of CN -was 7.4 × 10 -5 mol L -1 and CTABr was used in a concentration of 3.0 × 10 -3 mol L -1 .(b) Curve showing the variation in the absorbance at 406 nm of 1a with the addition of increasing amounts of CN -.

Figure 9 .Figure 10 . 1 HFigure 11 .
Figure 9. (a) Influence of the addition of increasing amounts of F -on the UV-Vis spectra of 2 (2 × 10 -5 mol L -1 ) in DMSO at 25 °C.The final concentration of F -was 8.0 × 10 -4 mol L -1 .(b) Curve showing the variation in the absorbance at 468 nm of 2 with the addition of increasing amounts of F -.