Dual Binding Site Assisted Chromogenic and Fluorogenic Recognition

Feb 26, 2014 - Compound 3 contains of two different Lewis acidic binding sites (Zn(II) and ..... fluorescent sensor for detection of cyanide in aqueou...
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Dual Binding Site Assisted Chromogenic and Fluorogenic Recognition and Discrimination of Fluoride and Cyanide by a Peripherally Borylated Metalloporphyrin: Overcoming Anion Interference in Organoboron Based Sensors P. Chinna Ayya Swamy, Sanjoy Mukherjee, and Pakkirisamy Thilagar* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, Karnataka 560 012, India S Supporting Information *

ABSTRACT: Peripherally triarylborane decorated porphyrin (2) and its Zn(II) complex (3) have been synthesized. Compound 3 contains of two different Lewis acidic binding sites (Zn(II) and boron center). Unlike all previously known triarylborane based sensors, the optical responses of 3 toward fluoride and cyanide are distinctively different, thus enabling the discrimination of these two interfering anions. Metalloporphyrin 3 shows a multiple channel fluorogenic response toward fluoride and cyanide and also a selective visual colorimetric response toward cyanide. By comparison with model systems and from detailed photophysical studies on 2 and 3, we conclude that the preferential binding of fluoride occurs at the peripheral borane moieties resulting in the cessation of the EET (electronic energy transfer) process from borane to porphyrin core and with negligible negetive cooperative effects. On the other hand, cyanide binding occurs at the Zn(II) core leading to drastic changes in its absorption behavior which can be followed by the naked eye. Such changes are not observed when the boryl substituent is absent (e.g., Zn-TPP and TPP). Compounds 2 and 3 were also found to be capable of extracting fluoride from aqueous medium. Detection of fluoride and cyanide ions is an active area of research because of their positive and negative impact on human health and environment, and their notorious association with chemical warfare agents.7 The most popular strategy employed for detection of fluoride and cyanide ions is the intermolecular hydrogen bonding interaction between the guest (anion) and the host (receptor).7g,8 The past decade has witnessed several reaction-based chemodosimeteric receptors for F− and CN− detection.9 In recent years, Lewis acid−base interactions have attracted much attention in this field. Among Lewis acids, triarylboranes have been increasingly exploited in this area of research.1,2,10 In spite of these efforts, single molecular sensors which can differentiate interfering anions are not well-defined in the literature.11 In 2010, Aldridge and coworkers12 studied structural and thermodynamic aspects of fluoride and cyanide binding by PhBMes2 (Mes = Mesityl). The binding affinities of the two anions in dichloromethane were found to be similar with nearly similar structural perturbations. Also, the cyanide binding was found to be marginally preferable with respect to fluoride. However, these trends are not followed for all borane based receptors, and there is no clear strategy for

he luminescent organoboron containing π-conjugated systems1,2 have attracted much attention because of the enormous potential of such molecules in the niche technology of organic light-emitting diodes (OLEDs),1b−e field-effect transistors (FETs),1g and NLO materials.1a Several boron containing donor−acceptor (D-A) dyads, boron containing polyaryls,1 and conjugated/nonconjugated boron containing polymers2 have been investigated in this regard. N-bonded boron complexes of oligopyrrolic systems like BODIPYs,3b−d subporphyrins,3a,g,h and porphyrins3e,f,i−k (boron in the tetrapyrrolic core of porphyrin) have been known for a long time. Considering the rich color chemistry associated with porphyrins and their expanded analogues (expanded porphyrin),4,5 organoboron conjugates of these families may present the opportunity of diverse applications. However, such triarylborane-porphyrin conjugates are seldom reported in literature. In fact, the only prior example of a triarylboraneporphyrin conjugate was that reported by Kubo and coworkers; the conjugate was prepared by multistep heavy metal catalyzed C−C bond forming reactions.6 One limitation to the even broader use of triarylborane-porphyrin conjugates has been the difficulty of their construction, which involves quite lengthy synthetic procedures. An important objective of this project is to devise a facile synthetic route for borane-porphyrin conjugates.

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© 2014 American Chemical Society

Received: January 18, 2014 Accepted: February 26, 2014 Published: February 26, 2014 3616

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Scheme 1. Synthesis of 2 and 3

Figure 1. Molecular structure of 2 and 3 as obtained from single crystal X-ray diffraction studies (50% probability for thermal ellipsoids). Hydrogen atoms are omitted for clarity.

shell d10 configuration. However, the axial affinity of Zn(II)porphyrins, e.g., Zn-TPP toward other ligands, is moderate. External stimulation or assistance of other building subunits is essential for designing efficient axial anion/ligand receptors with Zn(II)-porphyrins.17 In our design, the metal center in metalloporphyrin can easily accommodate axial coordination with no associated steric repulsions from neighboring sites, and this binding event should affect the absorption profile of porphyrin and could result in visual color changes.18 Concomitantly, the triarylborane moiety at the periphery can also accommodate Lewis bases, and thereby perturb the energy transfer from peripheral triarylborane unit to porphyrin core leading to fluorescence quenching upon excitation at boryl absorption region. Thus, by inserting suitable Lewis acidic metal ion at the porphyrin core, the triarylboryl-metalloporphyrin conjugate can be conveniently manipulated for colorimetric and fluorogenic differentiations of interfering anions like fluoride and cyanide. To evaluate this concept, we chose triarylborane appended porphyrin (2) and metalloporphyrin (3) systems (Scheme 1) for our studies, and the results are reported in this paper.

designing receptors selective toward any one of a number of interfering anions, e.g., either fluoride or cyanide. There is thus an urgent need for a single molecular sensor which can differentiate interfering cyanide and fluoride ions. The concept of “lab-on-a-molecule” can be used for designing sensors capable of distinguishing interfering species. Innovative molecular designs like “lab-on-a-molecule” and multichannel molecular probes for sensing multiple cations or anions are successfully demonstrated13 in the literature. However, no such design has been developed with boranebased anion receptors which may address the issue of discrimination of fluoride and cyanide. We reasoned that, if different Lewis acidic centers with different affinities toward dissimilar anions are tethered in a single molecule, the anions may preferentially bind to different receptor centers owing to their difference in polarizabilities and ligation behavior.14 In this context, a metalloporphyrin system peripherally decorated with triarylboryl units caught our attention. The basic chemistry of cyanide poisoning involves the binding of cyanide as a strong ligand to the Fe(III) center of the heme (porphyrin) unit in cytochrome c oxidase resulting in a highly stable complex and which terminates the oxidation process of the respiratory chain.15 Inspired by this biological system, porphyrin based cyanide receptors have been studied.16 The axial coordination in metalloporphyrins results in structural reorganizations, resulting in changes in optical absorption. To achieve fluorescence responses from such designs, the choice of central metal ion is important, as metal ions with unpaired electrons may quench the fluorescence of such porphyrins. In this respect, Zn(II) is a suitable choice as it contains a closed

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RESULTS AND DISCUSSION Synthesis and Characterizations. The key precursor for the synthesis of the porphyrin, 4-dimesitylboryl-benzaldehyde (1), was prepared according to the literature procedure reported by Thilagar and co-workers.19 The synthesis of triarylborane-porphyrin 2 and 3 follows conventional porphyrin synthesis route. Compound 2 was allowed to react with Zn(OAc)2·2H2O in MeOH to obtain 3 in quantitative yield 3617

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Figure 2. Comparison of UV−vis absorption spectra (above) and emission spectra (bottom left, λex = 335 nm; bottom right, λex = 425 nm; 10 μM DCM solutions) of compounds 2 and 3 with respect to H2TPP and Zn-TPP.

Figure 3. Overlap of emission spectra of Mes2BPh with UV−vis absorption spectra H2TPP and Zn-TPP (left). Possible mechanism of energy transfer (right).

both 2 and 3 has a trigonal planar configuration with the sum of angles around boron being 360°. The B−C (mesoaryl) bond length in 2 (1.585(2) Å) is slightly longer than bond length observed in 3 (1.568(3) Å). In contrast the B−C(Mes) bond lengths follow the opposite trend. The C(aryl)−C(meso carbon of porphyrin) bond length in 2 (1.490(2) Å) is comparatively shorter than the C(aryl)−C(meso carbon of porphyrin) length in 3 (1.501(3) Å). The dihedral angle between the mesophenyl unit and porphyrin core falls in the range ∼56° to ∼60° for 2 and 3, respectively. These structural parameters closely resemble those obtained from DFT optimized structures20 (see the Supporting Information).

(Scheme 1). Compounds 2 and 3 were characterized by NMR spectroscopy (1H and 13C) and MALDI-TOF mass spectrometry analysis. Further, their molecular structures were unambiguously confirmed by single crystal X-ray diffraction studies. Compounds 2 and 3 exhibit simple 1H NMR spectra. All the eight pyrrole C−H protons give rise to a single resonance at 8.87 and 9.01 ppm for 2 and 3, respectively. The C−H protons of −C6H4 moiety of 2 and 3 give rise to two distinct doublets (8.22 and 7.91 ppm for 2 and 7.94 and 6.99 ppm for 3). These symmetric resonances clearly indicate that the attachment of four bulky Mes2B groups at meso-phenyl unit do not alter the symmetry of the porphyrin core. The molecular structures of 2 and 3 are shown in Figure 1. The boron atom in 3618

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Figure 4. Fluorescence titration spectra of 2 (left) and 3 (right) with TBAF in 10 μM DCM solution at λex = 350 nm. Inset: Spectral changes on λex = 425 nm. (Imax − I)/(Imax − Imin) vs log[F−] plots for 2 (bottom left) and 3 (bottom right). The intercept at x-axis shows the lowest concentrations of [F] which can be detected by the respective compounds.

Photophysical Properties. To ascertain the effect of boryl substitution on the photophysical properties of the compounds (2 and 3), a thorough comparison of 2 with H2-TPP4h,j and 3 with Zn-TPP4i,j was performed in all aspects (Figure 2). The higher dihedral angle and the noncrossing FMOs of the two chromophores (boryl groups and porphyrin core, see the Supporting Information) clearly suggest that the absorption features of 2 and 3 should be similar to the additive spectrum of free Mes 2 B−C 6 H 5 and porphyrin unit. However, the fluorescence properties may be modified (e.g., energy transfer) due to the proximity effect. As expected, the absorption profile of 2 and 3 are similar to H2-TPP and Zn-TPP, respectively (except for a 5 nm red shift in Soret and Q bands, and an additional broad boryl absorption band at ∼335 nm). These results clearly indicate that there is minimum electronic communication between peripheral boryl unit and the porphyrin core in the ground state. As theorized vide supra, the fluorescence profiles of 2 and 3 are hardly comparable with those of H2-TPP and Zn-TPP. Upon excitation at 335 nm, both 2 and 3 exhibit distinctly different emission bands in the regions 400−700 nm. In addition to regular porphyrin emission bands, 2 and 3 showed an additional broad emission band at ∼375−550 nm (Figure 2). To understand the nature of these emission bands, we checked the emission of the model building block, Mes2BPh (dimesitylphenylborane), H2-TPP, and Zn-TPP. As shown in Figures 2 and 3, the ∼375−550 nm broad emission features of

2 and 3 directly corroborate the emission behavior of Mes2BPh. These results lead us to conclude that the broad emission bands observed for 2 and 3 arise from the pendant triarylborane moieties. The bifurcated structures of these triarylborane based emission bands result from the porphyrin centered selfabsorption in these molecules (see Figures S39 and S40 in the Supporting Information). Emission studies in different concentrations showed no effect on the relative intensities (or λmax) of these bifurcated emission features (in compounds 2 and 3) which led us to conclude that intramolecular (rather than intermolecular) self-absorption processes are operating in these molecules (see Figure S38 in the Supporting Information). As evident from Figures 2 and 3, the trenches at ∼420−430 nm for compounds 2 and 3 directly replicate the porphyrin based Soret bands (see Figures S39 and S40 in the Supporting Information). The observations of dual fluorescence from triarylborane moieties and the porphyrin core indicate the presence of partial (triarylborane to porphyrin) EET (electronic energy transfer) processes in compounds 2 and 3. The overall fluorescence quantum yields for 2 and 3 are much higher (see Suporting Information) than those observed for H2-TPP and Zn-TPP upon excitation at boryl dominated absorption band (λex 335 nm, see the Supporting Information). Further, comparison of photophysical properties of 2 and 3 with Mes2B−C6H5 indicated that the emission at 400 nm arises from the boryl unit. Now the relevant question is the origin of other lower 3619

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Figure 5. Left: UV−vis titration spectra of 2 with TBAF. Inset: Titration spectra of 2 with TBACN. Right: UV−vis titration spectra of 3 with TBACN. Inset: Titration spectra of 3 with TBAF.

contrast, upon addition of cyanide to 3, the Soret band at 430 nm was bathochromically shifted to 445 nm as shown in Figure 3. The Q-bands at 550 and 595 nm were also shifted to longer wavelengths, 580 and 625 nm, respectively. Previously, Valentine et al. and others have shown that binding of ligand to metal center in metalloporphyrin imparts pronounced spectral variations with readily observable color changes in solution.18 They have also observed that the magnitude of changes depends on the strength of the polarizability of the ligand. Thus, the binding of cyanide to the Lewis acidic Zn(II) center is exclusively responsible for the change in absorption profile of 3. Additionally, a color change from pale purple to dark green was observed by the naked eye upon the addition of cyanide to compound 3. The appearance of a new absorption peak at about 445 and 625 nm is in good agreement with this color. These results suggest that compound 3 can be used as a colorimetric sensor for cyanide ion. Further, to understand the apparent role of peripheral boryl group in 3, we carried out a similar cyanide titration studies with model compound ZnTPP, which showed no spectral change even upon addition of >100 equiv of cyanide. Thus, it can be concluded that the electron deficient boryl substituents at the periphery of the porphyrin ring play a crucial role in fine-tuning the Lewis acidity of the central Zn(II) ion. To the best of our knowledge this is the first report where the Lewis acidity of metal ions is modulated by triarylboranes. In general, metal complexes were used to fine-tune the lewis acidity of boryl groups.22 Binding constant (Ki) measurements from UV−vis titrations23 provided useful insights about the relative affinities of the porphyrins toward fluoride and cyanide. For compound 2, the binding constants (log K1F = 5.2, log K2F = 11.0, log K3F = 16.5, log K4F = 21.6) increase gradually with almost very low negative-cooperative binding nature (log K4F − log K3F = 5.1; log K3F − log K2F = 5.5; log K2F − log K1F = 5.8). In our view, as the borane units are pointing out away from each other and as their conjugation throughout the porphyrin core is limited, anion binding to any single borane unit only feebly disturbs the overall electronic affinity of other borane units toward subsequent anion binding. Similar behavior toward fluoride was also observed in the case of 3 (log K1F = 5.3, log K2F = 10.6, log K3F = 16.1, log K4F = 21.4). However, in this case the first binding constant was slightly higher compared to that of 2 whereas the consecutive binding constants were slightly less with respect to those of 2. This may be a result of the fact that Zn complexation slightly enhances the Lewis acidity of the borane units and also it promotes the electronic communica-

energy emissions when excited at the boryl band. The absorption of the porphyrin unit in the boryl band region is weak, and this cannot explain the intense emissions when excited at the boryl band. This may be due to singlet−singlet energy transfer from the triarylborane (donor) to the porphyrin (acceptor) unit. The principal requirements for energy transfer between the donor (D) and the acceptor (A) (overlap of spectra of D λem and A λabs, favorable orientation, and proximity between the D and A)21 are met in compounds 2 and 3 (Figure 3). The persistence of boryl emission at 400 nm indicates that the energy transfer is not 100% in both 2 and 3. The multiple emissions of 2 and 3 are very different from those observed for the only known monoborylporphyrin conjugate,6 where only porphyrin emission was observed upon excitation of the boryl band and the absence of boryl emission was attributed to 100% efficiency of energy transfer. Anion Binding Studies. Although 2 and 3 were designed to probe anions, the anion binding studies have helped in understanding the overall electronic structures of compounds 2 and 3. Upon addition of TBAF to CH2Cl2 solution of 2 and 3 and excitation at the boryl absorption dominated band at 330 nm, it was observed that the intensity of the boryl band at 400 nm and other emission bands in the region 450, 650, and 700 nm were quenched almost completely (Figure 4). Individually, it was verified that TBAF does not quench porphyrin emission (λex = 425 nm and above) in 2, 3, H2-TPP, and ZnTPP. Thus, the selective disappearance of porphyrin emission in fluoridebound 2 and 3 when the boryl unit is excited has been interpreted as an indication of a drastic decrease in energy transfer efficiency of the H4C6BMes2 unit to porphyrin in 2 and 3. Upon addition of fluoride, slight changes in the absorption profile of 2 and 3 were observed (slight reduction in intensity of Soret band and 5 nm red-shift in the lower energy bands, Figure 5). The unaffected absorption profile of 2 in the presence of F−/CN− certainly excludes the possibility that the anions deprotonate the N−H units (or form hydrogen-bond(s) with the porphyrin core) under titration conditions. This conclusion is also supported by the presence of the N−H signal at −2.8 ppm in the 1H NMR spectrum of 2, in the presence of 6 equiv of F− in CDCl3. The detection limits for the compounds 2 and 3 toward fluoride ions were found to be 0.09 and 0.10 ppm, respectively (Figure 4), demonstrating their remarkable sensitivity to the presence of fluoride ions. Unlike fluoride, CN− ion binds differently to 2 and 3. Upon addition of cyanide to a CH2Cl2 solution of 2, no significant change in its absorption spectrum was observed (Figure 5). In 3620

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Figure 6. Fluorescence titration spectra of 2 (left) and 3 (middle) with TBACN in 10 μM DCM solution at λex =350 nm. Inset: Spectral changes on λex = 425 nm. (Imax − I)/(Imax − Imin) vs log[CN−] plots for 3 (right). The intercept at x-axis shows the lowest concentrations of [CN−] which can be detected by 3.

compound 2 in the presence of CN− did not show any changes in the fluorescence spectra. In contrast, under similar conditions the emission profile of 3 changed drastically. For example, the emission band at 600 nm gradually vanished, while the band at 650 nm steadily gained in intensity with increasing amount of cyanide. In addition, a new emission band appeared at 690 nm. A significant enhancement in the fluorescence emission intensity was observed (Figure 6, right image inset, and Figure 7, right side image), which may be due to the more than 1.3-fold increase in the fluorescence quantum yield (ΦF for 3 is 19.5% and ΦF for 3 + CN‑ is 26.5%). The increased quantum yield can be ascribed to loss of flexibility of 3 upon binding to cyanide. The changes in the emission spectra are linear up to 5.0 equiv of cyanide; beyond 5 equiv, no appreciable changes in the emission profile were noted. In conclusion, the cyanide ion binds weakly to peripheral BMes2 units but strongly to the central Zn center. The detection limit of compound 3 in identifying cyanide was found to be 1.5 ppm in dichloromethane solution (Figure 6). The most intriguing feature of 3 is that its binding to fluoride and cyanide elicits different spectral responses. In general BAr3 responds similarly to F− and CN−. In several instances, a difference in the binding constants was observed, but significant changes in the spectral profiles were seldom noticed. The unprecedented spectral changes observed when 3 binds to F− and CN− certainly arise from the preferential binding of F− and CN− to one or the other of the two Lewis acidic centers viz. boron and Zn, respectively. To rationalize these findings, ESP surface images were generated for the model compound 3m by DFT calculations. A high positive electrostatic potential value at the Zn center was observed. The minimum electronic communication (confirmed from UV−vis studies and DFT,

tion throughout the porphyrin core. Surprisingly, the negativecooperative binding effect is also absent in the case of 3 (log K4F − log K3F = 5.3; log K3F − log K2F = 5.5; log K2F − log K1F = 5.3). As per our expectations, the cyanide binding affinity of 3 (log KCN = 5.4) was found to be much weaker with respect to the overall binding affinity of fluoride. In emission studies, unlike fluoride, cyanide binding led to completely contrasting responses. Upon excitation at 350 nm, cyanide bound 3 showed weak changes in fluorescence profile ∼375−550 nm (Figures 6 and 7). The apparent changes

Figure 7. Digital photographs of solution of 3 in CH2Cl2 (1 × 10−5 M) in the presence of F− and CN−. From left to right: no anion, fluoride, and cyanide under ambient light (A) and UV light (B). Color change from pale purple to dark green was observed by the naked eye upon the addition of cyanide to compound 3 (left). Under UV light the binding of fluoride quenches the fluorescence, while the binding of cyanide exhibits brighter emission.

observed at ∼425−450 nm (i.e., the trenches of the emission bands) essentially reflect the changes in the absorbance profile upon addition of cyanide (see Figures 5and 6 and Figure S40 in the Supporting Information). When excited at 425 nm,

Figure 8. From left to right: (A) fluorescence changes in a 10 μM DCM solution of 2 upon addition of water and aqueous TBAF, (B) fluorescence changes in a 10 μM DCM solution of 3 upon addition of water and aqueous TBAF, (C) visual changes observed for a 10 μM DCM solution of 3 upon addition of TBACN and its regeneration after addition of water. 3621

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were prepared using a microbalance (±0.1 mg) and volumetric glassware and then charged in quartz cuvettes with sealing screw caps. Fluorescence emission studies were carried out on a Horiba JOBIN YVON Fluoromax-4 spectrometer. All DFT and TD-DFT computations were performed using B3LYP hybrid functional (considering LANL2DZ basis set for all atoms) as incorporated in Gaussian 09.20 X-ray Crystallographic Studies. Single crystals of 2 and 3 were grown by diffusing hexane vapors into chloroform solutions of respective compounds. The diffraction data of 2 and 3 were collected on a Bruker SMART APEX CCD diffractometer using the SMART/SAINT software.24 Intensity data were collected using graphite-monochromatic Mo Kα radiation (0.7107 Å) at 160(2) K on a crystal as obtained after several attempts. The structures were solved by direct methods using the SHELX-97 incorporated in WinGX. Empirical absorption corrections were applied with SADABS. The solvent molecules in the crystal structures possess disorder and were refined accordingly using PART commands as incorporated in SHELX-97 software.24 All non-hydrogen atoms were refined with anisotropic displacement coefficients. Hydrogen atoms were assigned isotropic displacement coefficients, U(H) = 1.2U(C) or 1.5U (C-methyl), and their coordinates were allowed to ride on their respective carbons.24 Caution! Cyanide and fluoride are extremely toxic species and should be handled with extreme care and attention. Although very small quantities of cyanide/fluoride salts were used in this study, thorough laboratory safety protocols were strictly followed while handling such compounds (tetrabutylammonium cyanide and tetrabutylammonium fluoride). Synthesis of Compound 2. (5,10,15,20-Tetrakis((4′bismesitylboryl)phenyl)porphyrin). To a solution of 4dimesitylborylbenzaldehyde (1.0 g, 2.82 mmol) in chloroform (150 mL) was added freshly distilled pyrrole (0.25 mL, 3.43 mmol), and the resulting solution was degassed by purging N2 for 30 min. Later, BF3·Et2O (30 μL, 0.28 mmol) was added dropwise, and the reaction mixture was stirred until 4dimesitylborylbenzaldehyde was completely consumed (as observed in TLC). At this stage, a solution of DDQ (0.63 g, 2.82 mmol) in benzene (10 mL) was added to reaction mixture over 10 min, and the stirring was continued for 2 h at room temperature. The resulting mixture was filtered, concentrated, and purified using a neutral alumina gel column. The product 2 was obtained as a purple solid. Yield: 0.35 g, 31%. 1H NMR (400 MHz, CDCl3 δ ppm) 8.87 (s, 8H), 8.22 (d, J = 8.0 Hz, 8H), 7.91 (d, J = 8.0 Hz, 8H), 6.95 (s, 16H) 2.38 (s, 24H), 2.31 (s, 48H). 13C NMR (100.00 MHz, CDCl3, δ ppm) 146.03, 142.42, 141.49, 139.42, 134.97, 129.53, 128.88, 125.79, 120.71, 24.18, 21.79.11B NMR (160 MHz, CDCl3, δ ppm) 72.4. MALDI m/z (M + H) calcd for C116H114N4B4 1608.9528, found 1608.9051. Synthesis of Compound 3. (Zinc(II)-5,10,15,20-Tetrakis((4′-bismesitylboryl)phenyl)porphyrin). A solution of Zn(OAc)2·2H2O (11 mg, 62 μmol) in MeOH (2 mL) was added to a solution of 2 (20 mg, 12 μmol) in 15 mL of chloroform, and the reaction mixture was stirred for 6 h at room temperature. The reaction mixture was concentrated in vacuo and further purified by using basic alumina gel column (petroleum ether/dichloromethane). The desired product 3 was obtained as a purple color solid. Yield: 22 mg, 96%. 1H NMR (400 MHz, CDCl3) δ 9.01 (s, 8H), 8.26 (d, J = 8.0 Hz, 8H), 7.94 (d, J = 8.0 Hz, 8H), 6.99 (s, 16H), 2.42 (s, 24H), 2.32 (s, 48H). 13C NMR (100.00 MHz, CDCl3, δ ppm) 150.36,

see the Supporting Information) between peripheral BMes2 and porphyrin core is sufficient to modulate the Lewis acidity of Zn center in 3. The difference in Lewis acidity between boron and Zn centers enables 3 to discriminate F− and CN− (polarizability of these ligands also may play a role). Fluorescence titration studies of 2 and 3 with other anions (Cl−, Br−, I−, NO3−, −OAc, and ClO4−) did not show any affinity toward other anions. Thus, 3 can be employed for chromogenic and multichannel fluorogenic differentiations of interfering anions like fluoride and cyanide. Further competitive experiments by simultaneous addition of both CN− and F− to 2 and 3 support this conclusion (see the Supporting Information). The ability of 2 and 3 to extract fluoride and cyanide ions from water was explored. An aqueous solution (2 mL) of NaF was vigorously shaken for a few seconds with CH2Cl2 solution of 2 (10−5 M, 1 mL) and the system left aside. The aqueous and organic layers separated. Under UV light the organic phase displayed quenching of red color fluorescence (Figure 8A,B). This experiment clearly demonstrates that fluoride ions can be successfully extracted from the aqueous layer into the organic phase. These initial results encouraged us to evaluate the potential of 3 to extract cyanide from aqueous media. Under similar conditions, 3 failed to extract CN− from aqueous phase. Nevertheless, the treatment of cyanide bound 3 with water showed complete recovery of free 3 (Figure 8C). Similarly, treatment of aqueous solution of CaSO4 with fluoride bound 2 showed complete recoveries of free 2. These results suggest that the interaction between borylporphyrin conjugates and F− and CN− are reversible.

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CONCLUSIONS In conclusion, the design, synthesis, and structural characterization of a novel triarylboryl decorated porphyrin (2) and its zinc complex (3) were reported. Compounds 2 and 3 exhibited interesting multiple emissions when excited at C6H4BMes2 centered high energy absorption band. Metalloporphyrin 3 shows high selectivity and unprecedented multichannel fluorogenic and colorimetric response toward F− and CN−. The excitation wavelength dependent emission response of 3 toward F− and CN− affords a new way of sensing interfering anions. The conjugates 2 and 3 showed reversible binding interaction toward CN− and F−, and they are capable of extracting fluoride from aqueous media. Considering the rich ligand chemistry of porphyrin, further exploitation of 2 in the fields of materials and coordination chemistry is under investigation.

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EXPERIMENTAL SECTION Materials and Methods. n-Butyllithium (1.6 M in hexane), 4-bromobenzaldehyde, and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) were purchased from Aldrich, and pyrrole was purchased from SRL (India). All reactions were carried under an atmosphere of purified nitrogen using Schlenck techniques. THF, pyrrole, and diethylether were distilled over sodium. Chlorinated solvents were distilled over CaH2 and subsequently stored over 3 Å molecular sieves. The 400 MHz 1H NMR, 100 MHz 13C NMR, and 160.4 MHz 11B NMR were recorded on a Bruker Advance 400 MHz NMR spectrometer. All solution 1H and 13C spectra were referenced internally to the solvent signal. 11 B spectra were referenced externally to BF3·Et2O (δ = 0) in C6D6. Electronic absorption spectra were recorded on a PerkinElmer LAMBDA 750 UV−vis spectrophotometer. Solution 3622

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146.82, 142.41, 141.50, 139.38, 134.85, 132.49, 128.86, 121.64, 24.20, 21.80.11B NMR (160 MHz, CDCl3, δ ppm) 77.0. MALDI m/z (M+) calcd for C116H114N4B4Zn 1668.8550, found 1668.6750.

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ASSOCIATED CONTENT

S Supporting Information *

Additional characterization and experimental data. Crystallographic data in CIF format (CCDC numbers 938931− 938932).This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 0091-80-23601552. Phone: 0091-80-22933353. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.T. thanks the DST, New Delhi, and CSIR, New Delhi and ISRO-STC for the financial support. C.A.S.P. thanks IISc for SRF, and S.M. thanks CSIR, New Delhi, for Shyama Prasad Mukherjee Fellowship.

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