A pyrylium–coumarin dyad as a colorimetric receptor for ratiometric detection of cyanide anions by two absorption bands in the visible region

Abstract

We synthesized a pyrylium–coumarin dyad (1) and used it for colorimetric detection of cyanide anions (CN⁻) in aqueous media. The receptor itself exhibits a long-wavelength absorption band at 643 nm due to the strong intramolecular charge transfer (ICT) from the diethylaminocoumarin to the pyrylium moiety. Selective nucleophilic addition of CN⁻ to the pyrylium ring triggers the ring cleavage rapidly (within 8 min). This suppresses the ICT and decreases the intensity of the 643 nm band, along with an appearance of a new shorter-wavelength band at 472 nm assigned to the π,π* transition of the diethylaminocoumarin moiety. This drastic absorption change (643 nm → 472 nm) facilitates ratiometric detection of CN⁻ based on the intensities of these two absorption bands in the visible region and successfully detects CN⁻ as low as 8 μM.

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Introduction

Cyanide anions (CN⁻) are extremely toxic to the human body.¹ Strong binding of CN⁻ to a heme unit of the cytochrome c paralyzes cellular respiration² and causes serious damages to the central nervous system.³ Cyanides are, however, widely employed in industry for the synthesis⁴ and metallurgy,⁵ inevitably resulting in accidental release of CN⁻ into the environment. The cyanide level in industrial waste is therefore strictly regulated: for example, Japan’s Ministry of Environment sets its permissive level at 38.5 μM.⁶ Accurate quantification of CN⁻ in the environmental samples is therefore necessary. Several analytical methods such as voltammetry, potentiometry, and chromatography have been used for CN⁻ detection.⁷ These methods successfully detect very low levels of CN⁻ (<0.1 μM), but need tedious sample pretreatment or expensive instrumentation.

The design of a colorimetric receptor for CN⁻ has attracted a great deal of attention because it facilitates quantitative CN⁻ detection by simple absorption analysis on a common UV-vis spectrophotometer. A number of colorimetric CN⁻ receptors have been proposed so far. Most of the receptors show a single-wavelength absorption band, and the change in its absorbance is used for quantification of CN⁻.⁸ It is, however, well known that absorbance is strongly affected by factors such as instruments and receptor amounts, and a tedious calibration procedure is necessary for accurate quantification.⁹

In contrast, ratiometric receptors, which exhibit an absorption band in addition to the inherent one upon interaction with CN⁻, enable quantification just by monitoring the intensities of the two absorption bands. This enables accurate CN⁻ quantification without the need to consider the effects of instruments and receptor amounts. A number of ratiometric CN⁻ receptors have been proposed. Several receptors containing metal complexes (Zn²⁺, Ni²⁺, or Cu²⁺) detect CN⁻via the removal of the metal cation by the reaction with CN⁻, but produce inorganic salts as wastes.¹⁰ Many organic receptors display poor water solubility,¹¹ poor CN⁻ selectivity,¹² or a relatively long time (>10 min) for CN⁻ detection.¹³ Several receptors overcome these problems;¹⁴ however, some show an absorption band in the UV region (λ < 400 nm), which often overlaps with those of many components in the environmental samples; some also exhibit two absorption bands at very close wavelengths (Δ = ∼50 nm), which hinders accurate determination of the absorbance. To the best of our knowledge, there is only one report of ratiometric CN⁻ receptor that facilitates rapid and selective CN⁻ detection by two well-separated absorption bands in the visible region.¹⁵ Exploiting a new receptor creates many options in effective ratiometric CN⁻ detection.

A 2,4,6-trisubstituted pyrylium moiety is one powerful platform for colorimetric CN⁻ detection.¹⁶ As shown in Scheme 1a, nucleophilic addition of CN⁻ to the carbon atom at the ortho position of the pyrylium moiety leads to a drastic conformational change via a ring opening and promotes significant absorption change. Early reported pyrylium-based receptors (Scheme 1b–d),¹⁶ dissolved in aqueous media, show a long-wavelength absorption band at 450–600 nm due to the intramolecular charge transfer (ICT) from the substituent at the para position to the pyrylium moiety. Selective nucleophilic addition of CN⁻ to the pyrylium moiety leads to a decrease in these bands. They, however, scarcely show a new absorption band in the visible region; therefore, they cannot be used for ratiometric CN⁻ detection.

Scheme 1 (a) Nucleophilic addition of CN⁻ to 2,4,6-trisubstituted pyrylium cations. (b–d) Structures of earlier reported pyrylium-based CN⁻ receptors.

The purpose of the present work is to design a pyrylium-based receptor for ratiometric detection of CN⁻. As shown in Scheme 2, we synthesized a simple diethylaminocoumarin–pyrylium dyad (1). The receptor dissolved in aqueous media shows a long-wavelength absorption band at 643 nm due to the strong ICT from the diethylaminocoumarin to the pyrylium moiety. Relatively rapid nucleophilic addition of CN⁻ to 1 (within 8 min) selectively triggers the cleavage of the pyrylium ring. This suppresses ICT and creates a new short-wavelength absorption at 472 nm assigned to the π,π* transition of the diethylaminocoumarin moiety itself. This drastic absorption change (Δ = 171 nm) therefore facilitates ratiometric detection of CN⁻ and successfully detects very low levels of CN⁻ (detection limit: 8 μM).

Scheme 2 Synthesis of receptor 1.

Results and discussion

Synthesis and absorption properties

Receptor 1 was synthesized by the procedure depicted in Scheme 2, with 2,6-diphenylpyrylium tetrafluoroborate (3)¹⁷ as a starting material. Reaction of 3 with CH₃MgI in diethyl ether for 10 h followed by reaction with tetraphenylphosphonium tetrafluoroborate (TPP⁺BF₄⁻) in MeCN for 3 h gave 4-methyl-2,6-diphenylpyrylium tetrafluoroborate (2)¹⁸ as a brown powder with moderate yield (59%). Condensation of 2 with 7-diethylaminocoumarin-3-aldehyde¹⁹ in MeCN for 2 h gave 1 with 91% yield as a dark blue solid. The purity of the products was confirmed by ¹H, ¹³C NMR and FAB-MS analyses (Fig. S1–S4, ESI†).

The absorption spectra of 1 (10 μM) was measured in a buffered water/MeCN mixture (1/9 v/v; Tris–HCl 10 mM, pH 9.0) at 25 °C. As shown in Fig. 1a, the aqua blue solution containing 1 shows a distinctive absorption band at 643 nm. Addition of 20 equiv. of each respective anions (F⁻, Cl⁻, Br⁻, I⁻, H₂PO₄⁻, NO₃⁻, AcO⁻, ClO₄⁻, SCN⁻, and HSO₄⁻) to the solution followed by stirring for 10 min scarcely changes the spectrum. In contrast, as shown by the red line, addition of CN⁻ leads to a decrease in the 643 nm band, along with an appearance of significantly blue-shifted band at 472 nm. This suggests that 1 selectively reacts with CN⁻ and promotes drastic spectral change. In addition, as shown in Fig. 1b, the CN⁻-induced spectral change is scarcely affected by the presence of other anions. These data clearly suggest that 1 selectively detects CN⁻ even in the presence of these contaminating anions. It must also be noted that 1 rapidly detects CN⁻. Fig. 2 shows the time-dependent change in the absorption spectra of 1 measured after addition of 20 equiv. of CN⁻. The absorption change is mostly completed within 8 min, which is much faster than that for early reported receptors (>10 min).^{10c,11a,12f,g,13,14a}

Fig. 1 (a) Absorption spectra of 1 (10 μM) measured with 20 equiv. of each respective anions in a buffered water/MeCN mixture (1/9 v/v; Tris–HCl 10 mM, pH 9.0) at 25 °C. Photographs of the solutions obtained with or without CN⁻ are also shown in the figure. (b) Absorption spectra of 1 (10 μM) measured with 20 equiv. of CN⁻ together with 20 equiv. of other each respective anions (X⁻). All of the measurements were performed after stirring the solution for 10 min.

Fig. 2 Time-dependent change in the absorption spectra of 1 (10 μM) measured with 20 equiv. of CN⁻ in a buffered water/MeCN mixture (1/9 v/v; Tris–HCl 10 mM, pH 9.0) at 25 °C. The spectra obtained at 0 and 8 min are indicated by blue and red lines, respectively.

Ratiometric sensing of CN⁻

Fig. 3 shows the results of absorption titration of 1 with CN⁻. Stepwise addition of CN⁻ to the solution leads to a decrease in the 643 nm band, along with an increase in the 472 nm band. These clearly-separated two bands of 1 in the visible region facilitate ratiometric detection of CN⁻. Fig. 4 shows the change in the absorbance ratio of the two bands (A₆₄₃/A₄₇₂), when plotted as a function of CN⁻ concentration. A linear relationship is observed in the range of 8–200 μM CN⁻ (R² = 0.998). This indicates that 1 facilitates accurate CN⁻ sensing in this range. The detection limit (8 μM) is below the maximum permissive level of cyanide in the industrial effluents (38.5 μM), set by Japan’s Ministry of Environment. The receptor therefore allows sensitive CN⁻ detection in aqueous media.²⁰

Fig. 3 Result of absorption titration of 1 (10 μM) with CN⁻ in a buffered water/MeCN mixture (1/9 v/v; Tris-HCl 10 mM, pH 9.0) at 25 °C. All of the measurements were performed after stirring the solution for 10 min. Photographs of the solutions obtained with different amounts of CN⁻ (equiv.) are shown at the bottom.

Fig. 4 Change in the absorbance ratio (A₆₄₃/A₄₇₂) of 1 (10 μM) with CN⁻ concentration in a buffered water/MeCN mixture (1/9 v/v; Tris-HCl 10 mM, pH 9.0) at 25 °C.

Mechanism for nucleophilic addition of CN⁻

As shown in Fig. 3, during the titration of 1 with CN⁻, three isosbestic points are clearly observed at 352, 427, and 540 nm, respectively, suggesting that the reaction of CN⁻ with 1 produces a single component. To clarify the stoichiometry for the reaction, an MeCN (10 mL) solution containing 1 (1 mg, 1.8 μmol) and CN⁻ (0.48 mg, 1.8 μmol) was stirred for 30 min, and subjected to FAB-MS analysis. As shown in Fig. S5 (ESI†), the solution shows a peak at m/z 501.2, assigned to [1 + CN⁻−BF₄⁻ + H⁺]⁺ species, suggesting that 1 reacts with CN⁻ in a 1 : 1 stoichiometry.

As shown in Scheme 3, the reaction of CN⁻ with 1 occurs via the nucleophilic addition of CN⁻ to the ortho position of the pyrylium ring of 1, as is the case for related pyrylium-based receptors (Scheme 1a),¹⁶ leading to the formation of the ring-opened 1–CN⁻ species. To clarify this, ¹H NMR analysis was performed with compound 3 (Scheme 2) as a model compound in place of 1.²¹ It is noted that 3 shows properties similar to those of 1; as shown in Fig. S6 (ESI†), absorption titration of 3 with CN⁻ leads to a decrease in a 401 nm band and an increase in a 340 nm band, with an isosbestic point at 364 nm. FAB-MS analysis of the product obtained by the reaction of 3 with 1 equiv. of CN⁻ shows a peak at m/z 260.1, assigned to [3 + CN⁻−BF₄⁻ + H⁺]⁺ species (Fig. S7, ESI†), indicative of 1 : 1 interaction of 3 with CN⁻, as is the case for 1.

Scheme 3 Proposed mechanism for nucleophilic addition of CN⁻ to 1.

Fig. 5 shows the ¹H NMR charts of 3 and the 3–CN⁻ species in CDCl₃, where the chemical shifts of 3–CN⁻ were assigned based on the 2D COSY spectra (Fig. S8, ESI†). Addition of CN⁻ to 3 leads to significant upfield shift of the pyrylium protons (H^a, H^b), while the phenyl ring protons (H^c, H^d, H^e) scarcely shift. The triplet H^a proton (9.24 ppm) changes to double doublet (7.92 ppm), and doublet H^b protons (8.64 ppm) separate into two doublets (7.51 and 7.33 ppm). This suggests that, as shown in Fig. 5 (top), nucleophilic addition of CN⁻ to the ortho position of the pyrylium ring leads to ring cleavage. In addition, coupling constant of the double doublet H^a protons of 3–CN⁻ is 15.1 and 11.4 Hz. It is well known that trans-coupling of protons in diene derivatives usually exhibits a large coupling constant (12–18 Hz).²² The large coupling constant (15.1 Hz) is therefore due to the trans-coupling between H^a and H^b2 protons. This suggests that the diene moiety of 3–CN⁻ exists in a trans form (Fig. 5, top). It must also be noted that, as shown in Fig. S9 (ESI†), IR analysis of 3–CN⁻ on a KBr disk shows a characteristic band at 1653 cm⁻¹, assigned to C=O stretching vibration. These ¹H NMR and IR data support the formation of ring-opened 3–CN⁻ species, as shown in Fig. 5 (top).

Fig. 5 ¹H NMR chart (400 MHz, 30 °C, CDCl₃) for (a) 3 (2 mM) and (b) 3–CN⁻ species (10 mM). The peaks denoted by asterisks are assigned to the residual CHCl₃.

¹³C NMR analysis of the 3–CN⁻ species further confirms the reaction mechanism (Fig. 5, top). As shown in Fig. 6b, addition of CN⁻ to 3 creates a new peak at a higher magnetic field (115.64 ppm), assigned to the –C≡N group added to 3. As shown in Fig. 6a, the carbon atoms at the ortho position of the pyrylium ring of 3 (C2 and C6) appear at 173.3 ppm. Addition of CN⁻ separates it into two peaks at 121.5 and 189.7 ppm, respectively. The significant downfield shift of the C6 carbon is due to the formation of the –C=O group via the ring-opening reaction. The results agree with the ¹H NMR (Fig. 5) and IR data (Fig. S9, ESI†). The above findings obtained with compound 3 as a model compound clearly suggest that, as proposed in Scheme 3, nucleophilic addition of CN⁻ to the ortho position of the pyrylium ring of 1 produces ring-opened 1–CN⁻ species. This drastic conformational change therefore leads to a significant change in absorption spectra (Fig. 3).

Fig. 6 ¹³C NMR chart of (a) 3 (10 mM, 100 MHz, 30 °C, CD₃CN) and (b) 3–CN⁻ species (10 mM, 100 MHz, 30 °C, CDCl₃). The respective chemical shifts were assigned based on ab initio calculations (B3LYP/6-311+G(2d,p)) using PCM with MeCN and CHCl₃, respectively.

Mechanism for ratiometric absorption response

The significant blue shift of the absorption spectrum of 1 upon addition of CN⁻ is due to the localization of π-electrons on the diethylaminocoumarin moiety. This is confirmed by ab initio calculation based on time-dependent density functional theory (TD-DFT)²³ performed within the Gaussian 03 program²⁴ using the B3LYP/6-31G* basis set. As summarized in Table S1 (ESI†), singlet electronic transition of 1 mainly consists of the HOMO → LUMO (S₀ → S₁) transition. Its transition energy is determined to be 2.14 eV (580 nm), which is close to the observed λ_max (643 nm) of the absorption spectrum of 1 (Fig. 3). As shown in Fig. 7 (left), π-electrons of the HOMO of 1 are localized on the diethylaminocoumarin moiety, while those of the LUMO are delocalized over the entire molecule. The total change in the dipole moment for the S₀ → S₁ transition is relatively large (9.6 Debye). This indicates that the 643 nm absorption band of 1 (Fig. 3) is due to the intramolecular charge transfer (ICT)²⁵ from the diethylaminocoumarin to the pyrylium moiety.

Fig. 7 Energy diagrams for main orbitals of (left) 1 and (right) 1–CN⁻ species, calculated at the DFT level (B3LYP/6-31G*) using PCM with MeCN as a solvent. Gray, white, blue, and red atoms indicate C, H, N, and O atoms, respectively. Green and deep red parts on the molecular orbitals refer to the different phases of molecular wave functions (isovalue: 0.02 a.u.).

As shown in Table S1 (ESI†), singlet electronic transition of 1–CN⁻ species is mainly contributed by the HOMO → LUMO+1 (S₀ → S₄) transition. Its energy is determined to be 3.14 eV (395 nm), which is close to λ_max of the absorption spectrum of the 1–CN⁻ species (472 nm, Fig. 3). As shown in Fig. 7 (right), π-electrons of both the HOMO and LUMO+1 for 1–CN⁻ are localized on the diethylaminocoumarin moiety. The total change in the dipole moment for the S₀ → S₄ transition is very small (1.2 Debye). This indicates that the 472 nm absorption band is due to the π,π* transition of the diethylaminocoumarin moiety, and the transition has almost no ICT character. The nucleophilic addition of CN⁻ to 1 promotes cleavage of the pyrylium ring. This suppresses ICT from the diethylaminocoumarin to the pyrylium moiety and leads to localization of the electrons on the diethylaminocoumarin moiety. The disappearance of the ICT character therefore facilitates significant blue shift of the absorption spectra (643 nm → 472 nm) and, hence, facilitates ratiometric detection of CN⁻.

Conclusions

We found that a pyrylium–coumarin dyad (1) behaves as a colorimetric receptor for ratiometric detection of CN⁻ in aqueous media. The receptor itself shows a long-wavelength-absorption band (643 nm) due to the ICT from the coumarin to the pyrylium moiety. Nucleophilic addition of CN⁻ to the receptor triggers selective cleavage of the pyrylium ring. This suppresses ICT and creates a new shorter-wavelength-absorption band (472 nm) due to the π,π* transition of the coumarin moiety. This facilitates selective, sensitive (detection limit: 8 μM), and rapid (assay time: 8 min) ratiometric detection of CN⁻ by two absorption bands in the visible wavelength region. Application of the present receptor to real sample sensing is still difficult because it involves several problems that must be addressed: it needs organic solvents due to the low solubility in pure water; and it shows a relatively high detection limit. Nevertheless, the basic concept presented here based on the change in two absorption bands via the nucleophilic addition of CN⁻ to the pyrylium moiety may contribute to the design of more efficient CN⁻ receptors.

Experimental

General

All of the anions were used as n-Bu₄N⁺ salts. All of the reagents used were purchased from Wako, Aldrich, and Tokyo Kasei, and used without further purification. Water was purified using the Milli-Q system. Absorption spectra were measured on an UV-visible photodiode-array spectrophotometer (Shimadzu; Multispec-1500) equipped with a temperature controller (Shimadzu; S-1700) using a 10 mm path length quartz cell under aerated conditions. ¹H and ¹³C NMR charts were obtained using a JEOL JNM-ECS400 spectrometer. FAB-MS analysis was performed on a JEOL JMS 700 Mass Spectrometer. Compound 3 was synthesized according to the literature procedure.¹⁷

Synthesis

Compound 2. This was synthesized according to the literature procedure.¹⁸3 (186 mg, 0.55 mmol) was stirred in diethyl ether (10 mL) containing CH₃MgI (1.66 g, 10 mmol) at 25 °C for 10 h under N₂. The solution was washed with saturated NH₄Cl solution (10 mL × 2) and water (10 mL × 2), and concentrated by evaporation. The resultant and TPP⁺BF₄⁻ (280 mg, 1.1 mmol) were stirred in MeCN (4 mL) at 25 °C for 3 h. The resultant was concentrated by evaporation and dissolved in a small amount of acetone (1 mL). The precipitate formed by the addition of n-hexane (3 mL) was recovered by filtration, affording 2 as a brown powder (103.8 mg, 59%). ¹H-NMR (DMSO-d₆) δ (ppm): 8.84 (2H, s), 8.43 (4H, d, J = 7.8 Hz), 7.86–7.78 (6H, m), 2.84 (3H, s).

Compound 1. 2 (30 mg, 0.09 mmol) and 7-diethylaminocoumarin-3-aldehyde¹⁹ (28 mg, 0.11 mmol) were dissolved in MeCN (7 mL) and stirred at 25 °C for 2 h. The color of the solution changes immediately to dark green. The solution was concentrated by evaporation, and EtOH (10 mL) was added to the resultant. The solids formed were recovered by filtration and washed thoroughly with EtOH (20 mL), affording 1 as a dark blue solid (40.9 mg, 91%). ¹H-NMR (DMSO-d₆) δ (ppm): 8.69 (2H, s), 8.50 (1H, d, J = 15.6 Hz), 8.36 (4H, d, J = 7.3 Hz), 8.24 (1H, s), 7.86 (1H, d, J = 15.1 Hz), 7.79–7.71 (6H, m), 7.60 (1H, d, J = 9.2 Hz), 6.86 (1H, d, J = 8.2 Hz), 6.63 (1H, s), 3.54 (4H, d, J = 6.9 Hz), 1.18 (6H, t, J = 6.9 Hz). ¹³C NMR (100 MHz, DMSO-d₆, TMS) δ (ppm): 166.40, 162.30, 158.75, 157.26, 153.69, 149.39, 146.89, 134.02, 132.15, 129.63, 129.28, 127.65, 121.93, 113.24, 113.03, 111.04, 109.55, 96.38, 44.78, 12.41. FAB-MS: m/z: calcd for C₃₂H₂₈NO₃⁺ ([1−BF₄⁻]⁺) 474.6; found: 474.2; HR-MS (FAB+): m/z: calcd for C₃₂H₂₈NO₃⁺ ([1−BF₄⁻]⁺): 474.2064; found: 474.2069.

3–CN⁻ species. 3 (112 mg, 0.34 mmol) and n-Bu₄N⁺CN⁻ (92 mg, 0.34 mmol) were dissolved in MeCN (4 mL) and stirred at 25 °C for 5 h. The solution was concentrated by evaporation. The resultant (204 mg) was adsorbed onto a silica gel column (100 g) and eluted using a mixture of toluene/CH₂Cl₂/n-hexane (1/1/3 v/v/v, 600 mL), (3/2/3 v/v/v, 600 mL), and then (0/1/0 v/v/v, 600 mL). Condensation of the final eluent afforded 3–CN⁻ as a yellow solid (3.2 mg, 4%). ¹H-NMR (CDCl₃) δ: 7.98 (2H, d, J = 5.3 Hz), 7.92 (1H, dd, J = 15.1, 11.4 Hz), 7.70–7.68 (2H, m), 7.61 (1H, t, J = 7.6 Hz), 7.51 (2H, t, J = 7.6 Hz), 7.47–7.43 (4H, m), 7.33 (1H, d, J = 15.1 Hz). ¹³C NMR (100 MHz, CDCl₃, TMS) δ (ppm): 189.66, 138.55, 138.20, 137.41, 133.35, 132.45, 132.24, 130.62, 129.30, 128.82, 128.58, 126.34, 121.53, 115.64. FAB-MS: m/z: calcd for C₁₈H₁₄NO⁺ ([3−BF₄⁻ + CN⁻ + H⁺]⁺) 260.3; found: 260.1; HR-MS (FAB+): m/z: calcd for C₁₈H₁₄NO⁺ ([3−BF₄⁻ + CN⁻ + H⁺]⁺): 260.1070; found: 260.1075.

Calculation details

Ab initio calculations were performed with tight convergence criteria at the DFT level with the Gaussian 03 package,²⁴ using the B3LYP/6-31G* basis set for all atoms. The excitation energies and oscillator strengths of the compounds were calculated by TD-DFT²³ at the same level of optimization. Calculated NMR charts were obtained using the DFT GIAO/B3LYP method with the 6-311+G(2d,p) basis set,²⁶ using the polarizable continuum model (PCM) with MeCN or CHCl₃ as a solvent.²⁷ Cartesian coordinates are summarized at the end of the ESI.†

Acknowledgements

This work was supported by the Grant-in Aid for Scientific Research (No. 15K06556) from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) and the Precursory Research for Embryonic Science and Technology (PRESTO) from Japan Science and Technology Agency (JST).

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Footnote

† Electronic supplementary information (ESI) available: Table S1, Fig. S1–S10, and Cartesian coordinates for the compounds. See DOI: 10.1039/c5nj02219c

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