Selective naked-eye cyanide detection in aqueous media using a carbazole-derived fluorescent dye

Abstract

A Michael addition activated chemodosimeter was designed with a carbazole donor and a dicyanovinyl acceptor. The design features an acceptor–donor–acceptor (A–D–A) molecular configuration and the cyanide addition on the electron-deficient alkene bridge breaks the donor–acceptor interaction and produces a blue shift in the absorption and emission profiles. The ratiometric and colorimetric changes of the probes were monitored by the changes in absorption and fluorescence spectra. The probes exhibited excellent selectivity towards CN⁻ over other anions such as F⁻, OAc⁻, NO₃⁻, NO₂⁻, S₂O₃⁻, H₂PO₄⁻, SCN⁻, I⁻, Br⁻, and SO₄⁻. The action of the chemodosimeter was investigated by ¹H-NMR titrations and DFT/TDDFT theoretical calculations which supported the formation of dicyanide addition adducts (CN–probes–CN)⁻² with the probes. Paper strips developed using the dyes displayed excellent response for the detection of CN⁻ in aqueous solutions. The detection limits of the probes 3a (0.126 μM) and 3b (0.140 μM) for cyanide are lower than the maximum permissible level of CN⁻ (1.9 μM) in drinking water.

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Introduction

Though anions such as phosphate, carboxylate and sulphate are essential for biological function, cyanide is extremely poisonous to living organisms.¹ Due to the strong affinity of cyanide towards metal ions, it binds irreversibly with the iron in the heme unit and the active site of cytochrome-c.² Such cyanide actions completely stop the oxygen transport in the blood and the mitochondrial electron-transport chain. This affects the oxidative metabolism in living organisms due to the non-availability of oxygen.² So, the exposure of cyanide to humans over the permissible level (1.9 μM) prescribed by World Health Organization (WHO) is dangerous.³ However, the increased usage of cyanide in gold mining, electroplating, petrochemicals and organic synthesis, etc.⁴ often leads to cyanide contaminated wastes entering the environmental stream. In order to avoid the release of cyanide into the environment, the effluents in the related industrial processes must be qualitatively/quantitatively analysed for the presence of cyanide. Several modular approaches have been proposed for the detection and quantification of cyanide in the aqueous solution.⁵ Firstly, accurate detection of cyanide in aqueous solution requires a molecular sensor to be soluble in aqueous solution. Majority of the chemical sensors developed so far are predominantly soluble in organic solvents which jeopardizes its practical utility and very few are functional in aqueous solutions.⁶ However, the paper strip approach is generally successful for the analysis of aqueous wastes.⁷ It serves as a potential text tool for the “in-the-field” detection of cyanide that could avoid additional equipment and labour in remote areas. Generally, colorimetric chemodosimeters function by adopting two basic phenomena such as intramolecular charge transfer (ICT)⁸ and fluorescence resonance energy transfer (FRET)⁹ operational in designed molecular materials. Indeed, ICT is advantageous over FRET due to its functional and structural simplicity and easy synthetic access.¹⁰

Molecular receptors used for the cyanide sensing work by the addition reaction on the activated carbon–carbon/heteroatom double bond,^8,10 displacement reactions on metal centres,¹¹ hydrogen bonding interactions with proton donors,¹² nucleophilic addition on boron derivatives¹³ and CdSe quantum dots,¹⁴ etc. Also, most of the cyanide sensors function by the fluorescence ON–OFF or OFF–ON mechanisms. Practically, such probes cannot be deployed in remote places without extra equipment/light source. Also, the selectivity, sensitivity, and visible changes in colour of the probes are still unacceptable. Thus, undoubtedly development of an efficient sensing probe which is cost effective, sensitive enough, highly selective, fast response time and good signal to noise ratio to detect the cyanide anion is desirable. A probe showing colorimetric response to naked eyes is desirable for qualitative detection.^8a In this regard, reaction based probes were developed which are advantages over the other approaches developed so far with its unique selectivity, effective suppression of unwanted hydrogen bonding interactions of analyte with probe, and adverse environmental effects of media.^8d Among the reaction based probes developed so far, dicyanovinyl based probes are exceptional because of its strong electron withdrawing nature which is beneficial to extend spectral response. It also helps to improve both selectivity and sensitivity towards low concentrations of CN⁻ anion in the presence of other interfering ions. Moreover, the effective reaction based colorimetric probes for cyanide detection are scarce. Also, most of the reaction based probes featuring dicyanovinyl are in principle capable of interacting with one equivalent of cyanide anion.^8c–k We envisaged that the presence of multiple sensing sites in a probe will enhance the detection scope. Also a distinctive colour change in the absorption and emission profiles is desirous to facilitate the naked eye observation. If the conjugation alternation occurring during the interaction of cyanide with the probe is large then the corresponding absorption and emission changes will be also substantial. In this work, we have designed two probes featuring differences in the conjugation pathway. The thiophene containing dye, 3b possess more planar arrangement in the conjugation bridge while the phenyl analogue, 3a has tilted geometry and thereby reduced π-conjugation. We have investigated the effect of the conjugation pathway in the cyanide sensing properties. As expected, here we observed dramatic hypsochromic shift in absorption (80–106 nm) and in emission (140–142 nm) profiles of both the probes 3a and 3b on reaction with cyanide, which is rarely found in the related literature. The photophysical, electrochemical and optoelectronic properties of dicyanovinyl based derivatives of carbazole were well documented in the literature.¹⁵ To the best of our knowledge, carbazole-based dicyanovinyl derivative have not been exploited for the detection of cyanide anion.

Results and discussion

Synthesis and characterization

The probes containing carbazole donor and dicyanovinyl acceptors and different conjugation pathway, i.e., phenyl or thiophene were synthesized by the protocol shown in Scheme 1. The aldehydes were accessed by Suzuki¹⁶ or Stille¹⁷ cross coupling reactions of the corresponding reagents with 2,7-dibromo-9-(2-ethylhexyl)-9H-carbazole. Subsequently, the aldehydes were converted to the desired probes by Knoevenagel condensation with two equivalents of malononitrile in ethanol in the presence of piperidine. 2,7-Disubstituted carbazole was chosen over the 3,6-disubstituted carbazole as the former will lead to effective conjugation extension beneficial for the intense π–π* electronic excitation besides the built-in ICT.¹⁸ Interaction of cyanide would reduce the ICT while retaining the π–π* absorption. The dyes are reasonably soluble in common organic solvents such as dichloromethane, CHCl₃, THF, toluene, acetonitrile, dimethylsulfoxide and dimethylformamide and partially soluble in alcohols. The dyes were thoroughly characterized by ¹H and ¹³C NMR and high resolution mass spectral methods.

Scheme 1 Synthesis of the cyanide probes 3a and 3b.

Photophysical properties

The absorption and emission spectra of the probes were analysed in different solvents. The representative spectra collected for toluene solutions are displayed in Fig. 1. The dye 3a is yellow in colour and exhibited absorption maxima at 405 nm while the dye 3b inherited orange colour and displayed absorption maxima at 466 nm attributable to the intramolecular charge transfer (ICT) from carbazole donor to dicyanovinyl acceptor on both ends. The red-shifted absorption for 3b than the phenylene derivative (3a) is in accordance with the effective and comparatively electron-rich conjugation pathway present in 3b with the aid of electron rich thiophene bridge. The nonplanar arrangement of phenyl units in 3a probably reduces the conjugation. Cyan and green emissions were observed for the dyes 3a and 3b respectively in toluene solutions (Fig. 1) (Table 1).

Fig. 1 Absorption and emission spectra of the dyes recorded in toluene.

Table 1 Optical data for the dyes

Probe	λmax nm (εmax M^−1 cm^−1 × 10^3)			λem^a nm (ΦF)
	TOL	ACN	ACN + 5% H2O	TOL	ACN	ACN + 5% H2O
a Relative quantum yield measured by using coumarin-6 (ΦF = 0.78 in ethanol)^19 as standard.
3a	413 (60.0), 292 (24.2)	405 (68.2)	405 (68.6)	480 (0.48)	560 (0.10)	560 (0.03)
3b	446 (58.6), 305 (12.8)	466 (79.3)	466 (77.2)	523 (0.21)	576 (0.07)	568 (0.01)

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Sensor properties

Colorimetric detection. The cyanide sensing capability of the dyes 3a and 3b was investigated in aqueous acetonitrile (ACN) solution (ACN : H₂O = 95 : 5). The dyes were treated with 30 equiv. of CN⁻ and other interfering anions such as F⁻, Br⁻, I⁻, OAc⁻, SO₄²⁻, NO₂⁻, NO₃⁻, SCN⁻, S₂⁻, S₂O₃²⁻ and H₂PO₄⁻. Addition of CN⁻ to the dyes 3a and 3b (Fig. S1†) alone led to considerable hypsochromic shift in the absorption spectra (Fig. 2). On addition of increasing amounts of cyanide the absorption bands of the dyes 3a and 3b (Fig. S2†) at 405 and 466 nm decreased in intensity with a concomitant appearance of a new blue-shifted absorption profile centred at 327 and 360 nm respectively (Fig. 3). The blue-shift of the absorption spectra featuring isobestic points at 354 nm, and 389 nm in 3a and 3b respectively, suggests that the π-conjugation and intramolecular charge transfer, if any, were interrupted by the nucleophilic addition of CN⁻ at α-carbon of the dicyanovinyl end caps and also implies that its clear transformation into the new species.

Fig. 2 Absorption spectra of 3a in aqueous acetonitrile the presence of different anions.

Fig. 3 Absorption changes for 3a on addition of cyanide in aqueous acetonitrile.

Further, the complete bleaching of 3b required more amounts of cyanide than 3a. This indicates the reduced reactivity of the dye with cyanide ion. Since, it is a nucleophilic addition reaction, the increased electron-richness of the thiophene derivative, 3b, hinders the rate of the reaction (vide supra). This was further confirmed by the time-dependence of the sensor action (Fig. 4). Two equivalents of cyanide reacted completely with the dye 3a in ca. 6 min, while the dye 3b (Fig. S3†) required more time and excess of cyanide (10 equivalents) to complete the bleaching action. The rate constants were further measured under pseudo-first-order approximation. The reactions between the cyanide and the dyes 3a and 3b were monitored in aqueous acetonitrile solution at room temperature by recording the absorption intensities at 405 nm for 3a and 466 nm for 3b at various time intervals. A plot of ln[(A_max − A_t)/A_max], where A_max and A_t are the absorbance at time, t and end of the reaction respectively, against time gave a linear fit with the slope corresponding to the pseudo-first-order rate constant of 0.058 min⁻¹ and 0.033 min⁻¹ for the dyes 3a and 3b (Fig. S4†) respectively. A slightly larger rate constant for 3a is in accordance with its fast response for cyanide in solution.

Fig. 4 Kinetic data for the interaction of cyanide with 3a.

Interestingly, the most commonly interfering anions such as fluoride and acetate did not show any response with the probes. We have performed the competition experiments to ascertain the selectivity of the probes in the presence of other interfering anions. In this measurements, 2 equivalents of cyanide and 10 equivalents of the interfering anions were used. A representative selectivity bar chart observed for the probe 3a is displayed in Fig. 5. It clearly confirms tolerance of the probes toward the potentially interfering anions. This establishes that the newly developed carbazole-based probes can be used to quantitatively detect cyanide presence with high selectivity.

Fig. 5 Ratiometric absorbance changes (A₃₃₀/A₄₀₅) of 3a on addition of 2 equiv. of CN⁻ and 10 equiv. of other anions. Black bars indicate the blank and various anions, and red bars indicate the addition of CN⁻ to the interfering anions.

The detection limits of the probes 3a (0.126 μM) and 3b (0.140 μM) for cyanide are comparable to the maximum permissible level of CN⁻ declared by WHO in drinking water (1.9 μM). This clearly indicates that the new dyes can effectively detect very low concentrations of CN⁻.

To confirm the hypothesis of cyanide addition across the vinyl linkage, ¹H NMR spectral changes induced upon the addition of CN⁻ to the DMSO-d₆ solutions of the dyes were measured. On addition of cyanide to the dye, the signals corresponding to the vinyl protons at 8.60 and 8.64 ppm disappeared and new upfield signals appeared at 4.6 and 4.9 ppm respectively for the dyes 3a (Fig. 6) and 3b (Fig. S6†). The new signal is assignable to the tricyanoethyl proton. Also, the aromatic protons showed an upfield shift. All these observations are in accordance with the nucleophilic addition of cyanide at the π-carbon of the vinylic linkage. Upfield shift observed for the aromatic protons may be due to the removal of electron-withdrawing effect originating from the dicyanovinyl moiety and accumulation of negative charge on the tricyanoethyl unit. Finally, Jobs plots for 3a (Fig. 7) and 3b (Fig. S5†) constructed from the absorption titrations for the dyes confirmed the formation of 1 : 2 adduct with the cyanide.

Fig. 6 NMR spectral changes observed for the dye 3a on addition of various amounts of cyanide in DMSO-d₆.

Fig. 7 Jobs plot for the probe 3a.

Fluorimetric detection. The excitation of the dye 3a at 405 nm produced a dual emission with peaks at 480 and 560 nm. However, the excitation of the dye 3b at 460 nm gave a greenish emission peaking at 568 nm (Fig. 8). We presume that the appearance of dual emission attributable to the localized π–π* (shorter wavelength) and CT (longer wavelength) excited states originates from the tilted molecular arrangement. The non-planar structure is not suitable for charge transfer, however; on excitation undergoes a geometrical change to a more planar state and relaxes to the CT state. Such a twisted intramolecular charge transfer (TICT) is interesting and unique for the dye 3a. Addition of cyanide to 3a decreased the intensity of the peak at 560 nm with a concomitant formation of a new emission peak at 420 nm (Fig. 8). Similarly, for the dye 3b, the emission at the longer wavelength was replaced by a blue-shifted emission at 426 nm. No appreciable changes in the emission spectra were observed for the other interfering anions. On increasing the concentration of cyanide anions, the emission intensity of the peaks at 568 nm for the dye 3b gradually decreased with the evolution of emission at 426 nm. After the addition of 30 equiv. of cyanide anions, the peak at 568 nm was completely quenched. It led to a blue emission which is clearly visible by the naked eye under the irradiation of a hand held UV-lamp at 365 nm.

Fig. 8 Fluorescence changes for the dye 3b on interaction with various ions (top), change in emission intensity of 3b upon progressive addition cyanide anion at r.t, λ_ex = 460 nm (bottom).

Theoretical computations

To get more insight into the electronic structure of the dyes and rationalize the changes in the optical properties on interaction with cyanide, density functional theoretical computations were performed on the dyes and their cyanide adducts at the B3LYP/6-31G(d,p) level using the Gaussian 09 program suite.²⁰ The thiophene or phenyl groups were nearly coplanar with the dicyanovinyl unit. But the phenyl groups in 3a were slightly tilted (39.48°) from the central carbazole unit when compared to the thiophene unit (25.36°) in 3b. Overall the carbazole donors were reasonably conjugated with the terminal dicyanovinyl acceptors via the bridging phenyl and thiophene units in both the dyes. It is also understandable that due to the more planarized structure, the dye 3b possess red-shifted absorption.

In 3a HOMO is mainly localized on the carbazole while in 3b it is spread over the dithienylcarbazole segment (Fig. 9). This further attests the more planar arrangement of conjugated segment in 3b. LUMO is contributed by the whole molecule with major contribution from the phenyl/thienyl dicyanovinyl segment (Fig. 9). The HOMO to LUMO excitation in 3a is forbidden. It is predicted to exhibit a HOMO-1 to LUMO electronic transition with high oscillator strength. Interestingly, the HOMO-1 of 3a is similar to the HOMO of 3b and is constituted by the diphenylcarbazole fragment. Thus, the longer wavelength absorption in these dyes cannot be termed as a pure charge transfer transition. It is best described as a π–π* transition involving HOMO and LUMO orbitals in 3b and HOMO-1 and LUMO orbitals in 3a. On the contrary in both the cyanide adducts (3a–CN⁻ and 3b–CN⁻) HOMO is localized on the tricyanoethyl group this may be due to transformation of electron deficient dicyanovinyl segment into a relatively electron rich tricyanoethylate group on addition of cyanide at the vinylic carbon⁸ⁱ and the LUMO is contributed by the central core comprising phenyl/thiophene and carbazole units. And the primary absorption in the cyanide adducts is a carbazole and phenyl/thiophene localized π–π* transition. The cyanide adducts of 3a and 3b are predicted to show absorptions at 330.1 nm (f = 1.24) and 373.6 nm (f = 1.43) respectively (Table S1†). This is in agreement with the observed values.

Fig. 9 Electronic distribution in the frontier molecular orbitals of the dyes and their cyanide adducts.

Sensor strips

Finally, we have also fabricated paper strips for the detection of cyanide ion in aqueous solutions. The paper strip approach is quite useful for the detection of cyanide ion in environmental conditions. We have prepared the test strips coated with the dyes by immersing the Whatman filter papers in the dye solutions (2 × 10⁻³ M) in CH₃CN. Then the test strips coated with the dyes were exposed to the aqueous sodium cyanide solutions containing different cyanide strengths. The paper strips changed colour immediately (Fig. 10). Interestingly, the colour change is sharper for the dye 3b under illumination conditions. The paper strip almost acts as a pH indicator. Depending on the colour exhibited by the strip on illumination, the amount of cyanide present in the solution can be roughly estimated. To the best of our knowledge cyanide detection paper kit exhibiting emission colour response correlated to the concentration of cyanide have not been demonstrated.^7,21

Fig. 10 Response of (a) 3a and (b) 3b coated paper strips for the various concentrations (0 to 1 M) of aqueous cyanide solutions (under illumination with hand held UV-Vis lamp).

Conclusions

In summary, we have designed and synthesized two new colorimetric optical probes containing carbazole donor and dicyanovinyl acceptor. The dyes could interact, in principle, with two equivalents of cyanide ion and the interaction of cyanide with the probe inhibited the intramolecular charge transfer and led to the disappearance of the corresponding absorption band. Both the probes exhibited a remarkable selectivity over the other coexisting anions in the aqueous acetonitrile solutions. Due to the presence of two recognition sites, the sensitivity of the probes increased significantly. Particularly the dyes 3a displayed a maximum blue shift in the absorption (90 nm) on reaction with two equivalents of cyanide and exhibited a fast response time of 6 min. The irreversible binding nature of the probes with cyanide in 1 : 2 stoichiometric ratios induced a large hypsochromic shift in absorption (90–110 nm) and emission (140–145 nm) profiles. The test trips fabricated (“dip-stick” approach) showed visible colour changes on exposure to aqueous cyanide solutions under ‘day light’ and UV-Vis illumination. The dual channel detection approach on dicyanovinyl reactive probes presented here highlights the requirement of electron-deficient conjugation pathway for favourable interaction with cyanide ion.

Experimental section

General

All the chemicals were procured form commercial sources used as received. All the solvents were dried by following standard procedures and distilled prior to use. Doubly distilled water was used in all aqueous solution preparations. Mass spectral data were collected on ESI TOF High resolution spectrometer in positive mode. ¹H and ¹³C NMR spectral data were collected on Bruker spectrometer operating at 500.13 MHz and 125.77 MHz respectively, with TMS as internal standard. The absorption data were collected on Cary 100 UV-Vis spectrophotometer and the emission data on Shimadzu spectrofluorophotometer at room temperature. To perform the time dependent titrations and selectivity measurements, stock solution (1 × 10⁻⁵ M) of probes was prepared in CH₃CN + 5% H₂O at room temperature. The guest anion solutions (2 × 10⁻³ M) were prepared in the same solvent mixture.

The precursor (1) was synthesized by following literature procedure.²²

4,4′-(9-(2-Ethylhexyl)-9H-carbazole-2,7-diyl)dibenzaldehyde (2a)

A mixture of 1 (2.00 g, 5.25 mmol), 4-formylphenylboronic acid (1.73 g, 11.55 mmol) and Pd(PPh₃)₄ (0.33 mg, 0.29 mmol) were suspended in a inert mixture of THF and 2 M aqueous K₂CO₃ (3 : 1 volume ratio). The reaction mixture was refluxed at 80 °C under N₂ atmosphere for 24 h. After completion of the reaction, it was cooled to room temperature and extracted with dichloromethane (CH₂Cl₂), dried over anhydrous sodium sulphate. The solvent was removed under reduced pressure, the residue was purified with column chromatography on 100–200 mesh silica using hexane–CH₂Cl₂ (2 : 3) mixture as eluant to yield pale yellow solid, yield 1.30 g (60%); m.p 110–120 °C; ¹H NMR (CDCl₃, 500.13 MHz, δ ppm): 10.09 (s, 2H), 8.21 (d, J = 10.0 Hz, 2H), 8.01 (dd, J = 1.5, 6.7 Hz, 4H), 7.88 (d, J = 8.0 Hz, 4H), 7.63 (d, J = 0.5 Hz, 2H), 7.55 (dd, J = 1.5, 6.5 Hz, 2H), 4.33–4.24 (m, 2H), 2.17–2.12 (m, 1H), 1.50–1.27 (m, 8H), 1.96 (t, J = 7.4 Hz, 3H), 0.87 (t, J = 7.5 Hz, 3H); ¹³C NMR (CDCl₃, 100 MHz, δ ppm): 191.94, 148.13, 142.10, 137.83, 135.09, 130.36, 128.04, 122.63, 121.06, 118.94, 107.93, 47.45, 39.50, 31.00, 28.81, 24.53, 23.07, 14.05, 11.01; HRMS calcd for C₃₄H₃₃NO₂Na [M + Na]⁺ m/z 510.240, found 510.2439.

5,5′-(9-(2-Ethylhexyl)-9H-carbazole-2,7-diyl)bis(thiophene-2-carbaldehyde) (2b)

A mixture of 1 (2.30 g, 6.04 mmol), 5-(tributyl stannyl)thiophene-2-carbaldehyde (6.60 g, 13.28 mmol) were dissolved in 10 mL of dry dimethylformamide and degassed with N₂ followed by the addition of Pd(PPh₃)₂Cl₂ (0.96 g, 1 mol%) in 100 mL round bottom flask. The reaction mixture was heated at 90 °C for 24 h under N₂ atmosphere. After completion of reaction, extracted with CH₂Cl₂, organic layer were dried over anhydrous sodium sulphate. The solvent was removed under the residue pressure, the resultant crude product were further purified with column chromatography on 100–200 mesh silica using CH₂Cl₂–hexane (3 : 2) mixture as eluant to give orange solid, yield 1.20 g (70%); m.p 175–180 °C; ¹H NMR (CDCl₃, 500.13 MHz, δ ppm): 9.92 (s, 2H), 8.12 (d, J = 10.0 Hz, 2H), 7.79 (d, J = 4.0 Hz, 2H), 7.66 (d, J = 1.0 Hz, 2H), 7.58 (dd, J = 1.0, 8.2 Hz, 2H), 7.51 (d, J = 4.0 Hz, 2H), 4.24–4.21(m, 2H), 2.11–2.07 (m, 1H), 1.46–1.27 (m, 8H), 0.96 (t, J = 7.4 Hz, 3H), 0.87 (t, J = 7.5 Hz, 3H); ¹³C NMR (CDCl₃, 100 MHz, δ ppm): 182.76, 155.38, 148.38, 142.26, 137.59, 131.11, 124.11, 123.36, 121.23, 118.10, 107.00, 47.39, 39.49, 31.00, 28.74, 24.46, 23.03, 14.03, 10.97; HRMS calcd for C₃₀H₂₉NO₂S₂Na [M + Na]⁺ m/z 522.1538, found 522.1542.

2,2′-(((9-(2-Ethylhexyl)-9H-carbazole-2,7-diyl)bis(4,1-phenylene))bis(methanylylidene))dimalononitrile (3a)

A mixture of 2a (1.10 g, 2.26 mmol), malononitrile (0.44 g, 6.78 mmol), 3–4 drops of piperidine were together taken in ethanol (15 mL) and refluxed at 80 °C for 1 h. After completion of reaction, reaction mixture was allowed to cool to the room temperature, filtered and washed thoroughly with ethanol and dried. The resultant crude product was purified with column chromatography on 100–200 mesh silica using CH₂Cl₂–hexane (3 : 1) as eluant to give yellow solid, yield 0.94 g (78%); m.p 215–220 °C; ¹H NMR (CDCl₃, 500.13 MHz, δ ppm): 8.21 (d, J = 8.0 Hz, 2H), 8.05 (d, J = 8.5 Hz, 4H), 7.89 (dd, J = 1.6, 8.2 Hz, 4H), 7.82 (s, 2H), 7.64 (d, J = 0.5 Hz, 2H), 7.55 (dd, J = 1.3, 8.1 Hz, 2H), 4.29 (t, J = 6.5 Hz, 2H), 2.14 (t, J = 6.0 Hz, 1H), 1.47–1.25 (m, 8H), 0.98 (t, J = 7.0 Hz, 3H), 0.83 (t, J = 7.0 Hz, 3H); ¹³C NMR (CDCl₃, 125.77 MHz, δ ppm): 159.17, 148.21, 137.12, 131.49, 129.68, 128.35, 122.98, 121.32, 118.85, 113.97, 112.91, 107.86, 99.97, 47.50, 39.50, 30.98, 28.78, 24.45, 23.04, 14.03, 11.00; HRMS calcd for C₄₀H₃₃N₅ [M + H]⁺ m/z 583.2736, found 583.2743.

2,2′-((5,5′-(9-(2-Ethylhexyl)-9H-carbazole-2,7-diyl)bis(thiophene-5,2-diyl))bis(methanylylidene))dimalononitrile (3b)

It was synthesized by treating 2b (1 g, 2 mmol) and malononitrile (0.39 g, 6 mmol) with same protocol as followed for 3a synthesis as described above. Quantitative yield were isolated 1.2 g; m.p 210–220 °C; ¹H NMR (CDCl₃, 500.13 MHz, δ ppm): 8.12 (d, J = 8.5 Hz, 2H), 7.81 (s, 2H), 7.77 (d, J = 4.0 Hz, 2H), 7.67 (d, J = 1.0 Hz, 2H), 7.59 (dd, J = 1.5, 6.5 Hz, 2H), 7.55 (d, J = 4.5 Hz, 2H), 4.27–4.25 (m, 2H), 2.09–2.04 (m, 1H), 1.47–1.27 (m, 8H), 0.98 (t, J = 7.0 Hz, 3H), 0.87 (t, J = 7.0 Hz, 3H); ¹³C NMR (CDCl₃, 125.77 MHz, δ ppm): 157.47, 150.42, 142.09, 140.11, 134.20, 130.44, 124.71, 123.86, 121.59, 118.47, 114.25, 113.42, 107.17, 99.96, 47.33, 39.63, 31.07, 28.79, 24.51, 22.95, 13.98, 11.00; HRMS calcd for C₃₆H₂₉N₂S₂ [M + H]⁺ m/z 595.1864, found 595.1867.

Acknowledgements

Financial support to KRJT from Council of Scientific and Industrial Research (CSIR), New Delhi is gratefully acknowledged. RKK acknowledges a Junior Research Fellowship from CSIR, New Delhi.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Absorption spectra, Job plots, optimized structures, computed absorption parameters, ¹H & ¹³C NMR spectra. See DOI: 10.1039/c4ra02636e

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