1-(2-Naphthyl)benzimidazolium based tripod for fluorescence enhancement based recognition of surfactants in water

Abstract

1-(2-Naphthyl)benzimidazolium based fluorogenic tripod TRI-BINAP-1 exhibits highly selective and sensitive recognition of surfactants SDBS and SDS in water amongst a series of inorganic and organic anions. The titration studies, Job's plot, ¹H NMR studies and HRMS conspicuously reveal the co-operative participation of the three arms of the tripod for encapsulation of SDS or SDBS in the tripod cavity. The long chain carboxylates exhibit change in fluorescence at much higher concentrations and Job's plot shows the non-cooperative binding of the three arms of the tripod to form a 1 : 3 (tripod : carboxylate) stoichiometric complex. The lowest detection limit for SDBS and SDS by fluorescence is 0.25 μM. The reduced flipping of the naphthyl rings on complexation of TRI-BINAP-1 with SDBS or SDS decreases thermal deactivation processes and leads to enhanced fluorescence.

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Sodium dodecylbenzenesulfonate (SDBS) and sodium dodecylsulfate (SDS) are one of the most widely used anionic surfactants in domestic as well as industrial formulations.¹ As a result of their anthropogenic origin and extensive use, their presence in natural waters can cause serious environmental problems² by inhibiting biological activity and due to foam formation and transfer of other pollutants i.e. petroleum products, oil, pesticides etc. to the water. So, there is a need for the simple and effective analytical methods that enable the analysis of these surfactants in water at trace levels for environment control.³

A variety of procedures such as direct spectrophotometric determination,⁴ high-performance liquid chromatography (HPLC),⁵ gas chromatography-mass spectrometry (GC–MS),⁶ and capillary electrophoresis⁷ have been developed to determine SDBS, SDS and their homologues. The ion-selective electrodes⁸ have provided an alternate direct method but lack the reproducibility and signal stability.

More recently, imidazolium salts anchored hybrid materials⁹viz. MCM-41 or silica nano-particles based on their complimentary interaction with negatively charged carboxylate, sulfonate or sulfate anion and consequent release of the pre-adsorbed dye into the solution to signal the presence of the surfactant have been developed but exhibit many limitations. However, single molecule based fluorescent probes for selective recognition of surfactants have eluded the chemists though such probes are available for other anions¹⁰ are found to be sensitive and cost effective.

The positively charged imidazolium and its benzo derived molecular architects¹¹ due to their electrostatic and hydrogen bonding interactions with negatively charged species have been widely used for the recognition of anions. Usually, in these researches, sp³ hybridized carbon atoms are attached at both nitrogen atoms of the (benz)imidazolium¹² moiety. More recently, the presence of the combination of sp³ alkyl and sp² aryl substituents at the nitrogen atoms of the (benz)imidazolium core¹³ has been beneficially used for understanding the mesomorphic properties of ionic crystals¹⁴ and the aggregation behavior of surfactants¹⁵etc.

During our continuing interest in developing anion chemosensors, the significance of this sp³, sp² carbon atom combination on nitrogen atoms of the (benz)imidazolium moiety in the development of new anion chemosensors¹⁶ has been reported, especially with their high tolerance to the competitive protic solvents. However, in all these cases the 1-arylbenzimidazolium core is linked to chromogenic or fluorogenic moiety to achieve the sensing behaviour.

Now, we have synthesized 1-(2-naphthyl)benzimidazolium based tripod TRI-BINAP-1 which on excitation at 270 nm exhibits emission at 450 nm with a Stokes shift of 180 nm and does not require the presence of any additional fluorescent moieties to study the sensing behaviour. TRI-BINAP-1 exhibits highly selective fluorescence enhancement with SDBS and SDS in 95% aqueous medium (5% DMSO required for solubility of the tripod). Titration based stoichiometry determination, Job's plot, ¹H NMR titration studies and HRMS of its complex with SDBS and SDS confirm the encapsulation of SDBS and SDS in the tripodal cavity of TRI-BINAP-1 to form a 1 : 1 stoichiometric complex. To the best of our knowledge, this is the first example of molecular recognition based chemosensor for SDBS and SDS.

1-(2-Naphthyl)benzimidazole (2) was prepared by CuI and benzotriazole catalyzed N-arylation of benzimidazole with 2-bromonaphthalene by the procedure reported earlier for the synthesis of other N-arylbenzimidazoles.¹⁷ The solution of 1-(2-naphthyl)benzimidazole 2 and tribromide 3 in DMF on heating at 90 °C under nitrogen, and subsequent exchange of Br⁻ with hexafluorophosphate gave TRI-BINAP-1 in 79% overall yield (Scheme 1). The monopod 3-butyl-1-(2-naphthyl)benz-imidazolium PF₆⁻ (4) was prepared by heating 2 with 1-bromobutane at 100 °C followed by exchange of Br⁻ with PF₆⁻. ¹H NMR, ¹³C NMR, HRMS and CHN analysis confirmed the structures of TRI-BINAP-1 and 4.

Scheme 1 Synthesis of TRI-BINAP-1.

The solution of TRI-BINAP-1 in 95% aqueous DMSO gave an absorption band at 270 nm (Fig. 1) and on excitation at 290 nm gave the emission band centered at 450 nm with Stokes shift of 160 nm. The addition of only 2 equiv. (10 μM) of SDBS and SDS to the solution of TRI-BINAP-1 (5 μM) in 95% aqueous DMSO resulted in >300% increase in the absorbance at 330 nm (Fig. 1) along with the increase in fluorescence intensity associated with a blue shift of the fluorescence band from 450 nm to 415 nm (Fig. 3). The addition of excess of various inorganic anions viz. F⁻, Cl⁻, Br⁻, I⁻, H₂PO₄⁻, NO₃⁻, HSO₄⁻, ClO₄⁻, CN⁻, and organic anions viz. ethanoate, butyrate (C4), hexanoate (C6), octanoate (C8), decanoate (C10), benzoate etc. (1 mM) to the solution of TRI-BINAP-1 (5 μM) did not cause any change in its UV-Vis and fluorescence spectrum (Fig. 2). The addition of sodium salts of long chain carboxylates viz. laurate (C12, SL), myristate (C14, SM), palmitate (C16) up to 80 μM did not cause any change in the UV-Vis or fluorescence spectrum of TRI-BINAP-1 but at higher concentrations (>200 μM) lead to ∼200% increase in the fluorescence intensity at 415 nm. The addition of other alkyl sulfonate viz. methyl sulfonate (MeSO₃^–), butyl sulfonate (BuSO₃^–), octyl sulfonate (OctSO₃^–), decyl sulfonate (DecSO₃^–), octyl sulfate (OctOSO₃^–), decyl sulfate (DecSO₄^–) etc. even up to 1 mM did not cause any change in the fluorescence intensity of the solution of TRI-BINAP-1. The monopod 4 on excitation at 270 nm gave the emission band at 450 nm quite similar to the one observed in the case of TRI-BINAP-1 but this emission band remained unaffected even on addition of 1 mM solutions of all the anions discussed above and highlights the significance of the molecular architect of TRI-BINAP-1 in the recognition of SDBS and SDS.

Fig. 1 Effect of gradual addition of SDBS on the UV-Vis spectrum of TRI-BINAP-1 (5 μM) in 95% aqueous DMSO.

Fig. 2 Fluorescence intensity of the solution of TRI-BINAP-1 in 95% aqueous DMSO at 415 nm before and after addition of sodium salts of various carboxylates (C4–C16), sulfonates and sulfates.

Fig. 3 Effect of gradual addition of SDBS on the emission spectrum of TRI-BINAP-1 (5 μM) in 95% aqueous DMSO. Inset shows Job's plot pointing to a 1 : 1 stoichiometry of TRI-BINAP-1 and SDBS complex.

The absorption spectrum of TRI-BINAP-1 (5 μM) in 95% aqueous DMSO underwent an increase in absorption intensity at 330 nm upon gradual addition of the aqueous solution of SDBS and SDS and a plateau was achieved at ∼15 μM of SDBS and SDS. However, in case of sodium laurate (SL), the plateau was achieved on its addition of >200 μM to the solution of TRI-BINAP-1. The spectral fitting of the titration data by non-linear regression analysis program (SPECFIT-32) shows binding constant values of log β_L(SDBS) = 4.83 ± 0.13 (L : SDBS, 1 : 1); log β_L(SDS) = 5.28 ± 0.11 (L : SDS, 1 : 1); and log β_L(SL)3 = 12.13 ± 0.07 (L : SL, 1 : 3), respectively for SDBS, SDS and SL. Therefore, SDBS and SDS form a 1 : 1 stoichiometric complex with TRI-BINAP-1 but sodium laurate forms a 1 : 3 (TRI-BINAP-1 : SL) complex.

Similarly, gradual addition of aliquots of SDBS to the solution of TRI-BINAP-1 (5 μM) in 95% aqueous DMSO showed gradual increase in the fluorescence intensity and achieved plateau after addition of 2 equiv. of SDBS. The spectral fitting of the fluorescence data using non-linear regression analysis (SPECFIT-32) shows a stoichiometry of 1 : 1 with log β = 5.75 ± 0.09. The analysis of the Job's plot of the fluorescence titration profile in 95% aqueous DMSO showed the inflexion point near to 0.5 and further confirmed the existence of 1 : 1 stoichiometric (TRI-BINAP-1 : SDBS) complex (Fig. 3). Similarly, SDS forms a 1 : 1 complex with TRI-BINAP-1 with logβ = 5.41 ± 0.08 (see ESI†). The fluorescence of TRI-BINAP-1 increases linearly with the increase in concentration of SDBS and SDS between 0–10 μM and the lowest detection limit for both SDBS and SDS is 2.5 × 10⁻⁷ M (Fig. 4). The addition of sodium laurate, sodium myristate and sodium palmitate even up to 80 μM did not affect the fluorescence of TRI-BINAP-1 but at higher concentrations induced an increase in fluorescence with the plateau at ∼200 μM (see ESI†). The spectral fitting of these data shows the formation of a 1 : 3 (TRI-BINAP-1 : SL) complex with log β_L(SL)3 of 11.52 ± 0.06. This stoichiometry is also confirmed by the inflexion point near to 0.7 in the Job's plot (see ESI†).

Fig. 4 Concentration vs. emission intensity plots of SDBS, SDS, SL and SM on titration against TRI-BINAP-1.

These data clearly point that in case of SDBS and SDS, the three arms of the tripod TRI-BINAP-1 act co-operatively to interact with the three oxygen atoms of the sulfonate/sulfate moiety to form 1 : 1 complex with SDBS and SDS but in the case of long alkyl chain carboxylate ions due to unsymmetrical nature of the cavity with respect to Y-shaped carboxylate ion, the three arms act separately and individually to form 1 : 1 complex each with one carboxylate anion.

Mechanism for sensing SDS and SDBS : ¹H NMR, HRMS and theoretical studies

In order to rationalize the mechanism of interaction of TRI-BINAP-1 with these surfactants, a series of experiments were performed. The ¹H NMR spectrum of TRI-BINAP-1 in the presence of SDS and SDBS separately confirms the molecular cavity based recognition of these surfactant anions. The titrations were performed both by the addition of TRI-BINAP-1 to the solution of SDBS or SDS and by the addition of SDBS and SDS to the solution of TRI-BINAP-1. The results were same in both the cases. The addition of 1 equiv. of SDBS to a 5 mM solution of TRI-BINAP-1 in CD₃CN–H₂O (1 : 1) resulted in downfield shift of the BimC2–H proton by 0.28 ppm and points to the electrostatic interaction of sulfonate/sulfate anion with highly polarized C2–H protons. However, on performing these titrations in CD₃CN–D₂O (1 : 1) in place of CD₃CN–H₂O (1 : 1), the C2–H proton underwent deuterium exchange and disappeared without affecting the rest of the spectrum but provided more neat and clean ¹H NMR titration data. The aromatic protons of SDBS in CD₃CN–D₂O (1 : 1) solution appeared at δ 7.33 and 7.73 which were shifted up-field by 0.61 and 0.55 ppm resepctively¹⁸ to δ 6.72 and 7.18, on addition of 1 equiv. of TRI-BINAP-1 (Fig. 5a). The aliphatic protons corresponding to SDBS were also shifted up-field (Fig. 5b). However, the aromatic protons of the TRI-BINAP-1 exhibited change in their chemical-shifts only to a small extent. Similarly, on addition of 1 equiv. of TRI-BINAP-1 to the solution of SDS, the ⁻O₃S–OCH₂CH₂ protons of SDS were shifted up-field by 0.37 ppm from δ 3.98 to 3.61 and δ 1.67 to 1.30 (Fig. 6b). The aromatic protons corresponding to TRI-BINAP-1 exhibited change in their chemical shifts to a relatively smaller extent (Fig. 6a). The addition of higher equiv. of TRI-BINAP-1 to the solution of SDS or SDBS did not further cause any change in the chemical-shift of proton signals. Clearly, SDBS and SDS form a 1 : 1 compelx with TRI-BINAP-1 and the aromatic and some of aliphatic protons of SDBS and ⁻O₃SOCH₂CH₂ protons of SDS are under the influence of ring currents of the naphthyl rings of TRI-BINAP-1. The addition of sodium laurate did not cause any change in the ¹H NMR spectrum of TRI-BINAP-1 (see ESI†).

Fig. 5 ¹H NMR spectrum of TRI-BINAP-1 (5 mM) before and after addition of SDBS in CD₃CN–D₂O (1 : 1) (a) aromatic region (b) aliphatic region.

Fig. 6 ¹H NMR spectrum of TRI-BINAP-1 (5 mM) before and after addition of SDS in CD₃CN-D₂O (1 : 1), (a) aromatic region (b) aliphatic region.

The insight into the encapsulation of the SDBS and SDS anions in the cavity of TRI-BINAP-1 has been further revealed by the geometrical optimization of 1 : 1 stoichiometric complexes of TRI-BINAP-1 with SDBS and SDS. Both the structures of these complexes were optimized at DFT B3LYP 6-31G(d,p) basis set using Gaussian 09W package. The optimized structures clearly show that sulfonate and sulfate anions are stabilised through interactions with BIMC2–H and aromatic C–H moieties. The phenyl ring of SDBS and the CH₂ group directly attached to the phenyl ring are facing the ring currents of naphthyl rings and support the observed up-field shift of SDBS's aryl and CH₂ protons in the above discussed ¹H NMR titration data. Similarly, in case of the optimized structure of a 1 : 1 stoichiometric complex of TRI-BINAP-1 with SDS, 4–5 CH₂ units attached to sulfonate groups face the π currents (Fig. 7) of the three naphthyl rings and again confirm the up-field shift of these protons in the ¹HNMR spectrum of a 1 : 1 solution of TRI-BINAP-1 and SDS.

Fig. 7 The optimized structures of 1 : 1 complexes of TRI-BINAP-1 with SDBS and SDS. For clarity the hydrogen atoms have been omitted.

The HRMS of 1 : 1 solutions of TRI-BINAP-1 with SDBS and SDS gave the parent ion peaks respectively at 1403.6240 [trication + PF₆⁻ + dodecylbenzenesulfonate (DBS)] and 1343.5683 [trication +PF₆⁻ + dodecyl sulfate (DS)] (see ESI†) and further corroborated that TRI-BINAP-1 forms 1 : 1 complexes with both dodecylbenzenesufonate and dodecylsulfate.

Further, to provide insight into the fluorescence mechanism involved in this sensing process, the geometrical optimization of 2 and 4 were made at DFT B3LYP 6-31G(d,p) level and the electron density distribution in HOMO and LUMO was analysed (Fig. 8). It is well documented that in compound 2, on excitation to first excited state, the internal charge transfer (ICT) takes place from benzimidazole core to the aryl ring.¹⁹ The present study of monopod 4 shows that on excitation during transfer of an electron from HOMO to LUMO, the ICT takes place from the naphthyl ring to the electron-deficient benzimidazolium cation core. This results in conversion of the benzimidazolium cation core to a neutral radical and naphthyl ring to a radical cationic species. In the excited state, the naphthyl ring is able to accommodate the positive charge more effectively than the benzimidazolium cation and thus leads to reorganization of the excited state to a more stabilized excited state (Scheme 2). This is quite similar to the well documented internal charge transfer in heterocycles including imidazolium moieties.²⁰ The emission from this new lower energy excited state occurs at longer wavelength and the amount of red-shift of the emission depends on the charge stabilization ability of the naphthyl ring.

Fig. 8 Molecular orbital diagrams of 1-(2-naphthyl)benzimidazole (2) and its benzimidazolium salt 4 showing the localization of charge in the HOMO's and LUMO's.

Scheme 2 Proposed schematic presentation of ICT phenomenon in N-arylbenzimidazolium derivatives.

The encapsulation of SDS or SDBS into the tripod cavity restricts the flipping of the naphthyl rings and decreases the participation of non-radiative stabilization processes and thus increases the efficiency of fluorescence emission. The interaction of sulfonate/sulfate anion decreases the positive charge on the benzimidazolium core and results in blue-shift of the emission band.

In order to understand the energetics of the association process between TRI-BINAP-1 and surfactants, enthalpy and entropy factors in binding of TRI-BINAP-1 with SDBS/SDS were determined (Fig. 9). The −ΔG^o values increased linearly with the increase in temperature between 293 K to 308 K. The results clearly show that the increase in entropy (TΔS^o 27.21 Kcal mol⁻¹ for SDS and 17.88 Kcal mol⁻¹ for SDBS) is the major contributing factor for these complexation processes (ΔG^o −7.21 and −6.59 Kcal mol⁻¹ for SDS and SDBS). The release of water molecules from the cavity of the TRI-BINAP-1 and the surroundings of the SDBS and SDS molecules during complex formation contributes positively to the molecular recognition of SDBS and SDS.²¹

Fig. 9 Free energy change vs. temperature plots of TRI-BINAP-1 against SDBS and SDS.

In conclusion, we have prepared a tripod TRI-BINAP-1 which selectively recognizes SDBS and SDS in 95% aqueous medium. The cooperative binding of three imidazolium C–H protons with symmetrically placed three oxygen atoms of the surfactant anion and the hydrophobic interactions of naphthyl rings with aryl or alkyl protons of SDBS and SDS lead to high selectivity.

Experimental

General experimental conditions

All reagents were purchased from commercial suppliers and used without further purification. Solvents used were purified and dried by standard methods prior to use. TLC analyses were performed on silica gel plates and column chromatography was conducted over silica gel (mesh 100–200). ¹H NMR spectra and titrations were carried out using JEOL A1 spectrometer operating at 300 MHz at GNDU, Amritsar or using Brucker 400 MHz machine at RSIC, Chandigarh. ¹³C NMR sepctra were recorded at 75 MHz. All chemical shifts are reported in ppm relative to the TMS as an internal reference. UV-Vis studies were carried out on a Shimadzu UV-1601 PC or Shimadzu UV-2400 machines using slit width of 1.0 nm and matched quartz cells. HRMS spectra were recorded on Brucker MicroToff/QII. The fluorescence experiments were performed on a Shimadzu 1501 fluorescence spectrophotometer. Elemental analysis was performed on a Flash EA-1112 series CHNS–O analyser instrument.

Synthesis of 1-(2-naphthyl)benzimidazole (2)¹⁷. 2-Bromonaphthalene (4.0 g, 19.4 mmol), CuI (160 mg, 5 mol%, 0.84 mmol) and benzotriazole (202 mg, 10 mol%, 1.7 mmol) were dissolved in DMSO (10 mL). To this stirred solution was added benzimidazole (2.0 g, 16.9 mmol) and K–OBu^t (2.3 g, 20.4 mmol) under N₂ atm. and resulting reaction mixture was heated at 110 °C until the completion of reaction as monitored by TLC. The reaction mixture was cooled to room temperature and water (20 mL) was added, followed by the addition of a saturated solution of sodium sulfide (10 mL). The compound was extracted with diethyl ether (3 × 30 mL). The solvent was distilled off, and purification of the residue via column chromatography with hexane/CHCl₃ (9 : 1) as eluent gave pure compound 2 as a thick liquid (2.28 g). Yield 75%.¹H NMR (CDCl₃): δ 7.34–7.96 (m, 10H, ArH), 8.04 (d, J = 8.7 Hz, 1H, ArH), 8.21 (s, 1H, BIM C2–H). ¹³C NMR (CDCl₃): δ 110.5, 120.6, 122.1, 122.2, 122.8, 123.7, 126.8, 127.3, 127.9, 130.2, 132.4, 133.6, 133.7, 133.8, 142.5, 144.0.

Synthesis of TRI-BINAP-1. 1,3,5-Tris(bromomethyl)-2,4,6-triethylbenzene 3 (0.5 mmol, 220.5 mg) and 1-(2-naphthyl)benzimidazole 2 (2 mmol, 488 mg) were dissolved in DMF (20 mL) and heated at 90 °C under N₂ for 12 h. Then, the solvent was removed under vacuum. The crude solid was dissolved in methanol (10 mL) and was filtered to remove any insoluble impurities. The aqueous NH₄PF₆ (1.5 mmol, 244.5 mg) was added drop wise with continuous stirring and stirring was continued for 24 h. The solid separated was filtered and dried under vacuum to get TRI-BINAP (1) as an off-white solid. The compound was crystallized from acetonitrile–isopropanol (8 : 2) mixture. Yield 79%; M. pt. 216 °C; ¹H NMR (DMSO-d₆, 400 MHz): δ 1.08 (t, J = 6.7 Hz, 9H, 3 × CH₃), 2.88 (q, J = 7.5 Hz, 6H, 3 × CH₂), 5.89 (s, 6H, 3 × CH₂), 7.67–7.71 (m, 6H, ArH), 7.77 (t, J = 7.6 Hz, 3H, ArH), 7.83 (t, J = 7.8 Hz, 3H, ArH), 7.91 (d, J = 7.6 Hz, 3H, ArH), 7.94–8.01 (m, 6H, ArH), 8.09 (d, J = 8.2 Hz, 3H, ArH), 8.35 (d, J = 1.9 Hz, 3H, ArH), 8.48 (d, J = 8.4 Hz, 3H, ArH), 9.65 (s, 3H, BIM C2–H); ¹³C NMR (DMSO-d₆, 100 MHz): δ 15.3, 23.6, 45.3, 113.9, 114.3, 123.0, 124.8, 127.4, 127.8, 127.9, 128.0, 128.1, 128.2, 128.4, 130.1, 130.2, 131.6, 131.9, 132.5, 133.0, 141.1, 148.5; HRMS m/z (TOF MS ES⁺) calculated for C₆₆H₅₇N₆F₁₂P₂⁺ [M⁺], 1223.3928; found 1223.3926. Found C, 57.78; H, 4.31; N, 6.11%; C₆₆H₅₇N₆P₃F₁₈ requires C, 57.90; H, 4.20; N, 6.14%.

Synthesis of compound 4. 1-(2-Naphtyl)benzimidazole 2 (1 mmol) and butyl bromide (10 mmol) were dissolved in DMF (1 mL) and heated at 100 °C under N₂ for 12 h. Then, the solvent was removed under vacuum. Methanol was added to dissolve the crude product and filtered to remove any suspended impurity. The aqueous NH₄PF₆ (1 mmol, 163 mg) was added dropwise with continuous stirring and stirring was continued for 12 h. Solid separated and was filtered and dried to get monopod 4 as an off-white solid. The compound was recrystallized from acetonitrile–isopropanol (8 : 2) mixture. Yield 86%; M. pt. 98 °C; ¹H NMR (CDCl₃, 300 MHz): δ 0.92 (t, 3H, J = 7.2 Hz, CH₃), 1.39 (hext, 2H, J = 7.2 Hz, CH₂), 1.99 (quin, 2H, CH₂), 4.52 (t, 2H, J = 7.2 Hz, CH₂), 7.59–7.68 (m, 6H, ArH), 7.81 (d, 1H, J = 7.8 Hz, ArH), 7.91 (d, 1H, J = 7.5 Hz, ArH), 7.96 (d, 1H, J = 7.2 Hz, ArH), 8.04 (d, 1H, J = 8.4 Hz, ArH), 8.20 (s, 1H, ArH), 9.26 (s, 1H, Bim C2–H). ¹³C NMR (CDCl₃, 75 MHz): δ 13.3, 19.7, 31.0, 47.9, 113.5, 113.6, 121.3, 124.5, 127.7, 127.9, 128.2, 128.7, 129.8, 130.9, 131.4, 131.7, 133.2, 133.5, 140.3. Found C, 56.55; H, 4.79; N, 6.19%; C₂₁H₂₁N₂PF₆ requires C, 56.51; H, 4.74; N, 6.28%.

Procedure for surfactant sensing

Stock solution of chemosensor TRI-BINAP-1 and control compound 4 were prepared at 10⁻³ M in DMSO and was then diluted to 5 μm by DMSO–Water mixture (5 : 95). The resulting solution was shaken well before recording the absorption or fluorescence spectra. The solutions of all the anions including surfactants were prepared in de-ionized water. Titration experiments were performed by placing 3 mL of a solution of chemosensor in a quartz cuvette of 1 cm optical path length, and then adding the surfactant or long chain carboxylate stock solution incrementally by means of a micro-pipette. Spectra were recorded 3 min after the addition.

All absorption scans and fluorescence spectra were saved as ACS II files and further processed in Excel™ to produce all graphs shown. Stability constants were determined by fitting the absorption and emission spectra recorded during the titrations of the chemosensor with surfactants and long chain carboxylates. The data was fitted with the global analysis program SPECFIT-32. For determining the stoichiometry of the various complexes, Job's plot²² method was used. For this the ratio of intensities I_o/I (initial intensity/intensity at specific conc. of SDS/SDBS) vs. mole fraction of SDS/SDBS were plotted. The inflexion point in the Job's plot analysis gives the stoichiometry of the complex.

Computational details

The ground state structural calculations for the 1 : 1 stoichiometric complexes of chemosensor TRI-BINAP-1 with SDBS and SDS and monopod 4 and naphthylbenzimidazole (2) were computed using Becke's three-parameter hybrid functional and the Lee–Yang–Parr correlation (B3LYP) with 6-31G(d,p) basis set using GAUSSIAN 09W suite of programmes.²³ The optimized structures and MO diagrams were generated with the aid of Gauss view 5.0 package.

Acknowledgements

We thank UGC, New Delhi for CAS programme and CSIR, New Delhi for fellowship to Sandeep Kumar and DST, New Delhi for financial assistance.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2ra21247a

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