Effective fluorescent chemosensors for the detection of Zn2+ metal ion

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 143–147

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Effective fluorescent chemosensors for the detection of Zn2+ metal ion J. Jayabharathi ⇑, V. Thanikachalam, K. Jayamoorthy Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamil Nadu, India

g r a p h i c a l a b s t r a c t

h i g h l i g h t s " Photo induced electron transfer

(PET) mechanism plays main role. " FT-IR spectra confirm the binding of

ligand with Zn2+ metal ion. 2+

2+

2+

2+

" In the case of Pb , Fe , Co , Ni ,

Ca2+ and Hg2+ metal ions the fluorescence enhancement is very low. " Crystalline benzimidazole derivatives (5) are triclinic crystal.

a r t i c l e

i n f o

Article history: Received 16 February 2012 Received in revised form 28 March 2012 Accepted 5 April 2012 Available online 3 May 2012 Keywords: Sensing Chemosensor Benzimidazole XRD Photoinduced electron transfer

a b s t r a c t Benzimidazole derivatives synthesized from three components assembling condensation reaction behaves as a selective fluorescent sensor for Zn2+ metal ion. These benzimidazole derivatives were characterized by 1H, 13C NMR, mass and elemental analysis. XRD analysis was carried out for 1-(4-methylbenzyl)-2-p-tolyl-1H-benzo[d]imidazole. The increase in the fluorescence enhancement can be explained on the basis of photo induced electron transfer (PET) mechanism. The blockage of the photoinduced electron transfer process from benzimidazole ring to aryl fluorophore induced by Zn2+ co-ordination induced emission enhancement. Ó 2012 Elsevier B.V. All rights reserved.

Introduction Despite much effort in trying to unravel the role of metal ions in biology, much remains to be learned about how metal cations are trafficked and stored prior to their incorporation into different metalloproteins [1]. Of particular interest is zinc, which is the second most abundant d-block metal ion in humans [2]. Historically, Zn2+ was detected using dithizone as a histochemical stain, which revealed much about the spatial distribution of Zn2+ in various tissues. Today, fluorescent probes have attracted considerable attention for the direct visualization of biological Zn2+ at the molecular ⇑ Corresponding author. Tel.: +91 9443940735. E-mail address: [email protected] (J. Jayabharathi). 1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.04.038

level. The construction of fluorescent devices for sensing and reporting of chemical events is currently of significant importance for both chemistry and biology [3–5]. Fluorescence proffers high sensitivity among the many signal types available. Therefore, this mode of signal transduction has been used widely in the detection of a number of transition metal ions [6–8]. The imidazole ring system is one of the most important substructures found in a large number of natural products and pharmacologically active compounds [9]. In recent years, substituted imidazoles are substantially used in ionic liquids [10] that have been given a new approach to ‘Green Chemistry’. Triarylimidazole derivatives have many biological activities, namely herbicidal [11], fungicidal [12], anti-inflammatory [13] and antithrombotic activities [14]. In addition, they are used in photography as photosensitive compound

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[15]. In continuation of our research work in developing fluorescent sensor [16–19] herein, we report benzimidazole derivatives as sensor for Zn2+ metal ion by solution spectral studies. Experimental Spectral measurements The ultraviolet–visible (UV–vis) spectra of the benzimidazole derivatives were measured in an UV–vis spectrophotometer (Perkin Elmer Lambda 35) and corrected for background absorption due to solvent. Photoluminescence (PL) spectra were recorded on a (Perkin Elmer LS55) fluorescence spectrometer. NMR spectra were recorded on Bruker 400 MHz NMR spectrometer. Mass spectra were recorded on a Varian Saturn 2200 GCMS spectrometer. General procedure for the synthesis of ligands

Fig. 2. FT-IR spectra of benzimidazole derivative (1) (solid line) and benzimidazole derivatives bound with Zn2+ metal ion (broken line).

A mixture of corresponding aldehyde (2 mmol), o-phenylenediamine (1 mmol) and ammonium acetate (2.5 mmol) has been refluxed at 80 °C in ethanol. The reaction was monitored by TLC and purified by column chromatography using petroleum ether:ethyl acetate (9:1) as the eluent.

Zn2+ metal ion (Scheme S1). This is because of effective transfer of electron from excited state of benzimidazole derivatives to Zn2+ metal ion (Scheme S2).

Results and discussion

FT-IR characteristics of benzimidazole derivative–Zn2+ metal ion

Absorption characteristics of benzimidazole derivatives–Zn2+ metal ion

The UV–visible absorption study is inadequate to throw light on the molecular structure of the binding of benzimidazole derivatives with Zn2+ metal ion. Fourier transform infrared (FT-IR) technique may add further information about the nature of interaction between the benzimidazole derivatives with Zn2+ metal ion. Fig. 2 presents comparison of the FT-IR spectra of bare benzimidazole derivative (1) (solid line) and benzimidazole derivative–Zn2+ metal ion (broken line). The spectrum of bare benzimidazole derivative shows the >C=N stretching vibration at 1600 cm1. This band is shifted to 1629 cm1 for the benzimidazole derivative bound to Zn2+ metal ion. This confirms the binding

The absorption spectra of benzimidazole derivatives in presence of Zn2+ ion at different concentration and also in their absence are displayed in Fig. 1. The Zn2+ ion enhance the absorbance of benzimidazole derivatives remarkably without shifting its absorption maximum appeared around 290 nm. This indicates that Zn2+ ion do not modify the excitation process of the ligand. The enhanced absorption observed around 290 nm with different concentration of Zn2+ ion is due to binding of benzimidazole derivatives with

Fig. 1. Absorption spectra of benzimidazole derivatives (1–6) in presence and absence of Zn2+ metal ion.

J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 143–147

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Fig. 3. Emission spectra of benzimidazole derivatives (1–6) in presence and absence of Zn2+ metal ion.

Fig. 4. Bar diagram of emission intensity of benzimidazole derivatives (1–6) in presence of Pb2+, Fe2+, Co2+, Ni2+, Ca2+ and Hg2+ metal ions.

of benzimidazole derivative with Zn2+ metal ion. Similar trend was observed for all other benzimidazole derivatives (2–6). Fluorescence enhancement by Zn2+ metal ion The emission spectra of benzimidazole derivatives in presence of Zn2+ metal ion at different concentration and also in their absence are displayed in Fig. 3. The Zn2+ metal ion enhance the emission of benzimidazole derivatives remarkably without shifting its emission maximum around 350 nm. This indicates that the Zn2+ metal ion do not modify the excitation process of the ligand. The enhanced emission at 350 nm observed with Zn2+ metal ion is due to binding of the benzimidazole derivatives with Zn2+ metal ion. Fluorescence enhancement arises due to the formation of benzimidazole–Zn2+ metal ion complex. In the case of Pb2+, Fe2+, Co2+, Ni2+, Ca2+ and Hg2+ metal ions the fluorescence enhancement is very low (Fig. 4). Fig. 5 shows the linear variation of 1/F0  F

versus 1/[Q]. This clearly shows that each ligand is bound to Zn2+ metal ion. Calculation of fluorescence quantum yield and overlap integral According to Forster’s energy transfer theory, the energy transfer efficiency is related not only to the distance between the acceptor and donor (r0), but also to the critical energy transfer distance (R0). That is

E ¼ R60 =ðR60 þ r60 Þ where R0 is the critical distance when the transfer efficiency is 50%.

R60 ¼ 8:8  1025 K 2 N4 uJ where K2 is the spatial orientation factor of the dipole, N is the refractive index of the medium, u is the fluorescence quantum yield of the donor and J is the overlap integral of the fluorescence

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Fig. 5. Plot of 1/F0  F versus 1/[Q].

emission spectrum of the donor and the absorption spectrum of the acceptor. The value of J can be calculated by using the equation given below:

J¼

Z

is in accordance with the conditions of Forster’s energy transfer theory [22] and suggests that electron transfer occurs between the Zn2+ metal ion and benzimidazole derivatives (1–6) with high probability [23].

FðkÞeðkÞk4 dk=FðkÞdk Photoinduced electron transfer mechanism

where F(k) is the fluorescence intensity of the donor and e(k) is molar absorptivity of the acceptor. These experimental conditions, the calculated values of E, R0 and r0 are 0.13–0.45, 1.73–2.37 and 2.05–2.88, respectively. The literature values of K2(=2/3) and N(=1.3467) are used for the calculation [20,21] and the corresponding u values for 1–6 from the present study. Obviously, the calculated value of R0 is in the range of maximal critical distance. This

The enhancement in fluorescence intensity of benzimidazole derivatives on interaction with Zn2+ ion may be explained on the basis thermodynamically favourable photoinduced electron transfer mechanism (PET) [24] between benzimidazole derivatives and Zn2+ ion. The Zn2+ ion is likely to bind to benzimidazole derivatives via the nitrogen atoms. The process occurs due to transfer of

Table 1 Selected bond lengths (Å), bond angles (°) and torsional angles (°) of 5. Bond lengths (Å)

Experimental XRD (Å)

Bond angles (°)

Experimental XRD (°)

Torsional angles (°)

Experimental XRD (°)

N1A–C2A N1A–C8A N1A–C1A N3A–C2A N3A–C9A N1B–C2B N1B–C8B N1B–C1B N3B–C2B N3B–C9B C1A–C11A C2A–C21A C7A–C8A C8A–C9A C14A–C17A C24A–C27A

1.3782(1.4972) 1.3833(1.4584) 1.4537(1.4700) 1.3163(1.3671) 1.3881(1.4606) 1.3795(1.4972) 1.3863(1.4584) 1.4550(1.4700) 1.3174(1.3671) 1.3866(1.4606) 1.5135(1.5400) 1.4729(1.5400) 1.3921(1.3862) 1.3999(1.4763) 1.5089(1.5400) 1.5078(1.5400)

C2A–N1A–C8A C2A–N1A–C1A C8A–N1A–C1A C2A–N3A–C9A C2B–N1B–C8B C2B–N1B–C1B C8B–N1B–C1B C2B–N3B–C9B N1A–C1A–C11A N3A–C2A–N1A N3A–C2A–C21A N1A–C2A–C21A N1A–C8A–C3A N1A–C8A–C9A N3A–C9A–C4A N3A–C9A–C8A

106.23(101.69) 128.33(113.71) 124.79(113.33) 104.77(105.49) 106.27(101.69) 129.32(113.71) 123.98(113.33) 105.12(105.41) 115.07(109.47) 113.25(113.53) 123.44(123.23) 123.30(123.24) 131.76(130.72) 103.49(108.25) 129.91(130.69) 110.26(108.25)

C2A–N1A–C1A–C11A C8A–N1A–C1A–C11A C9A–N3A–C2A–N1A C9A–N3A–C2A–C21A C8A–N1A–C2A–N3A C1A–N1A–C2A–N3A C8A–N1A–C2A–C21A C1A–N1A–C2A–C21A C9A–C4A–C5A–C6A C2A–N1A–C8A–C7A C1A–N1A–C8A–C7A C2A–N1A–C8A–C9A C1A–N1A–C8A–C9A C6A–C7A–C8A–N1A C2B–N1B–C1B–C11B C8B–N1B–C1B–C11B C9B–N3B–C2B–N1B C9B–N3B–C2B–C21B C8B–N1B–C2B–N3B C1B–N1B–C2B–N3B C8B–N1B–C2B–C21B C1B–N1B–C2B–C21B C9B–C4B–C5B–C6B C2B–N1B–C8B–C7B C1B–N1B–C8B–C7B C2B–N1B–C8B–C9B C1B–N1B–C8B–C9B C6B–C7B–C8B–N1B

109.45(177.75) 81.12(62.25) 0.02(13.69) 178.71(165.91) 0.59(17.54) 171.55(139.72) 179.28(162.06) 9.75(39.88) 0.1(0.1581) 178.45(166.15) 7.08(43.71) 0.88(13.70) 172.25(136.15) 179.75(176.19) 108.10(177.75) 79.66(62.25) 0.86(13.69) 177.35(165.91) 1.24(17.54) 173.85(139.72) 176.90(162.06) 4.29(39.88) 0.25(0.1581) 176.39(166.15) 3.28(43.71) 1.05(13.70) 174.15(136.15) 177.76(176.19)

Values within the parenthesis corresponds to theoretical values.

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electron density from lone pair of electrons of the nitrogen atom to the LUMO of the benzimidazole derivatives. Binding of Zn2+ ion with benzimidazole derivatives through the nitrogen atom lone pairs will obviously hindered the PET process leading to enhancement of fluorescence of benzimidazole derivatives on interaction with Zn2+ ion. For a PET chemosensor, a fluorophore is usually connected to a receptor containing a relatively high-energy non-bonding electron pair, such as nitrogen atom, which can transfer an electron to the excited fluorophore and as a result quench the fluorescence. When this electron pair is bound by coordination of a cation, the redox potential of the receptor is raised so that the HOMO of the receptor becomes lower in energy than that of the fluorophore. Thus, the PET process from the receptor to the fluorophore is blocked and the fluorescence is switched on. Most of the PET type chemosensors sense Zn2+ with a fluorescence enhancement signal. Crystal structure Crystalline benzimidazole derivative (5) [25] are triclinic crys The cell dimensions are tal. It crystallizes in the space group P1. a = 9.6610 Å, b = 10.2900 Å, c = 17.7271 Å. ORTEP diagram of (5) (Fig. S6) shows that the benzimidazole ring is essentially planar. The dihedral angles between the planes of the benzimidazole and the benzene rings of the 4-methylbenzyl and the p-tolyl groups are 76.64 (3)° and 46.87 (4)°, respectively, in molecule A. The corresponding values in molecule B are 86.31 (2)° and 39.14 (4)°. The dihedral angle between the planes of the two benzene rings is 73.73 (3)° and 80.69 (4)° in molecules A and B, respectively. Optimization of 5 have been performed by DFT at B3LYP/6-31G(d,p) using Gaussian-03. All these XRD data are in good agreement with the theoretical values (Table 1). However, from the theoretical values it can be found that most of the optimized bond lengths, bond angles and dihedral angles are slightly higher than that of XRD values. These deviations can be attributed to the fact that the theoretical calculations were aimed at the isolated molecule in the gaseous phase and the XRD results were aimed at the molecule in the solid state. Conclusion We have developed a benzimidazole based fluorescent sensors that showed preferential binding for Zn2+ over Pb2+, Fe2+, Co2+, Ni2+, Ca2+ and Hg2+ metal ions. It is excited around 290 nm and emits around 350 nm with fluorescence enhancement after and before complexation with Zn2+ metal ion. The high selectivity of Zn2+ is marked by a significant fluorescent enhancement. The enhancement in fluorescence of benzimidazole derivatives on binding with Zn2+ ion via nitrogen atom is due to photo induced electron transfer process.

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Acknowledgment One of the authors Prof. J. Jayabharathi is thankful to Department of Science and Technology [No. SR/S1/IC-73/2010], University Grants commission (F. No. 36-21/2008 (SR)) and Defence Research and Development Organisation (DRDO) (NRB-213/MAT/10-11) for providing funds to this research study. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2012.04.038. References [1] R. McRae, P. Bagchi, S. Sumalekshmy, C.J. Fahrni, Chem. Rev. 109 (2009) 4780– 4827 [PubMed: 19772288]. [2] E.M. Nolan, S.J. Lippard, Acc Chem Res. 42 (2008) 193–203 [PubMed: 18989940]. [3] B. Bodenant, T. Weil, M. Businelli-Pourcel, F. Fages, B. Barbe, I. Pianet, M. Laguerre, J. Org. Chem. 64 (1999) 7034–7039. [4] A. Torrado, G.K. Walkup, B. Imperiali, J. Am. Chem. Soc. 120 (1998) 609–610. [5] R. Krämer, Angew. Chem. Int. Ed. 37 (1998) 772–773. [6] L. Fabbrizi, M. Licchelli, P. Pallavicini, L. Parodi, Angew. Chem. Int. Ed. 37 (1998) 800–802. [7] D.P. Gates, P.S. White, M. Brookhart, Chem. Commun. (2000) 47–48. [8] S. Bhattacharya, M. Thomas, Tetrahedron Lett. 41 (2000) 10313–10317. [9] J. Heers, L.J.J. Backx, J.H. Mostmans, J. Van Cutsem, J. Med. Chem. 22 (1979) 1003–1005. [10] P. Wasserscheid, W. Keim, Angew. Chem. Int. Ed. Eng. 39 (2000) 37872–37877. [11] R. Liebl, R. Randte, H. Mildenberger, K. Bauer, H. Bieringer, Chem. Abstr. 108 (1987) 6018g. [12] A.F. Pozherskii, A.T. Soldatenkov, A.Y. Katritzky, Heterocycles in Life and Society, Wiley, New York, 1997. p. 179. [13] J.G. Lambardino, E.H. Wiseman, J. Med. Chem. 17 (1974) 1182. [14] A.P. Phillips, H.L. White, S. Rosen, Chem. Abstr. 98 (1982) 53894z. [15] I. Satoru, J.P. Japn Kokkai Tokyo Koho, Chem. Abstr. 111 (1989) 01. 117, 867, 10 May, 214482. [16] J. Jayabharathi, V. Thanikachalam, K. Jayamoorthy, Spectrochim. Acta Part A 89 (2012) 168–176. [17] J. Jayabharathi, V. Thanikachalam, K. Brindha Devi, N. Srinivasan, Spectrochim. Acta A 82 (2011) 513–520. [18] J. Jayabharathi, V. Thanikachalam, K. Jayamoorthy, M. Venkatesh Perumal, Spectrochim. Acta A 79 (2011) 6–16. [19] K. Saravanan, N. Srinivasan, V. Thanikachalam, J. Jayabharathi, J. Fluoresc. 21 (2011) 65–80. [20] B. Lin, Z. Fu, Y. Ji, Appl. Phys. Lett. 79 (1991) (1991) 943–945. [21] L. Cyril, J.K. Earl, W.M. Sperry, Biochemists’ Handbook, E. & F. N. Spon, London, 1961. pp. 84–88. [22] G.Z. Chen, X.Z. Huang, J.G. Xu, Z.B. Wang, Z.Z. Zheng, Method of Fluorescent Analysis, 2nd ed., Science Press, Beijing, 1990. p. 126 (Chapter 4). [23] W.Y. He, Y. Li, C. Xue, Z.D. Hu, X.G. Chen, F.L. Sheng, Bioorg. Med. Chem. 13 (2005) 1837–1845. [24] C.P. Kulatilleke, S.A. Silva, Y. Eliav, Polyhedron 25 (2006) 2593–2596. [25] S. Rosepriya, A. Thiruvalluvar, K. Jayamoorthy, J. Jayabharathi, Antony Linden, Acta Cryst. E67 (2011) o3519.