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Colorimetric and fluorescence sensing of fluoride anions with potential salicylaldimine based schiff base receptors
Spectrochimica Acta Part A 75 (2010) 1146–1151
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Colorimetric and fluorescence sensing of fluoride anions with potential salicylaldimine based schiff base receptors Radhakrishnan Sivakumar, Vijayaragavan Reena, Nallamuthu Ananthi, Muthiya Babu, Sambandam Anandan ∗ , Sivan Velmathi ∗∗ Department of Chemistry, National Institute of Technology, Tiruchirappalli 620 015, India
a r t i c l e
i n f o
Article history: Received 16 July 2009 Received in revised form 15 December 2009 Accepted 31 December 2009 Keywords: Schiff base Anions Fluoride sensors Fluorescence enhancement
a b s t r a c t Salicylaldimine based schiff base receptors with different substituents showing fluorescent enhancement in the presence of fluoride anion was visualized through naked eye as well as by change in spectral properties (UV–vis and fluorescent techniques). The reason for such fluorescence enhancement may be due to hydrogen bond interaction between receptor recognition site and fluoride anion. Such a hydrogen bond interaction creates a six-membered transition state, which avoids quenching processes. To support this, fluorescence enhancement factor (FEF) was calculated and it was found to be more (FEF = 652) for –NO2 substituted receptor compared to other receptors. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Designing ion-selective molecular sensors for sensing of anions and cations is a challenging object in supramolecular chemistry, in view of the fact that ions play a major role in many chemical and biological processes. An advance in the field of sensor comprises investigation of molecular recognition at model interfaces which provides great insights into biological phenomena and modern applications such as high selectivity sensors [1]. Among the biologically important ions, fluoride anion is a common ingredient in hypnotics, anesthetics, psychiatric drugs and cockroach poisons and is a contaminant in drinking water. Excess fluoride ion exposure causes fluorosis, thyroid activity depression, bone disorders and immune system disruption [2]. Besides the above significance, detection of fluoride ion with the use of simple preparation and minimal instrumental assistance is desirable toward practical applications. Colorimetric chemosensors based on optical and spectral changes upon recognition with ions are particularly attractive because of their simplicity and high detection limit of fluorescence detection methods [3–7]. Nevertheless in most of the cases [8–11] fluorescence quenching rather than fluorescent enhancement was also observed. The reason for fluorescence quenching was mainly attributed to the nature of the adduct formed upon
∗ Corresponding author. Tel.: +91 431 2503639; fax: +91 431 2500133. ∗∗ Corresponding author. Tel.: +91 431 2503640; fax: +91 431 2500133. E-mail addresses: [email protected] (S. Anandan), [email protected] (S. Velmathi). 1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2009.12.077
interaction with anions which influence the photo-physical and electronic transitions within the receptors. It has thus become a demand for the development of fluorescent “turn on” sensors for anions of biological importance. Kim and Ahn [12] observed the fact that, the fluorescence enhancement of the receptors in the presence of anions was likely owing to the elimination of n–* transitions within the receptor which seems to intervene the –* transition levels which are responsible for emission. Hence, recognition of receptors towards anions not only involves color changes but also affects the electronic transitions within the molecule. For this reason, the receptors have to be suitably designed in such a way that it contains both anion recognition site and fluorophore moiety. In biological systems, anion recognition is carried out through hydrogen bonding by proteins with sterically well-defined complex cites in the interior of proteins [13,14]. The neutral type anion sensors generally consist of heterocyclic units such as pyrrole, pyridine and indole and other units like amide, amine and phenol in the recognition sites for hydrogen bonding [15,16]. Many reports have appeared describing the synthesis and study of very sophisticated anion sensors [17–21]. This is because, anions seems to interact with a hydrogen bond donors of a receptor (NH2 , –OH, –C(O)NHR) more strongly. Among the available anions F− anion shows better interaction than any other anions. To the best of our knowledge, only few research groups were working in this field with receptors having –OH group as a binding site [22–30]. On the basis of above mentioned background, here we report the fluorescence sensing of fluoride anions with receptors having –OH group as binding site (salicylaldimine based schiff base receptors with different substituents).
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application. In the FT-IR spectroscopy we get information about the presence of functional groups in the compound such as C C, CH N, etc. by the position of absorption peaks which arise due to stretching vibration of the bonds in the groups. The presence of C N in the molecule is confirmed by the vibrations between 1690 and 1640 cm−1 and aromatic C C between 1600 and 1473 cm−1 . These vibrations are seen in the IR spectra of all the compounds synthesized and the data obtained are given in the experimental section. NMR spectroscopy is one of the principal techniques which give us the structural information about molecules. NMR spectra were obtained using CDCl3 as the solvent. All the three compounds gave a singlet at around ı 8.6–9.0 ppm corresponding to the CH N proton indicating the formation of imine and the aromatic protons resonate in the ı 6.9–8.2 region In addition the OH proton of aromatic ring is shown as a singlet at around ı 12.3–12.6 ppm. Fig. 1. Structure of the receptors 1–3.
2. Experimental 2.1. Materials and methods All reagents were purchased from Sigma Aldrich and used as received. Analytical Grade solvents were used as such. 1 H NMR spectra were recorded on a JEOL LA600 (600 MHz) in CDCl3 at 298 K with TMS as internal standard. FT-IR spectra were measured on a Perkin–Elmer FT-IR spectrometer using KBr plates. UV–visible and Fluorescence spectra were recorded in 1 cm path length quartz cell on a Perkin Elmer EZ301 spectrophotometer and Shimadzu RF-5301 PC spectrofluorophotometer, respectively. 2.2. General experimental procedure for the synthesis of the receptors Typically to 4-nitro aniline (0.2762 g, 2 mmol) dissolved in 10 ml methanol, salicylaldehyde (0.2426 g, 2 mmol) dissolved in 5 ml was slowly added and stirred for 3 h at room temperature. Completion of the reaction was monitored through TLC for the disappearance of the starting compounds. Then the solvent was removed through rotovac yielding the reddish yellow crystals of N(4-nitrophenyl) salicylaldimine (receptor 3) yield: 98.3%. The solid thus obtained was dried in vacuum oven. M. Pt 130–132 ◦ C. Similarly following the above procedure the receptors 1 and 2 were synthesized. 2.2.1. Receptor 1 1 H NMR (CDCl , ı ppm ) 7.0–7.2 m 2H, 7.3–7.5 dd 4H, 8.2–8.4 d 3 2H (aromatic), 8.6 s, 1H (CH N), 12.5 s, 1H (OH). IR (KBr plates, cm−1 ): 1267, 1345, 1463, 1630, 3070, 3427. 2.2.2. Receptor 2 1 H NMR (CDCl , ı ppm ) 6.9–7.1 m 2H, 7.2 m 2H, 7.4 m 4H (aro3 matic), 8.6 s 1H, 12.6 s, 1H (OH). IR (KBr plates, cm−1 ): 1272, 1374, 1484, 1600, 3071, 3481. 2.2.3. Receptor 3 1 H NMR (CDCl , ı 3 ppm ) (aromatic) 6.9 m 2H, 7.2 m 1H, 7.4 m 5H, 7.6 d 1H (aromatic), 9.0 s 1H (CH N), 12.3 s, 1H (OH). IR (KBr plates, cm−1 ): 1272, 1395, 1485, 1611, 3056, 3450. 3. Results and discussion The chromogenic receptors 1–3 (Fig. 1) were synthesized by schiff’s base condensation of salicylaldehyde and aniline (1) or 4chloroaniline (2) or 4-nitroaniline (3). They were well characterized by FT-IR, 1 H NMR spectroscopic methods before using them in
3.1.
1H
NMR spectroscopic studies
To understand the molecular interactions between the receptor recognition site and fluoride anion, 1 H NMR titration experiments were done with receptor 3 in the presence and in the absence of TBAF in DMSO-d6 . Fig. 2 shows the 1 H NMR spectrum of the receptor 3 in the absence and in the presence of 1 and 3 equiv. TBAF. In the case of neat receptor 3 i.e. in the absence of TBAF, –OH proton appears as a singlet at 12.3 ppm and the imine proton (CH N) comes at 9.0 ppm as a sharp singlet, whereas in the presence of 1 equiv. of TBAF, the singlet at 12.3 ppm disappeared immediately indicating the hydrogen bond interaction between fluoride anion and –OH group, consequently an up field shift from 9.0 to 8.9 ppm in the imine proton was also observed. The disappearance of the phenolic OH upon the addition of 1 equiv. of fluoride anion may be due to the simultaneous formation of the alkoxide and its hydrogen bonding with the F− ion. In the presence of 3 equiv. of fluoride ions an additional sharp peak at 16 ppm appeared which is due to the deprotonation of the chromogenic receptor and the subsequent formation of [HF2 − ] species. From the NMR spectra, we inferred that complete disappearance of the –OH singlet upon addition of 1 equiv. of TBAF indicates strong H-bond interaction between the receptor recognition site and fluoride anion. This may be due to high sensing ability of the receptors. 3.2. Colorimetric analysis In order to deduce the anion sensing ability of the receptors 1–3 with halide anions (F− , Cl− , Br− and I− ) titrations were carried out in different solvents namely CHCl3 , CH3 CN and DMSO. The change in optical and optoelectronic properties was monitored by visual (naked-eye), absorption and fluorescence techniques. First, the halide anions (F− , Cl− , Br− and I− ) were added as tetrabutylammonium salts to 5 × 10−5 M solutions of the receptors in acetonitrile. In the naked eye experiments, the receptors 1, 2 and 3 (5 × 10−5 M) showed dramatic color change from colorless to pale yellow, fluorescent yellow and orange color, respectively in the presence of tetrabutyl ammonium fluoride (0.2 equiv.) in acetonitrile. The reason for this color change was probably due to the formation of hydrogen bond interactions between the –OH groups of the phenyl ring of salicylaldimine and fluoride ions. This is because, fluoride ions have higher electro negativity and small size [31,32] interacts with hydroxyl group through intermolecular hydrogen bond (O–H–F) which will affect the optical properties of the receptors. However the receptors 2 and 3 showed remarkable color change compared to the receptor 1 as seen in Fig. 3a–c. These observations are due to the presence of chromogenic signaling units (−Cl and −NO2 ) in the aniline ring of the receptors 2 and 3 while compared to the receptor 1. All the receptors were found
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Fig. 2.
1
H NMR (DMSO-d6 ) spectra of the receptor 3 in the presence (a) 3 equiv., (b) 1 equiv., and (c) absence of tetrabutyl ammonium fluoride.
to be insensitive even on addition of large excess of Cl− , Br− and I− (up to 100 equiv.). Similar observable color changes also took place in CHCl3 and DMSO solutions of the receptors. The receptors 1 and 2 became pale and fluorescent yellow in both chloroform and DMSO medium upon addition of fluoride ions, while for receptor 3 the color changes to pink color in DMSO and orange color in CHCl3 . No color change (CHCl3 and DMSO medium) was observed in the presence of chloride, bromide and iodide ions as similar to acetonitrile medium. 3.3. UV–vis spectroscopic studies The change in optoelectronic properties of receptors 1–3 in the presence of fluoride anion was investigated by UV–vis spectroscopic methods. The titrations were carried out in acetonitrile medium at 5.0 × 10−5 M concentrations of receptors 1–3 upon the addition of incremental amounts of 0.02 ml (1.5 × 10−3 M) of tetrabutylammonium fluoride, the spectra of the receptors are shown in Fig. 4a–c. The UV–vis absorption data of the receptors 1–3 were listed in Table 1. From the electronic absorption spectra of the receptors 1–3 in the absence of anions, it is found that the maxi-
Table 1 UV–visible spectral data for neat receptors 1–3 in acetonitrile medium (5 × 10−5 M). Receptor
max (nm)
1 2 3
336, 316, 300, 269, 225, 207 340, 318, 270, 230 355, 321, 214
mum wavelength of –* transitions for the compounds 1–3 are all almost within 207–355 nm, there is no absorption band in the visible region. The bands in the region 207–230 nm could be assigned to excitation of the electrons of the aromatic system. The band at 300 nm for receptors (1 and 2) and at 320 nm for receptor 3 is due to the transition between the orbital localized on the azomethine group (C N). The band in the region around 340 nm is due to existence of charge transfer transition within the molecule [33]. However, upon addition of fluoride anion to all the receptors immediate color change was noticed through naked eye, which on electronic (UV–vis) spectroscopic analysis we find a new peak in the visible region with noticeable peak intensity changes in the UV region. The new peak above 400 nm for the receptors 1–3 were mainly attributed to the formation of keto form [34] of the receptors resulting from hydrogen bonding interaction with fluoride anion as shown in Fig. 5. In case of receptors 2 and 3 the color of the solution become more intense upon increasing the concentration of the fluoride anion whereas for receptor 1 the color change was not appreciable due to the absence of chromogenic signaling unit. However, for all the receptors the increase in the intensity of the peak in visible region reaches a limit after the addition of 2 equiv. of F− . Upon successive addition of fluoride anion (from 0.2 to 2 equiv.) to receptor 1, the intensity of the peaks at 300, 316 and 336 nm gradually decreases, while the intensity of the peaks at 225 and 269 nm gradually increases. A new peak (420 nm) in the visible region was noticed upon addition of 0.6 equiv. of the fluoride anion for receptor 1 before that no peaks in the visible region. Similarly for receptor 2, with the addition of 0.4 equiv. of fluoride anion causes a new peak at 420 nm and it increased stepwise up to 2 equiv. Also from the absorption spectrum, we noticed the intensity of the peaks at 270, 300, 318 and 340 nm were decreased and at 230 nm get increased
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Fig. 4. Absorption spectra of the receptors 1, 2 and 3 (a, b and c, respectively) recorded in acetonitrile (5.0 × 10−5 M) after addition of 0–2 equiv. of tetrabutylammonium fluoride. Fig. 3. (a) Color changes of receptor 1 in acetonitrile (5.0 × 10−5 M) before and after the addition of 2 equiv. of representative anions (from left to the right: R, R + F− , R + Cl− , R + Br− , R + I− ). (b) Color changes of receptor 2 in acetonitrile (5.0 × 10−5 M) before and after the addition of 2 equiv. of representative anions (from left to the right: R, R + F− , R + Cl− , R + Br− , R + I− ). (c) Color changes of receptor 3 in acetonitrile (5.0 × 10−5 M) before and after the addition of 2 equiv. of representative anions (from left to the right: R, R + F− , R + Cl− , R + Br− , R + I− ).
upon addition of fluoride anion. However, for receptor 3, a new peak (487 nm) was observed just upon addition of 0.2 equiv. of fluoride anion with decrease in the intensity of absorption bands at UV region. This suggests that, the presence of nitro group in the receptor 3 acts as an excellent signaling unit compared to other receptors. Exposure of receptors with other anions (chloride, bromide and iodide) did not result in any spectral changes in the absorption spectrum. From the UV–vis absorption measurements the binding constant (Ka ) of the fluoride complexes of the receptors 1, 2 and 3 were calculated from the variation in the absorbance at 420, 420 and 48l nm, respectively and were found to be 2.787 × 103 , 5.684 × 103 and 2.53 × 104 . This suggests that the receptor 3 forms strong binding with fluoride anion compared to other receptors.
3.4. Fluorescence spectroscopic studies In order to learn more about the sensing ability of the receptors, fluorescence measurements were carried out similar to UV–visible measurements. The emission maximum (emi ) of the receptors (1 × 10−3 M) was around 490 nm for all the three receptors. It was reported that the enol form of the schiff base shows only very weak fluorescence [35–37]. For this reason, the concentration dependence of fluorescence spectra was investigated between 1 × 10−3 and 5 × 10−5 M for all receptors in study. At 5 × 10−5 M, the emission maximum of the receptors was red shifted and show broad
Fig. 5. The possible structure of the complex formed between receptor 3 and fluoride anion.
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Fig. 7. Dependence of fluorescence intensity (I/Io ) with respect to [F− ]o /[R]o . Inset shows the fluorescence response of the receptors 1–3 with 2 equiv. of TBAF.
Fig. 6. Emission spectra of the receptors 1, 2 and 3 (a, b and c, respectively) recorded in acetonitrile (5.0 × 10−5 M) after addition of 0–2 equiv. of tetrabutylammonium fluoride. Excitation wavelength (exi ) for the receptors 1, 2 and 3 was 358, 380 and 360 nm, respectively.
emission above 500 nm for all the three receptors with diminished fluorescence intensity. However, the addition of fluoride anion to the receptors markedly enhances the emission intensity as shown in Fig. 6a–c (upon adding 0.02 ml of fluoride anion to the receptors 1, 2 and 3 showed emission maxima at 475, 475 and 472 nm in acetonitrile solution, respectively) whereas no emission was observed with other anions. Further, the emission intensity increases on increasing the concentration of the fluoride anions with marginal changes in their emission maxima. This observed enhancement in the fluorescent intensity upon addition of fluoride anion may be due to the formation of new geometrically restricted six-membered transition state as shown in Fig. 5. That is upon addition of fluoride anion to the receptors, an increase in the conformational restriction occurs due to delocalization of the charge created with in the molecule. Thus, the increase in rigidity of the system makes the non-radiative decay from the excited state less probable, consequently, the emission intensity increases. Fig. 7 shows the dependence of fluorescence intensity (I/Io ) with respect to [F− ]o /[R]o for the three receptors, where I and Io are
the fluorescence intensity of the receptors in the presence and in the absence of the anion and [F− ]o and [R]o are the concentration of the anion and receptors, respectively. From this plot, the fluorescent enhancement factor was calculated. Particularly, the –NO2 substituted receptor show large enhancement factor of 652, when compared to receptors 1 [FEF = 79] and 2 [FEF = 16]. This suggests that, among the three receptors, Receptor 3 shows maximum sensitivity to the anion which may due to the presence of strong electron withdrawing substituent (−NO2 ). The reason for fluorescence enhancement in the presence of nitro group may be explained on the basis of electron transfer mechanism as follows: it has generally been recognized that emission was usually accompanied by the delocalization of the electrons within the molecules. In the case of salicylaldimine based schiff base receptors the excited state of the fluorophore was primarily quenched by reduced electron transfer from receptor (–OH) to the fluorophore (–C N) unit. However, upon interaction with anions, the electron transfer from the electron rich –OH moiety bonded with anion to the electron deficient –NO2 became more feasible and hence fluorescence enhancement was observed. Upon further addition of anion it appeared that the deprotonated species being more electron rich compared to the hydrogen bonded complex with fluoride, undergo very fast electron transfer showed up greater fluorescence enhancement. These findings open up the way to design and synthesis a new efficient ion-selective molecular sensor for biological molecules containing fluoride ions. 4. Conclusion In conclusion, we have systematically prepared and studied the fluorescence sensing properties of salicylaldimine based schiff base receptors via optical and spectroscopic techniques. Among the three receptors –NO2 substituted receptor shows high fluorescence enhancement. Fluorescence enhancement factor (FEF) calculations lend further support for the strong affinity and high selectivity towards fluoride ion for salicylaldimine based schiff base receptors. The molecular interactions between schiff base receptors and anions are really important because the knowledge base in the field of chemosensors and fluorescence “turn on” sensors still needs to be improved before going for applications. Acknowledgements Authors thank Dr. M. Chidambaram, Director, NITT for his constant support and encouragement for research. Authors also, thank DST, New Delhi (SR/FST/CSI-066/2008 dated 12.01.2009) for the sanction of major research fund for improvement of Science and Technology (FIST) for the development of the department.
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Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Colorimetric and fluorescence sensing of fluoride anions with potential salicylaldimine based schiff base receptors Radhakrishnan Sivakumar, Vijayaragavan Reena, Nallamuthu Ananthi, Muthiya Babu, Sambandam Anandan ∗ , Sivan Velmathi ∗∗ Department of Chemistry, National Institute of Technology, Tiruchirappalli 620 015, India
a r t i c l e
i n f o
Article history: Received 16 July 2009 Received in revised form 15 December 2009 Accepted 31 December 2009 Keywords: Schiff base Anions Fluoride sensors Fluorescence enhancement
a b s t r a c t Salicylaldimine based schiff base receptors with different substituents showing fluorescent enhancement in the presence of fluoride anion was visualized through naked eye as well as by change in spectral properties (UV–vis and fluorescent techniques). The reason for such fluorescence enhancement may be due to hydrogen bond interaction between receptor recognition site and fluoride anion. Such a hydrogen bond interaction creates a six-membered transition state, which avoids quenching processes. To support this, fluorescence enhancement factor (FEF) was calculated and it was found to be more (FEF = 652) for –NO2 substituted receptor compared to other receptors. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Designing ion-selective molecular sensors for sensing of anions and cations is a challenging object in supramolecular chemistry, in view of the fact that ions play a major role in many chemical and biological processes. An advance in the field of sensor comprises investigation of molecular recognition at model interfaces which provides great insights into biological phenomena and modern applications such as high selectivity sensors [1]. Among the biologically important ions, fluoride anion is a common ingredient in hypnotics, anesthetics, psychiatric drugs and cockroach poisons and is a contaminant in drinking water. Excess fluoride ion exposure causes fluorosis, thyroid activity depression, bone disorders and immune system disruption [2]. Besides the above significance, detection of fluoride ion with the use of simple preparation and minimal instrumental assistance is desirable toward practical applications. Colorimetric chemosensors based on optical and spectral changes upon recognition with ions are particularly attractive because of their simplicity and high detection limit of fluorescence detection methods [3–7]. Nevertheless in most of the cases [8–11] fluorescence quenching rather than fluorescent enhancement was also observed. The reason for fluorescence quenching was mainly attributed to the nature of the adduct formed upon
∗ Corresponding author. Tel.: +91 431 2503639; fax: +91 431 2500133. ∗∗ Corresponding author. Tel.: +91 431 2503640; fax: +91 431 2500133. E-mail addresses: [email protected] (S. Anandan), [email protected] (S. Velmathi). 1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2009.12.077
interaction with anions which influence the photo-physical and electronic transitions within the receptors. It has thus become a demand for the development of fluorescent “turn on” sensors for anions of biological importance. Kim and Ahn [12] observed the fact that, the fluorescence enhancement of the receptors in the presence of anions was likely owing to the elimination of n–* transitions within the receptor which seems to intervene the –* transition levels which are responsible for emission. Hence, recognition of receptors towards anions not only involves color changes but also affects the electronic transitions within the molecule. For this reason, the receptors have to be suitably designed in such a way that it contains both anion recognition site and fluorophore moiety. In biological systems, anion recognition is carried out through hydrogen bonding by proteins with sterically well-defined complex cites in the interior of proteins [13,14]. The neutral type anion sensors generally consist of heterocyclic units such as pyrrole, pyridine and indole and other units like amide, amine and phenol in the recognition sites for hydrogen bonding [15,16]. Many reports have appeared describing the synthesis and study of very sophisticated anion sensors [17–21]. This is because, anions seems to interact with a hydrogen bond donors of a receptor (NH2 , –OH, –C(O)NHR) more strongly. Among the available anions F− anion shows better interaction than any other anions. To the best of our knowledge, only few research groups were working in this field with receptors having –OH group as a binding site [22–30]. On the basis of above mentioned background, here we report the fluorescence sensing of fluoride anions with receptors having –OH group as binding site (salicylaldimine based schiff base receptors with different substituents).
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application. In the FT-IR spectroscopy we get information about the presence of functional groups in the compound such as C C, CH N, etc. by the position of absorption peaks which arise due to stretching vibration of the bonds in the groups. The presence of C N in the molecule is confirmed by the vibrations between 1690 and 1640 cm−1 and aromatic C C between 1600 and 1473 cm−1 . These vibrations are seen in the IR spectra of all the compounds synthesized and the data obtained are given in the experimental section. NMR spectroscopy is one of the principal techniques which give us the structural information about molecules. NMR spectra were obtained using CDCl3 as the solvent. All the three compounds gave a singlet at around ı 8.6–9.0 ppm corresponding to the CH N proton indicating the formation of imine and the aromatic protons resonate in the ı 6.9–8.2 region In addition the OH proton of aromatic ring is shown as a singlet at around ı 12.3–12.6 ppm. Fig. 1. Structure of the receptors 1–3.
2. Experimental 2.1. Materials and methods All reagents were purchased from Sigma Aldrich and used as received. Analytical Grade solvents were used as such. 1 H NMR spectra were recorded on a JEOL LA600 (600 MHz) in CDCl3 at 298 K with TMS as internal standard. FT-IR spectra were measured on a Perkin–Elmer FT-IR spectrometer using KBr plates. UV–visible and Fluorescence spectra were recorded in 1 cm path length quartz cell on a Perkin Elmer EZ301 spectrophotometer and Shimadzu RF-5301 PC spectrofluorophotometer, respectively. 2.2. General experimental procedure for the synthesis of the receptors Typically to 4-nitro aniline (0.2762 g, 2 mmol) dissolved in 10 ml methanol, salicylaldehyde (0.2426 g, 2 mmol) dissolved in 5 ml was slowly added and stirred for 3 h at room temperature. Completion of the reaction was monitored through TLC for the disappearance of the starting compounds. Then the solvent was removed through rotovac yielding the reddish yellow crystals of N(4-nitrophenyl) salicylaldimine (receptor 3) yield: 98.3%. The solid thus obtained was dried in vacuum oven. M. Pt 130–132 ◦ C. Similarly following the above procedure the receptors 1 and 2 were synthesized. 2.2.1. Receptor 1 1 H NMR (CDCl , ı ppm ) 7.0–7.2 m 2H, 7.3–7.5 dd 4H, 8.2–8.4 d 3 2H (aromatic), 8.6 s, 1H (CH N), 12.5 s, 1H (OH). IR (KBr plates, cm−1 ): 1267, 1345, 1463, 1630, 3070, 3427. 2.2.2. Receptor 2 1 H NMR (CDCl , ı ppm ) 6.9–7.1 m 2H, 7.2 m 2H, 7.4 m 4H (aro3 matic), 8.6 s 1H, 12.6 s, 1H (OH). IR (KBr plates, cm−1 ): 1272, 1374, 1484, 1600, 3071, 3481. 2.2.3. Receptor 3 1 H NMR (CDCl , ı 3 ppm ) (aromatic) 6.9 m 2H, 7.2 m 1H, 7.4 m 5H, 7.6 d 1H (aromatic), 9.0 s 1H (CH N), 12.3 s, 1H (OH). IR (KBr plates, cm−1 ): 1272, 1395, 1485, 1611, 3056, 3450. 3. Results and discussion The chromogenic receptors 1–3 (Fig. 1) were synthesized by schiff’s base condensation of salicylaldehyde and aniline (1) or 4chloroaniline (2) or 4-nitroaniline (3). They were well characterized by FT-IR, 1 H NMR spectroscopic methods before using them in
3.1.
1H
NMR spectroscopic studies
To understand the molecular interactions between the receptor recognition site and fluoride anion, 1 H NMR titration experiments were done with receptor 3 in the presence and in the absence of TBAF in DMSO-d6 . Fig. 2 shows the 1 H NMR spectrum of the receptor 3 in the absence and in the presence of 1 and 3 equiv. TBAF. In the case of neat receptor 3 i.e. in the absence of TBAF, –OH proton appears as a singlet at 12.3 ppm and the imine proton (CH N) comes at 9.0 ppm as a sharp singlet, whereas in the presence of 1 equiv. of TBAF, the singlet at 12.3 ppm disappeared immediately indicating the hydrogen bond interaction between fluoride anion and –OH group, consequently an up field shift from 9.0 to 8.9 ppm in the imine proton was also observed. The disappearance of the phenolic OH upon the addition of 1 equiv. of fluoride anion may be due to the simultaneous formation of the alkoxide and its hydrogen bonding with the F− ion. In the presence of 3 equiv. of fluoride ions an additional sharp peak at 16 ppm appeared which is due to the deprotonation of the chromogenic receptor and the subsequent formation of [HF2 − ] species. From the NMR spectra, we inferred that complete disappearance of the –OH singlet upon addition of 1 equiv. of TBAF indicates strong H-bond interaction between the receptor recognition site and fluoride anion. This may be due to high sensing ability of the receptors. 3.2. Colorimetric analysis In order to deduce the anion sensing ability of the receptors 1–3 with halide anions (F− , Cl− , Br− and I− ) titrations were carried out in different solvents namely CHCl3 , CH3 CN and DMSO. The change in optical and optoelectronic properties was monitored by visual (naked-eye), absorption and fluorescence techniques. First, the halide anions (F− , Cl− , Br− and I− ) were added as tetrabutylammonium salts to 5 × 10−5 M solutions of the receptors in acetonitrile. In the naked eye experiments, the receptors 1, 2 and 3 (5 × 10−5 M) showed dramatic color change from colorless to pale yellow, fluorescent yellow and orange color, respectively in the presence of tetrabutyl ammonium fluoride (0.2 equiv.) in acetonitrile. The reason for this color change was probably due to the formation of hydrogen bond interactions between the –OH groups of the phenyl ring of salicylaldimine and fluoride ions. This is because, fluoride ions have higher electro negativity and small size [31,32] interacts with hydroxyl group through intermolecular hydrogen bond (O–H–F) which will affect the optical properties of the receptors. However the receptors 2 and 3 showed remarkable color change compared to the receptor 1 as seen in Fig. 3a–c. These observations are due to the presence of chromogenic signaling units (−Cl and −NO2 ) in the aniline ring of the receptors 2 and 3 while compared to the receptor 1. All the receptors were found
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Fig. 2.
1
H NMR (DMSO-d6 ) spectra of the receptor 3 in the presence (a) 3 equiv., (b) 1 equiv., and (c) absence of tetrabutyl ammonium fluoride.
to be insensitive even on addition of large excess of Cl− , Br− and I− (up to 100 equiv.). Similar observable color changes also took place in CHCl3 and DMSO solutions of the receptors. The receptors 1 and 2 became pale and fluorescent yellow in both chloroform and DMSO medium upon addition of fluoride ions, while for receptor 3 the color changes to pink color in DMSO and orange color in CHCl3 . No color change (CHCl3 and DMSO medium) was observed in the presence of chloride, bromide and iodide ions as similar to acetonitrile medium. 3.3. UV–vis spectroscopic studies The change in optoelectronic properties of receptors 1–3 in the presence of fluoride anion was investigated by UV–vis spectroscopic methods. The titrations were carried out in acetonitrile medium at 5.0 × 10−5 M concentrations of receptors 1–3 upon the addition of incremental amounts of 0.02 ml (1.5 × 10−3 M) of tetrabutylammonium fluoride, the spectra of the receptors are shown in Fig. 4a–c. The UV–vis absorption data of the receptors 1–3 were listed in Table 1. From the electronic absorption spectra of the receptors 1–3 in the absence of anions, it is found that the maxi-
Table 1 UV–visible spectral data for neat receptors 1–3 in acetonitrile medium (5 × 10−5 M). Receptor
max (nm)
1 2 3
336, 316, 300, 269, 225, 207 340, 318, 270, 230 355, 321, 214
mum wavelength of –* transitions for the compounds 1–3 are all almost within 207–355 nm, there is no absorption band in the visible region. The bands in the region 207–230 nm could be assigned to excitation of the electrons of the aromatic system. The band at 300 nm for receptors (1 and 2) and at 320 nm for receptor 3 is due to the transition between the orbital localized on the azomethine group (C N). The band in the region around 340 nm is due to existence of charge transfer transition within the molecule [33]. However, upon addition of fluoride anion to all the receptors immediate color change was noticed through naked eye, which on electronic (UV–vis) spectroscopic analysis we find a new peak in the visible region with noticeable peak intensity changes in the UV region. The new peak above 400 nm for the receptors 1–3 were mainly attributed to the formation of keto form [34] of the receptors resulting from hydrogen bonding interaction with fluoride anion as shown in Fig. 5. In case of receptors 2 and 3 the color of the solution become more intense upon increasing the concentration of the fluoride anion whereas for receptor 1 the color change was not appreciable due to the absence of chromogenic signaling unit. However, for all the receptors the increase in the intensity of the peak in visible region reaches a limit after the addition of 2 equiv. of F− . Upon successive addition of fluoride anion (from 0.2 to 2 equiv.) to receptor 1, the intensity of the peaks at 300, 316 and 336 nm gradually decreases, while the intensity of the peaks at 225 and 269 nm gradually increases. A new peak (420 nm) in the visible region was noticed upon addition of 0.6 equiv. of the fluoride anion for receptor 1 before that no peaks in the visible region. Similarly for receptor 2, with the addition of 0.4 equiv. of fluoride anion causes a new peak at 420 nm and it increased stepwise up to 2 equiv. Also from the absorption spectrum, we noticed the intensity of the peaks at 270, 300, 318 and 340 nm were decreased and at 230 nm get increased
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Fig. 4. Absorption spectra of the receptors 1, 2 and 3 (a, b and c, respectively) recorded in acetonitrile (5.0 × 10−5 M) after addition of 0–2 equiv. of tetrabutylammonium fluoride. Fig. 3. (a) Color changes of receptor 1 in acetonitrile (5.0 × 10−5 M) before and after the addition of 2 equiv. of representative anions (from left to the right: R, R + F− , R + Cl− , R + Br− , R + I− ). (b) Color changes of receptor 2 in acetonitrile (5.0 × 10−5 M) before and after the addition of 2 equiv. of representative anions (from left to the right: R, R + F− , R + Cl− , R + Br− , R + I− ). (c) Color changes of receptor 3 in acetonitrile (5.0 × 10−5 M) before and after the addition of 2 equiv. of representative anions (from left to the right: R, R + F− , R + Cl− , R + Br− , R + I− ).
upon addition of fluoride anion. However, for receptor 3, a new peak (487 nm) was observed just upon addition of 0.2 equiv. of fluoride anion with decrease in the intensity of absorption bands at UV region. This suggests that, the presence of nitro group in the receptor 3 acts as an excellent signaling unit compared to other receptors. Exposure of receptors with other anions (chloride, bromide and iodide) did not result in any spectral changes in the absorption spectrum. From the UV–vis absorption measurements the binding constant (Ka ) of the fluoride complexes of the receptors 1, 2 and 3 were calculated from the variation in the absorbance at 420, 420 and 48l nm, respectively and were found to be 2.787 × 103 , 5.684 × 103 and 2.53 × 104 . This suggests that the receptor 3 forms strong binding with fluoride anion compared to other receptors.
3.4. Fluorescence spectroscopic studies In order to learn more about the sensing ability of the receptors, fluorescence measurements were carried out similar to UV–visible measurements. The emission maximum (emi ) of the receptors (1 × 10−3 M) was around 490 nm for all the three receptors. It was reported that the enol form of the schiff base shows only very weak fluorescence [35–37]. For this reason, the concentration dependence of fluorescence spectra was investigated between 1 × 10−3 and 5 × 10−5 M for all receptors in study. At 5 × 10−5 M, the emission maximum of the receptors was red shifted and show broad
Fig. 5. The possible structure of the complex formed between receptor 3 and fluoride anion.
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Fig. 7. Dependence of fluorescence intensity (I/Io ) with respect to [F− ]o /[R]o . Inset shows the fluorescence response of the receptors 1–3 with 2 equiv. of TBAF.
Fig. 6. Emission spectra of the receptors 1, 2 and 3 (a, b and c, respectively) recorded in acetonitrile (5.0 × 10−5 M) after addition of 0–2 equiv. of tetrabutylammonium fluoride. Excitation wavelength (exi ) for the receptors 1, 2 and 3 was 358, 380 and 360 nm, respectively.
emission above 500 nm for all the three receptors with diminished fluorescence intensity. However, the addition of fluoride anion to the receptors markedly enhances the emission intensity as shown in Fig. 6a–c (upon adding 0.02 ml of fluoride anion to the receptors 1, 2 and 3 showed emission maxima at 475, 475 and 472 nm in acetonitrile solution, respectively) whereas no emission was observed with other anions. Further, the emission intensity increases on increasing the concentration of the fluoride anions with marginal changes in their emission maxima. This observed enhancement in the fluorescent intensity upon addition of fluoride anion may be due to the formation of new geometrically restricted six-membered transition state as shown in Fig. 5. That is upon addition of fluoride anion to the receptors, an increase in the conformational restriction occurs due to delocalization of the charge created with in the molecule. Thus, the increase in rigidity of the system makes the non-radiative decay from the excited state less probable, consequently, the emission intensity increases. Fig. 7 shows the dependence of fluorescence intensity (I/Io ) with respect to [F− ]o /[R]o for the three receptors, where I and Io are
the fluorescence intensity of the receptors in the presence and in the absence of the anion and [F− ]o and [R]o are the concentration of the anion and receptors, respectively. From this plot, the fluorescent enhancement factor was calculated. Particularly, the –NO2 substituted receptor show large enhancement factor of 652, when compared to receptors 1 [FEF = 79] and 2 [FEF = 16]. This suggests that, among the three receptors, Receptor 3 shows maximum sensitivity to the anion which may due to the presence of strong electron withdrawing substituent (−NO2 ). The reason for fluorescence enhancement in the presence of nitro group may be explained on the basis of electron transfer mechanism as follows: it has generally been recognized that emission was usually accompanied by the delocalization of the electrons within the molecules. In the case of salicylaldimine based schiff base receptors the excited state of the fluorophore was primarily quenched by reduced electron transfer from receptor (–OH) to the fluorophore (–C N) unit. However, upon interaction with anions, the electron transfer from the electron rich –OH moiety bonded with anion to the electron deficient –NO2 became more feasible and hence fluorescence enhancement was observed. Upon further addition of anion it appeared that the deprotonated species being more electron rich compared to the hydrogen bonded complex with fluoride, undergo very fast electron transfer showed up greater fluorescence enhancement. These findings open up the way to design and synthesis a new efficient ion-selective molecular sensor for biological molecules containing fluoride ions. 4. Conclusion In conclusion, we have systematically prepared and studied the fluorescence sensing properties of salicylaldimine based schiff base receptors via optical and spectroscopic techniques. Among the three receptors –NO2 substituted receptor shows high fluorescence enhancement. Fluorescence enhancement factor (FEF) calculations lend further support for the strong affinity and high selectivity towards fluoride ion for salicylaldimine based schiff base receptors. The molecular interactions between schiff base receptors and anions are really important because the knowledge base in the field of chemosensors and fluorescence “turn on” sensors still needs to be improved before going for applications. Acknowledgements Authors thank Dr. M. Chidambaram, Director, NITT for his constant support and encouragement for research. Authors also, thank DST, New Delhi (SR/FST/CSI-066/2008 dated 12.01.2009) for the sanction of major research fund for improvement of Science and Technology (FIST) for the development of the department.
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