ARTICLE pubs.acs.org/JPCA
Sensitive Fluorescent Detection and Lewis Basicity of Aliphatic Amines Ivan Pietro Oliveri and Santo Di Bella* Dipartimento di Scienze Chimiche, Universita di Catania, I-95125 Catania, Italy
bS Supporting Information ABSTRACT: In this contribution is reported the sensitive properties of the ZnII Schiff base complex, 1, in dichloromethane with respect a series of primary, secondary, and tertiary aliphatic amines through the study of fluorescence enhancement upon amine coordination to the Lewis acidic ZnII metal center with formation of 1:1 adducts. It is found that complex 1 exhibits selectivity and nanomolar sensitivity for primary and alicyclic amines. A distinct selectivity is also observed along the series of secondary or tertiary amines, paralleling the increasing steric hindrance at the nitrogen atom. The binding interaction can be related to the Lewis basicity of the coordinating amine; thus, complex 1 represents a suitable reference Lewis acid, and estimated binding constants within the investigated amine series can be related to their relative Lewis basicity. A relative order of the Lewis basicity can be established for acyclic amines, primary > secondary > tertiary, while an inverted order, tertiary > secondary ≈ primary (acyclic), is found in the case of alicyclic amines. The present approach represents a simple, suitable method to ranking the relative Lewis basicity of aliphatic amines in low-polarity, nonprotogenic solvents.
’ INTRODUCTION Aliphatic amines are widespread compounds in nature.1 They are also involved in many organic syntheses and catalysis.2 In this view, the molecular recognition and sensitive optical detection of low-molecular-weight amines3 is critical in the medical field, in environmental science, in food safety, and in organic chemistry.4 The most important property of amines is certainly related to their intrinsic Brønsted or Lewis basicity. However, the evaluation of basicity in solution is of most importance for their actual properties in solvent media. In particular, the Lewis basicity represents the relevant parameter to predict and rationalize, in terms of basicity, the chemical behavior of amines.5,6 Tetracoordinated Schiff base ZnII complexes are Lewis acidic species capable of saturating their coordination sphere by coordinating a large variety of neutral substrates including nitrogenbased donor Lewis bases,7 or in their absence, they can be stabilized through intermolecular Zn 3 3 3 O axial coordination involving Lewis basic atoms of the ligand framework.8 Moreover, they possess variegate fluorescent features, related to the structure of the Schiff base template9 and the axial coordination.7,10 We have recently demonstrated that some amphiphilic ZnII Schiff base complexes in solution of dichloromethane (DCM) exhibit substantial optical variations and a dramatic enhancement of the fluorescence emission upon addition of a coordinating species.11 As this process occurs because of the axial coordination to the acidic ZnII ion, it is expected to be selective and sensitive to the Lewis basicity of the coordinating species. In this contribution, we report on the sensitive properties of the ZnII complex, 1, with respect a series of primary, secondary and tertiary aliphatic amines, by changing the steric hindrance of r 2011 American Chemical Society
N-alkyl substituents (Scheme 1), through the study of optical absorption and fluorescence changes upon amine coordination to the Lewis acidic ZnII metal center.
Various chromogenic12 and fluorogenic13 approaches are reported in the literature for the selective detection of aliphatic amines. These approaches are generally based on either Brønsted basicity of amines or formation of guesthost adducts. In our case, detection of investigated amines is based on their actual Lewis basicity in the low-polarity, nonprotogenic, noncoordinating solvent medium such as DCM, hence avoiding relevant specific solvation effects. Thus, the calculated binding constants of 1 3 amine adducts can properly be related to the relative Lewis basicity of involved amines.5,14
’ EXPERIMENTAL SECTION Materials and General Procedures. The ZnII complex, 1, was
synthesized and fully characterized as previously reported.11 Received: July 12, 2011 Revised: November 8, 2011 Published: November 08, 2011 14325
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Scheme 1. Structure of the Investigated Amines
Amines 215 (Aldrich) were used without further purifications. Dichloromethane (Aldrich) stabilized with amylene was used to prepare solutions of 1. Fresh prepared DCM solutions of 1, obtained from 1.0 103 M stock solutions, were used for spectrophotometric and fluorimetric measurements. Measurements. Optical absorption spectra were recorded at 25 °C with a Varian Cary 500 UVvisNIR spectrophotometer. Fluorescence spectra were recorded at 25 °C with a Fluorolog-3 (Jobin Yvon Horiba) spectrofluorimeter. Spectrophotometric and fluorimetric titrations were performed with a 1 cm path cell using DCM solutions of 1. Involved amines in DCM solutions were added to the solution of the complex 1 via a 25 μL Hamilton syringe. At least three replicate titrations were performed for each amine. In each fluorimetric titration, the wavelength of excitation was chosen in an isosbestic point. Calculation of the Binding Constants. The 1 3 amine binding constants, K, were calculated from fluorescence titration data by the nonlinear curve fitting analysis of F versus cp in eq 115 F ¼ F0 þ
Flim F0 2c0
½c0 þ cp þ 1=K ½ðc0 þ cp þ 1=KÞ2 4c0 cp 1=2
ð1Þ
where F0 is the initial fluorescence of the solution having a concentration c0, F is the fluorescence intensity after addition of a given amount of amine at a concentration cp, and Flim is the limiting fluorescence reached in the presence of an excess of amine (see Supporting Information for further details). Calculation of the Limit of Detection. The LOD was calculated from fluorescence data, according to IUPAC recommendations.16 In particular, it was calculated using eq 2 LOD ¼ K Sb =S
ð2Þ
where K = 3, Sb is the standard deviation of the blank solution, and S is the slope of the calibration curve. In our case, because the DCM solution of 1 gives a fluorescence signal without the presence of the amine, this signal is taken as the blank. Fifteen blank replicates were considered. The calibration curve was obtained from plots of the fluorescence intensity of 1 versus the concentration of the amine added. Each point is related to the mean value obtained from at least three replicate measurements (see Supporting Information for further details).
’ RESULTS AND DISCUSSION The amphiphilic complex 1 represents a Lewis acidic system capable of axially coordinating donor Lewis bases.11 In particular,
Figure 1. UVvis absorption (top) and fluorescence (bottom) (λexc = 467 nm) titration curves of 1 (10 μM solution in DCM) with the addition of propylamine. The concentration of propylamine added varied from 0 to 12.0 μM. (Inset) Variation of the fluorescence intensity at 600 nm as a function of the concentration of propylamine added. The solid line represents the curve fitting analysis with eq 1.
the binding interaction between 1 and the involved amines always implies formation of 1:1 adducts, as established by Job’s plot analysis, 1H NMR spectroscopy, and ESI mass spectrometry (see Supporting Information). Because this process is accompanied by appreciable optical variations, it can be investigated on a quantitative ground simply by means of spectrophotometric and spectrofluorimetric titrations. Spectrophotometric and spectrofluorimetric titrations of 10 μM DCM solutions of 1 were performed using DCM solutions of the amines 215 as titrants. As a representative example, the titration with propylamine, 2, is reported in Figure 1. Optical absorption spectra upon titration indicate some absorbance changes in the UVvis region with the existence of multiple isosbestic points and an appreciable increase of the absorbance (λmax = 557 nm) in the region between 530 and 600 nm. Spectrophotometric titrations of all investigated amines (see Supporting Information) show almost identical optical changes in the region >330 nm, in terms of intensity, λmax values (Table 1), and isosbestic points, thus indicating that coordination of the amine to the ZnII atom similarly affects the low-lying electronic states of 1. Moreover, the existence of multiple isosbestic points in optical absorption spectra upon titration is in agreement with the formation of defined adducts. Titration with propylamine is accompanied by an enhancement of the fluorescence emission, almost an order of magnitude larger. Spectrofluorimetric titrations of all investigated amines 14326
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Table 1. Optical Absorption and Fluorescence Emission Maxima, Binding Constants, and Limits of Detection for the 1 3 Amine Adducts λmax,abs
λmax,em
(nm)
(nm)
log K
2, propylamine
557
600
6.1 ( 0.2
0.17 (10)
3, n-butylamine
557
600
6.1 ( 0.2
0.17 (12)
4, isopropylamine
557
600
6.3 ( 0.3
0.19 (11)
5, tert-amylamine 6, ethylmethylamine
556 557
599 600
6.3 ( 0.2 6.1 ( 0.2
0.21 (18) 0.18 (11)
7, diethylamine
557
600
5.7 ( 0.1
0.27 (20)
8, diisopropylamine
554
599
3.0 ( 0.2
5.0 (506)
9, di-tert-amylamine
556
598
2.8 ( 0.1
27 (420)
10, piperidine
558
599
6.1 ( 0.2
0.16 (14)
adduct
LODa (μM)
11, dimethylethylamine
557
599
6.0 ( 0.1
0.21 (16)
12, triethylamine
558
603
3.7 ( 0.1
4.8 (480)
13, triisopropylamine 14, tris(2-ethylhexyl)
558 558
602 602
2.3 ( 0.1 1.7 ( 0.3
27 (380) 50 (18 103)
558
600
6.9 ( 0.3
0.19 (21)
amine 15, quinuclidine a
Values in parentheses refer to LOD values in (μg L1).
Figure 2. Profile of the calculated binding constants for the investigated amines.
(see Supporting Information) involve rather constant fluorescence emission λmax values (Table 1), independent from the excitation wavelength, and almost the same fluorescence enhancement upon reaching of the saturation point. Analogously to optical absorption data, the almost constant fluorescence λmax values upon formation of 1 3 amine adducts indicate that the same excited electronic state is involved for all of the investigated aliphatic amines. Although both optical absorption and fluorescence emission changes can be easily detected upon titration of 1 with the involved amines, we have chosen the latter observable because of the higher sensitivity of detection.17 Thus, fluorescence titration data are used to calculate the limit of detection16 (LOD) of 1 for the investigated aliphatic amines and, eventually, to explore its selective sensing. Moreover, as we are interested to variations of 1 3 amine binding constants along the considered series, rather than in their absolute values, we have estimated them from
fluorescence titration data by the nonlinear curve fitting analysis of fluorescence intensity versus amine concentration.15 Relevant optical absorption and fluorescence emission data, calculated binding constants, and LODs are collected in Table 1. On the basis of above considerations, formation of defined 1:1 Lewis acidbase adducts whose optical properties are almost independent by the nature of the base complex 1 represents a suitable reference Lewis acid, and the observed variations of estimated binding constants within the investigated amine series can be properly related to their relative Lewis basicity.5,14 The calculated binding constants for 1 3 amine adducts span over several orders of magnitude (Table 1, Figure 2). In particular, while binding constants are almost invariable along primary amines, exhibiting the largest values among the considered acyclic species, they show a distinct decrease along the series of secondary or tertiary amines, strictly related to the increasing steric hindrance at the nitrogen amine atom. In line with this view, the alicyclic piperidine, 10, having a low steric hindrance within the series of secondary amines, possesses a binding constant comparable to that of the ethylmethylamine, 6, the latter of which is the least sterically encumbered species within the series secondary acyclic amines. Analogously, quinuclidine, 15, representing the least sterically encumbered tertiary amine,18 exhibits the largest binding constant value, even with respect primary amines (Table 1). Moreover, from calculated binding constants, a relative order, primary > secondary > tertiary, is found for amines having an analogous N-alkyl chain, especially for those with more branched N-alkyls (e.g., 5, 9, 14), while a leveling effect is observed for amines with linear N-alkyls (e.g., 2, 6, 11). Overall, these data indicate a relative order of the Lewis basicity for acyclic amines, primary > secondary > tertiary, according to the relative enthalpies of formation for amineborane adducts with a series of primary, secondary, and tertiary amines19 or to the relative basicity deduced by competition experiments using B(Me)3 as a reference acid,20 although gas-phase basicities indicate the opposite.21,22 This order is however inverted, tertiary > secondary ≈ primary (acyclic), in the case of alicyclic amines. Thus, it turns out that the Lewis basicity of aliphatic amines with respect to the reference Lewis acid 1 is dominated by steric effects and can be attributed to poorer overlap of orbitals because of the additional steric constraints in the case of more encumbered species.23 In fact, the less sterically crowded primary amines show the highest, almost unchanging, binding constants. Alicyclic amines, possessing a reduced sterical hindrance, exhibit the larger binding constant values. Unfortunately, the relevant Lewis basicity scales reported in the literature5,6 include just very few aliphatic amines, thus precluding any useful comparison with present data. A possibility of comparison is, however, offered by a scale of hydrogen bond basicity, involving 4-fluorophenol as a reference acid, developed by Graton et al.,24 which includes most of the present investigated aliphatic amines, although the resulting hydrogen-bonded complexes can be considered as a special case of Lewis acidbase interactions.5 The comparison of the present Gibbs energy for the 1 3 amine adducts with that for hydrogen-bonded complexes is reported in Figure 3. It shows a roughly linear correlation, with the Gibbs energy for the 1 3 amine adducts approximately twice than that found for hydrogen-bonded complexes, thus indicating a stronger acidbase interaction in the former case. A divergence of this rough linearity is observed in the case of diisopropylamine, 8, representing one of the most encumbered species among the 14327
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Figure 3. Comparison of the Lewis basicity for 1 3 amine adducts and the 4-fluorophenol Lewis basicity. 4-Fluorophenol data in CCl4 are from ref 24.
Figure 4. Plots of the relative fluorescence changes versus the concentration of the secondary amines 610 in DCM added to 1 (10 μM solution in DCM), monitored at 600 nm.
amines involved in this comparison. This further indicates the major role of the steric hindrance on the stability of 1 3 amine adducts and, hence, on the relative Lewis basicity. Present results are in full agreement with the relative Lewis basicities of aliphatic amines deduced from spectrophotometric titrations using complex 1 as a reference acid and a least-squares nonlinear regression of multiwavelength spectrophotometric data.14 The plots of the relative fluorescence change versus the amine concentration, in the micromolar range, can be related to the selectivity of 1 with respect to the investigated amines. As expected, 1 is not selective within primary amines, according to the aforementioned almost identical estimated binding constants. In contrast, a distinct selectivity is observed along the series of secondary or tertiary amines and parallels the increasing steric hindrance at the nitrogen atom (Figures 4 and 5). Thus, among the secondary amines 610, piperidine and ethylmethylamine can selectively be detected with respect to the remaining amine series. This can be easily verified by competitive experiments. For example, fluorimetric titrations of 1 with piperidine performed with and without the presence of an equimolar concentration of diisopropylamine indicate negligible variations of fluorescence intensity (see Supporting Information, Figure S8). To observe an analogous fluorescence response to that of piperidine, it needs a concentration of diisopropylamine more than 2 orders of magnitude larger (see Supporting Information, Figure S9). Analogously, quinuclidine and dimethylethylamine can selectively be detected with respect to any other more sterically encumbered tertiary amine, as can be also easily verified by competitive titration experiments. Again, to observe an analogous fluorescence response to that of quinuclidine or dimethylethylamine,
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Figure 5. Plots of the relative fluorescence changes versus the concentration of the tertiary amines 1115 in DCM added to 1 (10 μM solution in DCM), monitored at 600 nm.
Figure 6. Plots of the relative fluorescence changes versus the concentration of the amines 4, 8, and 13 in DCM added to 1 (10 μM solution in DCM), monitored at 600 nm.
it needs, for example, a concentration of triisopropylamine about 600 times larger, while for triethylamine, a concentration 50 times larger is required. Selectivity of 1 becomes also evident upon comparing the fluorescence response, again in the micromolar range, of primary versus secondary and tertiary acyclic amines having an analogous N-alkyl chain (Figure 6). For example, within the amines 4, 8, and 13, to observe an analogous fluorescence response to that of isopropylamine, it needs a concentration of diisopropylamine more than 1 order of magnitude larger, while for triisopropylamine, a concentration almost 2 orders of magnitude larger is required. Therefore, except for the secondary and tertiary linear N-alkyl amines, 6 and 11, primary amines can selectively be detected over acyclic secondary and tertiary amines, especially those with branched Nalkyl chains. The limit of detection of a 10 μM DCM solution of 1, calculated according to IUPAC recommendations,16 indicates very low values for the series of investigated amines, falling into the nanomolar range for primary and alicyclic amines (Table 1). Thus, complex 1 is useful for fast detection in solutions of lowpolarity solvents of primary and alicyclic amines in the range of trace amounts (μg L1). For secondary and tertiary amines, even if LOD values are definitely higher, they again evidence an appreciable sensitivity of 1 for all involved amines. The sensitivity of complex 1 for primary and alicyclic amines exceeds that reported in the literature for the selective chromogenic12a,b,f or fluorogenic13ac,h detection of aliphatic amines, rivaling that reported for fluorescent sensors of aliphatic amines in traces.25 14328
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The Journal of Physical Chemistry A Preliminary data suggest that complex 1 can potentially be applied for the detection of amines even in aqueous solutions. However, given the insolubility of 1 in water, a two-phase (DCM/H2O) system should be used. Thus, the addition of defined amounts of amine in aqueous solution to a DCM solution of 1 produces a fluorescence response that can be compared to that achieved for addition of the same mole amounts of amine in DCM (see Supporting Information, Figure S10).
’ CONCLUSIONS In this contribution, we have successfully developed a sensitive fluorescent probe for aliphatic amines, which exhibits fluorescence enhancement upon formation of 1:1 supramolecular adducts. It is found that complex 1 shows selectivity and nanomolar sensitivity for primary and alicyclic amines. Moreover, a distinct selectivity is also observed along the series of secondary or tertiary amines, paralleling the increasing steric hindrance at the nitrogen atom. Because complex 1 represents a suitable reference Lewis acid, the observed variations of estimated binding constants within the investigated amine series can be properly related to their relative Lewis basicity. Thus, present results indicate a relative order of the Lewis base strength for acyclic amines, primary > secondary > tertiary. This order is however inverted, tertiary > secondary ≈ primary (acyclic), in the case of alicyclic amines, thus indicating a major role of the steric hindrance on the stability of 1 3 amine adducts and, hence, on the relative Lewis basicity. These findings are in agreement with available literature data. Therefore, the present approach represents a simple, suitable method to ranking the relative Lewis basicity of aliphatic amines in low-polarity, nonprotogenic solvents. ’ ASSOCIATED CONTENT
bS
Supporting Information. Job’s plot analysis, ESI mass spectrometry, and 1H NMR data for 1 3 amine adducts. Additional spectrophotometric and spectrofluorimetric titrations. Calculation of the binding constants and the detection limits. Preliminary studies in DCM/H2O. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We gratefully thank Prof. G. Maccarrone (Universita di Catania) for useful comments and suggestions. This research was supported by the MIUR and PRA (Progetti di Ricerca di Ateneo). ’ REFERENCES (1) See, for example:Lawrence, S. A. Amines: Synthesis, Properties and Applications; University Press: Cambridge, U.K., 2004. (2) See, for example: (a) Song, C. E. Cinchona Alkaloids in Synthesis & Catalysis: Ligands, Immobilization and Organocatalysis; Wiley-VCH: Weinheim, Germany, 2009. (b) Nugent, T. C. Chiral Amine Synthesis; Wiley-VCH: Weinheim, Germany, 2010. (c) France, S.; Guerin, D. J.; Miller, S. J.; Lectka, T. Chem. Rev. 2003, 103, 2985–3012.
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