Amine Sensing with Distyrylbenzenes and Their Hexamethylene

Article pubs.acs.org/Macromolecules

Amine Sensing with Distyrylbenzenes and Their HexamethyleneLinked Polymers: Spraying Them On Jan Kumpf,† Jan Freudenberg,† S. Thimon Schwaebel,† and Uwe H. F. Bunz†,‡,* †

Organisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany Centre of Advanced Materials, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany

‡

S Supporting Information *

ABSTRACT: We herein describe synthesis and property evaluation of two aldehyde-appended nonconjugated distyrylbenzene polymers as amine sensing platforms and compare their performance to that of their monomers. The monomers and polymers were dissolved in organic solvents and spraydeposited onto silica gel and neutral alox TLC plates so as to give small strip-shaped sensor arrays. With these, differential amine detection was achieved; we find that a neutral silica gel support is better able to discern the amines than alox TLC plates.

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INTRODUCTION In this paper, we demonstrate that aldehyde-appended distyrylbenzene (DSBs) polymers P2 and P3 and their monomers M2−M4 discern and detect different amines in the vapor phase when the dyes are spray-coated onto silica gel or alox TLC plates. Amines are almost omnipresent analytes of environmental, biological, and clinical importance. Their presence signals environmental hazards, food spoilage, or unwanted industrial effluvia. Monitoring of amines in breath even allows one to identify disease states.1,2 Amine sensing is an interesting scientific task that has generated clever solutions. A number of concepts for amine sensing, including Suslick’s colorimetric arrays, Lavigne’s polythiophene carboxylic acids, Anslyn’s receptors, Miljanic’s cruciforms, and Kaneda’s cyclodextrin-dye was successfully implemented.3−7 Water-soluble conjugated polymers, artificial receptor libraries, collections of hydrophobic porphyrines,8 but also highly active trifluoromethyl-substituted ketones and some 1,3-diketones were employed. Most of the investigated species show color and/or fluorescence changes in organic solvents, which is problematic if one wants to prepare functioning sensory or chemodosimetric devices. One exception is the work published by Suslick, who looks at the color change of solvatochromic dyes on a solid support. We demonstrated that amine sensing is also facile in aqueous phase, if oligoethylene glycol-substituted distyrylbenzenes are employed.9 These dialdehydes react with primary, secondary, or 1,n-diamines under significant change of their fluorescence properties due to the formation of imines, aminals, and hemiaminals, respectively. Here, in this contribution, we have prepared monomers M2−M4 (Figure 1) and polymers P2−P3, all of which contain distyrylbenzene aldehydes. We have sprayed solutions of M2−M4 and P2−P3 onto solid supports and exposed the dried preparations toward vapors of different amines. The amines were easily discerned in a strip-type assay © 2014 American Chemical Society

using all of the monomers and polymers. We compare the amine responses of the DSB-aldehydes as monomers and their polymers on solid supports with amine responses observed for the same materials in solution.

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RESULTS AND DISCUSSION Starting from the monomers M1−M3 (synthesissee Supporting Information) polymers P1−P3 were obtained as shown in Scheme 1 by treatment of the monomers with potassium carbonate and 1,6-diiodohexane in 74−86% yield as yellow to orange solids. It was not necessary to protect the monomer aldehydes as an acetal, as polymerization of M1 and M2 give polymers of comparable molecular weight (Table 1). Polydispersities for P1−P3 range from 1.5 to 2.1 and Mn from 20 kDa to 56 kDa. The optical properties of P1 and P2 are surprisingly similar, suggesting that the meta-connected aldehyde group does not change the electronic properties of the DSB much. However, the emission quantum yield drops from ϕ = 0.76 to ϕ = 0.45 when going from P1 to P2 in THF, as the carbonyl groups act as internal quenchers. The solution quantum yields of P2 and P3 are similar but P3 shows significantly red-shifted absorption and emission features−testimony to the presence of the para-positioned aldehyde groups, which expand the π-system. Sensing. M3 shows some reactions with amines in buffered solution (pH 9), while M2 does not show any response (Supporting Information); the monomers are unimpressive chemodosimeters in solution.10 Figure 2 shows the normalized absorption and emission spectra of P1−P3 in THF, where the red shift in the case of the para-aldehyde P3 is well visible. Solutions of P2 and P3 are Received: March 5, 2014 Revised: April 2, 2014 Published: April 11, 2014 2569

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Article

Figure 1. Chemical structures of the monomers M1−M4.

Scheme 1. Synthesis of Polymers P1−P3

Figure 2. Absorption and emission spectra of polymers P1−P3 in THF.

ethylenediamine is shown. A blue shift is always observed due to either the formation of imines (for normal primary amines) or the formation of aminals for 1,n-diamines such as ethylenediamine, 1,3-diaminopropane or cadaverine. Sensing and chemodosimetry of amines in organic solutions is not very practical and not feasible for any real life application, so we envisioned to put all of our dyes onto solid supports giving attractive chemodosimeters. We decided to spray organic solutions of M2−M4 and P2−P3 onto silica gel or neutral alox TLC plates with simple perfume vaporizers. All of the dye solutions were 20 μM in dichloromethane. We spray-coated every 20 cm × 20 cm plate with spots of the three monomeric and two polymeric dyes to obtain a simple five-dye strip. We exposed the dried strips overnight toward amine vapors. The exposed strips were then photographed under a hand-held black light (λmax em 365 nm) and compared to the photographs of the unexposed sensor strips taken under identical conditions. Alox and silica gel as solid supports show different colors for the fluorescence of the dyes both before and after the exposure to the amines. In some cases (Figure 5) the color differences are small, so we looked at the autocorrelation plots of their responses. For these plots we used the brightnessindependent color coordinates rg of the RAW data of the photographs.11 These rg values were determined for every square in the panel (Figure 5) and were treated with MANOVA statistics:12

Table 1. Molecular Weight and Optical Properties of Polymers P1-P3 polymer

Mna (kDa)

Mw/Mna

abs λmax (nm)

em λmax (nm)

Stokes shift (cm−1)

Φf ± 10b (%)

P1 P2 P3

42.4 55.9 19.0

1.8 2.1 1.5

388 388 409

443 449 510

3200 3501 4042

76 45 49

a

GPC data in CHCl3 versus polystyrene standards. bQuantum yields were obtained in THF by the comparative method using quinine sulfate in 0.1 M sulfuric acid as reference.

amine reactive, P2 to our surprise is much more so than P3 (Figure 3), which also takes much longer to show just slight variations of green emission color. P2 is much more amine reactive than P3, for unknown and not easily understood reasons; potentially, P3 is deactivated toward nucleophilic attack due to the resonance donation from its para substituents. Figure 4 shows the normalized absorption and emission spectra of P2 and P3 in dichloromethane before and after the addition of different amines. For P3 only the addition of

dye5

σm , n(r , g ) = 2570

∑dye1 (rn − rm)2 + (gn − gm)2 3×5 dx.doi.org/10.1021/ma500486u | Macromolecules 2014, 47, 2569−2573

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Figure 3. Photographs of solutions of polymers P2 (top) and P3 (bottom) in dichloromethane (concentration = 5 μg/mL) upon addition of amines 2−12 (left to right). Columns: (1) polymer reference, (2) butylamine, (3) tert-butylamine, (4) benzylamine, (5) cyclohexylamine, (6) ethylenediamine, (7) 1,3-diaminopropane, (8) cadaverine, (9) morpholine, (10) ephedrine, (11) 4-aminopyridine, and (12) ethanolamine. The samples were illuminated using a hand-held UV-lamp at an emission wavelength of 365 nm. Photographs were taken with fixed settings of the camera (JPEG format, shutter speed 0.05 s, ISO value 100, aperture F2.8, white balance 6500K, and Adobe RGB 1986 color space).

Figure 4. Normalized absorption spectra (left) and emission spectra (right) of solutions of P2 (top) and P3 (bottom) in dichloromethane upon addition of different amines.

In this correlation, the photographic data were classified into a silica gel or neutral alox based sensor array. The autocorrelation for each sensor is shown in Figure 6. All of the amines are differentiated, but silica gel is a better support for doing that than alox. Not unexpectedly, tert-butylamine (3) and benzylamine (4) are difficult to differentiate, but butylamine (2) is easily discerned from both columns 3 and 4 and can be differentiated against either of the other amines. Ethylenediamine and 1,3-diaminopropane are hard to discern, which is not too surprising, as they both tend to form aminals, and therefore have similar color responses. It is facile to prepare these solid state array-strips that allow to discern amines in the vapor phase. We will look at other solid state supports to find if the surface chemistry can modulate and improve recognition of amines. In this realm it will also be interesting to use highly lipophilic supports combined with lipophilic fluorophores to allow amine sensing in water but on a solid support.

We note that amine dosimetric action of our dyes works better on solid support than it does in solution (M2 and M3 were tested in buffered aqueous solution, M4 in DCM; see Supporting Information). Probably, the immobilization increases the overall quantum yield of all of the investigated species, with and without amines, perhaps due to site isolation or by rigidification of the dye structures or both. We are currently investigating these issues.

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CONCLUSIONS In this contribution, we have shown that solutions of M2−M4 and P2−P3 can be sprayed onto silica gel or alox TLC plates to give simple strip-type arrays of chemodosimeters for primary, secondary, and 1,n-diamines. The strips were exposed to amine vapors in air and analyzed using photography. Mathematical evaluation allowed to discern most of the amines. We found that 2571

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Figure 5. Photographs of samples of monomers M2−M4 and polymers P2−P3 on silica gel and alox TLC plates after 20 h of exposure amine vapors 2−10 (left to right). Columns: (1) reference, (2) butylamine, (3) tert-butylamine, (4) benzylamine, (5) cyclohexylamine, (6) ethylenediamine, (7) 1,3-diaminopropane, (8) cadaverine, (9) morpholine, and (10) ethanolamine. The samples were illuminated using a hand-held UV-lamp at an emission wavelength of 365 nm and photographs were taken with fixed settings of the camera (JPEG format, shutter speed 0.05 s, ISO value 100, aperture F2.8, white balance 6500K, and Adobe RGB 1986 color space).

Figure 6. Autocorrelation plot (RAW rg values) of fluorescent dyes M2−M4 and P2−P3 on silica gel (left) and neutral alox (right) after exposure to amines recorded with a digital camera. When color information on identical amines + dyes are correlated the deviation σn,m disappears (black squares on the diagonal).

hydroxydistyrylbenzenes,13 but its practical evaluation and implications are far from clear and possibly important. In future we will attempt to evaluate combinations of other solid supports and functional fluorophores as amine chemodosimeters with modulated fluorescence. Issues of sensitivity and selectivity combined with strong emission color changes will be investigated and optimized.

silica gel is a better support than alox and amines are easier differentiated on that support than in solution. The differences of the amine reactivity and response depend upon the solid support, suggesting that we can modulate the sensory responses through changes of the environment the fluorophores are in. That might not be too surprising in the light of our recent studies with 2572

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(8) Rakow, N. A.; Sen, A.; Janzen, M. C.; Ponder, J. B.; Suslick, K. S. Angew. Chem. 2005, 44, 4528. (9) (a) Kumpf, J.; Bunz, U. H. F. Chem.Eur. J. 2012, 18, 8921. (b) Freudenberg, J.; Kumpf, J.; Schäfer, V.; Sauter, E.; Wörner, S. J.; Brödner, K.; Dreuw, A.; Bunz, U. H. F. J. Org. Chem. 2013, 78, 4949. (10) (a) Solntsev, K. M.; McGrier, P. L.; Fahrni, C. J.; Tolbert, L. M.; Bunz, U. H. F. Org. Lett. 2008, 10, 2429. (b) McGrier, P. L.; Solntsev, K. M.; Schönhaber, J.; Brombosz, S. M.; Tolbert, L. M.; Bunz, U. H. F. Chem. Commun. 2007, 43, 2127. (11) Schwaebel, T.; Menning, S.; Bunz, U. H. F. Chem. Sci. 2014, 5, 1422. (12) (a) Schwaebel, T.; Trapp, O.; Bunz, U. H. F. Chem. Sci. 2013, 4, 273. (b) Davey, E. A.; Zucchero, A. J.; Trapp, O.; Bunz, U. H. F. J. Am. Chem. Soc. 2011, 133, 7716. (c) Patze, C.; Brödner, K.; Rominger, F.; Trapp, O.; Bunz, U. H. F. Chem.Eur. J. 2011, 17, 13720. (13) McGrier, P. L.; Solntsev, K. M.; Miao, S.; Tolbert, L. M.; Miranda, O. R.; Rotello, V. M.; Bunz, U. H. F. Chem.Eur. J. 2008, 14, 4503.

We think that this concept is attractive to also modulate the fluorescence response of one or a few different fluorophores by putting them down on any number of chemically different supports. As fluorescence is so sensitive toward the surrounding, this type of modulation should give powerfully differentiated and modularly constructed sensory arrays in which “sweet spots” with high selectivity, sensitivity and response might be realized.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis, 1H and 13C NMR spectra of M1−M3 and P1−P3, gel permeation chromatography of P1−P3, absorption and emission spectra, fluorescence quantum yields, and photographs of sensing responses of M1−M3. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(U.H.F.B.) E-mail: [email protected]. Author Contributions

J.K. and J.F. synthesized all of the monomers and polymers. J.K. performed the spectroscopic measurements and J.K. and S.T.S. performed the mathematical evaluation of the sensing events under color change. The experiments were conceived by U.H.F.B. and J.K. and the manuscript was written by U.H.F.B. and J.K. through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ABBREVIATIONS DSB, Distyrylbenzene; TLC, thin layer chromatography; THF, tetrahydrofuran; GPC, gel permeation chromatography; rg value, red, green normalized color coordinates

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REFERENCES

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