Langmuir 2006, 22, 5541-5543
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Amplified Fluorescence Quenching of a Conjugated Polyelectrolyte Mediated by Ca2+ Hui Jiang, Xiaoyong Zhao, and Kirk S. Schanze* Department of Chemistry, UniVersity of Florida, P.O. Box 117200, GainesVille, Florida 32611-7200 ReceiVed February 14, 2006. In Final Form: May 2, 2006 The fluorescence of conjugated polyelectrolytes (CPEs) is quenched with very high efficiency by small molecule quenchers. This effect has been referred to as amplified quenching. In the present communication, we demonstrate that aggregation of a poly(phenylene ethynylene)-type CPE (PPE-CO2-) induced by Ca2+ has a pronounced effect on the amplified quenching of the polymer by the dication methyl viologen (MV2+). In particular, absorption and fluorescence spectroscopy of PPE-CO2- in methanol solution indicate that addition of a low concentration of Ca2+ induces aggregation of the polymer chains. The range of MV2+ concentrations within which linear Stern-Volmer quenching behavior is observed systematically decreases with increasing Ca2+ concentration to a point where superlinear quenching is observed immediately upon addition of MV2+. This finding is unequivocal evidence that the superlinear Stern-Volmer quenching behavior typically observed in CPE-quencher systems arises due to quencher-induced aggregation of the CPE chains.
Conjugated polyelectrolytes (CPEs) have been studied extensively during the past several years because of their potential for application in chemo- and biosensors.1-8 The utility of CPEs as sensors is due in part to the extraordinary efficiency by which their fluorescence is quenched by small molecule quenchers with opposite charges.1,9-17 This property has been referred to as “superquenching”1 or “amplified quenching”,9,18 and it was first observed in an investigation of fluorescence quenching of poly(2-methoxy-5-propyloxysulfonate phenylene vinylene) (MPSPPV) by methyl viologen (MV2+) where a Stern-Volmer (SV) quenching constant (KSV) of 107 M-1 was observed.1 In most CPE-quencher systems that have been investigated, the SV quenching efficiency (I0/I) initially increases linearly at low quencher concentrations, followed by superlinear (upward curvature) behavior at high quencher concentrations.10,12,14,16 Several phenomena have been proposed to account for the * To whom correspondence should be addressed. E-mail: kschanze@ chem.ufl.edu. Tel: 352-392-9133. Fax: 352-392-2395. Web site: http:// chem.ufl.edu/∼kschanze. (1) Chen, L.; McBranch, D. W.; Wang, H.-L.; Helgeson, R.; Wudl, F.; Whitten, D. G. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 12287-12292. (2) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. ReV. 2000, 100, 2537-2574. (3) Pinto, M. R.; Schanze, K. S. Synthesis 2002, 1293-1309. (4) Gaylord, B. S.; Heeger, A. J.; Bazan, G. C. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 10954-10957. (5) Zheng, J.; Swager, T. M. AdV. Polym. Sci. 2005, 177, 151-179. (6) Achyuthan, K. E.; Bergstedt, T. S.; Chen, L.; Jones, R. M.; Kumaraswamy, S.; Kushon, S. A.; Ley, K. D.; Lu, L.; McBranch, D.; Mukundan, H.; Rininsland, F.; Shi, X.; Xia, W.; Whitten, D. G. J. Mater. Chem. 2005, 15, 2648-2656. (7) Kim, I.-B.; Dunkhorst, A.; Gilbert, J.; Bunz, U. H. F. Macromolecules 2005, 38, 4560-4562. (8) Cabarcos, E. L.; Carter, S. A. Macromolecules 2005, 38, 10537-10541. (9) Harrison, B. S.; Ramey, M. B.; Reynolds, J. R.; Schanze, K. S. J. Am. Chem. Soc. 2000, 122, 8561-8562. (10) Wang, J.; Wang, D.; Miller, E. K.; Moses, D.; Bazan, G. C.; Heeger, A. J. Macromolecules 2000, 33, 5153-5158. (11) Gaylord, B. S.; Wang, S.; Heeger, A. J.; Bazan, G. C. J. Am. Chem. Soc. 2001, 123, 6417-6418. (12) Wang, D.; Wang, J.; Moses, D.; Bazan, G. C.; Heeger, A. J. Langmuir 2001, 17, 1262-1266. (13) Fan, C.; Hirasa, T.; Plaxco, K. W.; Heeger, A. J. Langmuir 2003, 19, 3554-3556. (14) Fan, C.; Wang, S.; Hong, J. W.; Bazan, G. C.; Plaxco, K. W.; Heeger, A. J. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 6297-6301. (15) Tan, C.; Pinto, M. R.; Schanze, K. S. Chem. Commun. 2002, 446-447. (16) Tan, C.; Atas, E.; Muller, J. G.; Pinto, M. R.; Kleiman, V. D.; Schanze, K. S. J. Am. Chem. Soc. 2004, 126, 13685-13694. (17) Dalvi-Malhotra, J.; Chen, L. J. Phys. Chem. B 2005, 109, 3873-3878. (18) Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 7017-7018.
Chart 1
superlinear SV quenching behavior, including ion-pair complex formation between the polymer and the quencher,1,12,14,15 efficient singlet exciton migration within the polymer,1,11,12,14,19 efficient long-range Fo¨rster energy transfer between the polymer and quencher,14 and aggregation of polymer chains induced by the quencher.1,11-13,15,16 Although each of these mechanisms is supported by specific lines of experimental evidence, the issue remains unresolved and is the subject of active investigation. As part of an ongoing investigation into the mechanism of amplified fluorescence quenching in CPE-quencher systems,15,16 we examined the effect of Ca2+ on the fluorescence quenching of the anionic conjugated polyelectrolyte PPE-CO2- by methyl viologen (MV2+) (Chart 1), and the results of this study are reported in the present communication. Absorption and fluorescence spectroscopy of PPE-CO2- in methanol solution indicate that addition of low concentrations of Ca2+ induces aggregation of the polymer chains. Remarkably, the range of MV2+ concentrations within which linear SV behavior is observed systematically decreases with increasing Ca2+ concentration to a point where superlinear quenching is observed immediately upon addition of MV2+. This finding is unequivocal evidence that the superlinear SV behavior typically observed in CPEquencher systems arises due to quencher-induced aggregation of the CPE chains. The conjugated polyelectrolyte PPE-CO2- was prepared via a “precursor route” in which a Sonagashira coupling reaction was used to polymerize a stoichiometric mixture of 2,5-bis(dodecyloxy-carbonylmethoxy)-1,4-diiodobenzene and 1,4-diethynylbenzene to afford a poly(phenylene ethynylene) in which
10.1021/la060429p CCC: $33.50 © 2006 American Chemical Society Published on Web 05/19/2006
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Table 1. Photophysical Properties of PPE-CO2water methanol a
λab max (nm)
(M-1‚cm-1)
λem max (nm)
φa
441 433
1.45 × 105 1.08 × 105
524 436
0.075 0.24
[PPE-CO2-] ≈ 2.0 µM.
the carboxyl group was protected as a dodecyl ester. Gel permeation chromatography (GPC) of the ester precursor polymer showed that the polymer has Mn ≈ 127 000 g‚mol-1 (polydispersity index ) 2.25) corresponding to a number average degree of polymerization Xn ≈ 185. The ester polymer precursor was subsequently hydrolyzed with (n-Bu)4N+OH- to afford the watersoluble conjugated polyelectrolyte PPE-CO2-, and the final polymer was purified by dialysis against DI water for 4 days. (Complete details concerning the synthesis and characterization of PPE-CO2- are available in the Supporting Information.) The absorption and fluorescence of PPE-CO2- in methanol or water at low concentration (see Figure S1 in the Supporting Information) are similar to those for the bis-(3-propyloxysulfonate) substituted poly(phenylene ethynylene) (PPE-SO3-) which has the same backbone structure as PPE-CO2-.15,16 However, for PPE-CO2-, both the absorption and fluorescence spectra exhibit a stronger shoulder peak alongside of the maximum peak in methanol, which may be related to the difference in polymer chain length between the two materials.20 Photophysical data for PPE-CO2- in methanol and water is summarized in Table 1. As reported previously for PPE-SO3-,15,16 in water PPECO2- exists as an aggregate, as evidenced by a broad and redshifted fluorescence band (λem max ) 524 nm) combined with a comparatively low quantum yield (φf ) 0.075).21-24 By contrast, at low concentration in methanol, the polymer is not aggregated, as assessed by the fact that it features a structured fluorescence spectrum with λem max at 436 nm and a comparatively high quantum yield (φf ) 0.24). When the polymer concentration in methanol is increased to 10 µM (repeat unit concentration), as used in the quenching experiments described below, a small fraction of the polymer aggregates, as evidenced by the fact that its fluorescence intensity is about the same as that observed for a solution of 5 µM. In addition, due to the large absorption extinction coefficient ( ) 1.08 × 105 M-1‚cm-1 at 433 nm) and the small Stokes shift em (λab max ) 433 nm and λmax ) 436 nm), the apparent fluorescence band shape of the 10 µM polymer solution is distorted by selfabsorption (the 0-0 band in the emission spectrum is attenuated and the shoulder at 464 nm becomes dominant, see below). Figure 1 illustrates a series of SV plots for MV2+ quenching of PPE-CO2- fluorescence in various solutions. First, as reported previously for other CPE-quencher systems,15,16 MV2+ quenches PPE-CO2- fluorescence with very different efficiency in water and methanol. For the aggregated form in water, the quenching (monitored at 524 nm) is very efficient; less than 1 µM MV2+ is required to quench the fluorescence 10-fold. Importantly, the SV plot in water is superlinear even at very low quencher concentrations. By contrast, in methanol where the polymer is not aggregated, the SV plot features a distinct “induction region” (19) Chen, L.; McBranch, D.; Wang, R.; Whitten, D. Chem. Phys. Lett. 2000, 330, 27-33. (20) Chang, R.; Hsu, J. H.; Fann, W. S.; Liang, K. K.; Chang, C. H.; Hayashi, M.; Yu, J.; Lin, S. H.; Chang, E. C.; Chuang, K. R.; Chen, S. A. Chem. Phys. Lett. 2000, 317, 142-152. (21) Kim, J.; McQuade, D. T.; McHugh, S. K.; Swager, T. M. Angew. Chem., Int. Ed. 2000, 39, 3868-3872. (22) Kim, J.; Swager, T. M. Nature 2001, 411, 1030-1034. (23) Bunz, U. H. F.; Imhof, J. M.; Bly, R. K.; Bangcuyo, C. G.; Rozanski, L.; Bout, D. A. V. Macromoleucles 2005, 38, 5892-5896. (24) Halkyard, C. E.; Rampey, M. E.; Kloppenburg, L.; Studer-Martinez, S. L.; Bunz, U. H. F. Macromoleucles 1998, 31, 8655-8659.
Figure 1. Quenching of 10 µM PPE-CO2- emission by MV2+ in water (9) and in methanol with 0 µM (0), 2.5 µM (O), 5.0 µM (]), 7.5 µM (4), or 10.0 µM (3) CaCl2.
Figure 2. Absorption of 10 µM PPE-CO2-in the presence of 0 µM (s), 1.56 µM (- -), 3.12 µM (- b -), or 5.46 µM (‚‚‚) MV2+ in methanol.
wherein the correlation is nearly linear and the slope (KSV) is significantly less than at higher MV2+ concentration (c > 3 µM) where the correlation becomes superlinear. As shown in the absorption spectra in Figure 2 for methanol solution, over approximately the same concentration range (0-5 µM), addition of MV2+ induces a red-shift and narrowing of the PPE-CO2absorption spectrum. Similar quencher-induced changes in absorption have been seen in other CPE-quencher systems and have been interpreted as aggregation of the polymer induced by ionic bridging between chains by polyvalent quencher ions.1,12,15 Taken together, the absorption and fluorescence data on the PPECO2-/MV2+/methanol system suggest that there may be a connection between the quencher-induced polymer aggregation and the “induction region” in the SV plot. The divalent cation Ca2+ is known to induce aggregation of anionic polyelectrolytes.25,26 In a previous study, Ca2+ was found to quench the fluorescence of MPS-PPV, and this effect was attributed to aggregation of the CPE chains induced by the divalent metal ion.1 To investigate the effect of aggregation on amplified quenching, we used Ca2+ as an adjuvant in a study of the quenching of PPE-CO2- by MV2+ in methanol. Initially, the effect of Ca2+ on the absorption and fluorescence of PPE-CO2in methanol was examined, and as demonstrated by the spectra shown in Figure 3, addition of Ca2+ induces aggregation of PPECO2-.7,26 Specifically, concomitant to addition of Ca2+, the (25) Gregor, H. P.; Luttinger, L. B.; Loebl, E. M. J. Phys. Chem. 1955, 59, 980-981. (26) Kim, I.-B.; Bunz, U. H. F. J. Am. Chem. Soc. 2006, 128, 2818-2819.
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Figure 3. Absorption (upper) and emission (lower) of 10 µM PPECO2- in the presence of 0 µM (s), 2.5 µM (- -), 5.0 µM (- b -), 7.5 µM (- bb -), or 10.0 µM (‚‚‚) Ca2+ in methanol.
absorption of the polymer red-shifts and narrows, in a fashion similar to that seen upon addition of MV2+ (see Figure 2). More pronounced effects are seen in the fluorescence spectrum upon addition of Ca2+ (Figure 3). Thus, addition of one molar equivalent of Ca2+ to PPE-CO2- leads to a significant red-shift, band broadening, and decrease in the fluorescence intensity. Since Ca2+ is a closed-shell ion and cannot act as an electron or energy acceptor, the fluorescence quenching and red-shift must be due to aggregation of the polymer chains.21-24 The quenching arises because the emission from the aggregates is dominated by an interchain excimer like state which has a lower radiative rate than the intrachain exciton.27 It is evident that Ca2+ is able to effectively cross-link PPE-CO2- chains by complexing with the carboxyl-side groups of the polymer. Since each polymer repeat unit has two negatively charged carboxyl groups and Ca2+ is (27) Rothberg, L. J.; Yan, M.; Papadimitrakopoulos, F.; Galvin, M. E.; Kwock, E. W.; Miller, T. M. Synth. Met. 1996, 80, 41-58.
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dipositive, it is not surprising that the polymer’s fluorescence is dominated by the aggregate band when the PPE-CO2- to Ca2+ molar ratio is 1:1 (10 µM each). In a second set of studies, the effect of addition of various amounts of Ca2+ on the quenching of PPE-CO2- by MV2+ in methanol solution was examined. As shown in Figure 1 addition of Ca2+ has a pronounced effect on the efficiency by which MV2+ quenches the PPE-CO2- fluorescence. In particular, the range of MV2+ concentrations corresponding to the “induction range” in the SV plot is compressed in the presence of Ca2+. Remarkably, for methanol solutions that contain >7.5 µM Ca2+ (corresponding to approximately 1 equiv of Ca2+ per PPE-CO2repeat unit), the “induction range” in the SV plots is virtually eliminated, and MV2+ quenches PPE-CO2- emission with efficiency comparable to that seen in aqueous solution. These results clearly demonstrate the connection between cation-induced CPE aggregation and the superlinear quenching response typical of CPE-quencher systems. Although this connection has been pointed out previously,1,11,12,15,16,28 the findings of this study provide some of the best evidence presented to date showing that aggregation and/or conformational changes of CPEs induced by addition of polyvalent quencher ions play a very important role in contributing to amplified quenching. The results of this work also indicate that the “sphere-of-action” quenching model, which has been applied to analyze the SV quenching data in CPE-quencher systems,12 is likely not valid with polyvalent quencher ions because the state of the system (i.e., effective exciton radius, effective CPE-quencher ion-pair binding constant) is strongly dependent on the relative concentrations of the CPE and quencher. Analysis of the quenching efficiency and dynamics by using a model that incorporates the possibility of 3-dimensional exciton diffusion within a polymer aggregate that contains many individual CPE chains is clearly more appropriate for the situation.29 More detailed investigations of the effects of conjugated polyelectrolyte aggregation on quenching response are underway and will be reported in a forthcoming full paper. Acknowledgment. We acknowledge the United States Department of Energy, Office of Basic Energy Sciences (DEFG-02-96ER14617) for support of this work. Supporting Information Available: Details concerning the synthesis and structural characterization of PPE-CO2- and normalized absorption and emission of PPE-CO2- in water and methanol. This material is available free of charge via the Internet at http://pubs.acs.org. LA060429P (28) Haskins-Glusac, K.; Pinto, M. R.; Tan, C.; Schanze, K. S. J. Am. Chem. Soc. 2004, 126, 14964-14971. (29) Levitsky, I. A.; Kim, J.; Swager, T. M. J. Am. Chem. Soc. 1999, 121, 1466-1472.
