J. Phys. Chem. B 2008, 112, 12681–12685
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Enhance Effect of Surfactants on the Photoluminescence and Photostability of Water-Soluble Poly(phenylene ethynylene) Wenchao Dou, Chao Wang, Guannan Wang, Qiang Ma, and Xingguang Su* Department of Analytical Chemistry, College of Chemistry, Jilin UniVersity, Changchun, 130012, P.R. China ReceiVed: June 17, 2008; ReVised Manuscript ReceiVed: August 1, 2008
The interaction between the water-soluble anionic fluorescence conjugated polyelectrolytes PPESO3 (poly[2,5bis(3-sulfonatopropoxy)-1,4-phenylethynylene-alt- 1,4-poly(phenylene ethynylene)]) and various surfactants has been studied in aqueous solution by UV-vis absorption spectra and fluorescence spectra. With the addition of surfactants, the aggregations of polymers are broken up. For eliminating the self-quenching effect of the fluorescent polymers, the photoluminescence of PPESO3 is dramatically enhanced. The photoluminescence of PPESO3 can be enhanced 6- to 12-fold with the addition of different surfactants, and at higher concentration of surfactants, the photostability of PPESO3 is also highly increased. A “micelle incorporation model” is proposed to explain the enhancement of photostability. To deeply understand the interaction processes between PPESO3 and surfactants, we systematically studied the fluorescence spectra changes of PPESO3 and the dynamic processes at different CTAB concentrations. All results prove the surfactants can simultaneously enhance the photoluminescence and photostability of water-soluble conjugated polyelectrolytes, and this method is very simple and powerful. Introduction Water-soluble conjugated polyelectrolytes (WSCP) have been widely used for fabrication of sensors for ions,1-5 protease,6-9 proteins,10-18 and nucleic acids.19-23 However, these polymers suffer from two shortcomings: one is relatively low photoluminescence (PL) quantum yield in water solution compared with that of similar polymers soluble in organic media.24 Another major drawback for WSCP is its tendency to photo-oxidize.25 These two shortcomings limit their applications as sensors. If there is a method which could not only increase the photostability of WSCP but also enhance their photoluminescence, it will represent great progress. Surfactants can form complexes with conjugated polyelectrolytes and can strongly enhance the PL quantum yield of the polymer. Recently, a lot of studies have been performed to improve the optical and electronic properties of WSCP using different surfactants.25-27 Whitten and co-workers first demonstrated that by controlling conjugated polyelectrolytes with appropriate surfactants, one may control the geometric conformation of the polymer and thereby tune its optical and chemical properties.26 Bunz and co-workers studied the interaction of nonionic water-soluble conjugated polymers with nonionic surfactants. The addition of surfactants led to a blue-shifted emission and 20-fold enhanced emission, which the authors termed “surfactochromicity”.28 Burrows and co-workers systematically studied the interactions between the water-soluble anionic conjugated copolymer PBS-PFP and various surfactants, and they found that nonionic penta(ethylene glycol) monododecyl ether (C12E5) could blue-shift the fluorescence emission, dramatically increasing the fluorescence quantum yield of the polymer.29-31 Waldeck and co-workers found that cationic surfactant, octadecyltrimethyl ammonium, can break the PPESO3 aggregates and increase the fluorescence quantum yield.27 Many other reports also demonstrated that the interaction between * Corresponding author: Tel.: +86-431-85168352, E-mail: suxg@ mail.jlu.edu.cn.
conjugated polymers and appropriate functionalized surfactants provides a simple yet powerful way to tune the optical and electronic properties of these materials.3,32 Although much success in demonstrating the application of surfactant to improve the emissive properties of WSCP, but to the best of our knowledge, nobody has yet utilized surfactants to improve the photostability of conjugated polyelectrolytes. The photostability of a dye is of major importance, since it leads to the irreversible loss of fluorescence, which limits the statistical accuracy of the detection.33 Given the particular importance of dye photostability for fluorescence spectroscopy investigations, refined strategies were explored to chemically retard dye photobleaching.7,34 However, for compounds having a retarding effect on the bleaching of fluorophores, a corresponding or even stronger fluorescence quenching often occurs. As a consequence, bleaching retardation by fluorescence quenching can indeed maintain a certain fluorescence intensity level from a sample over a longer time, but the total amount of emitted fluorescence photons from the sample will decrease.34 We have an interest in developing a simple and effective method to improve both the photostability and photoluminescence of WSCP in aqueous solution. Sulfonate-substituted poly(phenylene ethynylene) (PPESO3) is a water- and alcoholsoluble polymer which exhibits a strong blue fluorescence in alcohol solution but weak green fluorescence in water solution.6,24 In order to study the mechanism and dynamics processes of the interaction between conjugated polyelectrolytes and surfactants and how it influences the photophysical and photochemical properties of polymer, a detailed investigation on the interaction of PPESO3 with a series of surfactants was carried out by using optical absorption and fluorescence spectroscopy. The results show that upon addition of surfactant, the photoluminescence of PPESO3 was increased 6- to 12-fold, and the photostabiity is also dramatically enhanced. A “micelle incorporation model” is proposed to explain the enhancement of photostability. This study provides a potential method for fabrication of sensors with the integration of higher photoluminescence and higher photo-
10.1021/jp805345d CCC: $40.75 2008 American Chemical Society Published on Web 09/13/2008
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Dou et al.
SCHEME 1: Structure of PPESO3, CTAB, Triton X 100, P123, F127
Figure 1. UV-visible absorption spectra of PPESO3 in ethanol (red line) and water (black line).
stability. The conjugated polyelectrolyte and the type of surfactants used in this study are the same as that used by Waldeck et al.27 Our work was mainly focused on developing a simple and effective method to improve both the photostability and photoluminescence of conjugated polyelectrolytes in aqueous solution, and a different interaction mechanism between conjugated polyelectrolytes and surfactants was proposed. Experimental Section The polymer PPESO3 was synthesized according to a method previously described.7 The polymer PPESO3 features a poly(phenylene ethynylene) backbone substituted with anionic 3-sulfonatopropyloxy groups (structures in Scheme 1). The polymer stock solution concentration was 1 mM (all concentrations are provided as polymer repeat unit concentration, [PRU]). The stock solution was diluted as need to prepare the solution used for spectroscopic experiments. The PPESO3 concentration used in all of our experiments is 5 µM ([PRU]). The surfactants cetyltrimethylammonium bromide (CTAB), Triton X-100 (4(C8H17)C6H4(OCH2CH2)n OH, n ∼ 10), Pluronic P123(triblock copolymer surfactant EO20 PO70 EO20), Pluronic F127(triblock copolymer surfactant EO106 PO70 EO106), were obtained from Aldrich Chemical, and they were used as received. All solutions were prepared by using water with a resistivity higher than 18 MΩ · cm. The absorption and fluorescence emission spectra were measured by using a Varian GBC Cintra 10e UV-visible spectrometer and a Shimadzu RF-5301 PC spectrofluorophotometer, respectively. In both experiments, a 1 cm path-length quartz cuvette was used to measure the absorption or fluorescence spectrum. All optical measurements were carried out at room temperature under ambient conditions. Results and Discussion Spectroscopic Properties of PPESO3 in Water and Ethanol. Figure 1 illustrates absorption spectra of PPESO3 in H2O and C2H5OH. It can be seen that the spectroscopic properties of the polymer vary strongly with solvent composition. In water, the absorption spectrum has a very well-defined red peak at 436 nm, whereas in C2H5OH the absorption band blue-shifts and the absorption maximum of the polymer is 419 nm. This feature suggests that the conjugation length along the backbone is higher in water solution than it is in ethanol. More information relating to the effect of solvent on the solution state of the polymer can be obtained from the fluorescence spectra, which are presented in Figure 2. In ethanol, the fluorescence of the PPESO3 polymer is strong and features
Figure 2. Fluorescence emission spectra of PPESO3 in ethanol (red line) and water (black line). The excitation wavelength was 401 nm.
SCHEME 2: Schematic Illustration for the Interaction Mechanism of the “Micelle Incorporation Model”
a well-defined 0s0 band with λmax ) 447 nm, along with a clearly resolved vibrational progression at lower energy. The fluorescence spectrum in C2H5OH is very similar in appearance to that of organic-soluble PPEs in ‘good’ solvents where the degree of polymer aggregation is minimal.35 However, in water, the characteristic unaggregated PPESO3 emission band disappears and a new broad structureless fluorescence band appears at longer wavelengths λmax) 535 nm. In addition, the photoluminescence of PPESO3 polymer decreases substantially from C2H5OH to H2O. The fluorescence spectra indicate that the polymer exists in a strongly aggregated form in aqueous solutions. In water the adjacent PPE chains aggregate with π-stacking, and the phenylene rings adopt an all planar conformation, effectively increasing interchain conjugation. By contrast, in
Water-Soluble Poly(phenylene ethynylene)
Figure 3. Evolution of the absorption spectra of PPESO3 (5 µM in repeat units) in the presence of different concentrations of CTAB: 0 mM (black line), 0.1 mM (red line), 0.2 mM (green line), 0.4 mM (blue line).
Figure 4. Evolution of the fluorescence emission spectra of PPESO3 (5 µM in repeat units) in the presence of different concentrations of CTAB: 0 mM (black line), 0.1 mM (red line), 0.2 mM (green line), 0.4 mM (blue line), 0.6 mM (orange line).
C2H5OH solution, the polymer is more strongly solvated, and the polymer exists as an isolated single chain (Scheme 2).6 Effect of CTAB on the Photoluminescence and Photostability of PPESO3. The effect of CTAB on the photoluminescence and photostability of PPESO3 aqueous solution was studied by UV-visible absorption spectroscopy and steady-state fluorescence spectroscopy. Upon addition of CTAB, the spectrum’s absorbance maximum of PPESO3 aqueous solution shifts blue by about 17 nm (Figure 3), and the color of the PPESO3 solution changes from yellow to light yellow. This blue shift is attributed to a decrease in conjugation length, which arises from dissociation of the aggregates.27 Obvious luminescence enhancement as a function of the increasing concentration of surfactant can be observed in Figure 4. As the CTAB concentration increases, the sharp peak at 447 nm appears, and the spectra are similar to that observed in C2H5OH solution. The fluorescence intensity and the fluorescence intensity ratio of the shoulder and the sharp peak remain constant when the concentration of CTAB is higher than 0.6 mM. The photoluminescence of the polymer is increased approximately 8-fold upon the addition of the surfactant. These results are in agreement with the previous report of Waldeck et al.27 PPESO3 is a photosensitive compound especially in dilute solution. To investigate whether the PPESO3 still remain photosensitive after the addition of the surfactant CTAB, the fluorescence spectra of PPESO3 aqueous solution before and after irradiation by a 125 W Hg lamp for 0.5 h were measured. We define a parameter of photobleaching, Y, to characterize the photostablility of PPESO3 under different conditions. Y is defined as
Y ) 1 - I1 ⁄I0 where I0 is the photoluminescence intensity before irradiation, and I 1 is the photoluminescence intensity after irradiation. The
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Figure 5. The fluorescence emission spectra of PPESO3 (5 µM in repeat units) in aqueous solutions before (black line) and after irradiation for 0.5 h (red line).
Figure 6. The fluorescence emission spectra of PPESO3 (5 µM in repeat units) and 1 mM CTAB mixture solutions before (black line) and after irradiation for 0.5 h (red line).
excitation wavelength is 401 nm. For the mixture solutions of PPESO3 and surfactant, the measurement of I0 is taken after the equilibrium is established (70 min after mixing). The Y for PPESO3 aqueous solution without surfactant is 0.91, and blueshift of the emission was observed, which indicates that the conjugated backbone of PPESO3 is destroyed after photobleaching (Figure 5). However, the photobleaching effect observed for PPESO3 in 1 mM CTAB solution was much weaker; Y in this situation equals 0.26, which shows that the photoluminescence of PPESO3 exposed to the Hg lamp decreases much less than the pure PPESO3 solution after irradiation. No blue shift of the emission at 447 nm was observed; only the shoulder around 480 nm decreases a lot after irradiation (Figure 6). We proposed a “micelle incorporation model” to explain the enhancement of photoluminescence and photostability of PPESO3 by CTAB. Although the famous “pearl-necklace model” could well explain the photoluminescence enchancement,30 it cannot well explain the enhancement of photostability upon the addition of surfactant. We presume that the polymer is incorporated into the long cylindrical surfactant micelles of CTAB (scheme 2). There are four points of evidence to support this postulate: The first one is the photobleaching experiment. The increasing photostability demonstrates that the conjugated polymer must be protected by the surfactant. The oxygen in solution is separated from the conjugated polyelectrolytes so the oxygen could not destroy the conjugated bond of the polymer under irradiation. The second reason is the rigid-rod backbone of the PPESO3, which does not allow PPESO3 to coil up around the micelle of the surfactant like that of the flexible polyelectrolytes in surfactant solution. The third reason is that the mixture of PPE and CTAB can be used as structure-directing agents for periodic nanoscale silica composite materials,36 which suggests that the PPE most likely exists in the micelles of CTAB. Finally, with the addition of the surfactant into the solution of PPESO3, the oppositely charged surfactant first forms complexes with the negatively charged sulfonate of polyelectrolyte via Coulombic force. After the negatively charged
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Dou et al. TABLE 1: Effect of Different Surfactants on the Photoluminescence and Photostability of PPESO3a
surfactant
cmc MW (in water), enhancement factor of (Da) mM photoluminescenceb
CTAB 364 Triton X-100 649 F127 12600 P123 5800 Figure 7. Temporal evolution of the fluorescence emission spectra of PPESO3 (5 µM in repeat units) and 0.1 mM CTAB solutions. 0 min (black line); 10 min (red line); 20 min (orange line); 30 min (yellow line); 40 min (green line); 50 min (cyan line); 60 min (blue line); 70 min (purple line); 80 min (olive line).
Figure 8. Temporal evolution of the fluorescence emission spectra of PPESO3 (5 µM in repeat units) and 1 mM CTAB solutions: 0 min (black line); 10 min (red line); 20 min (orange line); 30 min (yellow line); 40 min (green line); 50 min (cyan line); 60 min (blue line); 70 min (purple line); 80 min (olive line). The inset shows the normalized fluorescence emission spectra of mixture solutions taken immediately after addition of CTAB (black line) and 70 min after addition (purple line).
sulfonate on the side chain is saturated by cationic CTAB, the “stoichiometric polyelectrolyte-surfactant complexes” are completely hydrophobic37 so the aliphatic chains of excess CTAB will interact with the complex through hydrophobic interaction. With the increase of the CTAB concentration, it is likely to incorporate the complex into the micelles of CTAB by hydrophobic force. In order to deeply study the interaction processes between PPESO3 and CTAB, we systematically studied the dynamic change in PL upon the addition of different concentrations of CTAB. Fluorescence spectra of polymer and CTAB mixture solutions also change with time, and the behaviors are different for different CTAB concentrations. Figures 7 and 8 show the fluorescence spectra changes of PPESO3 and CTAB mixture solutions with time. The measurements were taken immediately after mixing and every 10 min time interval. At lower concentrations of CTAB (lower than 0.2 mM), it is evident from Figure 7 that the emission peak at 447 nm decreases dramatically with time, and the emission band at 510 nm becomes narrow and shows a red-shift. The fluorescence spectra show that it needs 70 min for the equilibrium to be established after mixing. However, at higher concentrations of CTAB (higher than 0.2 mM), the fluorescence intensity at 447 and 480 nm both increases with time, but the fluorescence intensity at 480 nm increases slowly compared with that at 447 nm so in the normalized emission spectra of PPESO3 and CTAB solution
0.8 0.2 3.2 0.3
8 (1 mM) 12 (10 mM) 7 (4 mM) 6 (10 mM)
Y 0.26 (1 mM) 0.06 (10 mM) 0.18 (4 mM) 0.15 (10 mM)
a The concentrations in parentheses are the concentrations of the surfactant. b The photoluminescence of PPESO3 was presumed to be 1, the photoluminescence of PPESO3 and surfactant mixture solution was obtained by comparing its integrated fluorescence intensities with that of PPESO3.
(the inset in Figure 8) it can be seen that the fluorescence intensity at 480 nm becomes lower after the equilibrium is established (70 min after mixing). It is interesting to notice that when the CTAB concentration is 0.2 mM, the fluorescence spectra of the PPESO3 and CTAB mixture almost does not change with time. When it is lower than 0.2 mM, the fluorescence intensity decreases with time. When it is higher than 0.2 mM, the fluorescence intensity increases with time. These behaviors are very useful for studying the dynamic processes of the interaction between conjugated polyelectrolytes and CTAB in aqueous solution. The changes in emission shape with time demonstrate that, in the mixture solution, the interaction between surfactant and polymer is a dynamic process. When the concentration of CTAB equals 0.1 mM, the surfactant-to-polymer ratio reaches a value around 20 (surfactant molecules per repeat unit of the polymer). One unit of the PPESO3 carries two anionic charges so it can only attract two cationic CTAB molecules by electrostatic interaction; there are still more CTAB molecules in the system, which will interact with PPESO3 by hydrophobic interaction and break up the aggregation of polymer. Meanwhile, the surfactant molecules also tend to assemble at the air-water interface. So the surfactant molecules in the aqueous solution will become fewer with time, and the CTAB molecules surrounding PPESO3 decrease, too. Therefore, some single-root PPESO3 will aggregate again, and the peak at 447 nm, which represents the single PPESO3, will decrease with time. When the concentration of CTAB equals 0.2 mM, the surfactant molecules surrounding the PPESO3 keep constant, and the spectrum at this point does not change with time. At CTAB concentrations over 0.2 mM, the surfactant molecules surrounding the PPESO3 will increase with time, the aggregation of PPE band will be further broken up, or the micelles incorporating the PPESO3 will become tighter. The micelles exclude interfacial water molecules surrounding the polymer, which will prevent fluorescent quenching. So the shoulder around 480 nm will decrease with time compared with the peak at 447 nm. It needs to be pointed out that the presence of PPESO3 will greatly decrease the cmc (critical micelle concentration) of CTAB because the PPESO3 is like a core, which makes the CTAB molecules easily gather around PPESO3 to form micelles at a low concentration of surfactants. We propose that 0.2 mM is the cmc of CTAB in the mixture solution. Other Surfactants. Besides the cationic surfactant CTAB, we also investigated the effect of other surfactants on the photoluminescence and photostability of PPESO3, including nonionic surfactant Triton X-100 and nonionic copolymer surfactants Pluronic P123 and Pluronic F127. The effects of these surfactants on the photoluminescence and photostability of PPESO3 are similar to that of CTAB. The photoluminescence
Water-Soluble Poly(phenylene ethynylene)
J. Phys. Chem. B, Vol. 112, No. 40, 2008 12685 References and Notes
Figure 9. Fluorescence emission spectra of PPESO3 (5 µM in repeat units) in different surfactant solution: without surfactant (black line), CTAB (1 mM) (red line), P123 (10 mM) (green line), F127 (4 mM) (blue line), Triton X-100 (10 mM) (orange line), ethanol (cyan line).
is enhanced; photostability is also dramatically increased (see Table 1). In the presence of surfactants, besides the increase in photoluminescence and photostability, the fluorescence emission spectra of PPESO3 also blue-shift, as shown in Figure 9. In all the cases, upon the addition of surfactant, the emission at 447 nm appears and the emission at 535 nm fades. It demonstrates that all these surfactants can breakup the aggregation of PPESO3, and incorporate the polymer into their micelles to protect the conjugated band from the oxygen, increasing the photoluminescence and photostability of PPESO3. We also found that the Triton X-100 is the most efficient surfactant on breaking up the aggregation of the polymer because the shoulder on the long wavelength peak is the lowest compared to the others. Conclusion In this article we studied the enhanced effect of several surfactants on the photoluminescence and photostability of water-soluble anionic conjugated polyelectrolytes PPESO3 in this article. Upon the addition of surfactants, the fluorescence spectra of PPESO3 blue shift obviously, the emission which represents the aggregation of the polymer disappears and the emission which represents the isolate single polymer chains appears. What more important is both the photoluminescence and photostability of PPESO3 are dramatically enhanced. We proposed a “micelle incorporation” model for explaining the enhancement on both photoluminescence and photostability of the conjugated polymer. We also investigated the changes in fluorescence spectra with the surfactant concentration and time in detail. From these results, we studied the surfactant behaviors in PPESO3 solution. And we could conclude that the surfactant can efficiently increase the photoluminescence and phtotostabilty by breaking up the polymer aggregation and incorporating the polymer into the surfactant micelles. This article has provided an easy and virtual method to simultaneously enhance the photostability and photoluminescence of conjugated polyectrolytes in aqueous solution. These findings provide the potential to facilitate the water-soluble conjugated polymer as more outstanding chemical sensors. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (No. 20475020, No. 20075009). The authors thank Dr. Yonglai Zhang and Dr. Shanpeng Wen (College of Chemistry, Jilin University) for helpful suggestions.
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