LETTER pubs.acs.org/JPCL
Complete Suppression of Blinking and Reduced Photobleaching in Single MEH-PPV Chains in Solution Suguru Onda, Hiroyuki Kobayashi, Tatsuhiko Hatano, Shu Furumaki, Satoshi Habuchi, and Martin Vacha* Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Ookayama 2-12-1-S8, Meguro-ku, Tokyo 152-8552, Japan
bS Supporting Information ABSTRACT: We report photophysical properties of individual chains of the conjugated polymer MEH-PPV in a concentrated solution of polystyrene. MEH-PPV chains diffusing freely in the solution show no fluorescence blinking and reduced photobleaching. Adsorption of the chains onto a substrate in the solution leads to 34% decrease in fluorescence intensity (quantum efficiency), fivefold increase in the rate of photobleaching and appearance of blinking. The blinking is often a twostate process caused by a reversible interaction with a single quencher. The interaction causes quenching of a substantial length of the extended MEH-PPV chain. SECTION: Macromolecules, Soft Matter
C
onjugated polymers continue to attract attention of physicists, chemists, and material scientists both because of their fascinating photophysical properties and because of their potential for applications in organic optoelectronic devices and sensors. The study of the photophysical properties is complicated by the amorphous nature of most conjugated polymers, which produces a wide distribution of chain conformations and resulting microscopic properties and interactions. The optical properties are determined by conjugated segments over which the πelectrons are delocalized. Near proximity of segments located on different chains or on different parts of the same chain can result in photophysical interactions, such as energy transfer, ground- or excited-state aggregate formation, or charge transfer. Single-molecule spectroscopy has been providing exceptional insight into the physical properties of polymers and other soft and complex matter.15 Basic understanding of the photophysics of conjugated polymers has been formed based on the early studies on single chains of the prototypical conjugated polymer, poly[2-methoxy-5-(20 -ethylhexyl)oxy-1,4-phenylenevinylene] (MEH-PPV). Fluorescence from single molecules of MEH-PPV showed step-like intermittency (blinking) and photobleaching.6 These are features that have been previously observed for single small dye molecules but were not expected for a conjugated polymer chain of the molecular weight of 900 000. The blinking has been ascribed to the localization of the exciton on one or a few conjugated segments, which effectively reemit the energy. The localization is caused by efficient energy transfer within the polymer chain due to small intersegment distances in a compact defect-cylinder conformation.7 Later, it has been shown that the blinking can be partially suppressed in single MEH-PPV chains, which retain extended random coil conformation when cast from a good solvent.8 This basic picture of “blinking = compact conformation (poor solvent)” and “nonblinking = extended r 2011 American Chemical Society
conformation (good solvent)” has been later confirmed by other groups on various conjugated polymer systems.917 At the same time, there are indications that the picture may be more complex. The blinking depends not only on the polarity but also on the molecular weight of the matrix polymer in which single conjugated chains are dispersed.9 Also, it was found that two different extended conformations can coexist in the same good-solvent matrix, and relatively subtle differences between the conformations can lead to the appearance of the blinking.18 There is also the fundamental question of what determines the solvent quality for MEH-PPV. Aromatic solvents such as toluene interact preferentially with the aromatic backbone of the main chain, whereas nonaromatic solvents have a preferential interaction with the side groups.19 This has resulted in conflicting reports on the solvent quality of polystyrene (PS) for MEH-PPV and other conjugated polymers.15,18,20 An answer on many of these controversies could be provided by measuring of a single MEH-PPV chain in its thermodynamically relaxed state, that is, in a solution of a good solvent. The study of single molecules in a solution is possible by fluorescence correlation spectroscopy (FCS). However, this method is based on detecting bursts of fluorescence signal of fast diffusing molecules and provides only physical parameters averaged over large numbers of individual chains. To observe differences between individual polymer chains, one needs to follow a particular chain for a time interval long enough to accumulate sufficient data. Here we overcame the problem of fast diffusion by dispersing individual MEH-PPV chains in a highly concentrated toluene solution of low-molecular-weight PS. Both toluene and PS have Received: September 15, 2011 Accepted: October 18, 2011 Published: October 18, 2011 2827
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The Journal of Physical Chemistry Letters been argued to be environments resembling good solvents for MEH-PPV. The PS concentration is well above the critical overlap concentration, and the resulting high viscosity of the PS solution slows the diffusion to such extent that we are able to study a single diffusing chain for tens of seconds. In addition, we are able to distinguish chains that were adsorbed on the substrate. We have found qualitatively new photophysical behavior of single MEH-PPV chains, including lack of blinking and increased fluorescence quantum efficiency in the diffusing chains and a new type of blinking in the adsorbed chains. MEH-PPV was purchased from Aldrich. THF solution of the polymer was fractionated using gel permation chromatography (Japan Analytical Industry, LC-9204R/U) and separated into fractions of narrow molecular weight distribution. The fraction of Mn = 541 000 and Mw/Mn = 1.28 was chosen for the singlemolecule experiments. The MEH-PPV sample was diluted to a final concentration of 5 1012 M in a 80 wt % toluene solution of low-molecular-weight PS (Polymer Standards, Mw = 12 000 g/mol). A 10 μL aliquot of the solution was sandwiched between a pair of cleaned microscope cover glasses and used immediately for experiments. The thickness of the solution layer was on the order of few micrometers. Control experiments were performed in a high-molecular-weight PS (Aldrich, Mw = 140 000 g/mol) solution of the same concentration and in a thin spin-cast film (thickness of ∼100 nm) of the low Mw PS. Because there is no easy criterion available to identify individual large multichromophoric systems such as MEH-PPV in solution, we use solution concentrations that are well below the values at which individual chains are dispersed and isolated in solid matrices. Similar to solid matrices, we also observe concentration-dependent changes in the density of emitting spots. Also, analysis of the emission intensity data provides single-mode distributions for both diffusing and adsorbed chains (see below). The estimate of the emission intensity based on the number of monomer units in a single chain further agrees well with the measured values. All of these arguments indicate that with high probability we look at single MEH-PPV chains. The experiments were carried out using an inverted fluorescence microscope (Olympus, IX71) equipped with a high N.A. objective lens (Olympus, 100, N.A. = 1.3). A 488 nm line from an Ar ion laser (Coherent, Innova 70, excitation power 23 W cm2) was used for excitation with circularly polarized light in an epi-illumination configuration. The fluorescence signal was collected by the same objective and was focused on an electron multiplication (EM) CCD camera (Andor, iXon+). Fluorescence images were recorded with 100 ms exposure time. For measurements of fluorescence spectra, the signal was dispersed using an imaging spectrograph (Bunkou Keiki CLP-50, 0.5 nm resolution) and detected with the same EM-CCD camera. The thickness of the samples exceeds several times the focal depth of the objective lens, and this fact makes it possible by adjusting the focus of the microscope to distinguish unambiguously between MEH-PPV chains freely diffusing in the solution and those adsorbed on the surface, as shown schematically in Figure 1a. An example of a diffusing chain is presented in Figure 1b where three snapshots show diffusion over several micrometers in the span of 10 s. Full trace is available as a video file in the Supporting Information. In the video, it is clear that during the diffusion the single-chain image can differ from a diffractionlimited spot. At certain time intervals, the spot is partially defocused and elongated, with changing shape and orientation. These are signs of extended conformation of the chain.
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Figure 1. Single-molecule fluorescence characterization of MEH-PPV in a solution of polystyrene. (a) Schematic depiction of the sample with freely diffusing and adsorbed chains (orange). (b) Snapshots of a single diffusing MEH-PPV chain at three different times. Size of each image is 5 5 μm. (c) Fluorescence intensity time traces of 2 diffusing chains. (d) Fluorescence intensity time traces of 4 adsorbed chains showing different patterns of fluorescence blinking. (e) Fluorescence intensity histogram of diffusing (red, 45 molecules) and adsorbed (black, 280 molecules) MEH-PPV chains.
To overcome the problem of 3D diffusion, we measured large numbers of diffusing molecules and analyzed only those that stayed more-or-less in focus for the period of 20 s or more. An example of an intensity trace of two diffusing molecules is shown in Figure 1c. Neither molecules 1 or 2 showed any effect of blinking. The small intensity changes of 2 are due to the 3D diffusion-related slight defocusing. The most surprising finding is that none of the 65 molecules studied showed any sign of blinking. In addition, as seen for molecule 2, many of the diffusing molecules underwent very little photobleaching. Even for those molecules that did photobleach (such as molecule 1) the process was much slower than that observed usually in matrix-isolated conjugated polymer chains. When the focus of the microscope is adjusted on the cover glass surface, the MEH-PPV molecules are immobile because of adsorption, and their photophysical behavior considerably differs from the diffusing ones. Figure 1d presents examples of intensity traces of four adsorbed molecules. Molecule 1 shows a smooth single-exponential photobleaching. This type of continuous bleaching is usually considered to be a signature of noninteracting conjugated segments that photobleach independently and randomly. In molecules 2 and 3, there are two intensity levels, each decaying exponentially, and the molecule is continuously switching between them. The relative intensity of the lowerintensity “dim” level can differ between different molecules. (It is lower for molecule 3 in Figure 1d.) Molecule 4 shows a complex blinking behavior typically found for single conjugated polymer chains isolated in solid matrices. Overall, of the 170 adsorbed molecules analyzed for blinking, 10 (6%) were of type 1 (of Figure 1d), 58 (34%) were of types 2 and 3, and the remaining ones (60%) were of type 4. Apart from blinking and photobleaching, the diffusing and adsorbed molecules were also analyzed in terms of emission intensity. The intensity was taken from the initial few frames of the intensity traces from an unexposed area of the sample. The results are summarized in the histograms in Figure 1e. Both the 2828
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Table 1. Photophysical Characteristics of MEH-PPV in Different Environments low Mw PS diffusing
relative emission intensity (standard deviation)
1.0 (0.35)
0.66 (0.34)
0.52 (0.31)
0.37 (0.45)
fraction of blinking molecules (%)
0
94
15
87
average photobleaching rate (s1)
0.009
0.050
0.038
0.019
diffusing and adsorbed molecules show similar broad distributions of the intensity, but the peak of the diffusing molecules is shifted to higher values. The average intensities calculated as numerical means of the two distributions were 7630 photons/ 100 ms (3320 photon s1 W cm2) for the diffusing molecules and 5060 photons/100 ms (2200 photon s1 W cm2) for the adsorbed ones; that is, the adsorption causes an average drop of the emission intensity of ∼34%. To estimate the quantum efficiency of individual chains, we calibrated the setup using single small molecules of perylene diimide (PDI) dispersed in a PMMA film, as described in detail in the Supporting Information. The calibration provided an overall detection efficiency of 21.6%, which is comparable to other reported values.21 Using the detection efficiency, the MEHPPV molecular weight and an approximate absorption crosssection of a single conjugated segment,22 we obtained the quantum efficiency of 0.05 for a single MEH-PPV chain diffusing in solution. This value is ∼4.4 times lower than the quantum efficiency found in bulk solution of a good solvent.22 The difference can be attributed to orientation factor. Whereas the calibration of the setup was carried out using a spin-coated film in which the single PDI molecules are mostly aligned parallel with the substrate23 and efficiently absorb the in-plane polarized excitation light, the diffusing chains are oriented randomly with respect to the excitation polarization. Taking into account the random orientation, the value of 0.05 should be multiplied by a factor of 3 to obtain an orientation-corrected value of the emission quantum efficiency of 0.15. This value now agrees reasonably well with the bulk solution value of 0.22. We note that there has been recently a report on decreasing quantum efficiency in single MEH-PPV chains with increasing molecular weight.21 The data presented here as well as our previous study22 do not confirm the generality of such phenomenon; the appearance of the so-called dark matter is likely related to specific samples, either by synthetic methods, number of defects, or sample preparation, and its effect on the present data is not substantial. As a reference, the emission intensity, blinking, and photobleaching were also compared with single MEH-PPV suspended in a viscous solution of high Mw PS as well as with MEH-PPV dispersed in a thin film of low Mw PS. The results are summarized in Table 1. The emission intensity is normalized by the value for diffusing chains in low Mw PS. The values are taken as an average of intensity distributions (such as the ones in Figure 1e), and the standard deviations of the distributions are provided in parentheses. There is a clear trend of a decrease in emission intensity (quantum efficiency) with decreasing conformational freedom of MEH-PPV. In the highly viscous high Mw PS solution, the intensity drops to 0.52, and it further decreases to 0.37 in a solid film. Similar changes were observed recently for single MEHPPV chains in a film during solvent vapor annealing.24 The loss of the conformational freedom also causes an increased probability of blinking from 0% in the low Mw PS solution to 15% in the high Mw PS solution to 87% in the solid film.
low Mw PS adsorbed
high Mw PS suspended
low Mw PS film
MEH-PPV in
As discussed further later, quenchers on the glass surface are considered to be a prominent cause of blinking for the adsorbed molecules. In the high Mw PS solution blinking is observed for molecules that are far from the surface. For these molecules, the high viscosity of the solvent causes restrictions on the free motion of the MEH-PPV chain. This fact enabled us to separate the effect of surface quenchers from the restricted conformation freedom. Partial folding of the chains in the viscous solution causes an increase in segmentsegment interactions and the formation of reversible quenchers that are known to cause blinking. This is further true in a solid film where the partial folding occurs during the spin-coating process, and the chain is fixed in a particular conformation by the solid matrix. The partial folding and formation of permanent quenchers on the chain also explains the systematic decrease in fluorescence intensity (quantum efficiency) with decreasing conformational freedom. An interesting result from the present work is that a simple adsorption of the chain onto the substrate leads to a marked increase in the continuous photobleaching rate, as seen in Figure 1c, lines 1 and 2, and Figure 1d, line 1. To quantify further this observation, we summed up separately all fluorescence time traces of the diffusing and adsorbed molecules. The traces were fit with single-exponential functions, and the photobleaching rates obtained from the fits are listed in Table 1. It is evident that the adsorption alone causes a five-fold increase in the photobleaching rate. This difference could result from a restriction on the chain relaxation of the adsorbed molecules or from other unspecified interactions with the substrate. For comparison (as seen from Table 1), the MEH-PPV chains suspended in high Mw PS solution undergo photobleaching similar to the adsorbed chains in the low Mw PS (about four times faster compared with the diffusing ones). This comparison indicates that the increased photobleaching rate of the adsorbed chains can be mainly attributed to the restricted conformational freedom, whereas the blinking is probably due to reversible interaction with quenchers on the substrate. Whereas in terms of blinking, photobleaching, and emission intensity there are clear differences between the diffusing and adsorbed molecules in the low Mw PS solution, spectroscopically, they look very similar. Figure 2 shows examples of fluorescence spectra of the two types of MEH-PPV molecules. The spectra were measured for a statistical ensemble of molecules, and each was fitted with a sum of Gaussians to determine the peak wavelength. The peak wavelengths are summarized in the histogram in Figure 2. Both types of molecules show spectral peak distribution with similar width and position. An average peak wavelength for the diffusing molecules obtained as numerical mean of 19 spectra was 576.9 nm, compared with 578.3 nm for the adsorbed molecules (obtained from 20 spectra). We have recently shown25 that subtle differences in conformation of single MEH-PPV in solid PS lead to large changes in positions of fluorescence maxima due to differences in conjugated segment lengths. The fact that we have not seen such differences in this work provides a supporting argument for our assumption that free unquenched 2829
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Figure 2. Examples of fluorescence spectra of single MEH-PPV chains: (a) freely diffusing and (b) adsorbed. Blue lines are Gaussian fits to the data. (c) Histogram of the spectral peak wavelengths obtained from the Gaussian fits for diffusing (red) and adsorbed (black) molecules.
parts of the adsorbed chains have similar extended conformation as the diffusing chains. The adsorbed molecules were also checked for spectral changes during the type-2 blinking. There have been no changes in spectral positions during the low-intensity “dim” periods of the blinking. We now turn to a more detailed analysis of the specific blinking phenomenon observed on the adsorbed molecules (type 2). Figure 3a shows an intensity trace of a blinking molecule in which the bright (“on”) times as well as the low-intensity “dim” periods have been each approximated with a single-exponential function (red and blue lines, respectively). The intensity trace has been normalized by the “on” time exponential decay and the first 10 s have been plotted in Figure 3b. It is clear from this Figure that the relative amount of intensity decrease due to the blinking remains the same during this time interval; that is, the blinking most likely reflects a dynamic transition between only two states of different emission intensity (Imax and Imin), and this process occurs on the background of continuous photobleaching. Whereas the relative amount of the intensity decrease due to blinking is constant for a given molecule, there are differences between different chains (as shown already in Figure 1d). To quantify these differences, we evaluated the extent of the intensity decrease (reversible quenching) during blinking as q = (Imax Imin)/Imax. The q values obtained for the 58 molecules that showed this type of blinking are summarized in the histogram in Figure 3c. The distribution is broad, and its mean is 0.43. Looking at the origin of the two-state blinking process, one of the conceivable mechanisms is proposed in Figure 3d. The “on” state (top) corresponds to a molecule that is adsorbed on the substrate at one or a few positions on the chain, and the remaining part is freely fluctuating in the solution. The adsorption results in the formation of a quencher, probably a positively charged segment formed by electron transfer to the substrate,26,27 and part of the chain adjacent to the quencher is nonemissive. This leads to
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Figure 3. (a) Fluorescence intensity time trace of an adsorbed MEHPPV chain. The red and blue lines are single-exponentials. (b) First 10 s of the trace in panel a normalized by the red line. Imax and Imin denote two states (intensity levels) between which the blinking occurs. (c) Histogram of the extent of quenching q obtained for 58 molecules that show the two-state blinking process. (d) Schematic depiction of the proposed two-state blinking mechanism.
the observed difference in the emission intensity between the diffusing and adsorbed chains. The “dim” state (bottom) corresponds to a reversible interaction of yet another part of the chain with a quencher. The quencher could be located again on the substrate (as suggested in Figure 3d) or could be on a different part of the same chain. The two-state nature of the blinking phenomenon indicates that only a single quencher interacting with a freely fluctuating part of the chain is involved in the process. The average difference between the “on” and the “dim” state intensity corresponds to an average 43% quenching of the extended chain emission by the single quencher. This relatively large value points to the presence of long-range exciton diffusion along the extended chain. Whereas it has been shown28 that in compact chain conformations the excitation is quenched in domains of an average size of 10 nm,29,30 spatial jumps of the apparent emitting sites on a single chain as large as 70 nm have also been observed.22 In solution, long-range intrachain energy transfer has been reported31 for conjugated polymers of poly(phenylene ethynylene) with exciton diffusion length of >90 nm. Recently it was found that in single chains of β-phase polyfluorene the excited-state delocalization can be on the order of 500 monomer units.32 These studies support the feasibility of long-range exciton diffusion in MEH-PPV in solution. The main conclusions from the present work are that: (1) MEH-PPV chains in a relaxed state show efficient emission, no blinking, and reduced photobleaching. Restrictions on the conformational freedom of the chains lead to decrease in emission intensity, increase in photobleaching rate, and high probability of fluorescence blinking. (2) In an extended conformational state in solution and in the absence of interchain interactions, long-range exciton diffusion occurs along the main chain of the MEH-PPV polymer.
’ ASSOCIATED CONTENT
bS
Supporting Information. Video file of the full diffusion trace of a single MEH-PPV chain and calibration of the microscope
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The Journal of Physical Chemistry Letters for quantitative estimate of single MEH-PPV molecule quantum efficiency. 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 thank Ms. Hye Ryun Tark from KAIST, Korea for help with some of the experiments and analysis. This work was supported by a Grant-in-Aid for Scientific Research No. 20340109 (M.V.) and No. 22750122 (S.H.) of the Japan Society for the Promotion of Science and by a Research Grant of Ogasawara Foundation. ’ REFERENCES (1) Moerner, W. E. New Directions in Single-Molecule Imaging and Analysis. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 12596–12602. (2) W€oll, D.; Braeken, E.; Deres, A.; De Schryver, F. C.; Uji-I, H. Hofkens. Polymers and Single Molecule Fluorescence Spectroscopy, What Can We Learn? Chem Soc. Rev. 2009, 38, 313–328. (3) Kulzer, F.; Xia, T.; Orrit, M. Single Molecules as Optical Nanoprobes for Soft and Complex Matter. Angew. Chem., Int. Ed. 2010, 49, 854–866. (4) Lupton, J. M. Single-Molecule Spectroscopy for Plastic Electronics: Materials Analysis from the Bottom-Up. Adv. Mater. 2010, 22, 1689–1721. (5) Vacha, M.; Habuchi, S. Conformation and Physics of Polymer Chains: A Single-Molecule Perspective. NPG Asia Mater. 2010, 2, 134–142. (6) Hu, D.; Yu, J.; Barbara, P. F. Single-Molecule Spectroscopy of the Conjugated Polymer MEH-PPV. J. Am. Chem. Soc. 1999, 121, 6936–6937. (7) Hu, D.; Yu, J.; Wong, K.; Bagchi, B.; Rossky, P. J.; Barbara, P. F. Collapse of Stiff Conjugated Polymers with Chemical Defects into Ordered, Cylindrical Conformations. Nature 2000, 405, 1030–1033. (8) Huser, T.; Yan, M.; Rothberg, L. J. Single Chain Spectroscopy of Conformational Dependence of Conjugated Polymer Photophysics. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 11187–11191. (9) Sartori, S. S.; De Feyter, S.; Hofkens, J.; Van der Auweraer, M.; De Schryver, F.; Brunner, K.; Hofstraat, J. W. Host Matrix Dependence on the Photophysical Properties of Individual Conjugated Polymer Chains. Macromolecules 2003, 36, 500–507. (10) Sun, W. Y.; Yang, S.-Ch; White, J. D.; Hsu, J.-H.; Peng, K.-Y.; Chen, S. A.; Fann, W. Conformation and Energy Transfer in a Single Luminescent Conjugated Polymer. Macromolecules 2005, 38, 2966–2973. (11) Liang, J. J.; White, J. D.; Chen, Y. C.; Wang, C. F.; Hsiang, J. C.; Lim, T. S.; Sun, W. Y.; Hsu, J. H.; Hsu, C. P.; Hayashi, M.; et al. Heterogeneous Energy Landscapes of Individual Luminescent Conjugated Polymers. Phys. Rev. B 2006, 74, 085209. (12) Sugimoto, T.; Ebihara, Y.; Ogino, K.; Vacha, M. StructureDependent Photophysics Studied in Single Molecules of Polythiophene Derivatives. ChemPhysChem 2007, 8, 1623–1628. (13) Sugimoto, T.; Habuchi, S.; Ogino, K.; Vacha, M. ConformationRelated Exciton Localization and Charge-Pair Formation in Polythiophenes: Ensemble and Single-Molecule Study. J. Phys. Chem. B 2009, 113, 12220–12226. (14) Becker, K.; Gaefke, G.; Rolffs, J.; H€oger, S.; Lupton, J. M. Quantitative Mass Determination of Conjugated Polymers for Single Molecule Conformation Analysis: Enhancing Rigidity with Macrocycles. Chem. Commun. 2010, 46, 4686–4688. (15) Khalil, G. E.; Adawi, A. M.; Fox, A. M.; Iraqi, A.; Lidzey, D. G. Single Molecule Spectroscopy of Red- And Green-Emitting FluoreneBased Copolymers. J. Chem. Phys. 2009, 130, 044903.
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