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A single-molecule approach to conformation and photophysics of conjugated polymers
Chemical Physics Letters 528 (2012) 1–6
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FRONTIERS ARTICLE
A single-molecule approach to conformation and photophysics of conjugated polymers Hiroyuki Kobayashi, Suguru Onda, Shu Furumaki, Satoshi Habuchi, Martin Vacha ⇑ Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Ookayama 2-12-1-S8, Meguro-ku, Tokyo 152-8552, Japan
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Article history: Available online 1 December 2011
a b s t r a c t This Letter provides a short overview of the current status and some recent developments in the study of photophysical properties of conjugated polymers on the level of individual chains by single-molecule spectroscopy. The emphasis is put on the relationship between conformation which is determined directly for individual chains and the resulting photophysics. Some of the phenomena covered include the processes of exciton localization and subsequent exciton dissociation, fluorescence blinking and its possible origins, and direct imaging of exciton localization domains. Most of the work has been done on poly(phenylene vinylene) and polythiophene families of conjugated polymers at room temperature. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Study of organic semiconductors is one of the fastest growing research fields in materials science and engineering. Among them, conjugated polymers are the focus of continued attention because they combine good optical and electrical properties with low cost, easy processability and mechanical flexibility. The broad potential for applications of conjugated polymers includes lightemitting diodes, field-effect transistors and photovoltaic cells. The optical and electrical properties of conjugated polymers are determined by conjugated segments which are parts of the polymer chain over which the p-electron is delocalized in its ground state. Conjugated polymers absorb light in the UV/visible part of the spectrum and often show efficient luminescence. Interactions between conjugated segments give rise to various photophysical phenomena which further modify the polymer optical properties. Photophysical interactions strongly depend on distances between segments in the range of nanometers and on the relative orientations. These factors are determined by conformation of an individual polymer chain and by the packing between chains in polymer films [1]. The amorphous nature of most conjugated polymers and the variety of possible conformational states make the study of the photophysical properties of thin films complicated. An alternative ‘bottom-up’ approach to the study of conjugated polymer photophysics has recently emerged together with the technique of single molecule spectroscopy. Single-molecule spectroscopy has been providing exceptional insight into the physical properties of polymers and other soft and complex
⇑ Corresponding author. Fax: +81 3 5734 2425. E-mail address: [email protected] (M. Vacha). 0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2011.11.064
matter [2–6]. In case of conjugated polymers the inter-chain packing interactions are removed by dispersing individual chains in an inert matrix at very dilute concentrations. Photophysics can be then studied in direct relation to the conformation of individual chains. The early studies on single chains of the prototypical conjugated polymer, poly[2-methoxy-5-(20 -ethylhexyl)oxy-1,4-phenylenevinylene] (MEH-PPV) provided a basic understanding of the photophysics of isolated conjugated polymer chains. Fluorescence from single molecules of MEH-PPV showed fluorescence intermittency (blinking) and step-like photobleaching [7]. These features have been previously observed for single dye molecules but were not expected for a conjugated polymer chain containing tens or hundreds of conjugated segments. The blinking has been explained as due to localization of the excitation energy from the whole polymer chain on one or a few conjugated segments. Only these segments effectively reemit the energy and any reversible photochemical reaction occurring on these segments gives rise to the observed blinking. The localization is caused by efficient energy transfer within the polymer chain due to small inter-segment distances in a compact chain conformation [8]. Later, it was found that the blinking and single-step bleaching can be partly suppressed if the single MEH-PPV chains are cast from a good solvent, in which case they retain extended random coil conformation with larger distances between the segments [9]. In this Letter, we will review some recent development in the study of conjugated polymer photophysics on the single molecule level. We will concentrate, where possible, on work where the photophysical properties of an individual polymer chain could be directly linked to the conformation of the same chain. The focus of the Letter will be on single molecule spectroscopy using far-field fluorescence microscopy at room temperature.
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2. How to measure conformation of a single conjugated polymer chain The existing gap between the resolution of conventional optical microscopy (on the order of hundreds of nanometers) and the typical size of a polymer chain (tens of nanometers even for highmolecular weight polymers) does not enable direct imaging of the polymer chain shape. Instead, indirect methods that make use of the relationship between conjugated chain conformation and the resulting absorption anisotropy have been proposed and used. The anisotropy, which is given by a sum of the absorption strengths (transition dipole moments) of individual conjugated segments, can be characterized by an absorption ellipsoid, as shown in Figure 1a. For a simplification the ellipsoid can be rotationally symmetric in which case it is fully determined by the ratio of its two axes. In single molecule experiments the absorption anisotropy is monitored in fluorescence.
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One of the methods to measure conformation of a single conjugated chain uses polarization-modulated excitation to measure projection of the 3-dimensional (3D) absorption ellipsoid onto the sample plane [10] (Figure 1b). This method is very simple but the 2D projection does not uniquely reflect the conformation of each chain because of two random variables, the ellipsoid shape and its spatial orientation. To obtain the conformation of the chains it is necessary to measure a modulation depth of a statistical ensemble of individual chains and then use Monte Carlo simulations to reconstruct the conformation via the modulation depth distribution. This method revealed that, on average, MEH-PPV has a defect cylinder conformation in the matrix of polycarbonate or PMMA [10]. The drawback of this method is that it does not measure directly the conformation of a specific chain but rather a prevailing conformation for a statistical sub-ensemble of individual chains. To overcome this shortcoming we proposed a method to measure the absolute shape of a randomly oriented 3D rotational absorption ellipsoid [11]. This method uses evanescent wave at total internal reflection (TIR) between the substrate and the sample. The component of the evanescent electric field perpendicular to the sample plane is used to measure projection of the ellipsoid into a plane perpendicular to the sample (Figure 1c). This projection and the projection onto the sample plane (Figure 1b) provide the full 3D shape and spatial orientation of the absorption ellipsoid for a specific single chain. The conformation of the chain is then reconstructed by molecular dynamics (MD) simulations. Using this method, conformation of MEH-PPV single chains in different polymer matrices has been studied [12]. The method can be also modified to measure the shape of a general ellipsoid of nanoparticles for which the spatial orientation is known or determined independently. The ratios of the three axes of a general ellipsoid then provide a 3D linear dichroism for single nanoparticles [13]. This method has recently been used to study the excitonic structure of single light-harvesting antennae of photosynthetic bacteria [14]. Apart from the above methods, there has been great progress recently in single-molecule imaging techniques that overcome the optical resolution of conventional optical microscopy [15,16] and some of them might find applications in direct imaging of single polymer chains and other nanostructures. One of such applications will be introduced later in this Letter.
Polar. angle 3. Exciton localization vs. exciton dissociation
epi-illumination
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z Polar. angle total internal reflection Figure 1. Principle of determining a single conjugated polymer chain conformation by measuring its absorption anisotropy. (a) Schematic relationship between a chain conformation and absorption ellipsoid; (b) projection of the ellipsoid on sample plane by epi-illumination (left) and a typical fluorescence image of single MEH-PPV chains (right). Inset: Excitation polarization angle modulated fluorescence intensity of the chain marked by circle in the image. (c) Projection of the ellipsoid on a plane perpendicular to the sample plane by total internal reflection illumination (left) and a typical fluorescence image of single MEH-PPV chains (right). Inset: Excitation polarization angle modulated fluorescence intensity of the chain marked by circle in the image.
Starting with the first single-molecule studies of poly(p-phenylene vinylene) (PPV) and poly(p-pyridylene vinylene) (PPyV) copolymer [8], fluorescence blinking has been interpreted as a signature of exciton localization on one or several conjugated segments. The localization is a result of efficient energy transfer of the absorbed light energy. Absorption in a conjugated polymer chain occurs on all individual conjugated segments and leads to the formation of a Frenkel-type singlet exciton which is initially located on the original segment. The exciton can be transferred by dipole–dipole interactions to other conjugated segments, either to neighbors along the polymer chain (intrachain transfer) or to segments located on different parts of the same chain (interchain transfer). In a collapsed conformation such as the defect cylinder [10] the interchain energy transfer is more efficient compared to the intrachain transfer [17] due to smaller inter-segment distances. The intrachain transfer is assumed to occur only on short length scales corresponding to a few conjugated segments [17]. This balance between the intra- and inter-chain energy transfer can be reversed in extended chain conformations such as random coil. The disappearance of blinking in single MEH-PPV chains cast from good solvent [9] can be explained by suppression of the interchain
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energy transfer due to increased interchain segment-segment distances. In chains with such conformations the exciton decays radiatively or non-radiatively before it can arrive at the segment with the lowest energy. Recently, a measurement of spectral linewidths in single collapsed MEH-PPV chains in PMMA matrix at lowtemperatures provided concrete values of the energy transfer times. At 1.2 K the energy transfer occurs with a mean value of 3.9 ps [18]. In a solid matrix of an inert polymer the conjugated polymer chain can form either collapsed or extended conformations, depending on the solvent and the matrix polymer. The solvent and matrix that support the extended conformation of poly(2methoxy-5-(20 ,60 -dimethyloctyloxy)-p-phenylenevinylene (OC1C10-PPV) are toluene and low molecular weight polystyrene (PS) [19]. In this matrix the fluorescence intensity shows no blinking, in contrast to a more polar poly(vinyl butyral). However, the blinking in the OC1C10-PPV single chains reappears in high molecular weight PS, which indicates that strain and distortions of the original extended conformation which are present during the spin-coating sample preparation process are also important factors in the resulting chain conformation. We have looked at the effect of molecular weight of the matrix polymer systematically and found that for single MEH-PPV chains in PS, the fraction of blinking molecules increases from 59% to 87% when the molecular weight of PS changes from 870 to 12,000 g/mol (Onda, unpublished results). Apart from the solid matrix, the environment of a conjugated polymer chain can be also controlled by incorporating the chains into nanostructures such as hollow silica spheres [20] or lipid vesicles [21]. Conformation of a polymer chain is determined and can be actively controlled by its chemical structure. Polymers with rigid main chain, such as methyl substituted ladder-type poly(paraphenylene) (MeLPPP) are effectively prevented from coiling or collapsing onto themselves. As a result, single chains of MeLPPP showed both emission from a single conjugated segment due to efficient uni-directional energy transfer as well as emission from multiple segments due to bi-directional energy transfer along the rigid chain [22]. Apart from using rigid main chain, the collapsed conformation can be prevented by decorating the main chain with various functional groups. We have shown that grafting bulky PS side
a
chains on the flexible main chain of polythiophene (PT) completely suppresses fluorescence blinking which is otherwise observed on the un-substituted PT [23], as seen in Figure 2. This result is well supported by coarse-grain molecular dynamics (MD) simulations which show that the un-substituted PT forms a compact conformation in which any pair of conjugated segments is within the Förster radius for efficient energy transfer [24]. On the other hand, the PS-substituted PT main chain is forced into an extended coil conformation in which the energy transfer is efficient over a few neighboring segments along the chain, as evidenced also by the MD simulations. Similarly, the rigidity of the main chain can be enhanced by decorating with macrocycles, as shown for the conjugated polymer of poly(p-phenylene-ethynylene-butadiynylene) [25]. One of the ways in which the exciton can decay non-radiatively is dissociation into a pair of charges. This process is determined by the exciton binding energy which for conjugated polymers is generally large (on the order of 0.4–0.6 eV) [26]. An excess energy, either by excitation into hot vibrational states [27] or by externally applied electric field [28] is usually necessary to separate the exciton into positive and negative charges. However, it has been also reported that for polythiophenes the charge pair formation can occur from relaxed excitons even in the absence of an external electric field [29]. An important factor in the exciton dissociation is the interchain interaction which can give rise to p-p stacking favorable for the charge separation. We have used electric-field induced fluorescence quenching [28,30] on the model compounds of the un-substituted and PS-substituted PT to study systematically the effect of iterchain interactions on the exciton dissociation. Examples of field-induced quenching on single PT chains are shown in Figure 2. For quenching of well-isolated chains on an ensemble level there is no difference in the quenching efficiency between the substituted and un-substituted chains. This result indicates that interchain intramolecular interactions are not a prerequisite for efficient exciton dissociation and that the process primarily occurs along the main PT chain. On the other hand, fluorescence from single un-substituted PT chains is much more likely to be quenched completely as compared to a partial quenching of the PS-substituted compounds. This observation can be explained assuming that in the compact conformation of the un-substituted chains the exciton is first localized on a low-energy segment by
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Figure 2. Single molecule detection and electric field-induced quenching of single chains of polythiophene (PT) derivatives. Fluorescence intensity traces of unsubstituted PT (a) and PS-substituted PT (b) single chains; the right sides show molecular dynamics simulations of the chain conformations; electric field induced quenching of unsubstituted PT (c) and PS-substituted PT (d) single chains.
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Intensity (a.u.)
energy transfer and after that the dissociation process takes place. In summary, the conclusion and implication for device engineering is that the pair of charges resulting from a dissociated exciton is located on the same main chain and that the dissociation itself is relatively slow process compared to exciton migration in a compact chain conformation.
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It has been now quite well established that efficient energy transfer and exciton localization on limited number of segments leads to the appearance of fluorescence blinking. This process is very effective for chains with collapsed conformation in a poor solvent or in a solid matrix that resembles a poor solvent. In other environments the factors that determine the efficiency of exciton localization can be more subtle. As mentioned above, in the same matrix of low molecular weight PS there can be both blinking and non-blinking chains present, with their relative fractions given by the PS molecular weight, spin-coating conditions, PS concentration and others. To clarify this point we used the method of 3D absorption ellipsoid measurement for single MEH-PPV chains in low molecular weight PS [11] and extracted chain conformations by MD simulations. It was found that along with two types of blinking behavior there are two distinct types of coil-like chain conformations, a more compact ‘oblong defect coil’ and a more extended ‘disk-like defect coil’, and that the two conformations are well correlated with the two types of blinking (see Figure 3a). In the extended chain conformations there is only continuous decrease of fluorescence intensity due to continuous photobleaching of individual non-interacting conjugated segments. In the more compact chain conformations there are locations with higher density where interchain interactions can occur and which likely serve as energy traps and exciton localization centers. The chain is effectively divided into a few domains and exciton localization in the domains gives rise to the observed blinking. The actual photochemical reaction that causes the reversible quenching of the emitting segment and the blinking has been originally ascribed to photooxidation [8]. Later, this assumption has been studied on a model system consisting of MEH-PPV chains dispersed in thin layer of PMMA in a device capable of repeatedly inserting and extracting positive charges (holes) into the layer [31]. It was found that hole injection leads to bias-dependent quenching of fluorescence from single MEH-PPV chains and that fluorescence can be recovered by applying a reverse bias on the device. The implication for an unbiased system is that formation of a complex MEH-PPV+/O 2 by photoinduced electron transfer from MEH-PPV to a nearby oxygen could be the mechanism behind the blinking in single MEH-PPV chains. Complete or partial exciton localization has been a necessary assumption in all of the above explanations of fluorescence blinking. Recently, we have studied the behavior of single MEH-PPV chains in a relaxed state in a solution. To prevent fast diffusion, a concentrated low molecular weight PS in toluene has been used as the solvent for single MEH-PPV molecules. As seen from the results in Figure 3b, single MEH-PPV chains freely and slowly diffusing in the solution do not show any effects of blinking and only reduced photobleaching [32]. These effects are due to presumably an extended coil conformation. In the same solution, the chains that are immobilized by adsorption on the substrate exhibit much faster (often purely exponential) photobleaching and blinking. In a substantial fraction of the chains the blinking is a two-state process, as depicted by the dashed and solid blue lines in Figure 3b. The adsorbed chains have basically the same conformation of an extended coil only slightly modified by the adsorption site. In such conformation the exciton localization is not an effective process.
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Figure 3. Origin of fluorescence blinking in MEH-PPV. (a) Dependence of blinking on defect coil conformation in a matrix of good solvent; (b) fluorescence intensity traces (left) of MEH-PPV molecules freely diffusing in solution (top) and adsorbed on the substrate (bottom). The dashed and solid blue lines in the bottom trace are single exponentials. Schematic depiction of the sample is shown in the right; (c) time changes in fluorescence spectra (spectral jumps) in single octamers of PV; the structure is shown in the right.
The extent of fluorescence quenching during the two-state blinking varies between different chains and is on average 0.43. In the absence of exciton localization the blinking process causes a quenching of a large number of conjugated segments along extended parts of the polymer chain. The above average extent of quenching would correspond to quenching of tens nanometers of the chain. The two-state blinking itself might be caused by repeated adsorption and release of a free part of the MEH-PPV chain on the substrate or by reversible interaction between two free parts of the chain in the solution. Apart from the conformation of the chain and inter-segment interactions, the photophysical properties of conjugated polymers are also influenced by properties and dynamics of the constituting conjugated segments themselves. Recently, it has been suggested that the shape of a conjugated phenylenevinylene (PV) oligomer is crucial in determining its spectral properties at low temperatures. Oligomers that are bent are characterized by red-shifted fluorescence spectra [33]. We have looked at spectral properties and dynamics of octamers of PV with various side groups at room
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location of the sites is highly dynamic – individual domains on a single chain can be switched on and off, resulting in jumps of the site along the chain. An example of a chain which during the course of the experiment changed the position of emission between 8 sites is shown in Figure 4a. The changes are often correlated with intensity changes during the blinking. The contour of all emission sites is related to the actual shape of the conjugated polymer chain. The contour in Figure 4a is elongated, as expected for the oblong defect coil conformation. While in the matrix of Zeonex most molecules show the elongated contour of the emitting sites, in the matrix of low molecular weight PS there are two types of contours observed. In addition to the elongated contour, a substantial number of single MEH-PPV chains contain only a single apparent emitting site which does show step-like change of position. This fact can be understood by considering emission of many non-interacting conjugated segments on an extended chain the superposition of which forms the apparent emission site. Thus, the results of the emission localization experiments in PS nicely correspond to the two types of chain conformation and two types of blinking of MEH-PPV in the same matrix, as described above. Recently, change of the position of apparent emitting sites has been observed also as a result of controlled quenching of single MEH-PPV by injection of holes in a device structure [37]. The mean jump size of 13.7 nm is similar to the values obtained for the spontaneous quenching [36]. The existence of independent exciton-localization domains is only one of the possible explanations for the observation of the apparent emission site and its spatial jumps. Another possibility is that the exciton would be sampling many locations throughout the whole polymer chain, and emitting from the lowest energy site. Photobleaching of this site would lead to emission from the next lowest energy site and to a jump in the apparent emission site. We tried to distinguish these cases by statistical analysis of the dynamics of the emitting sites. The Figure 4b shows a histogram
temperature (Kobayashi, unpublished results). An example of the results is shown in Figure 3c. The fluorescence spectra of a single PV octamer show jumps in time between two spectral forms, a blue one and a red one. The dynamics of the jumps does not depend on the nature of the side groups. However, the red-to-blue change is about 3.8 times faster than the blue-to-red transition. The spectral jumps are tentatively assigned to torsional motion of an unsubstituted phenyl ring between two energetically stable configurations. In one of the configurations the phenyl ring is more coplanar with the remaining part of the octamer causing extension of the conjugation and red shift of the fluorescence spectrum. Similar dynamics can be assumed to occur in conjugated polymer chains as well. Change of conjugation length affects not only fluorescence but also absorption spectra and as such can be viewed as another origin of fluorescence blinking in conjugated polymers.
5. Exciton migration – domains vs. whole chain Localization of the exciton in independent domains on the conjugated polymer chain has been used to explain the blinking behavior of the compact defect coil conformations, as mentioned in the previous section. At the same time, such domains were also postulated as a result of time-resolved experiments on single MEHPPV chains [34]. Later, we used a super-resolution imaging method of emission centroid localization in an attempt to confirm the existence of the domains in single MEH-PPV chains in the matrix of PS [35]. In the method, the position of an apparent emission site which at any time is a superposition of all emitting domains can be determined with the precision of a few nanometers. Depending on the molecular weight of MEH-PPV and the kind of the matrix polymer, each conjugated polymer chain can contain anywhere between one and more than ten apparent emitting sites [36]. The
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Figure 4. Super-resolution imaging of single MEH-PPV chains (molecular weight 1,440,000) in PS matrix. (a) Apparent emitting site locations (in different colors) observed on one chain during the course of the experiment; (b) distribution of emitting site jumps obtained on 200-molecules; red line – simulation of the jump distribution by domainlimited exciton migration model; blue line – simulation of the jump distribution by chain-limited exciton migration model; (c) dependence of the jump size on time during the course of experiment; blue line – average of the experimental points; red line – simulated time dependence based on a model of independent domains.
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of the jump size obtained on MEH-PPV of the molecular weight 1440,000 in PS. The jumps span a range between a few and 40 nm, with a mean value of 9.6 nm. The figure also shows simulation of the jump size obtained using two models, domain limited exciton migration and chain-limited exciton migration. In the domain-limited model, it is assumed that initially all domains (in total 20 in Figure 4b, the size of each domain in the simulation is set to 13 nm) emit at the same time, and in the course of time individual domains photobleach randomly, causing jumps in the apparent emitting sites. In the chain-limited model we assume that of all the domains only 1 is emitting at each time and after photobleaching another randomly located domain starts emitting. The photobleaching again causes a spatial jump of the apparent emitting site. The simulated distribution of the jump sizes obtained for 10,000 chains are plotted in Figure 4b as red (domain-limited) and blue (chain-limited) lines. It is evident that the domain-limited model provides much better simulation of the distribution of spatial jumps. The jumps were analyzed also in terms of time-dependence of the jump size, as seen in Figure 4c. The experimental points and their average (blue line) show no marked dependence on time. On contrary, the simulated time dependence of jumps due to emission from independent domains (red line) predicts a strong increase in the jump size with increasing time. This result points to the fact that even though the emission does come from excitons that are localized in domains, the domains are not completely independent. This conclusion is also supported by the observed blue shift in fluorescence spectra upon photobleaching [9,18,38] which can be explained only by assuming the existence of energetically correlated domains. 6. Conclusions Over the past 10 years, single molecule spectroscopy has provided a level of insight into the photophysics of conjugated polymers that would not have been possible by any other method. Some of the recent developments have been covered here, for others the reader can refer to other recent review Letters [3–6]. One can assume that this exciting development will continue. There are other sophisticated single-molecule techniques being developed, such as absorption imaging and spectroscopy [39], optically advanced microscopic methods [40], time-resolved excitation [41] or circular dichroism [42], that have a great potential in further studies of single conjugated polymers. Another progress can be expected from combinations of optical microscopy with probe techniques such as atomic force microscopy or scanning tunneling microscopy on single conjugated chains [43], or from active control of the polymer conformation by confinement in low-dimensional nanostructures [44]. Acknowledgments This work was supported by a Grant-in-Aid for Scientific Research Nos. 20340109 (M. Vacha) and 22750122 (S. Habuchi) of the Japan Society for the Promotion of Science and by a Research Grant of Ogasawara Foundation. References [1] B.J. Schwartz, Annu. Rev. Phys. Chem. 54 (2003) 141. [2] W.E. Moerner, Proc. Natl. Acad. Sci. USA 104 (2007) 12596. [3] D. Wöll, E. Braeken, A. Deres, F.C. De Schryver, H. Uji-I, J. Hofkens, Chem. Soc. Rev. 38 (2009) 313. [4] F. Kulzer, T. Xia, M. Orrit, Angew. Chem. Int. Ed. 49 (2010) 854. [5] J.M. Lupton, Adv. Mater. 22 (2010) 1689. [6] M. Vacha, S. Habuchi, NPG Asia Mater. 2 (2010) 134. [7] D. Hu, J. Yu, P.F. Barbara, J. Am. Chem. Soc. 121 (1999) 6936. [8] D.A. Vanden Bout, W.T. Yip, D. Hu, D.K. Fu, T.M. Swager, P.F. Barbara, Science 277 (1997) 1074.
[9] T. Huser, M. Yan, L.J. Rothberg, Proc. Natl. Acad. Sci. USA 97 (2000) 11187. [10] D. Hu, J. Yu, K. Wong, B. Bagchi, P.J. Rossky, P.F. Barbara, Nature 405 (2000) 1030. [11] Y. Ebihara, M. Vacha, J. Chem. Phys. 123 (2005) 244710. [12] Y. Ebihara, M. Vacha, J. Phys. Chem. B 112 (2008) 12575. [13] S. Furumaki, S. Habuchi, M. Vacha, Chem. Phys. Lett. 487 (2010) 312. [14] S. Furumaki, F. Vacha, S. Habuchi, Y. Tsukatani, D.A. Bryant, M. Vacha, J. Am. Chem. Soc. 133 (2011) 6703. [15] X. Zhuang, Nature Phot. 3 (2009) 365. [16] S.W. Hell, R. Schmidt, A. Egner, Nature Phot. 3 (2009) 381. [17] E. Hennebicq et al., J. Am. Chem. Soc. 127 (2005) 4744. [18] F.A. Feist, M.F. Zickler, Th. Basche, ChemPhysChem 12 (2011) 1499. [19] S. Sartori, S. De Feyter, J. Hofkens, M. Van der Auweraer, F. De Schryver, K. Brunner, J.W. Hofstraat, Macromolecules 36 (2003) 500. [20] E. Peris, J. Hernando, F.X. Llabres i Xamena, N.F. van Hulst, J.L. Bourdelande, H. Garcia, J. Phys. Chem. C 112 (2008) 4104. [21] P. Karam, A.T. Ngo, I. Rouiller, G. Cosa, Proc. Natl. Acad. Sci. USA 107 (2010) 17480. [22] J.G. Müller, J.M. Lupton, J. Feldmann, U. Lemmer, U. Scherf, Appl. Phys. Lett. 84 (2004) 1183. [23] T. Sugimoto, Y. Ebihara, K. Ogino, M. Vacha, ChemPhysChem 8 (2007) (1623). [24] T. Sugimoto, S. Habuchi, K. Ogino, M. Vacha, J. Phys. Chem. B 113 (2009) 12220. [25] K. Becker, G. Gaefke, J. Rolffs, S. Höger, J.M. Lupton, Chem. Commun. 46 (2010) 4686. [26] J.W. van der Horst, P.A. Bobbert, M.A.J. Michels, H. Bässler, J. Chem. Phys. 114 (2001) 6950. [27] M. Yan, L.J. Rothberg, F. Papadimitrakopoulos, M.E. Galvin, T. Miller, Phys. Rev. Lett. 72 (1994) 1104. [28] V.I. Arkhipov, H. Bässler, Phys. Stat. Sol. (a) 6 (2004) 1152. [29] Y. Zaushitsyn, V. Gulbinas, D. Zigmantas, F. Zhang, O. Inganas, V. Sundström, A. Yartsev, Phys. Rev. B 70 (2004) 075202. [30] P.R. Hania, D. Thomsson, I.G. Scheblykin, J. Phys. Chem. B 110 (2006) 25895. [31] S.J. Park, A.J. Gesquiere, J. Yu, P.F. Barbara, J. Am. Chem. Soc. 126 (2004) 4116. [32] S. Onda, H. Kobayashi, T. Hatano, S. Furumaki, S. Habuchi, M. Vacha, J. Phys. Chem. Lett. 2 (2011) 2827. [33] K. Becker, E. Da Como, J. Feldmann, F. Scheliga, E. Thorn Csanyi, S. Tretiak, J.M. Lupton, J. Chem. Phys. B 112 (2008) 4859. [34] H. Lin, S.R. Tabaei, D. Thomsson, O. Mirzov, P.O. Larsson, I.G. Scheblykin, J. Am. Chem. Soc. 130 (2008) 7042. [35] S. Habuchi, S. Onda, M. Vacha, Chem. Commun. (2009) 4868. [36] S. Habuchi, S. Onda, M. Vacha, Phys. Chem. Chem. Phys. 13 (2011) 1743. [37] J.C. Bolinger, M.C. Traub, T. Adachi, P.F. Barbara, Science 331 (2011) 565. [38] J.J. Liang et al., Phys. Rev. B 74 (2006) 085209. [39] M. Celebrano, P. Kukura, A. Renn, V. Sandoghdar, Nature Phot. 5 (2011) 95. [40] T. Zuchner, A. VirgilioFailla, A.J. Meixner, Angew. Chem. Int. Ed. 50 (2011) 5274. [41] D. Brinks et al., Nature 465 (2010) 905. [42] Y. Tang, A.E. Cohen, Science 332 (2011) 333. [43] L. Lafferentz, F. Ample, H. Yu, S. Hecht, C. Joachim, L. Grill, Science 323 (2009) 1193. [44] T.Q. Nguyen, J. Wu, V. Doan, B.J. Schwartz, S.H. Tolbert, Science 288 (2000) 652.
Satoshi Habuchi received his Ph.D. in chemistry in 2001 from Hokkaido University, Japan. After receiving his Ph.D., he conducted postdoctoral research on singlemolecule photophysics at Katholieke Universiteit Leuven, Belgium and on single-molecule DNA–protein interaction at Harvard Medical School, USA. During that time, he was awarded a Japan Society for the Promotion of Science Fellowship for Research Abroad. In 2011, he was appointed as an Associate Professor in the Department of Organic and Polymeric Materials at Tokyo Institute of Technology, Japan. His main research interests include single-molecule fluorescence microscopy, polymeric materials, fluorescent proteins, DNA– protein interaction.
Martin Vacha is an Associate Professor in the Department of Organic and Polymeric Materials at Tokyo Institute of Technology. He received his education from Charles University in Prague, Czech Republic, where he also obtained his Ph.D. in 1991 for work on low temperature optical spectroscopy of photosynthetic systems. He has extensive experience in the fields of holeburning and single-molecule spectroscopy of organic molecules and molecular complexes gained during stays at academic and government research institutions in Japan. His main research interests are nanoscale physical properties of organic materials and biomaterials studied by single-molecule techniques.
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
FRONTIERS ARTICLE
A single-molecule approach to conformation and photophysics of conjugated polymers Hiroyuki Kobayashi, Suguru Onda, Shu Furumaki, Satoshi Habuchi, Martin Vacha ⇑ Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Ookayama 2-12-1-S8, Meguro-ku, Tokyo 152-8552, Japan
a r t i c l e
i n f o
Article history: Available online 1 December 2011
a b s t r a c t This Letter provides a short overview of the current status and some recent developments in the study of photophysical properties of conjugated polymers on the level of individual chains by single-molecule spectroscopy. The emphasis is put on the relationship between conformation which is determined directly for individual chains and the resulting photophysics. Some of the phenomena covered include the processes of exciton localization and subsequent exciton dissociation, fluorescence blinking and its possible origins, and direct imaging of exciton localization domains. Most of the work has been done on poly(phenylene vinylene) and polythiophene families of conjugated polymers at room temperature. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Study of organic semiconductors is one of the fastest growing research fields in materials science and engineering. Among them, conjugated polymers are the focus of continued attention because they combine good optical and electrical properties with low cost, easy processability and mechanical flexibility. The broad potential for applications of conjugated polymers includes lightemitting diodes, field-effect transistors and photovoltaic cells. The optical and electrical properties of conjugated polymers are determined by conjugated segments which are parts of the polymer chain over which the p-electron is delocalized in its ground state. Conjugated polymers absorb light in the UV/visible part of the spectrum and often show efficient luminescence. Interactions between conjugated segments give rise to various photophysical phenomena which further modify the polymer optical properties. Photophysical interactions strongly depend on distances between segments in the range of nanometers and on the relative orientations. These factors are determined by conformation of an individual polymer chain and by the packing between chains in polymer films [1]. The amorphous nature of most conjugated polymers and the variety of possible conformational states make the study of the photophysical properties of thin films complicated. An alternative ‘bottom-up’ approach to the study of conjugated polymer photophysics has recently emerged together with the technique of single molecule spectroscopy. Single-molecule spectroscopy has been providing exceptional insight into the physical properties of polymers and other soft and complex
⇑ Corresponding author. Fax: +81 3 5734 2425. E-mail address: [email protected] (M. Vacha). 0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2011.11.064
matter [2–6]. In case of conjugated polymers the inter-chain packing interactions are removed by dispersing individual chains in an inert matrix at very dilute concentrations. Photophysics can be then studied in direct relation to the conformation of individual chains. The early studies on single chains of the prototypical conjugated polymer, poly[2-methoxy-5-(20 -ethylhexyl)oxy-1,4-phenylenevinylene] (MEH-PPV) provided a basic understanding of the photophysics of isolated conjugated polymer chains. Fluorescence from single molecules of MEH-PPV showed fluorescence intermittency (blinking) and step-like photobleaching [7]. These features have been previously observed for single dye molecules but were not expected for a conjugated polymer chain containing tens or hundreds of conjugated segments. The blinking has been explained as due to localization of the excitation energy from the whole polymer chain on one or a few conjugated segments. Only these segments effectively reemit the energy and any reversible photochemical reaction occurring on these segments gives rise to the observed blinking. The localization is caused by efficient energy transfer within the polymer chain due to small inter-segment distances in a compact chain conformation [8]. Later, it was found that the blinking and single-step bleaching can be partly suppressed if the single MEH-PPV chains are cast from a good solvent, in which case they retain extended random coil conformation with larger distances between the segments [9]. In this Letter, we will review some recent development in the study of conjugated polymer photophysics on the single molecule level. We will concentrate, where possible, on work where the photophysical properties of an individual polymer chain could be directly linked to the conformation of the same chain. The focus of the Letter will be on single molecule spectroscopy using far-field fluorescence microscopy at room temperature.
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2. How to measure conformation of a single conjugated polymer chain The existing gap between the resolution of conventional optical microscopy (on the order of hundreds of nanometers) and the typical size of a polymer chain (tens of nanometers even for highmolecular weight polymers) does not enable direct imaging of the polymer chain shape. Instead, indirect methods that make use of the relationship between conjugated chain conformation and the resulting absorption anisotropy have been proposed and used. The anisotropy, which is given by a sum of the absorption strengths (transition dipole moments) of individual conjugated segments, can be characterized by an absorption ellipsoid, as shown in Figure 1a. For a simplification the ellipsoid can be rotationally symmetric in which case it is fully determined by the ratio of its two axes. In single molecule experiments the absorption anisotropy is monitored in fluorescence.
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One of the methods to measure conformation of a single conjugated chain uses polarization-modulated excitation to measure projection of the 3-dimensional (3D) absorption ellipsoid onto the sample plane [10] (Figure 1b). This method is very simple but the 2D projection does not uniquely reflect the conformation of each chain because of two random variables, the ellipsoid shape and its spatial orientation. To obtain the conformation of the chains it is necessary to measure a modulation depth of a statistical ensemble of individual chains and then use Monte Carlo simulations to reconstruct the conformation via the modulation depth distribution. This method revealed that, on average, MEH-PPV has a defect cylinder conformation in the matrix of polycarbonate or PMMA [10]. The drawback of this method is that it does not measure directly the conformation of a specific chain but rather a prevailing conformation for a statistical sub-ensemble of individual chains. To overcome this shortcoming we proposed a method to measure the absolute shape of a randomly oriented 3D rotational absorption ellipsoid [11]. This method uses evanescent wave at total internal reflection (TIR) between the substrate and the sample. The component of the evanescent electric field perpendicular to the sample plane is used to measure projection of the ellipsoid into a plane perpendicular to the sample (Figure 1c). This projection and the projection onto the sample plane (Figure 1b) provide the full 3D shape and spatial orientation of the absorption ellipsoid for a specific single chain. The conformation of the chain is then reconstructed by molecular dynamics (MD) simulations. Using this method, conformation of MEH-PPV single chains in different polymer matrices has been studied [12]. The method can be also modified to measure the shape of a general ellipsoid of nanoparticles for which the spatial orientation is known or determined independently. The ratios of the three axes of a general ellipsoid then provide a 3D linear dichroism for single nanoparticles [13]. This method has recently been used to study the excitonic structure of single light-harvesting antennae of photosynthetic bacteria [14]. Apart from the above methods, there has been great progress recently in single-molecule imaging techniques that overcome the optical resolution of conventional optical microscopy [15,16] and some of them might find applications in direct imaging of single polymer chains and other nanostructures. One of such applications will be introduced later in this Letter.
Polar. angle 3. Exciton localization vs. exciton dissociation
epi-illumination
c laser
z Polar. angle total internal reflection Figure 1. Principle of determining a single conjugated polymer chain conformation by measuring its absorption anisotropy. (a) Schematic relationship between a chain conformation and absorption ellipsoid; (b) projection of the ellipsoid on sample plane by epi-illumination (left) and a typical fluorescence image of single MEH-PPV chains (right). Inset: Excitation polarization angle modulated fluorescence intensity of the chain marked by circle in the image. (c) Projection of the ellipsoid on a plane perpendicular to the sample plane by total internal reflection illumination (left) and a typical fluorescence image of single MEH-PPV chains (right). Inset: Excitation polarization angle modulated fluorescence intensity of the chain marked by circle in the image.
Starting with the first single-molecule studies of poly(p-phenylene vinylene) (PPV) and poly(p-pyridylene vinylene) (PPyV) copolymer [8], fluorescence blinking has been interpreted as a signature of exciton localization on one or several conjugated segments. The localization is a result of efficient energy transfer of the absorbed light energy. Absorption in a conjugated polymer chain occurs on all individual conjugated segments and leads to the formation of a Frenkel-type singlet exciton which is initially located on the original segment. The exciton can be transferred by dipole–dipole interactions to other conjugated segments, either to neighbors along the polymer chain (intrachain transfer) or to segments located on different parts of the same chain (interchain transfer). In a collapsed conformation such as the defect cylinder [10] the interchain energy transfer is more efficient compared to the intrachain transfer [17] due to smaller inter-segment distances. The intrachain transfer is assumed to occur only on short length scales corresponding to a few conjugated segments [17]. This balance between the intra- and inter-chain energy transfer can be reversed in extended chain conformations such as random coil. The disappearance of blinking in single MEH-PPV chains cast from good solvent [9] can be explained by suppression of the interchain
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energy transfer due to increased interchain segment-segment distances. In chains with such conformations the exciton decays radiatively or non-radiatively before it can arrive at the segment with the lowest energy. Recently, a measurement of spectral linewidths in single collapsed MEH-PPV chains in PMMA matrix at lowtemperatures provided concrete values of the energy transfer times. At 1.2 K the energy transfer occurs with a mean value of 3.9 ps [18]. In a solid matrix of an inert polymer the conjugated polymer chain can form either collapsed or extended conformations, depending on the solvent and the matrix polymer. The solvent and matrix that support the extended conformation of poly(2methoxy-5-(20 ,60 -dimethyloctyloxy)-p-phenylenevinylene (OC1C10-PPV) are toluene and low molecular weight polystyrene (PS) [19]. In this matrix the fluorescence intensity shows no blinking, in contrast to a more polar poly(vinyl butyral). However, the blinking in the OC1C10-PPV single chains reappears in high molecular weight PS, which indicates that strain and distortions of the original extended conformation which are present during the spin-coating sample preparation process are also important factors in the resulting chain conformation. We have looked at the effect of molecular weight of the matrix polymer systematically and found that for single MEH-PPV chains in PS, the fraction of blinking molecules increases from 59% to 87% when the molecular weight of PS changes from 870 to 12,000 g/mol (Onda, unpublished results). Apart from the solid matrix, the environment of a conjugated polymer chain can be also controlled by incorporating the chains into nanostructures such as hollow silica spheres [20] or lipid vesicles [21]. Conformation of a polymer chain is determined and can be actively controlled by its chemical structure. Polymers with rigid main chain, such as methyl substituted ladder-type poly(paraphenylene) (MeLPPP) are effectively prevented from coiling or collapsing onto themselves. As a result, single chains of MeLPPP showed both emission from a single conjugated segment due to efficient uni-directional energy transfer as well as emission from multiple segments due to bi-directional energy transfer along the rigid chain [22]. Apart from using rigid main chain, the collapsed conformation can be prevented by decorating the main chain with various functional groups. We have shown that grafting bulky PS side
a
chains on the flexible main chain of polythiophene (PT) completely suppresses fluorescence blinking which is otherwise observed on the un-substituted PT [23], as seen in Figure 2. This result is well supported by coarse-grain molecular dynamics (MD) simulations which show that the un-substituted PT forms a compact conformation in which any pair of conjugated segments is within the Förster radius for efficient energy transfer [24]. On the other hand, the PS-substituted PT main chain is forced into an extended coil conformation in which the energy transfer is efficient over a few neighboring segments along the chain, as evidenced also by the MD simulations. Similarly, the rigidity of the main chain can be enhanced by decorating with macrocycles, as shown for the conjugated polymer of poly(p-phenylene-ethynylene-butadiynylene) [25]. One of the ways in which the exciton can decay non-radiatively is dissociation into a pair of charges. This process is determined by the exciton binding energy which for conjugated polymers is generally large (on the order of 0.4–0.6 eV) [26]. An excess energy, either by excitation into hot vibrational states [27] or by externally applied electric field [28] is usually necessary to separate the exciton into positive and negative charges. However, it has been also reported that for polythiophenes the charge pair formation can occur from relaxed excitons even in the absence of an external electric field [29]. An important factor in the exciton dissociation is the interchain interaction which can give rise to p-p stacking favorable for the charge separation. We have used electric-field induced fluorescence quenching [28,30] on the model compounds of the un-substituted and PS-substituted PT to study systematically the effect of iterchain interactions on the exciton dissociation. Examples of field-induced quenching on single PT chains are shown in Figure 2. For quenching of well-isolated chains on an ensemble level there is no difference in the quenching efficiency between the substituted and un-substituted chains. This result indicates that interchain intramolecular interactions are not a prerequisite for efficient exciton dissociation and that the process primarily occurs along the main PT chain. On the other hand, fluorescence from single un-substituted PT chains is much more likely to be quenched completely as compared to a partial quenching of the PS-substituted compounds. This observation can be explained assuming that in the compact conformation of the un-substituted chains the exciton is first localized on a low-energy segment by
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Figure 2. Single molecule detection and electric field-induced quenching of single chains of polythiophene (PT) derivatives. Fluorescence intensity traces of unsubstituted PT (a) and PS-substituted PT (b) single chains; the right sides show molecular dynamics simulations of the chain conformations; electric field induced quenching of unsubstituted PT (c) and PS-substituted PT (d) single chains.
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Intensity (a.u.)
energy transfer and after that the dissociation process takes place. In summary, the conclusion and implication for device engineering is that the pair of charges resulting from a dissociated exciton is located on the same main chain and that the dissociation itself is relatively slow process compared to exciton migration in a compact chain conformation.
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It has been now quite well established that efficient energy transfer and exciton localization on limited number of segments leads to the appearance of fluorescence blinking. This process is very effective for chains with collapsed conformation in a poor solvent or in a solid matrix that resembles a poor solvent. In other environments the factors that determine the efficiency of exciton localization can be more subtle. As mentioned above, in the same matrix of low molecular weight PS there can be both blinking and non-blinking chains present, with their relative fractions given by the PS molecular weight, spin-coating conditions, PS concentration and others. To clarify this point we used the method of 3D absorption ellipsoid measurement for single MEH-PPV chains in low molecular weight PS [11] and extracted chain conformations by MD simulations. It was found that along with two types of blinking behavior there are two distinct types of coil-like chain conformations, a more compact ‘oblong defect coil’ and a more extended ‘disk-like defect coil’, and that the two conformations are well correlated with the two types of blinking (see Figure 3a). In the extended chain conformations there is only continuous decrease of fluorescence intensity due to continuous photobleaching of individual non-interacting conjugated segments. In the more compact chain conformations there are locations with higher density where interchain interactions can occur and which likely serve as energy traps and exciton localization centers. The chain is effectively divided into a few domains and exciton localization in the domains gives rise to the observed blinking. The actual photochemical reaction that causes the reversible quenching of the emitting segment and the blinking has been originally ascribed to photooxidation [8]. Later, this assumption has been studied on a model system consisting of MEH-PPV chains dispersed in thin layer of PMMA in a device capable of repeatedly inserting and extracting positive charges (holes) into the layer [31]. It was found that hole injection leads to bias-dependent quenching of fluorescence from single MEH-PPV chains and that fluorescence can be recovered by applying a reverse bias on the device. The implication for an unbiased system is that formation of a complex MEH-PPV+/O 2 by photoinduced electron transfer from MEH-PPV to a nearby oxygen could be the mechanism behind the blinking in single MEH-PPV chains. Complete or partial exciton localization has been a necessary assumption in all of the above explanations of fluorescence blinking. Recently, we have studied the behavior of single MEH-PPV chains in a relaxed state in a solution. To prevent fast diffusion, a concentrated low molecular weight PS in toluene has been used as the solvent for single MEH-PPV molecules. As seen from the results in Figure 3b, single MEH-PPV chains freely and slowly diffusing in the solution do not show any effects of blinking and only reduced photobleaching [32]. These effects are due to presumably an extended coil conformation. In the same solution, the chains that are immobilized by adsorption on the substrate exhibit much faster (often purely exponential) photobleaching and blinking. In a substantial fraction of the chains the blinking is a two-state process, as depicted by the dashed and solid blue lines in Figure 3b. The adsorbed chains have basically the same conformation of an extended coil only slightly modified by the adsorption site. In such conformation the exciton localization is not an effective process.
5
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Figure 3. Origin of fluorescence blinking in MEH-PPV. (a) Dependence of blinking on defect coil conformation in a matrix of good solvent; (b) fluorescence intensity traces (left) of MEH-PPV molecules freely diffusing in solution (top) and adsorbed on the substrate (bottom). The dashed and solid blue lines in the bottom trace are single exponentials. Schematic depiction of the sample is shown in the right; (c) time changes in fluorescence spectra (spectral jumps) in single octamers of PV; the structure is shown in the right.
The extent of fluorescence quenching during the two-state blinking varies between different chains and is on average 0.43. In the absence of exciton localization the blinking process causes a quenching of a large number of conjugated segments along extended parts of the polymer chain. The above average extent of quenching would correspond to quenching of tens nanometers of the chain. The two-state blinking itself might be caused by repeated adsorption and release of a free part of the MEH-PPV chain on the substrate or by reversible interaction between two free parts of the chain in the solution. Apart from the conformation of the chain and inter-segment interactions, the photophysical properties of conjugated polymers are also influenced by properties and dynamics of the constituting conjugated segments themselves. Recently, it has been suggested that the shape of a conjugated phenylenevinylene (PV) oligomer is crucial in determining its spectral properties at low temperatures. Oligomers that are bent are characterized by red-shifted fluorescence spectra [33]. We have looked at spectral properties and dynamics of octamers of PV with various side groups at room
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location of the sites is highly dynamic – individual domains on a single chain can be switched on and off, resulting in jumps of the site along the chain. An example of a chain which during the course of the experiment changed the position of emission between 8 sites is shown in Figure 4a. The changes are often correlated with intensity changes during the blinking. The contour of all emission sites is related to the actual shape of the conjugated polymer chain. The contour in Figure 4a is elongated, as expected for the oblong defect coil conformation. While in the matrix of Zeonex most molecules show the elongated contour of the emitting sites, in the matrix of low molecular weight PS there are two types of contours observed. In addition to the elongated contour, a substantial number of single MEH-PPV chains contain only a single apparent emitting site which does show step-like change of position. This fact can be understood by considering emission of many non-interacting conjugated segments on an extended chain the superposition of which forms the apparent emission site. Thus, the results of the emission localization experiments in PS nicely correspond to the two types of chain conformation and two types of blinking of MEH-PPV in the same matrix, as described above. Recently, change of the position of apparent emitting sites has been observed also as a result of controlled quenching of single MEH-PPV by injection of holes in a device structure [37]. The mean jump size of 13.7 nm is similar to the values obtained for the spontaneous quenching [36]. The existence of independent exciton-localization domains is only one of the possible explanations for the observation of the apparent emission site and its spatial jumps. Another possibility is that the exciton would be sampling many locations throughout the whole polymer chain, and emitting from the lowest energy site. Photobleaching of this site would lead to emission from the next lowest energy site and to a jump in the apparent emission site. We tried to distinguish these cases by statistical analysis of the dynamics of the emitting sites. The Figure 4b shows a histogram
temperature (Kobayashi, unpublished results). An example of the results is shown in Figure 3c. The fluorescence spectra of a single PV octamer show jumps in time between two spectral forms, a blue one and a red one. The dynamics of the jumps does not depend on the nature of the side groups. However, the red-to-blue change is about 3.8 times faster than the blue-to-red transition. The spectral jumps are tentatively assigned to torsional motion of an unsubstituted phenyl ring between two energetically stable configurations. In one of the configurations the phenyl ring is more coplanar with the remaining part of the octamer causing extension of the conjugation and red shift of the fluorescence spectrum. Similar dynamics can be assumed to occur in conjugated polymer chains as well. Change of conjugation length affects not only fluorescence but also absorption spectra and as such can be viewed as another origin of fluorescence blinking in conjugated polymers.
5. Exciton migration – domains vs. whole chain Localization of the exciton in independent domains on the conjugated polymer chain has been used to explain the blinking behavior of the compact defect coil conformations, as mentioned in the previous section. At the same time, such domains were also postulated as a result of time-resolved experiments on single MEHPPV chains [34]. Later, we used a super-resolution imaging method of emission centroid localization in an attempt to confirm the existence of the domains in single MEH-PPV chains in the matrix of PS [35]. In the method, the position of an apparent emission site which at any time is a superposition of all emitting domains can be determined with the precision of a few nanometers. Depending on the molecular weight of MEH-PPV and the kind of the matrix polymer, each conjugated polymer chain can contain anywhere between one and more than ten apparent emitting sites [36]. The
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Figure 4. Super-resolution imaging of single MEH-PPV chains (molecular weight 1,440,000) in PS matrix. (a) Apparent emitting site locations (in different colors) observed on one chain during the course of the experiment; (b) distribution of emitting site jumps obtained on 200-molecules; red line – simulation of the jump distribution by domainlimited exciton migration model; blue line – simulation of the jump distribution by chain-limited exciton migration model; (c) dependence of the jump size on time during the course of experiment; blue line – average of the experimental points; red line – simulated time dependence based on a model of independent domains.
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of the jump size obtained on MEH-PPV of the molecular weight 1440,000 in PS. The jumps span a range between a few and 40 nm, with a mean value of 9.6 nm. The figure also shows simulation of the jump size obtained using two models, domain limited exciton migration and chain-limited exciton migration. In the domain-limited model, it is assumed that initially all domains (in total 20 in Figure 4b, the size of each domain in the simulation is set to 13 nm) emit at the same time, and in the course of time individual domains photobleach randomly, causing jumps in the apparent emitting sites. In the chain-limited model we assume that of all the domains only 1 is emitting at each time and after photobleaching another randomly located domain starts emitting. The photobleaching again causes a spatial jump of the apparent emitting site. The simulated distribution of the jump sizes obtained for 10,000 chains are plotted in Figure 4b as red (domain-limited) and blue (chain-limited) lines. It is evident that the domain-limited model provides much better simulation of the distribution of spatial jumps. The jumps were analyzed also in terms of time-dependence of the jump size, as seen in Figure 4c. The experimental points and their average (blue line) show no marked dependence on time. On contrary, the simulated time dependence of jumps due to emission from independent domains (red line) predicts a strong increase in the jump size with increasing time. This result points to the fact that even though the emission does come from excitons that are localized in domains, the domains are not completely independent. This conclusion is also supported by the observed blue shift in fluorescence spectra upon photobleaching [9,18,38] which can be explained only by assuming the existence of energetically correlated domains. 6. Conclusions Over the past 10 years, single molecule spectroscopy has provided a level of insight into the photophysics of conjugated polymers that would not have been possible by any other method. Some of the recent developments have been covered here, for others the reader can refer to other recent review Letters [3–6]. One can assume that this exciting development will continue. There are other sophisticated single-molecule techniques being developed, such as absorption imaging and spectroscopy [39], optically advanced microscopic methods [40], time-resolved excitation [41] or circular dichroism [42], that have a great potential in further studies of single conjugated polymers. Another progress can be expected from combinations of optical microscopy with probe techniques such as atomic force microscopy or scanning tunneling microscopy on single conjugated chains [43], or from active control of the polymer conformation by confinement in low-dimensional nanostructures [44]. Acknowledgments This work was supported by a Grant-in-Aid for Scientific Research Nos. 20340109 (M. Vacha) and 22750122 (S. Habuchi) of the Japan Society for the Promotion of Science and by a Research Grant of Ogasawara Foundation. References [1] B.J. Schwartz, Annu. Rev. Phys. Chem. 54 (2003) 141. [2] W.E. Moerner, Proc. Natl. Acad. Sci. USA 104 (2007) 12596. [3] D. Wöll, E. Braeken, A. Deres, F.C. De Schryver, H. Uji-I, J. Hofkens, Chem. Soc. Rev. 38 (2009) 313. [4] F. Kulzer, T. Xia, M. Orrit, Angew. Chem. Int. Ed. 49 (2010) 854. [5] J.M. Lupton, Adv. Mater. 22 (2010) 1689. [6] M. Vacha, S. Habuchi, NPG Asia Mater. 2 (2010) 134. [7] D. Hu, J. Yu, P.F. Barbara, J. Am. Chem. Soc. 121 (1999) 6936. [8] D.A. Vanden Bout, W.T. Yip, D. Hu, D.K. Fu, T.M. Swager, P.F. Barbara, Science 277 (1997) 1074.
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Satoshi Habuchi received his Ph.D. in chemistry in 2001 from Hokkaido University, Japan. After receiving his Ph.D., he conducted postdoctoral research on singlemolecule photophysics at Katholieke Universiteit Leuven, Belgium and on single-molecule DNA–protein interaction at Harvard Medical School, USA. During that time, he was awarded a Japan Society for the Promotion of Science Fellowship for Research Abroad. In 2011, he was appointed as an Associate Professor in the Department of Organic and Polymeric Materials at Tokyo Institute of Technology, Japan. His main research interests include single-molecule fluorescence microscopy, polymeric materials, fluorescent proteins, DNA– protein interaction.
Martin Vacha is an Associate Professor in the Department of Organic and Polymeric Materials at Tokyo Institute of Technology. He received his education from Charles University in Prague, Czech Republic, where he also obtained his Ph.D. in 1991 for work on low temperature optical spectroscopy of photosynthetic systems. He has extensive experience in the fields of holeburning and single-molecule spectroscopy of organic molecules and molecular complexes gained during stays at academic and government research institutions in Japan. His main research interests are nanoscale physical properties of organic materials and biomaterials studied by single-molecule techniques.