pubs.acs.org/NanoLett
Electrothermal Manipulation of Individual Chromophores in Single Conjugated Polymer Chains: Controlling Intrachain FRET, Blinking, and Spectral Diffusion Florian Schindler,† and John M. Lupton*,‡ †
Photonics and Optoelectronics Group, Department of Physics, Ludwig-Maximilians University of Munich, D-80799 Munich, Germany, and ‡ Department of Physics and Astronomy, University of Utah, Salt Lake City, Utah 84112 ABSTRACT Single molecule spectroscopy of individual chains of a conjugated polymer opens up deep insight into electronic localization phenomena, which govern the underlying optical properties of these complex and disordered materials. We explore the nature of a single chromophore arising in a delocalized π-electron system by applying periodic electrothermal perturbations at low temperatures. Brief heating of the chromophore leads to a dramatic increase in the transition line width and is generally accompanied by a random jump of the emission energy. This observation demonstrates that chromophores on a polymer chain are not only defined by structural disorder but also by the subtleties of the local dielectric environment. The effect of thermal perturbation becomes more complex when long polymer chains are considered, which can potentially support the formation of multiple chromophores. Here, a momentary increase in temperature can promote intrachain energy transfer to quenching sites, leading to a strong modulation of emission intensity with temperature. Unexpectedly, such energy transfer can serve to either raise or lower the transition line width and quantum yield of the ensemble with increasing temperature, depending on the specific energetics of the chromophores in the system, which in turn vary with time. The controlled perturbation of both the emission spectrum and the intensity by brief heating of the polymer chain offers insight into possible microscopic origins of fluorescence blinking and spectral diffusion, which ultimately impact on the efficiency and spectral purity of devices. KEYWORDS Conjugated polymers, FRET, OLEDs, single molecule spectroscopy
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emissive unit.3 In fact, rather surprisingly, the narrowest transitions in single chains have been reported for the most flexible polymer backbones, which conversely exhibit the broadest spectra in the ensemble.12 When relating lowtemperature spectroscopy to the characteristics of a single chain in a real device at room temperature, the question arises as to how predetermined the observed transition energy of a single chromophore actually is: is a single chromophore static? Spectral lines broaden dramatically with increasing temperature both due to stronger electronphonon coupling and due to an increase in dynamic disorder arising from conformational fluctuations.13-15 In analogy to results from single molecule transport spectroscopy,16,17 it is therefore conceivable that repeated heating and cooling of the same chromophore will not return it to the same transition energy at low temperature, implying that the chromophore itself is more of a dynamic entity. Such changes in the electronic structure of the chromophore could in turn impact on intramolecular energy transfer processes, which are limited by the degree of electronic resonance between adjacent chromophores on a chain as required for dipole-dipole coupling.18 Building on the detailed picture of individual chromophores derived from single chain spectroscopy,2,3,19 we demonstrate here the effect of periodic electrothermal heat-
ingle molecule spectroscopy of conjugated polymers has opened up remarkable insight into the underlying electronic properties of these inherently disordered materials.1-3 Conjugated polymers consist of long chains of carbon atoms forming a delocalized π-electron system, which is disrupted at random intervals giving rise to more localized optically active units called chromophores.4,5 While much of the recent interest in single chain spectroscopy has focused on characterization of materials,1 several avenues have been explored to manipulate the optical properties of a single chain in a controlled fashion. Examples include the application of electric fields, which can quench the emission through redistribution of photogenerated charges,6-8 or shift the emission energy by means of the Stark effect;9 and the injection of external charges, leading to pronounced spectroelectrochemical effects such as highly reversible fluorescence quenching.10,11 Cryogenic single chain spectroscopy has been instrumental in uncovering how the shape of a particular molecular structure, whether rigid or flexible, tends to have a greater impact on the (inhomogeneous) distribution of transition energies between different chains than on the actual intrinsic transition width of a single * To whom correspondence should be addressed. E-mail:
[email protected]. Received for review: 04/29/2010 Published on Web: 06/10/2010 © 2010 American Chemical Society
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ing of a molecule on the transition energy, emission intensity, and line width of a prototypical conjugated polymer. The transition line width can serve as an accurate gauge of the local temperature of the chain.15,18,20 In short chains, which typically only support a single chromophore, heating appears to have little influence on the emission intensity and thus the fluorescence quantum yield but tends to trigger substantial random spectral jumps of order of the transition line width. The situation becomes more complicated in longer-chain polymers that can support multiple chromophores. Here, the chromophore line width can control the contribution of interchromophoric coupling, which results in energy transfer. When multiple localized chromophores contribute to the emission at low temperatures, heating of the chain can promote exciton migration to one of the chromophores, leading to an effective narrowing of the spectrum. Heating and line width modulation then occur out of phase. Likewise, exciton migration may promote energy harvesting by on-chain transient quenching sites, such as charge transfer excitations,21 leading to a pronounced dependence of emission intensity on the heating cycle. To study the effect of temperature on the formation of a single chromophore and the interaction between chromophores, it is necessary to cycle one and the same molecule repeatedly in temperature. Such a temperature modulation is hard to achieve in a conventional cryostat setup due to the limited photochemical lifetime of the molecule and the fact that the cryostat can shift out of focus of the single molecule fluorescence microscope during variation of the temperature. We remove both obstacles by heating only the immediate vicinity of the single molecule by means of a narrow wire deposited on the substrate, which in turn acts as a heat sink, so that the molecules thermalize again after termination of the heating current. Single chains of the conjugated polymer ladder-type poly(para-phenylene) (LPPP) were dispersed in polystyrene, deposited on a quartz substrate by spin coating in a nitrogen glovebox and mounted to a coldfinger cryostat in a microscope setup described in detail previously.19 We used two different molecular weight samples of the polymer of an average weight of ∼25 kDa (∼31 repeat units) and ∼66 kDa (∼83 units). Further details on these samples are given in ref 19. Figure 1a shows a microscope image recorded under wide-field illumination by a broad-band frequency-doubled femtosecond laser at 426 nm. Single molecules are clearly visible as bright spots. The horizontal strip arises from a lithographically defined aluminum wire, approximately 2 µm wide, 100 nm thick, and 1 cm long. A potential of 65 V is applied periodically to the wire for 1 s every 10 s, resulting in a current of approximately 100 mA (the current decreases with time as the wire heats up). Panels (b-d) display characteristic single molecule spectra of a conjugated polymer chain in proximity to the heating wire, such as the one labeled in the image by a white circle. The spectra were recorded at a nominal cryostat temperature of 5 K and each show only one single © 2010 American Chemical Society
FIGURE 1. Electrothermal switching of the emission from a single chromophore on an individual polymer chain at 5 K. (a) Single molecules, dispersed in a polystyrene matrix, are visible as bright spots in the fluorescence microscope image. A 2 µm wide aluminum wire passes over the quartz glass substrate. Application of a voltage pulse results in current flow and electrothermal heating of the wire and its immediate environment. (b-d) Examples of emission spectra of single polymer chromophores 1 s before, during, and 2 s after the heating pulse, recorded in 1 s time windows (1 s heat pulse applied every 10 s). (e) Average line width of a single chromophore transition as a function of temperature. The temperature was set using a cryostat.
peak, indicating that only one single chromophore contributes to emission from the chain. Note that the spectra are truncated to the 0-0 transition so as not to show the vibronic progression in the emission.3 As the chain heats up following application of the voltage pulse, the spectrum broadens. Removal of the voltage leads to a reversal of the process. As the heating pulse induces a measurable change in the cryostat’s coldfinger temperature, we assume that heating on this time scale gives rise to a fairly uniform thermal profile on the substrate. In contrast, using molecular thermometers, heat within a thin polymer film has been shown to dissipate on the millisecond time scale following thermal perturbation in excess of 100 K.22 Such molecular thermometry could provide further insight into the spatiotemporal heating profile in the present experiment. Fluorescence spectroscopy is often used to provide noninvasive access to the local temperature of a material.22-26 As ensemble spectra are strongly disorder broadened, mere spectral shifts in the emission are generally weak.25,26 On the single molecule level, however, the average line width of a single chromophore spectrum can be used as an accurate measure of the local temperature as it is free from disorder broadening effects.18 Figure 1e displays the spectral width (full width at half-maximum) averaged over ∼30 different single polymer chain emission spectra for each 2684
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3. However, in the present case the spectral fluctuations observed are clearly not random but rather correlate with the heating pulses; while there is some random spectral jitter between pulses, the large spectral jumps tend to arise from heating, giving rise to a parceled appearance of the trace. The fluorescence line width shows a clear modulation synchronous with the heating. The temporal variation of the line width is not symmetrical, indicating that the heating occurs faster than the cooling of the molecule. It is therefore concluded that the electrothermal pulse forces the chromophore into a new configuration, which is maintained upon refreezing of the chain: a single chromophore freezethaw cycle. From the line width variation it is concluded that the molecule is heated from 5 K to approximately 50 K in the present case. All short-chain single polymer molecules we studied displayed this reversible line width modulation. Figure 2b displays a close-up of the temporal region of the trace between 60 and 210 s to exemplify the triggered nature of spectral jumps. Heating of the molecule can result in a change of the mean chromophore energy, but need not necessarily do so, as in the time range 110-140 s. Spectral diffusion is an optically driven process arising from minute local rearrangements in the dielectric environment and the local charge density.14 Such rearrangements can be promoted by heating of the molecule. We note some marked analogies of the present situation with the differentiation between spectral jumps and spectral jitter in the case of single nanocrystal quantum dots.30,31 Spectral diffusion is primarily photochemical rather than photothermal in origin, but can be promoted by the influx of additional thermal energy.31 Close inspection of the fluorescence trace suggests an additional intensity modulation at twice the electrothermal pulse frequency: immediately after the maximum line width is recorded, the fluorescence intensity appears highest, and subsequently decreases again between heating pulses. We assign this effect to the build-up of metastable quenching species, such as charge transfer excitations, an effect often referred to as statistical aging.1,32 We have previously shown that these quenchers can be annihilated by rapid exposure to oxygen, increasing the fluorescence yield.33 Here, we propose that a rapid increase of temperature leads to a destruction of quenchers, raising the intensity. Subsequently, as the molecule has cooled again, quenchers build up again, lowering the emission intensity. Such a build-up of quenchers has been studied in detail by Barbara et al.1 As the chain length increases, the average number of chromophores on the polymer increases.19 This effect can be easily visualized at low temperatures, where single chromophores are identified as narrow optical transitions such as those shown in Figure 1. For longer polymer chains, the complexity of the emission spectrum increases as more and more chromophore units contribute to the emission.19 The question then arises as to how the individual chromophores on a chain interact with each other.18 Given spectral overlap
FIGURE 2. Electrothermal modulation (1 s heating every 10 s) of the emission of a short polymer chain containing just one chromophore, at 5 K (1 s integration time). (a) Plot of the emission spectrum as a function of time. Only the 0-0 transition intensity around 459 nm is shown. The heating pulses are indicated by the black lines superimposed on the trace. The emission shows a constant spectral jitter, characteristic of single molecule luminescence, with large spectral jumps occurring in response to the heating pulses. The spectral line width is also plotted, displaying a sudden rise triggered by the heating, followed by a gradual decay. (b) Close-up section of the trace revealing both spectral jitter and electrothermally induced spectral jumps. Every heating pulse results in a reversible increase in line width but not every pulse triggers a random jump in the emission spectrum.
cryostat temperature. As reported previously, the line width increases approximately linearly with rising temperature.18 Note that the ladder-type polymer is characterized by an extremely rigid backbone conformation, which does not change measurably with temperature.27 Thermal effects arise primarily due to an increase in interactions with the polystyrene environment13,16,28 as well as possible slight motion of the backbone’s side groups. Periodic modulation of the heating voltage allows a direct investigation of the effect of thermal perturbation on the single chromophore emission at the same temperature: the chromophore is heated up briefly and subsequently returns to the ambient temperature of 5 K as monitored by the transition line width. Figure 2a displays a trace of the single chromophore fluorescence, where the emission intensity is color coded and plotted as a function of time and wavelength. Single-step photobleaching of the emission occurs at time t ) 308 s, offering confirmation of the presence of just one single emitting species. The diagram also shows the voltage pulses (marked in black), and the line width (magenta). In the absence of electrothermal perturbation, single molecules exhibit random spectral diffusion of the emission spectrum to shorter and longer wavelengths with no discernible temporal structure in the fluorescence trace.3,29 An example of a trace of a single LPPP molecule is given in ref © 2010 American Chemical Society
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between the emission of one chromophore and the absorption of an adjacent unit, energy transfer may occur following the Fo¨rster dipole-dipole coupling process. In these prototypical rigid-rod ladder-type polymers, intrachain energy transfer has been shown to be thermally activated due to the increase of single chromophore line width, and concomitantly of interchromophoric spectral overlap with increasing temperature.18,34 Besides the singlet exciton, photoexcitation of the polymer may also generate long-lived transient species such as triplet and charge transfer excitations.1,21,35 These excitations in turn induce new absorption features in the polymer chain and can act as efficient quenchers of excitation energy.36-38 Single molecule spectroscopy has established that it is primarily these transient quenching species that lead to the overall limitation of the luminescence quantum yield.11 In single molecules, the transient appearance of quenchers gives rise to luminescence intensity fluctuations, which are known as blinking.39,40 The sensitivity of a single polymer molecule to quenching species increases with chain length, as intrachain light harvesting by quenching centers is promoted.41 One may therefore expect that fluorescence quenching in the form of blinking is enhanced at elevated temperatures.18 On the other hand, increasing the temperature will reduce the lifetime of the transient quenching species,21,35 which could in turn raise the quantum yield again. Whereas the data in Figure 2 were obtained for a short polymer chain (∼31 repeat units), Figure 3 displays similar measurements for a longer chain (∼83 units). As the complexity of the temporal emission dynamics increases with chain length, we use these data as an illustrative example of the intrachain processes which can arise but are generally masked in the ensemble, and refrain from attempting to formulate a statistical picture which would relate the single chain experiment to the ensemble. Figure 3a shows the modulation of the fluorescence spectrum as a function of time with the extracted line width (magenta line) and the voltage pulse (7.5 s duration every 30 s, black line) overlaid. From first inspection of the graph, it appears that the emission correlates with the application of the heating pulse. Indeed, Fourier transformation of the temporal evolution of both the line width and the emission intensity yield a clear frequency component consistent with the periodic thermal modulation. However, close inspection of the trace shows that neither the intensity nor the line width appear to vary systematically with the heating pulse for the duration of the measurement. Whereas the emission intensity is increased for some heating pulses (such as around t ) 750 s), it appears to be reduced for others (e.g., around t ) 1250 s). Likewise for the line width; the first few cycles of the trace (0-250 s) show a clear sudden increase of the transition width coinciding with heating, followed by a slow reduction in the width as the molecule and its environment cool again. However, at time t ) 375 s the emission vanishes entirely, returning in the next heating cycle with a line width modula© 2010 American Chemical Society
FIGURE 3. Electrothermal modulation (7.5 s heating every 30 s) of the emission of a long polymer chain with multiple chromophores at 5 K (2.5 s integration time). (a) The modulation of the fluorescence trace with heating (black line) is not as systematic as for the single chromophore (Figure 2). Strong variations in intensity occur, reminiscent of blinking, but both maximum and minimum intensity can coincide with heating. The line width (magenta) tracks the heating at short times but is subsequently modulated out of phase. (b) Closeup of the response of line width (magenta) and intensity (blue). At first, intensity modulation is weak, although a line width modulation is discernible. Subsequently, a strong intensity modulation arises with the onset of heating congruent with an increase in luminescence. In contrast to Figure 2, the emission line width decreases during heating. (c) Two representative spectra taken at the times marked by red and blue arrows in (b). The cold molecule spectrum (blue, 5 s before heating pulse following 17.5 s of cooling) has more structure than that of the heated molecule (red, shifted by -0.7 nm; 2.5 s into the heating pulse) with greater overall spectral width. (d) Schematics of interchromophoric interactions. Heating the molecule raises the chromophore line width, promoting on-chain dipole-dipole coupling between donor (D) and acceptor (A) units. At low temperatures, the chromophores may act independently of each other, but luminescence can still be partially quenched by, for example, a transient charge transfer state. At elevated temperatures, spectral overlap between the emission of one chromophore on the chain and the absorption of another increases, so that intramolecular energy transfer can occur. This effect may promote quenching, but can also serve to move excitation energy away from a quencher to a chromophore of higher quantum yield. Energy transfer on the chain may result in thermally activated spectral narrowing and brightening due to a smaller selection of chromophores contributing to emission.
tion out of phase with the heating pulse; for subsequent cycles, the maximum line width is recorded between pulses. 2686
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The expected correlation between heating and line width returns again for times beyond 1250 s. Figure 3b shows a close-up of the main trace, plotting the luminescence intensity and the line width as a function of time. At first, there appears to be only a weak effect of heating on intensity, as labeled in the figure. Toward the end of the transient, however, a strong modulation is observed with the intensity dropping to coincide with the rise in the line width. The rise in intensity mirrors the reduction in line width. In this case, the line width modulation occurs out of phase with the heating pulse; heating reduces the single molecule spectral width. Such thermal narrowing of the emission may seem an unlikely phenomenon but can arise if the size of the ensemble contributing to the emission is reduced upon heating. In long-chain polymers, the single molecule emission spectrum at low temperature can often be deconvoluted to reveal signatures of different chromophores.19 The presence of multiple chromophores on the chain then gives rise to additional inhomogeneous broadening in the emission. Figure 3c shows two single molecule spectra taken as vertical slices from the trace in panel a at the times indicated by the blue and red arrows in panel b, just before and during the heating pulse at t ) 820 s. The cold molecule spectrum displays three peaks (marked by arrows), which can be interpreted to arise due to the contribution of three chromophores to the emission.19 In contrast, the heated molecule reveals only one peak. This peak was blue shifted by 0.7 nm to overlay the dominant peak of the cold molecule spectrum. On the other hand, overlaying the two peaks reveals that the line shape of the red spectrum (heated molecule) is broader than that of the cold molecule (blue), as would be expected from the single chromophore heating spectra discussed in Figure 2. In interpreting the thermally modulated luminescence data it is necessary to distinguish between the single molecule and the single chromophore spectral line width.19 Such a distinction may not be immediately apparent from simply considering one emission spectrum at a single temperature, but is greatly aided by the modulation technique presented here. As Figure 3c shows, the cold molecule spectrum appears broader overall due to the contribution of multiple emitting units. The cold spectrum is approximately 25% as intense as the warm spectrum, indicating a reduction of luminescence quenching upon heating. In contrast, in bulk films luminescence quenching is generally thermally activated, increasing with rising temperature.4 The modulation of line width and intensity with temperature can be rationalized following the schematic depicted in Figure 3d. For the cold polymer chain, the chromophores on the molecule generally emit independently of each other. The appearance of a quenching species will only affect the chromophore emission in the immediate vicinity of the quencher. Heating raises the single chromophore line widths, thus enhancing interchromophoric coupling and energy © 2010 American Chemical Society
transfer. This enhanced coupling can promote quenching of more distant chromophores, lowering the overall quantum yield. Signatures of this model have been identified in the temperature dependence of fluorescence blinking of these structurally stiff and planar polymers, which do not tend to collapse. Whereas single polymer chains at low temperature show more continuous random intensity variations, strong intermittency is observed at elevated temperatures.18 However, it is important to note that quenching need not necessarily be discrete in nature.39 Energy transfer to a nonemissive absorber depends sensitively on distance, so that radiative and nonradiative recombination compete, enabling partial quenching of even one single emitting unit.42 Alternatively, increased interchromophoric coupling may serve to guide the excitation energy away from the quencher. This process could occur if a particularly strongly absorbing chromophore exists, defined by a highly delocalized π-electron system with correspondingly large oscillator strength.19 Such a delocalized chromophore would be characterized by an increased radiative fluorescence rate. Whether or not such a strongly absorbing acceptor chromophore influences the molecular fluorescence dynamics would depend on the precise location of the quencher on the chain, explaining the striking variation with time of the effect of electrothermal line width and intensity modulation as discussed in Figure 3a. In addition, it is conceivable that the actual lifetime of a particular quenching species can be reduced upon heating, which in turn would raise the chain’s quantum yield with increasing temperatures. Finally, it is also important to recall that chromophores undergo random spectral diffusion with time, changing their emission energy.3 They may be isoenergetic at one time, responding identically to a thermal perturbation; or drift apart so as to give rise to the spectral focusing effect shown in Figure 3c. Whether or not a correlation or an anticorrelation of intensity and line width with heating is observed therefore depends on the location of the quencher on the chain, the nature of the quenching species, and the energetics of the individual chromophores. These three parameters can vary with time. Scheblykin et al. recently offered time-resolved single chain luminescence spectroscopy as a route to studying the effect of transient quenchers at room temperature.39 Importantly, it was found that fluorescence quenching is not complete, so that the nonradiative rate in luminescence varies with time. A discrete on-chain quenching site could be identified through polarization-anisotropy measurements, which offer insight into the chain conformation.39,41 Future studies should involve spectrally resolved low-temperature fluorescence lifetime measurements to reveal the influence of a quencher on individual chromophores. It is anticipated that the single chromophore lifetimes will differ widely due to the distribution of single chromophore oscillator strengths, which in turn depend on the size of the π-system.19,43 In addition, measurements of fluorescence photon statistics could be employed to gain further insight 2687
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into the number of chromophores contributing to the single chain emission spectrum.44 Such studies have not previously been performed as a function of temperature. Periodic thermal perturbation of the cryogenic single chain fluorescence opens a new path to illuminating the interplay between chromophores in conjugated polymers, which is mostly covered in conventional spectroscopy at a static temperature.1,2,29 Analogies of these optical experiments exist to thermal effects revealed in single molecule electrical transport measurements, which are often interpreted in terms of miniscule conformational fluctuations in the molecule and its embedding matrix.16,17 Whereas chromophores are usually pictured as a result of static scissions in the π-electron system,4 we find that it is most appropriate to think of them as partially dynamic entities, which can be frozen in and thawed out reversibly through interactions with the surrounding matrix. The quantum yield of an organic semiconductor film generally decreases with rising temperature, as the excitons become more likely to diffuse to a quenching site.4,45 However, the single chain spectroscopy reveals that the opposite can also occur on the microscopic scale: the quantum yield of a single chain rises with temperature when interchromophoric coupling becomes such that excitations are lured away from the quenchers. The modulation technique opens up many exciting avenues to studying both the fundamental photophysics as well as other relevant material properties - such as thermal conductivity46 - on the single chain level. For example, one may expect heat transduction to depend sensitively on chain conformation. In polymers with strong spectral dependencies on conformation, such as polyfluorene,47 periodic thermal perturbation could provide more detailed insight into the conformational dependence of the underlying interaction of the polymer’s electronic system with the environment. Such microscopic insight is crucial to optimizing the photophysical stability of optoelectronic materials from the bottom up.
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Acknowledgment. The authors are indebted to Professor J. Feldmann for support and thank Professor U. Scherf for helpful discussions and the kind provision of the ladder-type polymer, as well as A. Helfrich and W. Stadler for technical assistance. This work was funded by the SFB 486 of the German Science Foundation. J.M.L. gratefully acknowledges support through a David & Lucile Packard Fellowship.
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DOI: 10.1021/nl101526p | Nano Lett. 2010, 10, 2683-–2689
