Light-Harvesting Action Spectroscopy of Single Conjugated Polymer

NANO LETTERS

Light-Harvesting Action Spectroscopy of Single Conjugated Polymer Nanowires

2008 Vol. 8, No. 10 3330-3335

Manfred J. Walter, Nicholas J. Borys, Kipp J. van Schooten, and John M. Lupton* Department of Physics, UniVersity of Utah, Salt Lake City, Utah 84112 Received June 18, 2008; Revised Manuscript Received August 8, 2008

ABSTRACT We study exciton migration in single molecular nanowires, dye-endcapped multichromophoric conjugated polymers, as a function of excitation energy. This approach reveals the actual molecular absorption properties, uncovering the molecules within an ensemble and the chromophores within a molecule which contribute to absorption at a given wavelength. As the excitation energy is raised, an increasing number of polymers exhibit energy transfer suggesting that, in contrast to the emission spectrum, the absorption of a single chain under energy transfer conditions can be very broad even at 5 K. At the same time, the polarization anisotropy in excitation decreases due to an increase in the number of noncolinear chromophores involved in absorption. Power and wavelength-dependent measurements clearly discern the exciton blockade effect that gives rise to strong fluctuations of energy transfer. Although the polymer and endcap constitute nominally discrete spectroscopic entities, we are able to identify a subtle influence of the primary backbone exciton energy on the ultimate endcap emission. This demonstration of interchromophoric cooperativity provides a direct realization of how nonradiative energy dissipation in one nanoscale unit influences the spectroscopy of another.

Photoaction spectroscopy, the wavelength dependence of photoconductivity, has been a powerful tool to unravel charge generation and transport in molecular semiconductors, providing insight into the properties of higher-lying states.1-5 Although a microscopic understanding of light harvesting is crucial to improving photovoltaic applications, little is known about the excitation energy-dependent activity of the primary photoexcitation, the exciton. Conjugated polymers are unique model systems to study the subtleties of light harvesting in multichromophoric assemblies. There are major differences between intra- and intermolecular energy transport phenomena; on average, that is, in the ensemble, the latter occurs on shorter timescales.6-9 Whereas ensemble measurements foremost provide insight into the occurrence of intermolecular energy transport phenomena,10 the true diversity of intramolecular couplings11,12 between chromophores is not revealed until the single molecule level is accessed spectroscopically.13-31 Developing a microscopic picture of intramolecular energy transfer on the experimental level is crucial to formulating a complete theoretical framework, which is intrinsically microscopic in nature.6,8,32-35 Besides the appealing intellectual challenge, light harvesting in molecular solids is vital to the operation of a variety of devices,36,37 ranging from color-tunable light-emitting diodes, over solar cells, luminescent solar concentrators, and lasers to novel biochemical sensors. * Correspondingauthor.E-mail:[email protected].+18015816408. Fax +18015814801. 10.1021/nl801757p CCC: $40.75 Published on Web 09/11/2008

 2008 American Chemical Society

Conjugated polymers are generally thought of as extended π-electron systems, interrupted at irregular intervals by topological or chemical defects to form more or less discrete chromophoric entities.38 Excitation energy can then be passed along the polymer chain through resonant dipole-dipole coupling between adjacent chromophores. This process is understood in terms of internal conversion preceding energy transfer, so that dipolar coupling occurs between vibrationally relaxed S1 excited states.10,18,38 As the energy difference between the absorbed and the emitted photon can be quite substantial, sometimes in excess of 1 eV, internal degrees of freedom such as structural or vibrational relaxation must dissipate a large amount of energy on ultrashort timescales. In the conventional picture of relaxed excitons, this energy dissipation within the molecule is not thought to perturb the spectroscopy of the ultimate emissive state, not least due to the inherent stability of the primary photoexcitation, the tightly bound exciton.1,2,5,38 To address the question of the role of intramolecular relaxation in energy transfer, we investigated the excitation energy-dependent light-harvesting properties of a model conjugated polymer system, a rigid blue-emitting polyindenofluorene (PIF) capped with red-emitting perylene dyes at either end (structure shown in Figure 1a, n ∼ 50-100).6,33,39,40 A detailed discussion of molecular weight, chain length and polydispersity is given in ref 39. By changing the excitation wavelength and monitoring the emission from the endcap, we can track the molecules and chromophores involved in the light absorption process and consequently the intra-

Figure 1 displays fluorescence microscope images of the same sample region under excitation at different wavelengths. The absorption of the backbone and the endcap are well separated spectrally, so that the two species can be excited independently.39 Here, we consider the case of energy transfer, where the incident radiation is absorbed by the backbone and only the emission of the endcap is monitored through a 545 nm long-pass filter. Panel b shows the spatial density of emitting endcaps under excitation at 410 nm. As the excitation is tuned to longer wavelengths, more and more single molecules disappear from the images (panels c-f). The highest density of spots is observed under direct excitation of the endcap at 530 nm (panel g) due to the fact that approximately 80% of all chains do not display energy transfer to the endcap.39 Remarkably, we find that if a particular molecule absorbs at long wavelengths it will also absorb at shorter wavelengths, suggesting that the single chain absorption spectrum is very broad. An example of this effect is indicated by the molecule circled red in the images. Because of the light-harvesting properties of the polymer, direct excitation of the endcap in panel g leads to a lower emission intensity per molecule.39

Figure 1. Single chain photoluminescence (PL) excitation imaging at 5 K. Only the emission of the endcap is considered. (a) Structure of the perylene-endcapped polyindenofluorene studied. (b-f) Fluorescence microscope images of the same molecules for different excitation wavelengths, shown on the same intensity scale at a constant excitation power of 2 mW. (g) Direct endcap excitation reveals the highest molecular density (intensity scale reduced by a factor of 2.5). (h) The density of observable molecules (red) characterizes the probability of exciting the polymer backbone and transferring the energy to the endcap. This probability scales directly with the (solution) absorption spectrum39 of the polymer backbone (black).

molecular light-harvesting trajectory. Whereas energy transfer coupling between the PIF and the endcap is, on average, inefficient,6 we recently demonstrated that approximately one in five molecules exhibits near unity coupling efficiency.39 It is these single molecules displaying highly efficient light harvesting that we focus on in the following. The polymer was dispersed in a Zeonex matrix in toluene solution (5 mg/ ml) at a concentration of ∼100 ng/ml, spin-coated on a quartz substrate, and mounted in a coldfinger helium cryostat at ∼5 K. A tunable, frequency-doubled Ti:sapphire laser (140 fs pulses at 80 MHz repetition rate) provided linearly polarized excitation in the range from 360 to 530 nm in widefield illumination. The single molecule fluorescence was collected by a long working distance microscope objective (0.55 numerical aperture), dispersed in a 50 cm spectrometer and detected by a cooled, back-illuminated charge-coupled device camera. Nano Lett., Vol. 8, No. 10, 2008

While the average emission intensity per molecule remains approximately constant, the number of molecules detected as a function of excitation wavelength corresponds directly to the ensemble absorption as shown in panel h for a total of 1698 single chains. It is not the overall strength of the single molecule absorption that scales with the ensemble absorption, but the probability of being able to observe energy transfer in a single chain. The shorter the excitation wavelength, the more polymer chains have a chromophore that overlaps spectrally with the excitation. The actual absorption process itself is more of a digital nature; either the polymer chain absorbs, or it does not. Considering that we only probe a specific subset of the polymer-endcap system (displaying efficient energy transfer), it is remarkable that the single chain PL excitation spectroscopy agrees so well with the ensemble solution absorption. On the one hand, we expect the individual chromophore absorption to be much narrower than the ensemble absorption at 5 K, as has been demonstrated in high-resolution PL excitation spectroscopy of single MEH-PPV chromophores.41 On the other hand, we are monitoring the particularly interesting situation of efficient energy transfer from the backbone to the endcap. From considerations of the polymer chain length, we expect between ∼5-10 chromophores per chain.6,33,39,42 If the individual chromophore absorption were indeed narrow, one should observe molecules moving in and out of resonance as the laser wavelength is tuned. The single PIF chain line width in emission provides an upper bound for the absorption line width of a chromophore that does not exhibit energy transfer: roughly 10 meV at 5 K.39 Such a small line width would lead to a significant change of the subset of chromophores involved in absorption as the laser wavelength is tuned, in conflict with observation. We propose that the PL excitation experiment may be dominated by the photophysics of the final chromophore in the energy transfer cascade, the polymer unit linked to the endcap. The polymer 3331

coupling to the endcap is presumably purely electronic in nature, that is, of through-bond character, distinguishing it from the other chromophores on the chain. Efficient polymerendcap coupling is a prerequisite for detecting intrachain transport as we are doing here. A helpful analogy to these photonic wires20,43 is the intrinsic barrier in an electronic device between a metal contact and a semiconductor. If the polymer backbone and the endcap are efficiently coupled, one may liken the situation to an ohmic contact: excitation energy will always flow, independent of the “bias applied” or the excess energy given to the excitation on the backbone. In the case of an “injection contact”, one would expect the light-harvesting efficiency to depend on photon energy, and individual molecules should shift in and out of resonance in analogy to a single molecule tunnel junction.44 In effect, light harvesting in conjugated polymers appears to be “ohmic”. Although the PIF polymer is nominally rather rigid, individual chromophores on the chain can differ significantly in orientation, leading to a reduction in polarization anisotropy in excitation.39,40 If shorter excitation wavelengths imply a greater probability of exciting a single chain that exhibits energy transfer, we also expect more and more chromophores to be involved in the absorption process as the wavelength is reduced. Following ref 45, the emission intensity I(R) was recorded for at least two full rotations of the laser polarization angle R yielding I(R) ∝ 1 + M cos 2(R - φ), where M denotes the polarization anisotropy and φ is the phase angle of each individual molecule. M ) 1 describes a fully extended chain, whereas M ) 0 corresponds to a molecule that can absorb light of all polarizations, that is, is strongly deformed. Figure 2 plots histograms of the polarization anisotropy M at three different excitation wavelengths, recorded for 100 molecules at each wavelength. In all cases, the backbone was excited and the emission from the endcap was detected. As the excitation wavelength is reduced, more chromophores, whose emission energies scatter statistically,15,39 absorb radiation of a given polarization. The polarization anisotropy decreases as the higher states of the inhomogeneously broadened absorption spectrum are probed.38 As this is a strictly statistical process, the decrease in anisotropy is witnessed in a shift of the histogram to lower M values at shorter excitation wavelengths. As the excitation energy is increased, more nonparallel chromophores participate in the absorption process and the overall length of the energy transfer cascade consequently increases. Combined with the results for the wavelength dependence in Figure 1 we conclude that it is the endcap coupling which foremost limits light harvesting, not energy transfer along the polymer backbone. As long as the last chromophore in the polymer chain is coupled strongly to the endcap, light harvesting also occurs from other chromophores on the polymer chain. The involvement of multiple chromophores implies that excitation energy is passed between individual chromophores before it reaches the fluorescent endcap. It has previously been reported that such intramolecular light harvesting can lead to strong fluorescence intermittency23,24,39,46-49 as the 3332

Figure 2. Wavelength dependence of light harvesting manifested in the polarization anisotropy in excitation. As the excitation wavelength is reduced, it becomes possible to address more chromophores within a single polymer chain, thereby lowering the polarization anisotropy in excitation. Gaussian curves are overlaid as a guide to the eye.

temporary deactivation of a chromophore, for example by the formation of a charge transfer state, promotes strong quenching of the excitation energy.50 This intermittency, studied in depth by De Schryver et al., has been termed “exciton blockade”23,51,52 in analogy to Coulomb blockade in mesoscopic systems, implying that no two excited-state species (charge transfer state and exciton) can pass a chromophore at once. The effect is distinct from conventional exciton-exciton annihilation31 as it does not necessarily display a nonlinear excitation intensity dependence. In the PIF light-harvesting system, it is not immediately obvious whether the difference in fluorescence intermittency of the endcap between direct and energy transfer excitation39 simply results from a difference in absorption strength and excitation density, as an increase in photon cycling rate promotes blinking.48,50 Figure 3a displays emission time traces of two single endcapped polymer chains, which were excited at two different wavelengths (backbone excitation, left, versus endcap excitation, right). The power dependence was studied on one and the same single chain (low powers, top, versus high powers, bottom). Only in the case of energy transfer does the fluorescence intermittency increase strongly as the power is raised 5-fold. The traces for 180 molecules for each case are analyzed systematically by plotting a histogram of the standard deviation normalized to the average emission intensity of a trace (a measure of fluorescence intermittency) in panel b. The spread of the histogram for high powers is significantly narrower for direct endcap excitation than for the energy transfer case, confirming that fluctuations in the endcap emission are not merely due to higher excitation densities. Nano Lett., Vol. 8, No. 10, 2008

Figure 4. Scatter of the endcap luminescence spectra at 5 K under direct and energy transfer excitation. (a) Example spectra; (b) plot of the transition line width versus peak energy for a total of 103 (direct excitation, red) and 112 (indirect excitation, blue) molecules. Note that the perylene emission is cut off by a filter above 2.27 eV under direct excitation. Figure 3. Excitation power dependence of single molecule endcap emission under different excitation conditions: left, backbone (400 nm); right, endcap (510 nm). (a) Examples of endcap emission dynamics at two different powers. (b) The fluorescence intermittency of a single time trace can be quantified by the normalized standard deviation of a blinking trace. Histograms are shown for 180 molecules each for the two excitation conditions.

As above, it appears to be the condition of efficient coupling of the backbone to the endcap which is the key issue in controlling light harvesting. We were not able to identify a significant dependence of intermittency on excitation wavelength between 380 and 420 nm. We therefore conclude that the excess energy deposited on the backbone at shorter excitation wavelengths does not immediately influence exciton quenching. Exciton quenchers arise as a consequence of photoexcitation, independent of the amount of excitation energy dissipated nonradiatively within the molecule, although we note that at even shorter wavelengths exciton fission events may promote the formation of charge separated states. Ultimate verification of a (lack of a) correlation between excitation and emission, in effect, “site” selective fluorescence spectroscopy38 on a single emitter, is not trivial as the endcap exhibits a vast range of emission characteristics. Figure 4a displays four examples of endcap emission under direct (510 nm) and indirect (400 nm) excitation. The spectral line width scatters by over a factor of 20. Panel b plots line width against peak energy for direct (red) and indirect (blue) excitation. The graph suggests a rather surprising grouping of emission linewidths, scattering around 60 and 15 meV, but provides no indication of a correlation between transition line width and peak energy, in contrast to recent observations in some polymeric materials.53 No significant difference is observed in the spectroscopy of the endcap under the two excitation conditions, although indirect excitation requires a rapid internal energy dissipation of ∼1 eV per photon emitted. Considering that the dominant spectral broadening Nano Lett., Vol. 8, No. 10, 2008

mechanism in the emission of single molecules lies in random spectral fluctuations (spectral diffusion),15,53-55 one would anticipate that the endcap emission under direct excitation is generally narrower than under energy transfer excitation. This is evidently not the case; in contrast, for example, to the situation for single quantum dots, where the line width scales with the excitation photon energy.56 Although the two measurements of direct and indirect excitation probe different subsets of molecules (direct perylene excitation also reports on molecules which are weakly bound to the backbone and do not exhibit energy transfer), there is no systematic difference in terms of the endcap spectroscopy. We conclude that spectral diffusion andothersourcesofspectralbroadening,suchaselectron-phonon coupling,57 correlate neither with the energy transfer efficiency nor with the total amount of energy dissipated nonradiatively in the molecule. As a variety of potential spectral broadening mechanisms exist, the quest to uncover an influence of excess energy dissipation on endcap emission must be pursued on one single molecule at a time. The experimental difficulty lies in the fact that the endcap itself displays significant spectral diffusion, random spectral fluctuations with time. The histogram of 81 line width changes (∆Γ) observed from one spectrum to the next under 30 s integration time, compiled for a total of 20 single molecules, is shown in Figure 5a. The distribution is symmetric, demonstrating that random line width changes occur in either direction. In order to identify an influence of excitation energy on emission, we need to repeatedly vary the excitation wavelength for one single molecule, monitor changes in the emission characteristics of this molecule (peak position and line width), and draw statistics from a collection of several single chains. The inset in Figure 5b indicates the general trend observed with two example endcap spectra, one taken at short excitation wavelength (370 nm, blue curve), the other at longer 3333

Figure 5. Influence of backbone excitation on the endcap emission. (a) Histogram of line width changes ∆Γ from one spectrum to the next, compiled for 20 molecules excited at 400 nm. (b) Switching excitation wavelength tends to lead to a small spectral shift in the endcap emission, opposed to the shift in excitation and accompanied by a line width change. Two example spectra of one and the same polymer chain are shown in the inset. The ∆Γ histogram for 29 single molecules exhibiting such a spectral shift is shown (light blue, switch between backbone and endcap excitation; dark blue, change of backbone excitation wavelength. Note that for a blue shift in excitation accompanied by a red shift in emission, negative ∆Γ values in panel b correspond to a spectral broadening). (c) A simple two-level system model of the influence of backbone excitation on endcap emission. For low energy excitation, only one endcap configuration exists (left). Higher excitation energies lead to increased excess energy dissipation on the backbone (right), inducing random fluctuations between two endcap configurations. This gives rise to increased spectral diffusion, leading to spectral broadening and a spectral red shift.

wavelength (420 nm, red curve). Whenever a blue shift in excitation leads to a red shift in emission, the shift is accompanied by spectral broadening: the more energy is dissipated on the backbone, the broader the endcap emission spectrum. Panel b shows a systematic investigation of the influence of excitation energy change on the emission line width change ∆Γ by plotting a histogram of ∆Γ for cases where the wavelength shift in emission is opposed to that in absorption. The link between endcap emission and backbone excitation can be rationalized by a simple two-level system model as sketched in Figure 5c. The occurrence of spectral diffusion in molecular compounds is often interpreted in terms of transitions occurring between different local energetic minima.28,54,55,58 Excitation at low photon energy only probes the emission from one local minimum. As the excitation energy is raised, the higher photon energies lead to the creation of a further, lower-energy potential minimum 3334

(dashed curve), for example, by polarization of the environment of the emitting molecule. A random averaging over emission from these different states gives rise to both spectral broadening and an apparent red shift in emission. Lowering the photon energy in excitation then leads to a blue shift in emission accompanied by spectral narrowing. The photophysics of organic semiconductors is generally interpreted in the framework of thermalized tightly bound excitons.5,38 In a photoluminescence excitation experiment, one would not usually expect to observe a wavelength dependence of the emitting species. The present case of endcapped polymers is rather intriguing: as the backbone excitation energy is varied, the energy difference is deposited on the backbone. The energetic gap between backbone and endcap, the energy dissipated in the final step of energy transfer, remains virtually constant. Evidently, increased energy dissipation on the polymer backbone drives spectral diffusion of the spatially and energetically remote endcap. Consequently, the time scale for thermal equilibration is smaller or comparable to that of intrachain energy transfer (j100 ps39), which clearly relates to recent observations of intramolecular thermalisation in peptides59 and alkane chains.60 This observation is significant in terms of understanding the thermodynamics of nanoscale systems in general, such as nanoparticle aggregates employed for surface-enhanced Raman scattering or coupled quantum dots explored in the context of quantum computing. Although the absorbing and emitting units on the polymer chain may be separated by tens of nanometers in the present case,6 energy dissipation in the absorber can significantly influence the overall energetics of the emitter. This form of remote interchromophoric coupling offers an additional conceptional subtlety to developing a complete microscopic understanding of intramolecular interactions in macromolecular aggregates. We have demonstrated photoaction spectroscopy of exciton migration in single polymer nanowires. The overall polymer-endcap donor-acceptor coupling appears to be dominated by the coupling strength of the last chromophore in the chain to the endcap. The technique provides access to relaxation processes in coupled nanoscale systems, revealing that it is inappropriate to picture polymer and endcap as strictly isolated systems; nonradiative energy dissipation on the backbone perturbs the endcap emission. Acknowledgment. The authors are indebted to Professor K. Mu¨llen, Professor A. Grimsdale, and Dr. S. Satayesh for the kind provision of the endcapped PIF polymer and would like to thank Dr. F. Schindler for helpful discussions and assistance with setting up the single molecule microscope. Funding by the Petroleum Research Fund (Grant 46795) and the National Science Foundation (Grant CHE-ASC 748473) is gratefully acknowledged. References (1) Arkhipov, V. I.; Emelianova, E. V.; Ba¨ssler, H. Phys. ReV. Lett. 1999, 82, 1321. (2) Barth, S.; Ba¨ssler, H. Phys. ReV. Lett. 1997, 79, 4445. (3) Ko¨hler, A.; dos Santos, D. A.; Beljonne, D.; Shuai, Z.; Bre´das, J. L.; Holmes, A. B.; Kraus, A.; Mu¨llen, K.; Friend, R. H. Nature 1998, 392, 903. Nano Lett., Vol. 8, No. 10, 2008

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