16140
J. Phys. Chem. C 2008, 112, 16140–16147
Energy Transfer Dynamics in a Series of Conjugated Polyelectrolytes with Varying Chain Length Lindsay M. Hardison,† Xiaoyong Zhao,‡ Hui Jiang, Kirk S. Schanze, and Valeria D. Kleiman* Department of Chemistry, UniVersity of Florida, GainesVille, Florida 32611-7200 ReceiVed: May 16, 2008; ReVised Manuscript ReceiVed: July 28, 2008
A time-resolved and steady state photophysical study of a family of conjugated polyelectrolytes (CPEs) with variable chain lengths (ranging from 8 to 108 polymer repeat units per chain) is reported. The CPEs investigated are poly(phenylene ethynelene)s substituted with two carboxylate groups per polymer repeat unit to provide water and methanol soluble conjugated polyelectrolytes. Steady state and ultrafast time-resolved fluorescence and anisotropy measurements were performed to explore the role of chain lengths on the energy transfer processes. We find that the CPEs aggregate under almost all conditions, with the degree of aggregation depending on the length of the conjugated polyelectrolyte chains. These CPEs are highly rigid and planar and present a very small loss of anisotropy during their emission lifetime. Introduction π-Conjugated polymers are an interesting class of materials with unique physical characteristics that make them excellent candidates for lasers,1 LEDs,2 photovolatics,3 and transistors.4 To be useful for any application, a fundamental understanding of their photophysical properties is necessary in order to continue to improve their efficiency and efficacy. In recent years, π-conjugated polyelectrolytes (CPEs) have been synthesized incorporating ionic solubilizing side groups enabling the polymer to be dissolved in water and other polar solvents while preserving the photophysical properties associated with the polymer backbone.5-7 In an effort to reduce exposure of the nonionic components to the environment, when CPEs are dissolved in a polar solvent such as water they self-assemble into aggregates due to the interaction between the charged functional groups and hydrophobic backbone.8-14 The degree of aggregation has been shown to depend on the charge density in the CPE,13 with the intra- or intermolecular π-π stacking creating new, red-shifted absorption and emission bands,15 decreasing the overall fluorescence quantum yield and competing with radiative emission processes from the isolated chains.16 Aggregates can also form in concentrated polymer solutions with nonpolar solvents.17-21 Addition of a metal cation such as Ca2+ acts as a cross-linking agent and it has been shown to induce aggregation in methanol, improving the amplified quenching properties of CPEs.15,22-24 In interchain interactions, π-electron density is delocalized among numerous conjugated segments in different polymer chains. Depending on the physical conformation of the chains, it is possible that the two interacting species be located on the same chain. If the polymer chains are very long, the conjugation segments from the same chain can interact spatially as a result of π-π stacking due to back folding forming excimers and or aggregates (excitedstate or ground-state interactions, respectively).24-27 Aggregate formations that can interact electronically will cause a significant change in the absorption spectra due to an elongation of the * Corresponding author. Tel: 352-392-4656. Fax: 352-392-0872. E-mail:
[email protected]. † Current address: Intel Corporation. ‡ Current address: University of California, Berkeley.
π-electron delocalization resulting in lower energy peaks compared to isolated chains. At room temperature, detection and identification of interchain interactions with fluorescence measurements is further complicated due to the large numbers of nonradiative trap sites in conjugated polymers resulting in very low emission quantum yields.24,25,28 Correct identification of the types of interchains species is important when considering charge transport and light emission applications of conjugated polymers. In order to fully understand a system and be able to make synthetic improvements it is necessary to characterize each of the species accordingly. Several studies have investigated the influence aggregates have on the kinetics within water soluble conjugated polymer systems.1,24,26,29-35 For example, Fakis et al. have shown that energy transfer from isolated poly(fluorenevinylene-co-phenylenevinylene) (PFV-co-PV)16 to aggregated chains is very rapid and efficient. They determined the isolated chain fluorescence, aggregate emission and energy transfer contributions to the overall decay, and the correlation between concentration and energy transfer efficiency. In most cases, conjugated polymer chains are not frozen in one conformation, instead they have a proclivity to twist and coil. A series of chromophores can be linked resulting in different degrees of π-electron delocalization depending on the planarity of the conjugated segments. Even if there are slight twists or bends along the polymer backbone, it is possible for the conjugation to not completely break, resulting in larger delocalization lengths.24,36 Just as in semiconductor nanoparticles, a particle-in-a-box model (1-D for polymers) is used to explain the delocalization of excitons along the polymer backbone. Conjugation lengths that are long have lower π f π* transition energies and vice versa.24 Longer conjugation lengths can be due to polymer rigidity which can create smaller shifts between the absorption and emission maximums (Stoke’s shift). We have reported previously the synthesis and characterization of a series of variable band gap poly(arylene ethynylene) (PAE) water soluble conjugated polyelectrolytes dissolved in methanol, water and methanol/water mixtures.5 Gap tunability within the visible region was attained by varying the repeat unit structure of the conjugated backbone and photophysical data
10.1021/jp804361w CCC: $40.75 2008 American Chemical Society Published on Web 09/23/2008
Conjugated Polyelectrolytes
Figure 1. PECO2Na repeat unit. PPECO2Na in methanol (blue) and water (green).
was collected to correlate the CPE side chain structure to the extent of polymer aggregation when dissolved in each solvent. In this work, we focus on the role aggregation plays in the intraand intermolecular energy transport in the conjugated polyelectrolyte carboxylated poly(phenylene) ethynylene (PPE-CO2Na) with varying polymer chain length. Photoluminescence from solutions with low concentrations of CPEs can be superlinearly quenched when placed in the presence of an oppositely charged electron- or energy quencher molecule (superquenching;1 amplified quenching5,37).9,10,12,22,23,27,28,38-40 This effect is not only due to ion-pairing between the polymer and quenchers22,28,39,41 as in typical Stern-Volmer kinetics, but also to inter- and intrachain energy transport mechanisms, i.e., random walk diffusion of the excitation energy along the polymer backbone,12,22,36,42-44 energy transfer between the polymer and quencher42 and energy transfer between the isolated polymer species and aggregated chains.8,11,43-46 Energy transfer is strongly dependent on the spectral overlap between the donor emission and acceptor absorption. If an aggregate inducer or quencher is added to the polymer solution, conformation changes can result in the spatial redistribution of several chromophores. This enables the excitation located on the polymer backbone to easily migrate to the quencher located at a particular site, lower in energy. Therefore, one quencher molecule can have the ability to reduce the emission from a large number of chromophores.22,46 Intrachain random walk, which leads to excitation migration toward the quencher molecule, is strongly dependent on the conjugation, polymer chain lengths and transition dipole orientations. Following absorption, the excited state “hops”/migrates from shorter (high energy segments) to longer (low energy ones) and depolarizes along the way, reducing the anisotropy value. The exciton will continue to funnel through the cascade of chromophores until it is either quenched (traps) or it reaches the lowest energy level where it can fluoresce or undergo nonradiative decay.36 In this article, we investigate the influence of polymer chain length, solvent and metal cations have on the ultrafast emission of a carboxylated poly(phenylene) ethynylene (PPE-CO2Na) shown in Figure 1. We explore the effect of polymer length by comparing results from CPEs with very short (very few polymer repeat units) and long polymer chains (many PRU). The level of aggregation and polymer rigidity is expected to vary with number of PRU and to have an influence on photophysical processes. Of particular interest is the energy transfer mechanism between isolated and aggregated chains within the PPE-CO2Na polymer. Experimental Methods Synthesis of Variable Chain Lengths of PPE-CO2-. The synthesis of these CPEs has been previously published.35 Briefly, to polymerize a stoichiometric mixture of 2,5-bis-(dodecyloxycarbonylmethoxy)-1,4-diiodobenzene and 1,4-diethynylbenzene a precursor route in which a Sonagashira coupling reaction is used to produce a poly(phenylene ethynylene) with a dodecyl
J. Phys. Chem. C, Vol. 112, No. 41, 2008 16141 ester protecting the carboxyl group. Gel permeation chromatography of the ester precursor polymers showed that the molecular weight (Mn) for the four polymer chain lengths investigated in this article are ≈ 5000, 24000, 74000 and 127000 g · mol-1 corresponding to average degrees of polymerizations (Xn) of 8, 35, 108, 185, respectively. The protected ester polymer precursor was then hydrolyzed with (n-Bu)4NOH to provide for the water-soluble conjugated polyelectrolyte PPE-CO2Na. The final polymer product was purified using dialysis against DI water for 4 days. All of the polymers have polydispersity indices of ∼ 2.35 Photophysical Methods. UV-visible absorption spectra were recorded using a Lambda 25 spectrophotometer from PerkinElmer. Steady-state excitation and emission spectra were obtained with a Fluorolog-3 spectrofluorometer from Jobin Yvon. A 1-cm square quartz cuvette was used for all steady state spectral measurements. The CPEs were dissolved in spectroscopic grade methanol with concentrations typically between 10 and 30 µM. Time-resolved anisotropy and fluorescence dynamics measurements were performed using a femtosecond upconversion apparatus. An optical parametric amplifier (OPA) pumped by a commercial Ti:Sa laser system consisting of a Ti:Sa oscillator (Spectra-Physics, Tsunami) and a subsequent Ti:Sa amplifier (Spectra-Physics, Spitfire) with a repetition rate of 1 kHz is used to produce excitation pulses. More specifically, the output of the Ti:Sa amplifier feeds an OPA, and the fourth harmonic of the signal is tuned to 375 nm. The excitation beam is fed through a prism compressor, yielding an instrument response function of 225 fs. The instrument response function (IRF) is determined by the cross-correlation of the excitation and gate pulses. The upconversion apparatus used for these experiments is described in detail elsewhere.29,30 Briefly, a fraction of the 800 nm Ti:Sa amplifier that is leftover from the OPA is used as a time delayed gate pulse (30 µJ/pulse). After excitation, the sample fluorescence is collected using a pair of off-axis parabolic mirrors and focused and spatially overlapped with the gate pulse in a nonlinear crystal (0.5 mm β-BBO), resulting in the sum frequency of the two electromagnetic fields (Figure 2A). Detection wavelength is chosen by tuning the nonlinear crystal to a particular angle. The resultant signal is then focused into a monochromator, detected with a photomultiplier and integrated with a boxcar integrator. When the gate pulse is temporally and spatially overlapped with the fluorescence signal, the nonlinear crystal behaves as an optical gate. Scanning the arrival time of the gate pulse with respect to the excitation pulses enables this optical gate to integrate different windows of time. The fluorescence signal is temporally mapped at these varying time delays (Figure 2B).29 The upconverted fluorescence signal intensity is determined by the convolution of the fluorescence and gate pulse intensities:
Isum(τ) )
∫-∞∞ Ifluo(t)Igate(τ - t)dt
(1)
where τ represents the time delay between the arrival of the gate pulse with respect to the sample fluorescence. This optical gating technique is very advantageous because the time resolution is dependent only on the width of the gate and pump pulses, not the detection system.29 The optical path length was 2 mm and the concentration of samples did not exceed 30 µM yielding an optical density of ∼0.45/mm. A homemade circulating cell was used to ensure that a fresh volume of sample was excited with every laser shot and a maximum of 100 nJ of excitation energy per shot were used to avoid photobleaching.
16142 J. Phys. Chem. C, Vol. 112, No. 41, 2008
Hardison et al.
Figure 2. Fluorescence upconversion technique. (A) Illustration of the upconversion principle. (B) Upconverted fluorescence signal is generated in a nonlinear crystal only while the delayed gate pulse is present.29
Time-resolved anisotropy measures the extent of polarization that a material maintains at a given time after being excited with polarized light. If the orientation of the absorption and emission transition dipole moments are identical, anisotropy will not be lost; however, if they differ, the anisotropy value will change with time. Anisotropy measurements become very useful tools to extract information concerning molecular size, shape and flexibility in addition to the viscosity of the solvent. More importantly, the temporal behavior of the anisotropy can provide useful information regarding the polarization loss mechanism.47 For relatively small molecules, molecular rotation is the primary source of loss of anisotropy. When the system under investigation is very large, rotational motion can become slower than the emission lifetime and time-dependent anisotropy becomes a useful tool to follow the scrambling of the transition dipole moment orientation due to energy transfer among chromophores with different orientations. Fluorescence anisotropy decay measurements were conducted by rotating the polarization of the excitation pulses with respect to a fixed polarization detection scheme. A Berek compensator (New Focus 5540) is used to excite the molecule with a beam polarized parallel and perpendicular with respect to the direction of polarization of the detected fluorescence or with the polarizations set to magic angle to measure isotropic (not influence by molecular rotation) fluorescence decay curves. The temporal evolution of the anisotropy is obtained by evaluation of
r(t) )
I|(t) - I⊥(t) I|(t) + 2I⊥(t)
(2)
where I| and I⊥correspond to the upconverted fluorescence of the polarization oriented parallel or perpendicular to the excitation beam polarization, respectively. Results and Discussion Steady State Characterization. Preliminary steady state photophysics of water soluble PPE-CO2Na polyelectrolytes of variable chain lengths has been previously reported.35 Figure 3 shows broadband absorption for CPEs with chain lengths of 8, 35, 108 and 185 PRU respectively. In each case, the broad bandwidths are due to a distribution of excitation energies resulting from absorptions of various segments with different conjugation lengths.48,49 A comparison of the different CPEs shows that as the length of the polymer chain increases the π f π* peak shifts toward longer wavelengths. The contribution from segments with longer conjugation lengths and the length of the longest segments increases and the absorption band
Figure 3. Absorption spectra of PPE-CO2Na in CH3OH. Polymer repeat units 8 (blue), 35 (green), 108 (black), and 185 (red).
becomes red-shifted.33 The absorption maximums for the 8 PRU and 35 PRU are 404 and 415 nm respectively. What appears as a shoulder on the absorption of the 35 PRU sample at 434 nm becomes the absorption maximum for the 108 and 185 PRU polymers and it is assigned to aggregated species. The π-π stacking leading to aggregation causes planarization of the chains and longer conjugation lengths; these lower energy segments absorb at longer wavelengths. Emission spectra of these CPEs present two distinctive bands: a high energy band, with clear vibronic structure which corresponds to isolated chain emission and a broader, low energy emission arising from aggregate states. Emission spectra do not display the similar red shift seen in the absorption, instead the fluorescence peak shifts are very small and decrease as the chain length is extended. The S0 r S1 (0-0) transition corresponding to the 35 PRU appears at 436 nm, shifted by ∼20 nm with respect to the absorption maximum observed at 415 nm. Meanwhile, the 185 PRU sample displays a shift of only 4 nm between the S0 r S1 (0-0) transition and the peak at the red edge of the absorption band (434 nm). A similar behavior has been observed in poly-(para)-phenylene-ladder-type (LPPP)50 in which the bridging present within the polymer prevents the phenyl rings to twist, maintaining conjugation. PPE-CO2Na polymers are geometrically rigid resulting in comparable shifts to the LPPP polymer. As the chain length increases the weight of the aggregate emission increases due to more aggregates present in solution. The conformational restrictions induced by
Conjugated Polyelectrolytes
J. Phys. Chem. C, Vol. 112, No. 41, 2008 16143
Figure 4. Normalized excitation spectra of 10 µM solutions in CPEs PRU for (a) 8 PRU in CH3OH, (b) 35 PRU in CH3OH, (c) 35 PRU in H2O, and (d) 35 PRU in CH3OH with ∼6 µM Ca2+. Detection wavelengths are 430 (blue), 475 (green), 510 (black), and 590 (red) nm. Notice that for detection at 430 nm, excitation spectra were collected only up to λ ) 420 nm.
this rigidity give rise to conjugation lengths longer than expected36,49,51ultimately reducing the degrees of freedom within the polymer. Emission from PPE-CO2Na is sensitive to solvent polarity and the presence of multivalent ions.52 Comparison of emission from the CPE with 35 PRU in CH3OH, H2O and in the presence of Ca2+ ions shows that changing environments leads to more or less aggregation. Ca2+ acts as an effective cross-linker with the 2 carboxyl groups inducing aggregation of the PPE-CO2Na15 and the emission from the aggregate can be identified as a broadband shoulder on the red side of the spectrum. When the CPE is dissolved in water, emission is observed mostly from the aggregated CPE. Overall, red shift, quenching and band broadening are due to aggregate formation of the polymer chains since Ca2+ is a closed-shell ion and does not act as an electron or energy acceptor.53-56 A small Stoke’s shift results in excellent overlap of the isolated chain emission with the aggregate absorption enhancing energy transfer from the higher energy isolated species to the lower energy aggregates. Despite the direct relationship between the increase in molar extinction values and the increase in repeat units, the quantum yield for fluorescence decreases from ∼0.6 (8 PRU) to ∼0.1 (108 PRU). Excitation spectra detected at different wavelengths is a more sensitive technique to detect the presence of mixtures of aggregates and isolated chains. Even though absorption spectra suggest that no aggregates are present in the shorter polymer samples (neither a shoulder nor broadening are observed) excitation spectra indicate that this is not the case. Figure 4 presents the excitation spectra of each CPE in methanol, detected at four different emission wavelengths (430, 475, 510 and 590 nm). Panel a (top left) shows the shorter polymer chains with only 8 PRU. Detection at 430 (blue line) and 475 nm (green line) show broad, featureless excitation spectra peaked at 396 nm. Upon shifting the detection wavelength to 510 nm the excitation spectrum becomes even broader
and a small red-edge shift begins to appear (440 nm). By shifting detection to 590 nm (red line), this red edge shift (due to the aggregate species) appears more pronounced. For PPE-CO2Na dissolved in methanol with a larger number of PRU per chain, the distinction between isolated and aggregate emission bands based on the excitation spectra becomes clearer. Panel b (bottom left) shows the excitation spectra of PPE-CO2Na with 35 PRU. Detection at 430 nm results in a distinct excitation peak at 390 nm. As the detection wavelength increases, the peak broadens and shifts to the red. Detection at 590 nm clearly shows a new peak at 430 nm and this peak is attributed to the direct excitation of aggregated species. Effects due to different solvation are observed in panel c, which presents the excitation spectra of the 35 PRU CPE dissolved in water. Detection at 430 nm shows broad, featureless excitation spectra peaked at 390 nm and due to isolated chains. As the detection wavelength increases, the peak broadens and shifts to the red. Upon shifting the detection wavelength to 510 nm the excitation spectra becomes even broader and a new peak appears (436 nm). Detection at 510 and 590 nm exhibit welldefined peaks at 436 nm, comparable to the very long chain (185 PRU) absorption spectrum observed in neat methanol (see Figure 3). A similar trend is observed after addition of 60% Ca2+ to the methanol solution (panel d). Changes within the excitation spectra indicate that the calcium induces more aggregation within the polymer solution. Contribution from aggregates is clearly evident when detecting at 475, 510 nm, and 590 nm. The excitation spectra results presented here provide evidence to support the presence of multiple species contributing to the detected emission, both in steady state and in time-resolved experiments. Even in dilute solutions of CPE with short polymer lengths some aggregate is present and this will become important when we look at their contribution to the overall energy transfer mechanism. A small amount of the short, rigid chains are likely
16144 J. Phys. Chem. C, Vol. 112, No. 41, 2008
Hardison et al.
TABLE 1: Results from Time-Resolved Fluorescence (See Text for Details) chain lengths (PRU) 8
35
108
35 with 50% Ca2+
detection amplitude amplitude λ (nm) τ1 ( σ (ps) (%) τ2 ( σ (ps) (%) 430 436 450 430 450 500 550 430 450 500 550 430 450 550
27 ( 3 14 ( 5
36 29
15 ( 1 37 ( 3 33 ( 5 35 ( 2 14 ( 2 43 ( 4 42 (fixed) 39 ( 6 15 ( 2 43 ( 5 26 ( 4
63 39 39 53 62 39 39 55 62 29 53
490 ( 10 624 ( 19 531 ( 7 180 ( 6 363 ( 10 402 ( 17 454 ( 16 201 ( 9 395 ( 11 333 ( 25 551 ( 49 430 ( 17 468 ( 11 611 ( 45
64 71 100 37 59 61 47 38 63 62 45 38 71 47
to stack on top of each other rather than cluster up creating a red edge shift. We mentioned before that fluorescence from the 35 PRU CPE in water arises mostly from aggregates. Excitation spectra confirm that it is present in both forms, as aggregated and as isolated chains although the isolated chain contribution to the overall steady state fluorescence is reduced due to fewer free chains in solution, and to energy transfer from isolated chains to aggregate states (see below). Time-Resolved Fluorescence. Fluorescence at Magic Angle. PPE-CO2Na solutions with polymer chain lengths equal to 8, 35, and 108 PRU in CH3OH were excited at 375 nm and their time-resolved fluorescence detected at magic angle at several different wavelengths. Data were fit with a sum of exponentials using the following equation:
I(t) )
()
∑ Aiexp τti i
(3)
where Ai represents the weight of each rate constant and τi is the associated time constant. Data from these fits are summarized in Table 1. Figure 5 shows time-resolved fluorescence decays of 8 PRU PPE-CO2Na- in CH3OH detected at three different wavelengths (430, 436 and 450 nm) after excitation at 375 nm. Detection at 430 nm shows a multiexponential behavior which disappears as the detection wavelength increases to 450 nm. The isolated chain time constant of 531 ps (Table 1) is extracted from the monoexponential decay of the 8 PRU. As the detection wavelength is increased from 450 to 550 nm (not shown) the behavior of the exponential decay does not change. At all detection wavelengths the rise times are comparable to our instrument response function.
Figure 5. Time-resolved fluorescence decay of PPE-CO2Na (8PRU) in CH3OH detected at 430 (blue), 436 (black), and 450 (red) nm. The inset shows early time decays. Black lines correspond to fits. (See text for details).
Figure 6. Emission of PPE-CO2Na with 35 PRU (blue) and 108 PRU (red) (30 µM in CH3OH) with fits (black) at various detection wavelengths (a) 430 nm, (b) 450 nm, and (c) 550 nm.
Figure 5 (inset) shows the same detection wavelengths on a shorter time scale. An extremely fast decay (∼0.7 ps) at the blue end of the spectrum (430 nm) is observed and its contribution to the overall signal diminishes as the wavelength increases from 430 to 436 to 450 nm. Emission at wavelengths redder than 450 spectrally overlaps with the aggregate species absorption resulting in efficient energy transfer from isolated chains to aggregates. Interestingly, detection at 590 nm does not yield a significant change in the risetime (not shown) or decay compared to 450 nm which had been expected if detecting emission from the aggregates. Dynamics observed at wavelengths between 430 and 450 nm exhibit an additional intermediate decay time of 15 to 45 ps. The amplitude of these time constants decreases as the wavelength increases. From the excitation spectra it is clear that when detection occurs at energies lower than 450 nm there is a small yet significant amount of aggregate in solution. These aggregates facilitate energy transfer hence, the intermediate time constant is assigned to the energy transfer from the ensemble of isolated CPE chains to the aggregated chains. Figure 6 shows a comparison of the time-resolved emission of two CPEs with different chain lengths, 35 and 108 PRU, at three different detection wavelengths (430, 450 and 550 nm). Detecting at 430 nm, the contribution from the subpicosecond component (inset) is larger for the 35 PRU sample compared to the 108 PRU sample. As the detection window is shifted to lower energies, contribution from this fast component to the overall signal is reduced. The changes are more pronounced on the 35 PRU than on the 108 PRU sample. Detecting at 550 nm, there is no contribution from the ultrafast component to the 35 PRU signal, but it is still present in the 108 PRU signal. The photoluminescence rise follows the response function of the experimental setup. At very long wavelengths one would expect to see a build up of population if the energy was transferred from isolated chains to aggregates.16,57 The absence of a slower rise indicates singlet exciton transfer from shorter (high energy) chains to traps within the isolated chains, not to aggregates. Intermediate and long decays times are also shown in Figure 6. Detection at 430 nm presents an intermediate decay constant of 10-15 ps and a long decay of 180-200 ps. For longer detection wavelengths, the intermediate time constant becomes∼ 35-40 ps while the long time decay changes to 350 and 500
Conjugated Polyelectrolytes
Figure 7. Photoluminescence from a solution of PPE-CO2Na (35 PRU, 30 µM, in CH3OH) with (red) and without (black) Ca2+. Detection wavelength is 430 nm.
ps. Results are summarized in Table 1. The qualitative trend observed for the intermediate component is the same for the 35 and 108 PRU though different from the 8 PRU sample. At detection wavelengths longer than 500 nm, the amplitude of the intermediate decay increases but the lifetimes do not change significantly. These time-resolved emission signals result from an ensemble of isolated CPE chains with segments with different conjugation lengths. After an initial energy transfer (100 PRU) and therefore many hops were possible before the exciton were emitted or trapped. In the new family of CPEs
16146 J. Phys. Chem. C, Vol. 112, No. 41, 2008 SCHEME 1
presented here, the rigidity and short chain length of the polymer is such that even if the exciton can undergo a few hops before being trapped (or emitted), the polarization is maintained. For the very short polymer chains (8PRU) there are not enough segments with different orientation to induce complete loss of anisotropy. Measurements in the longer PRU samples (35 PRU) showed a similar behavior, with an instantaneous small loss of polarization, followed by emission from polarized excitons throughout the lifetime of the emission. In addition to fewer hops, these CPEs are more rigid and the distribution of segment orientations must be narrower. Kinetic Model. Scheme 1 depicts a proposed model for the energy transfer steps in PPE-CO2Na. The 8 PRU consists of mostly isolated chains with a very small contribution from aggregates. After excitation at 375 nm, energy is transferred in an ultrafast time scale from the shorter conjugation lengths segments to traps located in the isolated chains. These detected emission signals are the convolution of decays from an ensemble of chains with segments with different conjugation lengths, and other nonradiative recombination pathways within the isolated species. From the excitation spectra of the 8 PRU (Figure 4) we can see that the red edge shift observed at 440 nm and interpreted as absorption from aggregates does not overlap significantly with segments whose emission will be detected at 450 nm or longer wavelengths. Under these conditions, energy transfer from the isolated chains to the aggregates is not efficient resulting in a monoexponential fluorescence lifetime from an ensemble of isolated chains. CPEs with longer chains (35, 108, and 185 PRU) contain a larger mix of isolated and aggregated chains. After excitation at 375 nm, energy is transferred from the shorter conjugation lengths to traps located in the isolated chains similar to the 8 PRU. Subsequently, energy transfer from this ensemble to the aggregate species is observed within a wide spectral range. The number of traps is increased upon addition of calcium resulting in a sharp increase in the ultrafast time component but the energy transfer to the aggregate states does not change significantly. Following the energy transfer, the detected emission is dominated by the ensemble of decays from isolated chains and traps located along the polymer backbone in addition to competing with nonradiative decay channels. In summary, upon excitation of the aggregates from energetically higher lying isolated chains, the fluorescence lifetimes result in multiexponential behavior due to the competition between the radiative and nonradiative decay. The integrated fluorescence collected for these experiments does show a low emission from aggregates but this emission cannot be observed in the ultrafast time-resolved experiments because of their very long decay time constants and small contribution to the overall signal. Conclusions The ultrafast time-resolved fluorescence of a series of PPECO2Na conjugated polyelectrolytes with varying chain length
Hardison et al. is presented. Using steady state UV-vis, photoluminescence and excitation spectra we distinguished the species present in each solution. It was shown that even dilute, short PRU chains exhibit small amounts of aggregation. The addition of calcium or using water as the solvent induces aggregation resulting in broad absorption/excitation spectra and the growth of a red shoulder in the emission. To investigate the influence aggregation has on the fluorescence of the polymers, we conducted a detection wavelength study using fluorescence upconversion. The isolated chain emission was extracted from the experiments with the shortest CPE chains under conditions of minimum aggregate absorption. In the presence of aggregates, an intermediate time constant on the order of 15 to 45 ps is observed and assigned to the energy transfer from the isolated to aggregate species. At shorter wavelengths, an ultrafast decay (
