Single-Molecule Spectroscopy of MEH-PPV Polymer Molecules in

Jun 8, 2009 - Florian A. Feist, Giovanni Tommaseo, and Thomas Basché*. Institut für Physikalische Chemie, Johannes Gutenberg-UniVersität, 55099 Mai...
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Single-Molecule Spectroscopy of MEH-PPV Polymer Molecules in Different Host Matrices† Florian A. Feist, Giovanni Tommaseo, and Thomas Basche´* Institut fu¨r Physikalische Chemie, Johannes Gutenberg-UniVersita¨t, 55099 Mainz, Germany ReceiVed: February 27, 2009; ReVised Manuscript ReceiVed: April 21, 2009

Fluorescence emission and excitation spectra of single MEH-PPV molecules dispersed in three different host polymers (PMMA, PS, and Zeonex) have been recorded at 1.2 K. We observed only minor effects of the host matrix on the following parameters: the ratios of single-chromophoric to multichromophoric emission, the widths of the distributions of emission maxima, and the (generally very low) fraction of emission spectra with sharp zero-phonon lines. The differences are tentatively attributed to different conformations of MEHPPV chains, subtle variations in local chromophore-matrix interactions, and/or different distributions of conjugation lengths of emitting chromophores, respectively. Using excitation spectroscopy, in all samples we found narrow zero-phonon lines, providing an upper limit for the homogeneous line width and indicating that in each of the host polymers nearly lifetime limited widths can be reached at low temperatures. While our investigations point to some influence of the host polymer on the chain conformations, the latter seem to be of only minor importance for those photophysical properties of the chromophores of individual MEH-PPV chains studied herein. 1. Introduction Conjugated polymers have become a field of vivid scientific interest ever since electroluminescence was discovered in these materials.1 According to our present understanding, they can be regarded as multichromophoric systems2 which are characterized by rich photophysical dynamics often related to electronic coupling between the different chromophoric units.3 Optical spectroscopy has provided profound insights into the photophysics of the widely used poly(2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene-vinylene) (MEH-PPV) studied herein, both on the ensemble4-9 and on the single-molecule level.10-21 A number of spectroscopic signatures in single-chain studies (e.g., intermittent fluorescence22-24 or shape and spectral position of emission spectra23,24) are thought to crucially depend on the chain conformation due to its effect on the efficiency of intramolecular energy transfer. Taking advantage of this fact, chain conformations have been controlled by varying polarity of solvents22,23 or molecular weight and polarity of the host matrix.24 Consequently, the spectroscopic properties of conjugated polymers are considered to be very sensitive to the details of experimental conditions and sample preparation. This fact probably accounts for a number of apparent discrepancies reported in the literature.15,19,20,25 The choice of the host matrix in which the conjugated polymer molecules are dispersed is one parameter that distinguishes single-molecule experiments conducted by different groups. Experiments have been performed with MEH-PPV embedded e.g. in poly(methylmethacrylate) (PMMA),15,21,25-30 polystyrene (PS),10,15,16,19,20 Zeonex,31 and polycarbonate32 and with cap layers of poly(vinyl alcohol)11,13,18 or poly(vinyl butyral).22,23 While the choice of the host matrix has been shown to influence the chain conformation of the conjugated polymer,24,33 other host matrix-induced effects on the photophysics of MEHPPV are hardly specified, so far. †

Part of the “Hiroshi Masuhara Festschrift”. * Corresponding author. E-mail: [email protected]. Telephone: + 49 (0) 6131 39 22707. Fax: + 49 (0) 6131 39 23953.

When conducting single-chain experiments at very low temperature, such investigations are most profitable if one succeeds in observing sharp zero-phonon lines (ZPLs) of the vibronic transitions of the chromophores along the chain, because the widths of the ZPLs, in principle, contain important information on the population and coherence decay of electronic excitations not easily accessible by other means.34 In particular, such information may be extracted for individual chromophores in the conjugated polymer, leading the way to determination of energy transfer rates between chromophores. Therefore, it is an important endeavor to find out under which conditions ZPLs prevail. Their availability depends on extrinsic parameters such as temperature and intrinsic parameters such as the strength of electron-phonon coupling. With regard to temperature, we could recently demonstrate by comparing single-molecule spectra at different temperatures26 that ZPLs of MEH-PPV in PMMA typically have disappeared at 20 K. The fraction of polymer molecules in which ZPLs could be observed even at temperatures as low as 1.2 K, however, was small, which has been attributed to the combined action of electron-phonon coupling and spectral diffusion.26 For a given combination of the dopant (i.e. the conjugated polymer) and the host polymer, the situation is difficult to improve. In another study of MEHPPV in polystyrene (PS), the observation of ZPLs was reported to be the typical case,19,20 in strong contrast to our results in PMMA.26,27 So far, it is unclear if this difference is caused by different properties of the host matrices (e.g., weaker electronphonon coupling or less common spectral diffusion in PS) or if other possible reasons, such as different MEH-PPV sources, have to be taken into account. Here, we present a study in which results obtained by fluorescence emission and excitation spectroscopy of single MEH-PPV molecules at 1.2 K embedded in three different host polymers (PMMA, PS, and Zeonex; see Chart 1 for chemical structures) are compared. To avoid ambiguities caused by sample preparation as far as possible, the same MW ) 50-60 kDa fraction of MEH-PPV was used for the preparation of the host/guest polymer films. The basic photophysical properties

10.1021/jp901816q CCC: $40.75  2009 American Chemical Society Published on Web 06/08/2009

MEH-PPV Polymer Molecules in Different Host Matrices

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CHART 1: Chemical Structures of MEH-PPV and the Three Host Polymers Studied

studied proved to be very similar in the three different matrix environments, suggesting that the spectroscopic behavior of MEH-PPV chains can only be controlled to a minor extent by the choice of the host polymer. We observed single-chromophoric as well as multichromophoric emission with ratios depending on the host polymer. The influence of the host also appeared in the widths of the distribution of the maxima of emission spectra. Furthermore, the fraction of emission spectra with sharp ZPLs, while generally being very low, varied in the three samples studied. In excitation experiments, ZPLs of laser line limited width (∼0.1 cm-1) were found for all samples. 2. Experimental Details MEH-PPV with an averaged molar weight of MW ∼ 200 kDa and large polydispersity (PD ) 5) was purchased from Aldrich. Using gel permeation chromatography, a fraction of MW ∼ 50-60 kDa and PD ) 1.1-1.3 was chosen for the singlemolecule experiments. The MEH-PPV was first completely dissolved in tetrahydrofuran (THF), and afterward appropriate concentrations were prepared by dilution with toluene containing poly(methylmethacrylate) (PMMA) at a concentration of 20 g/L, toluene containing Zeonex 330R at a concentration of 6 g/L, or toluene containing PS at a concentration of 12 g/L. From these solutions, thin films (thickness: ∼ 100 nm) were spin-coated onto thoroughly cleaned glass cover slides. During and after the spin coating process, the samples were constantly kept under argon. The sample to be investigated was transferred into an optical cryostat under a constant argon stream and kept under vacuum conditions. For the measurements at 1.2 K, the sample was immersed in superfluid liquid helium. The fluorescence imaging and spectroscopy of single polymer molecules was conducted using a home-built confocal microscope.35 The fluorescence emission spectra were taken with an excitation wavelength λexc ) 488 nm and a laser intensity of 270 W/cm2 to 4 kW/cm2. All emission light transmitted by a 500 nm long pass filter was collected. A spectrograph equipped with a LN2 cooled CCD camera was operated in low (150 grooves/mm grating; resolution 20 cm-1) and high (1800 grooves/mm; resolution 2 cm-1) resolution mode. The fluorescence excitation spectra were recorded by using a tunable, linearly polarized ring dye-laser (line width ∼ 0.1 cm-1) operated with pyrromethene 546. While scanning the dye laser across its tuning range (525-545 nm), all emission light transmitted by a 555 nm long pass filter was collected with an avalanche photodiode (APD). The excitation spectra were taken with an intensity of ∼5-150 W/cm2.

Figure 1. Emission spectra of single MEH-PPV chains (λex ) 488 nm) exhibiting different numbers of emitting chromophoric units: (a) one chromophore; (b) two chromophores; (c) three chromophores. In part d, the exact number of emitting chromophores is not determinable, due to their strong spectral overlapping. All examples are taken from the Zeonex sample but are also representative for the other samples. (λex ) 488 nm, Iexc ) 2 kW/cm2, tint ) 30 s).

3. Results and Discussion In previous investigations of MEH-PPV at low temperature, it has been concluded that the multitude of chromophores present in a single conjugated polymer chain gives rise to a large variety of emission spectra,15,26,27,36 to be analyzed in terms of singlechromophore or multichromophore emission. Accordingly, fluorescence from single chains is thought to originate from several or even only one lowest energy chromophore.15 The same conclusions can be straightforwardly drawn for the data shown in Figure 1. While these examples have been chosen from the Zeonex sample, the appearance of the spectra is also representative for the other samples. The spectrum displayed in Figure 1a is assigned to fluorescence of one particular chromophore. In this case, it is assumed that the multitude of chromophores excited by the laser funnel their electronic excitation energy to one single low-energy chromophore, which then relaxes to the ground state through fluorescence. This behavior could only be found for a minor fraction of investigated molecules (42% PMMA, 31% PS, 21% Zeonex; see Table 1). In the majority of cases, either certain chromophores could not transfer their excitation energy despite the presence of lowerenergy sites on the chain (e.g., for orientational reasons) or even different “energy funnels” were present on a single polymer chain.10 In both cases, this would give rise to multichromophoric emission: the superposition of two or more one-chromophore spectra. Examples for this behavior are shown in Figure 1b-d, exhibiting two, three, or an undeterminable number of emitting chromophoric units, respectively. While the basic observation of single-chromophoric and multichromophoric emission could be made in all samples, the different ratios in the PMMA, PS, and Zeonex samples (see Table 1) indicate some influence of the host matrix. It has been shown that conjugated polymers to a certain extent retain chain conformations adopted in solution during the spincasting process.22,23 All samples investigated in the present study have been spin-cast from toluene solutions and, hence, are likely to possess a rather coiled conformation.22,23 In addition, there are also indications for an influence of the polymer host on chain morphology.24 Accordingly, one possible explanation for the

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TABLE 1: Comparison of Experimental Results between MEH-PPV Samples in Different Host Polymers peak emission distribution PMMA PS Zeonex a

maximum/cm-1

width/cm-1

fraction of single chromophore emission

ZPLs in emissiona/ minimum width

ZPLs in excitation/ minimum width

18250 ( 10 18250 ( 30 18150 ( 20

850 ( 20 1050 ( 70 1350 ( 50

42% 31% 21%

12%/3 cm-1 5%/-c 13%/4 cm-1

6%/0.1 cm-1 b 5%/0.1 cm-1 b 13%/0.1 cm-1 b

Fractions related to the total number of chromophores. b Resolution-limited. c No high resolution emission spectra taken.

different average numbers of emitting units seems to be related to this morphology: A higher degree of multichromophoric emission could be caused by chain conformations less favorable for efficient energy transfer to the lowest-energy site(s). If this perception is correct, less frequent occurrence of singlechromophoric spectra would hint to less collapsed chain morphology, inducing larger average chromophore-to-chromophore distances and higher probabilities for unfavorable relative orientations. Although all samples have been spin-cast from a solvent favoring coiled conformations, this reasoning suggests different degrees of chain coiling in the three host matrices investigated, with PMMA leading to the tightest coiling and Zeonex to the most extended chains, while the conformation adopted in PS represents an intermediate case. We would like to point out that our finding is in contrast to a recent study comparing the photophysical properties of MEH-PPV embedded in Zeonex and PS.33 There, based on observations of absorption anisotropy and fluorescence intermittency in combination with numerical simulations, the authors concluded that in Zeonex the interchromophoric distances are, on average, smaller than those in PS. Most likely, the reason for the conflicting observations is the different molecular weight of PS. While in the work of Ebihara et al.33 extremely low molecular weight PS (MW ) 10,000) was used in order to simulate a good solvent for MEH-PPV, in the present study we used PS with MW ) 113,000. In a study by Sartori et al.24 low molecular weight PS (MW ) 4,500 and 44,000) caused a relatively extended chain conformation of OC1C10-PPV, a conjugated polymer of similar molecular structure as MEH-PPV, while high molecular weight PS (MW ) 240,000) led to a less extended conformation and thus to decreased interchromphoric distances. Taking this influence of the PS molecular weight into account, it is comprehensible that Ebihara et al. found larger interchromophoric distances for their low MW PS compared to Zeonex, while our data suggested a tighter coiling in the higher MW PS than in Zeonex. Moreover, in contrast to our study, Ebihara et al. used MEH-PPV of large polydispersity (PD ) 5), i.e. a large distribution of chain lengths, which might also have an impact on the observed chain conformations. The distributions of peak maxima of emitting chromophoric units observed in the three samples are depicted in Figure 2. The histograms are based on emission spectra of 266 MEHPPV molecules in the PMMA sample, 97 in the Zeonex sample, and 107 in the PS sample. The distributions of all samples are monomodal and are well described by Gaussians with maxima at 18250 cm-1 (PMMA), 18150 cm-1 (Zeonex), and 18250 cm-1 (PS). The slightly different values of the distribution maxima are close to the statistical inaccuracies of the fits. Furthermore, these values are roughly centered around the ensemble emission maximum of MEH-PPV in dilute toluene solution (not shown), which suggests a similar average solvent shift for the emitting chromophores in each of the host matrices. In this context, it would be interesting to study single chains without polymer embedding, too, because this might allow us to estimate the

Figure 2. Distributions of peak emission maxima observed for MEHPPV at 1.2 K in different host matrices (gray bars): (a) PMMA; (b) PS; (c) Zeonex. The drawn lines are Gaussian fits to the data. The black bars give the subpopulations of molecules for which ZPLs could be detected.

contribution of (dispersion) interactions between the emitting chromophores and the generic MEH-PPV chain to the solvent shift. Experiments by Mirzov et al.30 have suggested only a small shift between MEH-PPV emission with and without polymer capping. As mentioned before, the distributions in Figure 2 are monomodal. For higher molecular weight samples of MEHPPV in PMMA, bimodal distributions with an additional fraction of red-shifted emission maxima have been reported.25,26,28 The occurrence of “red” sites has been tentatively attributed to interchromophoric contacts leading to red-shifted aggregate emission. Adopting this point of view, one might argue that the relatively low molecular weight of the MEH-PPV investigated in this study, and thus the small number of chromophores per chain, prohibits interchromophoric aggregate formation. We would like to mention, however, that more recent studies have questioned the aggregate hypothesis and suggested an alternative explanation in terms of larger, i.e. red-shifted, chromophores.30,37 Further investigations are urgently needed to clarify the issue. There exists a significant difference in the widths (fwhm) of the distributions, which increases from 850 ( 20 cm-1 in PMMA to 1050 ( 70 cm-1 in PS and 1350 ( 50 cm-1 in Zeonex. Since the transition energy of a MEH-PPV chromophore is not only determined by the size of the conjugated segment (and hence the π-delocalization) but also by its local environment, which shifts the energy of ground and excited

MEH-PPV Polymer Molecules in Different Host Matrices

Figure 3. Emission spectra of single MEH-PPV chains in different host matrices (λex ) 488 nm): (a, b) PMMA (Iexc ) 2.7 kW/cm2, tint ) 30 s); (c, d) PS (Iexc ) 0.7 kW/cm2, tint ) 20 s); (e, f) Zeonex (Iexc ) 2 kW/cm2, tint ) 30 s). All spectra can be assigned to multichromophoric emission, with one chromophore showing a sharp ZPL.

states through dispersion interactions, the observed differences in distribution widths (static disorder) for the host polymers studied are likely to be partly caused by specific interactions between the emitting chromophores and the particular host. Above, we have argued based on the unequal ratios of multichromophoric emission that the MEH-PPV chains adopt the least coiled conformation in Zeonex and the tightest coils in PMMA. This finding might be connected with the widths of the spectral distributions: A more extended conformation is likely to lead to chromophoric units on average being more exposed to matrix influences and hence to enforce variations of the transition frequencies by a multitude of local environments. Furthermore, the efficiency of energy transfer to the lowest energy sites of a chain is reduced due to increased interchromophoric distances in a less collapsed chain, and the spectral distribution might reflect a larger distribution of conjugation lengths. In the past, we have reported the observation of narrow ZPLs in emission spectra of MEH-PPV samples of different molecular weight dispersed in PMMA.26,27 These sharp spectral features did only appear in a minority of cases (≈20% of molecules), with most emission spectra exhibiting a broad shape with large variations of the apparent line widths. We have argued that the low probability of observing ZPLs even at very low temperatures is due to linear electron-phonon coupling, in accordance with earlier literature data.38,39 In addition, we found evidence for spectral diffusion (on a time scale shorter than the integration time of the emission spectra) contributing to the substantial variations of spectral shapes.26 The same basic behavior of coexisting sharp and broadened emission spectra was observed for all samples studied herein, while the fractions of ZPLs slightly varied among the different samples (18% (PMMA), 8% (PS), and 28% (Zeonex) of molecules; 12% (PMMA), 5% (PS), and 13% (Zeonex) of chromophores; see Table 1). Spectra containing ZPLs are presented in Figure 3 for the three samples. The examples in Figure 3 were chosen

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Figure 4. Fluorescence emission (a, c; λex ) 488 nm) and excitation spectra (b, d) of single MEH-PPV molecules from the Zeonex (a, b) and the PS (c, d) samples. Please note the differences in wavelength scale. The insets show the ZPLs with increased resolution and Gaussian fits, giving the instrumentally limited fwhm of 0.1 cm-1. (a: λex ) 488 nm, Iexc ) 2 kW/cm2, tint ) 30 s. b: Iexc ) 150 W/cm2. inset: Iexc ) 5 W/cm2. c: λex ) 488 nm, Iexc ) 1.3 kW/cm2, tint ) 20 s. d: Iexc ) 70 W/cm2. inset: Iexc ) 7 W/cm2).

to emphasize the large variations in emission spectra of single MEH-PPV chains. Furthermore, these data show that, in the case of multichromophoric emission, the observation of a ZPL was not limited to a specific chromophore (e.g., the one with the lowest transition energy) but that any chromophore could exhibit a sharp feature. We have also highlighted the ZPL positions in the distributions shown in Figure 2; for none of the samples is the observation of narrow spectral features related to the spectral position. Therefore, we rule out the possibility of photodegradation as the cause for the narrow emission lines, which should lead to a substantial blue shift of the sharp spectra, as has recently been argued.29 The widths of the ZPLs in our emission experiments approach the experimental resolution that can be reached in the high-resolution mode of the spectrograph (cf. Table 1). In addition to emission spectra, we have also measured the excitation spectra of single MEH-PPV chains. For MEHPPV in PMMA, we have already reported the observation of ZPLs in excitation experiments elsewhere,26,27 with their width being limited by the experimental resolution of ∼0.1 cm-1. Thus, we could obtain a new upper limit for the homogeneous line width. We could find a very similar behavior in the PS and Zeonex samples. In Figure 4, fluorescence excitation and emission spectra of MEH-PPV chains embedded in the latter two matrices are presented. The emission spectrum of the particular example from the Zeonex sample (Figure 4a) shows fluorescence from three chromophoric units with peak wavelengths at 531, 544, and 563 nm. The chromophore at 544 nm exhibits a ZPL. The same transition appears in the corresponding excitation spectrum (Figure 4b), there giving rise to two lines, caused by spectral diffusion. The emission spectrum of the MEHPPV molecule chosen from the PS sample (Figure 4c) shows only one fluorescing chromophore without a ZPL (peak wavelength 541 nm, onset at ∼530 nm). Interestingly, in the excitation spectrum (Figure 4d), a sharp transition can be seen at 533.2 nm. Apparently, this transition belongs to the same chromophoric unit that appeared in the emission

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spectrum, there lacking a ZPL. While such kind of observation was made in several cases, the opposite (ZPL only in emission and not in excitation spectra) was even more common. We can only speculate about the origin of this lack of correspondence, which might be caused, e.g., by differences in excitation and detection conditions, spectral resolution, or simply stochastic spectral diffusion. The intensity jumps in the blue region (below 533 nm) of the excitation spectrum in Figure 4d are caused by temporal fluctuations of the fluorescence intensity, i.e. spectral diffusion. Analysis of the line widths of the two excitation spectra shown in Figure 4 by fitting Gaussians (shown in the insets) yielded the laser limited value of 0.1 cm-1 in both cases. As for MEHPPV in PMMA,26,27 only a very small number of ZPLs could be observed during the excitation experiments of the PS and the Zeonex sample (PMMA, 6%; Zeonex, 13%; PS, 5%). Those lines were subject to pronounced spectral diffusion, which resulted in frequent changes of their spectral position or even transient absence in a series of consecutive excitation scans. The few ZPLs that were reproducibly registered always marked the excitation of those low-energy chromophores, which also appeared in the corresponding emission spectrum. In no case could we observe excitation lines of higher energy donor chromophores which transferred their excitation energy completely to lower energy sites. This kind of “donor” ZPLs is expected to be appreciably broadened by rapid energy transfer, making it very difficult to detect them with a sufficient signal-to-noise ratio. Our results show that for the given MEH-PPV sample in none of the three host matrices can a large fraction of ZPLs be found. We therefore conclude that the large value of ∆µ accompanying the lowest energy optical transition in MEH-PPV and giving rise to appreciably strong linear electron-phonon coupling ultimately limits the cases in which ZPLs can be observed.39,44 Being aware of the limited statistical significance because of the small number of ZPL observations, we hesitate to draw definite conclusions from the small differences in fractions of spectra with ZPLs in the different samples. However, the observed tendency (i.e., highest probability to find a ZPL in Zeonex and lowest one in PS, while PMMA lies in between) qualitatively meets the expectations one can derive from earlier literature studies. The finding of the largest fraction of ZPLs in the aliphatic hydrocarbon polymer Zeonex seems to be in line with the well-known fact that poly(ethylene) or poly(isobutylene) is a suitable host for particularly stable and narrow singlemolecule ZPLs of organic dye molecules.40-42 On the other hand, ZPLs of dye molecules are on average less stable and broader in PMMA and PS.42 In addition, earlier bulk line narrowing experiments, such as, e.g., spectral hole-burning, have found that both linear and quadratic electron-phonon coupling are stronger in PS as compared to PMMA.43 These findings qualitatively agree with our observation that ZPLs of MEHPPV chromophores are more often observed in PMMA. There remains still a large discrepancy to a recent low temperature (T ) 5 K) single-molecule study of MEH-PPV in PS by Schindler et al.,19,20 where the finding of ZPLs represented the typical case. According to the results presented here, this discrepancy cannot simply be explained by the inherent properties of PS (weaker electron phonon coupling strength, less common spectral diffusion). As already mentioned above, rich spectral dynamics has been observed in emission as well as excitation spectra. Apart from the different probabilities to observe ZPLs in the spectra, which are at least partly related to spectral diffusion, there were no

Feist et al.

Figure 5. Series of emission spectra of two MEH-PPV chains showing dynamics with respect to the number of emitting chromophores: In part a, the number of emitting units is reduced during the measurement from three to one in a stepwise manner. In part b, in addition to the continuously emitting chromophore, a second one appears temporarily. (λex ) 488 nm, Iexc ) 2 kW/cm2, tint ) 30 s, Zeonex sample).

obvious qualitative differences in the spectral diffusion behavior in the three host matrices. In the remainder, we want to exemplarily highlight two typical cases of spectral dynamics which were often observed independent from the host matrix and appear as a common signature of the time dependent spectral response of a multichromophoric system. In Figure 5 intensity variations and the (dis)appearance of particular chromophores as tracked in a series of consecutive emission spectra are presented. The emission of the molecule from Figure 5a initially originated from three chromophoric units, two of which disappeared during the measurement one after the other. Most likely, this observation is due to photobleaching either of the emitting units themselves or of the “pool” of chromophores that had transferred their excitation energy to the emitter. Frequently, in the course of an emission experiment not only a successive reduction of the emitting chromophores could be observed. In the spectra series from Figure 5b, the chromophore with a peak maximum at around 560 nm fluoresces continuously throughout the complete measurement with only slight intensity fluctuations, while temporarily a second chromophore appears with a peak maximum around 535 nm (spectra nos. 3 and 4). Since the emission intensity of the chromophoric unit emitting at 560 nm remains basically unaffected by the appearance of the blue emitter, it is unlikely that the latter results from certain “energy donor” chromophores changing their acceptor temporarily (e.g., caused by chain reorientation). Probably, there exists a domain independent from the chromophore emitting at 560 nm, where energy is funneled efficiently to the “blue” emitter only part of the time, while mostly, either photoexcitation of the contributing chromophores is inefficient or the emission from the chromophore at 535 nm is quenched (e.g., through charge-transfer states).

MEH-PPV Polymer Molecules in Different Host Matrices 4. Conclusion We have compared the low temperature photophysical properties of single MEH-PPV molecules embedded in three different host polymers (PMMA, PS, and Zeonex). For all samples, we observed single-chromophoric as well as multichromophoric emission, with their ratio depending on the host polymer. Since not only the choice of the solvent, from which the samples are cast, but also the host matrices affect the chain morphology of conjugated polymers, these differences might arise from different conformations of the MEH-PPV chains, with PMMA leading to the tightest and Zeonex to the loosest coiling. The widths of the spectral distributions of emission maxima depended on the host matrix, probably resulting from the influence of the different environments on the transition energies of MEH-PPV chromophores. The distribution widths appear to be connected to the different conformations suggested above, with more extended conformations leading to chromophoric units being more exposed to the surrounding matrix and/or generating a larger distribution of conjugation lengths of emitting chromophoric units through less efficient energy transfer to the lowest energy sites. While it would be highly interesting to learn about the driving forces of the degree of coiling in a given host matrix, such studies were beyond the scope of the present investigation. The shape of emission spectra of single chromophores was subject to pronounced variations, ranging from spectra with sharp, purely electronic ZPLs of spectrograph-limited width to very broad ones. We attribute these variations to quite strong linear electron-phonon coupling and spectral diffusion. Both effects should be influenced by the host matrix, and although of limited statistical significance because of the small number of ZPL observations, we indeed observed trends concerning different ratios of spectra with ZPLs for the three samples investigated, in reasonable qualitative agreement with earlier literature data: with Zeonex leading to the highest and PS to the lowest probability of the occurrence of ZPLs. We could also find ZPLs in excitation spectra of single MEHPPV chains for all samples. These observations were very rare, however, and confined to emitting low-energy chromophores. For these transitions, line widths down to the instrumentally limited value of ∼0.1 cm-1 were measured in all host matrices, giving an upper limit of the homogeneous line width of MEH-PPV chromophores. In general, the overall probability of detecting zero-phonon transitions was relatively small, with none of the host matrices studied herein enabling the observation of a large fraction of chromophores with stable ZPLs. While the results of this study indicate a weak influence of the host polymers on optical spectra of isolated MEH-PPV chains at low temperature, the choice of the host polymer did not allow, or only to a minor extent allowed, us to modify or control the general spectroscopic behavior of the individual chromophores in a MEH-PPV chain. In particular, rich spectral dynamics, which actually may be already an inherent property of the disordered MEH-PPV chains, has been found in each of the host polymers. Given this fact, it is a difficult endeavor to study single-chromophore ZPLs over longer time scales and/or at different temperatures. The situation may be improved for the “red” emission observed in higher molecular weight MEHPPV.26 Both of the apparently contradicting interpretations in terms of aggregates or larger chromophores seem to rely on the common assumption that the phenomenon of “red” emission is due to somewhat more ordered regions in part of the chain. In such environments, ZPLs of MEH-PPV chromophores may

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