Structural basis for the spectroscopy and photophysics of solution-aggregated conjugated polymers

Synthetic Metals 141 (2004) 197–202

Structural basis for the spectroscopy and photophysics of solution-aggregated conjugated polymers Anoop Menon a , Mary Galvin a , Kenneth A. Walz b , Lewis Rothberg b,∗ a

Department of Materials Science, University of Delaware, Newark, DE 19716, USA b Department of Chemistry, University of Rochester, Rochester, NY 14627, USA Received 23 September 2003; accepted 29 September 2003

Abstract We present fluorescence spectra, quantum yields and structural data from 1 H NMR studies on solutions of a di-alkoxy substituted pentamer of p-phenylenevinylene (5PV) and the corresponding polymer poly-(p-phenylenevinylene) (PPV). We vary the solvent quality continuously using miscible good and poor solvents to study the effects of polymer aggregation as is relevant to properties of films and of the concentrated solutions from which films are formed. We observe a large drop in quantum yield with reduced solvent quality that we attribute to formation of interchain excitations with negligible luminescence when the polymer aggregates. Concomitant with the drop in fluorescence, we observe a red shift in the spectra. The NMR data suggest the red shift is due to an increase in the steric hindrance of the backbone motion that leads to an increase in effective conjugation length. In each material, our data can be described as a linear combination of only two species with distinct spectroscopy and quantum yield, an isolated and aggregated form of the polymer. Studies of the properties of mixtures of the pentamer and polymer demonstrate conclusively that these species mix and that the spectroscopic changes are due to aggregation and not conformational changes of single chains. They also provide insight into recent results from electroluminescent devices based on analogous conjugated polymer blends. © 2003 Elsevier B.V. All rights reserved. Keywords: Photophysics; Polymer; Spectroscopy

1. Introduction The progress in applications of conjugated polymers for light-emitting diodes and other optoelectronic applications is largely dependent on understanding the effects of material design and processing parameters on morphology and structure within films. There are many precedents for polymer modifications such as addition of solubilizing groups to rigid backbone polymers [1,2], design of molecules with two-dimensional conjugation [3] and incorporation of hole and electron transporting moieties [4]. Film morphology can be influenced by side group design [5], nature of solvent [6], polymer concentration, and other variables inherent in spin casting such as solvent evaporation rates and spin speed. The dramatic reduction in quantum yield upon going from solution to films has been attributed to photogeneration of interchain excitations facilitated by efficient chain packing [7]. Attaching bulky side chains to the polymer

∗ Corresponding author. Tel.: +1-585-273-4725; fax: +1-585-506-0205. E-mail address: [email protected] (L. Rothberg).

0379-6779/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2003.09.021

backbone to prevent aggregation quenching has been reasonably successful [8] but the charge transport properties become degraded by this strategy so that it does not appear viable for electroluminescent device applications. A number of researchers have studied the changes in conjugated polymer photophysics upon aggregation in solution [5,9–14]. This approach has been used to document bimodal inhomogeneity in the spectroscopy of polymers [8,14,15]. Working in solution has the advantages that it prevents damage, makes quantitative fluorescence quantum yield measurements straightforward and allows the study of sufficient amounts of material to make magnetic resonance spectroscopy practical. In the present work, we study solution aggregation of conjugated polymers where we have deliberately introduced polydispersity by mixing oligomers and polymers of similar phenylenevinylene based materials with different HOMO-LUMO gaps. We have seen that analogous polydispersity in conjugation length can have large effects on electroluminescent device efficiencies [16]. Our intent is to address ubiquitous phenomena such as phase segregation, quantum yield variation upon aggregation and excited state transfer to low energy sites when polydispersity is present.

A. Menon et al. / Synthetic Metals 141 (2004) 197–202

2. Experimental The chemical structures of the oligomer and polymer used in this study are shown in Fig. 1. The substituted di-alkoxy p-phenylenevinylene (5PV) was prepared via minor modifications to the orthogonal approach developed by Maddux et al. [17]. The di-alkoxy poly-(p-phenylenevinylene) (PPV) was synthesized via a Wittig-Horner reaction, but with a shortened reaction time to reduce the molecular weight. To prepare the very low molecular weight PPV, we added 1% of a mono-aldehyde to the polymerization reaction to terminate chain growth. The detailed properties of both materials are described elsewhere [18]. Spectrophotometric grade 1,4-dioxane used for the PL experiments was purchased from JT Baker and Acros Chemicals and used as received. The solvents were stored under nitrogen. The deionized water used in the fluorescence experiments was obtained from a Millipore purification system. Deuterated 1,4-dioxane, chloroform (CDCl3 ), methanol and deuterium oxide (D2 O) for the 1 H NMR were procured from Cambridge Isotope Labs. 1,4-Dioxane was dried by treating with activated molecular sieves. For the PL measurements, stock solutions were made up by dissolving the PPV and 5PV in dioxane. The solutions were then filtered through a 0.45 ␮m filter. We seek to elucidate the effect of adding a bad solvent (deionized water) in the present case to a solution of 5PV and PPV in 1,4-dioxane. Upon addition of water an observable change in color to the solutions was observed which was stable over an extended period. The change in color was instantaneous which indicates very rapid formation of aggregates. The optical measurements involved making samples with equal chromophore concentrations. Optical densities were maintained at 0.1 to minimize aggregation and reduce artifacts introduced by self-absorption in fluorescence. Absorp-

3 5

O C 8H17 6

7 C8H17O

1

C8H17O

10

3. Results and discussion We systematically investigated the change in photophysical properties of PPV and 5PV with solvent quality in dioxane–water mixtures. In Fig. 2a we present the emission spectrum of 5PV in different solvent mixtures ranging from 100% dioxane to 10% dioxane and 90% water. We observe a dramatic drop in quantum yield along with a red shift in

100 : 0 90 : 10 70 : 30 50 : 50 30 : 70 10 : 90

9

1x10

0 400

500

600

700

Wavelength (nm)

2 O 8/9 H 2 C 12 H2C C5H10 C H3 11 13

OC8H17

(b)

C8H17O

dioxane : water (%) 9

2x10

(a)

Photoluminescence (a.u.)

4

tion was recorded using a Hewlett-Packard 8452A diode array spectrometer and Beckman DU 640 spectrophotometer. Photoluminescence spectra were recorded on a SPEX Fl spectrofluorimeter using DM3000F software with a built-in reference quantum counter to correct for excitation intensity fluctuations. Quantum efficiencies of the same sample in different solvent mixtures were measured relative to Coumarin 480 in dilute ethanol solution according to the procedure of Williams et al. [19]. NMR spectra were recorded on a Bruker AM 400 MHz spectrometer with 512 scans per sample. One milligram of material was weighed in a 5 mm NMR tube followed by addition of requisite amount of deuterated solvents. This allowed use of single 5 mm tube such that conditions could be kept constant during the course of the experiment. Integration of the peaks was done by Bruker XWINNMR software. To achieve adequate signal to noise ratio, the NMR solutions were about an order of magnitude more concentrated than those used for the optical studies.

Photoluminescence (a.u.)

198

n

Fig. 1. Chemical structures of 5PV and PPV. Labeling of the protons 1–13 is for comparison with the magnetic resonance data in Table 1.

dioxane : water (%) 100 : 0 90 : 10 70 : 30 50 : 50 10 : 90

9

1x10

0 400

500

600

700

Wavelength (nm)

Fig. 2. (a) Photoluminescence spectra of 5PV in mixed dioxane/water solvents with varying solvent composition as noted in the legend. (b) Photoluminescence spectra of PPV in mixed dioxane/water solvents with varying solvent composition as noted in the legend. Excitation is at 400 nm.

A. Menon et al. / Synthetic Metals 141 (2004) 197–202

Fraction isolated species

emission spectra as water is added. The absorption spectra undergo a comparable red shift but do not change very much in magnitude of extinction indicating that the emission quantum yield is greatly reduced. This phenomenon is similar to what is observed in spin cast conjugated polymer films as compared to solution. In Fig. 2b we show data from a similar experiment with PPV dissolved in dioxane–water mixtures. Relative to the pentamer, the good solvent spectrum of the polymer is more “vertical” (i.e. has a larger ratio of 0–0 to 0–1 intensity) showing extended planarity as expected for the longer ␲-delocalization. By and large, however, the results are extremely similar to those for the pentamer even though 5PV is a five-ring rigid while the polymer has chain length ranging from 20 to 30 phenyl rings per chain. The main difference between the conformations of the molecules arises from the possibility of a chain folding on to itself upon bad solvent addition. In a short chain rigid oligomer like 5PV it is unlikely to occur. Hence the similarity in the photophysical behavior between the PPV and 5PV indicates interchain interaction between different molecules in both the experiments, consistent with previous studies on MEH-PPV trimers [20] and polymers [21,22]. We will confirm this conclusion below by observing energy transfer between chains directly and we will address the chain packing geometry with data from nuclear magnetic spectroscopy. A common feature in Figs. 2a and b is that the red shift and the drop in quantum yield occur at a well-defined solvent composition for both PPV and 5PV. The aggregation in the polymer case is triggered at a smaller fraction of poor solvent as expected. The intermediate cases of solvent mixtures have radically different PL band shapes from both the spectra in good solvent and those in the poorest solvent conditions. This results not from a change in Franck-Condon envelope but rather because isolated solution-like chains and aggregated film-like chains coexist in the solution under these conditions. In fact, every intermediate case PL spectrum can be fit with a linear superposition of the best and worst solvent PL spectra as has been shown in previous work on other phenylenevinylene polymers [9,14]. The magnitudes of fitting coefficients for the isolated (100% dioxane) component are plotted with respect to solvent quality for both PPV and 5PV in Fig. 3. The same fitting coefficients used to account

199

PPV 5PV

0.8 0.6 0.4 0.2 0.0

20

40

60

80

100

% Good Solvent Fig. 3. The calculated fraction of isolated segments contributing to the mixed solvent spectra of 5PV and PPV vs. solvent quality. The isolated segment spectra are assumed to be those in the 100% dioxane case and the aggregated spectra to be those in the 10% dioxane/90% water case.

for the spectroscopy of the intermediate cases can also be used to predict the quantum yield of the solutions based on the number of isolated and aggregated species in the solution. This prediction agrees exactly with the quantum yields we have measured directly. To gain information related to the conformational changes that occur when PPV and 5PV aggregate, we did NMR experiments in deuterated dioxane–D2 O mixtures analogous to the above solutions. The 1 H NMR assignments of 5PV in are shown in Table 1 corresponding to the labeling of the protons in Fig. 1. The 5PV was dissolved in 100% dioxane initially and, after each set of scans, D2 O was added to the same NMR tube thus progressively deteriorating the solvent quality in the tube. The concentration of good solvent in each solvent mixture decreases at exactly the same rate as the concentration of 5PV. Yet there is a decrease in 5PV integrated intensity relative to good solvent peaks through successive solvent mixtures. In several repetitions of this experiment with different solutions prepared in slightly different ways the results are the same. This gives a clear indication of the degree of aggregation with addition of poor solvent. Upon aggregation, configuration averaging is reduced, and a given proton under study fails to sample many of its possible positions on the time scale of the nuclear spin relaxation.

Table 1 Proton magnetic resonance peak assignments for 5PV Proton label 1

2

3

4

5

6

7

8

9

10

11

12

13

δ (ppm) Multiplicity J (Hz)

7.53 S

7.50 D(16.1)

7.44 D(8.0)

7.17 D(8.0)

7.13 S

7.12 D(16.1)

7.11 D(16.1)

4.06 T(6.4)

4.057 T(6.3)

2.37 S

1.78, 1.96 M

1.23, 1.62 M

0.82, 0.96 M

Number of protons

4H Arom

2H Vinyl

4H Arom

2H Vinyl

2H Arom

2H Vinyl

2H Vinyl

4H OCH2

4H OCH2

6H CH3

8H OCH2 , CH2

40H CH2

12H CH3

Labeled proton positions are as in Fig. 1. Chemical shift δ and multiplicity S with spin–spin splittings J in Hz are listed in the second row. Protons are grouped in terms of the assignments in the third row for intensity comparisons in Fig. 4.

A. Menon et al. / Synthetic Metals 141 (2004) 197–202

Aromatic Core

Alkoxy Side chain

Methyl

Alkoxy Side chain

Methyl

(b)

Aromatic Core

Fig. 4. Comparison of the relative integrated areas of the 1 H NMR signal from different protons in the mixed solvent spectra of 5PV. Proton assignments are as in Table 1: (a) 50% dioxane/50% water solvent; (b) 50% chloroform/50% methanol solvent. White bars are for the good solvent while shaded bars are for the mixtures. The proton intensities are normalized to the aromatic core protons for comparison.

A more detailed analysis of the proton intensities provides some insight into the microscopic nature of the aggregate formation. Of particular interest is the variation of relative strength of the aromatic proton upon bad solvent addition [14]. Fig. 4 illustrates the changes in integrated intensity of protons attached to different positions on the 5PV in good solvents (1,4-dioxane-d6, CDCl3 ) upon controlled titration with bad solvent (D2 O, CD3 OD). We have done two different solvent mixtures to rule out special complexation that might occur in a particular set of solvents. We take the strong singlet at 7.53 ppm in CDCl3 from the central ring protons in 5PV to be a probe of backbone motion. The reasons for selecting this proton signal as the indicator for studying multiple chain packing are two-fold. First, the signal is a strong singlet due to the symmetric environment around the protons, which means there is no splitting due to spin–spin coupling. Second, any intermolecular interaction would be most pronounced at the central ring of the five-ring oligomer since the packing of the chains may not always be a neat parallel stack and may be in a staggered arrangement. The downfield shifts of the aromatic ring protons upon addition of bad solvent indicate that the main chains probably form a ␲-stacked bilayer lamellar structure and these protons could be deshielded by a ring current effect [23]. In each poor solvent case, we observe that the integrated area per proton of the aromatic core degrades relative to that of the alkoxy side chain protons. We think this effect is less pronounced in the dioxane/water case because dioxane is a fairly poor solvent to begin with and there may already be significant aggregation due to the high 5PV concentration used in the NMR experiments. The relative diminution of backbone proton intensities indicates that the protons on the alkoxy chains have more freedom to move than the aromatic

protons as a result of which we observe this change in the ratios. We believe this to be indicative of reduction of configuration averaging in the backbone protons relative to the alkyl chain protons due to steric hindrance of ring motion caused by packing. Reduced torsion is consistent with better ␲-delocalization and the red shift we observe in absorption and emission. We can take advantage of our nearly monodispersed 5PV and PPV chains to introduce deliberate polydispersity and study its effects. For example, we can investigate the possibility of phase segregation that may occur due to differences in solubility of different molecular weight chains during the spin casting process. Polydispersity also introduces exciton traps since there is a corresponding distribution of HOMO-LUMO gaps. Our previous work on blends of 5PV and PPV shows that low gap chains in a matrix of high gap material had a deleterious effect on electroluminescent device performance [16]. We can simulate the optical properties of these blends using deliberate solution aggregation and investigate whether the issue in devices is reduced luminescence yield or charge imbalance. Fig. 5a depicts the emission spectrum of a 10% PPV: 90% 5PV blend in good solvent, 100% dioxane. The 5PV and PPV have a very low probability of communicating with each other and hence emit separately. As expected, the luminescence spectrum of the mixture is well described by sums of the individual spectra of PPV and 5PV in 100% dioxane in their proper quantitative ratios. In Fig. 5b, the variation of

Normalized PL

(a)

Pentamer 10% Blend Blend fit PPV

0.8

0.4

0.0

(a)

500

600

700

Wavelength (nm) 9

Photoluminescence (a.u.)

200

(b)

2x10

9

1x10

dioxane : water (%) 100 : 0 90 : 10 70 : 30 60 : 40 10 : 90

8

5x10

0

500 600 700 Wavelength (nm)

Fig. 5. (a) Normalized photoluminescence spectra of a 90% 5PV/10% PPV blend and its constituents in dioxane. A fit of the blend to a linear combination of its component spectra is also shown. (b) Photoluminescence spectra of the same blend as a function of dioxane/water solvent composition as noted in the ligand.

A. Menon et al. / Synthetic Metals 141 (2004) 197–202

blend emission upon addition of the non-solvent, water, is presented. As in case of 5PV or PPV alone, the PL is reduced upon addition of bad solvent. However, energy transfer to PPV is clearly observed since a fit to the most aggregated case in Fig. 5b gives approximately 30% aggregated 5PV emission and 70% PPV emission. Several conclusions can be drawn. First, phase segregation cannot be dramatic since it is clear from the relative paucity of 5PV aggregate emission that 5PV and PPV chains are mixed in the agglomerate. Second, the fact that energy transfer from 5PV to PPV appears at the same poor solvent fraction at which the spectral changes and yield drops are observed proves that these can be ascribed to aggregation of multiple chains and not to conformational changes of single chains in response to the changing solvent environment. This confirms previous work suggesting that interchain charge separation is the primary cause for reduced photoluminescence in aggregated conjugated polymers [7]. Third, we observe that the emission in the worst solvent case for the blend (Fig. 5b) appears to be reduced significantly less than that for either of the pure systems (Fig. 2). We consider this result tentative since it is very difficult to make the samples quantitatively reproducible but there appears to be almost twice as much remaining fluorescence in the aggregated blend case. This might be due to competition between rapid energy transfer to long segments with interchain charge generation. Once on the long segment sites in a host matrix of short chains, separation of the exciton into charges on adjacent chains may be more difficult, resulting in a higher quantum yield. Finally, the observation that the photoluminescence of the aggregated blend is at least high as for the aggregated constituents bears upon our recent results showing that similar blends make much less efficient light-emitting diodes [16]. Since the luminescence is not reduced by blending, it confirms our previous conclusion that the role of polydispersity in poor device performance has to do with degradation of charge balance and not reduction of excited state emissive efficiencies.

201

combinations, chloroform–methanol and dioxane–water, also support the generality of bimodal inhomogeneity in describing the polymer spectroscopy. From magnetic resonance data, we can infer something about the structural properties of these two different conformations. We conclude that isolated and well-packed chromophores are distinguished structurally by the freedom of the phenyl ring torsion. The protons on the aromatic backbone show a clear drop in integrated intensity due to this restriction when compared to the protons on the alkoxy side chains. We interpret this loss of intensity as lack of motional averaging when torsion is suppressed by aggregation-induced steric hindrance. Ring rigidity is also associated with long conjugation length and is consistent with the red shift in absorption and emission also observed with aggregation. The reduction in quantum yield with aggregation is also easily rationalized since interchain interactions enable formation of charge-separated species that do not lead to efficient emission. In the near future, we will publish results of transient spectroscopy that directly confirm the existence of these charge-separated species. Using mixed solvent aggregation, we have also simulated blend formation in a polydispersed system as is typical for conjugated polymers. These experiments verified that chain aggregation, not conformational change, is the root cause of the changes in spectroscopy and emissive efficiency. They also confirm our previous understanding that the reduction of efficiency observed due to similar polydispersity in electroluminescent devices based on 5PV and PPV is primarily a result of charge imbalance rather than changes in photophysics. Our conclusions may also have relevance to conformational sensitivity observed in many other experiments [21–25] and are clearly important in charge transport, charge photogeneration and lasing pertinent to device applications of conjugated polymers.

Acknowledgements 4. Conclusions In the current work we have shown that by controlling the solvent environment we can realize aggregated states in solution that can be analyzed by a range of spectroscopic techniques to correlate structural data with photophysical behavior. Upon poor solvent addition, both 5PV and PPV exhibit large drops in quantum yield and red shifts in emission resembling film behavior. While the kinetics of rapid solvent evaporation undoubtedly plays a role in film morphology, mixed solvent studies are useful as a tool to study conjugated polymer properties in films. We believe that treating the polymers as two distinct species, aggregated and isolated, is a useful picture, as it works quantitatively for both 5PV and PPV in addition to MEH-PPV [14], trimers of MEH-PPV [20] and dendritically substituted PPV [8]. The data from two different good/poor solvent

We are grateful to Dr. Chris Collison for helpful discussions and experimental assistance. We also thank the NSF for grant DMR-0208786 that supported this work and a NSF-RET grant to the University of Rochester Chemistry Department that supported Ken Walz.

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