Charge Transfer Absorption for π-Conjugated Polymers and

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ARTICLES Charge Transfer Absorption for π-Conjugated Polymers and Oligomers Mixed with Electron Acceptors Priyadarshi Panda,† Dirk Veldman,† Jo1 rgen Sweelssen,‡ Jolanda J. A. M. Bastiaansen,‡ Bea M. W. Langeveld-Voss,‡ and Stefan C. J. Meskers*,† Molecular Materials and Nanosystems, EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands, and TNO Science and Industry, P.O. Box 6235, 5600 HE EindhoVen, The Netherlands ReceiVed: January 30, 2007; In Final Form: March 13, 2007

π-conjugated polymers and oligomers show charge transfer (CT) absorption bands when mixed with electron acceptors in chloroform solution. This is attributed to the formation of (ground state) donor-acceptor complexes in solution. By varying the concentration of the donor and acceptor, the extinction coefficient for the CT absorption and the association constant of donor and acceptor are estimated. The spectral position of the CT bands correlates with the electrochemical oxidation potential of the π-conjugated donor and the reduction potential of the acceptor.

Introduction polymers1,2

π-conjugated are fascinating materials that can be applied as an active layer in various opto-electronic devices. This has spurred a large research effort on the electrical and photophysical properties of these materials. Initially, after conjugated polymers had become available in larger quantities, electrical conductivity was one of the properties investigated in detail. It was discovered that the electron acceptor iodine forms a charge transfer (CT) complex with a variety of π-conjugated polymers and doping polymer films with I2 can result in materials with very high conductivity.3-5 Later, the interaction of π-conjugated polymers with molecular oxygen was investigated, and this electron acceptor forms a complex with π-conjugated polymers.6,7 Because O2 has less electron accepting character than I2, the complex of O2 with π-conjugated polymers has only a limited ionic character in the ground state. Upon complexation with O2, a new optical absorption band arises, that corresponds to the transition from the (almost charge neutral) ground state to the charge separated state of the complex.6,8 The occurrence of such a charge transfer absorption band is well-known for small aromatic molecules upon complexation with electron acceptors.9-13 In the past decade it has become clear that mixing π-conjugated polymers possessing electron donating properties with molecular materials having an electron accepting character results in materials that can give rise to very efficient photoinduced charge separation.14-16 This allows for photovoltaic energy conversion with high quantum efficiency for those wavelengths which are absorbed effectively by the material. Recently, it has been shown that for a commonly used polymer-acceptor combination, a charge-transfer band can be * Corresponding author. E-mail: [email protected]. † Eindhoven University of Technology. ‡ TNO Science and Industry.

observed at the red edge of the allowed optical transitions of the polymer and that optical excitation via this transition contributes to charge generation.17-19 This indicates that the electronic structure of the complex formed between donor and acceptor may play an important role in the charge generation mechanism in these polymer-acceptor materials, which is at present only partially understood.20 In addition, for a number of donor-acceptor photovoltaic blends, the presence of a complex between donor and acceptor with its own special optoelectronicpropertiesisevidencedbycharge-transferluminescence.21-25 This emission corresponds to the transition from the ionic charge transfer state of the donor-acceptor pair back to the neutral ground state and is the emission analogue of the CT absorption band. The complex formation between an aromatic or π-conjugated donor and an acceptor can be considered as a supramolecular interaction and has been used to control the internal organization of materials or complex molecules.26-29 In turn, the relative position of donor and acceptor moieties and their interaction have a strong influence on photoinduced electron-transfer processes occurring in these materials.30,31 Here, the CT absorption band can be used as diagnostic for the donoracceptor interaction.31 The issues, described above, highlight the importance of CT interaction for π-conjugated molecules and materials. Much of our understanding of the charge-transfer complex formation is derived from studies of small aromatic molecules,9-13 but surprisingly, few systematic studies have been devoted to donor-acceptor complexes and CT absorption bands with π-conjugated oligomers or polymers with electron acceptors. Most attention has been paid to films using strong electron acceptors forming ionic complexes32-37 or to photoinduced electron transfer.38 Here, we investigate the CT absorption arising from ground state donor-acceptor complexes formed spontaneously in a chloroform solution. A set of acceptors (see

10.1021/jp070796p CCC: $37.00 © 2007 American Chemical Society Published on Web 04/19/2007

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Figure 1. Acceptors used.

J. Phys. Chem. B, Vol. 111, No. 19, 2007 5077 excited state. The energy of the charge separated state may be estimated independently from the electrochemical potentials (Eox - Ered), but obviously one needs also to take into account the electrostatic interaction between the charges in the charge separated state of the complex as well as the electrostatic interactions between the radical cation or anion formed in cyclic voltammetry (CV) and the counterions from the supporting electrolyte. In this work, we focus on the complexation of π-conjugated polymers and oligomers with electron acceptors in solution, monitoring the charge transfer absorption band of the complex. An important issue is to establish that this CT absorption band is different from the well-known cation (polaron) absorption bands of the π-conjugated material. At low concentration, one expects only formation of 1:1 complexes, and the association constant and molar extinction coefficient for the CT absorption band will be evaluated for a number of donor-acceptor combinations. A second issue addressed here is the correlation of the spectral position of the CT band and the difference (Eox - Ered). This correlation, when observed, provides evidence for the assignment of the CT absorption band to the transition from the (almost) charge neutral ground state donor (D)-acceptor (A) complex to the charge separated state of the complex (D+A-). The position of the charge-transfer band may provide independent information on the energetic position of the charge separated state in donor-acceptor combinations and may therefore be very relevant for further studies of photoinduced charge generation processes in polymer-acceptor blends used in, for example, photovoltaics. Experimental Section

Figure 2. Donors used.

Figure 1) with varying reduction potentials (Ered) is combined with π-conjugated oligomers and polymers (see Figure 2). Oligo-phenylenevinylenes (OPV3 and OPV4) together with a corresponding polymer (MDMO-PPV) and also two oligothiophenes (3T and 6T) with a structurally related polymer (regio-irregular P3HT) are investigated. In addition, polymers (1-3)39,40 and dyes (4 and 5)41,42 with less electron donating character are used as donors. This provides us with a set of donor molecules with different sizes and different oxidation potentials (Eox). From research on small (aromatic) molecules, the correlation between the photon energy for which the CT absorption reaches its maximum intensity (hνCTmax) and the difference between the oxidation potential of the donor and the reduction potential of the acceptor (Eox - Ered) is well-known. The rationale behind this correlation is that, for moderately strong electron donors and acceptors, the ground state complex is expected to be almost charge neutral so that hνCTmax reflects the energy difference between the neutral ground state and the charge separated

Absorption spectra were measured on a Perkin-Elmer Lambda 900 using a sample cell with small volume. Concentrations were determined by weighing. The solvent used in all experiments was chloroform. Cyclic voltammograms were recorded in an inert atmosphere (O2, H2O < 5 ppm) using degassed dichloromethane (DCM) as the solvent and tetrabutyl ammonium hexafluorophosphate (TBAPF6, 0.1 M) as the supporting electrolyte. The working electrode was a platinum disc (0.2 cm2), and the counter electrode was a platinum electrode. An Ag/ AgCl reference electrode was used, which was calibrated against ferrocene/ferrocenium (+0.47 V vs SCE). Estimated error in the electrochemical potentials was 0.01 eV. Some of the compounds were found to give irreversible voltammograms (e.g., 3T, which is known to dimerize). In these cases, high scan speeds (2 V/s and 5 V/s) were used, and we report the onset of the oxidation wave Eoxonset, as determined by taking the intersection of the extrapolated steeply rising edge of the voltammogram and the baseline. Polymers 1-3 have been described before.39,40 Compounds 4 and 5 were synthesized at TNO Science and Industry by slight modification of the method reported in refs 41 and 42. To synthesize 4, 5,5′′′′-diiodo-3′′,4′′-bis(2-ethylhexyl)-2,2′:5′,2′′: 5′′,2′′′:5′′′,2′′′′-quinquethiophene- 1′′,1′′-dioxide (0.50 gram, 0.54 mmol) was reacted with 5-trimethylstannyl-2-methoxythiophene (0.45 gram, 1.62 mmol) via Stille coupling. The product was purified by column chromatography (SiO2, hexane/dichloromethane, v/v, 1/1) and crystallizations. An orange crystalline solid was obtained in 43% yield. To synthesize 5, 5,5′′′′-diiodo3′′,4′′-bis(2-ethylhexyl)-2,2′:5′,2′′:5′′,2′′′:5′′′,2′′′′-quinquethiophene1′′,1′′-dioxide (1.0 gram, 1.08 mmol) was reacted with 5-trimethylstannyl-2-(9,9-dioctylfluoren-2-yl)thiophene (1.70 gram, 2.68 mmol) via Stille coupling. The product was purified by column chromatography (SiO2, hexane/dichloromethane, v/v,

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Figure 3. (A) Photoinduced absorption of OPV3 mixed with a fullerene derivative (MPC60) showing the cation absorption bands of OPV3+•.44 (B) Absorbance of OPV3 and TCNQ mixed in various ratios in chloroform. (C) Absorbance at 1.15 eV of the OPV3/TCNQ mixtures as a function of concentration (9) and a bimolecular association model fitted to the data (solid lines, K ) 186 ( 72 M-1, CT (1.15 eV) ) 144 ( 16 M-1cm-1.).

7/3) and crystallizations. An orange solid was obtained in 11% yield. Synthesis and optical characterization of OPVn,43,44 6T,45,46 and 3T47 oligomers and MDMO-PPV48 have been described. Regio-irregular P3HT was obtained by polymerization of 3-hexylthiophene using FeCl3 followed by Soxhlet extraction with hexane (Mw 11 kD, pD1.9). MPC60, a fullerene derivative, was obtained as a generous gift from Professor J. C. Hummelen. TCNQ and fluoranil were obtained from Aldrich, DCDCB and PMDA were obtained from Acros Oranics, and chloranil was obtained from Merck. Results and Discussion In Figure 3, we show absorption spectra for the OPV3TCNQ mixture in chloroform. As can be seen in Figure 3B, upon mixing, a new absorption band appears with a maximum intensity at a photon energy hνCTmax ) 1.15 eV. In addition, there is a second new absorption band at 1.7 eV, and these new bands are clearly different from the absorption of the isolated, neutral donor and acceptor compounds which absorb at photon energies >2.5 eV. Moreover, these new bands do not match with those of the OPV3+• cation. To illustrate this, we show the photoinduced absorption spectrum of a mixture of OPV3 and a fullerene derivative (MPC60) in Figure 3A.44. This system is known to give photoinduced charge generation upon excitation of the donor molecule, and the absorption bands of the OPV3+• and MPC60-• are indicated in Figure 3A. The new absorption bands of OPV3-TCNQ can also not be ascribed to TCNQ- 32 and are therefore most likely due to the transition from the (neutral) ground state of the OPV3-TCNQ complex to the charge separated state of this assembly. This assignment is supported by the concentration dependence of the intensity of the new absorption bands. At low concentration, an ap-

Figure 4. Charge transfer absorption bands for phenylene vinylene based donors, OPV3 (lines), OPV4 (-1-), and MDMO-PPV (-0-), mixed with various acceptors, DCDCB (top), TCNQ, chloranil, fluoranil, and PMDA (bottom) in a chloroform solution.

proximately linear dependence of the absorbance on both the total concentration of the donor (D0) and that of the acceptor (A0) can be observed. A bimolecular association model can be fitted to the data, including those at higher concentration, yielding values of CT(1.15 eV) ) 144 ( 16 M-1cm-1 for the molar decadic extinction coefficient and K ) 186 ( 72 M-1 for the association constant. We note that there is no change in the shape of the CT absorption band with concentration, which is consistent with formation of the 1:1 complex only. In addition, the intensity of the CT band is virtually the same for the mixtures containing 2.5:5 mM and 5:2.5 mM A0:D0 concentration, which is expected for a 1:1 complex formation. CT absorption bands for OPV3 are also observed with other electron acceptors, and this is illustrated in Figure 4. The spectral position of the CT band shifts to higher photon energies when acceptors with a lower Ered are mixed with OPV3. The reduction potential for the donors used has been determined with cyclic voltammetry, and values are listed in Table 1. In Figure 4, we also show data for the structurally similar OPV4 oligomer and MDMO-PPV polymer. The CT bands of the OPV4 and polymer with the stronger electron acceptors occur at slightly lower photon energies than for OPV3 indicating a correlation between hνCTmax and Eox of the donors (see Table 1). As can

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Figure 6. Correlation of the photon energy of maximum absorbance for the first CT band (hνCTmax) with the difference between oxidation potential of the donor and reduction potential of the acceptor (see Table 1) for various donors mixed with the acceptors: DCDCB (9), TCNQ (O), chloranil (1), fluoranil (∆), and PMDA (*). Individual datapoints are listed in Table 2.

Figure 5. Charge transfer absorption bands for thiophene based donors, 3T (lines), 6Τ (-1-), and ir-P3HT (-0-), mixed with various acceptors DCDCB (top), TCNQ, chloranil, and fluoranil (bottom) in a chloroform solution.

TABLE 1: Onset of the Oxidation Wave (Eonset ox ) and Oxidation Half Potential (E1/2 ox ) of the Donors and Reduction Half Potential (E1/2 red) of the Acceptors Versus SCE donor

Eonset ox (V)

E1/2 ox (V)

acceptor

E1/2 red (V)

MDMO-PPV OPV4 ir-P3HT 6T OPV3 4a 5a 3T 1 pyrene 2 9-Me-anthracene 3

+0.63 +0.69 +0.70 +0.73 +0.73 +0.81 +0.89 +0.95 +0.95 +0.98 +1.00 +1.08 +1.32

nd +0.75 nd +0.79 +0.80 +0.85 +0.93 nd nd nd nd nd nd

DCDCB TCNQ chloranil fluoranil PMDA

+0.48 +0.17 +0.05 -0.05 -0.57

E1/2 red (V) determined for 4 and 5 under the same experimental conditions are respectively -1.25 and -1.24 V (vs SCE). a

be seen, the CT band with DCDCB as the acceptor and PPV as the donor is unusually broad. The absence of any significant absorption at 0.4 eV excludes formation of a purely ionic D+A- complex, and thus, ground state charge transfer does not provide an explanation for the unusual band shape. We interpret the shape as a superposition of the first and second CT band with the second having a higher intensity. Deconvolution of the feature into two Gaussian bands gives 0.99 eV as the hνCTmax for the first CT band and 1.3 eV for the second CT band. In Figure 5, we illustrate the CT bands observed for the thiophene based donors combined with different acceptors.

Again, the correlation between hνCTmax and Ered of the acceptor and with Eox of the donor is apparent. For the stronger acceptors, TCNQ and DCDCB, a second CT band is also observed. We notice for both the phenylenevinylene and the thiophene donor series an increase in the intensity of this second CT band relative to the first when going from shorter to longer donor molecules. In Figure 6, we have plotted hνCTmax of the first chargetransfer band versus Eoxonset - Ered1/2 for each donor-acceptor couple. Eoxonset and Ered1/2 refer to the voltage at which the electrochemical oxidation of the donor sets on and the reduction half wave potential of the acceptor (see Table 1), respectively. The datapoints are also listed in Table 2. Looking at Figure 6, a correlation between hνCTmax and (Eoxonset - Ered1/2) is observed, and from a linear regression analysis, we find a slope of 0.94 ( 0.09 (eV/V) with an intercept of 0.84 ( 0.07 eV assuming a linear correlation between hνCTmax and (Eoxonset - Ered1/2). Previous studies examining CT bands in a KBr matrix have shown a similar correlation but with a considerably smaller intercept (0.4 eV).49 Part of the difference in the intercepts can be explained by our use of Eoxonset rather than the proper half wave potential, but this will reduce the intercept by at most 0.07 eV (see Table 1). The magnitude of the intercept is also influenced by, among other factors, electrostatic interactions between the charges in the charge separated state of the D-A complex. Also, electrostatic interactions in the CV measurements between the ionized molecules under study and the ions of the supporting electrolyte influence the intercept as these interactions affect Eox and Ered. Furthermore, the solvation of the DA complex in the ground and excited state need to be considered, as well as solvation of the radical cation and anion. Therefore, the value for the intercept may vary, depending on the choice of donor and acceptor molecules and the procedures used in the CV measurements. We note that in this study we have used the same solvent and supporting electrolyte for the determination of the redox potentials of all donor and acceptor molecules. One of the other factors contributing to the intercept is the shape of the CT bands. Following the Franck-Condon principle, the optical transition between the ground and the CT state is a vertical one, in which the ground state equilibrium nuclear geometry is projected onto the excited state. This ground state geometry most likely does not correspond to the energetically most favorable conformation in the CT state. The nuclear

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TABLE 2: Photon Energies of Maximum Absorbance (hνCTmax)for the First and Second CT Band of Various Donor-Acceptor Combinations

donor 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

MDMO-PPV ir-P3HT 6T 5 OPV4 OPV3 4 3T 1 2 3 ir-P3HT MDMO-PPV OPV4 OPV3 6T 4 5 1 2 9-Me-anthracene 3T pyrene 3 MDMO-PPV ir-P3HT 4 5 OPV4 OPV3 6T 3T ir-P3HT MDMO-PPV 5 4 OPV4 OPV3 6T 3T OPV4 OPV3

hνCTmax hνCTmax Eoxonset - first CT second CT acceptor Ered1/2 (V) band (eV) band (eV) DCDCB DCDCB DCDCB DCDCB DCDCB DCDCB DCDCB DCDCB DCDCB DCDCB DCDCB TCNQ TCNQ TCNQ TCNQ TCNQ TCNQ TCNQ TCNQ TCNQ TCNQ TCNQ TCNQ TCNQ chloranil chloranil chloranil chloranil chloranil chloranil chloranil chloranil fluoranil fluoranil fluoranil fluoranil fluoranil fluoranil fluoranil fluoranil PMDA PMDA

0.15 0.22 0.25 0.41 0.21 0.25 0.33 0.47 0.47 0.52 0.84 0.53 0.46 0.52 0.56 0.56 0.64 0.72 0.78 0.83 0.91 0.78 0.81 1.15 0.58 0.65 0.76 0.84 0.64 0.68 0.68 0.90 0.75 0.68 0.94 0.86 0.74 0.78 0.78 1.00 1.26 1.30

0.99 0.84 1.12 1.15 1.19 1.19 1.21 1.40 1.46 1.51 1.70 1.03 1.07 1.12 1.15 1.18 1.22 1.23 1.34 1.34 1.45 1.49 1.63 2.01 1.45 1.47 1.55 1.57 1.58 1.61 1.66 1.89 1.63 1.64 1.66 1.68 1.72 1.73 1.74 2.01 1.95 1.97

1.31 1.40 1.66 2.24 1.70

TABLE 3: Molar Decadic Extinction Coefficients at the Maximum of the First CT Band (ECTmax) and Bimolecular Association Constant K for Various Donor-Acceptor Combinations donor OPV3 OPV3 OPV4 OPV4 3T 3T 6T 6T

acceptor

CTmax (M-1cm-1)

K (M-1)

TCNQ chloranil TCNQ chloranil TCNQ chloranil TCNQ chloranil

144 ( 16 94 ( 26 202 ( 11 83 ( 52 61 ( 14 22 ( 6 76 ( 7 51 ( 50

(1.9 ( 0.7) × (1.2 ( 0.7) × 102 (2.1 ( 0.5) × 102 (3 ( 5) × 102 (3 ( 4) × 102 (4 ( 4) × 102 (2.2 ( 0.6) × 102 (1.0 ( 1.6) × 102

no of obs. 102

6 6 4 3 5 6 6 6

2.50

1.49 1.48 1.63 1.76 1.88 1.84

2.43

2.34

relaxation upon electronic excitation gives rise to a broad absorption band, of which the onset corresponds to the true energy difference between the energy minima of the ground and the CT states.50 Thus, one expects the energy of the charge separated state with respect to the ground state to be lower than hνCTmax and approximately equal to the onset of the CT band. As the CT band is rather featureless, the onset is difficult to determine precisely, but considering the experimental spectra, 0.4 eV for the difference between hνCTmax and the onset seems to be a reasonable estimate.51 Looking at Figure 6, it is evident that there is quite a large spread of the hνCTmax around the ideal linear correlation with (Eox - Ered). Interestingly, for datapoints obtained using the same acceptor, the correlation seems to be better; see, for example, the datapoints with TCNQ as the acceptor. A possible explanation for this is that the magnitude of hνCTmax is influenced by the electrostatic interaction between the positive and the negative charge in the excited, charge separated state of the complex (D+A-). The electrostatic interaction is strongly influenced by the distribution of charge over the ionized D+ and A- molecules. Using a single acceptor is then expected to reduce the variation in interaction energies. In addition, systematic errors in the

determination of Ered with cyclic voltammetry are canceled out when comparing data points obtained with the same acceptor. For the π-conjugated oligomers, with their well-defined molecular weight, the association constant for the donoracceptor complex formation may be evaluated by measuring the CT absorption for various total concentrations of the donor and acceptor. This we have done for the series with TCNQ and chloranil. The results for OPV3 and TCNQ are illustrated in Figure 3c. The absorbance values probed at hνCTmax and various concentrations for A0 and D0 can be analyzed assuming 1:1 association with the equilibrium constant K and an absorbance proportional to CT [AD], with [AD] the concentration of the complex. In our analysis, we have used a nonlinear least-squares fitting routine to determine K and CT. Results for the phenylenevinylene and thiophene oligomers are listed in Table 3. Values for CT are of the order of 102 M-1cm-1, and upon comparing the values obtained with TCNQ and chloranil for each oligomer, we notice that TCNQ gives generally higher values for the extinction coefficient compared with chloranil. This is in agreement with the relation between CT and the ionic character in the ground state complex proposed earlier,52 implying higher values for CT with increasing Eox - Ered. The values for K are generally of the order of 102 and do not seem to show any systematic trend. In some cases, estimated errors for the K and CT values are quite large. Possible reasons for this are the following. In the low concentration limit, the absorbance ≈ CT‚ K(A0,D0), and only the product CT‚K can be determined accurately. Any attempt to obtain separate values for K and CT results in numbers with large, correlated errors. Furthermore, there may be deviations from the assumed 1:1 complex stoichiometry. For some donor-acceptor systems, our experiments provide indications for this, because we observe that the CT absorbance measured with A0 ) x, D0 ) y is not equal to the absorbance for A0 ) y, D0 ) x (x > y). For a strictly bimolecular association, this equality should hold. Finally, problems in determining reliable equilibrium constants from CT data are well-known and may also arise from the nonideality of the solutions.53 Conclusions CT transfer absorption bands of ground state complexes of π-conjugated polymers and oligomers with electron acceptors have been identified. The dependence of the intensity on the concentration of the donor and acceptor provides compelling evidence for the assignment to a complex between the donor and the acceptor, while the spectral position and correlation with the redox potentials of the donor and acceptor support the assignment of the CT absorption to the transition from the ground state to the charge separated state of the complex. The CT band may be used to asses the energy of the charge separated state in organic donor-acceptor blends, and to this end, the

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