Miscibility between Differently Shaped Mesogens: Structural and

Mar 20, 2009 - Miscibility between Differently Shaped Mesogens: Structural and Morphological Study of a Phthalocyanine-Perylene Binary System. Gaël Z...
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J. Phys. Chem. B 2009, 113, 5448–5457

Miscibility between Differently Shaped Mesogens: Structural and Morphological Study of a Phthalocyanine-Perylene Binary System Gae¨l Zucchi,*,†,¶ Pascal Viville,‡ Bertrand Donnio,§ Alexandru Vlad,| Sorin Melinte,| Mihail Mondeshki,⊥ Robert Graf,⊥ Hans Wolfgang Spiess,⊥ Yves H. Geerts,† and Roberto Lazzaroni*,‡ Laboratoire de Chimie des Polyme`res, UniVersite´ Libre de Bruxelles, BouleVard du Triomphe, B-1050 Bruxelles, Belgium, SerVice de Chimie des Mate´riaux NouVeaux, UniVersite´ de Mons Hainaut/Materia NoVa, Place du Parc 20, B-7000 Mons, Belgium, Institut de Physique et Chimie des Mate´riaux de Strasbourg, UMR 7504 CNRS-UniVersite´ Louis Pasteur, 23 rue du Loess BP 43, 67034 Strasbourg Cedex 2, France, Microelectronics Laboratory (Unite´ DICE), UniVersite´ Catholique de LouVain, Place du LeVant 3, 1348 LouVain-la-NeuVe, Belgium, and Max-Planck-Institut fu¨r Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany ReceiVed: October 30, 2008; ReVised Manuscript ReceiVed: February 12, 2009

The thermotropic, structural, and morphological properties of blends of a disk-like liquid crystalline phthalocyanine derivative and a lath-shaped perylenetetracarboxidiimide mesogen derivative have been studied by combining differential scanning calorimetry, thermal polarized optical microscopy, X-ray diffraction, solidstate nuclear magnetic resonance, and atomic force microscopy. The two compounds are fully miscible for blends containing at least 60 mol % of the disk-like molecule. In such composition range, the homogeneous blends form a columnar hexagonal (Colh) mesophase for which the thermal stability is enhanced compared to that of the corresponding mesophase of the pure phthalocyanine. The miscible blends self-align homeotropically between two glass slides. For blends containing between 55 and 40 mol % of the diskshaped molecule, the two components are fully miscible at high temperature but the perylene derivative forms a separate crystalline phase when the temperature is decreased. Phase separation is systematically observed in blends containing less than 40 mol % of the discotic molecule. In this case, the resulting Colh mesophase is less stabilized compared to the blends containing a larger amount of the phthalocyanine derivative. These phase-separated blends do not show any homeotropic alignment. AFM investigations confirm the formation of a single columnar morphology in the phthalocyanine-rich blends, consistent with the full miscibility between the two compounds. Solid-state NMR measurements on the mixed phase show the influence of the presence of the perylene molecules on the molecular dynamics of the molecules; remarkably, the presence of the host molecules improves the local order parameter in the phthalocyanine columnar phase. Introduction Thin films made of electron-donor and electron-acceptor molecules and/or macromolecules are of prime importance for the development of large-area, structurally flexible, and lowtemperature processable solar cells.1-3 In this context, most studies have been devoted to the morphology of blends composed of an organo-soluble conjugated polymer and another component. The latter is either another conjugated polymer4-7 or a low molecular weight active molecule such as a soluble fullerene8-13 or a perylene derivative.14 When insoluble components are used, thin films of the blended materials can be fabricated by vapor phase deposition.15 To obtain efficient charge carrier mobility, the control of the microscopic morphology in thin films prepared either by solution processing or by vapor phase deposition is an essential parameter * To whom correspondence should be addressed. (G.A.) E-mail: [email protected]:+33169085879.(R.L.)E-mail:[email protected]. Ph: + 32 65 373860. † Universite´ Libre de Bruxelles. ‡ Universite´ de Mons Hainaut/Materia Nova. § UMR 7504 CNRS-Universite´ Louis Pasteur. | Universite´ Catholique de Louvain. ⊥ Max-Planck-Institut fu¨r Polymerforschung. ¶ Present address: CEA/IRAMIS/SCM/Laboratoire Claude Fre´jacques, CNRS URA 331, Baˆtiment 125, 91191 Gif-sur-Yvette, France.

in organic photovoltaics.16 Post-deposition treatments such as thermal1,17 or solvent annealing8,9 have been applied to thin films; however, the morphology control has not been dramatically improved. Other important features influencing the device performances are the degree of mixing5 and the control of the orientation,18 as well as the conditions for solution processing and thermal treatment. Blending a liquid crystalline semiconductor with a crystalline or another liquid crystalline active component constitutes a novel approach to this question. In such a case, the alignment of the donor and acceptor molecules can, in principle, be controlled in addition to the morphology.19,20 Moreover, the occurrence of several phase transitions for liquid crystalline materials makes it possible to probe the miscibility of donor and acceptor molecules as a function of composition and temperature. It is anticipated that the combined control of morphology, miscibility, and alignment in liquid crystalline films would pave the way to efficient solar cells and other electronic devices. In this context, we report hereafter a combined structural, thermotropic, and morphological study of blends made of a phthalocyanine (Pc), which is a disk-shaped electron donor, and a lath-shaped perylenetetracarboxidiimide derivative (PTCDI) as the electron acceptor. It has been shown that using these two molecules together would lead to efficient electron transfer.21 Their

10.1021/jp809591h CCC: $40.75  2009 American Chemical Society Published on Web 03/20/2009

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SCHEME 1: Molecular Structures of Pc (Left) and PTCDI (Right)a

a Pc exists as a mixture of four isomers: D2h:Cs:C2V:C4h with the statistical distribution 12.5:50:25:12.5, according to ref 36. Alkoxy side chains have been used as racemates.

TABLE 1: Thermotropic Properties of the Blends Pc:PTCDIa

phase behaviorb,c

100:0

Colr 60 (0.05) Colh 180 (3.3) I I 174 (-3.2) Colh 57 (-0.05) Colr Colh 260 (5.0) I I 258 (-3.2) Colh Colh + Cr 175 (0.7) Colh 277 (2.9) I I 270 (-2.2) Colh 168 (-0.9) Colh + LCol 137 (-2.2) Colh + Cr Colh + Cr 177 (6.0) Colh + LCol 230 (1.2) I + LCol 258 (3.3) I I 244 (-3.1) I + LCol 218 (-0.8) Colh + LCol 142 (-6.2) Colh + Cr Cr 178 (12.8) LCol 292 (21.6) I I 282 (-20.1) LCol 152 (-12.3) Cr

75:25 50:50 25:75 0:100

Molar ratio. b Phase transition temperatures (peak, °C) measured during the second heating and cooling runs (10 °C/min). Between parentheses: normalized transition enthalpy values (kJ/mol, ( 5%). c Phase assignment: Cr ) crystalline phase; Colr ) columnar rectangular phase; Colh ) columnar hexagonal phase, LCol ) lamello-columnar phase, I ) isotropic liquid. a

molecular structures are depicted in Scheme 1. Recent investigations22,23 have shown that Pc presents a columnar rectangular phase directly at room temperature up to 60 °C and a columnar hexagonal phase between 60 and 180 °C, whereas PTCDI is crystalline below 178 °C and behaves as a lamellocolumnar (LCol) mesophase between 178 and 292 °C. Furthermore, we have recently shown that Pc and PTCDI can be miscible:22 the blend containing 75 mol % phthalocyanine presents a single Colh mesophase in the 25-260 °C temperature range (Table 1). In this case, the columnar phase is more ordered (with a quasi-long-range order of the stacking repeat unit along the columnar axis), and the phase stability is extended by more than 100 °C with respect to the columnar hexagonal mesophase of pure Pc. It is believed that the phase is mainly formed by the random insertion of lath-like molecules inside columns of disks.

To better understand the conditions of phase formation, the molecular dynamics within the blends, and the microscopic morphology of thin films of those blends, we have carried out a study over an extended composition range. Here we report the results of a combined differential scanning calorimetry (DSC), polarized optical microscopy (POM), small-angle X-ray diffraction (XRD), solid-state nuclear magnetic resonance spectroscopy (NMR), and atomic force microscopy (AFM) investigation as a function of the Pc:PTCDI molar ratio and the temperature. Experimental Methods Materials. Pc and PTCDI have been synthesized as previously described.22,23 For DSC, X-ray, and optical microscopy analysis, all blends were prepared from the molten state, by mixing the two pure compounds during a thermal annealing above their transition to the isotropic state. This was carried out between two glass plates for the optical microscopy analysis. The mixtures can also be cast on a glass substrate for AFM studies of thick films (i.e., a few microns). For AFM studies of thin films (∼100 nm), the two components were dissolved in various proportions using toluene as the solvent. Thin films of these blends were obtained at ambient temperature either by spin-coating or by spin-casting the toluene solutions onto 1 × 1 cm2 pieces of commercially available ITO substrates from Merck KgaA and PGO (Praezisions Glas and Optik). The difference between these two deposition methods lies in the fact that for spin-coating the substrate is accelerated after deposition of the solution, whereas for spin-casting the substrate already rotates at constant speed during the solution deposition. Careful cleaning of the substrates was carried out prior to the film deposition and consisted of the stripping of the photoresist used as a protection layer during the dicing of the substrate (ca. 1 cm2), followed by ultrasonication in acetone. Rinsing in deionized water and washing in boiling acetone and in trichlorethylene was then carried out in order to remove possible organic

5450 J. Phys. Chem. B, Vol. 113, No. 16, 2009 impurities. Rinsing in isopropanol and drying under a nitrogen jet were finally performed to complete the cleaning process. Measurements. Differential scanning calorimetry experiments were run on a Mettler-Toledo heat-flux DSC-821 at a heating/cooling rate of 10 °C/min under a helium atmosphere. Optical microscopy studies were performed on a Nikon eclipse 80i microscope coupled to a Linkam heating stage and a digital sight DS-U1 camera. The cooling rate was 10 °C/min. The XRD patterns were obtained with two different experimental set-ups. In all cases, a linear monochromatic Cu KR1 beam (λ ) 1.5405 Å) from a sealed-tube generator (900 W) equipped with a bent quartz monochromator was used. In the first set of measurements, the transmission Guinier geometry was used, whereas a Debye-Scherrer-like geometry was used in the second experimental setup. In all cases, the powder was filled in Lindemann capillaries of 1 mm diameter and 10 µm wall thickness. An initial set of diffraction patterns was recorded on an image plate; periodicities up to 80 Å can be measured, and the sample temperature is controlled to within (0.3 °C from 20 to 350 °C. The second set of diffraction patterns was recorded with a curved Inel CPS 120 counter gas-filled detector linked to a data acquisition computer; periodicites up to 60 Å can be measured, and the sample temperature is controlled to within (0.05 °C from 20 to 200 °C. In each case, exposure times were varied from 1 to 24 h. Tapping mode (TM) AFM images were recorded with a Nanoscope IIIa microscope from Digital Instruments (Veeco, Santa Barbara, CA) operating in ambient atmosphere at room temperature. Pointprobe cantilevers (Nanoworld) with a resonant frequency of 320 kHz, a typical spring constant of 42 N/m, and an integrated Si tip with a curvature radius of ∼7 nm were used. The phase-lag signal was recorded simultaneously with the topographic response, since the phase signal is very sensitive to small changes in the tip-sample interaction, thereby providing complementary information to the height cartography. Solid-state magic angle spinning (MAS) NMR experiments were performed on a Bruker Avance DRX spectrometer with a 1 H Larmor frequency of 700.1 MHz and a 13C Larmor frequency of 176.05 MHz. A Bruker double resonance probe, supporting rotors of 4.0 mm outer diameter at spinning frequencies of 12.5 kHz, was used for the heteronuclear recoupling experiments. At those moderate spinning frequencies, additional heating effects caused by air friction reported in earlier investigations are not significant, and a correction of the effective sample temperature has not been performed. The REDOR-based recoupled polarization transfer heteronuclear dipolar order rotor encoding (REPT-HDOR)24,25 and rotor encoded rotational echo double resonance (REREDOR)26 were used for 1H-13C double quantum NMR to excite heteronuclear two-spin coherences under recoupling conditions. In both cases, analyses of the recorded sideband patterns yielded effective dipole-dipole couplings depending on internuclear distances and molecular dynamics. Results and Discussion Thermotropic Behavior and Structural Characterization of the Blends. As a first step, we were interested in determining the solubility of PTCDI in Pc over the whole composition range. For this purpose, we have prepared blends with a decreasing amount of the discotic molecule. We report hereafter the results obtained for the blends with the following Pc:PTCDI molar ratios: 75:2, 66:33, 60:40, 55:45, 50:50, 40:60, and 25:75. We describe in more detail the thermotropic and structural behavior of two representative blends, 50:50 and 25:75, and compare

Zucchi et al. them with the properties of the 75:25 blend, which we reported previously.22 The thermotropic behavior of the blends is summarized in Table 1. The transition temperatures discussed hereafter are given as the position of the peak maximum, for the second heating run. The DSC analysis of the 50:50 blend shows a first transition at 175 °C and a second broad transition with a maximum at 277 °C, indicating that this blend does not behave as simply as the 75:25 blend. The thermotropic behavior of the blend with the lowest Pc content (25:75) is also different from those of the 75:25 and 50:50 systems: the DSC curve shows a first transition at 177 °C and two consecutive broad transitions at 230 and 258 °C, respectively. For each mixture, the highest transition temperature corresponds to the isotropization. When examining the blends with intermediate compositions, it was found that the 66:33 and 60:40 systems show only one phase transition (as already observed for 75:25), whereas the 55:45 and 40:60 systems behave very similarly to the 50:50 blend (Table 1). The only difference is the temperature of the “Colh-to-isotropic” transition in the 40:60 blend, which is decreased by about 25 °C, as in the 25:75 blend. These results suggest that the columnar mesophase found previously in the 75:25 blend also exists in the other blends, even though the temperature range over which it is stable depends on the composition: when gradually increasing the amount of PTCDI, phase separation occurs in the low-temperature regime (from the 55:45 blend onward) and finally the transition temperature to the isotropic phase significantly decreases (from the 40:60 blend on). In all cases, we observe reproducible transitions when heating or cooling the samples but the “LCol-to-crystalline” transition of PTCDI is slightly supercooled on cooling (152 °C instead of 178 °C). This difference appears for each blend, as well as for pure PTCDI. It is a strong indication of the occurrence of a transition from (or to) a crystalline phase (transitions between two mesophases or between a mesophase and an isotropic phase show smaller hysteresis). We then used POM to investigate the thermal evolution of the morphology in thick films of these blends (the thickness is of the order of a few micrometers). Again, we show here the evolution for two typical blends (50:50 and 25:75) showing phase separation at low temperature. Cooling from the melt, the 50:50 blend shows a first texture appearing at 275 °C. The dendritic growth is typical of a columnar hexagonal mesophase (Figure 1a), as already observed for the 75:25 system. Below 153 °C, the dendritic texture can still be seen but small streaks appear (Figure 1b), which indicates the coexistence of at least two phases, one of those being a columnar hexagonal mesophase. No other change can be detected upon further cooling down to 25 °C. Under crossed polarizers, the dark areas are the signature of the homeotropic alignment of the Colh mesophase (Figure 1c). The behavior of the 25:75 blend is different. Upon cooling from the melt, a first texture appears around 250 °C (Figure 1d). Since it is similar to that observed in pure PTCDI, we can reasonably assign this texture to the presence of pure PTCDI in its liquid crystalline phase. At 226 °C, a second texture appears (bright areas in Figure 1e). This fan-shaped texture is representative of a columnar mesophase.27 However, in contrast to the behavior of the columnar hexagonal mesophase described above for the other blends, the Colh phase of the 25:75 blend does not form a homeotropic alignment, as shown by the absence of dark areas in the images obtained with crossed polarizers (Figure 1f). No significant eye-detectable change in

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Figure 1. Optical microscopy textures obtained for the 50:50 blend at 268 °C (a) and at 140 °C (b, normal light; c, crossed polarizers) and the 25:75 blend at 250 °C (d) and at 210 °C (e, normal light; f, crossed polarizers).

Figure 2. Diffractograms of the Pc:PTCDI blends (molar ratio): 50: 50 blend at 200 °C (a), 160 °C (b), and 40 °C (c); 25:75 blend at 250 °C (d), 200 °C (e), and 40 °C (f). The diffraction peaks of the Colh mesophase have been indexed for the sake of clarity.

the texture morphology is observed on further cooling down to 25 °C. In particular, the LCol f Cr phase transition of the perylene derivative is not detected, as is the case for the pure compound. To confirm these results and to firmly identify the different phases, we have performed a temperature-dependent X-ray study on the 50:50 and 25:75 blends. The diffractograms are reported in Figure 2, and the structural data are collected in Table 2. At 200 °C, the 50:50 blend shows a diffraction pattern typical of a Colh mesophase. The X-ray pattern presents a set of sharp reflections at small angles, with reciprocal spacings following the 1:3:2 ratio. They correspond to the indexation (hk) ) (10), (11), and (20) of the hexagonal lattice, respectively. In the wideangle region, the diffractogram shows a broad halo with a maximum at 4.77 Å, corresponding to the average distance between side chains in a column, which is representative of the liquid-like order of the molten chains. A sharper halo with a maximum at 3.4 Å is seen at larger angles. This value is attributed to the average stacking distance between the molecules in a column. As already observed for the 75:25 blend,22 the relative sharpness and high intensity of this signal is indicative of quasi-long-range stacking order along the columns. Thus,

the 50:50 blend behaves similarly to pure Pc at high temperature, which means that PTCDI completely incorporates into the Pc phase without modifying it, a clear indication that the two compounds are miscible. The fact that the diffraction pattern of the blend is similar to that of pure Pc clearly suggests that the PTCDI molecules incorporate within the columns of Pc molecules rather than between those columns (since that would lead to an increase in the spacing). The question of miscibility in mesophases of donor-acceptor blends has been investigated by Araya and Matsugana28 in the case of calamitic compounds. In those systems, an extended thermal stability is also observed for the mixed mesophases, which has been attributed to a donor-acceptor interaction. Similarly, a weak charge transfer interaction may take place between Pc and PTCDI molecules in the ground state (full charge transfer occurs only upon photoexcitation), which could explain the enhanced stability of the liquid crystalline phase in the 75:25 and 50:50 Pc:PTCDI blends with respect to pure Pc. A second set of diffraction peaks appears on the diffractogram of the 50:50 blend recorded at 160 °C, along with the reflections of the Colh phase. It corresponds to the diffractogram described for pure PTCDI, consistent with the occurrence of phase separation. At 40 °C, the intensities of the diffraction peaks related to the crystalline phase of PTCDI are higher, indicating that more PTCDI has come out of the initial columnar hexagonal mesophase. This is in a good agreement with the POM and DSC results reported above. The diffractogram of the 25:75 blend recorded at 250 °C (Figure 2d) shows the superposition of two sets of diffraction peaks: the sharp ones are attributed to the LCol mesophase of PTCDI, and the diffuse ones are attributed to an isotropic liquid. This confirms the coexistence of the liquid crystalline phase of PTCDI and the isotropic phase, as proposed above on the basis of the POM results. The diffractogram recorded at 200 °C shows two sets of diffraction peaks, consistent with the coexistence of a columnar hexagonal mesophase and the lamello-columnar mesophase of PTCDI. Since the I f Colh phase transition occurs at 174 °C for pure Pc, we can conclude that the columnar mesophase identified at 200 °C in the 25:75 blend not only is made of Pc but also contains a certain amount of PTCDI. At 160 °C, the diffraction peaks due to the crystalline phase of PTCDI appear, showing that the liquid crystalline-to-crystalline

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TABLE 2: X-ray Characterization of the Phases of Pc, PTCDI, and the 75:25, 50:50, and 25:75 Blends Pc:PTCDI 100:0 (40 °C) Colr

h k l 1 2 0 3 2 4

100:0 (100 °C) Colh 1 1 2 2 75:25 (100 °C) Colh 1 1 2 2

1 0 2 1 2 0

0 1 0 1

0 1 0 1

50:50 (200 °C) Colh 1 0 1 1 2 0 50:50 (40 °C) Colh

CrLam

1 1 2 2

dexpa

dcalca,b

27.6 26.05 16.1 15.15 13.75 13.15 4.6 (halo 1, broad) 3.5 (halo 2, broad)

27.6 26.05 16.3 15.3 13.8 13.0

27.75 15.85 13.7 10.3 4.75 (halo 1, broad) 3.55 (halo 2, broad)

27.45 15.85 13.7 10.35

27.9 16.0 13.8 10.45 4.6 (halo 1, broad) 3.4 (halo 2, sharp)

27.7 16.0 13.85 10.45

cell parametersac a ) 52.1 Å b ) 32.5 Å S ) 1700 Å2 V ) 3260 Å3 h ) 3.8 Å

a ) 31.7 Å S ) 870 Å2 V ) 3375 Å3 h ) 3.85 Å

a ) 32.0 Å S ) 885 Å2 〈V〉 ) 2860 Å3 〈h〉 ) 3.2 Å

0 0 0 0

0 0 0 0

1 2 3 4

25:75 (250 °C) LCol 0 0 0 1 0 1 1

0 0 0 0 0 0 0

1 2 3 0 4 1 2

27.05 15.64 13.53 10.21 4.56 (halo 1, broad) 3.37 (halo 2, sharp) 34.02 16.77 11.17 8.36 8.09 7.41 4.72 4.40

27.05 15.62 13.53 10.22

37.92 18.93 12.66 10.09 9.43 (peak massif) 8.45 4.6 (halo 1, broad) 3.5 (halo 2, sharp)

37.87 18.94 12.62 10.09 9.48 9.26 8.2

a ) 31.25 Å S ) 845 Å2 〈V〉 ) 2280 Å3 〈h〉 ) 2.7 Å

25:75 (40 °C) Colh

CrLam

dexpa

h k l

25:75 (200 °C) LCol 0 0 0 1

Colh

27.33 26.75 a ) 31.12 Å 15.44 15.55 S ) 840 Å2 13.40 13.47 〈V〉 ) 2470 Å3 4.77 (halo 1, broad) 〈h〉 ) 3.0 Å 3.40 (halo 2, broad)

0 1 0 1

Pc:PTCDI

0 0 0 0

1 2 3 0

38.21 18.63 12.39 10.19

1 0 1 1 2 0

27.08 15.49 13.35 4.92 (halo 1, broad) 3.53 (halo 2)

1 0

28.29 4.5 (halo, broad)

0 0 0 0

0 0 0 0

0:100 (200 °C) LCol 0 0 0 1 1 1 1 2

1 34.02 2 16.64 3 11.07 4 8.36 7.83 7.46

0 0 0 0 0 0 0 0

1 2 3 0 1 2 3 0

37.4 18.7 12.45 10.0 9.2 8.2 7.3 5.2 4.6 (halo 1, broad) 0 1 0 3.5 (halo 2)

0:100 (100 °C) CrLam 0 0 1 35.2 0 0 2 17.25 0 0 3 11.5

dcalca,b

cell parametersac

a ) 10.47Å b ) 3.5 Å c ) 38.58 Å β ) 103.3° V ) 1376 Å3 〈V〉 ) 1900 Å3 〈N〉 ) 1.4 Å 26.87 a ) 31.03 Å 15.51 S ) 834 Å2 13.43 〈h〉 ) 1.5 Å 37.55 18.77 12.5 10.19

28.3

a ) 32.67 Å S ) 924.5 Å2 〈V〉 ) 1790 Å3 〈h〉 ) 1.9 Å

37.4 18.7 12.45 10.0 9.25 8.2 7.2 5.0

a ) 10.2 Å b ) 3.5 Å c ) 38.1 Å β ) 100.85° V ) 1333 Å3 N≈1

3.40

a ) 10.34 Å b ) 3.5 Å c ) 38.8 Å β ) 102.47° Vcell ) 1370 Å3 〈V〉 ) 2000 Å3 〈N〉 ) 1.4 Å

a Distances and cell parameters are given in angstroms. b dexp and dcalc are the experimentally measured and calculated diffraction spacings. dcalc is deduced from the following mathematical expressions: for the LCol phase 〈d001〉 ) (1/Nl)(∑l d00l · l), where Nl is the number of 00l reflections, the other theoretical dh0l spacings were determined considering a monoclinic lattice; for the Colh phase, the lattice parameter a is given by a ) (2/Nhk3)(∑h,k dhk · (h2 + k2 + hk)), where Nhk is the number of hk reflections and the lattice area (i.e., columnar cross-section) S ) a231/2/2; for the Colr phase, the lattice parameters a and b are obtained from 〈dhk〉 ) 1/[(h2/a2 + k2/b2)1/2], and the lattice area S ) a × b. a, b, c, and β are the parameters of the monoclinic cell of the LCol phase.

phase transition of pure PTCDI is occurring. This transition corresponds to the signal detected at 176 °C during the DSC analysis. The difference between the DSC and the XRD measurements in the temperature at which this transition is observed is most probably due to the kinetics of the crystallization: the sample for XRD is maintained for a few hours at

the temperature of the measurement, while the cooling rate used for DSC analysis is much faster (10 °C/min). These results, along with our previous studies,22 lead us to the following conclusions. For the blend containing 75 mol % Pc, the perylene derivative is fully soluble in the phthalocyanine for temperatures ranging from 258 to at least 25 °C. The binary

Miscibility between Differently Shaped Mesogens system forms a single columnar hexagonal mesophase that is more ordered and thermally stabilized with respect to the columnar phase in pure Pc. This behavior is maintained up to the 60:40 Pc:PTCDI composition. For the blends having between 55 and 40 mol % Pc, the two components are totally miscible at high temperature, where they form a hexagonal mesophase similar to that described for pure Pc, and crystallization of PTCDI occurs when the sample is cooled. At lower temperatures, both X-ray and optical microscopy techniques show that there is coexistence of the two phases (the columnar phase and the crystalline PTCDI phase). These results thus indicate that the solubility of PTCDI in Pc is limited. This is further confirmed by the behavior of the 25:75 blend. For that composition, PTCDI is no longer fully soluble in Pc, even at high temperature. Molecular Dynamics in the Pure Components and the Blends. Miscibility of PTCDI in the Pc mesophase most probably implies that PTCDI molecules are inserted within the columns of phthalocyanine molecules. To understand the nature of the intermolecular interactions between Pc and PTCDI molecules in the mixed phases, which can explain the increased thermal stability of the columnar phase, we performed solidstate NMR studies of the pure components and the blends. Temperature-dependent studies of quadrupole couplings under static conditions29 and determinations of T1 relaxation times30 have indeed been proven to be powerful methods for siteselective studies of molecular dynamics. Recently, advanced NMR methods have been developed, providing site-selectivity by spectral resolution, which previously had been obtained by selective isotopic labeling, and increased sensitivity by means of fast MAS. In fact, instrumental improvements, in particular the high magnetic fields and high MAS spinning frequencies, contributed to the success of homo-31 and heteronuclear25 dipolar recoupling methods. These approaches have successfully been applied to investigations of the site-selective dynamics in discotic liquid-crystalline compounds.32 The molecular dynamics in the 72:25 Pc:PTCDI blend, as well as the dynamics in the pure components, were studied by analyzing the heteronuclear spinning sideband patterns, recorded in 1H-13C REPT-HDOR and 1H-13C REREDOR NMR experiments performed under moderate MAS conditions. The sideband pattern analysis provided effective heteronuclear 1H-13C dipolar coupling constants, which have been interpreted in terms of local dynamic order parameters, S, given in the form of the second order Legendre polynomial.33 A common way to determine the local order parameter S is to relate the motional averaged effective dipolar coupling constant Deff obtained experimentally to the static dipolar coupling constant Drigid, which depend only on the spin species involved and on their internuclear distance. In the case of 1H-13C dipolar couplings of CHn groups, where the distance is not averaged by molecular motions and the geometry of the spin system is known, Drigid can be computed from the distance between the spins involved. From the definition S ) Deff/Drigid, it is obvious that S ) 1 stands for segments that are totally rigid, whereas S ) 0 indicates fast isotropic molecular reorientations on the kHz time scale. Pure PTCDI. To probe the molecular dynamics of the pure PTCDI at ambient conditions, a 1H-13C REREDOR measurement at 25 °C and 12.5 kHz MAS with 160 µs recoupling time has been conducted. At elevated temperatures, 1H-13C REPTHDOR NMR experiments performed better than the REREDOR approach. REPT-HDOR sideband patterns at 12.5 kHz MAS were recorded at 100 °C using 320 µs recoupling time and at 140 °C with only 160 µs recoupling time.

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Figure 3. Schematic representation of the axial 180° flips and outof-plane fluctuations of the PTCDI core (the (CH2)8-CHdCH(CH2)7CH3 side chains are not displayed) about the symmetry axis the molecule at elevated temperatures.

The REREDOR measurements at 25 °C provided a value of 14 kHz for the 1H-13C couplings of the aromatic core CH moieties, a value that is reduced by a factor of 2/3 compared to the immobile CH coupling constant. This indicates that the perylene diimide core is not completely rigid even in the crystalline phase. It should be noted that the observed reduction factor is close to the value of 0.625 characteristic for phenyl flips,32 but the reduction could as well result from significant thermal fluctuations about the symmetry axis of the molecule. Upon heating, the effective dipolar 1H-13C couplings of the core CH groups are further reduced, reaching a value of 5.6 kHz corresponding to S ) 0.27 at 140 °C. Since the melting of the crystalline phase starts above 178 °C, the dynamic processes causing the motional averaging of the dipolar couplings have to preserve the symmetry of the crystal lattice. Thus, 180° flips and strong fluctuations about the symmetry axis of the molecule are the most likely processes (Figure 3). The dynamic behavior of the alkyl chains of PTCDI at ambient conditions reflects the characteristic gradient of mobilities along the chain, starting from an initial order parameter value of S ) 0.21 for CH2 units close to the perylene core down to S ) 0.13 and less than 0.1 for the methylene and the methyl groups at the chain ends, respectively. The chemical structure suggests that the dynamic local order parameter of the methyne group should be elevated in comparison to the other parts of the alkyl chain, because of the stiffer sp2-hybridization state of the double bond. However, this assumption was not confirmed by the experimental results at ambient temperature. Upon heating to T ) 100 °C, the effective couplings of segments in the alkyl chain apparently increase to an average dynamic order parameter value close to 0.2 and the gradient of mobility observed at ambient conditions vanishes. Increasing the temperature to T ) 140 °C does not further modify the order parameters. Only a minor decrease from S ) 0.26 at T ) 100 °C to S ) 0.21 at T ) 140 °C is observed at the methyne position. Remarkably, at these temperatures, the order parameter depends on the recoupling time for all positions along the alkyl chain, apart from the methyne position. This suggests that the complex dynamic behavior of the alkyl side chains might be caused by a coupling of the dynamics of the rigid core and the stiff CHdCH group in middle of the chain via the more flexible alkyl chain; further investigations are needed to confirm this assumption. Pure Pc. The 1H-13C REPT-HDOR NMR technique has been used to determine the 1H-13C dipolar couplings and thus local order parameters S from recorded sideband patterns. The experimental conditions were the same as for the perylene diimide derivative: 100 and 140 °C, 160 µs recoupling time, and 12.5 kHz spinning at the magic angle. REREDOR measurements at ambient conditions have been performed but did not provide any information because of very poor signal intensity and a lack of rotor modulation. The failure of the experiments

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Figure 4. REPT-HDOR sideband patterns of the 75:25 blend, recorded at 12.5 kHz MAS, 320 µs recoupling time at 100 °C, and 160 µs recoupling time at 140 °C, with the simulated patterns (blue) superimposed. The effective 1H-13C dipolar coupling constants, derived from the sideband patterns, together with the related local order parameters, S, are presented.

is attributed to molecular tumbling with a correlation time similar to the MAS rotor period, which leads to destructive interferences during the recoupling procedure. Thus, a siteselective study of the molecular dynamics, as described for the perylene derivative, is not feasible at ambient conditions. At T ) 100 °C, the effective dipolar couplings determined for the core CH moieties were in the range of 5.5-6.8 kHz, with the respective order parameter S ) 0.26-0.32, depending on the CH position in the phthalocyanine core. Two scenarios explaining this significant reduction of the measured couplings with respect to the rigid state values are possible: either a rapid rotation about the 4-fold molecular symmetry axis of the phthalocyanine in the mesophase with additional out-of-plane excursions with a σ ) 20°, assuming a Gaussian distribution of the fluctuations, or a stronger tumbling of the stacked phthalocyanine molecules (σ ) 30°) in the disk plane without rotation. Remarkably, further heating to 140 °C has no major influence on the effective dipole-dipole couplings for the core CH moieties: the effective coupling values determined at T ) 140 °C are in the range of 5.6 and 6.0 kHz and the derived local order parameters S ) 0.27 - 0.29. The smaller spread of S values for the aromatic CH groups observed at higher temperature is indicative of more uniform molecular dynamics of the Pc core, compared to its behavior at lower temperature. These results already suggest that further heating to the melting point of the sample (180 °C), which is not feasible because of NMR hardware limitations, would not change substantially the dynamic behavior of the core. The C14,10 side chains of the phthalocyanine derivative exhibit a gradient of mobility at 100 °C. The dynamic order parameter at the chain branching position Sbranch ) 0.29 corresponds to the value of the core, whereas the average order parameter of the methylene units along the chain has been determined to be Schain ) 0.17. As expected, raising the temperature from 100 to 140 °C does not change the gradient of mobility and causes only a minor reduction of these order parameters (Sbranch) 0.26, Schain) 0.12). 75:25 Pc:PTCDI Blend. This system has been studied using the REPT-HDOR NMR technique. To evaluate the mobility of

both components of the blend, spectral separation of the signals resulting from the different components is needed. This requirement, however, is not fulfilled here, because most of the signals of the PTCDI aromatic core overlap with signals from the Pc core. Nevertheless, two signals from the Pc core are unaffected by signals from the other component, so that the dynamic behavior of the Pc component in the blend can be investigated, in connection with the X-ray data that show that the symmetry of the Pc columnar hexagonal mesophase is not affected by the addition of PTCDI, which in fact stabilizes the phase and extends its existence to a broader temperature range. The measurements have been performed at 12.5 kHz MAS under conditions comparable to those for the pure Pc sample. At T ) 100 °C, the local order parameters are S ) 0.43 and S ) 0.44 for the two accessible sites of the Pc core (Figure 4). Comparing these values with the local order parameters of the same sites in the pure Pc sample (S ) 0.26 and S ) 0.30), an increase of the order parameter by about 50% is found upon addition of PTCDI. Moreover, the local order parameter of the Pc component in the blend slightly increases to S ) 0.46 upon raising the temperature to T ) 140 °C. Similar to the signals of the aromatic core, most of the signals of the aliphatic side chains of the Pc and PTCDI are superimposed and cannot provide further information on the dynamic behavior of the Pc molecules in the mixture. However, the signal of the Pc OCH2 group is well resolved and confirms the findings for the dynamic behavior of the aromatic Pc core in the mixture. At T ) 140 °C, the OCH2 group exhibits a dynamic order parameter as high as S ) 0.47 compared to S ) 0.26 in the pure compound at the same temperature. These results point to an increased degree of order in the Pc liquid crystalline phase when PTCDI is incorporated (a schematic representation of the structure of the Pc columns containing PTCDI molecules is shown in Figure 5). This is quite consistent with the calorimetric measurements showing an extended thermal stability of the columnar phase in the blend with respect to the pure Pc system. Microscopic Morphology. The phase behavior is expected to govern the morphology on the mesoscale in films of the Pc/

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Figure 5. Schematic representation of the molecular organization within the Pc columns, in the absence (left) and in the presence (right) of a PTCDI molecule (in green); the columns of pure Pc show a lower degree of order (since molecular mobility is higher, as indicated by the NMR data). The order is better in the mixed columns because the presence of the PTCDI molecules reduces such mobility.

Figure 6. AFM height images of thick deposits of the 75:25 blend [(a) 1 × 1 µm2]; the 50:50 blend [(b) 1 × 1 µm2]; pure PTCDI [(c) 15 × 15 µm2]; the 25:75 blend [(d) 40 × 40 µm2].

PTCDI blends, which in turn can strongly affect the performances of the photovoltaic devices. This issue has been investigated here with TMAFM. The microscopic morphology of a “thick” deposit (i.e., a few micrometers) of the 75:25 blend at room temperature is shown in Figure 6a. No specific structure is observed, and the surface of the film is smooth (the rootmean-square (rms) roughness is 0.3 nm). This is consistent with the absence of phase separation in this system (upon phase separation, very clear morphological features are present). In sharp contrast, the surface of the 50:50 blend (Figure 6b) is much rougher (the rms roughness is 10.9 nm), with elongated, needle-like features reminiscent of the crystalline domains in deposits of pure PTCDI (Figure 6c). This definitely points to the occurrence of phase separation, in agreement with the optical microscopy and XRD results. The formation of crystalline PTCDI domains is even clearer in blends richer in PTCDI; for instance, the 25:75 blend shows a very large number of PTCDI crystals (Figure 6d) and, consistently, the roughness is as high as 14.8 nm. We also have examined the morphology versus composition dependence for film thicknesses relevant to solar cells, i.e., in the 100 nm range.34 Those films were deposited from toluene solutions on ITO substrates. As a preliminary step, AFM analysis of the clean ITO surface was carried out (Figure 7a

Figure 7. AFM images of the bare ITO substrate [5 × 5 µm2 (a) and 1 × 1 µm2 (b) height images]; thin deposits of the 75:25 blend [2 × 2 µm2 (c) height andphase (d) images; 300 × 300 nm2 phase image (e)]; a thin deposit of the 50:50 blend [1 × 1 µm2 phase image (f)]. The vertical grayscale of the height images is 15 nm.

and b). For both types of ITO considered in this study, the surface exhibits a granular morphology (the average lateral size of the grains is in the 20-40 nm range). The grains are arranged inside terraces that appear at different heights (the “darker” terraces corresponding to those of lower height). On the micrometer scale, thin films of the 75:25 blend show a featureless morphology, as illustrated by the height image in Figure 7c. This structure is quite similar to that observed for the “thick” film with the same composition (Figure 6a), again suggesting the absence of phase separation. The corresponding phase image (Figure 7d) shows a pattern of darker and brighter

5456 J. Phys. Chem. B, Vol. 113, No. 16, 2009 domains, often with well-defined, straight boundaries. The size and shape of those domains are similar to those of the terraces found on the bare ITO. A closer view to the internal structure within those domains reveals the presence of very thin and very long objects (Figure 7e), which we attribute to single columns of mesogen molecules. A given domain is made of columns with the same orientation; it is the difference in the orientation of the columns from one domain to another that induces differences in contrast in the phase image of Figure 7d. Such differences probably originate from subtle variations in tip-sample interaction due for instance to asymmetry of the tip apex. The darker domains are made of columns that are organized perpendicularly to the scan direction, whereas the brighter ones are those for which the column orientation is somewhat tilted with respect to the scan direction. The thick dark lines appearing in the images are domain boundaries that mark a change in the column orientation. Since the size and shape of those domains are similar to those of the ITO terraces, we can reasonably propose that it is the topography of the ITO surface, in particular the presence of terraces, that sets the lateral extension of the domains with uniform column orientation. A sectional analysis over a given domain yields an intercolumn distance of about 3.5 nm. So, this type of uniform columnar organization is the AFM signature of a single Pc:PTCDI phase. We have observed this type of organization for films of the 75:25 blend prepared with different spinning speeds (from 1000 to 6000 RPM) and different thicknesses (from a few 10s to 100 nm). Interestingly, the same nanometer-scale morphology is observed with AFM in thin films of pure Pc with planar alignment (i.e., with the columns lying parallel to the surface).23,35 In particular, the intercolumnar distance observed for the 75:25 blend is almost identical to that of pure Pc, i.e., 3.5 versus 3.9 nm, respectively. It is also worth noting here that the intercolumnar distance measured with AFM in the case of the 75:25 blend is also in good agreement with the XRD data of Table 2. It thus appears that the presence of 25 mol % PTCDI does not disrupt the organization of Pc, which is a further indication that the PTCDI molecules intercalate between the Pc molecules in the stacks (as previously proposed22). The morphology of films of the 50:50 blend prepared in the same conditions is markedly different, as illustrated in Figure 7f. The deposits are made of an intimate mixing of small crystallites (appearing brighter in the phase image), which we attribute to the pure PTCDI phase, surrounded by darker regions attributed to the mixed mesophase. This observation is in agreement with the DSC, POM and XRD results, which show the occurrence of phase separation for this composition at room temperature. Finally, we have investigated whether we could induce and possibly control the phase separation process in systems showing complete mixing, e.g., in the 75:25 blend. For this purpose, we have first modified the conditions for film deposition by spincoating, by operating at low speeds. We have observed that films generated at 500 RPM indeed exhibit a clear phase separation (Figure 8a), with crystalline features typical of PTCDI surrounded by a matrix still constituted of a dense packing of 3.5 nm wide columns (Figure 8b). This phase separation, which does not occur in films prepared at higher speeds, most probably originates from the fact that during this ‘slow spin-coating’ the compounds stay for a longer time in solution before complete evaporation of the solvent. Since PTCDI is less soluble than Pc in toluene, it is more likely to precipitate into crystalline aggregates before complete removal of the solvent, yielding the phase-separated morphology. The fraction of the film area

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Figure 8. AFM images of thin deposits of 75:25 blends deposited by spin coating at 500 RPM [5 × 5 µm2 (a) and 200 × 200 nm2 (b) phase images]; deposited by spin coating at 4000 RPM and exposed to toluene vapors for 24 h [200 × 200 nm2 (c) and 20 × 20 µm2 (d) phase images].

corresponding to the PTCDI crystallites, as estimated by image analysis, is close to 6%, i.e., less than the amount of that compound in the blend (the mass fraction of PTCDI in this blend is 13.5%), which indicates that a significant part of the PTCDI molecules still form the mixed phase with Pc. To confirm the influence of the solvent on the development of the phase separation in those blends, a homogeneous sample of the 75: 25 system was exposed to toluene vapors for 24 h. Such treatment induces drastic morphological modifications: while the initial morphology is homogeneous, with densely packed molecular columns (as in Figure 7e), the annealed film exhibits a pronounced phase separation, which can be observed at different lengthscales. On the one hand, small bright objects (probably small crystalline domains) a few tens of nanometers in width and length appear in between the close-packed columns, as illustrated in Figure 8c. On the other hand, phase separation also takes place on a much larger lengthscale, with PTCDI crystals as large as a few microns (Figure 8d). It must be emphasized that single-phase 75:25 Pc:PTCDI deposits are absolutely stable at room temperature and do not show any tendency to spontaneously phase separate, even after a few months. Thus, it is the solvent that induces phase separation, probably by “plasticizing” the mixed mesophase and leading to local increase in PTCDI concentration and then nucleation of crystallites. Conclusion In this work, we have investigated and rationalized the thermotropic, structural, and morphological behavior of a binary system made of electron-donor and electron-acceptor mesogens with different shapes. Through a combined DSC, POM, XRD, solid-state NMR, and AFM study, we have determined (i) the miscibility domain between the disk-like phthalocyanine compound and a lath-like perylene diimide, (ii) the nature of the phases occurring in the various mixtures, and (iii) the dynamics of the Pc compound in the presence of PTCDI in the Pc

Miscibility between Differently Shaped Mesogens columnar phase. When containing at least 60 mol % of the discotic molecule, the blends show only one columnar hexagonal mesophase, which is strongly stabilized (more than 100 °C) and much more ordered with respect to the corresponding phase of pure Pc. Since the structure of the “mixed” phase and the microscopic morphology of its thin films are almost identical to that of pure Pc, we are led to the conclusion that the PTCDI molecules are not located between the Pc columns but are inserted within those columns. Quite interestingly, such incorporation appears to increase the degree of order of the Pc molecules, which is probably the reason for the increased stability of the columnar phase. For blends containing between 55 and 40 mol % Pc, the two components are fully miscible at high temperature and form a columnar hexagonal mesophase, but PTCDI crystallizes out when the temperature is decreased. The columnar mesophase is less stabilized when the amount of PTCDI is increased. Finally, for the 25:75 blend, part of the lath-shaped compound is not soluble in the phthalocyanine mesogen, but the Colh mesophase observed at high temperature is still formed of a mixture of both compounds. On the basis of these results, we believe that the blends having more than 40 mol % Pc should be promising as active layers for photovoltaic applications. First, the Colh mesophase aligns homeotropically between two substrates (e.g., two electrodes), leading to the desired orientation for optimal transport properties. Second, as the ideal morphology for photovoltaics would be a blend made of nanometer-scale monodomains of each active component, the fact that a part of the perylene derivative can crystallize out of the Colh mesophase is interesting. The amount of phase-separated PTCDI could be controlled by exposing a blend in which the two components are fully miscible to solvent vapors for a controlled time. This would create the bulk heterojunction with domains having the needed size. Solvent annealing could therefore be used to control the lengthscale of phase separation and therefore the spatial distribution of the interfaces between donor-rich and acceptor domains, in order to optimize the charge separation and transport properties, which would lead to efficient devices. Acknowledgment. This work was supported by the Walloon Region PIMENT program (SOLPLAST project), the Belgian Federal Science Policy Office (SOLTEX PAT project and IAP 6/27), the European Commission (NAIMO NMP4-CT-2004500355 and Phasing-out Hainaut), and the Belgian National Fund for Scientific Research (FNRS). BD thanks the Centre National de la Recherche Scientifique and the University Louis Pasteur for support. References and Notes (1) Peumans, P.; Uchida, S.; Forrest, S. R. Nature 2003, 425, 158. (2) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498. (3) Coakley, K. M.; McGehee, M. D. Chem. Mater. 2004, 16, 4533. (4) Halls, J. J. M.; Arias, A. C.; MacKenzie, J. D.; Wu, W.; Inbasekaran, M.; Woo, E. P.; Friend, R. H. AdV. Mater. 2000, 12, 498.

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