in Dilute Solutions and Spin-Coated Films - ACS Publications

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Aggregation Behaviors of Ladder-Type Poly(p‑phenylene) in Dilute Solutions and Spin-Coated Films Linlin Liu, Tao Han, Xiaoyan Wu, Song Qiu, Baoling Wang, Muddasir Hanif, Zengqi Xie, and Yuguang Ma* Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China ABSTRACT: The polymer aggregation morphology in thin films strongly depends on the state of the solution and the film processing conditions, which lead to important impacts on the optoelectronic properties. In this work, the aggregation behaviors of ladder-type poly(p-phenylene) (Me-LPF) in dilute toluene solutions and spin-coated films are investigated by concentration- and temperature-dependent fluorescence spectroscopy and atomic force microscopy (AFM) combined with X-ray diffraction. Spin coating is normally a typical method for amorphous and isotropic films. In Me-LPF, the preassembly in solution plays an important role in aggregation behaviors in spin-coated film. The driving force of aggregation would be weak side-chain entanglement in the absence of intermolecular π−π interactions, which are very sensitive to temperature. The polymer chain aggregation is observed in dilute toluene solutions (>8 μg/mL) and spincoated films at room temperature, which induces fluorescence quenching of the 0−0 emission band. It comes from enhanced energy transfer efficiency in aggregates by shortened intermolecular distance and ordered molecular orientation. These results supply a facile spin-coating process to prepare high order films, which show potential application in optoelectronic devices. the determination of excimer in fluorene-based polymer in fluorenone-free polyspirobifluorene has been reported by our group.13 Ladder-type poly(p-phenylene) (LPPP) derivatives with rigid planar backbone are an important model compound in photophysical research fields.14−20 Our research group reported high quality ladder-type PPPs (Me-LPF) (Chart 1),21

1. INTRODUCTION In the past few decades, conjugated polymers have attracted broad research interest due to their potential applications in polymer light-emitting diodes (PLED), polymer solar cells (PSCs), and field effect transistors (FETs). In all of these applications, the conjugated polymers are used in the thin-film state fabricated by solution processing. It is well-known that optoelectronic functions are not only from the nature of a single polymer chain but also from the aggregation morphology in condensed matter. The polymer aggregation morphology in thin film strongly depends on the state of the solution and the film processing conditions, leading to important impacts on the optoelectronic properties, i.e., the solid-state luminescence and the charge mobility properties.1−3 In most cases, spin coating is regarded as a typical method for the fabrication of amorphous and isotropic films. However, during the active-layer preparation of PSCs, scientists have observed that suitable solvents and additives can induce effective packing of a conjugated polymer chain in spin-coated films, which is a benefit for charge transport and PSC efficiency. The main driving force of this process would be π−π stacking of conjugated backbone and entanglement of side chain substitutions.4−6 To our knowledge, there is little reported on effective packing in spin-coated film for electroluminescent conjugated polymers.7 The collaboration between morphological and optical properties in electroluminescent conjugated polymers requires a high quality polymer because some emissive defect, like fluorenone in fluorene-based polymers, shows similar low energy emission as π-stacked aggregates.8−12 In previous work, © 2015 American Chemical Society

Chart 1. Molecular Structure of Me-LPF

which exhibit high efficient electroluminescence with the absence of low-energy emission and are ideal for the study of the aggregation behaviors and the optical properties. In this work, we investigate the aggregation of Me-LPF in dilute toluene solutions and spin-coated films with its role in optical properties. The aggregation behaviors and the optical properReceived: March 6, 2015 Revised: May 9, 2015 Published: May 12, 2015 11833

DOI: 10.1021/acs.jpcc.5b02224 J. Phys. Chem. C 2015, 119, 11833−11838

Article

The Journal of Physical Chemistry C

Figure 1. (a) Absorption and PL spectra of Me-LPF in dilute solution. The spectra are normalized by the 0−0 band. (b) The emission spectra of the Me-LPF in diluted toluene solutions with different concentrations at room temperature. The spectra are normalized by the 0−1 band. (c) The absolute PL intensity at 464 nm (the 0−0 band) and 493 nm (the 0−1 band) of the Me-LPF emission at the same spectral parameters depending on concentration. Excitation is at 400 nm for all PL measurements.

2.2−2.3 eV that is similar to the emission of the π-stacked aggregates or the excimer. To address this question, Me-LPF was cast on CaF2 for FT-IR measurement, and the band around 1720 cm−1 was not observed, which indicates the absence of such a fluorenone defect.

ties are demonstrated as functions of concentrations and temperatures. The relationship between aggregates and optical properties and the driving force of aggregation are discussed.

2. EXPERIMENTAL SECTION 2.1. Instrumentation. Absorption and fluorescence measurements were performed on a Shimadzu UV-3100 spectrophotometer and a Shimadzu RF-5301PC spectrophotometer, respectively. Atomic force microscopy (AFM) images were recorded under ambient conditions using a Digital Instrument Multimode Nanoscope IIIa operating in the tapping mode. Si cantilever tips (TESP) with a resonance frequency of approximately 300 kHz and a spring constant of about 40 N m−1 were used. The scanning rate is varied from 0.5 to 1.5 Hz. The X-ray diffraction was performed on a Rigaku X-ray diffractometer (XRD) (D/max-r A, using Cu Kα radiation of wavelength 1.542 Å), and the samples were prepared by a dropcasting method. 2.2. Materials. Me-LPF was synthesized according to the literature.21 The number-average molecular weight (Mn) of Me-LPF is 22 000 with a polydispersity index of 1.92 determined by gel permeation chromatography (GPC), corresponding to ∼70 para-phenylene subunits in the backbone. It was necessary to ensure that the fluorene copolymer is fluorenone-free at the 9-fluorene sites because the 9-fluorenone would lead to an unwanted low-energy emission band around

3. RESULTS AND DISCUSSION Figure 1a shows the absorption and PL spectra of Me-LPF in toluene. Me-LPF displays a steep absorption onset and wellresolved vibrational energy bands, which indicate a rigid πsystem. The PL spectra are mirror image to the absorption spectra with a very small Stokes shift and show a large overlap of 0−0 bands with absorption spectra. This large spectral overlap would induce an effective energy transfer process between polymer chains. The concentration-dependent emission spectra of Me-LPF in toluene solutions are shown in Figure 1b,c. In normalized spectra, the intensity of 0−0 emission bands decreases with a small red shift when the solution concentration is increased. The absolute PL intensity for the 0−0 emission band increases first and then decreases with a critical concentration nearby of 8 μg/mL (Figure 1c). For the 0−1 emission to still increase linearly during this concentration range (Figure 1c), this critical point would not be from concentration quenching. Because of the large overlap between absorption and PL spectra, the unlinear relation of the 0−0 emission band with concentration would be related to the 11834


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Figure 2. (a, b) Normalized PL spectra of the Me-LPF solutions as a function of the temperature (a) at 1 μg/mL and (b) at 10 μg/mL. (c) Absolute PL intensity at 464 nm (the 0−0 band) of the Me-LPF as functions of temperature. (d) PL spectra of Me-LPF film spin-coated from solutions with different temperatures.

processes on all wavelength ranges.22 However, when the concentration is increased above 5 μg/mL, especially above 8 μg/mL, the emission intensity unusually increases with the temperature increase (Figure 2c). When the spectra in these regions are normalized by the 0−1 band, the relative intensity of the 0−0 band increases, and there is a blue shift at high temperature (the results at 10 μg/mL are shown in Figure 2b), which indicates the decreased energy transfer process with increasing temperature. This phenomenon would be understood by dissolution of the aggregate at high temperature. After dissolution of the aggregate, the Förster energy transfer process weakens where the relative intensity of the 0−0 band increases and blue shifts. We emphasize that the effective 0−0 band quenching comes back when the solution goes back to room temperature, which indicates the aggregate is from the assembly of a polymer chain-like micelle but not bad solubility. We also characterize the aggregate by transferring it onto a suitable substrate and determine it by morphological methods. In order to keep the structure in solution as much as possible, a silicon wafer is inserted into 8 μg/mL of Me-LPF solution and peeled off after solution quenching occurs at 77 K, and the AFM image of the silicon wafer is shown in Figure 3a. Under the AFM images, the Me-LPF appears as circle particles with 30 nm and 130−200 nm diameter. On the basis of the numberaverage molecular weights of Me-LPF, the theoretical length of the polymer chain can be estimated as 20−30 nm, which is in agreement with the size in AFM images. Thus, the observed particles of 30 nm might be single-chain-like polymer. Particles

energy transfer process between different molecules, such as reabsorption and dipole−dipole energy transfer (Förster type). Reabsorption, which means the emission of one polymer chain is absorbed by another polymer chain, is ineffective radiant energy transfer with long working range. Thus, the reabsorption process would exist in all concentration ranges. The dipole− dipole energy transfer (Förster type) is an efficient energy transfer with short working range (the typical Förster radius of 5−10 nm),22 where additional Förster energy transfer in higher concentration would be on duty for effective 0−0 quenching after the critical concentration. However, for the 8 μg/mL solution of Me-LPF the average distance of the adjacent polymer chains is about 500 nm by assuming the polymer is molecularly soluble, which is too large for an efficient Förster energy transfer. On the basis of the above observations and analysis, we speculate the aggregates of Me-LPF to be at room temperature when the concentration is higher than 8 μg/mL. As the aggregations are normally a function of temperature, the temperature-dependent PL measurements were measured for the Me-LPF solutions with varied concentrations (Figure 2). When the concentrations are below 5 μg/mL, the relative intensity of the 0−0 band and 0−1 band remains the same at different temperatures (the result at 1 μg/mL is shown in Figure 2a), and the absolute PL intensity decreases with the increased temperature (Figure 2c). These spectral properties are very common in photophysics because the emission quenching is proportional to temperature, which means the increasing temperature will enlarge the emission-quenching 11835


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Figure 3. (a) AFM height image (2 × 2 μm2) of the aggregates in dilute solution (silicon wafer peeling off from 8 μg/mL of Me-LPF at 77 K, z scale 10 nm). (b−f) AFM height images of Me-LPF film spin-coated from toluene solution on quarts substrates. (b: 0.1 mg/mL r.t., 10 × 10 μm2, z scale 10 nm; c: 0.4 mg/mL r.t., 5 × 5 μm2, z scale 10 nm; d: 0.8 mg/mL r.t., 10 × 10 μm2, z scale 15 nm; e: 2 mg/mL r.t., 10 × 10 μm2, z scale 10 nm; f: 2 mg/mL at 50 °C, 5 × 5 μm2, z scale 15 nm.)

force of aggregates. Figure 4 shows the XRD patterns of the Me-LPF cast film. At 20 °C, the diffraction pattern of Me-LPF shows a broad peak at 19.0° and two sharp peaks at 6.6° and 2.0°, which correspond to a d spacing at 4.8, 13.5, and 40 Å, respectively. This type of pattern is similar to those observed in PFs or PIFs (polyindenofluorenes).23 The thickness of the single molecules is 40 Å. The origin of the diffraction peak at

with 130−200 nm diameter are aggregates consisting of tens of polymer chains. The critical aggregate concentration measured by concentration and temperature dependence of PL spectra would correspond to the formation of these aggregates. Furthermore, the AFM images of Me-LPF spin-coated films are compared from solutions with different concentrations. Figure 3b shows the AFM high images of the spin-coated film from 0.1 mg/mL of solution. The resulting Me-LPF film could not cover the whole substrate and shows a dendritic shape of Me-LPF islands, which are a few hundred nanometers wide and ∼4 nm in height. The thickness of the layer (4 nm) corresponds well to the thickness of the single molecules. At 0.4 mg/mL, the Me-LPF film still does not cover the whole substrate and shows a dendritic shape as in 0.1 mg/mL (Figure 3c). In this concentration, the substrates are covered more than that in 0.1 mg/mL, and the size of the dendritic islands is much more uniform than that in 0.1 mg/mL. When the concentration increases to 0.8 mg/mL, the dendritic shape is still observed, and some additional circle particles adsorb on it (Figure 3d). When the concentration is larger than 1 mg/mL, the polymer covers all the substrate, and we could not observe the dendritic shape at all (Figure 3e). The surface of the film is on average covered by circle particles with a few hundred nanometers diameter. These results are remarkable because of the regular packing mode from the spin-coating process, where the preassembly in solution would play an important role. The polymer structures in the solid state are studied by X-ray diffraction, in order to explore the packing mode and driving

Figure 4. X-ray diffraction pattern of Me-LPF cast films as a function of cast temperatures. 11836


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Figure 5. Proposed aggregation model of Me-LPF dendritic island.

The energy transfer efficiency in thin solid film strongly depends on the distribution of the dipole orientation in aggregates. In Förster energy transfer, it is proportional to the donor−acceptor dipole orientation factor (κ2). In the isotropic distribution of the donor and acceptor molecules, a value of 0.66 can be assigned to κ2, while κ2 is equal to 1 or 4, when the absorption and emission dipoles are parallel reversed or the same, respectively.22 Thus, the efficiency in ordered packing is bigger than that in the isotropic distribution system. This would be the reason that the emission intensity of the 0−0 band in spin-coated films increases with the dissolution of aggregates (Figure 2d).

13.5 Å is assigned to the average distance between polymeric backbones. The periodicity of 4.8 Å would reflect the spacing between adjacent side chains along the polymer backbone and corresponds to side chain segregation. For the efficient π−π interaction, the d spacing must be less than 3.5 Å, thus there is little π−π interaction in Me-LPF. It is interesting to observe the formation of the polymer chain aggregate in dilute solutions, even though there is no strong π−π interaction. The film morphology and emission properties are very sensitive to the temperature of the spin-coated solution. Increasing the solution temperature from 20 to 50 °C, the morphology difference between Figure 3e,f shows the role of preassembly in solution. The spin-coated film of Me-LPF from 20 °C solution shows small circular topographic features (Figure 3e, RMS 1.859). When the solution is stored at 50 °C, the aggregates disappear in the image of the corresponding spin-coated film, and the surface of the film becomes very smooth (Figure 3f, RMS 0.823). In the corresponding XRD pattern (Figure 4), the diffraction intensity becomes weaker than that at 20 °C, and the periodicity of 4 nm which reflects the thickness of a single molecule is not observed. This indicates that the aggregate is partly destroyed at high temperature. The emission intensity of the 0−0 band in spincoated films also increases with increasing temperature (Figure 2d), which is similar to the results in solutions. It indicates that the energy transfer efficiency strongly depends on the aggregate packing mode. On the basis of XRD and AFM results, the proposed aggregate model of the dendritic morphology is shown in Figure 5. In this model, the observed dendritic model is composed of several high molecular ordering domains. In each domain, the rigid planar backbones stand face to face, with a thickness of 4 nm and interbackbone distance 13.5 Å. Because the intersegment space is too far for π−π interaction, the sidechain entanglement with adjacent alkyl spacing ∼4.8 Å would be regarded as the main driving force of polymer aggregates. The interaction between the alkyl side chain would be much weaker without π−π interaction, which is the reason that aggregates are sensitive to temperature.

4. CONCLUSIONS In this work, an aggregate behavior in dilute solutions and spincoated film of Me-LPF is investigated by concentration- and temperature-dependent fluorescence spectroscopy combined with morphological investigation. The film morphology and emission property are strongly dependent on the solution temperature because the aggregate is sensitive to temperature. The side-chain entanglement would be regarded as the main driving force of polymer aggregates but not π−π interaction from a rigid planar backbone. In this work, we have used a rigid planar polymer with small Stokes shift, where obvious fluorescence quenching well assists the certificate of aggregation. However, side-chain entanglement would be general in many conjugated polymers for similar types of aggregates. In the cases where there are conjugated polymers with big Stokes shift, the high ordered films may be positive for charge transport without obvious fluorescence quenching. Even though spin coating is normally regarded as a typical method for the fabrication of amorphous and isotropic films, these results supply a facile spin-coating process to prepare highorder films, which show potential application in optoelectronic devices. 11837


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(14) Grüner, J.; Wttmann, H. F.; Hamer, P. J.; Friend, R. H.; Huber, J.; Scherf, U.; Müllen, K.; Moratti, S. C.; Holmes, A. B. Electroluminescence and Photoluminescence Investigations of the Yellow Emission of Devices Based on Ladder-Type Oligo(para-Phenylene)s. Synth. Met. 1994, 67, 181−185. (15) Tasch, S.; Niko, A.; Leising, G.; Scherf, U. Highly Efficient Electroluminescence of New Wide Band Gap Ladder-type Poly (paraPhenylenes). Appl. Phys. Lett. 1996, 68, 1090−1092. (16) Yang, X. H.; Neher, D.; Scherf, U.; Bagnich, S. A.; Bässler, H. Polymer Electrophosphorescent Devices Utilizing a Ladder-Type Poly (para-Phenylene) Host. J. Appl. Phys. 2003, 93, 4413−4419. (17) Lupton, J. M.; Pogantsch, A.; Piok, T.; List, J. W. E.; Patil, S.; Scherf, U. Intrinsic Room-Temperature Electrophosphorescence from a π-Conjugated Polymer. Phys. Rev. Lett. 2002, 89, 167401. (18) Lemmer, U.; Heun, S.; Mahrt, R. F.; Scherf, U.; Hopmeier, M.; Siegner, U.; Göbel, E. O.; Müllen, K.; Bässler, H. Femtosecond Dynamics of Stimulated Emission and Photoinduced Absorption in a PPP-Type Ladder Polymer. Chem. Phys. Lett. 1995, 244, 171−176. (19) Haugeneder, A.; Lemmer, U.; Scherf, U. Exciton Dissociation Dynamics in a Conjugated Polymer Containing Aggregate States. Chem. Phys. Lett. 2002, 351, 354−358. (20) Romaner, L.; Heimel, G.; Wiesenhofer, H.; Scandiucci de Freitas, P.; Scherf, U.; Brédas, J. L.; Zojer, E.; List, E. J. W. Ketonic Defects in Ladder-type Poly (p-Phenylene)s. Chem. Mater. 2004, 16, 4667−4674. (21) Qiu, S.; Lu, P.; Liu, X.; Shen, F. Z.; Liu, L. L.; Ma, Y. G.; Shen, J. C. New Ladder-Type Poly (p-Phenylene)s Containing Fluorene Unit Exhibiting High Efficient Electroluminescence. Macromolecules 2003, 36, 9823−9829. (22) Turro, N. J. Modern Molecular Photochemistry; University Science Books: Menlo Park, 1978. (23) Keivanidis, P. E.; Jacob, J.; Oldridge, L.; Sonar, P.; Carbonnier, B.; Baluschev, S.; Grimsdale, A. C.; Müllen, K.; Wegner, G. Photophysical Characterization of Light-Emitting Poly (indenofluorene)s. ChemPhysChem 2005, 6, 1650−1660.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-20-87110606. Tel.: +8620-22237036. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors express their thanks to the Natural Science Foundation of China (51303057, 91233113, 21334002, 51373054, 51473052), the Ministry of Science and Technology of China (2013CB834705, 2015CB655003), and Introduced Innovative R&D Team of Guangdong (201101C0105067115) for their support.

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