Electric-Field-Driven Alignment of Chiral Conductive Polymer Thin Films

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Electric-Field-Driven Alignment of Chiral Conductive Polymer Thin Films Francesco Tassinari,†,§ Shinto P. Mathew,§ Claudio Fontanesi,‡ Luisa Schenetti,† and Ron Naaman*,§ †

Department of Life Science and ‡Department of Chemical and Geological Science, University of Modena and Reggio Emilia, Via G. Campi 183, 41125 Modena, Italy § Department of Chemical Physics, The Weizmann Institute, Rehovot 76100, Israel S Supporting Information *

ABSTRACT: We investigated the effect of an electric field on the alignment and structural properties of thin films of a chiral polybithiophene-based conductive polymer, functionalized with a protected L-cysteine amino acid. Thin films were obtained by exploiting both drop-casting and spin-coating procedures. The electric properties, the polarized Raman spectrum, the UV−vis spectrum, and the CD spectra were measured as a function of the electric field intensity applied during film formation. It was found that beyond the enhancement of the conductivity observed when the electric field aligns the polymer, the electric field significantly affects the chiral properties and the effect depends on the method of deposition.

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INTRODUCTION It is known that the properties of thin films of conductive polymers are related both to the structure and the conformation of chains in the polymer lattice.1,2 Crystallinity, chain length, substituents, and doping level are just some of the many variables that have a large effect on the final electronic properties of the films.3−5 Since the polymer chains are anisotropic, the orientation of the macromolecular chains greatly affects the overall properties of the film and especially their conductivity.6,7 Different methods were applied to align polymer chains: shear-flow fields, magnetic fields,8 mechanical stresses on the cast films,9 laser beams,10 and electric fields.11 Applying an electric field during the deposition process of the polymer thin films is an effective and simple way of modifying the orientational order and affecting its electric properties. Owing to the high polarizability of the conductive polymer, it becomes oriented along the electric field even for relatively weak fields. Chiral conductive polymers have attracted much attention in recent years because of their possible application in electrochemical chiral sensing and electrochemical asymmetric synthesis.12 Another possible application is in devices based on the chiral-induced spin-selectivity effect.13 In the present work, we address the electric-field-effect-induced alignment of a chiral conductive polymer, polybithiophene, functionalized with a protected L-cysteine amino acid in the lateral chain (Figure 1). The polymer adapts a chiral structure owing to the presence of chiral substituents along the polythiophene backbone that induce the formation of helically shaped bundles of the polymer in a solid state. Thin films of this polymer, drop-cast from a chloroform solution, display a very intense circular dichroism © 2014 American Chemical Society

effect that depends on the aggregation of the chains and the structure of the aggregates.14 Here we investigated the effect of an electric field for films produced both by drop-casting and by spin-coating. The electric properties as well as the CD spectra were measured as a function of the applied field. It was found that beyond its effect on the conductivity of the film, the electric field also significantly affects the chiral properties, and the extent of the effect depends on the method of deposition.

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MATERIALS AND METHODS

Reagents. Poly{methyl(2R)-3-(2,2′-bithiophen-4-ylsulfanyl)-2[(tert-butoxycarbonyl)amino]propanoate} (PCT-L, Figure 1) was synthesized following the procedure described in ref 15. Ethanol, acetone, and chloroform were purchased from Bio-Lab Ltd. (Israel). All solvents were used as received. Single-crystal silicon wafers (orientation ⟨100⟩, p-doped 0−100 Ω/cm, 525 ± 25 μm thickness, 300 nm of thermal oxide on the surface) were purchased from University Wafers, Inc. (USA). Single-crystal quartz slides (thickness 0.3 mm) were purchased from Chemglass Life Sciences (USA). All metals used during evaporation were purchased from Kurtz J. Lesker Company (USA). All other chemicals used were purchased from Sigma-Aldrich (USA) and used as received. Electrochemistry. Cyclic voltammetry (CV) measurements were performed using a Princeton Applied Research (PAR) 263A potentiostat driven by the PAR PowerSuite 4.0 program, employing a typical three-electrode electrochemical cell arrangement. NaNO3 0.1 M was used as the supporting electrolyte in aqueous (Millipore reagent-grade water) solution. Ni surfaces, a Pt wire, and a saturated Received: February 19, 2014 Revised: March 30, 2014 Published: April 14, 2014 4838

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Figure 1. (a) Structure of poly{methyl (2R)-3-(2,2′-bithiophen-4-ylsulfanyl)-2-[(tert-butoxycarbonyl)amino]propanoate} (PCT-L). (b) Helical structure formed by PCT-L in the solid state, owing to the hydrogen bonding between the polar substituents and the π staking of the thiophene rings. (c) Hydrogen-bond-based interactions between substituents. calomel electrode (SCE) were used as the working, counter, and reference electrodes, respectively. Film Deposition. Thin polymer films were prepared by dropcasting and spin-coating. The spin-coated films were produced from 80 μL of a 30 mg/mL solution of PCT-L in CHCl3. The sample was spun at 750 rpm (with an acceleration of 3300 rpm/s) for 45 s. A chuck of the spin-coater was connected to the electrical ground, and an electrostatic field was applied between a metallic plate, placed 3 mm above the sample, and the substrate surface. Figure 3. Experimental setup for the alignment of the PCT-L polymer during drop-casting. The distance between the plates is 1 cm, whereas the distance between the substrate surface and the upper plate is 0.5 cm. nm nominal probe tip radius (manufacturer’s values). All measurements were conducted in ambient air and at room temperature. Images were recorded at an optimized linear scan rate of 0.5−1.0 Hz, with an image resolution of 256 pixels × 256 pixels and a variable scan size. Scratching experiments were performed using a Park CP instrument. NSG11 probes were purchased from NT-MDT (head office in Moscow, Russia) with resonance frequency in the range of 190−300 kHz. The microscope Z axis was calibrated using a standard grid provided by NT-MDT.

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RESULTS AND DISCUSSION Drop-Cast Films. Figure 4 shows the circular dichroism (CD) spectrum of a drop-cast thin film of PCT-L. This result is consistent with a previously published CD spectrum of bithiophene-based polymers,15 characterized by the presence of a strong absorption maximum at 570 nm. Note that the CD spectrum of the drop-cast film of PCT-L (Figure 4) displays two absorption peaks, the first part having a negative CD sign and the other having a positive one. This effect has already been interpreted as being caused by exciton coupling that occurs when the electric transition moments of two or more chromophores, close in space and in a chiral configuration, interact. This interaction results in the splitting of the energy level of the excited states related to these chromophores.16 In our case, the chromophore is the thiophene backbone itself, which adapts a chiral conformation derived by intra- and interchain interactions.14 The van der Waals interactions, occurring between the chiral side chains, cause the polymer to assemble in chiral (helical) structures, as qualitatively sketched in Figure 1. Note that the self-assembling of the helical aggregates occurs on a relatively long time scale. If the film is dried at a fast rate, then the circular dichroism signal is negligible as compared to the large CD signal observed in the case of a film obtained by drying the drop in a solvent-vapor-

Figure 2. (a) Scheme of the spin-coating setup in which an electric field is applied during spin-coating. (b) Picture of the instrument used. The drop-cast films were prepared on various surfaces under an electrostatic field having different values. They were produced by depositing a single drop (80 μL) of a 1 mg/mL solution of PCT-L in CHCl3 on the surface. The PCT-L solution was allowed to dry in a vapor-saturated atmosphere for about 2 h. The electrostatic field was applied perpendicular to the sample surface by two electrodes separated 1 cm from each another, when the sample was positioned on one of them. The electric field was varied from 0 to 1.2 MV m−1. The thickness of the films was probed using a DekTak profilometer. Characterization by Spectroscopy. Due to the differences in the structure of the various devices, not all of the configurations could be tested by all of the characterization methods. Raman spectra were obtained using a Raman spectrometer (Horiba LabRAM HR800) equipped with a 632 nm laser and a light polarizer. The circular dichroism measurements were performed using a Chirascan CD spectrometer (Applied Photophysics). AFM. AFM measurements were performed with an APE Research AFM A-10 microscope (Italy) operated in contact mode with a constant applied force of 1.3 nN. Images were obtained with an NSC19/NoAl uncoated silicon-etched probe (MikroMasch), with a cantilever length of 125 μm, a spring constant of 0.6 N/m, and a 10 4839

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coating. (ii) A significant change in the value is observed for films prepared at 0 and 3000 V m−1.

Figure 4. (a) UV−vis spectrum of a drop-cast thin film of PCT-L on quartz when no field is applied. (b) CD spectrum of the same film.

saturated atmosphere for 1 to 2 h. Drop-cast PCT-L thin films were prepared while applying an electrostatic field at various field strengths in the 0 to 4000 V m−1 range, after which the relevant CD spectra were recorded. Since the drop-cast procedure (as well as the spin-casting procedure) does not result in the formation of a strictly controlled and constant film thickness, the spectra recorded for different films were compared by normalizing the intensity of the circular dichroism absorption to the relevant UV/vis 570 nm absorbance value. Following this practice, the CD values, reported in Figure 5, are

Figure 6. PCT-L CD spectra as a function of the applied static electric field: 0.0, 62.5, 125.0, 187.5, and 250.0 kV m−1.

When the electric field increases, the absorption peak at 520 nm and the shoulder at 612 nm are red-shifted, indicating a more ordered polymer chain.17 The anisotropy of the PCT-L film, as a function of the electric field, was also probed using Raman spectroscopy.11 Figure 7a,b shows the experimental Raman spectra recorded, as a function of the electric field intensity, in two different geometrical configurations. Figure 7a,b shows the spectra when the laser is polarized either parallel (∥) or perpendicular (⊥) to the applied electric field, respectively. Note that the intensity of the 1470 cm−1 peak shows a much more pronounced electric field dependence in the parallel (|) configuration with respect to the perpendicular (⊥) one: the ratio of the peak intensity between 4 and 0 kV m−1 is 0.3 and 0.8 for the parallel and perpendicular configurations, respectively. Figure 7c shows the calculated Raman spectrum; note the good agreement between the experimental and theoretical results, in particular that concerning the major peak found at 1470 cm−1. This vibration corresponds to the combination of the single C−C (i.e., an inter-ring) bond stretching and ring C−C contracting and is referred to as “ring breathing” modes. (The relevant animation can found in the Supporting Information.) Thus, the dependency of the experimental peak height on the electric field strength (Figure 7a) suggests that the polymer backbone orientation varies as a function of the applied electric field. (Details of the dipole moment orientation with respect to the molecular frame can be found in the Supporting Information.) It is expected that the alignment along the electric field is due to the PCT-L dipole moment that interacts with the electric field. Because the dipole moment forms almost a 40° angle with the polythiophene backbone (Supporting Information), its alignment by the electric field distorts the backbone alignment. Note that the opposite is found for the perpendicular laser polarization/electric field configuration, as indicated in Figure 7b, where the 1470 cm−1 peak height is less influenced by the electric field intensity. Figure 8 shows the resistivity of the PCT-L film as a function of the electric field. The films were dried under vacuum overnight to eliminate any possible solvent effect. Clearly, as

Figure 5. The CD values (normalized to the absorption intensity at 570 nm) for thin PCT-L films. The films were prepared by dropcasting under an applied electric field. The solid line simply connects the experimental points.

independent of the film thickness (which was found to be in the range of 0.5 to 15 μm; thinner layers are obtained by spincoating with respect to the drop-cast ones). Figure 5 shows the normalized CD intensity (measured at 570 nm) of the dropcast films as a function of the electrostatic field strength applied during the drying (film-formation) process. Note the following: (i) The CD signal decreases as the electrostatic field increases, which is found for a large number of samples obtained by spin4840

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Figure 9. CD signal intensity of the spin-coated thin PCT-L films as a function of the applied electric field.

increases as the electric field increases and (ii) the variation of the CD signal in Figure 9A (a variation of ca. 50 m°/a.u. is found by comparing the 0 and 3000 kVm−1 signals) is almost negligible if compared to the variation observed for the dropcast films. Moreover, the nonlinear CD dependence on the applied electric field is consistent with the expected dependence of the electrostatic interaction energy applied on the molecule in the electric field, which is given by U (E) = μ·E + α·E2 + ...

(1)

where U(E) is the electrostatic interaction energy, E is the electric field, μ is the component of the molecular permanent dipole moment along the applied electric field, and α is the polarizability (higher-order hyperpolarizabilities are neglected). Indeed, our results show that the second-order term in eq 1 contributes to the effect. The quadratic dependence of the CD signal on the applied electric field (see Figure 8) is consistent with this statement. Hence, the interaction between the electric field and the polarizability cannot be neglected. Indeed, phase transition and ordering concerning 2D organic films in an electric field have been shown to depend quadratically on the electric field strength.18,19 Spin-coated films were also prepared on nickel surfaces to check the effect of the alignment on the properties of the films using electrochemistry. Cyclic voltammetry (CV) curves of the K4[Fe(CN)6 ]/K3[Fe(CN)6 ] redox probe were used to investigate the differences in charge transfer (electronic conduction ability) between spin-coated PCT-L films prepared with and without an applied electric field. Figure 10 shows the relevant results. The CV curve for the unaligned film shows a maximum current of about 50 nA and no peaks that can be related to the redox probe, indicating a coating layer that is a poor conductor. In the case of aligned films, two shoulders can be observed at about +0.24 V (oxidation of Fe2+) and +0.11 V (reduction of Fe3+) with a separation of roughly 130 mV. Larger currents are found for films grown at increasing electric field values. The higher current measured for the aligned films is consistent with the increased conductivity resulting from the electric-field-induced alignment of the polymer. In principle, the variation of the current in the CVs, as found in Figure 10, could also be related to a variation of the “effective” area of the sample. To clarify this point, atomic force microscopy (AFM) images were recorded to investigate the morphology of the electrode surface. Images of the different

Figure 7. Raman spectra of the (a) parallel and (b) perpendicular configurations. The orientation of the plane of the polarized light refers to the direction of the applied electric field (details in the text). (c) Theoretical Raman spectrum calculated for the PCT-L monomer at the B3LYP/cc-pVTZ level of the theory.

Figure 8. Resistance of PCT-L films drop-cast on an interdigitated gold electrode (the distance between electrodes is 2 μm) while applying an electric field. The resistance is measured parallel to the direction of the field.

the field increases, the resistivity decreases, as expected if the polymer is better aligned along the applied field. Spin-Coated Films. The electric field alignment procedure was also applied during spin-coating. Figure 9 shows the change in the CD signal at the 570 nm peak divided by the absorbance value at the same wavelength. Note that (i) the CD signal 4841

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Table 1. Effective Surface Area and Surface Roughness Determined through AFM Measurements, Relevant to Figure 10A−D Images Figure 10 images:

surface area (μm2)

surface roughness (nm)

A B C D

4.0125 4.0605 4.0143 4.1889

1.05 4.43 1.51 6.63

affect the film structure. A comparison of Figure 11A (dropcast, no field) and Figure 11B (spin-coated, no field) reveals differences in the topography. The spin-coated film is rougher (even if the overall value is still very small), and it appears to be less ordered. However, when the 10 kV electric field is applied during the spin-coating procedure, a very regular structure appears, resembling “pillars” (with a height of 20−30 nm) emerging from the surface (Figure 11D). This is the normal response of a polymer solution put inside a high electric field, and this kind of patterning has been reported in the literature. The change in surface area between the aligned and nonaligned spin-coated films is in the range of 3%; thus it is insufficient to explain the order-of-magnitude difference in the electrode conductivity. The results presented here strongly indicate that the change in the conductive properties of the adsorbed films is due to the alignment of the polymer chains induced by the electric field.

Figure 10. Cyclic voltammetry curves of the Fe2+/Fe3+ redox probe, measured when the working nickel electrode is coated with PCT-L films spin-coated under different electric fields. The reference electrode was a standard calomel electrode, and the counter electrode was a Pt wire. The electrolyte solution was 1:1 [K4Fe(CN)6]/ [K3Fe(CN)6] 0.4 mM + NaNO3 0.4 M in water. The inset graph is the CV recorded using a surface of bare nickel as the working electrode.

samples are shown in Figure 11, and the calculated area values and roughness parameter are presented in Table 1. A clear difference in morphology was found between the drop-cast and spin-coated films. Figure 11A shows a typical image of a drop-cast PCT-L film. The surface is smooth and regular, and a few “hills” of circular shape are present. In Figure 11C, the image of a drop-cast film is shown, when deposited under an applied electric field. Hills of the same type are present, but they are slightly larger and higher. The similar area and roughness values of these two surfaces (columns A and C in Table 1) suggest that the electric field does not dramatically

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CONCLUSIONS In this work, it was shown that the chiral characteristics of thin films of PCT-L conductive polymer are due to both the intrinsic chirality of the polythiopene backbone, through the (2R) chiral carbon center, as well as the formation of chiral polymer bundles (intermolecular, interchain-related).14 These two different contributions to the chirality of the polymeric film can be modulated by (i) an applied electric field and (ii) the

Figure 11. The images were all obtained in contact mode. (A) Drop-cast PCT-L film, (B) spin-coated PCT-L film, (C) drop-casted PCT-L film with an applied electric field of 1 kVm−1, and (D) spin-coated PCT-L film with an applied electric field of 10 kV m−1. 4842

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in Macroscopically Aligned Electrospun Polymer Nanofibers. J. Am. Chem. Soc. 2007, 129, 2777−2782. (12) Kane-Maguire, L. A. P.; Wallace, G. G. Chiral conducting polymers. Chem. Soc. Rev. 2010, 39, 2545−2576. (13) Naaman, R.; Waldeck, D. H. Chiral-Induced Spin Selectivity Effect. J. Phys. Chem. Lett. 2012, 3, 2178−2187. (14) Mucci, A.; Parenti, F.; Schenetti, L. A self-assembling polythiophene functionalised with a cysteine moiety. Macromol. Rapid Commun. 2003, 24 (9), 547−550. (15) Cagnoli, R.; Lanzi, M.; Mucci, A.; Parenti, F.; Schenetti, L. Polymerization of cysteine functionalized thiophenes. Polymer 2005, 46, 3588−3596. (16) Person, R. V.; Monde, K.; Humpf, H. U.; Berova, N.; Nakanishi, K. A new approach in exciton-coupled circular dichroism (ECCD) insertion of an auxiliary stereogenic center. Chirality 1995, 7 (3), 128− 135. (17) Iarossi, D.; Mucci, A.; Parenti, F.; Schenetti, L.; Seeber, R.; Zanardi, C.; Forni, A.; Tonelli, M. Synthesis and spectroscopic and electrochemical characterisation of a conducting polythiophene bearing a chiral β-substituent: polymerisation of (+)-4,4′-bis[(s)-2methylbutylsulfanyl]-2,2′-bithiophene. Chem.Eur. J. 2001, 7, 676− 685. (18) Fontanesi, C. Entropy change in the two-dimensional phase transition of adenine adsorbed at the Hg electrode/aqueous solution interface. J. Chem. Soc., Faraday Trans. 1998, 94, 2417−2422. (19) Marder, S. R.; Gorman, C. B.; Meyers, F.; Perry, J. W.; Bourhill, G.; Bredas, J.-L.; Pierce, B. M. A Unified description of linear and nonlinear polarization in organic polymethine dyes. Science 1994, 265, 632−635.

solvent evaporation rate; a slow evaporation rate favors an ordered alignment of the polymer. It was shown that although the chirality, governed by the 2R carbon center, can be driven by applying the electric field, the chirality that results from the alignment of the polymer bundles can be controlled by the spin-coating (fast-drying) or drop-cast (slow-drying) process. Film formation is controlled by two parameters, the solvent evaporation rate and the force applied by the electric field. The ratio between these parameters can account, qualitatively, for the opposite CD signal pattern observed as a function of the electric field when comparing spin-coating and drop-casting film-formation techniques.

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ASSOCIATED CONTENT

S Supporting Information *

B3LYP/cc-pVTZ-optimized PCT-L and monomer structures. Animation of the calculated vibrational mode at 1473.35 cm−1. Effective surface area and surface roughness determined through AFM measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Israel Science Foundation and by the Grand Center for Sensors and Security. REFERENCES

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