Electrical Chiral Assembly Switching of Soluble Conjugated Polymers

Oct 14, 2014 - A π-conjugated polymer of propylenedioxythiophene-phenylene (ProDOT-Ph) with a chiral alkyl substituent on the propylene bridge of Pro...
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Electrical Chiral Assembly Switching of Soluble Conjugated Polymers from Propylenedioxythiophene-Phenylene Copolymers Xu Yang, Seogjae Seo, Chihyun Park, and Eunkyoung Kim* Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, South Korea S Supporting Information *

ABSTRACT: A π-conjugated polymer of propylenedioxythiophene-phenylene (ProDOTPh) with a chiral alkyl substituent on the propylene bridge of ProDOT was prepared to explore reversible multiswitching chiral conjugated polymers (CCPs) capable of electrochromic, electrofluorochromic, and chirality switching. The chiral assembly of the yellow colored poly(ProDOT-Ph)s (YPCr) was optimized by annealing the YPCr films at 120 °C. From the thermally annealed YPCr films, we demonstrated the electrical switching of chirality, color, and fluorescence in a single device, for the first time, which was precisely controlled electrochemically to achieve highly efficient and reversible switching of chiral assembly at a low working potential. The YPCr films showed electrochromism with a high coloration efficiency of 690 cm2 C−1 by changing color between yellow and blue. The film also exhibited fluorescence switching with a quantum yield change from 3.8 to 0.21 during the reversible electrochemical switching. The chirality of the YPCr, which showed a negative bisignate Cotton effect, was switched reversibly with 97% change in the anisotropy factor (gabs). The multiswitching properties were correlated to the electrochemical switching in chiral assembly of conjugated polymers according to the electrochemical doping/dedoping process. To the best of our knowledge, the change in Cotton effect during the switching is the highest reported value of a chirality tunable polymer without any chiral inducer. In addition, the YPCr showed a sustainable memory effect for both chirality and coloration. witchable organic films have been collecting widespread interest, due to their high potential application in flexible and soft electronics.1−4 In this respect, many soluble πconjugated polymers (CPs) have received much attention for their tunable electrical and optical properties in solution processed thin films.5,6 These switching properties of CPs are strongly dependent on their chemical structure as well as structural assembly in films.7 The structural assembly of CPs in thin films is especially important as it is responsible for its charge transport and optical properties, critical in organic transistors, organic photovoltaics, organic emitting devices, and fluorochromic devices.7 Due to their unique chiroptical properties, chiral polymers have been applied to enhance optoelectric properties by optimizing structural assembly.8−22 The structural assembly of CPs is induced by the chiral assisted assembly, in which the assembly is originated from either additives such as chiral dopants23−25 or from the chiral groups substituted on the polymers.26−28 Although the solvent and temperature induced aggregation of soluble chiral macromolecules already exists in the literature,29−31 there are only a few studies on the tuning of chiral polymers in solid states. For example, H. Goto and K. Akagi32−34 reported on the electrochemical polymerization of achiral monomers in the presence of chiral dopants to obtain polymers with chiroptical properties. J. R. Reynolds reported the synthesis of conjugated polymers substituted with a chiral (2S)-ethylhexanol and (2S)methylbutyl unit.35 These polymers provide chemically switchable chirality to yield 80% change in anisotropy factor (gabs). However, it is rare to find reports on reversible switching of

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chiral assembly in conjugated polymers that can provide modulation of chirality accompanied by color and fluorescence switching. Since the assembly of conjugated polymers could enhance the electrical and optical properties of polymer films,1,36−40 it is an important challenge now to explore the reversible modulation of chiroptical properties from solution processed CCPs. 3,4-Propylenedioxythiophene (ProDOT) has been the monomer of choice to obtain cathodically coloring and highly stable electroactive CPs.41 The chiral alkyl chain in the ProDOT makes the polymer soluble and induces dense packing between the polymer chains,35 while the introduction of phenylene unit without side chain results in a relatively high degree of freedom around the internal rotation of the main chain, compared to the ProDOT homopolymers.42 Herein, we report a soluble yellow colored ProDOT-phenylene copolymer (YPCr) with a chiral substituent on the side chain. This electroactive chiral polymer yielded a thin film easily through a solution process and applied in the modulation of optical properties followed by chiroptical changes according to external stimuli. This is the first example of a multiple switching that modulates color, fluorescence, and chirality in a single active layer. It exhibits not only highly efficient electrochromism but also electrofluorochromism with a large change in the Received: May 21, 2014 Revised: September 27, 2014

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Figure 1. (a) and (b) CD (top, solid lines), UV/vis (bottom, solid lines), and fluorescence spectra (bottom, dashed lines) of (a) YPCr (concentration: 5 × 10−5 M) in chloroform−methanol (50:50, v/v) (red) and chloroform only (black) at 20 °C; (b) YPCr in chloroform−methanol (50:50, v/v) with different molar concentrations (red: 1 × 10−4 M, blue: 5 × 10−5 M black: 2.5 × 10−5 M). (c) CD (top) and UV/vis spectra (bottom) of YPCr solution (5 × 10−5 M) in chloroform−methanol (50:50, v/v) at different temperatures (red solid line at −10 °C, black solid line at 50 °C, dotted black lines at every 10 °C increase from 0 to 40 °C). (d) Chemical structure of YPCr.

thus, the polymers are aggregated to form an intermolecular helical packing structure upon addition of a nonsolvent (methanol).46,47 However, as the concentration increased, the fluorescence intensity decreased dramatically, possibly due to the aggregation induced fluorescence quenching via intermolecular energy transfer.1,48,49 Consequently, the chiroptical behavior of the YPCr can be summarized as follows. The broad band for YPCr, located at around 410 nm, arises from conjugated polymer chains, which are chirally oriented toward each other (chiral exciton coupling). As a consequence, the corresponding Cotton effect is bisignate with a zero-crossing near λmax. This process does not require macroscopic order and is therefore present in both amorphous and semicrystalline polymers. The fact that the actual chromophore is chiral accounts for the monosignate nature of the Cotton effect. Such aggregations that produce an intermolecular helical packing structure should be temperature dependent. Indeed, remarkable thermodriven changes in the CD spectrum were clearly observed (Figure 1c) when the solution was warmed from −10 to 50 °C. The YPCr was well aggregated at low temperatures and, thus, showed higher CD and absorption intensities. However, the CD and absorption intensities decreased dramatically upon heating the solution above 50 °C, possibly because the polymer aggregation was disturbed.50,51 Chiroptical Properties in Thin Film State. π-Conjugated polymers with chiral derivatives are well-known to exhibit a bisignate Cotton effect not only in solution but also in solid states, due to the exciton coupling of chiral oriented chromophores.52 The YPCr films showed a negative bisignate Cotton effect with a left-handed helical (M-helicity) sense,53 and the CD spectrum was found to be dependent on the thickness of film at room temperature (Figure S5). To obtain a thermally induced polymer assembly, YPCr films were fabricated by spin-coating onto a substrate, using a 5−10 mg/ mL polymer solution in chloroform, annealed for 2 h, and then cooled very slowly. A significant change in the CD and fluorescence was observed, but minimal change in the UV/vis absorption spectra was found within a wide range of annealing temperatures

anisotropy factor (gabs) during the electrochemical doping and dedoping process.

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RESULTS AND DISCUSSION Optical and Chiroptical Properties in Solution. A yellow colored chiral polymer of a thiophene-phenylene copolymer (YPCr, Figure 1d) was synthesized through the Suzuki coupling43 reaction (the full scheme and synthetic details are shown in the Supporting Information). The asprepared YPCr was readily soluble in chloroform and showed characteristic optical properties with a broad absorption in the UV/vis spectrum along with high fluorescence (quantum yield of ∼15%, referenced to quinine sulfate, excitation at 400 nm). However, the Cotton effect from the chloroform solution of YPCr at 20 °C was very low (Figure 1a, black lines), which indicates the polymers are molecularly dissolved without ordering. Upon addition of methanol to the chloroform solution (chloroform: methanol = 5:5 v/v), the circular dichroism (CD) response was dramatically changed in the vicinity of the absorption band of the polymer at 20 °C (Figure 1a, red lines). The absorption maximum was ∼10 nm blue-shifted, possibly due to the formation of chiral assembly of a helical type,44 and there was a sharp decrease in fluorescence (quantum yield decreased to 5%) as a result of such assembly in the presence of nonsolvent (methanol). In addition, the CD spectrum showed a strong negative bisignate Cotton effect45 in the absorption for the π−π* transition of the polymer. The CD intensity was negative at a longer wavelength region (470 nm) with a zero-crossing at λmax (410 nm) of the absorption and then positive at a shorter wavelength (380 nm). Bisignate Cotton effects with a zerocrossing at λmax have been attributed to the chiral exciton coupling of chirally aligned polymer strands.42 As the concentration of the YPCr was increased in the chloroform/methanol (v/v = 5/5) solution at 20 °C, the CD and UV/vis absorbance intensities also increased (Figure 1b). Clearly, the bisignate Cotton effect located at the π−π* transition is concentration dependent, indicating that the Cotton effect from YPCr is an intermolecular phenomenon, B

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morphology of the film before annealing at 120 °C had an average height of 9.3 nm, from top to bottom, and ∼170 nm wide bulges. After thermal annealing, the average height of polymer morphology decreased to 6.5 nm, and the average bulge width was decreased to 100 nm. In the previous chiral conjugated polymer study, ProDOT polymers stacked perpendicular to the flat surface was observed as kind of bulge morphology with a compatible height to that of a ProDOT unit.35 In Figure S7a, it seems that the height of bulge was increased with an increase of film thickness, forming high stacks of polymer chains. The roughness and surface area for the unannealed film were 3.4 nm and 25.5 μm2, respectively. After annealing at 120 °C for 2 h and cooling very slowly, the bulge structures had shrunk to show presence of interconnected nanoribbons (Figure S7b). The polymer chains were densely packed, to result in a higher CD intensity compared to the unannealed film. The morphology change in the surface was accompanied by the water contact angle change from 117° for the unannealed film, to 103° for the annealed film, and the surface energy gradually increased from 7 to 14 mN/m (Table S1), suggesting the denser packing of the YPCr by thermal annealing. The annealing effect on the structure of the polymer stacks was estimated from an X-ray diffraction (XRD) analysis (Figure S8). It suggests that the polymer orientation was not much changed, but the spacing of the polymer stacks was slightly decreased by the thermal annealing process, maintaining edge-on orientation. Electrical Modulation of the Optical and Chiroptical Properties. A sandwich type EC device was prepared in a 3electrode system, with an Ag wire as the reference electrode between a polymer coated ITO glass as the working electrode and a bare ITO glass as the counter electrode, as published in the previous report.55 An electrolyte solution, which is 0.2 M lithium bis(trifluoromethane)sulfonimide (LiBTI) in propylene carbonate (PC), was injected into the device to fill the gap between the two electrodes. The electrochemical modulations of the optical and chiroptical properties were demonstrated by applying potentials to the 3-electrode device. Fluorescence intensity changes of the

(Figure 2). As shown in Figure S6, the glass transition temperature (Tg) of the YPCr was observed at around 90 °C, as

Figure 2. CD, UV/vis absorbance, and fluorescence spectra of the YPCr film after annealing 2 h at different temperatures and cooling slowly.

determined by a differential scanning calorimetry (DSC) with a scan rate of 20 °C/min. When the annealing temperature was lower than Tg (50 °C, 70 °C), the CD and fluorescent intensities were relatively low. The fluorescence intensity was shown to increase, as the intermolecular packing become loose, by increasing the annealing temperature. When the annealing temperature was increased higher than Tg (100 °C, 120 °C) and the film was cooled slowly, the CD intensity was maximized, and the fluorescence intensity decreased significantly because of the fluorescence quenching by the intermolecular assembly in the conjugated polymer films.54 To explain this thermal annealing effect, we also examined the morphologies of the polymer films before and after thermal annealing at 120 °C. AFM study of the polymer films before annealing reveals bulge morphology. The bulge morphology could be formed when there is poor interconnection among the polymer chains in the polymer stacks.35 The polymer

Figure 3. (a) Cyclic voltammogram (black) of the YPCr film in a 3-electrode electrochemical switching device using an Ag wire as a reference electrode and the in situ fluorescence intensity change (red) during the 5 cycles of cyclic voltammogram from 0 to 1.0 V with a scan rate of 10 mV/s. (b) and (d) present the images at dedoped state (0 V), showing yellow colored and fluorescence ON, under room light and UV excitation of 365 nm, respectively. (c) and (e) present the images at doped state (+1.0 V), showing pale blue colored and fluorescence OFF, under room light and UV excitation of 365 nm, respectively. (f) The electrochromism and electrofluorochromism arise from the reversible redox reactions in the electrochemical device. C

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Figure 4. CD ((a) and (d)), UV/vis absorption ((b) and (e)), and fluorescence ((c) and (f)) spectra of a 3-electrode YPCr device using an asprepared YPCr film ((a)−(c)) and a YPCr film after thermal annealing ((d)−(f)), at different applied potentials from 0 V (vs Ag wire, black line) to +1.0 V (vs Ag wire, red line) with 0.1 V interval in between (dashed lines).

Figure 5. AFM images of the pristine YPCr films in (a) the dedoped (0 V) and (b) doped states (+1 V). (i) 5 × 5 μm2 scale image, (ii) magnified image, with 2 × 2 μm2 scale and the cross height profile. Schematic images of YPCr at (c) dedoped state and (d) doped state. HR-TEM images of (e, f) the YPCr film at dedoped state and (g, h) doped state.

YPCr films were recorded at 525 nm with a cyclic voltammetry (CV) upon a potential range from 0 to 1 V, as shown in Figure

3a. When the applied potential was changed from 0 to 1 V with a scan rate of 10 mV/s, an oxidation peak was obtained at 0.87 D

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distance for the π−π stacking on (100) plane (Figure S8).58,59 The (100) plane of YPCr was hardly observed in TEM, possibly due to the edge-on orientation where the π-conjugated chains are vertically aligned with respect to the substrate.58,59 Based on the length of the polymer chain and distances observed in TEM images, the possible helical structure could be estimated with ∼100 nm length and ∼30 nm widths (Figure S9). On the other hand, at a doped state the aligned structure was diminished, possibly because π−π stacking was widened by the addition of the counteranions (dopant), such as BTI−, into the interchains of the film via electrochemical doping, the cross image from SEM showed that the film thickness of the YPCr increased by over 30% (Figure S10) after doping process. Notably, AFM images showed the roughness of YPCr film was decreased from ∼4.2 nm to ∼3.4 nm during the doping process. Also, the dedoped YPCr films exhibited randomly assembled rod-like structure with ∼60 nm of grooves. In the doped YPCr film, however, the ∼60 nm of grooves disappeared and the width of the rod-structure was widened, indicating that the helical structure was also untangled. This structural change was also consistent with the peak shift observed in XRD (Figure S8). The π−π stacking (100) with 21 Å distances (4.2°) was shifted to the longer distance 23 Å (3.8°), which is the same tendency with the previous studies. The π−π stacking on the (100) plane observed in TEM images was also displaced by the addition of the counteranions, showing a distorted structure (Figure 5). The doped YPCr, however, still exhibited XRD reflection at 2θ = 3.8° and reversibly recovered to aligned structure by the electrochemical dedoping process, so it seems that the polymer assembly is widened but not fully disappeared at a doped state.42 Based on the structural changes observed in TEM and AFM images, the schematic diagrams of the helical πstacking structure at the dedoped state is presented in Figure 5c. The reversible chirality switching of YPCr is possible through the reversible assembly and disassembly of the interchain helical π-stacking by the transport of dopants ions, which cause swelling of the film and possibly reconfiguring polymer chains toward random orientation when they are inserted at a doped state (Figure 5d).60,61 Although CD peaks were observed at 573 and 705 nm, with a zero-crossing at λmax (620 nm) of the absorption at the doped state, their intensities are very weak due to the disruption of the helical structure by the incoming dopants. By the previously mentioned structural changes, YPCr could provide the reversible electrochirality, coloration, and fluorescence switching in single device. Although the switching contrast can be further maximized with a high oxidation potential over 1 V (Figure S11), absorbance decrease of YPCr at ∼600 nm was not large. Therefore, we maintained the highest applied potential as 1 V to examine the multiswitching properties of YPCr. When the potential was changed from 0 to 1 V with 20 s of switching time, the chirality change (ΔCD) was 3.84 and 6.98 mdeg, for 390 and 460 nm absorption, respectively (Figure S12a). Interestingly, the change in anisotropy factors (Δgabs) was 2 × 10−4 and −3 × 10−4 for 390 and 460 nm absorption, respectively. When these values are divided by the original gabs in the dedoped state to calculate the chirality contrast (γ), it showed a very large contrast (γ) of 75% and 98% for 390 and 460 nm absorption, respectively. This is one of the highest values for electrochemically tunable CD intensities among the other reported values for CPs without any additional chiral dopant (Table S2).

V (vs Ag wire) with a color change from transparent yellow to blue (Figure 3b, c). The bright yellow fluorescence from the YPCr film increased slightly at the beginning, then extinguished gradually from 0.5 V, which is the onset potential for oxidation of YPCr, and then completely extinguished when the potential increased beyond 1 V (Figure 3d, e). When the applied potential was reduced to 0 V, a reduction peak was observed at 0.77 V. The color returned reversibly to yellow, and the fluorescence intensity was also recovered without any decomposition even after several cycles. This is because the fluorescence quenching originated from the electrochemical oxidation of the polymer (Figure 3f), without the production of side products. Therefore, the fluorescence modulation by the applied potential for YPCr was quite reversible.56 The electrochirality, coloration, and fluorescence switching properties of the YPCr films were examined by monitoring their CD, absorbance, and photoluminescence spectra at different applied potentials, respectively. For the as-prepared unannealed YPCr film at 0 V, a strong negative bisignate Cotton effect was observed at 390 and 460 nm regions, with +9.4 mdeg and −13.4 mdeg of signal intensity, respectively (Figure 4a). When these values were calculated as a degree of thickness-independent circular polarization in absorption, as defined by anisotropy factors, gabs (Δε/ε), they were found to be 4.1 × 10−4 (λabs = 390 nm) and −5.3 × 10−4 (λabs = 460 nm). This bisignate CD signal is related to the ordered structure of the main chain in the dedoped state.57 The YPCr possesses a helically twisted structure, because of its benzonoid structure and a relatively high degree of freedom around the internal rotation of the main chain as observed for the similar chiral polymers.42 When the applied potential was increased from 0 to 1 V, the absorption band at 425 nm decreased, and a new band appeared at 620 nm (Figure 4b), with a color change from yellow to blue. In addition, the fluorescence emission band at 525 nm rapidly diminished with increased positive potential (Figure 4c), as the fluorescence quenching is accelerated by intermolecular energy transfer, especially in a solid state.49 The changes in CD intensity between the dedoped and doped states could be attributed to the structural change in the YPCr film. When the applied potential was increased over 0.8 V, the absorption band, characteristic of polaron formation, was observed in a longer wavelength region. Thus, chiroptical changes in the YPCr film can be explained by the electrochemical switching in the chiral assembly of YPCr from a neutral state to a polaronic state and vice versa. The electrochemical doping of the polymer chain can induce large changes in the structural assembly of the polymer due to the introduction of dopant anions. In the dedoped state, the polymer chains could be densely packed through an interchain interaction to form a helical structure, which shows a strong bisignate Cotton effect. On the other hand, in the doped state, the distance between the polymer chains is increased, due to the addition of dopants, which could alter the helical π-stacking, causing a decrease in the CD intensity.42 This structural change of YPCr film was clearly revealed by AFM (Figure 5 a-b), TEM images (Figure 5e-h), and X-ray diffraction (Figure S8). In the dedoped state, well ordered π−π conjugated stacks (average width of ∼20 Å) were observed in TEM image, which can be assigned as (100) reflection of YPCr. Since polymers that favor edge-on packing show intense crystallographic (100) reflection and rather weak (010) reflection in X-ray diffraction (XRD), it seems that the peak observed at 2θ = 4.2° corresponds to the E

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Figure 6. (a) Multiswitching of the annealed YPCr device. The chirality (CD, at 460 nm, red line) and color (absorbance, at 425 nm, blue line) switching was recorded with applied potential steps from 1 V to the different potentials (−1 V ∼ + 0.9 V) with 20 s switching time. The fluorescence (at 525 nm, black line) switching was recorded with applied potential step from 1 to 0 V with different switching time (60, 30, and 20 s). (b) Switching responses in chirality at 460 nm, color at 425 nm, and florescence at 525 nm to a step potential of +1/0V with a step duration time for 20 s (green dotted line). (c) CD (solid), absorbance (dashed line), and (d) Δgabs/gn of the annealed YPCr EC device with different thicknesses at different applied potentials was monitored at 460 nm. The doping potential was fixed at +1 V and dedoping potential varied from −1 to 0.9 V. (e) Long-term memory effect on CD of YPCr device without electricity at three different wavelengths.

Under the same experimental condition, the YPCr film annealed at 120 °C showed a relatively higher CD intensity but lower absorbance compared to the film without annealing. The bisignate Cotton effect in the dedoped state was obtained as −18.4 mdeg at 460 nm (Figure 6a), with gabs of −8.2 × 10−4 (λ = 460 nm), which are higher than the film without annealing. These results verify the formation of a highly ordered structure upon annealing. The EC cell with annealed YPCr film also showed a high gabs contrast (γ) as 73% and 97% at 390 and 460 nm, respectively. The change in CD, absorbance, gabs, and Δgabs at 390 and 460 nm with different applied potentials is summarized in Figure 6c and d (180 nm thickness, blue lines). The electrochromic switching properties of the YPCr device was examined using the absorbance change (EC contrast, ΔAbs), which determines the coloration efficiency (CE).41 The CE for the EC device from the YPCr film was determined as 462 cm2 C−1 (Table S3a, Figure S12a), which is much higher than that of PEDOT and PProDOT.62 Furthermore, this CE is about 50% higher than that of the achiral yellow polymer (CE = 310 cm2 C−1) which has a similar structure.3 Such a large difference in CE could be attributed to the formation of the assembled nanostructures in the chiral polymer (YPCr). Furthermore, the EC device from the YPCr film after annealing at 120 °C showed enhanced CE of 631 cm2 C−1 (Table S3b), which is 37% higher than the unannealed film device. These results could be attributed to the chiral assembly formed from molecular aggregation by thermal annealing, which could facilitate the charge transport required for the EC reaction. Another interesting feature on the chiral EC device was the memory effect (Figure 6e). The average CD and absorbance losses were less than 10% even after 30 h, indicating that the

YPCr device has a sustainable memory effect for both chirality and coloration. Such a long-term memory effect could be attributed to the dense structure at the colored (dedoped state) as well as relatively high oxidation potential of YPCr.63 As the doping process is accompanied by the structural change, the fluorescence of the film was also changed. The fluorescence contrast, which can be evaluated by the fluorescence intensity ratio at the on/off states, was varied from 3.3 to 2.2 under different switching times. The maximum on/off ratio was achieved as 3.3 with a 120 s potential step (Figure S12a).64 The change in polymer structure between the neutral and oxidized states was reversible and hence controllable by the electrochemical doping and dedoping procedures.65 The interchain π-stacking structures in YPCr could be reversibly recovered by electrochemical dedoping. Therefore, multiswitching in color, chirality, and fluorescence could be observed in one cell in response to the electrochemical reversible switching in chiral assembly. Figure 6a shows the switching properties of a multiswitching device with a thermally annealed YPCr film, under a step potential from a fixed oxidation potential (+1 V) to different reduction potentials of −1−0.9 V for chirality (CD, at 460 nm), color (absorbance, at 425 nm), and fluorescence (emission, at 525 nm), with a switching time of 20 s (∼60 s for electrofluorescence switching). Because the switching contrast was almost the same at the reduction potentials from 0.5 to −1.0 V, the YPCr device provides reproducible multiswitching properties in response to a very small change in applied potentials (from +1 to +0.5 V). This is clearly matched with the redox potentials of YPCr in Figure 3a. Importantly the interchain π-stacking structures in YPCr could F

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be reversibly recovered by electrochemical dedoping as the CD switching is quite reversible. The response time for these switchings is very fast (2−10 s). It was noteworthy that electrochromic switching is faster than chirality and fluorescence switching, under the same condition, possibly due to the required molecular rearrangements for the latter cases. In addition, the fluorescence quenching is faster than the evolution (Figure 6b, black line) because the remaining cation radical can contribute to the fluorescence quenching by electron transfer or energy transfer in a conjugated polymer.66 Thickness Modulation for Enhancing the Optical Properties. Because the optical properties of thin films are considerably dependent on the thickness,67 the electrochemical devices were fabricated using YPCr films with different thicknesses (20, 50, 80, 140, and 180 nm). As described above, the annealed films showed enhanced optical properties over those of the pristine films. Therefore, all the films with different thicknesses were annealed at 120 °C and cooled slowly. As summarized in Table S4, the 140 nm thick film showed a relatively high absorbance change but required smaller charge density for EC operation, resulting in the highest coloration efficiency of 687 cm2 C−1. The chiral properties of the YPCr with different thicknesses are shown in Figure 6c, d and Table S4. The 20 nm thick polymer film showed a very low CD signal because of its low absorption. On the other hand, the 180 nm thick film showed the highest CD intensity with −18.5 mdeg at 460 nm due to its high absorption. As a consequence, the highest Δg and g contrast (γ) was achieved with the180 nm thick YPCr film.

ASSOCIATED CONTENT

S Supporting Information *

Synthesis details, AFM, TEM images, and surface free energy changes of YPCr films before and after thermal annealing, comparison of optical properties between chiral polymers, electrochemical properties. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2007-0056091).

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REFERENCES

(1) You, J.; Kim, J.; Park, T.; Kim, B.; Kim, E. Highly Fluorescent Conjugated Polyelectrolyte Nanostructures: Synthesis, Self-Assembly, and Al3+ Ion Sensing. Adv. Funct. Mater. 2012, 22, 1417−1424. (2) Kim, Y.; Kim, Y.; Kim, S.; Kim, E. Electrochromic Diffraction from Nanopatterned Poly(3-hexylthiophene). ACS Nano 2010, 4, 5277−5284. (3) Bhuvana, T.; Kim, B.; Yang, X.; Shin, H.; Kim, E. Reversible FullColor Generation with Patterned Yellow Electrochromic Polymers. Angew. Chem., Int. Ed. 2013, 52, 1180−1184. (4) Seo, S.; Kim, Y.; You, J.; Sarwade, B. D.; Wadgaonkar, P. P.; Menon, S. K.; More, A. S.; Kim, E. Electrochemical Fluorescence Switching from a Patternable Poly(1,3,4-oxadiazole) Thin Film. Macromol. Rapid Commun. 2011, 32, 637−643. (5) Zaumseil, J.; Sirringhaus, H. Electron and Ambipolar Transport in Organic Field-Effect Transistors. Chem. Rev. 2007, 107, 1296−1323. (6) Unur, E.; Beaujuge, P. M.; Ellinger, S.; Jung, J.-H.; Reynolds, J. R. Black to Transmissive Switching in a Pseudo Three-Electrode Electrochromic Device. Chem. Mater. 2009, 21, 5145−5153. (7) Son, S.; Dodabalapur, A.; Lovinger, A. J.; Galvin, M. E. Luminescence Enhancement by the Introduction of Disorder into Poly(p-phenylene vinylene). Science 1995, 269, 376−378. (8) Langeveld-Voss, B. M. W.; Janssen, R. A. J.; Christiaans, M. P. T.; Meskers, S. C. J.; Dekkers, H. P. J. M.; Meijer, E. W. Circular Dichroism and Circular Polarization of Photoluminescence of Highly Ordered Poly{3,4-di[(S)-2-methylbutoxy]thiophene}. J. Am. Chem. Soc. 1996, 118, 4908−4909. (9) Peeters, E.; Christiaans, M. P.; Janssen, R. A.; Schoo, H. F.; Dekkers, H. P.; Meijer, E. Circularly polarized electroluminescence from a polymer light-emitting diode. J. Am. Chem. Soc. 1997, 119, 9909−9910. (10) Oda, M.; Nothofer, H. G.; Lieser, G.; Scherf, U.; Meskers, S. C. J.; Neher, D. Circularly Polarized Electroluminescence from LiquidCrystalline Chiral Polyfluorenes. Adv. Mater. 2000, 12, 362−365. (11) Moutet, J.-C.; Saintl-Aman, E.; Tran-Van, F.; Angibeaud, P.; Utille, J.-P. Poly(glucose-pyrrole) modified electrodes: A novel chiral electrode for enantioselective recognition. Adv. Mater. 1992, 4, 511− 513. (12) Huang, J.; Egan, V. M.; Guo, H.; Yoon, J. Y.; Briseno, A. L.; Rauda, I. E.; Garrell, R. L.; Knobler, C. M.; Zhou, F.; Kaner, R. B. Enantioselective Discrimination of D- and L-Phenylalanine by Chiral Polyaniline Thin Films. Adv. Mater. 2003, 15, 1158−1161.

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CONCLUSION In conclusion, a reversible switching of chiral assembly was obtained from a chiral soluble conjugated polymer of a dioxythiophene-phenylene copolymer to allow a multiswitching device for electrochromic, electrofluorescence, and chirality, for the first time. The modulation on chiroptical switching was conducted simultaneously in a single device because the switching occurs under the same mechanism, which is electrochemical doping and dedoping of the polymer. The polymer film showed a high CD intensity, and gabs contrast (γ) of 97%, which is the highest value compared to other chiral polymers without any additional chiral dopants. It also showed a very high coloration efficiency of 690 cm2 C−1 for electrochromism along with electrofluorescence switching, by optimizing the annealing process and film thickness of the chiral polymer film. The multiswitching properties of the chiral polymer film in an electrochemical device were reversible with a long-term memory effect even after the power supply was off.

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EXPERIMENTAL SECTION

The sandwich typed electrochemical switching device was prepared in a 3-electrode system with two ITO glasses separated by polyimide tape and Ag wire as a reference electrode. The liquid electrolyte was prepared by dissolving 0.2 M lithium bis(trifluoromethane)sulfonimide in propylene carbonate (LiBTI/PC). An YPCr coated transparent ITO glass was used as a working electrode, and another bare conductive electrode was used as a counter electrode. A silver wire was inserted between the two ITO glasses as a reference electrode. After injecting the electrolyte into the gap between two ITO glasses, the device was then sealed with epoxy resin. G

dx.doi.org/10.1021/ma501059z | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

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(13) Tigelaar, D. M.; Lee, W.; Bates, K. A.; Saprigin, A.; Prigodin, V. N.; Cao, X.; Nafie, L. A.; Platz, M. S.; Epstein, A. J. Role of Solvent and Secondary Doping in Polyaniline Films Doped with Chiral Camphorsulfonic Acid: Preparation of a Chiral Metal. Chem. Mater. 2002, 14, 1430−1438. (14) Dai, Z.; Lee, J.; Zhang, W. Chiroptical switches: applications in sensing and catalysis. Molecules 2012, 17, 1247−77. (15) Goto, H. Vortex fibril structure and chiroptical electrochromic effect of optically active poly(3,4-ethylenedioxythiophene) (PEDOT*) prepared by chiral transcription electrochemical polymerisation in cholesteric liquid crystal. J. Mater. Chem. 2009, 19, 4914. (16) Lemaire, M.; Delabouglise, D.; Garreau, R.; Guy, A.; Roncali, J. Enantioselective chiral poly(thiophenes). J. Chem. Soc., Chem. Commun. 1988, 0, 658−661. (17) Yashima, E.; Nimura, T.; Matsushima, T.; Okamoto, Y. Poly((4dihydroxyborophenyl)acetylene) as a Novel Probe for Chirality and Structural Assignments of Various Kinds of Molecules Including Carbohydrates and Steroids by Circular Dichroism. J. Am. Chem. Soc. 1996, 118, 9800−9801. (18) Yashima, E.; Matsushima, T.; Okamoto, Y. Chirality Assignment of Amines and Amino Alcohols Based on Circular Dichroism Induced by Helix Formation of a Stereoregular Poly((4-carboxyphenyl)acetylene) through Acid−Base Complexation. J. Am. Chem. Soc. 1997, 119, 6345−6359. (19) Onouchi, H.; Maeda, K.; Yashima, E. A Helical Polyelectrolyte Induced by Specific Interactions with Biomolecules in Water. J. Am. Chem. Soc. 2001, 123, 7441−7442. (20) Cao, H.; Zhu, X.; Liu, M. Self-Assembly of Racemic Alanine Derivatives: Unexpected Chiral Twist and Enhanced Capacity for the Discrimination of Chiral Species. Angew. Chem., Int. Ed. 2013, 52, 4122−4126. (21) Liu, X.; Wang, T.; Liu, M. Interfacial assembly of cinnamoylterminated bolaamphiphiles through the air/water interface: headgroup-dependent assembly, supramolecular nanotube and photochemical sewing. Phys. Chem. Chem. Phys. 2011, 13, 16520−16529. (22) Labuta, J.; Futera, Z.; Ishihara, S.; Kouřilová, H.; Tateyama, Y.; Ariga, K.; Hill, J. P. Chiral Guest Binding as a Probe of Macrocycle Dynamics and Tautomerism in a Conjugated Tetrapyrrole. J. Am. Chem. Soc. 2014, 136, 2112−2118. (23) Mori, T.; Kyotani, M.; Akagi, K. Horizontally and vertically aligned helical conjugated polymers: Comprehensive formation mechanisms of helical fibrillar morphologies in orientation-controlled asymmetric reaction fields consisting of chiral nematic liquid crystals. Chem. Sci. 2011, 2, 1389. (24) Whyte, J. R.; McQuaid, R. G. P.; Sharma, P.; Canalias, C.; Scott, J. F.; Gruverman, A.; Gregg, J. M. Ferroelectric Domain Wall Injection. Adv. Mater. 2014, 26, 293−298. (25) Weder, C.; Sarwa, C.; Montali, A.; Bastiaansen, C.; Smith, P. Incorporation of Photoluminescent Polarizers into Liquid Crystal Displays. Science 1998, 279, 835−837. (26) Zheng, Z.; Yim, K.-H.; Saifullah, M. S. M.; Welland, M. E.; Friend, R. H.; Kim, J.-S.; Huck, W. T. S. Uniaxial Alignment of LiquidCrystalline Conjugated Polymers by Nanoconfinement. Nano Lett. 2007, 7, 987−992. (27) Kim, B.-G.; Jeong, E. J.; Chung, J. W.; Seo, S.; Koo, B.; Kim, J. A molecular design principle of lyotropic liquid-crystalline conjugated polymers with directed alignment capability for plastic electronics. Nat. Mater. 2013, 12, 659−664. (28) Nowacki, B.; Oh, H.; Zanlorenzi, C.; Jee, H.; Baev, A.; Prasad, P. N.; Akcelrud, L. Design and Synthesis of Polymers for Chiral Photonics. Macromolecules 2013, 46, 7158−7165. (29) Zhang, Z.-B.; Fujiki, M.; Motonaga, M.; Nakashima, H.; Torimitsu, K.; Tang, H.-Z. Chiroptical Properties of Poly{3,4-bis[(S)2-methyloctyl]thiophene}. Macromolecules 2001, 35, 941−944. (30) Kilbinger, A. F. M.; Schenning, A. P. H. J.; Goldoni, F.; Feast, W. J.; Meijer, E. W. Chiral Aggregates of α,ω-Disubstituted Sexithiophenes in Protic and Aqueous Media. J. Am. Chem. Soc. 2000, 122, 1820−1821.

(31) Zahn, S.; Swager, T. M. Three-Dimensional Electronic Delocalization in Chiral Conjugated Polymers. Angew. Chem., Int. Ed. 2002, 41, 4225−4230. (32) Goto, H.; Akagi, K. Asymmetric Electrochemical Polymerization: Preparation of Polybithiophene in a Chiral Nematic Liquid Crystal Field and Optically Active Electrochromism. Macromolecules 2005, 38, 1091−1098. (33) Jeong, Y. S.; Goto, H.; Iimura, S.; Asano, T.; Akagi, K. Chiral BEDOT-based copolymers prepared by chemical and electrochemical polymerization. Curr. Appl. Phys. 2006, 6, 960−963. (34) Goto, H.; Jeong, Y. S.; Akagi, K. Electrochemical Synthesis and Optical Properties of a Chiral Poly[1,4-bis(3′,4′-ethylenedioxythienyl)phenylene] Derivative. Macromol. Rapid Commun. 2005, 26, 164−167. (35) Grenier, C. R. G.; George, S. J.; Joncheray, T. J.; Meijer, E. W.; Reynolds, J. R. Chiral Ethylhexyl Substituents for Optically Active Aggregates of π-Conjugated Polymers. J. Am. Chem. Soc. 2007, 129, 10694−10699. (36) You, J.; Yoon, J. A.; Kim, J.; Huang, C.-F.; Matyjaszewski, K.; Kim, E. Excimer Emission from Self-Assembly of Fluorescent Diblock Copolymer Prepared by Atom Transfer Radical Polymerization. Chem. Mater. 2010, 22, 4426−4434. (37) Kim, B.; Koh, J. K.; Kim, J.; Chi, W. S.; Kim, J. H.; Kim, E. Room Temperature Solid-State Synthesis of a Conductive Polymer for Applications in Stable I2-Free Dye-Sensitized Solar Cells. ChemSusChem 2012, 5, 2173−2180. (38) Kim, J.; Koh, J. K.; Kim, B.; Ahn, S. H.; Ahn, H.; Ryu, D. Y.; Kim, J. H.; Kim, E. Enhanced Performance of I2-Free Solid-State DyeSensitized Solar Cells with Conductive Polymer up to 6.8%. Adv. Funct. Mater. 2011, 21, 4633−4639. (39) Kim, J.; Koh, J. K.; Kim, B.; Kim, J. H.; Kim, E. Nanopatterning of Mesoporous Inorganic Oxide Films for Efficient Light Harvesting of Dye-Sensitized Solar Cells. Angew. Chem., Int. Ed. 2012, 51, 6864− 6869. (40) Koh, J. K.; Kim, J.; Kim, B.; Kim, J. H.; Kim, E. Highly Efficient, Iodine-Free Dye-Sensitized Solar Cells with Solid-State Synthesis of Conducting Polymers. Adv. Mater. 2011, 23, 1641−1646. (41) Beaujuge, P. M.; Reynolds, J. R. Color Control in π-Conjugated Organic Polymers for Use in Electrochromic Devices. Chem. Rev. 2010, 110, 268−320. (42) Jeong, Y. S.; Akagi, K. Control of Chirality and Electrochromism in Copolymer-Type Chiral PEDOT Derivatives by Means of Electrochemical Oxidation and Reduction. Macromolecules 2011, 44, 2418−2426. (43) Amb, C. M.; Kerszulis, J. A.; Thompson, E. J.; Dyer, A. L.; Reynolds, J. R. Propylenedioxythiophene (ProDOT)−phenylene copolymers allow a yellow-to-transmissive electrochrome. Polym. Chem. 2011, 2, 812. (44) Yan, Y.; Yu, Z.; Huang, Y. W.; Yuan, W. X.; Wei, Z. X. Helical Polyaniline Nanofibers Induced by Chiral Dopants by a Polymerization Process. Adv. Mater. 2007, 19, 3353−3357. (45) Ernest, L. Eliel, S. H. W. Stereochemistry of Organic Compounds; Wiley: 1994. (46) Vangheluwe, M.; Verbiest, T.; Koeckelberghs, G. Influence of the Substitution Pattern on the Chiroptical Properties of Regioregular Poly(3-alkoxythiophene)s. Macromolecules 2008, 41, 1041−1044. (47) Vandeleene, S.; Van den Bergh, K.; Verbiest, T.; Koeckelberghs, G. Influence of the Polymerization Methodology on the Regioregularity and Chiroptical Properties of Poly(alkylthiothiophene)s. Macromolecules 2008, 41, 5123−5131. (48) Chaudhuri, K. D. Concentration quenching of fluorescence in solutions. Z. Phys. 1959, 154, 34−42. (49) Seo, S.; Allain, C.; Na, J.; Kim, S.; Yang, X.; Park, C.; Malinge, J.; Audebert, P.; Kim, E. Electrofluorescence switching of tetrazinemodified TiO2 nanoparticles. Nanoscale 2013, 5, 7321−7. (50) Langeveld-Voss, B. M. W.; Janssen, R. A. J.; Meijer, E. W. On the origin of optical activity in polythiophenes. J. Mol. Struct. 2000, 521, 285−301. (51) Li, C.; Numata, M.; Takeuchi, M.; Shinkai, S. Unexpected chiroptical inversion observed for supramolecular complexes formed H

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Article

between an achiral polythiophene and ATP. Chem. - Asian J. 2006, 1, 95−101. (52) Kane-Maguire, L. A.; Wallace, G. G. Chiral conducting polymers. Chem. Soc. Rev. 2010, 39, 2545−76. (53) Watanabe, K.; Osaka, I.; Yorozuya, S.; Akagi, K. Helically πStacked Thiophene-Based Copolymers with Circularly Polarized Fluorescence: High Dissymmetry Factors Enhanced by Self-Ordering in Chiral Nematic Liquid Crystal Phase. Chem. Mater. 2012, 24, 1011− 1024. (54) Kawamura, Y.; Brooks, J.; Brown, J. J.; Sasabe, H.; Adachi, C. Intermolecular Interaction and a Concentration-Quenching Mechanism of Phosphorescent Ir(III) Complexes in a Solid Film. Phys. Rev. Lett. 2006, 96, 017404. (55) Seo, S.; Pascal, S.; Park, C.; Shin, K.; Yang, X.; Maury, O.; Sarwade, B. D.; Andraud, C.; Kim, E. NIR electrochemical fluorescence switching from polymethine dyes. Chem. Sci. 2014, 5, 1538−1544. (56) Seo, S.; Shin, H.; Park, C.; Lim, H.; Kim, E. Electrofluorescence switching of fluorescent polymer film. Macromol. Res. 2013, 21, 284− 289. (57) Koeckelberghs, G.; Vangheluwe, M.; Samyn, C.; Persoons, A.; Verbiest, T. Regioregular Poly(3-alkoxythiophene)s: Toward Soluble, Chiral Conjugated Polymers with a Stable Oxidized State. Macromolecules 2005, 38, 5554−5559. (58) Chen, M. S.; Niskala, J. R.; Unruh, D. A.; Chu, C. K.; Lee, O. P.; Fréchet, J. M. J. Control of Polymer-Packing Orientation in Thin Films through Synthetic Tailoring of Backbone Coplanarity. Chem. Mater. 2013, 25, 4088−4096. (59) Li, G.; Yao, Y.; Yang, H.; Shrotriya, V.; Yang, G.; Yang, Y. “Solvent Annealing” Effect in Polymer Solar Cells Based on Poly(3hexylthiophene) and Methanofullerenes. Adv. Funct. Mater. 2007, 17, 1636−1644. (60) Skompska, M.; Szkurlat, A.; Kowal, A.; Szklarczyk, M. Spectroelectrochemical and AFM Studies of Doping−Undoping of Poly(3-hexylthiophene) Films in Propylene Carbonate and Aqueous Solutions of LiClO4. Langmuir 2003, 19, 2318−2324. (61) Hillman, A. R.; Efimov, I.; Skompska, M. Dynamics of regioregular conducting polymer electrodes in response to electrochemical stimuli. Faraday Discuss. 2002, 121, 423−439. (62) Gaupp, C. L.; Welsh, D. M.; Rauh, R. D.; Reynolds, J. R. Composite Coloration Efficiency Measurements of Electrochromic Polymers Based on 3,4-Alkylenedioxythiophenes. Chem. Mater. 2002, 14, 3964−3970. (63) Padilla, J.; Seshadri, V.; Filloramo, J.; Mino, W. K.; Mishra, S. P.; Radmard, B.; Kumar, A.; Sotzing, G. A.; Otero, T. F. High contrast solid-state electrochromic devices from substituted 3,4-propylenedioxythiophenes using the dual conjugated polymer approach. Synth. Met. 2007, 157, 261−268. (64) Nakamura, K.; Kanazawa, K.; Kobayashi, N. Electrochemically controllable emission and coloration by using europium(III) complex and viologen derivatives. Chem. Commun. (Cambridge, U. K.) 2011, 47, 10064−6. (65) Brédas, J. L.; Chance, R. R.; Silbey, R. Comparative theoretical study of the doping of conjugated polymers: Polarons in polyacetylene and polyparaphenylene. Phys. Rev. B 1982, 26, 5843−5854. (66) Miomandre, F.; Lépicier, E.; Munteanu, S.; Galangau, O.; Audibert, J. F.; Méallet-Renault, R.; Audebert, P.; Pansu, R. B. Electrochemical Monitoring of the Fluorescence Emission of Tetrazine and Bodipy Dyes Using Total Internal Reflection Fluorescence Microscopy Coupled to Electrochemistry. ACS Appl. Mater. Interfaces 2011, 3, 690−696. (67) DeLongchamp, D. M.; Kastantin, M.; Hammond, P. T. HighContrast Electrochromism from Layer-By-Layer Polymer Films. Chem. Mater. 2003, 15, 1575−1586.

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dx.doi.org/10.1021/ma501059z | Macromolecules XXXX, XXX, XXX−XXX