Strong Bathochromic Shift of Conjugated Polymer Nanowires Assembled with a Liquid Crystalline Alkyl Benzoic Acid via a Film Dispersion Process.

We present aqueous dispersions of conjugated polymer nanowires (CPNWs) with improved light absorption properties aimed at aqueous-based applications. We assembled films of a donor-acceptor-type conjugated polymer and liquid crystalline 4-n-octylbenzoic acid by removing a cosolvent of their mixture solutions, followed by annealing of the films, and then formed aqueous-dispersed CPNWs with an aspect ratio >1000 by dispersing the films under ultrasonication at a basic pH. X-ray and spectroscopy studies showed that the polymer and liquid crystal molecules form independent domains in film assemblies and highly organized layer structures in CPNWs. Our ordered molecular assemblies in films and aqueous dispersions of CPNWs open up a new route to fabricate nanowires of low-band-gap linear conjugated polymers with the absorption maximum at 794 nm remarkably red-shifted from 666 nm of CPNWs prepared by an emulsion process. Our results suggest the presence of semicrystalline polymorphs β1 and β2 phases in CPNWs due to long-range π-π stacking of conjugated backbones in compactly organized lamellar structures. The resulting delocalization with a reduced energy bang gap should be beneficial for enhancing charge transfer and energy-conversion efficiencies in aqueous-based applications such as photocatalysis.

Introduction

Conjugated polymers are a popular material for optoelectronic,[1] biomedical,[2] and energy applications[3] because of their semiconducting properties and versatility in modulating their properties by adjusting their chemical structures. Through intra- and intermolecular charge transfer in assemblies of conjugated polymers, charge carriers can efficiently transport to enhance the efficiency of field-effect transistors (FETs),[4−6] light-emitting diodes (LEDs),[7−9] and photovoltaic cells.[10−12] In addition, they can efficiently harvest energy from a wide range of light, from ultraviolet (UV) to near-infrared (NIR) regions, by conveniently adjusting their band gaps, and either emit the energy as light or heat or transport it to other materials. Accordingly, they have been considered an ideal material for photoluminescence imaging[13−15] and sensing,[16−18] photoacoustic imaging[19−21] and photothermal therapy upon heat generation,[22−24] and photocatalysis reactions by efficient light harvesting.[3,25,26] The preparation of nanowires has been important for enhancing the performance in these applications. For example, charge transport efficiency in FETs can be significantly improved when conjugated backbones are assembled in one-dimensional (1D) nanowires, nanorods, or nanofibers due to the increased intermolecular charge transport.[27−29] In addition, intermolecular electron delocalization can narrow the band gaps of conjugated polymers, thereby enlarging the range of light absorption in the visible and NIR regions, thus enhancing light harvesting efficiency, which is useful when conjugated polymers are used as a photosensitizer in photocatalysis applications.[30−32]

Aqueous dispersions of conjugated polymer nanomaterials are particularly attractive in energy and environmental issues. As a photocatalyst or a photosensitizer combined with photocatalysts, conjugated polymer nanowires (CPNWs) can provide a useful method to harvest solar energy and utilize energy for photocatalysis reactions such as water splitting, hydrogen generation, and carbon dioxide conversion, and for degrading organic pollutants in aqueous media.[3,30] Charge transport along 1D assemblies can enhance the charge separation ability and increase the lifetime of a charge carrier when combined with other materials, resulting in increased photocatalytic activity. Although there are distinct advantages of CPNWs for energy harvesting and charge transport in such applications, fabricating 1D nanostructures of conjugated polymers in aqueous media has been challenging mainly because such aqueous dispersions of conjugated polymers typically require the use of amphiphiles that can partition the nanostructures of hydrophobic conjugated polymers and aqueous media at interfaces. Amphiphiles typically have alkyl tails and polar heads and are associated with conjugated polymers via lateral alkyl chain association, while 1D growth of CPNWs should occur by the π–π stacking of conjugated backbones. In most cases, the lateral association and surface tension minimization at interfaces between conjugated polymers and aqueous media overwhelm intermolecular π–π interactions between conjugated backbones, resulting in spherical morphologies.[33−35] Only a few studies have presented aqueous dispersions of CPNWs employing soft-templating[30] and emulsion processes.[36] In the in situ polymerization process using a mesophase as a soft template, monomers were confined to the cylindrical mesophase and converted to CPNWs by photoinitiated polymerization. The resultant CPNWs showed a significantly enhanced visible-light-active photocatalytic property.[30] In addition, we recently suggested the use of cylindrical alkyl benzoic acids as a soft template to guide the formation of CPNWs in an emulsification process.[36] However, it is difficult to form a closely packed structure of conjugated backbones via the emulsion process, which is indispensable for enhanced photophysical properties, and controlling molecular assemblies in 1D nanostructures of CPNWs remains challenging; thus, the development of new technologies is strongly required.

In this article, we present a method to prepare CPNWs as aqueous dispersions via the film assembly of a donor–acceptor-type conjugated polymer and liquid crystalline (LC) amphiphile, followed by a dispersion process of the film under ultrasonication. The donor–acceptor-type conjugated polymer, poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) can be assembled with 4-n-octylbenzoic acid (8BA), a representative nematic LC material, by hydrophobic associations between the ethylhexyl side chains of PCPDTBT and the octyls of 8BA due to its comparable length (Figure ). Meanwhile, the hydrophilic carboxylic acids of 8BA can form acid dimers via hydrogen bonding and be separated from the remaining hydrophobic moieties, resulting in ordered film assemblies. Unlike the conventional emulsion process in which we used the same PCPDTBT and 8BA to prepare CPNWs,[36] the phase-separated film dispersion process takes advantage of ordered molecular structures in LC 8BA films to direct assemblies of CPNWs and via an annealing process to enhance their optical properties. As a characteristic of LC materials, 8BA can form crystalline structures in which the donor–acceptor-type amorphous conjugated polymer can be co-associated. We show that aqueous dispersions of PCPDTBT nanowires can be formed when water molecules penetrate the polar regions of the crystalline film assemblies of PCPDTBT and 8BA, thereby dispersing the films and resulting in CPNWs with an aspect ratio >1000 dispersed in aqueous media.

Figure 1

(a) Chemical structures of PCPDTBT and 8BA and (b) a schematic illustration of the overall assembly process.

This article is organized as follows. We start with an analysis of thermal phase transitions, morphologies, and the formation of acid dimers in phase-separated films of PCPDTBT and 8BA assemblies compared to independent, pristine films of PCPDTBT and 8BA. The successful formation of CPNWs is demonstrated by preparing films at various molar mixing ratios of PCPDTBT to 8BA, followed by dispersing the films in basic conditions to break carboxylic acid dimers in the films. Our findings provide a basis for developing new CPNWs for aqueous-based applications including photoacoustic imaging, sensing, photothermal therapy, and photocatalysis.

Results and Discussion

Film Assemblies of PCPDTBT and 8BA

The key to the preparation of aqueous-dispersed CPNWs via a film dispersion process is to take advantage of the ordered assemblies of a liquid crystalline alkyl benzoic acid when a donor–acceptor-type conjugated polymer is co-associated with the LC material. First, we investigated the phase transition behaviors of three films: 8BA, PCPDTBT, and assemblies of PCPDTBT/8BA at a 1:5 mixing ratio. These films were prepared by removing the cosolvent, toluene, from their solution mixtures by continuous, mild nitrogen blowing, and their thermal phase transition behaviors were monitored by differential scanning calorimetry (DSC).

The alkyl benzoic acid 8BA is a representative LC material that shows the phase transitions from a nematic to isotropic crystalline state upon increasing the temperature. Reportedly, 8BA shows three characteristic temperatures for crystal/crystal (TCC), crystal/nematic (TCN, melting of solid crystalline phases), and nematic/isotropic (TISO) phase transitions.[37] When the thermal phase transition behavior of 8BA was analyzed by DSC, it clearly presented TCC, TCN, and TISO at 31.63, 100.23, and 111.74 °C, by enthalpy changes of 27.18, 73.54, and 16.25 J g–1, as shown in Figure . The polymorphic melting transition below TCN suggests a set of crystalline forms in the 8BA assembly. Obviously, three structural units of flexible aliphatic chains, stiff phenyl rings, and carboxylic acids contribute to the polymorphism in the 8BA assembly. In particular with numbers of methylene group higher than four for alkyl benzoic acids, the interactions between aliphatic chains and intrinsic conformational degrees of freedom significantly influence the molecular packing with the entropy increment per methylene group of 11 ± 1 J (mol·K)−1 and possibly predetermine the formation of polymorphic crystalline forms, as investigated in the literature.[38] On the other hand, it is well known that the LC property of 8BA originates from hydrogen bonds and that the resultant formation of acid dimers hinders the complete melting of crystals to an isotropic state.[39] In comparison, the PCPDTBT film did not show any phase transition behavior (Figure ) in the experiment window in our DSC study, indicating no phase transition. Meanwhile, the film assembly of PCPDTBT and 8BA at a 1:5 molar mixing ratio showed two peaks in its DSC thermogram (Figure ), at 31.66 and 93.56 °C, by enthalpy changes of 7.16 and 61.35 J g–1, respectively, which were transition temperatures close to TCC and TCN of the pristine 8BA. There was no transition peak around TISO of 8BA. These results indicate that the assembly of PCPDTBT and 8BA maintains crystalline molecular structures similar to those of 8BA, and the crystals melt at around 94 °C, which we denote as Tiso,mix of the PCPDTBT/8BA assembly. The existence of a liquid crystalline phase in the PCPDTBT/8BA assembly was not detected, which could be clarified by temperature-dependent X-ray diffraction (XRD) measurements later.

Figure 2

DSC thermograms of pristine 8BA, PCPDTBT, and PCPDTBT/8BA (1:5 molar mixing ratio) films.

Optical textures observed for films of 8BA and PCPDTBT/8BA assembly at a 1:5 mixing ratio show distinct variations near the transition temperatures measured by DSC, as shown in Figure a,b, respectively. Optical microscopy (OM) and polarized OM (POM) images of the 8BA film in Figure a show a crystalline texture up to 100 °C, the temperature around TCN, and then the film is dewetted from the substrate surface after TCN by melting the crystals into micro-droplets with a mesophase. In the droplets, the mesophase texture disappears at 115 °C beyond TISO, reappears at 110 °C upon cooling, and changes to a crystalline texture at 75 °C below TCN. The optical textures of the PCPDTBT/8BA assembly at the 1:5 molar mixing ratio present an interesting morphological transition with increasing temperature. At 25 °C below TCC, the film bears crystalline wires, as shown in the OM and POM images in Figure b; the wirelike crystalline structures disappear at 100 °C beyond Tiso,mix. Upon cooling, the wirelike crystalline structure is not recovered, and tiny crystals appear instead as shown in the OM and POM images below 90 °C (Figure b). Atomic force microscopy (AFM) analysis shows that the tiny crystals have a nanorod-like morphology, as shown in Figure . AFM images scanned in a height mode clearly show long wires before thermal treatment (Figure a) and rod-like morphology about 120 nm in length and 70 nm in width after thermal annealing (Figure b). These results show that PCPDTBT chains form a wirelike crystalline structure by assembling with 8BA LC molecules upon solvent drying. Wire morphology is transformed to nanorod morphology with heat treatment by surface-free-energy minimization upon melting of 8BA in the PCPDTBT/8BA assembly.

Figure 3

Optical textures of (a) 8BA and (b) PCPDTBT/8BA (1:5 molar mixing ratio) films with heating and cooling. Scale bars are 100 μm.

Figure 4

AFM images of PCPDTBT/8BA (1:5 molar mixing ratio) films at room temperature (a) before and (b) after heat treatment.

To elucidate the morphological changes with phase transitions, we analyzed the FT-IR spectra of three films. The Fourier transform infrared (FT-IR) spectrum of 8BA without thermal treatment in Figure a shows characteristic bands for O–H stretching (an amorphous hollow at 3300–2500 cm–1) with Fermi resonance coupling (2669.5 and 2549.0 cm–1), aromatic and aliphatic C–H stretching (3200–2800 cm–1), asymmetric C=O stretching (1678.3 cm–1), aromatic C=C stretching (1600–1400 cm–1), C–O stretching (1290.2 cm–1), and out-of-plane O–H bending (941.6 cm–1). The band positions of carbonyl stretching in the 8BA film are characteristic for closed dimers of aryl acids,[38,40] proving that hydrogen bonds form in the 8BA film. Also, the strong, broad O–H stretching band extended from 2500 to 3300 cm–1, which are overlapped with sharp C–H stretching peaks, is indicative of the existence of hydrogen-bonded acid dimers.[38] The FT-IR spectrum of the PCPDTBT film in Figure a presents characteristic peaks for aromatic and aliphatic C–H stretching (3200–2800 cm–1) and aromatic ring stretching (1600–1300 cm–1). The characteristic ring stretching peaks of thiophene and cyclopentadiene in PCPDTBT are indicated by red arrows at 1504.2 and 1396.2 cm–1 (Figure a), respectively, which are coincident with the literature.[41] Notably, the characteristic peaks for closed acid dimers, thiophene, and cyclopentadiene also appear in the FT-IR spectrum of PDPCTBT/8BA film as shown in Figure a. They are the OH stretching band (3300–2500 cm–1), the carbonyl stretching band (1678.8 cm–1), and ring stretching bands of thiophene and cyclopentadiene (1504.2 and 1397.2 cm–1). These results clearly indicate that 8BA molecules do form closed dimers of acyl acids via hydrogen bonding after assembling with PCPDTBT in the film and suggest that 8BA molecules might form crystal structures in assembly with PCPDTBT.

Figure 5

(a) FT-IR spectra of pristine 8BA, PCPDTBT, and PCPDTBT/8BA (1:5 molar mixing ratio) films at room temperature before heat treatment. (b) In situ FT-IR spectra of PCPDTBT/8BA (1:5 molar mixing ratio) film with temperature change to monitor O–H stretching with Fermi resonance coupling and carbonyl stretching bands.

In situ FT-IR study of the PCPDTBT/8BA film assembled at a 1:5 molar mixing ratio film reveals the origination of Tiso,mix. The strong, broad OH stretching band clearly shown at 25 °C in Figure b disappears above 100 °C. Also, the strong carbonyl stretching band characteristic for aryl acid dimers at 1678.8 cm–1 at 25 °C also disappears above 100 °C, and acid monomer bands at around 1730 and 1718 cm–1 only remain. These results indicate that Tiso,mix is the temperature at which hydrogen bonds between aryl acids are broken and free acid monomers are formed.

The temperature-dependent XRD measurements in Figure clearly show the crystallographic evolution corresponding to phase transitions confirmed by DSC thermograms and in situ FT-IR spectra. The XRD pattern of 8BA powder (Figure a) measured in a capillary tube clearly shows a triclinic structure at room temperature (space group P1, a = 13.64 Å, b = 21.92 Å, and c = 7.64 Å; α = 90.02°, β = 101.05°, γ = 108.02° at 20 °C; see Figure S1 and Table S1 for a representative Rietveld analysis of XRD data based on a whole-pattern profile matching method using the FullProf Suite and temperature-dependent analysis results, respectively). This crystal structure is transformed into the same triclinic structure of the identical crystal phase with different lattice constants and angles above TCC (P1, a = 13.50 Å, b = 22.79 Å, and c = 7.59 Å; α = 98.37°, β = 99.55°, γ = 101.43° at 60 °C) and a mixture of a mesophase and the crystal phase near TCN (P1, a = 9.74 Å, b = 21.11 Å, and c = 7.68 Å; α = 92.65°, β = 103.70°, γ = 93.71°; P1, a = 13.56 Å, b = 23.05 Å, and c = 7.60 Å; α = 99.90°, β = 99.08°, γ = 100.46° at 95 °C), single mesophase above TCN (P1, a = 9.74 Å, b = 21.13 Å, and c = 7.66 Å; α = 92.56°, β = 103.93°, γ = 93.79° at 105 °C) and disappears after Tiso.

Figure 6

Temperature-dependent XRD patterns of (a) pristine 8BA, (b) PCPDTBT, and (c) PCPDTBT/8BA assembly at a molar mixing ratio of 1:5. Temperature-dependent lattice constants and angles of (d, e) pristine 8BA and (f, g) PCPDTBT/8BA assembly, respectively.

In comparison, PCPDTBT scraped up from its film formed in a vial by solvent drying with nitrogen blowing did not show distinct crystal peaks in its XRD pattern (Figure b). It showed only three blunt diffraction peaks at around q = 0.598, 1.064, and 1.540 Å–1, respectively. These peak positions correspond to those of the (110), (130), and (150) planes in an orthorhombic cell of crystalline PCPDTBT[42,43] and did not change with an increase in temperature, indicating that there was a slight ordering and not a crystalline structure, and showed no structural variation up to 150 °C.

In the XRD pattern of the PCPDTBT/8BA assembly (Figure c), characteristic diffraction peaks for both 8BA and PCPDTBT appeared simultaneously. Interestingly, the temperature-dependent lattice constants and angles of the crystal structure in the PCPDTBT/8BA assembly at a 1:5 molar mixing ratio presented in Figure f,g are almost coincident with those of the pristine 8BA as shown in Figure d,e. The XRD pattern of PCPDTBT/8BA shows a triclinic structure at 20 °C (space group P1, a = 13.64 Å, b = 21.72 Å, and c = 7.63 Å; α = 89.38°, β = 101.08°, γ = 107.03°). This crystal structure is transformed into the same triclinic structure of the identical crystal phase with different lattice constants and angles above TCC (P1, a = 13.51 Å, b = 22.84 Å, and c = 7.61 Å; α = 98.08°, β = 99.37°, γ = 101.30° at 60 °C). A mixture of a mesophase and the crystal phase appears near 95 °C (P1, a = 9.73 Å, b = 21.12 Å, and c = 7.65 Å; α = 92.56°, β = 103.36°, γ = 93.74°; P1, a = 13.58 Å, b = 23.18 Å, and c = 7.58 Å; α = 100.20°, β = 98.43°, γ = 101.13° at 95 °C), and disappears above 95 °C. These results mean that 8BA and PCPDTBT exist as independent regions in the assembly of PCPDTBT and 8BA. Figure c looks almost like a superposition of Figure a,b, suggesting that the PCPDTBT/8BA assembly is a simple phase-separated blend of the pure polymer and pure 8BA. The differences in DSC transition temperatures can be attributed to finite size effects due to the existence of PCPDTBT domains in the excess 8BA domains in the film of the PCPDTBT/8BA assembly. Because more 8BA than PCPDTBT was used, it is plausible that PCPDTBT chains are associated with 8BA at the outmost surface of 8BA crystal domains and that the domains of PCPDTBT are inserted in the alkyl chain regions in between the 8BA crystal domains.

Both the DSC and temperature-dependent XRD results present decreases in energies required for melting transitions, TCN and Tiso, when PCPDTBT molecules are assembled with 8BA, indicating that the association between PCPDTBT and 8BA disrupts the original crystal structure of 8BA. Pristine 8BA assembly has three regions of alkyl chain packing, π–π stacking between phenyl rings, and hydrogen bonding between carboxylic acids, as described in studies regarding alkyl benzoic acids.[44] Three phase transition temperatures of TCC, TCN, and Tiso should be related to order/disorder transitions of three structural elements in 8BA. The melting enthalpy at TCC significantly decreased, from 31.63 to 7.16 J g–1, although the peak position for the melting transition is the same, at 31.6 °C. The melting enthalpies at TCN and Tiso also decreased from 89.79 to 61.35 J g–1. The transition temperatures, TCN and Tiso, subsequently decreased and merged from 100.23 and 111.74 to 93.56 °C, showing the influence of the finite size effect by blending the PCPDTBT domains in the 8BA assembly.

With the DSC, XRD, and in situ FT-IR results, the crystal structure of wires shown in POM and AFM images (Figures b and 4a) measured before thermal treatment provides us a motif of the PCPDTBT and 8BA assembly. The (150) plane shown in Figure b is a characteristic diffraction peak by π–π stacking of conjugated planes in the PCPDTBT assembly. Thus, it is also plausible that PCPDTBT chains are self-associated with each other via the π–π stacking forming their independent regions and that their alkyl side chains are associated with octyls of 8BA between 8BA crystal domains, forming the wirelike morphology before the heat treatment. It seems that the wirelike morphology is not in the thermodynamically most stable state, mainly because the assembly film is prepared by evaporating the solvent with nitrogen gas blowing. The long-wire-like morphology was transformed to the film with a fine texture upon heating by surface energy minimization above Tiso,mix, where the 8BA crystal melted and hydrogen bonds between aryl acids are broken (Figures b and 4b).

CPNWs of PCPDTBT and 8BA

The film assemblies of PCPDTBT and 8BA were dispersed in basic aqueous solutions under ultrasonication to form CPNWs. An optimal basic condition was required to obtain CPNWs that were independently dispersed with a narrow width of less than one micrometer. The concentration of 8BA in 2 mL of aqueous solution that contained PCPDTBT/8BA assembly at a 1:5 molar mixing ratio was 0.935 mM. After adding aqueous solutions at pHs 10, 11, 12, 13, and 14 followed by ultrasonication, the pHs of the resulting solutions were changed to 5.61, 6.24, 7.09, 9.71, and 12.46, respectively. With these concentrations of [H+] and pKa values of benzoic acid (pKa = 4.2),[45] the % degrees of ionization are estimated to be substantially high enough for dispersing CPNWs by electrostatic repulsion of carboxylate ions. Interestingly, a successful dispersing of the assembly occurred at pH 12, where the concentration of hydroxyl ion [OH−] was 1 mM. When the pHs of aqueous solutions that were added into vials with dry PCPDTBT/8BA assemblies were 10 and 11, micrometer-scale rods were shown after ultrasonication, indicating that the film dispersing into nanowires is inefficient (Figure S2a and S2b). At pHs 13 and 14, there exist significantly excessive amounts of NaOH (Figure S2c and S2d) in which the wires of conjugated polymers are embedded. Under the optimal condition of pH 12, it appears that carboxylic acids in the PCPDTBT/8BA assembly are sufficiently ionized to split off the film assembly into narrow nanowires. Meanwhile, these results indicate that the morphology of CPNWs is sensitive to changes in the pH of the aqueous solutions. The morphology might need to be stabilized through the curing reaction by introducing a photo-cross-linkable functional group into the chemical structure of LC alkyl benzoic acid.

To effectively assemble ethylhexyl side chains of PCPDTBT with octyls of 8BA in the crystalline structures of 8BA, it is presumed that the molar mixing ratio of PCPDTBT to 8BA should be high. As we previously reported on the assembly of PCPDTBT with a phospholipid, the molar mixing ratio of the phospholipid to PCPDTBT had to be higher than three for the successful inclusion of PCPDTBT chains into the alkyl regions of the lipid layers.[24,46] We investigated the effects of molar mixing ratios on the preparation of CPNWs by mixing PCPDTBT and 8BA at five different molar mixing ratios, from 1:1 to 1:5. Phase-separated films prepared by removing the toluene solvent were dispersed at pH 12 to facilitate the breakage of the film by ionizing the carboxylic acids at the basic condition with ultrasonication. It was clearly observed that CPNWs formed unsuccessfully at low mixing ratios, of 1:1 and 1:2 (Figure a,b) although the observed incompletely dispersed films at 1:1 and 1:2 lurk in curved nanowires. Separated CPNWs become straight and independent as the molar mixing ratios are increased (Figure c,d), presenting the complete isolation of CPNWs at the ratio of 1:5 (Figure e). Notably, the aspect ratio of CPNWs at 1:5 was beyond 1000 as shown in a wide view scanning electron microscopy (SEM) image (Figure S3). The formation of CPNWs at such a high aspect ratio as an aqueous dispersion occurred using both the PCPDTBT/8BA assemblies with and without thermal annealing above Tiso,mix at the 1:5 molar mixing ratio. AFM images of PCPDTBT/8BA wires at an intermediate stage of the film dispersion process without (Figure f) and with (Figure g) thermal annealing of the films presented flat top surfaces, indicating that the wires are dispersed from crystal assemblies. Further film dispersing of the PCPDTBT/8BA assembly at the 1:5 molar mixing ratio after thermal annealing by ultrasonication could produce narrow nanowires with a width of about 30 nm, as shown in a transmission electron microscopy (TEM) image (Figure h).

Figure 7

SEM images of CPNWs prepared by shattering film assemblies of PCPDTBT/8BA at molar mixing ratios of (a) 1:1, (b) 1:2, (c) 1:3, (d) 1:4, and (e) 1:5. AFM images of wires shattered from PCPDTBT/8BA assemblies at the 1:5 ratio (f) without and (g) with thermal annealing. (h) TEM image of a PCPDTBT/8BA nanowire shattered from PCPDTBT/8BA assembly at the 1:5 ratio after thermal annealing.

The structures of CPNWs were investigated using two-dimensional grazing-incident X-ray diffraction (2D-GIXD) in comparison to those of 8BA and PCPDTBT/8BA films assembled on silicon substrates (Figure ). Peak positions of the (010) plane (q = 0.295 and 0.282 Å–1, respectively) in the 2D-GIXD patterns of 8BA and PCPDTBT 8BA films slightly varied from peak positions in the powder patterns in Figure a,c (q = 0.302 and 0.302 Å–1, respectively) (Figure a,b). Interestingly, there exist (0h0) diffractions along the out-of-plane direction, indicating an ordered layer structure of 8BA (Figure d). These higher-order peaks in the (0h0) diffraction did not appear in the XRD patterns of the powder samples encased in the quartz tube after scraping from the bottom of the vials (Figures a,c and S4). These series of (0h0) diffractions become distinct when the PCPDTBT/8BA film assembled on a silicon substrate (Figure b) was directly measured by GIXD, showing that preferred orientation becomes prominent on the substrate. A representative 2D-GIXD pattern of CPNWs dispersed at pH 12 from film assembly of PCPDTBT and 8BA at the 1:5 molar mixing ratio after thermal annealing, and its corresponding 1D line cut along the out-of-plane direction (Figure c,d, respectively), also present the series of distinct (0h0) diffractions together with diffractions by NaOH. Characteristic diffraction peaks of 8BA crystals that appear in the 2D-GIXD patterns (Figure a,b) are not clearly shown after dispersing the film into CPNWs except the layer peaks, indicating the disruption of the crystal structure during the dispersion process. Most notably, the position of the (010) plane in the 2D-GIXD pattern of CPNWs and its line cut was significantly shifted to q = 0.215 Å–1 (Figure c,d). The d-spacing estimated by the (010) diffraction corresponds to a thickness of one 8BA bilayer. Thus, these results indicate that the bilayer thickness of 8BA increased from about 21 Å in the powder samples to about 29 Å in CPNWs. On the other hand, enlarged 1D GIXD patterns in Figure e shows that the positions of (010) diffractions by transmission (qTz) and reflection beam (qRz) significantly shifted to higher q values at molar mixing ratios of 1:3, 1:4, and 1:5 than those at 1:1 and 1:2 (Figure e). The d-spacing values estimated using qRz decreased from 37.7 and 32.7 Å at the 1:1 and 1:2 ratios to 28.8, 28.8, and 29.3 Å at the 1:3, 1:4, and 1:5 ratios, respectively (Figure f). These results indicate that PCPDTBT and 8BA in CPNWs are compactly assembled into a highly ordered layer structure at molar mixing ratios beyond 1:3.

Figure 8

2D-GIXD patterns of (a) 8BA film and (b) PCPDTBT/8BA film at a 1:5 molar mixing ratio before thermal annealing, and (c) CPNWs at a 1:5 molar mixing ratio dispersed after thermal annealing. (d) 1D line cut of 2D-GIXD of CPNWs along the out-of-plane direction. (e) Enlarged 1D patterns of the (010) plane at different molar mixing ratios. (f) d-Spacing of the layered structure estimated from qRz values of the (010) plane, where qTz and qRz are out-of-plane diffraction peaks by transmission and reflection, respectively.

The assembly structure of CPNWs was further examined by comparing the IR spectrum of the film assembly of PCPDTBT and 8BA at the 1:5 molar mixing ratio after thermal annealing with that of CPNWs cast and dried on a wafer (Figure a). In the IR spectrum of the film assembly, the integration ratio of the peak of the 8BA carbonyl at 1678.8 cm–1 to that at 1504.2 cm–1 indicative of thiophene ring stretching in PCPDTBT is about 4.7:1. We could observe the peak of the thiophene stretching at 1502.3 cm–1 in the IR spectrum of CPNWs and confirm the presence of PCPDTBT in CPNWs although other peaks of PCPDTBT are overlapped with those of 8BA. The integrated peak ratio of 8BA carbonyl to PCPDTBT thiophene stretching is 3.6:1 for CPNW, a slightly reduced value from that for film assembly. This result indicates that some 8BA molecules do not participate in CPNW formation during the film dispersion process and explains why many crystal peaks disappear except for the layering peaks and sodium-related peaks in Figure c. It should also be noted that no broad O–H stretching peak extending from 2500 to 3500 cm–1 is observed for CPNWs. This result manifests that no hydrogen bonds are present and that carboxylic acid in 8BA is in the form of sodium n-octylbenzoate.

Figure 9

IR absorption spectra of (a) PCPDTBT/8BA film assembly at a 1:5 molar mixing ratio and CPNW and (b) redshifts in absorption bands of aliphatic C–H stretching and thiophene ring stretching vibration.

On the other hand, by measuring any shifts in wavenumbers for C–H and thiophene vibrations, we were able to validate these improved intermolecular interactions. In enlarged spectra of aliphatic C–H stretching and thiophene ring stretching bands in Figure b, the asymmetric C–H stretching vibration in −CH3 and −CH2, and the symmetric stretching vibration in −CH3 and −CH2, respectively, are allocated to the IR bands of the PCPDTBT/8BA film assembly at 2954.0, 2920.2, 2869.1, and 2852.3 cm–1 in Figure a, respectively.[47] The asymmetric vibration band of −CH2 was red-shifted from 2920.2 to 2915.9 cm–1 in the IR spectra of CPNWs produced via thermal annealing. Meanwhile, the asymmetric stretching of −CH3 and symmetric stretching of CH2 band locations stay at the same wavenumber, and the band for symmetric stretching of −CH3 at 2869.1 cm–1 vanishes, probably incorporated in the other bands. The weakening of the C–H bond due to electron density transfer due to intermolecular interaction is indicated by the redshift of the band for the asymmetric stretching vibration in −CH2, which is suggestive of alkyl chain packing. We also discovered a redshift in the thiophene ring stretching vibration from 1504.2 cm–1 in the film assembly to 1502.3 cm–1 in CPNWs, indicating greater intermolecular interaction between conjugated planes.

From the results of GIXD and IR measurements, it is credible to suggest that ionization of 8BA in the basic condition induces a lyotropic state of 8BA and that the amphiphilic 8BA can plasticize the PCPDTBT chains and further enhance the layering structure of 8BA with PCPDTBT during 8BA dissolution. The distinct enhancement of the layer peaks in Figure c,d compared to that of Figure b supports an increase in the ordered layer assembly during the dissolution process. These results also indicate that PCPDTBT chains are incorporated into the alkyl domains between the 8BA domains via alkyl chain associations, leading to an increase in the 8BA layer thickness. The most plausible molecular assembly structure is that the sodium ions coordinate with the carboxylates to form a hydrophilic layer, and the alkyl tails of 8BA bind with the alkyl side chains of PCPDTBT via hydrophobic interactions to form a hydrophobic layer, and these hydrophilic and hydrophobic layers are repeated multiple times to form a highly ordered nanowire structure.

Room-temperature electronic absorption spectra were obtained to gain information on the effect of assembly structures in CPNWs on their optical properties. In Figure a, representative UV−visible (UV−vis) spectra of each CPNW prepared through the dispersion process of films with and without thermal annealing show absorption maxima at 794 and 779 nm (red and blue lines in Figure a), respectively. Previously, the absorption spectroscopic features of PCPDTBT and its aggregates were investigated both experimentally and theoretically.[48−51] The low-energy absorption band with a vibronic replica spanning from 600 to 900 nm has been ascribed to the S0 → S1 transition. In contrast to PCPDTBT in solution, which has the absorption peak maximum at about 710 nm, it is known that the formation of π–π stacks in PCPBTBT results in a substantial redshift of 40–70 nm depending on the microstructure.[48−50] In this regard, the red-shifted absorption band of the CPNW dispersions by the film dispersion process accounts for the well-ordered microstructures and is consistent with the structural analysis results above. In particular, PCPDTBT chains form long-range π-stacking in the in-plane direction, presenting an edge-on morphology of the conjugated planes in the out-of-plane layered structure of 8BA, as shown by the diffraction of the (110), (130), and (150) planes in a 1D line cut of the 2D-GIXD pattern of the PCPDTBT/8BA film assembly along the in-plane direction (Figure S5). The interchain stacking distance estimated by the diffraction of the (150) plane is 3.8 Å, which corresponds to that of semicrystalline PCPDTBT films in the literature.[52−61] The absorption spectra show that the thermal annealing of the film assembly can cause the hydrogen bonds in the 8BA crystal structure to completely break and become a molten state, contributing to the increase of the long-range π-stacking of PCPDTBT chains. On the other hand, our control PCPDTBT dispersion, prepared by an emulsion process (green in Figure a) that we used in our previous report,[36] displays a distinctive absorption spectrum with a band maximum at 666 nm, which is even a shorter wavelength than that of PCPDTBT single chain (∼710 nm). This substantial blueshift in absorption spectrum has been noticed in a highly disordered packing of PCPDTBT that disturbs π-conjugation in a polymer backbone.[62] These results manifest the influence of the film formation and dispersion process on the enhancement of the π–π stacking structure of conjugated backbones.

Figure 10

(a) UV−vis absorption spectra of CPNW aqueous solutions prepared with and without thermal annealing of PCPDTBT/8BA film assemblies at a 1:5 molar mixing ratio and via an emulsion process. All spectra were measured with an integrating sphere. (b) Peak deconvolution of CPNW’s UV−vis absorption spectrum prepared with thermal annealing.

The CPNW’s absorption spectra were further analyzed by peak deconvolution (Figure b). The absorption wavelength at 794 nm for CPNW prepared from the annealed film is the most red-shifted one for PCPDTBT. Even in thin films based on PCPDTBT for photovoltaic devices, the characteristic peaks are shown at around 780 nm.[51] PCPDTBT is a semicrystalline conjugated polymer with an amorphous phase (α) resulting from spaghetti-like conformation and an aggregate phase (β) resulting from strong intermolecular interactions between conjugated planes or aromatic functionalities.[50,63] Peak deconvolution shows that CPNWs have the α phase and semicrystalline polymorphs (β1 and β2). For CPNW produced with thermal annealing, the compositions of the α, β1, and β2 phases are 63.4, 31.5, and 5.1%, respectively, with center of gravity values at 685.8, 810.7, and 958.6 nm (Figure b). At 688.1, 806.4, and 960.1 nm, those for CPNWs without thermal annealing are 70.5, 25.7, and 3.8%, respectively. The redshift of the β1 wavelength from 780 nm in thin-film absorption spectra of photovoltaic devices to ∼810 nm in CPNW absorption spectra in our work suggests a broader delocalization due to long-range π stacking. Also, the existence of the β2 phase implies that conjugated planes have a more organized, compact assembly structure. As revealed in a recent study of a diketopyrrolopyrrole-based polymer, the β2 phase can be generated when intrachain disorder is reduced or interchain interaction between conjugated planes is promoted, perhaps due to a slightly smaller π-stacking distance.[64,65] Our findings suggest that lamellar layering and intermolecular alkyl chain packing during the film dispersion process cause the compact, structured assembly structure, resulting in the long-range delocalization and lower energy state.

Conclusions

In this study, we presented a fabrication process for nanowires of a donor–acceptor-type conjugated polymer, PCPDTBT, as an aqueous dispersion by forming its film with an amphiphilic liquid crystalline molecule, 8BA, followed by dispersing the film at a basic condition. We confirmed that the PCPDTBT and 8BA form independent phases and that the association of PCPDTBT domains with the 8BA domains decreases transition temperatures of the pristine 8BA assembly, seemingly due to a finite size effect, while the triclinic structure of the 8BA assembly is maintained. Film assemblies of PCPCDTB and 8BA could be split off into nanowires by ionization of carboxylic acids and dissolution of 8BA at a basic solution pH. GIXD and IR data showed that sodium n-octylbenzoate are compactly associated with alkyl side chains of PCPDTBT, resulting in a highly ordered layer structure of PCPDTBT and sodium n-octylbenzoate. Our experiments show that this ordered structure in CPNWs strongly enhances NIR absorption due to π–aggregates in the ordered assembly, suggesting their usefulness for light harvesting and charge transporting in aqueous-based applications. It should be noted that the morphology of nanowires is not stable with changes under ambient conditions, such as the pH of the solution, and a technology to improve the stability of the morphology of nanowires needs to be developed. Nevertheless, the absorption property and the final morphology of CPNWs prepared via the film dispersion process are truly unusual, unprecedented for PCPDTBT, showing a different domain shape, crystal habit, and optical spectrum from those polymers typically known for, and might play an important role in aqueous-based applications such as photocatalysis and biomedical applications utilizing conjugated polymers.

Experimental Section

Materials

PCPDTBT (Mw = 34 kDa, PDI = 2.1, MW on a repeat unit basis = 534.845 g mol–1) was purchased from One Materials, Inc. (Quebec, Canada), and 8BA (Mw = 234.33 g mol–1) was provided by Sigma-Aldrich (St. Louis, MO). All other chemicals were of analytical reagent grade and used as received.

Nanowire Assemblies

Phase-separated films of PCPDTBT and 8BA at five different molar mixing ratios, from 1:1 to 1:5, were prepared by dissolving both species in toluene in vials 20 mL in volume, followed by solvent removal with mild flowing of nitrogen. For example, to prepare a film at a 1:5 molar mixing ratio of PCPDTBT to 8BA, 0.200 mg (0.374 μmol on its repeat unit basis) of PCPDTBT and 0.438 mg (1.870 μmol) of 8BA were separately dissolved in 1 mL of toluene, and then both solutions were mixed in a vial and treated under ultrasonication for 5 min to completely mix both species. The toluene solvent was removed by nitrogen streaming, resulting in assembly films of PCPDTBT and 8BA on the bottom of the vials. The films were further dried in vacuum at room temperature overnight. To the vials with films on their bottom, 2 mL of pH-controlled water was added. The pH of the water was adjusted to five different values in the range 10–14, and each was then added to the vials to investigate the effect of solution pH on film shattering. The solutions were then treated under ultrasonication for 30 min with a bath-type sonicator (Branson 1510R-DTH, Branson, Newtown, CT), followed by another 30 min with a probe-type sonicator (Sonics VCX0750, Sonics & Materials, Newtown, CT) to shatter the films and obtain aqueous dispersions of CPNWs.

Characterization

The UV–vis absorption spectra of the CPNW dispersions were obtained using a UV–vis spectrometer (V-770, JASCO, Tokyo, Japan), equipped with an integrating sphere. Attenuated total reflection FT-IR spectra of 8BA, PCPDTBT, and PCPDTBT/8BA films were recorded using an FT-IR spectrometer (Nicolet 6700, Thermo Scientific, Waltham, MA). The films’ phase transitions were monitored by a DSC (214 Polyma, NETZSCH Co., Ltd., Selb, Germany) with a scanning rate of 10 °C min–1 in a nitrogen atmosphere. Data in the second heating were used for analysis. Morphological observations of the nanostructures were conducted using a field-emission SEM (SIGMA, Carl Zeiss, Oberkochen, Germany), a high-resolution TEM (JEM3010, JEOL, Akishima, Japan), and an AFM (XE-100, PSIA, Suwon-si, South Korea). Structural analysis using powder XRD and 2D-GIXD was conducted at a synchrotron facility (6D UNIST-PAL beamline of PLS-II at Pohang Accelerator Laboratory, Pohang, Republic of Korea). The X-rays coming from the bending magnet were monochromated at 11.6 keV (wavelength: 1.0688 Å) using Si (111) double crystals and were focused at the detector position using a combination of a sagittal-type monochromator crystal and a toroidal mirror system. The incidence angle of the X-ray beam was set to 0.13°, and the sample-to-detector distance was ca. 246 mm for the GIXD experiments. Temperature-dependent XRD studies were done while spinning the capillary after filling powder samples in a quartz capillary (diameter 200 μm). Scattering patterns were recorded with a 2D CCD detector (MX225-HS, Rayonix L.L.C.), and the diffraction angles were calibrated using a lanthanum hexaboride (LaB6) standard (NISTSRM 660b).