Article pubs.acs.org/crystal
Influence of Dissolution Status in Eutectic Mixtures on Crystallization Routes of Semiconductive Polyalkylthiophenes Bei-Kai Yang,† Chi-An Dai,‡ Chi-Ju Chiang,‡ Chieh-Nan Lai,† Chao-Cheng Hsu,† and Jrjeng Ruan*,† †
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan
‡
ABSTRACT: Upon the dissolution of comb-like polyalkylthiophenes in the melt of a crystalline hexamethylbenzene compound, eutectic molten mixtures were prepared, which were found to involve disparate mixing statuses of polyalkylthiophenes. When more than 2 wt % of polyalkylthiophenes is mixed in molten solutions, metastable face-on lamellae of polyalkylthiophenes widely spread upon efficient eutectic solidification, instead of stable edge-on crystalline lamellae. Beyond a critical concentration of polyalkylthiophene, the association of board-like thiophene backbones into stripe-like micelles in molten solutions was proposed as a preceding background. When the association tendency in molten mixture declines with the decrease of concentration or molecular weight, only edge-on lamellae are precipitated. Furthermore, as indicated by the structural analysis via electron diffraction patterns, the stacking of metastable face-on nanostructures is able to transform to stable edge-on crystals via collaborative rotation and lateral association of thiophene backbones. Nevertheless, with the attachment of a longer side chain, this structural evolution is hindered by enhanced kinetic barriers, and thus regular stacking of face-on nanostructures is able to persist and progress as an alternative organization route. For face-on nanostructures serving as a preordering state prior to the growth of edge-on crystals, annealing temperatures capable of initiating an overall structural transition within thin film have been explored and manifested as an indication of the metastability. might diversify involved crystallographic matching.18−22 Furthermore, many novel epitaxial relationships without strict crystallographic matching between epitaxy interfaces of organic molecules have been reported,23−25 including the epitaxially oriented preordering state prior to crystallization of polylactides and the directional eutectic epitaxy of block copolymer. Apparently the epitaxial organization of organic molecules is not only related to classic epitaxial relationships derived completely from the interactions of crystalline lattices. It is inferred that the assemblies and organization of organic molecules actually depend on a much more delicate balance of weak noncovalent interactions,26,27 which continuously triggers curiosity about the possible guiding effects of epitaxial relationships. In an eutectic molten mixture of conjugated poly(3hexylthiophene) (P3HT) and hexamethylbenzene (HMB), the epitaxial crystallization of P3HT on the surface of previously developed HMB crystals has been studied.17 This polyalkylthiophene is considered representative of an alternating inverse comb-like molecule with conjugated board-like thiophene backbone covalently attached with aliphatic side chains.28 With the development of epitaxial molecular
1. INTRODUCTION For the increased emphasis on the use of molecular-based optoelectrical thin film in devices, many challenges remain, such as the uniform orientation of crystalline phase, oriented arrangement of dipole moments, and favored routes of charge transportation.1−5 Regarding the influence of azimuthal orientation of molecular organization, edge-on stacking of conjugated backbones is acknowledged as helpful for optimizing the performance of an organic active layer in transistors, whereas the face-on organization is much needed for enhancing the efficiency of organic photovoltaics.6−9 Thus, manipulating the azimuthal orientation of molecular organization is also critical for the practical use of organic film in optoelectronic devices, but however is still puzzling. As one of the potential solutions, epitaxial relationships are of interest and widely studied in order to guide the organization of conjugated molecules10−14 and adjust the azimuthal orientation according to the need. The classic epitaxy relationships are derived from the crystallographic matching of contact crystalline lattices including the influence of packing symmetries initially upon the study on the epitaxy of inorganic crystals.15,16 The commensuration between contact crystal lattices was elucidated and classified in terms of epitaxial relationships.17 Nevertheless, for the epitaxy of long-chain molecules, both the helical conformation of molecular chains and the periodic arrangement of vinyl groups in crystal lattices © XXXX American Chemical Society
Received: December 14, 2015 Revised: April 24, 2016
A
DOI: 10.1021/acs.cgd.5b01765 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
regioregularity of these purchased samples are all larger than 90%. Moreover, in order to investigate the effect of molecular weight of P3ATs on the epitaxial crystallization, low molecular weight (LMW) P3HT (Mn = 11 800 g/mol and Mw/Mn = 1.23) and P3OT (Mn = 12 100 g/mol and Mw/Mn = 1.18) were synthesized by using the Grignard metathesis polymerization method. These synthesized samples are also highly regioregular with the regioregularity larger than 90%. The molecular weight and its distribution for synthesized polymers were measured by using GPC (Waters 2695) equipped with two Styragel columns (HR3 and HR4E), a refractive index detector (Waters 2414) and a photodiode array absorbance detector (Waters 2996). THF was used as the eluent at a flow rate of 1 mL/min and monodispersed polystyrenes were used as the calibration standard. The crystalline powders of hexamethylbenzene (HMB) with a melting point of 165 °C were purchased from Tokyo Chemical Industry and used without further purification. X-ray Diffraction. The two-dimensional X-ray diffraction patterns were performed by using a Bruker D8 DISCOVER with GADDS instrument. This X-ray facility is equipped with the ceramic X-ray diffraction tube for generating the incident X-ray source at a wavelength of 0.154 nm. The diffraction signals were recorded by use of a VANTEC-2000 area-detector with 2048 × 2048 pixels in an active area of 14 cm × 14 cm. For obtained 2D diffraction patterns, the distance between sample and detector has been calibrated by the standard sample Corundum SRM 1976a (2θ = 25.59°, 35.18°, 37.81°, 43.39°, 52.59°, 57.55°, 66.58°, 68.28°). Moreover, the reflection at the angles of at 19° and 33° was used to calibrate the location of center point of obtained diffraction patterns, and a resolution of 0.01° has been achieved. The radial intensity scan of diffraction signals on 2D diffraction pattern was adopted to obtain diffraction profiles for evaluating the intensity of diffraction peaks. For comparing these diffraction profiles yielded by samples annealed at selected annealing temperatures, the detected diffraction intensities have been multiplied by suitable factors, and thus the intensity of the HMB (200) diffraction peak of shown on each profile was adjusted to the same level. As this (200) diffraction peak of HMB triclinic crystal was adopted as a reference peak and adjusted to have the same intensity in each obtained diffraction profile, diffraction intensities of other diffraction peaks are to be normalized accordingly. Therefore, the variation of diffraction intensity of concerned diffraction peaks upon selected annealing temperatures can be evaluated in this research. Transmission Electron Microscopy. Transmission electron microscopy (TEM) observations were made using a JEOL JEM1400 instrument operated at 120 kV with tungsten filament. Brightfield images (BFI) were obtained using a Gatan digital detector, whereas selected-area electron diffraction (SAED) patterns were recorded also via a digital camera. Sample Preparation. Both of HMB and selected polythiophene samples were dissolved in chloroform solution with selected concentrations and mixing ratios. The solution casting via quick evaporation of chloroform solvent was conducted for the preparation of thin film. These resultant composite thin films were heated to 180 °C for 20 s in order to form homogeneous molten solutions, which is followed either by the quenching to room temperature or quickly cooled to selected temperatures for inducing eutectic solidification. The iron bar cooled by liquid nitrogen is used to quench the molten mixtures. By controlling the time of the contact of glass slide with this cooled iron bar, the molten mixture on glass slide can be quenched to selected temperatures within a few seconds. For the purpose of research, designed annealing treatment can be further applied to these solidified films. For preparing the TEM specimen, the sublimation of HMB crystals was conducted by placing prepared films in a vacuum oven overnight at room temperature. Afterward the film was coated with a thin layer of carbon through the method of vapor deposition in a vacuum, and this carbon-coated film was transferred to a copper grid for TEM observation.
organization, the wide facet of board-like thiophene backbone can be uniformly oriented either parallel or perpendicular to the substrate surface, recognized as face-on and edge-on azimuthal organization orientation within thin film, respectively. When molecular backbones are lain on the substrate, these two organization orientations basically reflect how molecular backbones prefer to interact with substrate, which nevertheless can be influenced by a thin-film fabrication process as well. As a result of eutectic precipitation under a sufficient extent of undercooling, oriented arrays of P3HT face-on lamellae were surprisingly found to spread on HMB crystalline platelets. However, it is widely known that edge-on organization of P3HT is more stable and frequently found to develop prevailingly within thin film as a major event.17 Thus, the initial epitaxial spread of less stable P3HT face-on lamellae upon the crystallization of HMB solvent is rather curious. This prevailing presence of face-on lamellae is further manifested by this research as a result of the precipitation of stripe-like nanostructures upon eutectic solidification. When the tendency of molecular association in molten mixture declines with the decrease of concentration or molecular weight, only edge-on crystalline lamellae are present via the precipitation of individual molecular chains and subsequent organization at selected solidification temperatures. Normally the dissolution status of conjugate polymers is difficult to be preserved and identified, making subsequent structural evolution and possible impacts on later crystal growth less understood for decades. In this research, the efficient eutectic solidification serves as a feasible mechanism to freeze nanostructures possibly developed in molten solution. Hence, there is a unique opportunity to identify the existence of nanostructures of polyalkylthiophene as a disparate dissolution status and explore subsequent structural evolution of precipitated nanostructures. In this research, the overall transition of stripe-like face-on nanostructures to edge-on crystals within thin film has been explored by designed annealing treatment, which provides a systematic evaluation of the metastability of precipitated faceon nanostructures as a preordering state. The stability of metastable face-on nanostructures was found to be modified by side-chain length. With the attachment of longer side chains, face-on nanostructures are able to persist as a stable preordering state upon annealing, which illustrates the kinetic barrier of structural evolution as the decisive factor of metastability. This effect of side-chain length on the metastability of the preordering state has not been unveiled before and has been fully elucidated by conducted structural analysis in this research. In summary, by studying the further development of precipitated face-on nanostructures, disparate structural evolution routes of polyalkylthiophenes have been disclosed and comprehensively elucidated, which helps to advance our understanding of crystallization behaviors of conjugated polymers with alkyl side chains
2. EXPERIMENTAL SECTION Materials. High molecular weight (HMW) poly(3-hexylthiophene) (P3HT) and poly(3-butylthiophene) (P3BT) powders were purchased from Sigma-Aldrich. The weight-average molecular weight (Mw) of the purchased HMW P3HT sample was measured to be around 87 000 g/mol. For the purchased powders of HMW P3BT, the Mw was estimated to be around 54 000 g/mol. HMW poly(3octylthiophene) (P3OT) powders with regioregularity of around 91− 93% were purchased from Rieke Metals Inc., and the Mw was measured to be around 70 000−90 000 g/mol. As indicated, the B
DOI: 10.1021/acs.cgd.5b01765 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
Figure 1. (a) After the quenching of molten mixture containing 2 wt % of P3HT to 95 °C, the extended stay at 95 °C for 1 h results in parallel stacking of black stripes within platelet domains as shown in the electron micrograph. The diffraction pattern yielded by the organization of P3HT segments is inset, which indicates the orientations of c*-axis and thus molecular chains to be perpendicular to these stripes in general. For enhancing the contrast of P3HT stripes, HMB crystals have been sublimated in a vacuum previously. (b) The composite film prepared by the quenching of molten mixture to 95 °C was further annealed at 120 °C for 1 h. Closer and more regular stacking of P3HT stripes was observed to develop upon this annealing process as being illustrated in the electron micrograph. The corresponding electron diffraction pattern is in the inset at the right upper corner, which indicates that this annealing treatment results in regular crystalline packing of P3HT stems with edge-on orientation. (c) The transformation from face-on organization of P3HT segments to edge-on crystalline packing is schematically illustrated, which demonstrates the necessary involvement of rotational and translational motions of thiophene backbones during this transformation process.
3. RESULTS The Route of Thin-Film Preparation and the Prevailing Presence of Face-on Nanostructures. Hexamethylbenzene (HMB) is a crystalline alkylaromatic compound with featured photoinduced electron transfer and interesting lowfrequency vibration of attached methyl groups.29−31 Upon the chemical affinity of HMB, the HMB melt was able to dissolve conjugated comb-like polyalkylthiophenes, yielding binary eutectic mixtures. Upon the solidification of molten mixtures at selected temperatures, feasible organization routes of polyalkylthiophene solutes were explored in this research. In the prepared molten mixture containing 2 wt % of P3HT, the crystallization of HMB solvent occurred first via the quenching of molten mixture to the isothermal temperature of 95 °C, which results in uniaxially extended crystalline platelets as a featured crystallization habit. With an extended stay at 95 °C for 1 h, the subsequent organization of precipitated P3HT molecules on HMB platelets resulted in oriented stacking of obscure black stripes as being shown in Figure 1a. For enhancing the contrast of these P3HT black stripes, HMB crystals were sublimated in a vacuum overnight prior to the preparation of TEM specimen, and only extended traces of HMB platelet crystals were left as being outlined by straight dashed lines drawn on Figure 1a.
On the corresponding electron diffraction (ED) pattern (the inset of Figure 1a), the pair of (100) diffraction arcs related to the d-spacing of 1.6 nm was clearly observed. This pair of (100) diffraction arcs can be observed only when the incident electron beam was diffracted by the face-on organization of P3HT segments and therefore is viewed as a pair of characteristic diffraction arcs specifically attributed to the growth of the faceon organization of polythiophenes. According to collected diffraction patterns of the parallel stacking of obscure stripes, the face-on organization of P3HT backbones was found to develop prevailingly. On the basis of the above results, a route of thin-film preparation able to prevailingly result in face-on organization of polythiophene has been recognized, and this research attempted to further explore the growth mechanism of this face-on organization and the thermal stability. The c*-axis on this diffraction pattern identifies the perpendicular alignment of polythiophene chains within morphological stripes. These obscure stripes are thus inferred to be related to the lateral association of polythiophene segments into layer-type structural units. Furthermore, within each platelet domain of stacked stripes, there is an individual preferred orientation of the parallel stacking of obscure stripes. This preferred deposition orientation of polythiophene segC
DOI: 10.1021/acs.cgd.5b01765 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
Figure 2. (a) The experimental setup for detecting the 2D WAXD pattern of composite thin film is schematically illustrated. Composite films containing 3 wt % of P3HT have been prepared by quenching the molten mixture to room temperature. The obtained 2D WAXD pattern of composite film being further annealed at 75 °C for 1 h is shown on the right of panel a as an example, and the diffraction arc circled by dashed line is actually composed of several disparate diffraction signals including the (020) diffraction of P3HT face-on organization. (b) The diffraction profiles obtained from radial scans of the meridian diffraction arc including the P3HT (020) diffraction are illustrated, which demonstrates the intensity variation of P3HT (020) diffraction upon selected annealing temperatures indicated on the right. The (020) diffraction peak of P3HT is represented by red curves, and other HMB diffraction peaks are shown by blue curves, which were identified based on peak deconvolution and structural analysis. The overall diffraction profiles are represented by black curves.
For indexing diffraction arcs in this diffraction pattern, the structure proposed by Prosa et al. and Kayunkid et al. has been adopted as the reference.28,32 On the meridian, the pair of strongest diffraction arcs is related to the periodicity of 0.38 nm, which can be caused by successive bonding of hexylthiophene units along extended segments. This pair of diffraction arcs is thus indexed as (002). On the equator, weak and diffuse diffraction arcs also related to the periodicity of 0.38 nm were found, which is inferred to be caused by the π−π stacking of thiophene backbones and therefore is indexed as (020). Considering the perpendicular incidence of electron beams through the specimen, the absence of (100) diffraction and presence of (020) diffraction manifest the development of crystalline packing with edge-on azimuthal orientation via the annealing at 120 °C. Hence the transformation from initial faceon organization of P3HT segments to edge-on crystalline packing is recognized, also classifying the face-organization as a metastable state.
ments and stacking feature of black stripes rationally suggests the existence of one-dimensional epitaxy. Evolution of Edge-On Crystalline Lamellae from Prior Metastable Face-On Nanostructures via Collaborative Rotation of Conjugated Backbones and Subsequent Thermal Association. For exploring the thermal stability of P3HT face-on organization developed via the isothermal stay at 95 °C, the annealing treatment at 120 °C was conducted subsequently. As shown in Figure 1b, this annealing treatment results in closer stacking of wavy black stripes in a more regular manner. The corresponding ED pattern is shown as the inset, and some diffraction arcs are present in quadrants. Apparently, on the basis of this adopted annealing process at 120 °C, better crystalline packing of P3HT stems has been developed from prior face-on organization. The closer regular packing of welldefined wavy black stripes signifies the stacking of crystalline lamellae and bright areas in between correspond to the amorphous regions. D
DOI: 10.1021/acs.cgd.5b01765 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
Figure 3. (a) After quenching of molten mixture containing 2 wt % of P3HT solute to room temperature, connected board-like domains and diffuse wavy P3HT strips were observed by TEM, which can be better recognized in the circled area. This result suggests the precipitation of P3HT layertype nanostructures upon the efficient crystallization of HMB solvent. (b) Upon further annealing at 75 °C, well-defined P3HT stripes were developed, which are oriented perpendicular to the edges of long board-like domains. The corresponding electron diffraction pattern is given as the inset. (c) After further annealing at 95 °C, more closely spaced strips of P3HT were observed within each elongated board-like domain.
Nevertheless, the (120)/(22̅0) and (32̅0) diffraction arcs are also present on the equator, and these diffraction arcs do not belong to the same [100] zone. The presence of extra diffraction from neighboring diffraction zones manifests the misalignment of the a-axis of P3HT crystal packing with incident electron beams. Prior to the annealing at 120 °C, this a-axis is parallel to the substrate as a result of initial development of face-on organization (inset ED pattern of Figure 1a). When the edge-on crystal packing is to be established through annealing treatment, this a-axis of crystal packing is required to be aligned with incident electron beams, yielding strong (002) and (020) diffraction.6 The a-axis is along the direction of side-chain interdigitation. This means the growth of edge-on crystalline packing from prior face-on molecular organization necessarily involves the collaborative rotation of thiophene backbones (Figure 1c). When this backbone rotation has only been partially accomplished, the aaxis is not perfectly aligned, which elucidates the presence of diffuse (020) reflection and additional diffraction arcs belonging to neighboring zones. On the basis of unveiled evolution of molecular packing orientation, this ED pattern is elucidated as a fiber pattern rather than a single-zone diffraction pattern, and the fiber axis is along the c-axis of molecular packing. As a matter of fact, this ED pattern is also similar to the fiber diffraction pattern obtained by Tashiro et al. from drawn P3HT thin films.33 In addition to the rotational motion along backbones, further lateral association of these hexylthiophene segments is also expected for achieving the π−π stacking of thiophene backbones and crystalline packing. This translational molecular
motion and encountered kinetic barriers elucidate the evident waviness of the resultant crystalline lamellae. According to above results, the kinetic evolution of metastable face-on organization to edge-on crystalline lamellae has been recognized as feasible for the first time. This evolution involves the collaborative rotation of rigid polythiophene conjugated backbones within stripe-like nanostructures, which makes these initial stripe-like nanostructures less qualified as a crystalline state. Hence these stripe-like nanostructures are inferred as a preordering state prior to the growth of edge-on crystals. The Metastability of Face-On Nanostructures and the Dependence on Annealing Temperatures. According to above discussion, the transition of metastable face-on organization to edge-on crystals needs to involve collaborative rotation and lateral association of thiophene backbones. The encountered kinetic barriers are inferred to link with the kinetic stability of metastable face-on nanostructures. When this evolution toward edge-on crystals is less feasible kinetically at lower annealing temperatures, metastable face-on nanostructures are able to persist. Thus, it is possible to evaluate the metastability of face-on nanostructures via designed annealing treatments. First of all, composite films containing 3 wt % of P3HT have been prepared by rapid solidification of molten mixture at room temperature via the crystallization of HMB solvent. The further organization of frozen P3HT nanostructures within thin film was initiated thereafter via subsequent annealing at several selected annealing temperatures. In order to identify the annealing effect on the structural evolution through thin film, 2D wide-angle X-ray diffraction patterns of annealed samples E
DOI: 10.1021/acs.cgd.5b01765 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
Figure 4. (a) The 2D X-ray diffraction pattern obtained from composite thin film containing 3 wt % of P3BT, which is prepared by the quenching of molten mixture to room temperature. Both the (020) and (010) diffraction arcs of P3BT crystal packing are present on the meridian, indicating the existence of both face-on and edge-on P3BT crystals. (b) The diffraction profiles obtained from radial scans of the meridian diffraction arc including the P3BT (020) diffraction are illustrated for demonstrating the intensity variation of P3BT (020) diffraction upon selected annealing temperatures, which are indicated on the right. The (002) diffraction peak of P3BT is represented by red curves, and other HMB diffraction peaks are indicated by blue curves, which were identified based on peak deconvolution and structural analysis. The overall diffraction profiles are represented by black curves.
is beyond 75 °C, the initiated structural evolution is to proceed with the alteration of azimuthal orientation, and the intensity of (020) diffraction peak starts to decrease. Thus, for the continuous development of the face-on organization without the alteration of azimuthal orientation, the optimum annealing temperatures is realized to be around 75 °C, reflecting the metastability of P3HT face-on nanostructures. For initially deposited face-on nanostructures, viewed as a preordering phase prior to the crystallization of polyalkylthiophenes, above results illustrate a feasible methodology to measure the overall metastability within thin film. Considering the distribution of growth orientation of HMB crystallites within thin film and nonorthogonal triclinic lattice packing, diffraction signals belonging to different zones are likely to be detected as well, which can slightly influence the location of (200) and (22̅1̅) diffraction peaks in obtained diffraction profile, especially the (200) diffraction of HMB crystals. However, the overlap of (200) diffraction with the concerned P3HT (020) diffraction is quite minor. Thus, the variation of (020) diffraction intensity with the increase of annealing temperature can be still evaluated unambiguously. The precipitation and evolution of P3HT nanostructures were further investigated by the TEM observation. As shown in Figure 3a, the first quenching step results in connected platelet domains upon the efficient solidification of HMB solvent from the molten mixture containing 2 wt % of P3HT. Within these board-like domains, there are diffuse wavy strips. The presence of short wavy strips suggests the precipitation of layer-type nanostructures of P3HT. These scattered wavy stripes are found to orient roughly perpendicular to the edges of platelet domains. Upon the second annealing step at 75 and 95 °C, longer wavy stripes were developed prevailingly and stacked perpendicular in principle to the domain edges of these precipitated platelet domains (Figure 3b,c). Accordingly, in addition to derived structural evolution via 2D X-ray diffraction pattern, the observed morphology change under TEM also
were recorded at room temperature via reflection geometry and grazing incidence of X-ray beam as shown in Figure 2a. The obtained 2D WAXD pattern of composite film annealed at 75 °C for 1 h is shown on the right of Figure 2a as an example. The (020) diffraction is projected on the meridian of obtained 2D X-ray diffraction pattern, which clearly indicates the survival of P3HT face-on organization upon annealing at 75 °C. As an attempt to evaluate the annealing-induced evolution of face-on organization, the radial intensity variation of (020) diffraction arc 2D X-ray diffraction pattern was further scanned, which resulted in the diffraction profiles shown in Figure 2b. Both the (200) and (22̅1̅) diffraction peaks of HMB triclinic crystal packing, at the diffraction angles of 23.15° and 23.24° respectively, are partially overlapped with the P3HT (020) diffraction peak located at the diffraction angle of 23.38 o (Figure 2b). The intensity ratios of these three specific diffraction peaks were thus clarified via peak deconvolution. Furthermore, the normalization of diffraction profiles has been conducted upon leveling the intensity of HMB (200) diffraction peak in order to compare the intensity of (020) diffraction peak of each annealed sample. This normalization process is stated in detail in the Experimental Section. For the film sample merely quenched from the molten state, the red (020) diffraction peak of bottom diffraction profile indicates that the face-on organization of P3HT segments is already present widely. This result elucidates the presence of P3HT face-on organization as a result of prior existence of nanostructures in molten solutions, and these P3HT nanostructures were frozen by rapid eutectic solidification. Through conducted annealing processes for 1 h at selected temperatures, the intensity of (020) diffraction arcs is illustrated to increase with the increase of annealing temperatures until reaching the maximum caused by the annealing at 75 °C. This result illustrates that it is feasible to widely enhance the ordering organization of P3HT molecules within these face-on nanostructures below 75 °C. When the annealing temperature F
DOI: 10.1021/acs.cgd.5b01765 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
Figure 5. (a) For the molten mixture containing only 1 wt % of P3HT, the isothermal stay at 95 °C results in regular stacking of long straight welldefined stripes in platelet domains. The corresponding electron diffraction pattern is inset at the right upper corner, which indicates the development of edge-on organization of P3HT segments. (b) When the fraction of P3HT solute in molten mixture is between 1 and 2 wt %, the isothermal stay at 95 °C occasionally results in local face-on P3HT domains as being indicated by the corresponding diffraction pattern inset at the right upper corner. (c) For the molten mixture containing only 0.6 wt % of P3HT, the isothermal stay at 95 °C results in only scattered edge-on stripes that gather together, which are roughly in parallel. The corresponding diffraction pattern is in the inset at the right upper corner.
solidification of molten mixture containing 3 wt % of P3HT solute at room temperature only results in the spread of face-on nanostructures of P3HT chains. Hence, with the attachment of shorter side chains, the presence of face-on nanostructures of polyalkylthiophene within thin film is realized to be less dominant. Furthermore, the annealing effect on the structural transformation of face-on organization of P3BT was explored. The annealing of composite thin film at 55 °C was found to yield the strongest (020) diffraction peak, which is related to the face-on organization (Figure 4b). As the adopted annealing temperature is beyond 55 °C, the intensity of (020) diffraction peak decreases. This result manifests that original face-on nanostructures of P3BT can survive only below 55 °C and are ready to transform to edge-on crystalline lamellae via the annealing above 55 °C. Compared with the survival of P3HT face-on nanostructures (Figure 2b), the (020) diffraction of P3BT face-on organization is present within a narrower range of annealing temperature. Hence, the decrease of side-chain length is identified to make face-on nanostructures of polyalkylthiophene less stable and more liable to transform to edge-on crystalline lamellae at lower temperatures. It is inferred
helps to recognize the occurrence of oriented association and parallel stacking of these face-on nanostructures of polythiophenes upon later annealing treatment. The Metastability of Face-On Nanostructures and the Dependence on Side-Chain Length. Regarding involved backbone rotation during the evolution of edge-on crystals from prior face-on nanostructures, attached alkyl side chains are assumed to cause steric hindrance and thus enhance the stability of face-on nanostructures. In order to further evaluate the influence of side-chain length on the metastability of faceon nanostructures of polythiophenes, the composite films with 3 wt % of poly(3-butylthiophene) (P3BT) were prepared by quenching the molten mixture to room temperature. Afterward the 2D WAXD patterns of composite films were recorded via the experimental setup shown in Figure 2a and illustrated in Figure 4a. For the thin film prepared via the solidification of molten mixture at room temperature, both (020) and (100) diffraction arcs of P3BT crystals are present on the meridian of 2D X-ray diffraction pattern obtained. Thus, both face-on and edge-on organization of P3BT are present within thin film upon the solidification process at room temperature. However, according to the results discussed previously, the eutectic G
DOI: 10.1021/acs.cgd.5b01765 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
Figure 6. (a) For the molten mixture containing only 2 wt % of P3OT, the isothermal stay at 105 °C results in regular parallel stacking of ribbon-like domains. The corresponding electron diffraction pattern is inset at the right upper corner, and in addition to the first-order diffraction located near the center beam, the weak second- and third-order diffraction arcs can be observed as well with careful examination. (b) For the composite thin film containing 2 wt % of P3OT with a lower molecular mass around 12 kDa, this electron micrograph illustrates parallel stacking of wavy P3OT edge-on lamellae as a result of the isothermal stay at 95 °C. The corresponding electron diffraction pattern is in the inset at the right upper corner.
regular stacking of shorter waving stripes. According to these results, it is realized that only when the concentration of P3HT solute in molten mixture is above 2.0 wt %, face-on nanostructures of P3HT is prevailingly spread at 95 °C. When the concentration of P3HT solute in molten mixture was further lowered to 0.6 wt %, only loosely stacked stripes of edge-on lamellae (black stripes) were observed upon the eutectic solidification at 95 °C (Figure 5c). As a matter of fact, regardless of the preparation temperature of composite thin film, only the growth of edge-on lamellae was found as the concentration of P3HT solute is below 1 wt % in molten solution. Apparently, for the presence of less stable face-on nanostructures of P3HT upon eutectic solidification, reaching a sufficient level of solute concentration in advance is necessary. Hence, the prior association of polyalkylthiophenes into stripelike nanostructures in molten mixtures is demonstrated as a possible cause for the spread of face-on nanostructures in thin films Exploration of the Growth Background of Polythiophene Face-On Nanostructure via Effects of Molecular Weight. The molecular mass is able to largely influence the solubility of long-chain molecules in solutions. In addition, the association tendency of molecular chains in solution can be enhanced as well based on the increase of molecular weight. For exploring the effect of molecular mass on the dissolution status of polythiophenes and later spread of face-on nanostructures within thin film, the molten mixture containing 2 wt % of P3OT with a higher molecular weight of 70−90 kDa was prepared first. With the eutectic solidification at 105 °C, as shown in Figure 6a, instead of featured stacking of stripes, wavy bands were present and stacked in a parallel manner. In the inset diffraction pattern, a set of diffraction arcs is present with the second- and third-order reflections. The pair of first-order diffraction arcs is related to the stacking periodicity of 2 nm, which can be only elucidated as a result of lateral stacking of thiophene backbones with the organization of alkyl side chains
that, when shorter side chains are attached with thiophene backbones, a lower extent of steric hindrance of collaborative backbone rotation is involved, making the structural transition more feasible. On the obtained diffraction profile of sample annealed at 95 °C (Figure 4b), diffraction peaks of HMB crystals belonging to different zones are inferred since crystallization within thin film is likely to progress with deviated orientations. Nevertheless the intensity of the concerned (020) diffraction peak is still quite weak, and thus the influence of these HMB diffraction peaks is relatively trivial. The structural evolution of P3HT organization can be still derived unambiguously via the observed (020) diffraction peak. Exploration of the Growth Background of Polythiophene Face-On Nanostructures via Concentration Effect. According to the above results, polyalkylthiophene nanostructures were conceived to develop in molten solutions prior to the eutectic solidification. As an attempt to explore the growth background of face-on nanostructures as a preordering phase in molten solution, the dissolution status of polythiophene has been further explored via the change of concentration level of P3HT solute. First of all, the molten solution comprising merely 1 wt % of P3HT at 180 °C was prepared. Afterward the eutectic solidification of prepared dilute molten solutions at 95 °C was studied. As shown in Figure 5a, regular stacking of long straight well-defined stripes in platelet domains was widely found within thin film. Furthermore, according to the corresponding diffraction pattern shown as the inset of Figure 5a, the development of edge-on organization was recognized by the presence of featured (020) diffraction arcs and also the absence of (100) diffraction.9,32,34 For prepared molten mixtures with P3HT concentration between 1 and 2 wt %, in addition to the edge-on organization, local domains of face-on organization were also found to develop occasionally at 95 °C as being identified by the inset diffraction pattern of Figure 5b, which resulted in less H
DOI: 10.1021/acs.cgd.5b01765 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
Figure 7. Drawing of the stripe-like nanostructures of polyalkylthiophenes proposed to develop in a molten mixture. The coarser black lines indicate extended segments of thiophene backbone stacked laterally within nanostructures. The curly thinner lines represent flexible parts of molecular chains responsible for connecting these nanostructures.
the spread of face-on nanostructures. With a lower molecular weight, better dissolution of polyalkylthiophene chains is available, and thus there is a weaker tendency of molecular assembly into nanostructures in molten mixture. Furthermore, the decrease of molecular weight is able to enhance crystallization efficiency within thin film at selected temperatures. Thereafter, stable edge-on crystalline lamellae of LMW P3OT are able to efficiently develop upon the stay at selected solidification temperature.
in between. This pair of diffraction arc is thus indexed as (100) diffraction, indicating the presence of a face-on nanostructure of P3OT on HMB substrate. The presence of second- and thirdorder reflections suggest that, with the attachment of longer side chains, a layer-stacking scheme of polythiophene develops preferentially. Since no other diffraction spot was yielded, the growth of the liquid crystalline (LC) phase rather than the crystalline phase is inferred. It is inferred that long alkyl side chains are able to modify the crystallization tendency, which has been widely acknowledged. Moreover, developed face-on LC nanostructures of P3OT remain unchanged upon a later annealing process at any selected temperature, manifesting the enhanced stability of LC face-on nanostructures of polythiophenes via the attachment of longer side chains. This result also supports the previously proposed mechanism of structural evolution. The molten mixture containing 2 wt % of P3OT with a lower molecular weight (LMW) of 12 kDa was prepared as well, and the isothermal eutectic solidification at 95 °C was conducted afterward. On the basis of the presence of quadrant diffraction arcs in the inset diffraction pattern of Figure 6b, instead of liquid crystalline phase, a true crystalline phase is recognized. With the growth of crystalline phase, the morphology of stacked stripes is present, which is different from that of the liquid crystalline phase of P3OT. Thus, the crystallization of shorter P3OT chains was found to progress efficiently within thin film at 95 °C. Furthermore, the (100) diffraction is absent, and instead, the (020)/(002) diffraction arcs are observed clearly. Accordingly, this diffraction pattern recognizes the presence of edge-on crystals of LMW P3OT upon the eutectic solidification at 95 °C. It should be noticed that mainly face-on LC nanostructures of longer P3OT chains were present and persist upon the eutectic solidification at a higher temperature of 105 °C. Hence, at a similar level of concentration of polyalkylthiophene solutes in molten solutions, molecular mass is manifested to be a critical factor for the presence of face-on nanostructures. According to the above results, the decrease of molecular weight was found able to cause the absence of metastable faceon nanostructures after eutectic solidification, similar to the effect of concentration. This unveiled effect of molecular weight further demonstrates the influence of molecular dissolution on
4. DISCUSSION Concerning the spread of metastable face-on stripes upon sudden solidification of eutectic mixture, reaching a critical concentration of polyalkylthiophenes in molten solution has been recognized as essential. When the mixing ratio of polyalkylthiophenes is well below a critical value, which is estimated to be around 2 wt %, mainly stable edge-on lamellae are developed. For the presence of metastable face-on stripes upon eutectic solidification, stripe-like nanostructures of polythiophene are perceived to develop as a disparate dissolution status of polythiophene solutes in molten solution beyond a critical concentration. This concentration effect on the growth of stripe-like nanostructures of polyalkylthiophenes in molten mixtures is similar to the formation of micelles of amphiphilic molecules beyond the critical micelle concentration.35−38 The dispersion of micelles can be viewed as one of dissolution statuses of amphiphilic molecules as well. However, the dissolution status of conjugated polymers is usually difficult to be identified experimentally and therefore is less understood. For studied eutectic mixtures, the efficient crystallization of HMB solvent compound upon cooling provides a feasible mechanism to preserve these nanostructures developed previously in studied eutectic molten solution. After the still sublimation of HMB crystals at room temperature without disturbing preserved nanostructures, the observed spread of face-on stripes directly reveal the presence of stripe-like nanostructures in molten solutions. Compared with the organization of individually precipitated thiophene chains, which is driven to progress with edge-on orientation only for achieving a lower energy state, the prior growth of stripe-like nanostructures in molten solution provides an alternative route of molecular organization. I
DOI: 10.1021/acs.cgd.5b01765 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
comb-like semiconductive polyalkylthiophenes. This result also renders face-on nanostructures as a metastable preordering state prior to the growth of edge-on crystals. For these unveiled organization routes of polyalkylthiophenes, several designed experiments have been conducted in this research in order to clarify the existence of metastable face-on nanostructures and the mechanism of later structural evolution. Metastability of Face-On Nanostructures. As being indicated by conducted structural analysis, the evolution of edge-on crystalline lamellae from initially deposited face-on nanostructures was understood to advance via collaborative rotation of thiophene backbones and subsequent lateral association within nanostructures. The survival of metastable face-on nanostructures, related to the level of metastability, therefore depends on encountered kinetic barriers encountered during this structural evolution. As an attempt to evaluate the metastability of face-on nanostructures, the annealing temperatures required for overall structural evolution within thin films of P3HT and P3BT have been explored. Regarding face-on nanostructures of P3HT, the prevalent transformation toward edge-on crystals occurs at 105 °C, whereas P3BT face-on nanostructures are liable to transform to edge-on crystals at 55 °C. The attachment of side chains unavoidably increases encountered steric hindrance during collaborative backbone rotation and therefore kinetic barriers of the structural evolution. With the presence of long octyl side chains, face-on nanostructures of P3OT chains were found to be quite stable and not able to transform to edge-on crystals. This effect of side-chain length on the transition of a metastable preordering state has not been unveiled before and further illustrates the kinetic barrier of structural evolution as the decisive factor of metastability. Usually it is difficult to assess the metastability of a preordering state prior to crystallization of long-chain molecules. Nevertheless, with designed annealing treatment and conducted X-ray experiments in this research, the selected annealing temperatures related to overall transition within thin film have been comprehensively explored and illustrated to serve as an indication of general metastability of the initially deposited preordering state. Growth Background of Face-On Nanostructures. For classifying face-on nanostructures as a preordering phase in molten solution, possible dissolution statuses of polyalkylthiophene have been further explored via the change of concentration level of P3HT solute. When the concentration of polythiophenes in HMB melt is below 1 wt %, mainly stable edge-on lamellae were found to spread on previously developed HMB crystalline platelets. With the inclusion of more than 2 wt % of polythiophenes in molten mixture, which is viewed as a critical concentration, metastable face-on nanostructures are prevailingly present upon the solidification of the eutectic mixture. According to this unveiled concentration effect, the spread of face-on nanostructures within thin films is manifested to be related to prior dissolution status of polyalkylthiophene in molten mixtures. Furthermore, by lowering the association tendency of polyalkylthiophene in molten mixtures via the decrease of molecular weight, mainly edge-on nanostructures were developed upon eutectic solidification. On the basis of studied effects of concentration and molecular weight, the association of polyalkylthiophene into stripe-like nanostructures in molten solutions has been rendered as the background of the spread of face-on nanostructures upon eutectic solidification. This unveiled influence of molecular weight on prior molecular
Indicated by observed diffraction patterns, thiophene backbones are laterally stacked along with the long axis of face-on nanostructures. Accordingly, lateral stacking of parallel extended thiophene backbones is conceived as the growth mechanism of stripe-like nanostructures (Figure 7), and a similar association and packing model of P3HT have been speculated in the literature as well.17,39,40 Furthermore, with parallel stacking of board-like thiophene backbones in stripelike nanostructures in molten solution, it is rational for constituent thiophene segments to deposit in face-on orientation upon quick eutectic solidification. The interactions between alkyl side chains are presumed to be lessened by involved thermal motions in molten solutions. Hence the stacking of extended board-like thiophene backbones within stripe-like nanostructures can be in a ragged manner merely with orientation order (Figure 7). This packing scheme of thiophene backbones is helpful for resulting in π−π interactions and appears as a result of simple lateral assembly of extended thiophene segments. The established π−π interactions appear helpful in reducing the system enthalpy, and therefore compensating the loss of entropy upon the involved transition from coil to rod-like molecular conformations. Furthermore, it is widely known that, for the crystallization of long chain molecules with flexible backbones, the adoption of chain folding essentially enhance crystallization efficiency. Nevertheless the kinetic barrier of chain folding is significantly enhanced by rigid thiophene backbones and long flexible side chains. Hence simple lateral association of board-like polythiophene segments without chain folding is likely to be kinetically favored. With the absence of chain folding, the development of edge-on crystalline packing from prior face-on organization of polythiophenes is more feasible also. Furthermore, when the reorganization of face-on nanostructures toward the stacking of stable edge-on lamellae is less feasible at selected annealing temperatures, both the structural evolution derived via 2D X-ray diffraction pattern and observed morphology change under TEM help to recognize the occurrence of oriented association and parallel stacking of these face-on nanostructures of polythiophenes upon later annealing treatment. This oriented reorganization of P3HT nanostructures is perceived to be guided by a certain type of epitaxial relationship with underneath crystal packing of HMB solvent, which is curious and is to be further studied in a separate research project. Furthermore, these stripe-like nanostructures might refer to the formation of polymer fringed fibrils, which has been debated as a possible route of polymer crystallization several decades ago.41−43 Instead of further addressing this argument, the attempt of this research is to propose a feasible experimental approach to decipher the controlling growth mechanism of metastable face-on organization of rigid polythiophene backbones.
5. CONCLUSIONS With the dissolution of inverse comb-like polyalkylthiophenes in the melt of crystalline hexamethylbenzene compound, eutectic binary mixtures were prepared in this research. Upon the rapid solidification of molten mixtures, arrays of oriented stripe-like face-on nanostructures of polyalkylthiophenes were found to spread within thin films. Subsequent annealing treatment causes either more regular packing of molecular stems within face-on nanostructures or the evolution of edgeon crystalline lamellae from initial face-on nanostructures, thus unveiling opportunities of achieving desired packing schemes of J
DOI: 10.1021/acs.cgd.5b01765 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
(13) Hartmann, L.; Tremel, K.; Uttiya, S.; Crossland, E.; Ludwigs, S.; Kayunkid, N.; Vergnat, C.; Brinkmann, M. Adv. Funct. Mater. 2011, 21, 4047−4057. (14) Brinkmann, M.; Wittmann, J. C. Adv. Mater. 2006, 18, 860−863. (15) Matthews, J. Epitaxial Growth; Academic Press: New York, 1975. (16) Forrest, S. R. Chem. Rev. 1997, 97, 1793. (17) Cheng, W. C.; Yang, C. Y.; Kang, B. Y.; Kuo, M. Y.; Ruan, J. Soft Matter 2013, 9, 10822−10831. (18) Stocker, W.; Schumacher, M.; Graff, S.; Thierry, A.; Wittmann, J. C.; Lotz, B. Macromolecules 1998, 31, 807−814. (19) Lovinger, A. J.; Davis, D. D.; Lotz, B. Macromolecules 1991, 24, 552−560. (20) Wittmann, J. C.; Lotz, B. Polymer 1989, 30, 27−34. (21) Yan, S.; Katzenberg, F.; Petermann, J.; Yang, D.; Shen, Y.; Straupé, C.; Wittmann, J. C.; Lotz, B. Polymer 2000, 41, 2613−2625. (22) Zhang, J.; Yang, D.; Thierry, A.; Wittmann, J. C.; Lotz, B. Macromolecules 2001, 34, 6261−6267. (23) Ho, R. M.; Hsieh, P. Y.; Tseng, W. H.; Lin, C. C.; Huang, B. H.; Lotz, B. Macromolecules 2003, 36, 9085−9092. (24) Yan, C.; Li, H. H.; Zhang, J. M.; Ozaki, Y.; Shen, D. Y.; Yan, D. D.; Shi, A. C.; Yan, S. K. Macromolecules 2006, 39, 8041−8048. (25) De Rosa, C.; Park, C.; Thomas, E. L.; Lotz, B. Nature 2000, 405, 433−437. (26) Strohmaier, R.; Ludwig, C.; Petersen, J.; Gompf, B.; Eisenmenger, W. Surf. Sci. 1994, 318, L1181. (27) Noda, K.; Ishida, K.; Horiuchi, T.; Matsushige, K.; Kubono, A. J. Appl. Phys. 1999, 86, 3688−3693. (28) Prosa, T. J.; Winokur, M. J.; Moulton, J.; Smith, P.; Heeger, A. J. Macromolecules 1992, 25, 4364−4372. (29) Markel, F.; Ferris, N. S.; Gould, I. R.; Myers, A. B. J. Am. Chem. Soc. 1992, 114, 6208−6219. (30) Hamilton, W. C.; Edmonds, J. W.; Tippe, A. Discuss. Faraday Soc. 1969, 48, 192−204. (31) Pauling, L.; Brockway, L. O. J. Am. Chem. Soc. 1937, 59, 1223− 1236. (32) Kayunkid, N.; Uttiya, S.; Brinkmann, M. Macromolecules 2010, 43, 4961−4967. (33) Tashiro, K.; Kobayashi, M.; Kawai, T.; Yoshino, K. Polymer 1997, 38, 2867−2879. (34) Brinkmann, M.; Chandezon, F.; Pansu, R. B.; Julien-Rabant, C. Adv. Funct. Mater. 2009, 19, 2759−2766. (35) Mukerjee, P. Adv. Colloid Interface Sci. 1967, 1, 242−275. (36) Corkhill, J. M.; Goodman, J. F. Adv. Colloid Interface Sci. 1969, 2, 298−330. (37) Shinoda, K.; Nakagawa, T.; Tamamushi, B.; Isemura, T. Colloidal Surfactants; Academic Press: New York, 1963. (38) Israelachvili, J. Intermolecular & Surface Force, 2nd ed.; Academic Press: New York, 1991; Chapter 16. (39) Grévin, B.; Rannou, P.; Payerne, R.; Pron, A.; Travers, J.-P. Adv. Mater. 2003, 15, 881−884. (40) Noriega, R.; Rivnay, J.; Vandewal, K.; Koch, F. P. V.; Stingelin, N.; Smith, P.; Toney, M. F.; Salleo, A. Nat. Mater. 2013, 12, 1038− 1044. (41) Keller, A. Makromol. Chem. 1959, 34, 1−28. (42) Hearle, J. W. S. J. Polym. Sci. 1958, 28, 432−435. (43) Lindenmeyer, P. H. Science 1965, 147, 1256−1262.
association also helps to provide an explanation of widely observed crystallization routes of polyalkylthiophene via the change of molecular weight. Conventionally the face-on orientation is simply realized as one of two possible organization orientations of polythiophene backbones relative to substrate. In this research, the spread of face-on nanostructures has been studied as a result related to prior association of polyalkylthiophene in molten mixtures. Depending on the metastability of precipitated face-on stripelike nanostructures, two disparate organization schemes of polyalkylthiophene are able to evolve in prepared thin film. Accordingly, a comprehensive elucidation of possible structural evolution routes of polyalkylthiophenes has been illustrated by this research, which might be related to crystallization behaviors of various conjugated polymers with alkyl side chains. For the prospective use of polythiophene thin films in optoelectronic devices, the organization scheme is related to the establishment of transportation routes of charge carriers and thus significant for device performances.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +886-6-2757575 ext: 62953. Address: No. 1, University Road, East district, Tainan 701, Taiwan. Department of Materials Science and Engineering, National Cheng Kung University. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work is financially supported by the National Science Council under Grant NSC 100-2221-E-006-086. Authors also thank Mr. Hsien-Tsan Lin of Regional Instruments Center at National Sun Yat-Sen University for his help in TEM experiments. The support and help from Prof. An-Chung Su of the Department of Chemical Engineering at National Tsing Hua University is highly appreciated.
■
REFERENCES
(1) Lee, M. J.; Gupta, D.; Zhao, N.; Heeney, M.; McCulloch, I.; Sirringhaus, S. Adv. Funct. Mater. 2011, 21, 932−940. (2) Zheng, Z.; Yim, K. H.; Saifullah, M. S. M.; Welland, M. E.; Friend, R. H.; Kim, J. S.; Huck, W. T. S. Nano Lett. 2007, 7, 987−992. (3) Sirringhaus, H.; Wilson, R. J.; Friend, R. H.; Inbasekaran, M.; Wu, W.; Woo, E. P.; Grell, M.; Bradley, D. D. C. Appl. Phys. Lett. 2000, 77, 406−408. (4) Yang, C. Y.; Soci, C.; Moses, D.; Heeger, A. J. Synth. Met. 2005, 155, 639−642. (5) Whitehead, K. S.; Grell, M.; Bradley, D. D. C.; Inbasekaran, M.; Woo, E. P. Synth. Met. 2000, 111−112, 181−185. (6) Brinkmann, M. J. Polym. Sci., Part B: Polym. Phys. 2011, 49, 1218−1233. (7) Yang, H.; Shin, T. J.; Yang, L.; Cho, K.; Ryu, C. Y.; Bao, Z. Adv. Funct. Mater. 2005, 15, 671−676. (8) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Nature 1999, 401, 685−688. (9) Yang, H.; LeFevre, S. W.; Ryu, C. Y.; Bao, Z. Appl. Phys. Lett. 2007, 90, 172116. (10) Brinkmann, M.; Aldakov, D.; Chandezon, F. Adv. Mater. 2007, 19, 3819−3823. (11) Brinkmann, M. Macromolecules 2007, 40, 7532−7541. (12) Brinkmann, M.; Rannou, P. Adv. Funct. Mater. 2007, 17, 101− 108. K
DOI: 10.1021/acs.cgd.5b01765 Cryst. Growth Des. XXXX, XXX, XXX−XXX
