Controlled self-assembly of alkylated-π compounds for soft materials — Towards optical and optoelectronic applications

COCIS-00896; No of Pages 9 Current Opinion in Colloid & Interface Science xxx (2014) xxx–xxx

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Controlled self-assembly of alkylated-π compounds for soft materials — Towards optical and optoelectronic applications Agnieszka Zielinska a,b, Marcin Leonowicz b, Hongguang Li c,⁎, Takashi Nakanishi a,b,⁎⁎ a b c

National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba 305-0047, Japan Faculty of Materials Science and Engineering, Warsaw University of Technology, ul. Woloska 141, 02-507 Warsaw, Poland Laboratory of Clean Energy Chemistry and Materials, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China

a r t i c l e

i n f o

Article history: Received 28 January 2014 Received in revised form 3 March 2014 Accepted 7 March 2014 Available online xxxx Keywords: Alkyl chains Hydrophobic amphiphiles Organic electronics π-Conjugated compounds Self-assembly

a b s t r a c t Organic and polymeric molecules based on π-conjugated units represent an important class of components for optical and optoelectronic functionalized soft materials. Inspired by the innovative molecular design made by synthetic chemists, new functions and applications of π-conjugated molecules are continuously emerging. However, a challenge that remains is to soften these molecules. Alkylation is a commonly employed synthetic strategy to achieve functionalization in order to improve processability, i.e., solubility in volatile solvents, for better utilization in the rapidly-developing field of organic electronics. In addition it is recognized as a powerful strategy to tune the interaction among the π-conjugated moieties. In a different interpretation of alkylation, alkylated-π compounds can be viewed as a class of hydrophobic amphiphiles, since the rigid π-conjugated moiety and flexible alkyl chains are intrinsically immiscible. Recent studies have shown that such compounds can form a variety of self-organized solid and thermotropic liquid crystalline structures as well as nonassembled liquid forms depending upon the position, number and kinds of attached alkyl chains. Here, we present a brief overview of recent developments of alkylated-π chemistry, with an emphasis on the relationships between molecular design, self-assembly behavior and applications in optical and optoelectronic devices. We hope this review can serve as a guide and reference for people working in different research areas, including self-assembly and colloid sciences, synthetic and materials chemistry was well as organic electronics. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction Surfactants compose a class of organic molecules that possess both a liphophilic moiety and a hydrophilic part. Typically the former consists of alkyl chains while the latter is an ionic head group. This molecular design induces so-called “amphiphilicity” for surfactants, which enables them to form a variety of self-assembled structures in aqueous solutions such as micelles, vesicles and lyotropic liquid crystals [1]. If it is not restricted to aqueous solutions, this definition of amphiphilicity can be broadened. Theoretically, as long as the segments of one molecule are different from each other, it could be viewed as an amphiphile. Typical examples are alkylated-π compounds where the rigid π-conjugated moieties are functionalized with flexible alkyl chains [2]. Due to their optical and optoelectronic properties such as light emitting and charge transporting capabilities, these alkylated-π compounds have received increasing attention in recent years for applications in flexible organic

⁎ Correspondence to: H. Li, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China. ⁎⁎ Correspondence to: T. Nakanishi, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba 305-0047, Japan. E-mail addresses: [email protected] (H. Li), [email protected] (T. Nakanishi).

electronics [3]. Generally, the skeleton of the π-conjugated moiety is composed of sp2 hybridized carbons while that of the alkyl chains consist of sp3 ones. This structural difference causes a varied electron affinity and hence a different polarity and interactions between the rigid π-conjugated moiety and the flexible alkyl chains, though both of them are considered hydrophobic in character. Driven by these differences, the π-conjugated moiety and the alkyl chains are mutually immiscible and tend to segregate from each other. In a special case where the compound has one head and one set of tails, a conceptual “hydrophobic amphiphilicity” has been elucidated [4]. Alkylated-π compounds may involve a class of molecules carrying additional interacting units, such as hydrogen bonding, mesogens in between the “alkyl” and “π” moieties, which are out of scope of the discussion. As seen in traditional surfactants, amphiphilicity can also induce remarkable self-assembly behavior of alkylated-π compounds when processed in solvent-free states and/or from solutions. In the former case, room temperature functional liquids and thermotropic semiconducting liquid crystals could be constructed while in the latter, hierarchicallyorganized solids with various morphologies can be obtained. The formation of self-assembled and non-assembled structures for alkylated-π compounds is of great interest from a viewpoint of fundamental research, since self-assembly control is considered a key issue in both colloid science and materials chemistry. Importantly, it has been found that

http://dx.doi.org/10.1016/j.cocis.2014.03.007 1359-0294/© 2014 Elsevier Ltd. All rights reserved.

Please cite this article as: Zielinska A, et al, Controlled self-assembly of alkylated-π compounds for soft materials — Towards optical and optoelectronic applications, Curr Opin Colloid Interface Sci (2014), http://dx.doi.org/10.1016/j.cocis.2014.03.007

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the self-assembly behavior of alkylated-π compounds can significantly influence their optical and optoelectronic properties by adjusting the interaction among the π-conjugated moieties. For example, as far as the charge transportation is concerned, well-ordered π-conjugated moieties surrounded by insulating alkyl chains are desired, while for lightemitting materials, a suitably-isolated π-conjugated moiety is preferred. During the past decade, great progress has been made in organic electronics where alkylated-π compounds have played important roles. Here, we give a brief overview of the main achievements in this research area, focusing on representative studies which ultimately led to end-up optical and optoelectronic applications. To provide clearer structure–property relationships, we limit the scope of this review to two- and three-dimensional structures of alkylated-π compounds. Other aspects of π-conjugated polymers substituted with alkyl chains have been discussed in other excellent review articles [5–7]. 2. Techniques in the investigation of alkylated-π compounds For researchers working in colloid and interface chemistry, the techniques used for the investigation of traditional surfactants are wellknown while those for alkylated-π compounds including hydrophobic amphiphiles may be unfamiliar. Thus before proceeding to the studies of structures and applications of alkylated-π compounds, we provide a brief introduction to commonly chosen methods for their characterization. The first question regarding these compounds relates to their physical state, i.e., solid, liquid crystal, or isotropic liquid, at a particular temperature or over a temperature range. A typical way to probe thermal behavior is with thermogravimetric analysis (TGA) which determines the temperature at which the molecule decomposes. Further important thermo-analysis is enabled by differential scanning calorimetry (DSC) which provides phase transition temperatures. In the case that the transition temperature from a crystalline phase (or a glassy state) to an isotropic phase is below around 25 °C, the bulk material is recognized as a room temperature fluid (i.e. liquid). For thermotropic liquid crystals (LC), further structural analysis should be necessary. The first tool commonly used is polarized optical microscopy (POM), which gives the textures of the LC. Normally, based on these observations, one can determine the phase of the LC since the relationships between the observed textures and typical thermotropic LC have been well summarized [8]. To get quantitative information regarding the LC such as lattice parameters, X-ray diffraction (XRD) is an essential analytical technique. For investigation of their optoelectronic properties such as charge transport features, pulse radiolysis timeresolved microwave conductivity (PR-TRMC) can be used since it is a facile electrode-less technique [9]. Transient photocurrent measurement using a time-of-flight (TOF) setup is also commonly adopted [10]. In the case of room temperature fluids, the first stage of analysis should be rheological measurements. This is not merely to provide evidence of a liquid state, but is also essential to guide the applications of these fluids as paintable and/or printable materials. To confirm that the π-conjugated moiety of the compounds are isolated from each other, small angle X-ray scattering (SAXS) [11] is applied. Since most of the π-conjugated compounds are strong chromophores, further characterization of these liquids is performed with various spectroscopic techniques such as UV–Vis absorption and fluorescence (FL) measurements. These provide important parameters such as excited state lifetime and absolute quantum yield. As a unique class of amphiphiles, alkylated-π compounds are expected to form self-assembled architectures in solutions. The nature of alkylatedπ compounds enables them to be soluble in various common organic solvents such as alkanes (suitable for alkyl chains), aromatics (suitable for π-conjugated moieties), alcohols, ethers and their combinations, which provide abundant opportunities to prepare self-assembled structures out of solution. These preparations have been illustrated by a heating-cooling method (namely recrystallization or reprecipitation method) from solution, and by precipitation by vapor diffusion of a poor

solvent into solution containing compounds in a good solvent [12]. A variety of imaging techniques including TEM, scanning electron microscopy (SEM), scanning tunneling microscopy (STM) and atomic force microscopy (AFM) can visualize hierarchically-organized structures. Further structural analysis of such solid assemblies is available from XRD, optical and FT-IR spectroscopies. The next important step is to evaluate the performance of alkylated π-compounds in optoelectronic devices including organic light emitting diodes (OLED), organic solar cells and organic field effect transistors (OFET). 3. Room-temperature solvent-free functional liquids Driven by π–π interaction among the cores, π-conjugated molecules alone usually exist as solids at room temperature. These solids, whether amorphous or crystalline, are difficult to process further for applications in the field of “flexible” optoelectronic devices. In addition, such bare π-conjugated molecules are less stable, due to their decomposition by oxygen attack or dimerization (polymerization) under external stimuli. In recent years, increasing attention has been paid to solvent-free molecular liquids containing functional π-conjugation [13]. Some typical molecules are summarized in Fig. 1. Adachi et al. adopted a carbazole with a 2-ethylhexyl chain linked to the nitrogen atom (1 in Fig. 1) [14]. When applied in an organic light emitting diode (OLED), 1 acts as both a liquid emitter and a charge transport agent. Due to the fluid character of 1, the corresponding OLED device can be easily refreshed by replacing the liquid emitter when it degenerates. However, this process is often slow due to the small gap between the glass in which the viscous functional liquid is placed. To address this issue, an OLED with a mesh-structured cathode was designed, which allows free circulation and fast recovery of the liquid emitter [15]. To introduce bulky, soft, multiple branched alkyl chains with tunable substitution positions, we have developed a new molecular design strategy in which the π-conjugated core moiety can be “wrapped” with the chains, thus resulting in core isolation. Based on this method, blue emitting room temperature liquids derived from oligo(p-phenylenevinylene) (OPV) and anthracene have been synthesized. This allows us to obtain molecular intrinsic optical properties in the neat liquid state. Our first route used liquid OPV derivatives (2 and 3 in Fig. 1) functionalized by attaching swallow tail type (i.e. 2-octyldodecyl) or hyperbranched alkyl chains with a (2-, 4-, 6-) or (3-, 5-) substitution pattern [16]. Rheological measurements confirmed the liquid nature of 2 and 3. UV–Vis spectroscopy showed the absorption characteristics of 2 and 3 in solvent-free state are in good agreement with those in dilute solutions. This result indicates the significant isolation of the OPV core by the branched alkyl chains. A similar strategy has been employed to obtain liquid anthracenes with eight (4) or four (5) branched alkyl chains (Fig. 1) [17]. The photo images of 5 taken under visible light, appearing as transparent liquid, and emitting blue-color under UV light, are shown in Fig. 2a and b respectively. SAXS and XRD analysis of the neat samples showed that the distance between the isolated anthracene cores is 21 Å for 4, due to its bulky substituents (see a schematic drawing of the chemical 3D model structure in an inset of Fig. 2d), which effectively disturbs the anthracene core–core interactions. This molecular design can stabilize the molecule under light irradiation. There is no dimerization and relatively slow oxidation. The liquid OPVs and anthracenes can not only be directly used as blue-emitting materials, but also as matrices for accommodation of acceptor type dopants to fabricate composite fluid materials. The ability to tune the color of light-emitting materials with dopants in a solventfree condition is generally limited. Such composites can be prepared by blending different components with a spatula. For liquid OPVs, when mixed with tris(8-hydroxyquinolinato)aluminium (Alq3) and 5,6,11,12-tetraphenylnaphthacene (rubrene) in a molar ratio of e.g. 1:1.65:0.23 (in case of 3b), white light emission with quantum yields over 35% was observed (Fig. 2c). The corresponding color coordinate

Please cite this article as: Zielinska A, et al, Controlled self-assembly of alkylated-π compounds for soft materials — Towards optical and optoelectronic applications, Curr Opin Colloid Interface Sci (2014), http://dx.doi.org/10.1016/j.cocis.2014.03.007

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Fig. 1. Molecular structures of room-temperature solvent-free functional liquids containing π-conjugated moieties, carbazole (1), OPV (2, 3), anthracene (4, 5), fullerene (6–8) and porphyrin (9).

value on Commission Internationale de lEclairage (CIE) 1931 chromaticity diagram was recorded as (0.33, 0.34). The results, as well as the fluid state of the compounds indicate that the liquid OPV-containing composites can find a possible application as white-light emitting inks (see coating on UV–LED in Fig. 2c). In the case of liquid anthracenes, two dopant dyes were chosen, i.e., 9,10-bis (phenylethynyl)anthracene (D1) and tris(1,3-diphenyl-1,3-propanedionato)(1,10-phenanthroline) europium(III) (D2). It was found that the fluorescence color emitted by the composite is very sensitive to the dopant concentration. For example, with the addition of 0.3 mol% of D1 to 4, a cyan fluorescence was observed (Fig. 2d (iv)). When the content of the dopant was increased to 0.5 mol%, the color of the fluorescence changed to green (Fig. 2d (v)), and at 5.0 mol% concentration of D1, a yellow (Fig. 2d (vii)) fluorescence emerged. The liquid matrix of 4 can be also doped with both dopants (D1 and D2) at the same time, which leads to a combined emission of red, at room temperature (Fig. 2d (viii)). The emitting color of this three-component composite is shown to be dependent on temperature, which reversibly changed to yellow at 50 °C (Fig. 2d (ix)) and further to green at 100 °C (Fig. 2d (x)). The photostable liquid anthracene molecular concept can also be applied to a photon upconversion (UP) system [18]. A liquid anthracene, as an acceptor, based on a 9,10-diphenylanthracene derivatized with tetra(2-octyldodecyloxy) chains can accommodate a Pt(II) porphyrin sensitizer and results in an UP property free from the effect of oxygen. All the previous UP systems were either employing organic solvents or solid polymer matrices for the protection of triplet states from deactivation by oxygen. This homogenous liquid dispersion led to a high quantum yield of ~28% and high stability. As the size of the π-conjugated moiety increases, π–π interaction becomes stronger, leading to an increased difficulty in the production of room-temperature liquids. An example is fullerene C60 which is a spherical π-conjugated molecule. Nevertheless liquid fullerenes with both linear (6) [19,20] and branched (7, 8) [21] alkyl chains have been successfully synthesized (Fig. 1). Initial studies focused on the N-methylfulleropyrrolidine modified with a (2-, 4-, 6-) linear alkyl chains substituted phenyl unit (6) [19,20]. Liquid fullerenes were obtained when the carbon number in the alkyl chain is above 12, due to

the effective disturbance of the π–π interaction among the adjacent C60 moieties. Increasing the chain length lowers the melting point of the liquid fullerenes; the lowest melting point was noticed for 6b, at −36.5 °C. Further increase in the chain length caused an upturn of the melting point, since the van der Waals forces among the alkyl chains begin to dominate [20]. The formation of liquid C70 used the same strategy [20]. Branched alkyl chains have been found to be more effective for lowering the viscosity of liquid fullerenes. The substitution position also has a significant effect, such as in two liquid fullerenes, where each possesses two swallow tail type alkyl chains at (3-, 5-) (7) or (2-, 5-) (8) positions of the phenyl group. Compound 7 exhibits a complex viscosity of ~1500 Pa·s, while that of 8 is only ~260 Pa·s, indicating that the orthoposition substitution is more effective to produce liquid fullerenes of lower viscosity. As an example of the practical applications of liquid fullerenes, 6d was selected to construct composite material by hosting CdSe nanocrystals (NC). The fluid composite results in a long-lived charge transfer state between 6d and CdSe NC amenable to optoelectronic applications such as photovoltaics [22]. Almost the same molecular design strategy as utilized in 6 has been adopted in the synthesis of liquid porphyrins (9) [23,24], and yet further applications are to be elucidated. 4. Thermotropic liquid crystals One of the most important properties of alkylated π-conjugated compounds is their ability to thermally organize into mesophase structures in solvent-free conditions, i.e., thermotropic liquid crystals (LC). The formation of mesophases is driven by the segregation of the rigid π-conjugated moieties and the alkyl chains, whose flexibility and volume are dependent on temperature. The phase transition temperatures for a specific LC system are thus determined by the number and type of attached alkyl chains. The geometry of the π-conjugated moiety also plays an important role in regulating the structure of the LC phases. While rod-like π-conjugated compounds can form both the orientationally-ordered nematic and positionally-ordered smectic phases, planar or discotic π-conjugated compounds have a strong tendency to organize into

Please cite this article as: Zielinska A, et al, Controlled self-assembly of alkylated-π compounds for soft materials — Towards optical and optoelectronic applications, Curr Opin Colloid Interface Sci (2014), http://dx.doi.org/10.1016/j.cocis.2014.03.007

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Fig. 2. Photographs of 5 under (a) visible and (b) UV light (365 nm) (reproduced from [17] under a Creative Commons Attribution 3.0 Unported (CC BY) license). (c) Illustration of the preparation method of white light emitting fluid composite based on liquid OPV (2b or 3b) and photographs of a uncoated commercially available UV–LED (375 nm) (left) and the same UV–LED after coating with the OPV composite (right) (Copyright @ 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Reproduced with permission [16]). (d) Scope of the color emissions obtained by blending of the 4 with D1 and D2 as follows: (i) 4 alone, (ii) 4 + D2 at 2.0 mol%, (iii) 4 + D2 at 5.0 mol%, (iv) 4 + D1 at 0.3 mol%, (v) 4 + D1 at 0.5 mol%, (vi) 4 + D1 at 2.0 mol%, (vii) 4 + D1 at 5.0 mol%, (viii-x) 4 + D1 at 0.5 mol% + D2 at 5.0 mol%, (viii) at 20 °C, (ix) at 50 °C, (x) at 100 °C. Inset is 3D chemical modeling structure of 4 (adapted from [17] under a Creative Commons Attribution 3.0 Unported (CC BY) license).

columnar phases [8]. Those discotic LC have many potential optoelectronic applications due to their intrinsic one-dimensional intracolumnar charge transport capability [25]. However, the optimization of the system by molecular design to achieve the best performance for optoelectronic applications remains a big challenge. Fig. 3 summarizes chemical structures that can form LC. Müllen et al. investigated a series of polycyclic aromatic hydrocarbons (PAHs) with both linear and branched alkyl chains attached in most cases directly on the aromatic rings or through a phenyl group [26–28]. In one of their recent works, applying Marcus theory, they investigated what conditions need to be optimized for the best charge transport in PAHs columnar LC [29]. It was shown that this can be achieved by the use of molecular design tools, either by increased steric hindrance of the alkyl chains at increased temperatures, or by hydrophobic/hydrophilic repulsion between the chains. In other examples, they have proven that changing the same type of alkyl chains can significantly affect self-alignment properties and different geometries can be obtained with hexa-peri-hexabenzocoronene (HBC) derivatives [28]. Self-assembled structures of swallow tail type branched alkyl chain modified HBC compounds (12) with edge-on and face-on columnar arrangements, crystallized from their molten state, were well characterized. Both 12b and 12c show LC behavior, with their crystalline to mesophase transitions at 24 °C and 97 °C respectively, while 12a

undergoes a glassy transition at 46 °C. Scale-like structures with a domain size that varies depending on the heat treatment were found for 12c. Synchrotron radiation two dimensional wide-angle X-ray scattering (2D-WAXS) measurements revealed a columnar face-on geometry, with the columnar axis perpendicular to the substrate's surface. The POM observation and 2D-WAXS measurements of 12a crystalline structures revealed spherulites formed by edge-on ordered columns. In 12b, LC behavior was found at room temperature, in which both the face-on and edge on orientations could be achieved depending on the fabricating conditions. When 12b was placed between two glass substrates, it aligned into columns perpendicular to the substrate's surface, while it crystallized with parallel orientation on glass surfaces. Semiconducting nature in such aligned HBC structures will be described in the next section. Unlike in the above described planar-discotic π-conjugated systems, the fullerene C60, with its spherical geometry, provides a unique building block for LC fabrication. Initial attempts to obtain C60 containing LC phases involved an attachment of mesogen groups directly onto the C60 moiety, as shown by Deschenaux et al. [30]. C60-LCs can be prepared by several additional strategies [31,32]. Although various nematic, smectic or columnar C60-LC can be obtained, in most of these cases, the potential of the utilization of those materials in organic electronic applications is limited and have not yet been deeply investigated.

Please cite this article as: Zielinska A, et al, Controlled self-assembly of alkylated-π compounds for soft materials — Towards optical and optoelectronic applications, Curr Opin Colloid Interface Sci (2014), http://dx.doi.org/10.1016/j.cocis.2014.03.007

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Fig. 3. Chemical structures of C60 (10, 11, 14) and HBC (12, 13) derivatives carrying branched (10–12) and linear (13, 14) alkyl chains that can form the liquid crystalline and/or solid assemblies.

This limitation is due to the destruction of the carbon–carbon double bonds in the C60 moiety or the low C60 content in the LC phase caused by the bulky and large mesogen attachments. As an alternative, we have presented a method that is the same as that applied for obtaining liquid fullerenes (6–8 in Fig. 1), alkylated C60 derivatives forming LC phases with high C60 content (10, 11, 14 in Fig. 3) [21,33]. For the compound bearing three eicosyl chains with a (3-, 4-, 5-) substitution pattern on the phenyl group (14b), a highly ordered smectic phase with a d-spacing of 5.59 nm was obtained between 62 °C and 193 °C. Transient photocurrent measurements on the mesophase formed by 14b using a TOF setup revealed a relatively high electron mobility of ~3 × 10−3 cm2 V−1 s−1 [33]. The molecular design can be altered by placing two swallow tail type alkyl chains at the (3-, 5-) substitution positions. For compound 10a, with the 2-octyldodecyl chains, the presence of smectic phase extends over room temperature, and the melting point (84 °C) is much lower compared to 14b. Reduction of the length of the chains (2-hexyldecyl, 10b) or the change of the substitution pattern from the (3-, 5-) position to the (3-, 4-) position (2-octyldodecyl, 11), both resulted in a dramatic increase of the melting point: 148 °C for 10b and 196.2 °C for 11 [21]. One of the most desirable applications of liquid crystals based on the alkylated C60 compounds is their utilization in flexible organic electronics such as solar cells. Possessing the LC performance extending to room temperature, 10a was selected to fabricate bulk heterojunction (BHJ) solar cells together with a commonly used p-type organic semiconductor poly(3-hexylthiophene) (P3HT) [21]. The device architecture is given in Fig. 4 and the active layer thickness is about 100 nm. At a 10a/ P3HT weight ratio of 1:1, an open circuit voltage (VOC) of 0.56 V, a short circuit current density (JSC) of 5.5 mA cm−2 and a fill factor (FF) of 50% were obtained under standard AM 1.5G solar simulated light (80 W) (Fig. 4c, curve b). This yielded a power conversion efficiency (PCE) of 1.6 ± 0.1%. Nevertheless it is already comparable to that obtained for the device constructed and measured under the same experimental conditions with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)/ P3HT as an active layer, which shows a PCE of 2.2 ± 0.1% (Fig. 4c, curve a). On the contrary, the 14b/P3HT system exhibits a PCE of only 0.5% (Fig. 4c, curve c). The improved performance of this 10a/P3HT system could be ascribed to the presence of the branched alkyl chains,

which may facilitate lowering the crystallinity (softening the system) and improving the charge transport. 5. Applications in assembled architectures out of solution There is another important state of self-assembled materials based on alkylated-π compounds that is the “solid” architecture produced out of solution. The application of alkylated-π compounds in their solid form, in particularly crystalline form, to OFET or solar cells requires precise control over the organization of the molecules on a substrate. In the case of OFET, the charge carriers travel between the electrodes parallel to the surface on which the organic layer is deposited. Therefore, an edge-on organization of the organic semiconducting molecule with respect to the surface is required. HBC compounds have already been noted for their good conductive features toward FET applications from their solution casting [34]. Yet the issue of optimizing a simple manufacturing method to obtain highly-organized molecular structures over large area still remains. In one attempt, a HBC compound carrying six dodecyl chains (13 in Fig. 3) was used, which shows a crystal-tocolumnar phase transition at 107 °C. When zone-cast (Fig. 5a) from a tetrahydrofuran solution onto hexamethyldisilazane treated silicon dioxide, uniaxially aligned columnar structures with long-range order were obtained as evidenced by AFM measurements (Fig. 5b) [35]. HRTEM observations revealed the homogeneous morphology of the zonecast film with an in-plane intercolumnar distance of 2.48 nm. These structural features make it an ideal candidate for applications in organic electronics such as FET (Fig. 5c). It was found the device exhibited good performance with an on-off ratio of 104, and a field-effect mobility in the saturation regime at 5 × 10−3 cm2 V−1 s−1. In addition, the zone-cast film of 13 also shows interesting optical properties, i.e., the optical anisotropy can be reversibly switched between a negligibly low level in the crystalline state and a high level in the mesophase. Although π-conjugated polymers are beyond the scope of this article, it should be noted that self-assembly tools have proven successful for the assembly control of alkylated π-conjugated polymers [5–7]. Kim and coworkers defined three factors essential to the self-assembly process and alignment: chain planarization induced by concentration, side-chains for the prevention of the chains' intercalation, and tetrahedral carbon

Please cite this article as: Zielinska A, et al, Controlled self-assembly of alkylated-π compounds for soft materials — Towards optical and optoelectronic applications, Curr Opin Colloid Interface Sci (2014), http://dx.doi.org/10.1016/j.cocis.2014.03.007

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Fig. 4. (a) POM image of liquid crystalline texture of 10a at 70 °C. (b) Scheme of the BHJ cell construction. (c) J (V) curves of binary mixtures PCBM/P3HT (curve a), 10a/P3HT (curve b,) and 14b/P3HT (curve c) ([21] — Reproduced by permission of The Royal Society of Chemistry).

Fig. 5. (a) Schematic illustration of the zone casting process. (b) Filtered Inverse Fast Fourier transform image showing columnar arrangement of 13 obtained in the zone processing technique (Copyright @ 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Reproduced with permission [35]). (c) Schematic illustration of the HBC-FET.

linkers with out-of-plane bonding [36]. When these conditions are met, the conjugated polymers can be aligned by shear flow or stress and manufactured using a coating technique. Very rich research examples and applications were reported about solid assemblies of alkylated-C60 compounds. The intrinsic optoelectronic properties of C60 are expected to be transferred or modulated in the film and/or bulk materials by forming various self-assembled architectures. For instance, with a self-assembled monolayer treatment on substrate, a high performance solution-processed TFT based on a single dodecyl phenyl N-methylfulleropyrrolidine was successfully fabricated [37]. In other examples for different applications (from electronics), a polymorphism phenomenon in a C60 derivative (14a in Fig. 3) with three hexadecyl chains on (3-, 4-, 5-) positions of the phenyl group has been observed. Different shapes of self-assembled nano- and microstructures of 14a were obtained by varying the solvent conditions: vesicles, fibers, tapes, and conical structures as well as left- and righthanded spirals [38,39]. Additionally, it was shown that the same compound 14a formed globular flower-like structures with sizes of several tens of micrometers by precipitation out of solution [39]. The assembled structural subunit contains bilayer structures formed by interdigitated alkyl chains, as confirmed by high resolution cryogenic TEM (HR-cryoTEM), XRD, UV–Vis, and FT-IR analysis. Additionally, a similar bilayer based self-assembly (Fig. 6a) behavior forming microparticles with nanoflake outer surface was observed for a C60 compound (14b) (Fig. 6b) with eicosyl chains [40]. In the assembled microparticles, the multi-lamellar structural subunit was confirmed with HR-cryo-TEM analysis (Fig. 6c) and with XRD. The cast film morphology prepared from the nanoflaked-microparticles mimics the leaf surface of the Lotus plant and displays superhydrophobicity (static water contact angle, 152.0° shown in Fig. 6d) [40]. The alkylated-C60 based superhydrophobic nature has been improved by hardening the self-assembled morphology via UV light irradiation, which causes topochemical polymerization in the case of an alkylated C60 derivative carrying diacetylene polymerizable

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Fig. 6. (a) Structural model of assembled bilayer of 14b. (b) SEM image of the morphology of a thin film prepared from the self-organized, nanoflake-featured microparticles of 14b. (c) HRcryo-TEM image of the multilamellar structure obtained from the “flake” of the microparticles of 14b. (d) An image of a water droplet, 152.0° of contact angle, placed on the film's surface of (b) indicating the superhydrophobic nature (Copyright @ 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Reproduced with permission [40]). (e) SEM image of a tilted-view of an Au nanoflake obtained in transcribing from the self-organized nanoflake-featured microparticle of 14b (Copyright @ 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Reproduced with permission [46]).

units [41]. The alkylated-π compounds' polymorphism and superhydrophobic nature are getting increasing attention. For instance, attractive examples reported by J. Pei [42,43] and J. Veciana [44,45], where truxenone and perchlorotriphenylmethyl radical as π-conjugated unit, respectively, have been selected. A nanoflake-featured film surface composed of a microparticle assembly of 14b can also be covered with metals, deposited through sputtering, and act as template for the design of nanoflake-featured metal structures (Fig. 6e) [46]. This approach has many important advantages over the traditional methods of metallic microfabrication. It allows the easy tunability of the morphology and almost any desired metal (Au, Pt, Ti, Ni, etc.) can be treated in this process. Additionally, this method is environmentally friendly and economical as it eliminates the use of harsh chemicals and high temperatures and allows the reuse of the template material (see Fig. 6). Nanoflake-featured Au structures obtained using this method proved to be of possible use as both a

superhydrophobic (water contact angle ~170°) and a superhydrophilic (water contact angle ~11°) surface, depending on the chemical modification by self-assembled monolayers with different terminal groups [46]. In addition, the nanostructured Au can be applied as a substrate for surface-enhanced Raman spectroscopy (SERS). Nearly the same methodology has been adopted by Li and co-workers, where only the starting self-assembling molecule was different (diphenylalanine) [47]. The self-assembled architectures of the alkylated C60 compounds have also been used as temperature indicators for photothermal conversion of single wall carbon nanotubes (SWCNT) under near-infrared (NIR) irradiation [48]. SWCNT are known for generating heat under NIR irradiation [49,50]. The strong interaction between the alkylatedC60 and graphitic structure [51] seen at the outer wall of SWCNT, and the different melting temperatures of the alkylated-C60 derivatives, make it possible to study their features further. SWCNT were mixed with the three types of alkylated-C60 compound (14a, 14b and 14c in

Please cite this article as: Zielinska A, et al, Controlled self-assembly of alkylated-π compounds for soft materials — Towards optical and optoelectronic applications, Curr Opin Colloid Interface Sci (2014), http://dx.doi.org/10.1016/j.cocis.2014.03.007

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Fig. 3). These hybrid structures maintained the morphology of the original self-assembled structure seen in the compounds. Irradiation by a NIR laser caused a local increase in temperature due to the photothermal conversion of the SWCNT. To determine the degree of heating, melt-induced morphology changes in the assembled structures were observed. A local temperature increase of the 14/SWCNT hybrid assemblies, greater than 220 °C, was determined. A similar photothermal system of alkylated-C60 compounds incorporating Au nanoparticles has been investigated with a view to tailoring local wettability and morphology [52]. 6. Conclusions With their intrinsic optical and electronic properties, π-conjugated compounds play a vital role in the rapidly-developing field of flexible organic electronics. To make the properties of individual π-conjugated moiety useful in devices, sophisticated functionalization of the π-core is highly important, with alkylation being the most prevalent and successful method. Besides simply improving the processibility of the compounds, the importance of the attached alkyl chains on the regulation of the self-organized structures as well as optical and optoelectronic properties of the alkylated π-conjugated compounds have been recognized. From a viewpoint of self-assembly, some of those alkylated π-conjugated compounds can be regarded as a unique class of amphiphiles, namely “hydrophobic amphiphiles”, with both parts hydrophobic. The difference in their intrinsic interactions between the alkyl chains and π-conjugated moiety shows less contrast compared to that between the hydrophobic tail and hydrophilic head group in a traditional surfactant. In many cases the “hydrophobic amphiphile” concept helps to explain the relatively complicated self-assembly behavior and various morphologies of these compounds. The simple combination of alkyl chains and π-conjugated unit can also regulate their assembly in solvent-free state behavior seen as thermotropic liquid crystals and room temperature liquids. The formation of liquid matter in this molecular design concept is very much facilitated by the attachment of branched type alkyl chains. The liquid crystals and liquids derived from alkylated π-conjugated compounds are promising components for flexible semiconductor and light emitting devices. Therefore, these alkyl group tuning concepts are expected to yield novel alkylated π-conjugated compounds with desired self-assembly behavior and new applications in optical and optoelectronic devices. Acknowledgements This work was partially supported by KAKENHI (23685033, 25620069, 25104011) from MEXT, Japan, Nagase Science and Technology Foundation. H. L. thanks the financial support of the Hundred Talents Program of Chinese Academy of Sciences (Y20245YBR1). References [1] Schwartz AM, Perry JW. Surface active agents. New York: Interscience Publishers Inc.; 1949. [2] Li H, Choi J, Nakanishi T. Optoelectronic functional materials based on alkylatedπ molecules: self-assembled architectures and nonassembled liquids. Langmuir 2013;29:5394–406. [3] Hollamby MJ, Nakanishi T. The power of branched chains: optimising functional molecular materials. J Mater Chem C 2013;1:6178–83. [4] Asanuma H, Li H, Nakanishi T, Möhwald H. Fullerene derivatives that bear aliphatic chains as unusual surfactants: hierarchical self-organization, diverse morphologies, and functions. Chem Eur J 2010;16:9330–8. [5] Lei T, Wang J-Y, Pei J. Roles of flexible chains in organic semiconducting materials. Chem Mater 2014;26:594–603. [6] Mei J, Bao Z. Side chain engineering in solution—processable conjugated polymers. Chem Mater 2014;26:604–15. [7] Uy RL, Price SC, You W. Structure–property optimizations in donor polymers via electronics, substituents, and side chains toward high efficiency solar cells. Macromol Rapid Commun 2012;33:1162–77. [8] Dierking I. Textures of liquid crystals. Wiley-VCH; 2003.

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Please cite this article as: Zielinska A, et al, Controlled self-assembly of alkylated-π compounds for soft materials — Towards optical and optoelectronic applications, Curr Opin Colloid Interface Sci (2014), http://dx.doi.org/10.1016/j.cocis.2014.03.007