Topologically Directed Assemblies of Semiconducting SphereâRod

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Topologically Directed Assemblies of Semiconducting Sphere-Rod Conjugates Zhiwei Lin, Xing Yang, HUI XU, Tsuneaki Sakurai, Wakana Matsuda, Shu Seki, Yangbin Zhou, Jian Sun, Kuan-Yi Wu, Xiaoyun Yan, Ruimeng Zhang, Mingjun Huang, Jialin Mao, Chrys Wesdemiotis, Takuzo Aida, Wei Zhang, and Stephen Z. D. Cheng J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10193 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 4, 2017

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Journal of the American Chemical Society

Topologically Directed Assemblies of Semiconducting Sphere-Rod Conjugates Zhiwei Lin1, Xing Yang1, Hui Xu1, Tsuneaki Sakurai2, Wakana Matsuda2, Shu Seki2, Yangbin Zhou1, Jian Sun1, Kuan-Yi Wu1, Xiao-Yun Yan1, Ruimeng Zhang1, Mingjun Huang1, Jialin Mao3, Chrys Wesdemiotis3, Takuzo Aida4, Wei Zhang5 and Stephen Z. D. Cheng1* 1

Department of Polymer Science, College of Polymer Science and Polymer Engineering, The University of Akron, Akron, Ohio 44325, USA 2

Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan

3 4

Department of Chemistry, The University of Akron, Akron, Ohio 44325, USA

Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

5

South China Advanced Institute for Soft Matter Science and Technology, South China University of Technology, Guangzhou 510640, China * To whom the correspondence should be addressed: [email protected]

ABSTRACT Spontaneous organizations of designed elements with explicit shape and symmetry are essential for developing useful structures and materials. We report the topologically directed assemblies of four categories (a total of 24) of sphere-rod conjugates, composed of a sphere-like fullerene (C60) derivative and a rod-like oligo-fluorene(s) (OF), both of which are promising organic semiconductor materials. Although the packing of either spheres or rods has been well-studied, conjugates having both shapes substantially enrich resultant assembled structures. Mandated by their shapes and topologies, directed assemblies of these conjugates result not only in diverse unconventional semiconducting supramolecular lattices with controlled domain sizes, but also in tunable charge transport properties of the resulting structures. These results demonstrate the importance of persistent molecular topology on hierarchically assembled structures and their final properties.

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Introduction Investigating how the molecular elements modularly assemble, transfer and amplify microscopic functionalities to macroscopic properties is of paramount importance for material designs since properties are critically dependent on hierarchical structures.1 The self-assemblies of designed elements with explicit shape and symmetry cooperated into pre-expected structures with tailored properties become a popular approach to manufacture materials for diverse applications in different fields.2 Among nanoscale shaped objects, sphere and rod are perhaps the most basic geometrical forms in nature.3 For example, most bacteria come in one of three basic shapes: sphere (coccus), rod (bacillus), and spiral.4 The bulk structures constructed from single sphere- or rod-shaped elements

5

or their mixtures3,

6

have been intensively

explored. Despite the recent progresses on the development of building elements with very complex shapes by computer simulation,2a experimental realizations are challenging. We ask a basic and intriguing question: can we create new structures based on simple building elements such as sphere-rod conjugates? This remains a largely unexplored question due probably to the challenges in designing an appropriate model system. When one designs these conjugates, three important parameters are needed to take into consideration: elemental composition, topology and interactions. The composition is associated with the size and number of each component in the element, while the topology deals with types of conjugations between the sphere and rod. Two of typical ways are a sphere connected with one end of a rod or with the middle junction of a rod. Structural formation of these conjugates may be analyzed based on different shapes of these two elements: a sphere likes to aggregate with spheres and a rod likes to stay with rods,6 namely, “like likes like”. It may also introduce another pair of interactions via functionalizing the surfaces of elements, such as multiple hydrogen bonding, to construct collective secondary interactions.7 As a result, the thermodynamic driving force of assembled pathway to reach supramolecular structures can be viewed as a


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contribution of enthalpy changes (collective secondary interactions) and entropy changes (shape driven self-aggregations).3 This generates a kinetically favorable pathway to reach a supramolecular structure having its overall lowest free energy. Based on this design principle, we prepared four categories (a total of 24) of sphere-rod conjugates (Figure 1), so-called shape amphiphiles8 in giant molecules.7, 9 They are composed of a fullerene (C60) derivative as a sphere and oligo-fluorene(s) (OF) as rod(s). Note that both C60 and OF are promising organic semiconductor materials that are applicable to various electronic systems.10 For example, C60 is an electron acceptor, while OF is an electron donor employed frequently in the organic solar cell.11 Both elements in the conjugates possess explicit shapes and different symmetry. In addition, at the periphery of C60, we also introduce collective hydrogen bonding among the C60 spheres. Directing hierarchical assemblies of these sphere-rod conjugates may lead not only to a variety of unconventional ordered structures, but also to tunable structure-dependent semiconducting behaviors. Results and discussion Sphere-rod Conjugate Design. The two basic units in the sphere-rod conjugate are hydroxylfunctionalized C60 (DC60) having 20 hydroxyl groups on the periphery of C60 and OF with a tunable and uniform length (Figure 1). Both the DC60 and OF were synthesized and unambiguously characterized as monodispersed molecular masses [see Supporting Information (SI) Figures S1-S17]. Incorporation of hydrophilic groups on DC60 enhances nanophase separation between DC60 and hydrophobic OF in addition to the sphere-rod shape immiscibility. The rigid conformations of DC60 and OF render the persistent shapes, while changing the linker in-between help tuning molecular shapes and topologies. By design, the DC60 sphere can be located at the end or central junction of the OF rod(s) (referred as I- or Tshape, respectively). Among each category, the OF rod length can be tuned via varying the repeating unit number (n) of fluorene (Figure S18). With the same n value, two series of these conjugates, DC60-IOFn 3 ACS Paragon Plus Environment


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and DC60-TOFn as well as DC60-2IOFn and DC60-2TOFn are pairs of topological isomers,12 which are also known as constitutional isomers.13

Figure 1. Chemical structures and molecular models of four categories of sphere-rod conjugates. Cartoons are their corresponding simplified molecular models, where blue spheres and purple rods represent hydroxyl-functionalized C60 and hydrophobic OF, respectively. n is the precise repeating unit number of fluorene. Topologically Directed Assemblies. To illustrate how shapes and topologies direct the assemblies of these sphere-rod conjugates, we first investigated the series of DC60-2TOFn. After thermal annealing the freeze-dried powder samples under nitrogen atmosphere at 110 °C for 2h (detailed annealing conditions for other samples are summarized in Table S1), DC60-2TOF2 self-organizes into a lamellar (LAM) supramolecular lattice, as revealed by small-angle X-ray scattering (SAXS) pattern with a scattering 4 ACS Paragon Plus Environment

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vector (q) value ratio of 1:2 in reciprocal space (Figure 2a). This LAM lattice was further validated by the bright-field (BF) transmission electron microscope (TEM) image of a microtomed thin-section of DC602TOF2 bulk sample in real space (Figure 2e). Within the LAM, the OF rods are arranged perpendicular to the layer normal, as confirmed by a 2D SAXS pattern and polarized optical microscopy (POM) images on the oriented DC60-2TOF2 sample (Figure S19). Note that in this case the diameter of DC60 is ∼1.5 nm with the rod length of ∼1.8 nm (Figure S18). Increasing the rod length of OF to DC60-2TOF4 resulted in the formation of a hexagonally packed cylinder (HEX) lattice with a space group of P6mm, as indicated by the SAXS pattern with a q-ratio of 1:√3:2 (Figure 2b). A convincing BF-TEM image shows that the hexagonal packing of dark cylindrical domains is embedded in the gray matrix (Figure 2f). Considering the higher electron density of DC60 than that of OF, the dark cylinders are attributed to the aggregation of the DC60 spheres and the gray matrices are attributed to the domains of OF rods. 2D SAXS and POM results (for the oriented sample) suggest that the OF rods are aligned along the cylindrical direction (detailed analysis see SI, Figure S20). In this case, the rod length is ∼3.6 nm, while the DC60 sphere diameter is ∼1.5 nm. To ensure close packing of the DC60 spheres in the core of cylinders, the rods that surround the cylindrical DC60 core possess the rodlong direction parallel to the cylinder direction with only an orientational order, identified to be a nematic liquid crystal-like matrix. 5b, 14 Upon further increasing the OF rod length to DC60-2TOF6, a SAXS pattern featuring twelve sharp scattering peaks can be observed (Figure 2c). The indexed q ratios for these scattering peaks are √2:√4:√5:√6:√8:√10:√13:√14:√16:√20:√21:√22, which is characteristic of the Frank-Kasper (F-K) A15 structure with a space group of Pm3തn.15 A cubic unit cell with a=13.6 nm can be deduced from d200 (a=2×d200). This A15 supramolecular lattice can also be confirmed by the BF-TEM image shown in Figure 2g, where the arrangement of spheres with a regular 44 tiling pattern along the zone is



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explicitly observed. The center distance between two closest neighboring spheres is measured to be ~13 nm, which is in good agreement with the value obtained in the SAXS result. This A15 structure is thermodynamically stable phase in a temperature range between 25 °C and 180 °C (Figure S21).

Figure 2. Supramolecular lattices assembled from two series of sphere-rod conjugates composed of one sphere and two rods. SAXS patterns (a to d) and thin-sectioned BF-TEM images (e to h) of DC602TOF2, DC60-2TOF4, DC60-2TOF6 and DC60-2TOF8, where lattices change from LAM to HEX, to A15 6 ACS Paragon Plus Environment

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and finally to BCC. (g) and (h) are Fourier filtering images, and the original ones are shown in Figure S23. SAXS patterns (i to l) and thin-sectioned BF-TEM images (m to p) of DC60-2IOF2, DC60-2IOF4, DC602IOF6 and DC60-2IOF8, where lamellar lattices with controlled thicknesses are formed. (q) Schematic illustration of ordered structures and packing models of two sets of sphere-rod conjugates, which are topological isomers when their OF rods length are identical. To simplify the models, the OF rods are not shown in A15 and BCC.

Originally, A15 phase was identified in metal alloys, and then discovered in soft materials systems including supramolecular dendrimers through a systematically constitutional isomeric library approach,13, 15-16

surfactants,17 polymers,18 and giant molecules.9 The general understanding is that molecules in soft

materials first assemble into spherical motifs. Due to the character of deformable spherical motifs to adjust their sizes, an A15 structure is formed.19 The spherical motifs in the current sphere-rod conjugates are formed through aggregating DC60 spheres together in the core via collective hydrogen bonding, while the rod orientation is scarified to form the shell of the spherical motif with tangential arrangements. These motifs are then assembled into the A15 lattice via deformation to finally become polyhedral shape for close packing. Based on the measured density (ρ) of the A15 sample (ρ=1.15 g·cm-3), it can be estimated that each polyhedral motif contains about 33 conjugate molecules on average. A body-centered cubic sphere (BCC, space group Im3m) supramolecular lattice has been achieved in the longest OF rods in this series (DC60-2TOF8), evidenced by the combination of a SAXS pattern exhibiting a q-ratio of 1:√2:√3 (Figure 2d), and the BF-TEM image of squarely packed spheres along zones (Figure 2h) as well as the hexagonally packed spheres along zones (Figure S24). The unit cell of BCC lattice is a=8.34 nm, and the density was measured to be ρ =1.10 g·cm-3. Our calculation indicated that each motif consists of around 24 conjugates. Figure 2q represents a cartoon to summarize the ordered supramolecular lattices and their molecular packing of the four pairs of topological isomers, DC60-2TOFn and DC60-2IOFn (n=2, 4, 6 and 8).



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The assembly behaviors of DC60-2TOFn and DC60-2IOFn (Figure 1), which are topologically isomeric and therefore compositionally identical, are completely different from one another. Regardless of their OF rod length (n=2, 4, 6 and 8), the LAM supramolecular lattices are commonly observed in DC602IOFn, evidenced by their sharp scattering peaks having a q ratio of 1:2:3 (up to the 7th order, Figures 2i2l) and BF-TEM images (Figures 2m-2p). We note that the third order scattering peak in Figure 2j is missing, which can be attributed to extinction (see detailed discussion in SI, Section 3.2). These LAM structures consist of double layer, head-to-head packing of the DC60 spheres, where OF rods are orientated parallel to layer normal within the layers (Figure S25, and see detailed discussion in SI, Section 3.2). In addition, the WAXS results suggest that the molecular packing within the lamellae only possesses an orientational order (Figure S25), revealing that the supramolecular structures are driven by the nanophase separation between intrinsically immiscible DC60 spheres and OF rods. For DC60-2IOFn (n=1, 3, 5 and 7) with odd-numbers of the repeating units in the OF rods, LAM structures were also observed (Figure S26). Of particular interest is the highly asymmetric LAM lattices from DC60-2IOF8 with very dissimilar sizes between the DC60 spherical diameter (∼1.5 nm) and the OF rod length (∼7.0 nm). It is generally known that a deviation from the symmetric volume fraction usually drives the fabrication of structures with curved interfaces such as HEX and BCC. For example, in the case of asymmetric di-block copolymers (e.g. polystyrene-polyisoprene, PS-PI) or giant surfactants (e.g. a hydrophilic polyhedral oligomeric silsesquioxane tethered with two PS tails, namely, DPOSS-2PSn), HEX and BCC were constructed within the following volume fraction of PS (VPS): 0.66 < VPS < 0.77 and VPS > 0.77 for PSPI,20 respectively, 0.71 < VPS < 0.84 and VPS > 0.84 for DPOSS-2PSn.9a In contrast, DC60-2IOF8 maintain a LAM lattice with a volume fraction of OF as high as 0.81. This can be attributed to the rigid conformations of the DC60 spheres and OF rods as well as close commensurate cross-section areas


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between the DC60 (1.76 nm2) and two OF rods (1.56 nm2, the projection along the long axis of the rods, see detailed calculation in SI). The curved interfaces between DC60 and OF are thus not able to form, and a double-layered molecular packing in the LAM lattice with the asymmetric and flat interface readily affords a close packing of the DC60 and oriented OF rods. Similar characterizations can be carried out in the cases of DC60-TOFn and DC60-IOFn (Figures S27 and S28). Briefly, LAM structures were created in DC60-TOF2 and DC60-TOF4 (Figures S28a, b and e, f). A double gyroid (DG, space group Ia3തd) lattice featured by a “wagon wheel” pattern was observed for DC60-TOF6 (Figure S28g), corresponding to the indexed q ratios of √6:√8:√14:√16:√20:√22 in SAXS pattern (Figure S28c). DC60-TOF8 assembles into the HEX structures consisting of DC60 as cylindrical cores and OF rods as matrix (Figure S28 d and h). For their I-shaped topological isomers, DC60-IOF1 and DC60-IOF2 possessing a very short OF rod self-organize into HEX structures, indicated by SAXS patterns in Figures S27a and S28i. Different from the HEX formed by DC60-2TOF2 in Figures 2b and 2f, this HEX is regarded as inversed hexagonally packed cylinders (i-HEX) in which the OF rods are wrapped into centers of columns and DC60 form continuous matrix as proved by TEM observation (Figure S28m). For DC60-IOFn (n=3-8), LAM structures were all generated as revealed by combining TEM (Figures S28n-p) and SAXS results (Figure S27 and Figures S28j-l). Considering the incommensurate cross-section areas between one OF and one DC60, an interdigitated molecular packing within the OF domains can be expected. The molecular packing and comparison of distinct ordered structures formed by these four pair of topological isomers is summarized in Figure S28q. Figure 3 and Table S1 summarize supramolecular lattices for the 24 sphere-rod conjugates in these four series. The driving force for the formation of these ordered structures is dictated by a balance of maximizing the collective hydrogen bonding interactions among DC60 spheres and the dense packing and orientation of OF rods. 9 ACS Paragon Plus Environment


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Figure 3. Schematic representation of diverse supramolecular lattices assembled from four series of 24 sphere-rod conjugates. (a) Inversed hexagonally packed cylinders (i-HEX), interdigitated layered lamellae (i-LAM) with controlled domain sizes are formed by DC60-IOFn; (b) double layered LAM with controlled domain sizes are formed by DC60-2IOFn; (c) LAM, double gyroid (DG, space group Ia3തd), and HEX (space group P6mm) are formed by DC60-TOFn; (d) LAM, HEX, Frank-Kasper A15 (space group Pm3ത n) and body-centered cubic sphere (BCC, space group Im3m) are formed by DC60-2TOFn. The anticlockwise arrows indicate the direction of increasing OF rod length, where n presents the repeating unit number of OF rod. Structure-dependent Semiconducting Behaviors. We use charge transport properties as a probe to demonstrate how the assembled supramolecular structures and their lattice domain sizes will influence the physical properties of materials. To avoid experimental artifacts as much as possible (the electronic state, see Figure S29), we decided to employ an electrodeless technique, flash-photolysis time-resolved 10 ACS Paragon Plus Environment

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microwave conductivity (FP-TRMC) to evaluate the charge carrier transport in a short distance (~10 nm). 21

We examined ten different conjugates [five pairs of topological isomers, DC60-2IOFn and DC60-2TOFn

(n=2, 4, 6 and 8), and DC60-IOF8 and DC60-TOF8]. All the conjugates gave rise to transient conductivity (φΣµ) with a prompt rise and a slow decay in the TRMC profiles (Figures 4a and 4b), indicating that they are photoconductive.22 Here φ and Σµ represent charge carrier generation efficiency (non-dimension) and sum of the hole and electron mobilities (cm2 V–1 s–1), respectively. For the five pairs of isomers, the conjugates forming LAM exhibit higher photoconductivity (in ×10–5 cm2 V–1s–1) than those forming other ordered structures (Figure 4c). It can be attributed to the better stacking and orientation of OF rods within the LAM, which facilitate the charge carrier transport. This can be further demonstrated by a trend of decreasing (φΣµ)max values in DC60-2TOFn from 7.1 to 1.4 with increasing the rod length from n=2 to n=8, accompanied by the supramolecular lattices changing from LAM to HEX, A15 and BCC, in which OF rods are oriented in LAM and HEX, but not in A15 and BCC. We also note that the maximum transient conductivity (φΣµ)max of DC60-2TOF2 is higher than that of DC60-2IOF2, implying that in comparison with parallel to layer normal, the orientation of OF perpendicular to layer normal may be more efficient to the charge carrier transport probably due to decreased transport distances. Of particular interest is the fact that the charge carriers generated in these conjugates are long-lived.21c, 22b For example, half-lifetime (τ1/2) of the generated charge carriers for DC60-2TOF2 was measured to be 77 µs (Figure S30). The long lifetime thus observed may result from the highly ordered alternative packing between DC60 and OF domains. Then the charge carrier mobility values were roughly estimated by selecting DC60-2TOF2 and DC602IOF2 as representative examples. In transient absorption spectroscopy (TAS), both films showed a moderate absorption band at 450–600 nm, where their kinetic profiles were correlated with the corresponding TRMC signals (Figure S31). We assigned the observed absorption peaks as radical cations



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of fluorene dimers. 23 Based on these results, the observed charge transport property dominantly relies on the hole transport among OF parts. Electron carriers generated on DC60 parts may not be mobile due to the surrounding insulating hydrophilic substituents. DC60 serves to increase the charge carrier generation efficiency through photoinduced charge separation between the DC60 and OF. It is worth noting that the length scale of these highly ordered nanostructures containing alternatively interdigitated donor-acceptor (D-A) domains fit with the typical carrier photoexcitation diffusion distance of ~10 nm in organic heterojunctions,24 which is expected to ensure the transport of highly mobile charge carriers with reduced recombination.25 The hole mobility values for DC60-2TOF2 and DC60-2IOF2 were roughly estimated by combining φmax values that were obtained from TAS and absorption coefficient of radical cations of fluorene dimer and (φΣµ)max values from FP-TRMC, leading to the 0.05 cm2 V–1 s–1 for both films constructing by D-A alternating LAM assembly. This value was smaller than that observed for polyfluorene films26 and that along polyfluorene single chains,27 both of which were evaluated by TRMC techniques. However, compared with the single chain mobility along oligofluorenes (n ~ 2–16),27b the present system was found to show larger hole mobility by self-organization of oligofluorene moieties.


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Figure 4. Influences of supramolecular ordered structures and their domain sizes on charge transport properties. FP-TRMC response of (a) I-shaped sphere-rod conjugates including DC60-2IOFn (n=2, 4, 6 and 8) and DC60-IOF8; (b) T-shaped sphere-rod conjugates including DC60-2TOFn (n=2, 4, 6 and 8) and DC60-TOF8; (c) Summarized maximum transient conductivities (φΣµ)max for all the samples showing in (a) and (b).

Conclusions In summary, we have presented the design of four series of precisely defined semiconducting sphererod conjugates based on fullerene derivative (DC60) and oligo-fluorenes (OF). The assembly of these sphere-rod conjugates result in the formation of a variety of unconventional highly ordered semiconducting lattices, which is mandated by their molecular shape and topologies. These semiconducting lattices with controlled domain sizes demonstrate dissimilar charge transport properties 13 ACS Paragon Plus Environment


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depending on their molecular packing and orientation. These results reveal that the precise control and change of the elemental composition, topologies and interactions could exert significant impact on the hierarchically assembled structures and final properties of materials. The highly ordered supramolecular lattices containing alternatively interdigitated donor-acceptor domains at the length scale of 10 nm provide models for ideal organic heterojunctions ensuring the transport of highly mobile charge carriers with reduced recombination. This work opens up exciting opportunities for guiding molecular design, supramolecular structures and domain size controls to develop functional materials for diversified applications.

Acknowledgement: This work was supported by the National Science Foundation (DMR-1408872). Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences. The authors thank Mrs. Ying Yu for the help with artwork design. Supporting information The experimental details, additional results and discussion are provided. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author Correspondence and requests for materials should be addressed to S. Z. D. C.

Competing financial interests The authors declare no competing financial interests.

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