Self-assembly and ring-opening metathesis

ChemComm COMMUNICATION

Cite this: Chem. Commun., 2018, 54, 5546 Received 20th March 2018, Accepted 30th April 2018

Self-assembly and ring-opening metathesis polymerization of cyclic conjugated molecules on highly ordered pyrolytic graphite† Shih-Ting Chiu,a Hsin-Yu Chiang,a Yen-Jen Lin,a Yun-Yung Lu,a Hirofumi Tanaka, *b Takuya Hosokai c and Masaki Horie *a

DOI: 10.1039/c8cc02224k rsc.li/chemcomm

Cyclic conjugated monomers comprising cyclopentadithiophenevinylene trimers and their polymers on HOPG are observed using STM and AFM. ROMP of the monomers is performed using a Grubbs catalyst. Their STM images exhibit single chains of planar polymers, whereas their AFM images show elongation of the polymer chains on HOPG.

Conjugated molecules have attracted great interest because of their unique rigid structures and optical and electronic properties.1–5 They have been used for the development of opto-electronic devices, such as organic light emitting diodes,1 organic photovoltaic devices,2,3 and organic field-effect transistors.4,5 Scanning tunneling microscopy (STM) has been used to observe individual conjugated molecules, such as polythiophenes,6–8 polyacetylenes,9 and some oligomers,10–14 at a molecular scale resolution on highly oriented pyrolytic graphite (HOPG). Cyclic conjugated molecules are of great interest because of their shape-persistent planar structures, which lead to self-assembled two-dimensional (2D) packing on HOPG.12,15–18 Furthermore, some of these cyclic conjugated molecules form inclusion complexes on HOPG with guest molecules, such as hexabenzocoronene.15,16 However, observation and manipulation of single molecules are challenging because single molecules are influenced by different factors, such as molecular backbone structures, length and chemical structures of side chains, molecule–substrate interactions, and solvent properties.19 Lately, conjugated polymers have been synthesized via cross coupling reactions, such as Suzuki, Stille, and direct arylation

polymerization, between functionalized monomers.20–26 However, the propagation process has rarely been observed via STM because these monomers are generally too small for direct observation on a substrate surface or because their interaction with the substrate is not significant. We recently reported a unique cyclic conjugated molecule, cyclophane triene, which comprises three cyclopentadithiophene-vinylene (CPDTV) units.27 This molecule was used for the synthesis of poly(cyclopentadithiophene-vinylene), poly(CPDTV), via ring-opening metathesis polymerization (ROMP). ROMP for cyclic olefins on a substrate surface is advantageous to tailor the surface morphology of polymers; however, molecular scale observations using STM are rarely reported. In some reports, atomic force microscopy (AFM) was used to observe the micrometer-scale morphology of the surface in a surface-initiated ROMP of cyclic olefins in a direction vertical to the substrate.28,29 In this work, we report the self-assembly of cyclophane trienes on HOPG (Fig. 1). ROMP of the cyclophane triene monomers was performed on either HOPG or in a reactor tube, and it resulted in a unique chemical structure and morphology of the polymer on HOPG. Utilization of these cyclophane trienes allowed the direct observation of individual monomers and polymers on a scale of

a

Department of Chemical Engineering, National Tsing Hua University, 101, Sec. 2, Kuang-Fu Road, Hsinchu 30013, Taiwan. E-mail: [email protected] b Department of Human Intelligent Systems, Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu, Kitakyushu 808-0196, Japan. E-mail: [email protected] c National Metrology Institute of Japan (NMIJ), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan † Electronic supplementary information (ESI) available: Experimental details, synthetic procedures, material characterization data and additional STM and AFM images. See DOI: 10.1039/c8cc02224k

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Fig. 1 Schematic illustration of the self-assembly and ROMP of cyclophane trienes on HOPG.

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few nanometers using STM and on a scale of few hundred nanometers using AFM. To the best of our knowledge, this is the first study on STM and AFM observations of the formation of conjugated polymers on a substrate surface via ROMP of cyclic conjugated molecules. Based on the STM observations, cyclic conjugated molecules are expected to have high tunneling efficiency because of their good adsorption ability on HOPG due to their planar structures. Here, we designed and synthesized cyclophane trienes containing CPDTV units with different side chains—hexyl (CPDTV-C6), dodecyl (CPDTV-C12), octadecyl (CPDTV-C18), and the branched 2-ethylhexyl (CPDTV-EH)—via McMurry coupling using TiCl4/Zn. The monomers were dissolved in 1-phenyloctane, and the solution was casted on HOPG to observe the assembly of the cyclophane trienes on the substrate (Method 1 in Fig. 1). Fig. 2 (also Fig. S1 in the ESI†) depicts the STM images of the cyclophane trienes adsorbed on HOPG. These molecules spontaneously produce uniform self-assembled structures. The bright dots are attributed to the conjugated backbone because of the electronic interaction of p-orbitals between the conjugated monomer and the substrate,8,18 resulting in a

Fig. 2 STM topographic images and cross-section profiles of (a) CPDTV-C6 and (b) CPDTV-C18 on HOPG. Molecular structures and packing models of (c) CPDTV-C6 and (d) CPDTV-C18 obtained from molecular modeling calculations.


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‘‘face-on’’ configuration. According to single-crystal X-ray crystallography, cyclophane triene with ethyl side chains has a C2v-like symmetry in the crystal state, in which three cyclopentadithiophene (CPDT) units form an isosceles triangle configuration.27 In contrast, the CPDT units are indistinguishable on HOPG, suggesting that they fluctuate on HOPG to produce a C3v-like symmetry. Based on these results, one of the stable molecular structures and its packing model were estimated by molecular modeling calculations (Fig. 2c, d and Fig. S2 in the ESI†). The packing models varied according to the side chains. For CPDTV-C6, the calculated molecular diameter (3.0 nm) including the stretched side chains was consistent with the observed intermolecular distance (3.0 nm, Fig. 2c). This is because the intermolecular repulsion force excluded neighboring molecules to form appropriate packing with high density (0.11 molecules per nm2). For CPDTV-C12, the intermolecular distance increased to 4.0 nm due to the longer alkyl chains to afford a lower density (0.06 molecules per nm2) than that of CPDTV-C6 (Fig. S2b in the ESI†). The calculated molecular diameter of CPDTV-C18 (5.8 nm) was significantly longer than the estimated intermolecular distance (4.0 nm, Fig. 2b and d). This implies that the alkyl chains were interdigitated with other alkyl chains of adjacent molecules. In contrast, CPDTV-EH showed the shortest intermolecular distance (2.5 nm) with the highest density (0.16 molecules per nm2) in all CPDTVs (Fig. S2d, ESI†). This suggests that the branched side chains have a lower intermolecular exclusion ability than linear chains. ROMP of the cyclophane trienes was performed in a reactor tube using a second-generation Grubbs catalyst in p-xylene at 120 1C for 6 min (Method 2 in Fig. 1). The reaction mixture was diluted using 1-phenyloctane solvent and directly casted on a bare HOPG without quenching the reaction; STM observation was immediately performed. The STM images and a cross-section profile after ROMP of CPDTV-C6 are shown in Fig. 3a and Fig. S4 in the ESI,† respectively. The bright features indicate that the polymer chains were adsorbed on top of the monomer layer, which resulted from the incomplete polymerization. It is worth noting that the planar polymer chains could only be observed on the monomer layer. This suggests that having the underlying monomer layer resulted to be beneficial for the observation of planar polymer chains because, generally, polymer chains tend to form random coils in solution. As shown in the magnified picture in Fig. 3a, the length of one polymer chain was approximately 11 nm, which corresponds to 4–6 monomer units according to either the trans-rich and cisrich possible structures (Fig. 3c). Previous studies showed that ROMP of cyclophane dienes, comprising two –CQC– bonds in the cyclic molecules, produced poly(phenylenevinylene)s with an alternating cis–trans sequence that can be photoisomerized to all-trans.30,31 Therefore, ROMP of the cyclophane trienes, comprising three –CQC– bonds, was expected to produce polymers mainly with a cis–cis–trans sequence. However, according to a previous study conducted by our group, only the all-trans conformation of poly(CPDTV) was obtained via ROMP because of thermal isomerization during polymerization and quenching

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Fig. 3 STM images after ROMP of (a) CPDTV-C6 via Method 2 and (b) CPDTV-C6 on HOPG via Method 3. (c) Approximate size of the possible chemical structures. (d) I–V curves of CPDTV-C6 and its ROMP product recorded during STM measurements.

in solution.27 In this study, the polymer chains probably remain in the cis-form because STM was performed immediately after ROMP. ROMP of CPDTV-C12 and CPDTV-C18 also yielded the corresponding polymer chains, which were observed on the monomer monolayer on HOPG (Fig. S3, S5 and S6 in the ESI†). In these STM images, defects of monomer alignment were found around the polymer chains. They were probably due to the formation of p–p interactions between the polymers and monomers, and these p–p interactions weakened the interactions between the polymer adjacent monomers and HOPG. In contrast, such defects are minor in CPDTV-C6 probably because of the robust and dense packing of this monomer compared to the others. As shown in Method 3 in Fig. 1, ROMP was performed for the self-assembled monolayer of CPDTV-C6 on HOPG by adding a p-xylene solution of the catalyst onto the monomer surface and heating at 120 1C for 1 min. However, the STM image shows only the aggregation of the polymer on the HOPG without the monomer layer (Fig. 3b). This is probably because the initial monomer was too dense on the HOPG surface. ROMP of such a densely packed monomer provided the aggregated polymer. A more detailed morphological study using AFM is described later.

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To compare the electronic properties of the cyclophane triene and the corresponding polymer at a local position, scanning tunneling spectroscopy (STS) was conducted. Fig. 3d shows the I–V curves of CPDTV-C6 and its polymer recorded during the STM measurement. The onset potentials are related to the HOMO and LUMO of the molecule.8 Thus, the significant change in the onset potentials for the ROMP product implies there was a decrease in the energy gap. To verify the electronic state, the molecular orbitals were calculated using density functional theory (DFT) for the simulated molecular models of the monomer and the predicted ROMP products (Fig. 4). The molecular orbital distributions for all the structures showed widely delocalized electronic states through the whole molecule. The cyclic monomer had a relatively high theoretical energy gap (Eg = 2.6 eV), which dramatically decreased to 2.1 eV for the ring-opened curled structure with cis–cis–trans conformation. The linear structure with all-trans conformation had a slightly lower energy gap (2.0 eV) compared with the curled structure. With an additional monomer unit (Fig. S9 in the ESI†), the energy gap slightly decreased. Therefore, the decrease in the energy gap can be strongly attributed to the first ring-opening. As shown in the STM, ROMP of CPDTV-C6 (Method 3) showed large-scale aggregation of the polymer. AFM was conducted to further study the morphology of the resulting polymer on HOPG at a scale of a few hundred nanometers (Fig. 5). Samples were prepared as follows: different concentrations of the catalyst in p-xylene were casted on the self-assembled monolayer of CPDTV-C6 on HOPG, and ROMP was conducted at 120 1C for 1 min until the solvent evaporated. With a lower catalyst concentration (4 mM), the polymer chains formed on spherical clusters (Fig. 5a). When the catalyst concentration was increased to 24 mM, polymer chains

Fig. 4 (a) Molecular orbital distributions for the monomer and predicted ROMP products estimated using DFT (B3LYP/6-31G(d)). The alkyl groups were simplified. (b) Calculated energy diagrams.


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Notes and references

Fig. 5 AFM topographic and phase images after ROMP of CPDTV-C6 using second-generation Grubbs catalyst with different concentrations (a) 4, (b) 24, (c) 33, and (d) 49 mM.

(length ca. 100–200 nm, width ca. 30 nm) were observed from a catalyst cluster (Fig. 5b). These results suggest that the active species can elongate the polymer chains with no inhibition from other polymer chains. When the catalyst concentration was further increased to 33 mM, elongated and linked polymer chains were observed (Fig. 5c). At the highest catalyst concentration (49 mM), the surface was completely covered by sphere-shaped aggregations of the polymer with a boundary distance of ca. 100 nm (Fig. 5d). These results suggest that the large amount of active species results in several polymer spheres at the same time, leading to tight packing of the polymer spheres on the substrate surface. Such unique morphology can only be achieved by ROMP using the self-assembled monomer layer of CPDTV-C6. AFM images of drop-casted films of the purified polymer on HOPG were significantly different from these results (Fig. S10 in the ESI†). In summary, we performed single molecule observation of cyclophane trienes and their ROMP products via STM and AFM, presenting unique surface morphologies on HOPG. These morphologies were quite different from the polymer brush films obtained from the surface-initiated ROMP of cyclic olefins in a direction vertical to the substrate.29 This is because our ROMP was conducted on HOPG or in the solution just above the self-assembled monomer layer on HOPG, leading to the polymer chain growth in a horizontal direction to the substrate. We anticipate that this single molecule observation approach and molecular assembly of the cyclic conjugated molecules on a substrate surface will be useful for applications such as fabrication of opto-electronic devices and in molecular structural analyses, tracing of reactions, molecular manipulations, and surface modifications based on the simple integration of molecular building blocks. This work was financially supported by Ministry of Science and Technology Taiwan.

Conflicts of interest There are no conflicts to declare.


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