DOI: 10.1002/asia.201500452

Focus Review

Molecular Electronics

Conjugated Oligomers and Polymers Sheathed with Designer Side Chains Chengjun Pan,[a] Chunhui Zhao,[b] Masayuki Takeuchi,*[a, b] and Kazunori Sugiyasu*[a]

Chem. Asian J. 2015, 10, 1820 – 1835

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Focus Review Abstract: Conjugated polymers (CPs) are often referred to as molecular wires because of their quasi one-dimensional electronic wavefunctions delocalized along the polymer chains. However, in the solid state, CPs tend to self-assemble through p-stacking, which greatly attenuates the one-dimensional nature. By molecular design, CPs can be molecu-

1. Introduction Conjugated polymers (CPs) function collectively in organic electronic devices; that is, the device performance, including electrical conductivity and emission quantum yield, is strongly influenced by the supramolecular assembly modes of the pconjugated systems. This concept is clearly illustrated by a film of poly(3-hexylthiophene) (P3HT), in which the P3HT backbones self-assemble to form a nanocrystalline lamellae structure.[1] In this film, the charge carriers are delocalized over adjacent P3HT backbones through interchain p-orbital overlap (i.e., p-stacking). Accordingly, the film behaves as a two-dimensional (2D) semiconductor despite the fact that P3HT itself is a one-dimensional (1D) chain.[2] The charge carrier mobility of the film can be enhanced by increasing the crystallinity of the 2D lamellae structure; therefore, it is of great significance to control the supramolecular assembly of CPs. To this end, optimization of both molecular design and processing conditions are crucial, as discussed in detail elsewhere.[3] Given this context, research on the 1D and 2D characteristics of CPs should provide deep insight into the conduction mechanisms and also reveal the unexplored potential of these useful polymeric materials, paving the way for new applications. In the course of developing CP-based optoelectronic devices, such as field-effect transistors (FETs), the 2D characteristics of CPs have been extensively studied from both theoretical and experimental point of views.[4] In contrast, the investigation of individual CPs, precluding the influence of interchain interactions, is experimentally challenging. Scanning tunneling microscopy (STM) enables isolated CPs to be accessed on a substrate;[5] however, such sophisticated techniques cannot generally be applied to a variety of CP structures. To address this issue, synthetic chemists can contribute. A single CP chain can be molecularly “insulated” by molecular design so that cross-talk or short-circuits between the polymer chains are effectively prevented. This molecular design concept allows us to isolate and investigate the electronic properties of a single [a] Dr. C. Pan, Prof. Dr. M. Takeuchi, Dr. K. Sugiyasu Organic Materials Group, Polymer Materials Unit National Institute for Materials Science 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 (Japan) E-mail: [email protected] [email protected] [b] C. Zhao, Prof. Dr. M. Takeuchi Department of Materials Science and Engineering Graduate School of Pure and Applied Sciences University of Tsukuba 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577 (Japan) Chem. Asian J. 2015, 10, 1820 – 1835

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larly insulated just like electric power cords, resulting in socalled “insulated” molecular wires (IMWs). In this Focus Review, we will discuss their unique photophysical, electronic, and mechanical properties which originate from the absence of p-stacking.

CP. Herein, we focus on such new types of CPs, which are often referred to as insulated molecular wires (IMWs) in a structural analogy with electric power cords.[6] IMWs have been synthesized through a (1) polyrotaxane approach, (2) polymer wrapping approach, or (3) covalent bond approach (Figure 1). In 2007, Frampton and Anderson reviewed IMWs comprehensively,[6a] focusing on these three synthetic approaches. For example, Anderson and co-workers[7] synthesized a CP-based polyrotaxane: a CP threaded through a series of cyclodextrins (CDs) and end-capped with bulky naphthalene-3,5-disulfonates at both the termini (Figure 1 a). Owing to the encapsulation, the CP showed a high fluorescence quantum yield and chemical stability, based on which efficient organic light-emitting diodes (OLEDs) were later fabricated.[8] Shinkai and co-workers,[9] through approach (2), prepared a chiral IMW: a CP wrapped with a helical polysaccharide, schizophyllan (Figure 1 b). The chiral sheath not only encapsulated the CP chain but also imposed a helical conformation on the conjugated backbone, thereby realizing circularly polarized luminescence (CPL).[9b] Approaches (1) and (2) are dependent on intermolecular non-covalent interactions between the CPs and insulating sheaths under thermodynamic control (i.e., a supramolecular approach). These processes spontaneously result in complex three-dimensional (3D) architectures, which is a strong advantage of supramolecular chemistry. However, the synthetic conditions are restricted to solvent systems where the non-covalent interactions are ensured to operate. In many cases, including the above two examples, the hydrophobic effect is relied on as the driving force for complexation; accordingly, both components should be soluble in aqueous media. The polar hydrophilic groups introduced for this purpose as well as trace amounts of water remaining from the synthetic procedure have, to a certain extent, hampered the use of IMWs in further optoelectronic applications. In addition, the formation of structural defects related to the stoichiometry between the CPs and insulating sheaths is inherently unavoidable, and thus authentication of the complex structures is difficult. To circumvent these problems, the covalent bond approach (3) has increasingly attracted much attention, and we will give an overview of the recent development in this field. A representative example of this approach is dendronized IMWs. For example, Schlìter and co-workers[10] reported poly(paraphenylene)s substituted with the so-called Fr¦chet-type dendrons (Figure 1 c). This molecular design strategy has led to IMWs with a variety of CP backbones, such as poly(phenylene vinylene),[11] poly(phenylene ethynylene),[12] polyfluorene,[13] polythiophene,[14] etc., because common polymerization condi-

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Focus Review tions can be applied. Although dendronized CPs are the archetypal IMWs obtained using the covalent bond approach, due to page limitations, they will not be discussed in this Focus Review, nor will CPs substituted with other simple bulky side chains. Herein, we focus on IMWs with p-conjugated backbones that are isolated by elaborately designed side chains. We also include p-conjugated oligomers that are encapsulated by unique sheaths and, in principle, capable of polymerization. Such IMWs were classified as “nonfractal dendronized IMWs” in the review by Frampton and Anderson;[6a] however, as of 2007, only a few examples existed.

2. IMWs Through the Covalent Bond Approach As an example of the IMWs that we will highlight herein, Figure 2 shows an insulated polythiophene (1p : hereafter, “p” represents polymer) reported by us in 2010.[15] The 3D structure of 1p resembles a polyrotaxane as the CP backbone is threaded through macrocyclic molecules. However, the ring and axis (i.e., polythiophene) are covalently linked; accordingly, all the repeating units are completely encapsulated without any “short-circuits”. The doubly strapped bithiophene monomer (11) was synthesized from known compounds through five steps with moderate yields. The key step was the ring-closing olefin metathesis (RCM) reaction, which produced the macrocycle quantitatively. We note that a conceptually related approach to IMWs based on a RCM reaction has also been reported by Cox et al. and Gladysz et al.[16] As confirmed by X-ray crystallographic analysis (Figure 2 b), the bithiophene backbone was encapsulated by its own cyclic side chain. More interestingly, the symmetrically fixed dihedral angle of the bithiophene resulted in a coplanar geometry, achieving developed conjugation. In fact, spectroscopic studies on 1n (hereafter, “n” is the number of the repeating units in oligomers; n was 1–5 in this study) revealed that the effective conjugation length of 1p was quite long and close to that of non-substituted oligothiophenes. Figure 3 compares the absorption spectra of P3HT and 1p in solution and in films. As is well known, the absorption maximum of P3HT shows a significant red shift upon film formation (Figure 3 a). The shoulder in region (i) is attributed to the formation of interchain p-stacking, while the peak in region (ii) is assigned to the 0–0 transition of planar P3HT conjugation.[17] Thus, the absorption spectra indicate that the polythiophene backbones of P3HT become planar and are stacked in the film. In stark contrast, 1p has almost identical absorption spectra in solution and in a film (Figure 3 b). Note that, even in the film, the shoulder in region (i) is missing, suggesting that p–p interactions are completely suppressed. However, the 0–0 transition of the planar polythiophene is observed in region (ii). These results clearly indicate that the polythiophene backbones of 1p are coplanar and isolated. Such a structure is inaccessible with conventional unsheathed polythiophenes (see illustrations in Figure 3), and using this unique material, we succeeded, for the first time, in determining the charge carrier mobility along a single planar polythiophene wire: 0.9 cm2 V¢1 s¢1 by the timeresolved microwave conductivity (TRMC) method.[15, 18] Chem. Asian J. 2015, 10, 1820 – 1835

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The structural features of 1p are noteworthy. As discussed above, the dihedral angle of the head-to-head linkage in 11 is fixed as coplanar. On the other hand, the tail-to-tail linkages between the monomers are free to rotate. However, the absence of substituents in the tail-to-tail linkage, akin to non-substituted oligothiophenes, allows a coplanar conformation. In addition, when a tail-to-tail linkage rotates, two phenyl groups

Chengjun Pan was born in Hunan, China, in 1983. He received his PhD degree of Materials Science and Engineering from the University of Tsukuba in 2014 under the supervision of Professors Masayuki Takeuchi and Kazunori Sugiyasu. In the same year, he joined the Organic Materials Group, National Institute for Materials Science as a postdoctoral researcher. His current research interest focuses on the design and creation of functional conjugated polymers with unique optical and electronic properties.

Kazunori Sugiyasu was born in Kagoshima, Japan in 1977. He earned his BS degree in 2000 and PhD degree in 2005 from Kyushu University under the supervision of Professor Seiji Shinkai. He then moved to MIT to work with Professor Timothy M. Swager as a postdoctoral researcher. Since 2008, he is a senior researcher in the Organic Materials Group, National Institute for Materials Science. He is also an Associate Professor of Kyushu University since 2013. His research interests are in the area of functional supramolecular and polymeric materials. Masayuki Takeuchi was born in Kyoto, Japan, in 1966. He earned his BS degree in 1990 and PhD degree in 1994 from Doshisha University under the supervision of Professor Koji Kano. In 1994, he was appointed as an Assistant Professor in the laboratory of Professor Seiji Shinkai, Kyushu University. He also worked with Professor Timothy M. Swager at MIT as a visiting scientist from 1999 to 2000. In 2007, he moved to the National Institute for Materials Science as a Group Leader. His research interests include supramolecular materials, functional conjugated polymers, and molecular machinery. Chunhui Zhao is a PhD student in the Graduate School of Pure and Applied Sciences, University of Tsukuba. He received his MS degree in 2012 from Xiamen University, where he studied polymer chemistry under the supervision of Professor Qinglin Liu. He is currently studying, under the supervision of Professors Masayuki Takeuchi and Kazunori Sugiyasu, the design and synthesis of insulated polythiophenes to investigate the charge carrier conduction mechanisms.

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Figure 1. Typical synthetic approaches to IMWs: (a) polyrotaxane approach,[7a] (b) polymer wrapping approach,[9a] and (c) covalent bond approach.[10]

Figure 2. (a) Synthetic scheme for an insulated polythiophene (1p). (b) X-ray crystal structure of the monomer (11, R = H).[15]

of the side chains sterically interfere. We expect that these characteristic head-to-head and tail-to-tail linkages in 1p dictate the coplanar transoid conformation of the polythiophene backbone (Figure 4 a), thereby achieving a long effective conjugation length. This molecular design is appealing in comparison with the common “insulation” strategy that utilizes bulky substituents and gives rise to a low degree of polymerization and heavily twisted conjugation because of steric hindrance. Another advantage of this molecular design is that the dihedral angle of the head-to-head linkage can be tuned. We synthesized 11(5,6), 11(6,6), 11(6,7), 11(7,7), 11(8,8), and 11(10,10) (Figure 4 b), and found that by increasing the length of the alkylene chain straps, the conformational fixation of the bithiophene frameChem. Asian J. 2015, 10, 1820 – 1835

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work is relaxed.[19] This fine structural tuning would enable control of the resistivity of IMWs, as the charge carrier mobility along the CP chains is a function of the dihedral angle.[20] Furthermore, we also synthesized a 11(6,6) family in which the hexylene straps were replaced with trans-3-hexene, cis-3-hexene, or 3-hexyne; in this case, we found that the electronic characteristics were preserved. The olefins and alkynes in these monomers can be used as a “buckle” to functionalize the surface of the IMW through, for example, epoxidation and 1,3-dipolar cycloaddition without disturbing the coplanar conformation, which we expect to be useful for designing advanced IMWs. As 1p illustrates, IMWs synthesized through the covalent bond approach have a rich designability, which will expand

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Focus Review nonradiative deactivation processes that compete with the fluorescence process are charge separation, intersystem crossing, and other bimolecular processes such as singlet–singlet annihilation. The isolation of CPs can suppress these processes to some extent (as in solution), and IMW strategies have been expected to improve the solid-state fluorescence properties of CPs. Swager and co-workers[22] developed a variety of CPs that have rigid iptycene scaffolds (Figure 5). The X-shaped pentiptycene in 2p prevents the poly(phenylene ethynylene) (PPE)

Figure 3. Absorption spectra of (a) P3HT and (b) 1p in solution (black lines) and in films (red lines).

Figure 5. IMW (2p) designed and synthesized by Swager and co-workers.[23]

Figure 4. Structural characteristics of 1p : (a) head-to-head and tail-to-tail linkages and (b) tuning the dihedral angle of the head-to-head linkage.

the structure–property relationships of CPs and lead to unprecedented polymeric materials. In the following sections, we discuss such IMWs with designer sheaths by categorizing them in terms of their unique properties and functions. 2.1. Photophysical Properties Many CPs are fluorescent, which is an attractive feature for a variety of applications. A milestone in this context was the invention of CP-based OLEDs in 1990,[21] which demonstrated the practical application of CPs in organic electronics. However, CPs show greatly reduced fluorescence quantum yields in the solid state relative to those obtained in dilute solution. The Chem. Asian J. 2015, 10, 1820 – 1835

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backbones from aggregating in the film state, thereby enhancing the fluorescence quantum yield.[23] However, on exposure to trace vapor of electron-deficient aromatics, such as 2,4,6-trinitrotoluene (TNT), the fluorescence was significantly attenuated, based on which a sensing scheme for such explosives was established. The fluorescence quenching was attributed to photoinduced electron transfer from excited PPE to TNT (Figure 5). Interestingly, the sensitivity of 2p was greater than that of unsheathed PPE. The rigid shape-persistent structure of 2p probably gives rise to porosity in the polymeric film, which facilitates diffusion of the analytes. Because excitons can migrate along the CP, the probability of encountering the analyte (i.e., quencher) is greater than that in small molecule-based sensing schemes, and CP-based fluorescence sensors show signal amplification. Inspired by this concept, many relevant sensing systems have been developed.[24] Schlìter and co-workers[25] reported a fully unsaturated allcarbon ladder polymer (3p) obtained through polyaddition based on the Diels–Alder reaction (Figure 6). With the increased solubility owing to the alkylene straps, they successfully obtained highly planar p-conjugated systems, which are intriguing as hypothetical open-chain, polymeric analogues of the belt-region of fullerenes. Scherf and co-workers[26] synthesized a poly(para-phenylene) (PPP) ladder polymer isolated by alkylene straps (4p) (Figure 6). The molecular design aimed at introducing chirality into the CP. In general, CPs are chiral when the conjugated backbone is helically twisted; however, 4p is unique in that the backbone is

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Figure 6. Strapped IMWs reported by Schlìter and co-workers: 3p,[25] Scherf and co-workers: 4p,[26] Swager and co-workers: 5p,[27] Chiavarone and coworkers: 6p,[28] Smith and co-workers: 7p,[30] Osuka and co-workers: 8n and 9n,[32] and Sugiyasu and Takeuchi: 101.[33]

undoubtedly nonhelical (it is a ladder) but has planar chirality. Because 4p is highly fluorescent, even in the solid state, it is a promising candidate that shows high chiroptical activity, such as CPL. Chem. Asian J. 2015, 10, 1820 – 1835

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To reduce interchain interactions, Swager and co-workers[27] also used alkylene straps. 5p is an electron poor isoelectronic analogue of poly(phenylene vinylene) (PPV) (Figure 6). Photoluminescent studies showed that 5p had improved efficiencies compared with the reference CP that lacked the alkylene straps. The electron-poor character of 5p could be useful in CPbased OLEDs with bilayer structures. Chiavarone et al.[28] investigated the photoluminescence properties of a strapped PPV (6p) (Figure 6). Using pulsed excitation with sufficiently high power (above the threshold), they observed luminescence line narrowing. The sharp luminescence was attributed to cooperative emission (also called superfluorescence).[29] Such spectral line narrowing was not observed for PPV with linear alkyl chains, suggesting that isolation is effective for generating cooperative emission. Smith and co-workers[30, 31] reported PPE incorporating oxacyclophane units as canopies (7p) (Figure 6). An interesting aspect of this molecular design stems from the through-space electronic interaction between the aromatic canopy and the PPE backbone, which could be applied for the rational design of materials with intriguing photophysical properties. The authors utilized one of these IMWs for TNT sensing. Osuka and co-workers[32] synthesized meso-meso-linked porphyrin arrays (8n) and triply linked porphyrin tapes (9n) from a doubly strapped porphyrin molecule (Figure 6). Long oligomers, such as 912, have absorption bands that reach into the infrared region, demonstrating that these IMWs have extremely narrow band gaps. In general, one often encounters serious problems in the synthesis of such extended p-conjugated systems in terms of chemical instability and poor solubility; however, the double straps were effective in overcoming these issues. We synthesized a doubly strapped porphyrin-based monomer (101) that has four bithiophene moieties protruding from the porphyrin core (Figure 6).[33] Through electrochemical polymerization, 101 yielded a CP network that was cross-linked by isolated porphyrin molecules on an electrode. Efficient energy transfer occurred from the oligothiophenes to the porphyrin, which we believe will be useful for light-harvesting applications, such as photocurrent generation. This molecular design does not provide molecular wires, but shows the possibility of incorporating isolated p-conjugated systems into 3D network polymers and covalent organic frameworks, which have recently attracted much attention. Terao and co-workers[34, 35] reported an elegant strategy to synthesize defect-free IMWs through the formation of pseudo[1]rotaxanes.[36] First, a permethylated a-cyclodextrin (PM aCD) was functionalized with a short p-conjugated oligomer. This molecule forms a self-inclusion complex (i.e., pseudo[1]rotaxane) in polar media (Figure 7 a,b); thereupon, the p-conjugation was extended through Suzuki–Miyaura coupling to lock the threaded rotaxane structure. This strategy allowed the synthesis of 111 in high yield (80 %). The IMWs (12p) thus obtained are highly soluble in common organic solvents, and interestingly, a concentrated chloroform solution exhibited chiral liquid crystalline phases. This unique mesophase originates from the rigid 1D structure and chirality of the cyclodextrin

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Focus Review zation/depolymerization can be reversibly controlled using UV irradiation and carbon monoxide addition. The authors anticipate that such reversible formation of IMWs could be useful for wiring applications in molecular electronics. We applied the molecular design concept of our insulated polythiophene (1p) to synthesize fluorescent IMWs (Figure 8 a,b).[41] Monomer 151 was efficiently synthesized from 1,4-dibromo2,5-diiodobenzene (> 80 % over 5 steps), and copolymerization with various p-conjugated molecules yielded solid-state emissive IMWs with distinct fluorescence colors (16p–19p, Figure 8 c). Interestingly, we found that these IMWs were highly miscible, allowing them to be blended without phase separation, and obtained a variety of fluorescence colors, including white. Despite the different CP backbones, all the polymers were sheathed by identical cyclic side chains, which we assert is the origin of the miscibility. In the blend, efficient fluorescence resonance energy transfer (FRET) between the IMWs occurred, and the overall fluorescence effi[34–39] (b) X-ray crystal structure Figure 7. (a, c, and d) Cyclodextrin-based IMWs reported by Terao and co-workers. ciency was improved.[41b] of 111. In addition to the photophysisheaths. Upon drying this solution, they obtained a highly fluocal properties, the thermophysical and mechanical properties rescent polymeric film with a fluorescence quantum yield of of these IMWs are noteworthy. Owing to the absence of crys0.23. tallinity, these polymers are thermoplastic and thus processible Based on a similar synthetic strategy, Terao and co-workthrough nano-imprinting methods (Figure 8 d). In addition, ers[37–39] recently explored conjugated metallopolymers (Figsome of these IMWs formed self-standing films; remarkably, ure 7 c,d). Platinum-acetylide-based polymers are known to a small piece of the film of 18p (80 mm thick and 5 mm wide) have high intersystem crossing efficiencies and display phoscould sustain a weight of more than 200 g (> 5 MPa tensile phorescence at room temperature. The basic concept to enstress), was stretchable (> 300 % tensile strain), and even foldahance the phosphorescence of such polymers by insulation ble without the formation of cracks.[41a] These intriguing prop[40] was originally reported by Schanze and co-workers using erties are also attributable to the unique 3D structure of these pentiptycene, and similarly, 13p was designed (Figure 7 c).[38] In IMWs. This study suggested that IMWs, obtained by elaborate 13p, cyclic sheaths restricted conformational fluctuations of the molecular design, have unexplored potential that should be CP backbone, which suppressed the thermal relaxation prouseful for plastic optoelectronics. cesses of the triplet excited state. Furthermore, the sheath not A similar doubly strapped structure was reported by Kobayaonly prevented the CP from aggregating but also protected shi and co-workers[42] for anthracene derivatives. Remarkably, against oxygen quenching. Consequently, 13p showed intense the strapped anthracenes were much more resistant to photophosphorescence, even in the film state under an air atmoschemical reaction than the parent anthracene. phere. The same authors also synthesized IMWs via coordinaAs polythiophenes (PTs) are key materials for the fabrication tion polymerization using a Rh(II) porphyrin complex (Figof organic photovoltaics (OPVs), the determination of the exciture 7 d).[39] The metallopolymer 14p is unique in that polymeried state deactivation mechanism is vital to the fundamental Chem. Asian J. 2015, 10, 1820 – 1835

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Figure 8. (a) Fluorescent IMWs reported by us.[41] (b) Crystallized form of 151. (c) Picture of the films of 16p–19p and their blended films (B1–B4, in which 16p/17p/18p/19p blending ratios are (B1) 100/10/0/0, (B2) 0/0/100/5, (B3) 100/0/0/5, and (B4) 200/0/3/1) taken under UV light: numbers shown under the samples are fluorescence quantum yields. (d) AFM image of the patterned film of 18p.

understating of these devices. Scheblykin and co-workers[43] studied the ultrafast exciton quenching process in a PT film by comparing P3HT with 1p. The excitation dynamics and fluorescence quantum yield were analyzed as a function of excitation power densities over the range from 10¢4 to 100 W cm¢2, which gave insight into the excitation power-dependent and -independent quenching processes individually. Chain insulation was found to prevent static (or ultrafast) fluorescence

quenching, but to have no effect on slow dynamic quenching at time scales longer than 10 ps. Thus, they concluded that static quenching is solely due to chain aggregation, whereas dynamic quenching is a consequence of intrachain processes. Hçger, Lupton, and co-workers[44] reported a fluorescent CP encapsulated by a series of p-conjugated macrocycles (21p) (Figure 9). The fluorescence spectrum of the macrocycle overlaps well with the absorption spectrum of the CP core and thus FRET occurred from the macrocycle (energy donor) to the CP (energy acceptor). Consequently, the photon energy absorbed by the macrocycles was funneled and concentrated to the CP wire. With sufficiently high excitation densities (above the threshold), excitons accumulated along the CP, allowing the observation of exciton–exciton interactions, such as singlet–singlet annihilation, on the CP. They found that the excitation power threshold was chain length-dependent; that is, as the chain length increased, the threshold power decreased. These results demonstrated the mobility of excitons along the CP to interact with one another. This light-harvesting function and the multichromophoric nature of CPs provide a route to accumulating excitation energy within a single molecular wire, which could lead to functional systems that efficiently utilize photon energy. 2.2. Electronic Properties Charge conduction is another important property of CPs, owing to which, CPs have received an increasing amount of attention as functional materials that are applicable to a variety of optoelectronic devices.[1–4] Despite the practical applications developed so far, the mechanism of charge carrier conduction through the polymeric materials is still a subject of great controversy. First, s-dimers, p-dimers, and polaron pairs have been proposed as the charge carrying species, in addition to the simple polaron–bipolaron model. These charge carriers usually coexist at a given doping level but are often indistinguishable, which makes elucidation of the conduction mechanism diffi-

Figure 9. Light-harvesting IMWs reported by Hçger, Lupton, and co-workers.[44] Chem. Asian J. 2015, 10, 1820 – 1835

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Focus Review cult. Second, there are two conceivable charge carrier transport pathways in the polymeric materials, namely, intrawire and interwire processes. Recalling Figure 3 a, chain conformational changes (relevant to the intrawire process) and p-stacking formation (interwire process) are simultaneously affected by self-assembly in conventional CPs. Therefore, in practice, the contributions of these two processes to electrical conductivity cannot be assessed independently. However, using IMWs, one can, in part, address these fundamental issues. Nishinaga, Komatsu, and co-workers[45] designed and synthesized oligothiophenes that were annelated with bicyclo[2.2.2]octene (BCO) units (Figure 10 a). The BCO units not only prevented the oxidized cationic oligothiophenes from forming p-dimers but also stabilized the cationic p-systems owing to electronic effects. They successfully characterized the oxidized (or doped) species by X-ray crystallographic analysis.[45] Based on the bond lengths, the oxidized species 2262 + was unambiguously shown to have quinoidal character, in ac-

cordance with planarization of the cationic oligothiophenes in solution. As such, the insulation strategy is very effective for determining the charge carrying species. In fact, they later showed an unprecedented “bent” p-dimer structure using similar oligothiophenes bearing BCO units only at the termini.[46] In 2003, Aso, Otsubo, and co-workers[47] reported structurally well-defined a-conjugated oligothiophenes (23n) (Figure 10 b). The elegant molecular design, which makes use of a five-membered ring to protect the reactive b-positions, allowed stepwise synthesis of very long oligothiophenes, up to 96-mer. In addition, solubilizing side chains could be substituted with minimal steric hindrance, thus achieving a long effective conjugation length. In 2009, they reported IMWs based on this molecular design where the n-butyl groups in 23n were replaced with bulky tert-butyldiphenylsilyl (TBDPS) groups to insulate the oligothiophene backbone (Figure 10 c).[48] Interestingly, the effective conjugation was preserved despite the bulky side chains. In addition, doped 24n (i.e., 24nC + ) was unable to form p-dimers in solution, even at very low temperatures (223 K), demonstrating the effective insulation. Further doping to the dicationic state (i.e., 24n2 + ) results in either a polaron pair or bipolaron depending on how the cationic charges are delocalized on the oligothiophene backbone. Navarrete, Casado, and coworkers[49] investigated the Raman spectra of 24n2 + . They found that there is a given length at which the stabilization energy gained by thiophene aromatization overcomes the energy required to break a double bond; namely, the polaron pair prevails over the bipolaron in longer oligothiophenes such as 24122 + . Extrapolation from such systematic oligomer studies can give insight into the charge carrying species in PTs. Aso and co-workers[50] reported insulated oligothiophenes with an electron-deficient character (25n). Similar to 24n, 25n consists of a five-membered ring, but has a large electron affinity owing to the carbonyl groups (Figure 10 d). Polymerization of 25n is thus expected to lead to n-type IMWs. In this context, Morisaki, Chujo, and co-workers[51] reported an intriguing molecular design based on benzocarborane (Figure 10 e). The electron-withdrawing character of carborane decreased the HOMO and LUMO levels of the fused bithiophene in 261. Spectroscopic studies showed that oligothiophenes consisting of 261 have well-developed conjugation owing to the coplanar conformation fixed by the benzocarborane moiety. Starting from 11, we synthesized an electrochemically polymerizable monomer (271) (Figure 11 a).[52] Electrochemical polymerization of 271 resulted in an insoluble purple polymeric film on the electrode (27p). By applying a controlled electrochemical potential (i.e., through electrochemical doping), charge carrying species were generated within the isolated Figure 10. Oligothiophene-based IMWs reported by (a) Nishinaga, Komatsu, and co-workpolythiophene wire of 27p, which were simultaneous[45] [48–50] [51] and (e) Morisaki, Chujo, and co-workers. (b) Strucers, (c,d) Aso and co-workers, ly assessed by ESR, Raman, and UV/Vis-NIR spectrosturally well-defined a-conjugated oligothiophene reported by Aso, Otsubo, and co-workers.[47] copies. The spectral data were plotted as a function Chem. Asian J. 2015, 10, 1820 – 1835

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Focus Review the deprotonation/protonation of the pyrrole that accompanied electrochemical doping occurred rapidly and reversibly in 28p. This was attributed to the porous nature of the material, which facilitates diffusion of acids and bases. As such, 28p is analogous to 2p if the charge carriers in 28p are considered as the excitons in 2p ; therefore, this molecular design should lead to sensing materials that show an amplified conductivity response to analytes. Terao, Tada, Seki, and co-workers[54] proposed a design strategy for increasing the charge Figure 11. Electrochemical generation and transformation of charge carriers in IMWs.[52] mobility of CPs and demonstrated the concept using IMWs. They compared their original of the electrochemical potential (or doping level) and clearly PPE-based linear IMW (29n) with its positional isomer, a zigzag showed the transformation of the charge carrying species from IMW (30n) (Figure 13). In the zigzag IMWs, conjugation is sega polaron to a polaron pair, and eventually to a bipolaron. The charge carrier transformations were found to occur at defined doping levels; for example, the transformation of the polaron pair to the bipolaron occurred at a doping level of 30 % (i.e., approximately two charges on a septithiophene (7-mer) unit) (Figure 11 b). These results were in good agreement with previously reported theoretical calculations. Importantly, the comparison of the sheathed PT (27p) with a conventional unsheathed PT suggested that p-stacking strongly affects the generation and stability of the charge carriers. Swager and co-workers[53] reported a pyrrole monomer that was isolated by a phenyl canopy (Figure 12). This molecular

Figure 13. Linear and zigzag IMWs reported by Terao, Tada, Seki, and coworkers.[54]

Figure 12. Canopied polypyrrole reported by Swager and co-workers.[53]

design minimized cross-communication between adjacent polymer chains and also provided free volume in the solid state. Owing to the reduced dimensionality of the charge transport pathways, 28p showed a markedly different conductivity profile than that of the parent polypyrrole; namely, 28p showed a drastic conductor-to-insulator transition, whereas the parent polypyrrole had a sluggish response. Furthermore, Chem. Asian J. 2015, 10, 1820 – 1835

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mented regularly at the meta-joints, which equalizes the distance and molecular orbital levels between hopping sites, resulting in a narrow density of states (DOS). In contrast, the DOS in linear IMWs is widely distributed because the orbital energies are not well defined owing to random structural deformations along the linear wire. In general, faster charge transfer occurs when the energy difference between the hopping sites is smaller. Accordingly, they hypothesized that the zigzag IMWs should have higher charge carrier mobility. In fact, the intrawire charge mobilities of the linear (29n) and zigzag IMWs (30n) were determined to be 0.75 and 2.1 cm2 V¢1 s¢1, respectively, using TRMC measurements, and theoretical calculations agreed reasonably with the experimental results. Based on this result, they increased the length of the oligo(phenylene

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Focus Review ethynylene) segments in 30n and achieved a very high charge mobility of 8.5 cm2 V¢1 s¢1. Considering that the boundary between the hopping and band transport mechanisms occurs when the mobility is in the range of 1– 10 cm2 V¢1 s¢1, the value achieved using this concept is near the highest limit for charge carriers transported via an inter-site hopping processes.[54] 2.3. Single-Molecule Conductance

Figure 14. IMWs reported by Tanaka and Yamashita,[57] which were investigated using the STM break junction method by Tada and co-workers.[56]

Owing to their structural resemblance with electric power cords, IMWs are expected to function as such to wire up molecular switches, memory, etc., in molecular electronics. To this end, single-molecule conductance should be assessed, and some research groups have investigated IMWs by the STM break junction method. It should be noted that, as demonstrated by Mayor, Calame, and co-workers,[55] intermolecular interactions are not negligible, even in the “molecular junction”, and the IMW strategy ensures single-molecule measurements. If the charge transport is based on a tunneling process, the resistance of a metal/single-molecule/metal junction (R) is expressed as R ¼ R0 expðblÞ where R0 is the contact resistance, b is the pre-exponential factor, called a decay constant, representing the efficiency of charge transmission through the molecule, and l is the molecular length. The coherent tunneling mechanism is predominant when the molecular length is short, but for longer molecular wires, the thermally activated hopping mechanism contributes to the charge transport. In the former mechanism, R shows an exponential dependence on the molecular length, as described by the above equation, whereas the latter mechanism shows a linear dependence. Tada and co-workers[56] investigated insulated oligothiophenes (31n), which were synthesized by Tanaka and Yamashita in 1999,[57] by the STM break junction method (Figure 14). In this study, they systematically compared insulated oligothiophenes consisting of 5, 8, 11, 14, 17, 20, and 23 repeating units and discussed the mechanism of charge transport in terms of molecular length and temperature. When the logarithm of the resistance (R) was plotted as a function of the number of thiophene units (or molecular length, l), the slope of the plot (i.e., the b value) changed from 0.16 A¢1 for the shorter oligothiophenes to 0.05 A¢1 for the longer ones. The change in the b value was rationalized by a change in the conduction mechanism from the tunneling mechanism to the hopping mechanism. Based on this length dependence, they concluded that Chem. Asian J. 2015, 10, 1820 – 1835

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the conduction mechanism changes at a molecular length of approximately 5.6 nm (3114)–6.8 nm (3117).[56a,b] They further investigated the temperature dependence of the conductance for 315, 3114, and 3117 in the temperature range from 100 to 470 K, motivated by the fact that the tunneling mechanism is temperature-independent, whilst the hopping mechanism is temperature-dependent.[56c] As expected, the conductance of the shortest oligothiophene, 315, was temperature-independent (tunneling mechanism); on the other hand, that of the longer oligothiophene, 3117, showed an Arrhenius-type temperature dependence (hopping mechanism). These results are consistent with the aforementioned relationship between the resistance (reciprocal of the conductance) and molecular length. Interestingly, 3114 showed a crossover between the two mechanisms in the Arrhenius plot with the tunneling and hopping mechanisms observed in the lower and higher temperature regimes, respectively. Based on this observation, the authors suggested the possibility of using temperature to control the charge transport mechanism in a single molecular wire. This in-depth investigation was of significance not only to realize molecular electronics but also to elucidate the conduction mechanisms of CPs. Tada and co-workers also investigated the insulated oligothiophenes (33n), which were designed by Aso and co-workers (Figure 15 a),[58] by the STM break junction technique. Like 24n, 33n is also based on a five-membered ring, but was synthesized using a different approach: zirconocenemediated transformation of a diyne into a thiophene ring. Thiol groups were introduced as anchoring points for the metal electrodes. The b value obtained for 332, 334, and 336 was 0.19 A¢1, which is in agreement with the value predicted for unsubstituted oligothiophenes using theoretical calculations (0.21 A¢1). Most molecular junctions show fluctuating conductance values originating from molecular motion, which is a drawback for applications in molecular electronics in terms of the warranty. Kiguchi, Tada, Terao, and co-workers[59] reported an interesting system that may address this issue (Figure 15 b). In 34, the cyclodextrin sheath not only isolates the conjugated back-

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Figure 15. IMWs studied using the STM break junction method by (a) Tada, Aso, and co-workers,[58] (b) Kiguchi, Tada, Terao, and co-workers,[59] and (c) Kiguchi, Nakamura, Sugiyasu, and co-workers.[60]

bone and eliminates the influence of intermolecular interactions but also fixes the backbone conformation. As a result, 34 showed stipulated electric conductance (a sharper conductance histogram in comparison with the unsheathed molecular wire, 35) in the STM break junction. In contrast to the above system, molecular junctions that show multiple conductance states have attracted much attention as molecular resistive switches. Recently, Kiguchi and Nakamura together with us[60] investigated an insulated quaterthiophene (36) by the STM break junction technique (Figure 15 c). The thiophene moieties in 36 are expected to act as the anchoring points for the metal electrodes. As 36 has two anchors at each end of the IMW, it can connect the electrodes in three different modes, i.e., T1-T1, T1-T2, and T2-T2 connections. In fact, 36 showed three-step conductance switching with conductance values of 0.05 (high-conductance state), 0.005 (medium-conductance state), and 0.0005 G0 (low-conductance state) upon pulling the two electrodes apart (Figure 15 c). Conductance switching was also achieved by repeated modulation of the distance between the two electrodes, with a displacement of 0.17 nm. Theoretical calculations suggested that by changing the contact points, the effective molecular wire length increases from bithiophene to terthiophene, up to quaterthiophene, and accordingly, the length dependence of the conductance can be observed with a reasonable b value (0.31 A¢1 by calculation). These results demonstrated a distinct strategy to dynamically switch the single molecular conductance via contact engineering. 2.4. Device Applications Given the unique optoelectronic properties of IMWs described so far, one can expect IMWs to contribute to the development of organic electronics. More than a decade ago, Cacialli, AnderChem. Asian J. 2015, 10, 1820 – 1835

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son, and co-workers[8a,b] made use of their polyrotaxane-based IMWs in OLEDs (Figure 16). Even without optimization of the device structure, the external quantum yield was doubled by formation of the rotaxane structure. Since then, they have further developed IMW-based OLEDs, to obtain white emission,[8h] amplified spontaneous emission, etc.[8f,i] These studies are an encouraging demonstration of the use of IMWs in practical applications. Tamao and co-workers[61–63] reported p-conjugated oligomers substituted with 1,1,3,3,5,5,7,7octa-R-substituted s-hydrindacen-4-yl groups, abbreviated as “Rind” groups, in which “R’’ can

Figure 16. IMW-based OLED reported by Cacialli, Anderson, and co-workers.[8a,b] (with permission; Copyright Nature Publishing Group)

be methyl, ethyl, etc., and these groups are accordingly called Mind, Eind, etc. Such sterically demanding groups can protect and kinetically stabilize reactive p-conjugated systems consisting of heavy main group elements. For example, they succeeded in synthesizing disilene (Si = Si) analogues of oligo(p-phenylenevinylene) (372) (Figure 17 a).[61a] Interestingly, the bulky side chains interlock with one another above and below the Si = Si moiety, which enforces coplanar geometry, thereby achieving well-developed conjugation. With 372, strong fluorescence from disilene derivatives at room temperature was observed for the first time. These unique photophysical properties were attributed to efficient p-delocalization over the skeleton consisting of the heavier main group elements. One of the disilene-based p-conjugated molecules was used to fabricate an OLED device.[63] Eind-protected disilene derivative 38[62] as an

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Focus Review ture of 39 resembles the oligothiophene-based dye known as MK-2,[65] but as discussed above, the bithiophene backbone of 11 is isolated and planar. Accordingly, 39 has a large molar absorption coefficient, which is advantageous for efficiently harvesting of photon energy. In addition, the insulation prevents the dyes from aggregating on the electrode and protects them from the acceptors (I3¢ ions). As a result, charge recombination was effectively suppressed, and dye 39 exhibited a good power conversion efficiency of 9.2 % under 100 mW cm¢2 simulated AM1.5 sunlight. Swager, Bulovic´, and co-workers[66] developed a sensing device that detects trace vapors of explosives such as TNT based on the lasing action of a highly fluorescent IMW film of 40p (Figure 19 a). They demonstrated that sensitivity enhance-

Figure 17. (a) Disilene-based p-conjugated molecules stabilized by isolation and (b) their application in OLEDs, reported by Tamao and co-workers.[61a, 63]

light-emitting material was doped in the host polyfluorene (PFO) matrix (Figure 17 b). The emitting layer was sandwiched with another functional layer between a transparent ITO (indium tin oxide) anode and metallic cathode (LiF/Al). Although the total performance was far from that required for practical applications, this first demonstration of electroluminescence from a disilene derivative showed the potential of functional ”elemento-organic“ materials and devices. Han and co-workers[64] used the sheathed bithiophene designed by us (11) as a component of a donor–acceptor conjugated dye for dye-sensitized solar cells (Figure 18). The struc-

Figure 18. Isolated donor–acceptor dyes (39) reported by Han and co-workers.[64] Chem. Asian J. 2015, 10, 1820 – 1835

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Figure 19. Highly sensitive TNT sensor based on the lasing action of IMWs reported by Swager, Bulovic´ and co-workers.[66] (Adapted from Ref. [66] with permission; Copyright Nature Publishing Group)

ment is most pronounced when the film was pumped at intensities near the lasing threshold (Figure 19 b,c). Consideration based on the standard four-level laser model suggested that the pump threshold energy increases in the presence of TNT and that the chemical sensitivity is determined by the excitation power. CPs already lend themselves to signal amplification owing to their collective properties, as discussed above for 2p, yet this combined lasing action can further enhance the sensitivity. Given the mechanism, incorporation of IMWs into high-Q optical feedback structures is expected to lead to reduction of the pump threshold energy, as well as unparalleled sensitivity. 1832

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Figure 20. (a) Synthesis of a diblock copolymer comprising unsheathed and sheathed polythiophenes, P3HT-b-P3FT (41p). (b) XRD and (c) AFM image of its microphase separated structure.[67]

3. Conclusions and Perspectives The most fascinating feature of organic materials is their high designability at the molecular level. In the case of CPs, the electronic structures, such as the HOMO–LUMO levels, have conventionally been designed by introducing electron-donating and -accepting groups on the polymer backbones. However, as interchain electronic interactions have been found to play a crucial role, it is now widely accepted that designing the molecular structure (e.g., regioregularity) and controlling the p-stacking are equally important. As an approach to this end, we have discussed the recent developments of IMWs. Through “insulation”, CPs are endowed with unique photophysical, electronic, mechanical, and thermophysical properties that clearly contrast with those of the conventional unsheathed CPs. The application of IMWs is still its infancy, yet one can expect unprecedented organic devices to be achieved using these unique materials. Undoubtedly, conventional unsheathed CPs are also useful materials owing to their charge transport ability, as illustrated by many recent studies related to FETs, OLEDs, and OPVs. Reflecting on the considerable developments in CP-based organic electronics, as well as the intriguing properties of IMWs, it occurred to us that combinations of unsheathed and sheathed CPs should lead to novel functional materials. With this simple idea in mind, we have recently designed a primitive system based on a diblock copolymer comprising unsheathed and sheathed polythiophenes (Figure 20).[67] The sheathed polythiophene, named picket-fence polythiophene (P3FT), has terphenyl picket side chains protruding from the polymer backbone in a regioregular manner. The block copolymer (41p) was synthesized through catalyst-transfer polycondensation (CTP) method;[68] namely, a quasi-living growth of a P3HT block was followed by a block extension of the picket-fence polythiophene (Figure 20 a). The unsheathed P3HT block self-assembled through p-stacking into a crystalline structure, whereas the sheathed P3FT block was amorphous (Figure 20 b). Accordingly, these two blocks underwent phase separation, and we succeeded for the first time in creating a microphase separation comprising an ensemble of stacked and isolated polythioChem. Asian J. 2015, 10, 1820 – 1835

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phenes in a single polymeric film (Figure 20 c: brighter and darker domains are attributed to P3HT and P3FT, respectively). Such sophisticated control over p-stacking will extend the use of these materials to various unexplored applications owing to the synergy of the contrasting properties of the two blocks. It should be emphasized that the CTP method, which was essential to realize the above P3HT-b-P3FT (41p) system, has just recently evolved to allow CP-based block copolymer chemistry.[69] Likewise, many other synthetic methodologies under development should also enable the synthesis of new types of IMWs that are otherwise unavailable. In the meantime, inspirational ideas developed in supramolecular chemistry can be incorporated into the molecular design principle. As illustrated in Figure 1, IMWs have indeed been developed in the interdisciplinary field of polymer chemistry and supramolecular chemistry. In this context, we believe that “p-system figuration”[70]—proposed as an approach to create superb p-electronic materials as well as discover new phenomena and functions through elaborating the beautility of molecules—opens a new door in conjugated polymer chemistry.

Acknowledgements The authors thank KAKENHI grants from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan (No. 23655108, No. 25620101, No. 26102009), Shorai Foundation for Science and Technology, and The Association for the Progress of New Chemistry. Keywords: conjugated polymers · fluorescent materials · molecular wires · organic electronics · p-stacking

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Manuscript received: May 1, 2015 Final Article published: July 14, 2015

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