Registered charity number: 207890

www.rsc.org/polymers

As featured in:

See Yohei Yamamoto et al., Polym. Chem., 2014, 5, 3583.

Showcasting the joint research from the labs of Yohei

Yamamoto, Junpei Kuwabara and Takaki Kanbara at University

of Tsukuba, Noriyuki Ishii at National Institute of Advanced

Industrial Science and Technology (AIST), Akinori Saeki and

Shu Seki at Osaka University and Seiichi Furumi at National

Institute for Materials Science (NIMS).

Title: Tetramethylbithiophene in π-conjugated alternating

copolymers as an eff ective structural component for the

formation of spherical assemblies

π-Conjugated alternating copolymers containing a

tetramethylbithiophene unit show a strong tendency to form

well-defi ned, sub- to several-micrometer-sized spheres,

which display extremely long photocarrier lifetimes up to

the millisecond-order.

PolymerChemistry

PAPER

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View Article OnlineView Journal | View Issue

aDivision of Materials Science and Tsukub

Materials Science (TIMS), Faculty of Pu

Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibarak

ims.tsukuba.ac.jp; Fax: +81 29 853 4490; TbBiomedical Research Institute, National Ins

Technology (AIST), Tsukuba Central-6, 1-1

JapancDepartment of Applied Chemistry, Graduate

2-1 Yamadaoka, Suita, Osaka 565-0871, JapdApplied Photonic Materials Group, Nationa

2-1, Sengen, Tsukuba, Ibaraki 305-0047, Jap

† Electronic supplementary informationFP-TRMC proles. See DOI: 10.1039/c4py0

Cite this: Polym. Chem., 2014, 5, 3583

Received 8th January 2014Accepted 16th February 2014

DOI: 10.1039/c4py00023d

www.rsc.org/polymers

This journal is © The Royal Society of C

Tetramethylbithiophene in p-conjugatedalternating copolymers as an effective structuralcomponent for the formation of sphericalassemblies†

Liang Tong,a Soh Kushida,a Junpei Kuwabara,a Takaki Kanbara,a Noriyuki Ishii,b

Akinori Saeki,c Shu Seki,c Seiichi Furumid and Yohei Yamamoto*a

p-Conjugated alternating copolymers containing a tetramethylbithiophene unit show a strong tendency to

form well-defined, sub- to several-micrometer-sized spheres. The twisted bithiophene unit inhibits

interchain stacking and anisotropic crystal growth of these copolymers, leading to the formation of

structurally isotropic spheres by means of a slow diffusion of a nonsolvent into a solution of the

copolymers. These micrometer-sized spheres display extremely long photocarrier lifetimes (�10�3 s) in

comparison with cast films from the solutions of the polymers and those of the irregular aggregates

(<10�6 s).

Introduction

Polymer colloids are utilized in various applications such ascatalyst support,1–4 drug and gene delivery,5–7 biosensors,8,9 gassensors,10,11 and so forth. In particular, colloids composed ofp-conjugated polymers are highly valuable for optical applica-tions such as uorescence imaging and cellular tracking.12,13

However, p-conjugated polymers are generally difficult toassemble into well-dened spheres because of their rigid andplanar backbones, thus limiting the number of reportedexamples of p-conjugated polymer spheres.14–16 Hence, thedevelopment of a general methodology for the quantitativeformation of colloidal particles from p-conjugated polymerswill further advance colloid science toward the application ofnot only single particles but also colloid assemblies in polymercolloid photonic crystals.17–19

In this context, we have recently reported that severalp-conjugated alternating copolymers having uorene andthiophene repeating units tend to form well-dened spheres

a Research Center for Interdisciplinary

re and Applied Sciences, University of

i 305-8573, Japan. E-mail: yamamoto@

el: +81 29 853 5030

titute of Advanced Industrial Science and

-1 Higashi, Tsukuba, Ibaraki 305-8566,

School of Engineering, Osaka University,

an

l Institute for Materials Science (NIMS), 1-

an

(ESI) available: SEM, PL spectra, and0023d

hemistry 2014

under certain self-assembling conditions.20 The yielded spheresexhibit extremely long photocarrier lifetimes, possibly becauseof the suppression of charge recombination. In this Article, wecomprehensively study structural factors related to the forma-tion of spherical assemblies from p-conjugated alternatingcopolymers, and show that the use of tetramethylbithiophene(TMT2) as one of their repeating units results in a strongtendency to form well-dened spheres that are sub- to several-micrometers in diameter. The steric hindrance due to fourmethyl groups on the bithiophene unit markedly lowers theplanarity of the polymer backbone, inhibiting the interchainstacking of the polymers and thus leading to the formation ofstructurally isotropic spheres. This nding provides a newmolecular design strategy toward the realization of photo- andelectroluminescent polymer colloids.

Results and discussionSelf-assembly of alternating copolymers

We previously showed that an alternating copolymer F8T2comprising dioctyluorene (F8) and bithiophene (T2) repeatingunits (Fig. 1) yielded irregular aggregates by the slow diffusionof a nonsolvent such as MeOH into a solution of the polymers.20

Conversely, F8TMT2 (ref. 21) containing F8 and TMT2 repeatingunits (Fig. 1) quantitatively formed spheres under identical self-assembling conditions.20 Herein, we investigate the self-assembling behaviors of additional ve alternating copolymers,DOPTMT2, PTTMT2, 3,6-CTMT2, 2,7-CTMT2, and DPPTMT2,22

in which all polymers possess the TMT2 unit as one of theirrepeating units, and a different arylene unit as its counterpart:para-dioctylphenylene (DOP), phenothiazine (PT), 3,6-carbazole

Polym. Chem., 2014, 5, 3583–3587 | 3583

Fig. 1 Molecular structures of seven p-conjugated alternatingcopolymers.

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(3,6-C), 2,7-carbazole (2,7-C), and diketopyrrolopyrrole (DPP),respectively. For self-assembly, a 5 mL vial containing a CHCl3,CH2Cl2, or THF solution of the polymers (1 mg mL�1, a totalamount of 2 mL) was placed in a 50mL vial containing 5mL of anonsolvent such as acetone or MeOH. The outer vial was cappedand kept in the dark at a constant temperature of 25 �C. Thenonsolvent vapor gradually diffused into the solution of thepolymers to give a suspension aer 3 days (Fig. S1†).

Fig. 2a–e show the scanning electron microscopy (SEM)images of the air-dried suspension of the polymers with asolvent combination of CHCl3 (good solvent) and MeOH

Fig. 2 (a–e) SEM micrographs of air-dried CHCl3/MeOH suspensionsof the spherical assemblies of DOPTMT2 (a), PTTMT2 (b), 3,6-CTMT2(c), 2,7-CTMT2 (d), and DPPTMT2 (e). Insets show histograms of thespheres' diameter. (f) TEM micrograph of an air-dried CH2Cl2/acetonegel of DPPTMT2. Inset shows an optical micrograph of the gel.

3584 | Polym. Chem., 2014, 5, 3583–3587

(nonsolvent). For DOPTMT2, PTTMT2, 3,6-CTMT2, and 2,7-CTMT2, well-dened spheres were observed with diameters (d)ranging from 0.2 to 8.0 mm (Fig. 2a–d). The average diameter(dav) and standard deviation (s) values were dependent on thesolubility of the polymers and the solvent combination. Table 1shows the morphologies of the precipitates afforded fromvarious solutions of the copolymers upon diffusion of theMeOH vapor. Spheres were formed from the CHCl3 and THFsolutions of the four aforementioned copolymers, while CH2Cl2solutions did not always afford spheres (Fig. S2–S5†). This ispossibly because the precipitation takes place too rapidly.However, the s values were rather small for rapid precipitationfrom CH2Cl2 solution (Table 1). The dav and s values of theobtained spheres are possibly determined by factors such as themixing rate of good solvent and nonsolvent, and the solubilityof the polymers.20 In addition, slow diffusion of the polar non-solvent is required for spherical assembly, because the hydro-phobic polymers tend to assemble while minimizing thecontact area with the polar nonsolvents.20

In contrast, DPPTMT2 showed a different self-assemblybehavior. For the CHCl3/MeOH solvent combination, sphericalassemblies were obtained with an average diameter (dav) of11.1 mm, which is apparently larger than that formed from othercopolymers (Fig. 2e). Furthermore, the surfaces of the sphereswere not as smooth as those of the other spheres, appearing tobe aggregates of bers (Fig. S6†). In fact, when acetone vaporwas diffused into a CH2Cl2 solution of DPPTMT2, a gel formed(Fig. 2f, inset). Transmission electron microscopy (TEM) of thegel, diluted using acetone and ultrasonicated for a short period,displayed thin nanobers with a width of 10–20 nm (Fig. 2f).Accordingly, the spheres formed from DPPTMT2 likely consistof aggregates of thin nanobers.

Structural characterization of the spheres

X-ray diffraction (XRD) studies of the self-assembled precipi-tates demonstrate a clear relationship between theirmorphology and crystallinity. The copolymers of irregularlyaggregated F8T2 and nanobrous assembly of DPPTMT2exhibited diffraction peaks at 2q � 7� (d � 12 A), which isattributed to the periodicity of the lateral arrays of the main

Table 1 Morphologies of the self-assembled precipitates formed fromp-conjugated alternating copolymers by a slow diffusion of MeOHvapor into various solutions.a Numerical values in parentheses indicatethe average diameter (dav) and standard deviation (s) of the spheres

CHCl3 (dav, s/mm) THF (dav, s/mm) CH2Cl2 (dav, s/mm)

F8T2b � � —F8TMT2b ◎ (2.4, 1.2) ◎ (1.9, 0.7) ◎ (2.7, 0.4)DOPTMT2 ◎ (3.7, 0.8) ◎ (1.8, 0.7) �PTTMT2 ◎ (1.3, 0.4) O (1.9, 0.6) ◎ (1.9, 0.5)3,6-CTMT2 ◎ (3.2, 0.8) ◎ (2.4, 1.0) �2,7-CTMT2 ◎ (4.3, 1.1) ◎ (1.7, 0.7) �DPPTMT2 O (11.1, 1.1) — �a ◎, well-dened spheres; O, distorted or fused spheres; �, irregularaggregates; —, low solubility. b Ref. 20.

This journal is © The Royal Society of Chemistry 2014

Fig. 3 (a) Powder XRD patterns of thin films of ill-aggregated F8T2,spherical assemblies of F8TMT2,DOPTMT2, PTTMT2, 3,6-CTMT2, and2,7-CTMT2, and self-assembled nanofibers of DPPTMT2. (b) DlPLmax

of the copolymers between solution and self-assembled spheres oraggregates.

Fig. 4 Calculated molecular configurations of F8T2 (a), F8TMT2 (b)and DPPTMT2 (c) by DFT calculations. Dihedral angles between theneighboring rings are drawn.

Fig. 5 (a–c) FP-TRMC profiles of thin films of DOPTMT2 (a), PTTMT2(b) and DPPTMT2 (c) cast from CHCl3 solution (black) and suspensionsof self-assembled spheres or aggregates (red). The green and bluearrows indicate s1/2 of thin films cast from solution and suspensions ofself-assembled spheres or aggregates, respectively. The s1/2 value ofthe self-assembled spheres of PTTMT2 was determined by a curvefitting of a single-exponential decay of the TRMC signal (blue curve inb). (d) Bar chart of the photocarrier lifetimes (s1/2) of thin films cast from

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chain of the polymers (Fig. 3a). In contrast, the other vecopolymers with spherical morphologies did not show anyparticular diffraction peaks in the whole 2q region (Fig. 3a),indicating that the spheres are composed of amorphousassembly of the copolymers.

Such clear differences were also observed in the photo-luminescence (PL) spectra, where a signicant red-shi wasobserved for suspensions of the self-assembled aggregates ofF8T2 andDPPTMT2 in comparison with their solutions with thePL peak shi, DlPLmax, of 46 and 57 nm, respectively (Fig. 3band S7†). Meanwhile, the DlPLmax values between the solutionand suspension of the spheres for the other ve copolymerswere comparatively small within the range of 5–21 nm (Fig. 3band S7†), implying that interchain p-electronic interactions arevery small in their spherical morphologies. These XRD and PLresults indicate that copolymers with low crystallinity tend toform spheres, while those having a high crystallinity rarely formspheres.

According to the density functional theory (DFT) calcu-lations,‡ the dihedral angles between the neighboring p-planesin F8T2 are less than 20� (Fig. 4a), but those in F8TMT2 areapparently larger (44–67�, Fig. 4b). Such a large torsion in theTMT2 unit possibly inhibits the interchain stacking andanisotropic crystal growth of the polymers, leading to theformation of amorphous spheres with an isotropic geometry.For the other alternating copolymers that form spheres, asimilar twisting of the TMT2 unit likely occurs. The onlyexception is the donor (D)–acceptor (A) copolymer DPPTMT2,where a strong interchain D–A interaction possibly takesplace,23,24 resulting in a nanobrous assembly of polymers, eventhough the polymer main chain involves a highly twisted TMT2unit (Fig. 4c). In addition, two phenylene groups attached onboth sides of the TMT2 group cause better planarity of the mainchain of DPPTMT2, which might help interchain stacking of thepolymer.

‡ Molecular geometries were optimized by B3LYP/6-31G level DFT calculationswith Gaussian09 (see ESI, ref. S2).

This journal is © The Royal Society of Chemistry 2014

Photocarrier lifetime studies

The photocarrier half-lifetime (s1/2) was greatly enhanced whenthe polymers form spheres. Fig. 5a–c show ash-photolysistime-resolved microwave conductivity (FP-TRMC)25,26 proles ofDOPTMT2, PTTMT2, and DPPTMT2 for cast lms from theirsolutions (black) and self-assembled aggregates (red). Uponlaser ash (lex ¼ 355 nm), rise and decay proles of a TRMCsignal, given by fSm, are observed, where f and Sm represent

CHCl3 solution (blue) and suspensions of self-assembled spheres oraggregates (red). The s1/2 values are defined by the time when the FP-TRMC transient decays down to a half of its maximum value. The dataof F8T2 and F8TMT2 are referred from ref. 20.

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photocarrier generation yield and sum of the mobilities ofgenerated charge carriers, respectively. It is clear that the decayof the TRMC signals for spherically assembled copolymers isquite slow in comparison with that of the lms from theirsolutions or irregular aggregates. For example, s1/2 of thespherically assembled DOPTMT2 is 280 ms, which is more than103 times longer than that of the solution-cast lm ofDOPTMT2(�0.1 ms, Fig. 5a). The degree of the enhancement of s1/2 ismuch greater for PTTMT2 (Fig. 5b), where the ratio of s1/2 for thelms of the spheres to that for the solution-cast lm reaches2.3� 104 (Table S1†). In contrast,DPPTMT2 does not show suchmarked enhancement of s1/2 between the thin lm from itssolution and that of nanobrous assembly (Fig. 5c). The otherpolymers show a similar tendency that the spherical assemblyexhibits at least 15 times longer s1/2 values than the solution-cast lms, while the irregularly aggregated polymers showsimilar s1/2 values to the corresponding solution-cast lms(Fig. S8 and Table S1†).

The elongation of s1/2 found in the spheres is likely due tothe suppression of bulk charge recombination, facilitated by theisolation of each sphere. In fact, spheres with the smallestdiameters, formed from PTTMT2, afford the largest s1/2 value.The investigation of the detailed elementary steps is ongoing inorder to know why the spherical geometry enhanced the pho-tocarrier lifetime so markedly.

Noteworthy is that the PL quantum yield (fPL) is slightlyenhanced by the spherical formation. Table S2† lists the abso-lute fPL values of the copolymers in the solid state. The spher-ically assembled samples displayed as well or better fPL incomparison with cast lms from their CHCl3 solutions. On theother hand, irregularly assembled samples (F8T2 andDPPTMT2) showed worse fPL than cast lms from their CHCl3solutions.

Conclusion

We have shown that tetramethylbithiophene is a powerfulstructural component in the formation of spherical structures,yielding a useful tool for developing strategies for convertingp-conjugated polymers into colloidal assemblies. Most copoly-mers having a tetramethylbithiophene unit form well-denedmicrospheres quantitatively, except for the case of donor–acceptor copolymer. By the formation of spherical geometry, thephotocarrier lifetime is extremely elongated, which will bebenecial for utilizing these colloids as optical and optoelec-tronic materials. The construction of colloidal crystals fromp-conjugated polymers will realize new optoelectronic devicessuch as light-emitting photonic crystals.27

Experimental sectionMaterials

Unless otherwise noted, reagents and solvents were used asreceived from Aldrich Chemical Co. Ltd and Nakarai TesqueCo., respectively. Alternating copolymers, poly[(9,9-dioctyl-uorenyl-2,7-diyl)-alt-(bithiophene-2,50-diyl)] (F8T2, number-averaged molecular weight (Mn) ¼ 26 600, polydispersion index

3586 | Polym. Chem., 2014, 5, 3583–3587

(Mw/Mn)¼ 2.66), poly[(9,9-dioctyluorenyl-2,7-diyl)-alt-(3,30,4,40-tetramethylbithiophene-2,50-diyl)] (F8TMT2, Mn ¼ 31 800,Mw/Mn ¼ 2.46), poly[(1,4-dioctylphenyl-2,5-diyl)-alt-(3,30,4,40-tetramethylbithiophene-2,50-diyl)] (DOPTMT2, Mn ¼ 15 000,Mw/Mn ¼ 1.69), poly[(N-(2-ethylhexyl)phenothiazine-3,7-diyl)-alt-(3,30,4,40-tetramethylbithiophene-2,50-diyl)] (PTTMT2, Mn ¼21 000, Mw/Mn ¼ 2.82), poly[(N-octadecylcarbazol-3,6-diyl)-alt-(3,30,4,40-tetramethylbithiophene-2,50-diyl)] (3,6-CTMT2, Mn ¼11 700, Mw/Mn ¼ 1.93), poly[(N-octadecylcarbazol-2,7-diyl)-alt-(3,30,4,40-tetramethylbithiophene-2,50-diyl)] (2,7-CTMT2, Mn ¼26 000, Mw/Mn ¼ 2.76), and poly[(2,5-bis(2-ethylhexyl)-3,6-bis(phenyl)pyrrolo[3,4-c]pyrrole-1,4-dione-40,40'-diyl)-alt-(3,30,4,40-tetramethylbithiophene-2,50-diyl)] (DPPTMT2, Mn ¼18 100, Mw/Mn ¼ 2.35), were synthesized according to thereported procedures.20–22

Measurements

SEM microscopies were performed at 25 �C on a JEOL modelJSM-5610 scanning electron microscope operating at 20 kV.Silicon was used as a substrate and 5 nm of Au was depositedonto the cast lms from the suspension of the polymers. TEMmicroscopy was recorded on a Philips model Tecnai F20 elec-tron microscope operating at 120 kV. Sample dispersions wereapplied onto a specimen grid covered with a thin carbonsupport lm, which had been hydrophilized by a JEOL modelHDT-400 ion bombardment device. Images were recorded on aGatan slow scan CCD camera (Retractable Multiscan Camera)under low dose conditions. Powder X-ray diffraction patternswere recorded at 298 K on a Rigaku model Miniex600 X-raydiffractometer with a CuKa radiation source (40 kV and 15 mA).Photoluminescence spectra were measured at 25 �C with aJASCO model FP-6200 spectrouorometer. Molecular congu-rations were optimized at the B3LYP/6-31G level DFT calcula-tions with the Gaussian09 program.‡ FP-TRMC measurementswere carried out at 25 �C in air, where the resonant frequencyand microwave power were set at �9.1 GHz and 10 mW,respectively, so that the electric eld of the microwave was smallenough not to disturb the thermal motion of charge carriers.28

The charge carriers were photochemically generated using thethird-harmonic generation (THG, 355 nm) light pulses from aSpectra Physics model INDI Nd:YAG laser (5–8 ns pulse dura-tion) with an incident photon density of 9.1 � 1015 photonscm�2. The TRMC signals, picked up by a diode (rise time <1 ns),were monitored by a Tektronics model TDS3052B digital oscil-loscope. The observed conductivities were converted intonormalized values, given by a photocarrier generation yield (f)multiplied by sum of the charge carrier mobilities (Sm),according to an equation fSm¼ (1/eAI0Flight)(DPr/Pr), where e, A,I0, Flight, Pr and DPr denote unit charge of a single electron,sensitivity factor (S�1 cm), incident photon density of excitationlaser (photon per cm2), lling factor (cm�1) and reectedmicrowave power and its change, respectively. The sample lmsfor FP-TRMCmeasurements were prepared by dropcasting ontoquartz plates of the precipitates prepared by the vapor diffusionmethod or CHCl3 solutions. Absolute PL quantum yields weremeasured with a Hamamatsu model C9920-02G absolute PL

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quantum yield measurement system equipped with a Hama-matsu model C10027-01 photonic multichannel analyzer.

Conflict of interest

The authors declare no competing nancial interest.

Acknowledgements

This work was partly supported by a Grant-in-Aid for YoungScientists A (25708020) from MEXT, Japan, the IndustrialTechnology Research Grant Program (2011, NEDO, Japan),Cooperative Research Program of “Network Joint ResearchCenter for Materials and Devices”, Asahi Glass Foundation, andTokuyama Science Foundation.

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