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Synthetic Metals 157 (2007) 318–322

Strong stacking behavior and large third-order nonlinear opticalsusceptibility χ(3) of head-to-head-type

poly(3-alkynylthiophene-2,5-diyl), HH-P3(C CR)Th

Takao Sato a, Hideo Kishida b, Arao Nakamura b, Takashi Fukuda c, Takakazu Yamamoto a,∗a Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan

b Department of Applied Physics, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japanc National Institute of Advanced Industrial Science and Technology (AIST), Central 5, Higashi 1-1-1, Tsukuba, Ibaraki 305-8565, Japan

Received 25 January 2007; received in revised form 16 March 2007; accepted 5 April 2007Available online 15 May 2007

bstract

The head-to-head-type poly(3-alkynylthiophene-2,5-diyl) HH-P3(C CR)Th has been shown to exhibit a strong tendency to self-assemble.he UV–vis spectrum of HH-P3(C CDec)Th (Dec = decyl) in 1,2-dichlorobenzene at 131 ◦C showed a peak at λmax = 520 nm, which shifted to

max = 553 nm at 25 ◦C, with a shoulder peak at 608 nm. However, the UV–vis spectrum of an HH-P3(C CDec)Th film showed a more pronouncedhift in its UV–vis absorption peak to a longer wavelength. These phenomena are the characteristics of the self-assembly of the polymer molecule.he HH-P3(C CDec)Th film had a large third-order nonlinear optical susceptibility χ(3) of 3.6 × 10−11 esu.2007 Elsevier B.V. All rights reserved.

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eywords: Conjugated polymers; Self-assembly; UV–vis spectroscopy; Nonlin

. Introduction

�-Conjugated polymers have been attracting much inter-st because of their promising applications in electronic andptical devices [1–4]. Among various �-conjugated polymers,hose that form �-stacked self-assembled structures have beenttracting considerable attention [5–12]. The regioregular head-o-tail-type poly(3-alkylthiophene-2,5-diyl), HT-P3RTh [5–14]

is a typical example of such polymers, and its self-assembled

nd ordered structure on the surface of substrates is important forhe excellent performance of HT-P3RTh in electronic devices,uch as field effect transistors.

∗ Corresponding author. Tel.: +81 45 924 5220; fax: +81 45 924 5976.E-mail address: tyamamot@res.titech.ac.jp (T. Yamamoto).

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379-6779/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2007.04.002

ptical susceptibility

Recently, it has been shown that the self-assembled structuref HT-P3RTh is important for obtaining a large third-order non-inear optical susceptibility, χ(3) [15]. �-Conjugated polymersith a large χ(3) are considered to be crucial for the rapid treat-ent of signals in optical communication systems using polymeraterials [16–19].We previously reported the preparation of a new type of

-conjugated polythiophene with alkynyl side chains; namely,he head-to-head-type poly(3-alkynylthiophene- 2,5-diyl), HH-3(C CR)Th [20], which is considered to have a coplanartructure.

HT-P3RTh is thought to assume an essentially coplanar struc-ure in its molecularly self-assembled solid state. However, thereeems to be a steric repulsion between the long alkyl side chainnd the S of the thiophene ring in the polythiophene main chain.he steric repulsion appears to be overcome by forming a stackedacking structure in the solid state, and the main chain is con-idered to have an essentially coplanar structure in the solid

tate.

However, polythiophenes with higher coplanarities are moreuited to form a coplanar self-assembled structure [20,21]. Inddition, HT-P3RTh seems to contain a regioirregular part,

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lthough at a small amount (e.g., 1.5% [8,12]). The HH-3(C CR)Th molecule shown in Chart 1 is considered to havemore fixed coplanar structure. Single-crystal analysis [20] and

he CPK molecular model of the starting dibromo monomerf HH-P3(C CR)Th (cf. Chart 1) revealed that the dibromoonomer assumes a coplanar structure and that there is no steric

epulsion between the alkynyl C CR group and the thiopheneing in the starting monomer. Consequently, HH-P3(C CR)Ths considered to have a more rigid coplanar main chain, and theegioregularity (head-to-head regularity) of HH-P3(C CR)Ths completely determined by the preparation method. The 1H-MR spectrum of HH-P3(C CDec)Th (R = decyl (Dec) inC CR) showed only one �-CH2 signal at δ 2.50, indicatinghigh regioregularity of HH-P3(C CDec)Th. On the basis of

hese findings, HH-P3(C CDec)Th is considered to be moreuited to the study of the self-assembly behavior of thiophene-ased polymers.

In this paper, we report that HH-P3(C CDec)Th has atronger tendency to self-assemble and a larger χ(3) than HT-3RTh.

. Experimental

The head-to-head-type poly(3-(1-dodecynyl)thiophene-2,5-iyl) HH-P3(C CDec)Th with a number average moleculareight (Mn) of 7900 and an Mw/Mn of 2.87 (Mw = weight aver-

ge molecular weight) was prepared as previously reported20]. The polymer was prepared by polycondensation, whichsually gives an Mw/Mn of 2.0 [22]; the larger Mw/Mn of HH-3(C CDec)Th may be due to the occurrence of two types of

ermination process (i.e., reductive elimination and a protonol-sis) during the work-up of the polymer [23,24].

A chloroform-soluble part of HH-P3(C CDec)Th with ann of 4400 [20] was obtained from a CHCl3 extract obtained

y stirring a mixture of chloroform and the original HH-3(C CDec)Th at room temperature; the CHCl3 extract wasried in vacuum after filtration through a 0.45 �m membrane.

UV–vis spectra at room temperature were obtained usingShimadzu UV-2550PC spectrometer. UV–vis spectra at ele-

ated temperatures were measured using a multi-channel fiberptics spectrometer equipped with an Ocean Optics HR2000etector. An HH-P3(C CDec)Th film with an absorbance of

.6 at the UV–vis absorption peak (λmax = 540 nm) was pre-ared by spin-coating a hot 1,2-dichlorobenzene solution of theolymer and annealing the spin-coated polymer film on a hotlate for 5 min at 140 ◦C. The thickness of the obtained film was

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ls 157 (2007) 318–322 319

14 nm, as determined using a surface profiler (Alfa Step 500,LA-tencor).Third-order nonlinear optical susceptibility (χ(3)(−3�;

,�,�), which is simply expressed as χ(3)) was determined byhe third-harmonic generation (THG) method using the spin-oated and annealed film. For excitation, light pulses producedy a Ti:sapphire regenerative amplifier system with a pulse widthf 120 fs (Spitfire Pro, Spectra Physics) and a tunable opticalarametric amplifier (TOPAS, Light conversion) were used. Tovaluate the absolute value of χ(3), we compared the THG mag-itudes of the sample and the reference SiO2, both of whichere determined by the Maker fringe method [25]. The χ(3)

pectrum of SiO2 is obtained by applying Miller’s rule [26,27]o the reported value (2.79 × 10−14 esu) at 1.907 �m [28]. Weook account of THG reabsorption. All the measurements wereerformed in vacuum to prevent errors caused by the THG ofir from being included in the results.

. Results and discussion

Among the HH-P3(C CR)Th-type polymers shown in Chart, HH-P3(C CDec)Th (Dec = decyl) had a high solubility inarm 1,2-dichlorobenzene and was suitable for optical studies.ther polymers had lower solubilities. Halogenated hydrocar-ons are good solvents for substituted polythiophenes, such asT-P3RTh [1–14], and the observation of the UV–vis spectrum

t high-temperatures is possible using 1,2-dichlorobenzene,hich has a high boiling point, as the solvent.The HH-P3(C CDec)Th used had a number average molec-

lar weight (Mn) of 7900 (or a degree of polymerization (DP)f about 16 with 32 thiophene (Th) units) as measured by GPCsing 1,2-dichlorobenzene at 135 ◦C. About half of the poly-er was soluble in chloroform at room temperature, and the

hloroform-soluble part had an Mn of 4400 (or a DP of 9 with8 Th) as measured by GPC using chloroform as the eluent.

It is reported that HT-P3HexTh (R = hexyl in HT-P3RTh) withn Mn of 33000 (or about 200 Th units) is soluble in chloroform29,30], and the lower solubility of HH-P3(C CDec)Th thanhat of HT-P3RTh implies a stronger intermolecular interactionf HH-P3(C CDec)Th than that of HT-P3RTh.

Fig. 1 shows the temperature-dependent UV–vis spectra ofH-P3(C CDec)Th (Mn = 7900) in 1,2-dichlorobenzene. As

hown in Fig. 1, the 1,2-dichlorobenzene solution of HH-3(C CDec)Th at 131 ◦C exhibits a UV–vis absorption peak atmax = 520 nm. The shape of the UV–vis absorption band resem-les that of a single molecule of HT-P3HexTh in chloroform5–12], and no molecular assembly, which often accompaniesubstructuring UV–vis peaks, is observed.

The λmax position is in agreement with the UV–vis peakosition of the chloroform-soluble part of HH-P3(C CDec)Thith a lower molecular weight (Mn = 4400) in chloroform

λmax = 513 nm) and 1,2-dichlorobenzene (�max = 523 nm) atoom temperature, whose UV–vis spectra are shown in the inset

f Fig. 1.

Because both (i) HH-P3(C CDec)Th with a higherolecular weight corresponding to 32 Th units and (ii) HH-3(C CDec)Th with a lower molecular weight corresponding

320 T. Sato et al. / Synthetic Meta

Fig. 1. Temperature dependence of UV–vis spectrum of HH-P3(C CDec)Th(Mn = 7900) in 1,2-dichlorobenzene. The inset shows the UV–vis spectra of HH-Pr

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swiobtained by changing the molecular weight was also observedfor HT-P3HexTh; however, the trend was not as obvious as thatobserved for HH-P3(C CDec)Th. The difference between theUV–vis spectra of HH-P3(C CDec)Th (Mn = 7900) and HH-

3(C CDec)Th (Mn = 4400) in (A) 1,2-dichlorobenzene and (B) chloroform atoom temperature.

o 18 Th units have UV–vis absorption peaks at positions nearach other (at about 520 nm; cf. Fig. 1 and the inset of Fig. 1),8 Th units are considered to be sufficient for the saturation ofhe λmax position of HH-P3(C CDec)Th.

The λmax position of oligothiophenes of the formula H-(Th)n-saturates at about n = 7 [31], to reach a λmax of 440 nm, and this

max is generally in agreement with the λmax of poly(thiophene-,5-diyl) at about 460 nm [1–4].

HT-P3HexTh has the same UV–vis absorption spectrum in,2-dichlorobenzene and chloroform, with a peak at 450 nm.owever, HH-P3(C CDec)Th has a UV–vis spectrum that

xhibits some dependence on the type of solvent used, as shownn the inset of Fig. 1.

As described above, the UV–vis absorption peak of HH-3(C CDec)Th at 520 nm observed at 131 ◦C is considered

o originate from a single HH-P3(C CDec)Th molecule, ands located at a wavelength 70 nm longer than that of a singleT-P3HexTh molecule at 450 nm in 1,2-dichlorobenzene and

hloroform [5–14]. This shift to a longer wavelength is attributedo the highly coplanar structure of the HH-P3(C CDec)Th mainhain and the expansion of the �-electron system induced by thearticipation of the side C CDec group. The band gap of HH-3(C CDec)Th (Eg = 2.0 eV) estimated from the onset positionf the UV–vis absorption band at 131 ◦C is smaller than that ofT-P3HexTh (Eg = 2.2 eV [5–10,12]).Lowering the temperature from 131 to 25 ◦C leads to a shift in

he UV–vis absorption peak to a longer wavelength and a changerom pink to purple, as shown in Fig. 1. At 25 ◦C, the mainV–vis absorption peak is observed at 553 nm with a shoul-er peak at 608 nm. A similar shift to a longer wavelength andsimilar color change have been observed when the CHCl3

nd CH2Cl2 solutions of HT-P3HexTh are cooled from 0 to40 ◦C and −59 ◦C, respectively [12]. The UV–vis absorption

rofiles of HT-P3HexTh observed at these temperatures are simi-ar to that of HH-P3(C CDec)Th (Mn = 7900) obtained at 25 ◦C,

Fg(

ls 157 (2007) 318–322

lthough the peak position of HH-P3(C CDec)Th is located atlonger wavelength.

These results and the data obtained from the HH-3(C CDec)Th film indicate that the molecular assembly ofH-P3(C CDec)Th occurs at a much lower temperature than

hat of HT-P3HexTh, in spite of the large Mw/Mn of HH-3(C CDec)Th.

The UV–vis spectrum of the HH-P3(C CDec)ThMn = 7900) film also shows a shift in the absorption band to aonger wavelength than that of HH-P3(C CDec)Th observed at31 ◦C in 1,2-dichlorobenzene, as shown in Fig. 2. The peak at34 nm was not distinct for a nonannealed film. However, afternnealing the film (cf., Section 2), the peak at 634 nm becameistinct.

The longest UV–vis peak of the HT-P3RTh (e.g., R = hexyl)lm appears at 610 nm [5–12], and the observation of the longestV–vis peak of the film of HH-P3(C CDec)Th at a longer wave-

ength of 634 nm is ascribed to the expansion of the �-electronystem induced by the participation of the side C CDec groupnd/or the stronger �-stacking intermolecular electronic interac-ion between HH-P3(C CDec)Th molecules. The UV–vis chartf the HH-P3(C CDec)Th (Mn = 4400) film is shown in the insetf Fig. 2.

As shown in Fig. 2 and its inset, the HH-P3(C CDec)ThMn = 7900) film undergoes a larger shift to a longer wavelengthhan the HH-P3(C CDec)Th (Mn = 4400) film, suggesting thatigher molecular weight HH-P3(C CDec)Th forms a well-tacked structure with a higher packing order, and that the packedtate generates new electronic state(s).

These data indicate that the self-assembly force becomestronger when the length of HH-P3(C CDec)Th increases,hich is achieved presumably by increasing the number of

nteracting sites between the polymer molecules. Such a trend

ig. 2. UV–vis spectrum of film of HH-P3(C CDec)Th (Mn = 7900) on quartzlass plate. The inset indicates the UV–vis spectrum of the HH-P3(C CDec)ThMn = 4400) film. Both films were annealed at 140 ◦C for 5 min.

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3(C CDec)Th (Mn = 4400) in 1,2-dichlorobenzene at 25 ◦Ccf., Fig. 1 and its inset) is also attributed to the stronger self-ssembly ability of HH-P3(C CDec)Th (Mn = 7900) than thatf HH-P3(C CDec)Th (Mn = 4400).

Because the number density of the side C CR chain in HH-3(C CR)Th is the same as that of the R chain in HT-P3RTh,H-P3(C CR)Th is considered to assume a stacked structureith an end-to-end packing [12] mode (not an interdigitationacking mode), similarly to HT-P3RTh. The stacking structuref HH-P3(C CR)Th is indicated by powder X-ray diffractionatterns and the densities of HH-P3(C CR)Th polymers [20].

As described above, the self-assembly of HH-3(C CDec)Th starts at a much higher temperature than

hat of HT-P3HexTh, supporting the hypothesis that HH-3(C CDec)Th has a much stronger tendency to self-assemble

han HT-P3HexTh, presumably because of the high coplanarityf its main chain and its highly regulated one-dimensionaltructure.

Fig. 3 shows a χ(3) profile of the film of HH-P3(C CDec)ThMn = 7900). The χ(3) profile has a main peak at a three-photonnergy of 1.96 eV (or 633 nm).

Because the peak energy for χ(3) is in agreement with that ofhe UV–vis peak (cf. Fig. 2), the peak is due to the three-photonesonance. A smaller subpeak is observed at a three-photonnergy of about 2.2 eV. This is similarly assigned to the three-hoton resonance of the phonon side band observed as theecond peak in the absorption spectrum. The maximum χ(3)

f 3.6 × 10−11 esu is larger than that (χ(3) = 2.7 × 10−11 esu)btained for an HT-P3HexTh film [15], although the volumeensity of the �-electron system is decreased by the large sideon-�-conjugated decyl group in HH-P3(C CDec)Th. Namely,n effective and significant increase in χ(3) is observed for HH-3(C CDec)Th. This is probably related to the increase in theffective conjugation length, which results from the highly reg-

lated one-dimensional structure of HH-P3(C CDec)Th.

As reported in our previous paper [15], a large increasen the conjugation length of the fully extended �-conjugatedhains can enhance nonlinear susceptibility, without signifi-

ig. 3. Comparison of UV–vis spectrum (solid line) and χ(3) profile (�) ofpin-coated and annealed film of HH-P3(C CDec)Th (Mn = 7900).

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ls 157 (2007) 318–322 321

antly affecting the linear optical properties. Although a detailediscussion of the origin of the increase in χ(3) requires the spec-roscopic analysis of χ(3) spectra, the present data suggest themportance of the well-stacked structure of the polymer for large(3) values.

. Conclusion

The head-to-head-type poly(3-(1-dodecynyl)thiophene-2,5-iyl) HH-P3(C CDec)Th forms a molecular assembly in theigh-temperature range of 80–25 ◦C in a solution. The poly-er is also considered to form a �-stacked molecular assembly

n its film, which shows a UV–vis absorption peak at a longavelength of 634 nm. The polymer film has a large χ(3) of.6 × 10−11 esu. These results are attributed to the coplanartructure of HH-P3(C CDec)Th and are expected to contributeo a better understanding of the chemical and physical propertiesf �-conjugated polythiophenes.

cknowledgments

This research was partly supported by a Grant-in-Aidor Science Research in a Priority Area “Super-Hierarchicaltructures” and 21st centrury program from the Ministry ofducation, Culture, Sports, Science and Technology, Japan.

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