Periodic Structure in a Fluorene-based Polymer
Prepared by Electrochemical Polymerization
Kohsuke Kawabata and Hiromasa Goto
Institute of Materials Science, Graduate School of Pure and Applied Sciences, University of Tsukuba,
1-1-1 Tennodai, Tsukuba 305-8573(Received April 20, 2009; CL-090393; E-mail: gotoh@ims.tsukuba.ac.jp)

A fluorene-based polymer film prepared by electrochemical polymerization in cholestericliquid crystal electrolyte solution displays a labyrinthine fingerprint texture, and diffraction
grating function.

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Vol.38 No.7 2009 p:706{707CMLTAG
July 5, 2009

The Chemical Society of Japan
Published on the web (Advance View) June 13, 2009; doi:10.1246/cl.2009.706

706

Chemistry Letters Vol.38, No.7 (2009)

Periodic Structure in a Fluorene-based Polymer
Prepared by Electrochemical Polymerization
Kohsuke Kawabata and Hiromasa Goto
Institute of Materials Science, Graduate School of Pure and Applied Sciences, University of Tsukuba,
1-1-1 Tennodai, Tsukuba 305-8573(Received April 20, 2009; CL-090393; E-mail: gotoh@ims.tsukuba.ac.jp)
A conjugated copolymer, poly[2,7-di(2-furyl)ﬂuorene],with periodic dielectric microstructure is prepared by electrochemical polymerization using cholesteric liquid crystal as anelectrolyte solution. The polymer ﬁlm thus prepared displays alabyrinthine ﬁngerprint texture formed by a convexo–concave
structure, and exhibits a random diﬀraction grating function.
This anisotropic electrochemical polymerization technique is a
novel bottom-up approach for preparing one-dimensional photonic crystals.

The manipulation of surface microstructures through micropatterning and other technique1 is a fundamental process in technologies such as integrated circuits, biosensors, and photonic
crystals.2 Photonic crystals, materials with highly useful opticalproperties, can be prepared by synthesizing a periodic dielectricsurface structure with dimensions comparable to the wavelengthof incident light. In recent years, numerous approaches to preparing photonic crystals with periodic nanostructures have beenproposed, based on processing methods such as photolithography, etching, and the self-assembly of colloidal crystals.3 Electrochemical deposition is another technique that can be employed to prepare periodic dielectric structures through the synthesis of various conjugated polymers with optical, electrical,and redox properties.4 Furthermore, photo-functional materials
have been developed.5,6 Polymerization using cholesteric liquid
crystal (CLC) aﬀords a one-dimensional self-assembled photonic structure7,8 that mimics biological iridescence such as thatexhibited by certain Coleoptera beetles.9 CLC is characterized by a continuous and periodic rotation of the director axis,
which results in a helically ordered periodic structure. CLC
as a reaction ﬁeld has thus been applied to the synthesis of photonic crystals for lasing10 and diﬀraction gratings. Polymerstabilized CLC and polymer-dispersed CLC, both polymer/
CLC composites, have also been developed as variants of thisapproach.
Our group has previously reported a method for the preparation of conjugated polymers having cholesteric-like order.11 The
conjugated polymers thus obtained exhibit circular dichroism
and a surﬁcial ﬁngerprint texture similar to that of the CLC electrolyte solution. The results of previous studies indicate that thepolymers develop a CLC-like helical structure by transcription
from the CLC reaction ﬁeld.
In the present study, a straightforward approach for thepreparation of a furan–ﬂuorene conjugated copolymer byelectrochemical polymerization using CLC is demonstrated(Scheme 1). The polymer thus synthesized exhibits a distinct
labyrinthine convexo–concave surface structure that acts asa periodic dielectric to provide a diﬀraction grating function.

OO

Electrochemicalpolymerization
CLC electrolyte

2,7-Di(2-furyl)fluoreneOO

nPoly[2,7-di(2-furyl)fluorene]

Scheme 1.
Table 1. Constituents of CLC electrolyte reaction solution

Films of the conjugated copolymer were prepared as follows. The monomer, 2,7-di(2-furyl)ﬂuorene, synthesized via
the Stille cross-coupling reaction, was dissolved in a CLC electrolyte solution consisting of 4-cyano-40 -n-hexylbiphenyl (6CB)
as a nematic liquid crystal (NLC) solvent, cholesteryl oleyl carbonate as a chiral inducer, and tetrabutylammonium perchlorate
as a supporting salt (Table 1).
Diﬀerential scanning calorimetry measurements conﬁrmedthat the CLC electrolyte solution exhibits a stable CLC phaseat temperatures in the range of 7–26  C. A polarizing optical
microscopy (POM) image of the CLC electrolyte solution(Figure 1a) shows the characteristic ﬁngerprint texture of theCLC. The polymer was formed on indium tin oxide (ITO) coatedglass (used as substrate electrodes) by injecting the CLC electrolyte solution between two ITO glass slides separated by a
190 mm-thick Teﬂon spacer. A direct-current voltage of 4.0 V
was applied between the electrodes for 60 min, resulting in theepitaxial deposition of the polymer ﬁlm on the anodic ITO glasselectrode. The temperature during polymerization was maintained at 20  C so as to preserve the stable CLC phase. Afterthe reaction, the residual CLC electrolyte solution and low molecular mass fractions were washed oﬀ using hexane to aﬀord astable green ﬁlm. POM observations under crossed Nicols revealthat the polymer ﬁlm exhibits birefringence with a ﬁngerprint
texture similar to that of the CLC electrolyte solution (Figure 1b).
The observation of birefringence indicates that the individual main chains are assembled so as to produce optical anisotro-Copyright  2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.7 (2009)707

Figure 3. Diﬀraction of blue laser light by poly[2,7-di(2-furyl)ﬂuorene] ﬁlm. (inset) Schematic of optical system.

Figure 1. POM image of (a) CLC electrolyte solution and(b) poly[2,7-di(2-furyl)ﬂuorene] ﬁlm. (c) SEM image of thepolymer ﬁlm.
py, and the ﬁngerprint texture indicates that the main chains of
the polymer conform to a CLC-like periodic helical ordering.
This helicity-based periodic structure is transcribed from theCLC to aﬀord a periodic dielectric microstructure.
Scanning electron microscopy (SEM) images of the polymerﬁlm taken at 45 from the surface normal reveal that the ﬁngerprint texture of the polymer is produced by a labyrinthine convexo–concave surface structure (Figure 1c).
Polymers prepared by electrochemical polymerization using
isotropic solutions such as acetonitrile do not exhibit CLC-likestructures. Polymerization using CLC therefore makes it possible to prepare anisotropic polymer ﬁlms with distinct helicalstructures. This anisotropic epitaxial deposition may occur due
to the eﬀect of the continuous change in electric ﬁeld intensity
in the CLC electrolyte solution, or by orientation of the growing
polymer according to the LC director. In the former case, the periodic rotation of the director gives rise to a periodic distributionof electrical conductivity within individual CLC molecules,
which may aﬀect the rate of polymerization at the molecularlevel. In the latter case, the rod-like aromatic monomer becomes
physically aligned with the helical director in each CLC molecule, causing the individual main chains of the polymer to growin a helical manner.
The convexo–concave structure of the polymer produces asequential alternation of dielectric constant between two values(n1 and n2 ), and thus acts as a diﬀraction grating (Figure 2).
Under normally incident blue laser light ( ¼ 473 nm), the ﬁlm
diﬀracts the transmitted beam into ﬁve directions (Figure 3).
The regularity of the labyrinthine convexo–concave structure produces second-order diﬀraction at 54 in addition toﬁrst-order diﬀraction at 24 . From these diﬀraction angles andFigure 2. Schematic of periodic dielectric structure forming thepolymer ﬁngerprint texture.

the laser wavelength, the calculated grating constant is 1.17mm, which corresponds to the distance between stripes of the ﬁngerprint texture (helical half pitch of the polymer). These results
demonstrate that the polymer ﬁlm prepared in this study has diffraction properties due to the periodic dielectric microstructure.
The grating function is random due to the random alignment ofstripes comprising the ﬁngerprint texture. The present synthesis
thus aﬀords a highly ordered microstructure through transcription from the three-dimensional helical continuum of the CLC
reaction ﬁeld.
In conclusion, electrochemical polymerization using a CLC
electrolyte solution was demonstrated to aﬀord anisotropic epitaxially deposited ﬁlms of a furan–ﬂuorene conjugated copolymer having a periodic microstructure observable by POM andSEM. The polymer ﬁlms thus obtained exhibit a CLC-like ﬁngerprint texture formed by a labyrinthine convexo–concavestructure. This surface structure acts as a diﬀraction grating,and blue laser light was demonstrated to be diﬀracted by the ﬁlm
in a manner similar to a one-dimensional photonic crystal. Thegrating constant of the ﬁlm was calculated to be 1.17 mm, corresponding to the helical half pitch of the ﬁngerprint texture. The
present bottom-up synthesis technique using CLC as an electrolyte solution is expected to be applicable for the preparation of arange of new functional conjugated polymer materials with useful optical properties, and provides an alternate route to topdown processes such as laser-based methods for the preparationof microstructured materials.
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Published on the web (Advance View) June 13, 2009; doi:10.1246/cl.2009.706