Photoluminescent liquid crystals based on trithienylphosphine oxides

Article abstract of DOI:10.1246/cl.2009.800

Trithienylphosphine oxides tethering three π-conjugated oligothiophene arms self-assemble into columnar liquid-crystalline structures that show high photoluminescent properties at ambient temperature. Copyright

Full text of DOI:10.1246/cl.2009.800

800
Chemistry Letters Vol.38, No.8 (2009)
Photoluminescent Liquid Crystals Based on Trithienylphosphine Oxides
Masaomi Kimura,1 Tsukasa Hatano,1 Takuma Yasuda,1 Jun Morita,2 Yusuke Akama,2
Kiyoshi Minoura,2 Takeshi Shimomura,2 and Takashi Katoꢀ1
1Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo,
Hongo, Bunkyo-ku, Tokyo 113-8656
2Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology,
Koganei, Tokyo 184-8588
(Received May 11, 2009; CL-090449; E-mail: kato@chiral.t.u-tokyo.ac.jp)
Trithienylphosphine oxides tethering three ꢀ-conjugated
oligothiophene arms self-assemble into columnar liquid-crystal-
line structures that show high photoluminescent properties at
ambient temperature.
Self-organization of liquid-crystalline (LC) molecules leads
to the preparation of functional materials with a variety of
ordered nanostructures.1,2 Columnar liquid crystals exhibit
photo-,2,3 electro-,2,4 and ion-active2,5 functions due to their
designed molecular organization.
Photoluminescent (PL) molecular assemblies such as liquid
crystals3 and fibrous aggregates6 composed of ꢀ-conjugated sys-
tems can be used for optical and sensing applications. For the en-
hancement of PL efficiency in the self-assembled states, modu-
lation of the supramolecular structures through specific interac-
tions is a key factor. The introduction of ꢀ-conjugated moieties
into sterically bulky molecular architectures could enhance the
PL efficiency of the materials in the ordered bulk states because
of the suppression of intermolecular ꢀ–ꢀ interactions.
Our intention here is to prepare new photofunctional colum-
nar LC oligothiophenes exhibiting high PL efficiency by intro-
ducing a triarylphosphine oxide moiety, which can act as a steri-
cally bulky building block. We previously prepared a columnar
liquid crystal containing a triarylphosphine oxide moiety to ob-
tain an ion-active material.7 An interesting aspect of the triaryl-
phosphine oxide molecules is that the stacking distance8 is quite
large compared to the ꢀ–ꢀ stacking distance of conventional
ꢀ-conjugated compounds.9 Herein, we report C3-symmetrical
molecules 1a, 1b, and 2 composed of a polar phosphine oxide
moiety, ꢀ-conjugated oligothiophene arms and lipophilic alkyl
chains (Figure 1).
Until now, a number of examples of the LC oligothiophenes
have been reported.10 The preparation of oligothiophene-based
columnar liquid crystals exhibiting high PL efficiency may lead
to the development of new photofunctional materials. Com-
pounds 1a and 1b were synthesized by palladium-catalyzed
coupling reactions of tris(50-bromo-2,20-bithiophene-5-yl)phos-
phine oxide with three equivalents of phenylboronic acid deriv-
atives.11 Compound 2 was synthesized through phosphination
of 5-bromo-500-(3,4,5-trimethoxyphenyl)-2,20:50,200-terthiophene
with phosphorus tribromide, followed by an oxidation with hy-
drogen peroxide.
Figure 1. Structures of liquid-crystalline compounds 1a, 1b,
and 2.
Table 1. Liquid-crystalline behavior of 1a, 1b, and 2
Compound
Phase transition behaviora
1a
1b
2
Cr ꢁ14 (31.1) Col1 25 (5.2) Col2 79 (2.6) Iso
Cr [ꢁ13 (8.3) Col1 25 (2.9) Col2 78 (2.3)]b 100 (50.2) Iso
Cr ꢁ14 (45.9) Col1 23 (6.9) Col2 109 (2.8) Iso
aTransition temperatures (ꢂC) and enthalpies (kJ molꢁ1, in parentheses)
determined by DSC on the second heating at 5 ꢂC minꢁ1. Cr: crystalline;
b
Col: columnar; Iso: isotropic. Monotropic phase transition.
moieties from the surrounding alkyl chains as well as dipolar in-
teractions of polar phosphine oxide moieties.
The X-ray diffraction pattern of 1a in the Col phase gives
˚
two peaks with d-spacing of 35.6 and 17.8 A, which correspond
to the (10) and (20) reflections, respectively. For compound 2,
the reflections of 40.3 (10) and 20.1 A (20) are observed.11 This
˚
increase in the intercolumnar distance can be attributed to the
elongation of the oligothiophene arms. A molecular modeling
study for 1a has revealed that the molecule adopts a conical-
shaped conformation and thereby possesses a large intrinsic di-
pole of approximately 6 debye along the longitudinal C3 axis.
In the Col phase, it is reasonable to assume that the molecules
stack to form the 1D columnar structure (Figure 2). It was report-
ed that triphenylphosphine oxide formed a similar 1D array with
˚
a stacking distance of 6.2 A due to the steric effects of the phos-
phine oxide moiety, in single crystals.8
It is noteworthy that compound 1a exhibits a high PL effi-
ciency in the Col phase. As shown in Figure 3, compounds 1a
and 2 in the thin films exhibit intense green and yellow-green
PL emissions with the peaks at 484 and 525 nm, respectively,
upon photoexcitation. From the PL spectroscopy with an inte-
grating sphere technique, the PL quantum yields of 1a and 2
in the thin films have been determined to be 10.3 and 3.2%, re-
As summarized in Table 1, compounds 1a and 2 exhibit
enantiotropic columnar (Col) LC phases in wide temperature
ranges, whereas 1b shows a Col phase only on cooling from
the isotropic melt. A key driving force in the formation of the
Col phases should be nanosegregation of the central aromatic
Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.8 (2009)
801
This work was partially supported by Grant-in-Aid for Cre-
ative Scientific Research of ‘‘Invention of Conjugated Electronic
Structures and Novel Functions’’ (No. 16GS0209; T.K.) from
JSPS and the Global COE Program for Chemistry Innovation
(T.K. and M.K.) from MEXT. M.K. acknowledges the Sasakawa
Scientific Research Grant from the Japan Science Society for
financial support.
References and Notes
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Figure 2. Representation of a possible assembled structure of
1a: (a) top view and (b) side view. Hydrogen atoms are omitted
for clarity. Green: ꢀ-conjugated moieties, red: phosphine oxide
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Figure 3. (a) Photograph of PL emission from a THF solution
and thin film of 1a under irradiation with UV light at 365 nm.
(b) UV–vis and (c) PL spectra of 1a in THF solution (red) and
the Col phase (blue). (d) Photograph of PL emission from a
THF solution and thin film of 2 under irradiation with UV light
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oligothiophenes in the solid states.13 The relatively high PL
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the aforementioned unique stacked structure (Figure 2) that can
prevent ꢀ–ꢀ electronic interactions of the oligothiophene arms
between the neighboring stacked molecules.
The UV–vis spectra of 1a both in the solution (the molecu-
larly dissolved state) and in the thin film (the self-assembled con-
densed state) show that the peaks appear at almost the same po-
sition (Figure 3b). For 2, a red shift of the absorption peak from
the solution to the thin film together with a broadening of the
peak is observed (Figure 3e). This observation suggests the pres-
ence of intermolecular electronic interactions in the thin film of
2, which may result in the lower PL quantum yield.
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11 Supporting Information is available electronically on the CSJ-Journal Web
site, http://www.csj.jp/journals/chem-lett/index.html.
In summary, we have developed ꢀ-conjugated oligothio-
phene-based columnar LC molecules having the phosphine ox-
ide moiety. We expect that thin films of these liquid crystals with
high luminescent functionalities are a promising candidate as
polarized light-emitting materials.
12 In THF solutions, PL quantum yields of 1a and 2 are determined to be 42
and 13%, respectively.
13 D. Oelkrag, H.-J. Egelhaaf, J. Gierschner, A. Tompert, Synth. Met. 1996,
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Published on the web (Advance View) July 4, 2009; doi:10.1246/cl.2009.800