Synthesis of anthracene-stacked oligomers and polymer

Article abstract of DOI:10.1021/ol1011295

(Figure Presented) Anthracene-stacked oligomers and a polymer were synthesized using a xanthene skeleton as the scaffold, and their structures and properties were fully characterized. Intramolecular π-π stacking of the anthracene rings in the ground state and excited state was observed.

Full text of DOI:10.1021/ol1011295

ORGANIC
LETTERS
2010
Vol. 12, No. 14
3188-3191
Synthesis of Anthracene-Stacked
Oligomers and Polymer
Yasuhiro Morisaki,* Toshiyuki Sawamura, Takuya Murakami, and Yoshiki Chujo*
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto UniVersity,
Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
ymo@chujo.synchem.kyoto-u.ac.jp; chujo@chujo.synchem.kyoto-u.ac.jp
Received May 16, 2010
ABSTRACT
Anthracene-stacked oligomers and a polymer were synthesized using a xanthene skeleton as the scaffold, and their structures and properties
were fully characterized. Intramolecular π-π stacking of the anthracene rings in the ground state and excited state was observed.
Currently, construction of oriented and π-stacked structures
is one of the topics in the field of synthetic polymer
chemistry. Cofacially oriented π-electron systems are of
importance in the fields of materials chemistry and biochem-
istry. This importance can be explained with the help of a
few examples. The performance of organic optoelectronic
devices strongly depends on the arrangement of the constitu-
ent π-electron systems. DNA has face-to-face base pairs
stacked in double-stranded main chains. Recently, novel
π-stacked polymers such as poly(dibenzofulvene)s,1 7,7-
diarylnorbornane-containing polymers,2 side-chain conju-
gated polymers,3 cyclophane-containing through-space con-
jugated polymers,4-7 and face-to-face ferrocene polymers8
have been synthesized, and their optical and electrochemical
properties have been studied. In addition, xanthene-9,10 and
anthracene-based polymers11 consisting of layered π-electron
systems and polymeric ladderphanes12 consisting of oriented
π-electron systems have also been prepared. These polymers
have potential applications in optoelectronic devices and
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10.1021/ol1011295  2010 American Chemical Society
Published on Web 06/14/2010

single-molecular devices such as single molecular wires. In
addition to these polymers, well-defined aromatic-system-
layered oligomers such as anthracene-based trimeric and
pentameric porphyrin arrays have been synthesized.13 We
recently synthesized aromatic-ring-layered polymers by using
xanthene compounds as scaffolds.9,10,14 Among them,
[2.2]paracyclophane-layered polymers9 exhibited a photo-
excited energy transfer from the layered [2.2]paracyclophanes
to the end-capping groups. However, effective π-π stacking
among the layered [2.2]paracyclophanes was not observed
in the ground state. This is because of the relatively long
distance between the 4- and 5-positions of the xanthene
skeleton. Thus, our next target was to construct layered and
π-stacked aromatic units in proximity in the polymer
backbone. In this study, we selected anthracene as the layered
aromatic unit and xanthene as the scaffold. Anthracenes are
expected to be facing each other in the polymer chain due
to their restricted rotary motion on xanthene and to exhibit
π-π interactions both in the ground state and in the excited
state. Herein, we report the synthesis, characterization, and
optical properties of a system of anthracene-stacked oligo-
mers and a polymer.
Scheme 1. Synthesis of Anthracene-Stacked Oligomers
We synthesized anthracene-stacked oligomers 2A1X,
3A2X, and 4A3X containing two, three, and four face-to-
face anthracenes, respectively. These oligomers were syn-
thesized by the Sonogashira-Hagihara coupling reaction,15
as outlined in Scheme 1. The two-anthracene-stacked com-
pound 2A1X was obtained in 25% isolated yield by the
reaction of 9,9-didodecyl-4,5-diiodoxanthene (1)10a,c with
9-(trimethylsilylethynyl)anthracene (2) in the presence of
nBu4NF. The treatment of a 9-ethynylanthracene-substituted
compound (3) with 9,10-diethynylanthracene (4) gave the
three-anthracene-stacked oligomer 3A2X in 61% yield. The
reaction of 9,9-didodecyl-4,5-diethynylxanthene (5) with
9-bromo-10-(trimethylsilylethynyl)anthracene (6) gave an-
other two-anthracene-stacked compound (7) in 28% yield;
then, the successive reaction of 7 with 3 in the presence of
nBu4NF afforded the corresponding four-anthracene-stacked
oligomer 4A3X in 33% yield.
(10) (a) Morisaki, Y.; Imoto, H.; Miyake, J.; Chujo, Y. Macromol. Rapid
Commun. 2009, 30, 1094–1100. (b) Morisaki, Y.; Fernandes, J. A.; Wada,
N.; Chujo, Y. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 4279–4288.
(c) Morisaki, Y.; Fernandes, J. A.; Chujo, Y. Macromol. Rapid Commun.
2009, 30, 2107–2111.
The synthesis of anthracene-stacked polymer P1 is shown
in Scheme 2. Monomers 1, 8, and 2 were polymerized by
the Pd(PPh3)4/CuI catalytic system in the presence of nBu4NF
to yield the corresponding P1 in 59% yield after repeated
reprecipitation from CHCl3/MeOH. The two dodecyl groups
at the 9-position of xanthene contribute to the solubility of
the oligomers and polymer in common organic solvents such
as CHCl3, CH2Cl2, toluene, and THF. The number-average
molecular weight (Mn) and weight-average molecular weight
(Mw) of P1 were calculated to be 4600 and 6200, respec-
tively, by gel permeation chromatography (GPC) with
polystyrene standards (eluent CHCl3).
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The structures of oligomers, polymer, and related com-
pounds were characterized by 1H and 13C NMR spectroscopy
Org. Lett., Vol. 12, No. 14, 2010
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Scheme 2. Synthesis of Anthracene-Stacked Polymer
(Figures S1-S21 in Supporting Information). 1H NMR
spectra of 2A1X, 3A2X, 4A3X, and P1 are summarized in
Figure S21. The chemical shifts of anthracene protons were
in higher magnetic field than those of common anthracene
compounds.16 Such a chemical shift to a shielding region is
attributed to the ring current shielding effect among neigh-
boring anthracene rings. Such an effect is observed in the
1H NMR spectra of a series of 1,8-anthrylene-ethynylene
cyclic tetramers17 as well as [2.2]anthracenophanes.18 In the
1H NMR spectrum of 2A1X, signals of the anthracene units
appeared at 6.9 ppm, whereas those of the inner anthracene
ring in 3A2X appeared at around 6.5 ppm, as shown in
Figure S21. An additional shift was observed in the 1H NMR
spectra of 4A3X and P1, and the signals appeared at around
6.3 ppm and 6 ppm (Figure S21), respectively. Thus, with
an increasing number of stacked anthracenes, the signals of
anthracene protons gradually shifted to the higher magnetic
field due to the buttressing effect of the anthracene rings.
In order to gain further insight into their π-stacked
structure, we designed and synthesized compound 10, which
has two ferrocenes and an inner anthracene, using two 9,9-
dimethylxanthene units as the scaffold, as shown in Figure
1. Ferrocenes and 9,9-dimethylxanthenes were employed for
the purpose of enhancing the crystallinity of the compound.
Figure 1 also depicts the X-ray structure of 10. The top view
of the ORTEP drawing clearly shows that the cyclopenta-
dienyl rings of the two ferrocenes and the anthracene ring
Figure 1. Synthesis and X-ray structure of model compound 10.
Hydrogen atoms are omitted for clarity.
are twisted and have a face-to-face structure. The dihedral
angles of the stacked aromatic rings and xanthene scaffolds
are 27.2°. The distance between the 4- and 5-positions of
xanthene is 4.643 Å, whereas the shortest distance between
the intramolecular face-to-face aromatic rings is 3.265 Å.
This distance is smaller than the sum of the van der Waals
radius of an sp2 carbon (3.40 Å), suggesting the presence of
π-π interactions among the two ferrocenes and the an-
thracene. Thus, the restricted rotaion of anthracenes in the
polymer can be expected, leading to their π-stacked structure.
UV-vis absorption spectra and fluorescence spectra of
2A1X, 3A2X, 4A3X, P1, and compound 11 in diluted CHCl3
solutions are shown in Figure 2. The molar extinction
coefficient of a strong absorption peak at around 250 nm
and a broad absorption band at around 400 nm for 2A1X,
3A2X, and 4A3X increased with the number of anthracene
rings. The absorption spectra of 2A1X, 3A2X, 4A3X, and
P1 were blue-shifted in comparison with that of 11, which
is derived from the less coplanarity between anthracene and
xanthene in the oligomers and polymer. According to the
studies on the conformation of 9,10-bis(phenylethynyl)an-
thracene,19 the absorption spectrum of 11 exhibits free
rotation of anthracene, whereas those of olgomers and
polymer show the contribution of the anthracene-twisted
structures. Upon normalization of the molar extinction
(16) In the 1H NMR spectrum of compound 11 (vide supra), signals of
the anthracene protons are observed at δ 7.06 and 8.82 ppm (Figure S19 in
Supporting Information).
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Org. Lett., Vol. 12, No. 14, 2010

featureless, and they appeared in the longer-wavelength
region. Fluorescence lifetime measurement of P1 was
performed (Figure S27 in Supporting Information), and it
was found that the emission comprised two components. A
biexponential fit of the decay afforded short and long decay
components with lifetimes (τ) of 4.9 and 34.8 ns, respec-
tively. In consideration of a previously reported observation
that the fluorescence lifetime of anthracene in solution is
approximately 5 ns,21 the longer lifetime (τ ) 34.8 ns) of
P1 suggests excimer formation in the polymer chain. Thus,
P1 as well as 3A2X and 4A3X emit mainly from the excimer
state because of the buttressing effect of the anthracene rings.
These phenomena are attributed to the intramolecular
π-stacked structure of the anthracene units in the excited
state.
As a preliminary experiment, the present system was
applied to a single molecular wire. As shown in the Scheme
S1 in Supporting Information, anthracene-stacked polymer
P2 end-capped with ferrocenes as a fluorescence quencher
was synthesized. In diluted CHCl3 solution (1.0 × 10-6 M),20
photoexcited energy or electron was intramolecularly trans-
ferred to both terminals, and the photoluminescence of
anthracenes was effectively quenched (Figure S28 in Sup-
porting Information).
In conclusion, anthracene-stacked oligomers and a polymer
were synthesized using a xanthene skeleton as the scaffold,
and their structures and properties were fully characterized.
NMR, UV-vis absorption spectroscopy, and X-ray crystal-
lography revealed that π-π interactions were present in the
anthracene layer in the ground state. The π-stacked structure
induced intramolecular excimer formation of the anthracene
rings. Thus, we succeeded in the construction of a π-stacked
structure using a xanthene scaffold. This class of polymers
consisting of stacked aromatic rings has potential application
to the single molecular wires, which was demonstrated
experimentally in the polymer with quencher units at the
polymer chain ends. We are currently extending our research
to the design and synthesis of π-stacked oligomers and
polymers, which transfer energy or electrons in one direction.
Figure 2. (A) UV-vis absorption and (B) fluorescence spectra of
2A1X, 3A2X, 4A3X, P1, and 11 in CHCl3 (1.0 × 10-5 M).
coefficients of the oligomers and polymer with the number
of anthracene rings, the coefficients were found to be smaller
than that of 11 due to the intramolecular π-stacked structure
of the anthracene units in the ground state.
Acknowledgment. This work was supported by Grant-
in-Aid for Young Scientists (A) (No. 21685012) from the
Ministry of Education, Culture, Sports, Science and Technol-
ogy, Japan.
Fluorescence spectra in CHCl3 (1.0 × 10-5 M)20 revealed
that 2A1X exhibited an emission peak at around 450 nm
with a clear vibrational structure similar to that of 11.
Excimer emission was not observed from 2A1X despite the
face-to-face orientation of the two anthracenes. In contrast,
the emission peaks of 3A2X, 4A3X, and P1 were broad and
Supporting Information Available: Experimental details
and an X-ray crystallographic files in CIF format. This
material is available free of charge via the Internet at
http://pubs.acs.org.
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OL1011295
(20) The concentration, 1.0 × 10-5 or 1.0 × 10-6 M, is sufficiently low
and the intermolecular interactions can be ignored; see Figures S23-S26
in Supporting Information.
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