Dibenzofulvene, a 1,1-diphenylethylene analogue, gives a π-stacked polymer by anionic, free-radical, and cationic catalysts [2]

Full text of DOI:10.1021/ja0111131

9182
J. Am. Chem. Soc. 2001, 123, 9182-9183
indicated that the THF-soluble parts have a vinyl polymer structure
(Supporting Information).9
Dibenzofulvene, a 1,1-Diphenylethylene Analogue,
Gives a π-Stacked Polymer by Anionic, Free-Radical,
and Cationic Catalysts
To obtain information on why DBF gives a polymer, while
DPE does not, the electron density of the vinyl group was first
considered. The 13C chemical shift of the â-vinyl carbon of DBF
was 107.7 ppm, while those of DPE and styrene were 114.3 and
113.6 ppm, respectively, as determined in CDCl3 at 23 °C using
a JEOL JNM-ECP600NK spectrometer (150 MHz), suggesting
that the â-carbon of DBF has a higher electron density compared
with those of DPE and styrene.10 However, this only supports
the reactivity of DBF during cationic polymerization. We next
examined the structure of DBF using semiempirical molecular
orbital calculations. The AM1 calculation11 using the Hyperchem
(Hypercube) software package suggested a nearly planar structure
for DBF which is not possible for DPE. The nearly planar
structure should not only reduce steric hindrance but also
effectively stabilize the anionic, cationic, or radical species at the
R-carbon through electron conjugation. In addition, the bond angle
between the two vinyl-to-phenyl bonds in DBF was 105.6°, which
is significantly deviated from 120°, implying that the vinyl group
of DBF contains a significant strain energy. Therefore, strain relief
may be a driving force of the DBF polymerization regardless of
the type of active species. This would reasonably explain the fact
that DBF polymerizes even with t-BuOK. The enhanced reactivity
of cyclic monomers having an exomethylene group compared with
the acyclic parent vinyl compound has been pointed out;12,13
however, the acyclic counterparts do slowly produce homopoly-
mers in the existing examples whereas DPE does not produce a
homopolymer.
Tamaki Nakano,*,†,‡ Kazuyuki Takewaki,‡ Tohru Yade,‡ and
Yoshio Okamoto§
PRESTO, Japan Science and Technology Corporation (JST)
Takayama-cho 8916-5, Ikoma, Nara 630-0101, Japan
Graduate School of Materials Science
Nara Institute of Science and Technology (NAIST)
Takayama-cho 8916-5, Ikoma, Nara 630-0101, Japan
Department of Applied Chemistry
Graduate School of Engineering, Nagoya UniVersity
Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
ReceiVed May 2, 2001
1,1-Diphenylethylene (DPE) is known as a representative vinyl
compound that does not produce a homopolymer through any
kind of catalysis, and its low reactivity has been ascribed to steric
reasons.1-3 Here we report that dibenzofulvene (DBF),4 whose
structure is closely related to that of DPE, affords a polymer by
anionic, radical, and cationic catalyses. DBF differs from DPE
only in the fact that the two phenyl groups are connected to each
other to form a fused ring structure. In addition, the absorption
and emission features of the polymerization products suggested
that they possess a novel “π-stacked structure” in which the
aromatic groups in the side chain are stacked on top of each other.
The absorption and emission spectra of the THF-soluble
product of the anionic polymerization (run 2 in Table 1) along
with those of fluorene as a model of the monomeric unit are shown
in Figure 1. The THF-soluble part was purified by SEC
fractionation and the chemical structure was identified as 1 (n )
3-17) by 1H NMR and MALDI-TOF mass analyses. The
absorption spectrum of the oligomers indicated a significant
hypochromism and had the broader, red-shifted peaks compared
with that of fluorene, suggesting that the side-chain fluorene
groups are stacked and the chromophore-to-choromophore dis-
tance is short enough to cause a π-π interaction in the ground
state. The degree of hypochromism is comparable to those
reported for DNAs and oligomers having fully overlapped
aromatic groups.14-16 Furthermore, in the emission spectrum of
the oligomers, the excimer band with a maximum at about 400
nm dominated over the monomer emission with negligible
intensity whose wavelength corresponds to the emission band of
fluorene. The emission spectrum of the oligomers was independent
of concentrations below 1.3 × 10-5 M in fluorene units, indicating
that the excimer is only formed intramolecularly. The excimer
DBF was synthesized from 9-hydroxymethylfluorene (Aldrich)
by the reaction in a methanol solution containing KOH according
to the literature5,6 and purified by recrystallization7 from hexane
(mp 50-52 °C (lit.5 48-49 °C)).
The conditions and results of the polymerization are sum-
marized in Table 1. In the anionic polymerization, the reactions
using n-butyllithium (n-BuLi) and 9-fluorenyllithium (FlLi) led
to nearly complete monomer consumption at -78 °C, while that
using CH3MgBr required room temperature to achieve a 73%
monomer conversion. It was interesting that t-BuOK, a weak
nucleophile, very effectively polymerized DBF at -78 °C. In this
case, generation of a 9-fluorenyl anion, a carbanion, from the
oxy anion was indicated by the deep red color of the reaction
mixture.8 Cationic and free-radical polymerizations also produced
a polymer. Under the reaction conditions shown in Table 1, DPE
did not produce a polymer. In all cases, the products were only
partially soluble in solvents such as tetrahydrofuran (THF) and
chloroform. The 1H NMR and MALDI-TOF mass analyses
† PRESTO, JST.
‡ Nara Institute of Science and Technology (NAIST).
§ Nagoya University.
(9) The IR spectra (Supporting Information) and elemental analysis
indicated that the insoluble polymers have the same chemical structure as the
soluble part. Because the anionic polymerization at [DBF]/[Li] ) 5 under
conditions similar to run 1 in Table 1 led to a higher content of THF-soluble
part (47%), the insoluble polymers probably have higher molecular weights
compared with the soluble parts.
(10) The Mulliken charges obtained by ab initio calculations supported this
conclusion (Supporting Information).
(11) Dewar, M. S. J.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am.
Chem. Soc. 1985, 107, 3902.
(1) (a) Evans, A.; George, D. J. Chem. Soc. 1961, 4653. (b) Alwyn, G.;
Evans, G.; George, D. B. J. Chem. Soc. 1962, 141.
(2) Yuki, H.; Hotta, J.; Okamoto, Y.; Murahashi, S. Bull. Chem. Soc. Jpn.
1967, 40, 2659.
(3) Richards, D. H.; Scilly, N. F. J. Polym. Sci., Polym. Lett. 1969, 7, 99.
(4) Although polymer formation from DBF was found in 1937, no details
of the reaction nor the structure of the products has been reported: Wieland,
H.; Probst, O. Liebigs Ann. Chem. 1937, 530, 274.
(5) Greenhow, E. J.; McNeil, D.; White, E. N. J. Chem. Soc. 1952, 986.
(6) More O’Ferrall, R. A.; Slae, S. J. Chem. Soc., Chem. Commun. 1969,
486.
(7) The monomer crystal changed into a polymeric product during storage,
suggesting the possibility of a solid-state polymerization. This aspect will be
reported elsewhere.
(8) Although the generation of a carbanion from an oxy anion has been
accomplished using Si-containing cyclic compounds, the method using a
hydrocarbon compound is unprecedented to the best of our knowledge: (a)
Sheikh, Md. R. K.; Imae, I.; Tharanikkarusu, K.; LeStrat, V. M.-J.; Kawakami,
Y. Polym. J. 2000, 32, 527. (b) Zundel, T.; Baran, J.; Mazureki, M.; Wang,
J.-S.; Jerome, R.; Teyssie, P. Macromolecules 1998, 31, 2724.
(12) (a) Ueda, M.; Mano, M,; Mori, H.; Ito, H. J. Polym. Sci., Part A:
Polym. Chem. 1991, 29, 1779. (b) Ueda, M.; Takahashi, M.; Suzuki, T. J.
Polym. Sci., Polym. Phys. Ed. 1983, 20, 1139. (c) Ueda, M.; Mori, H. J. Polym.
Sci., Part A.: Polym. Chem. 1990, 28, 1779.
(13) Suenaga, J.; Sutherlin, D. M.; Stille, J. K. Macromolecules 1984, 27,
2913.
(14) (a) Tinoco, I. J. Am. Chem. Soc. 1960, 82, 4785. (b) Rohdes, W. J.
Am. Chem. Soc. 1961, 83, 3609.
(15) Nelson, J. C.; Saven, J. G.; Moore, J. S.; Wolynes, P. G. Science 1997,
277, 1793.
(16) Okamoto, K.-I.; Itaya, A.; Kusabayashi, S. Chem. Lett. 1974, 1167.
10.1021/ja0111131 CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/25/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 37, 2001 9183
Table 1. Polymerization of Dibenzofulvenea
[M]0
(M)
[I]0
(M)
temp
(°C)
time
(h)
conv.b
(%)
THF-insoluble
part yield (%)
THF-soluble
part yield (%)
run
solvent
initiator
Mnd
1
2
3
4
5
6
7
8
THF
n-BuLi
1.0
0.10
0.04
0.04
0.04
0.05
0.20
0.048
0.076
-78
24
24
96e
24
24
24
24
48
>99
>99
73
87
<1
46
76
86
82
15
-
13
>99
27
770
810
THF
9-FlLi
0.21
0.20
0.20
1.0
-78
THF
THF
CH2Cl2
CH2Cl2
toulene
toulene
CH3MgBr
t-BuOK
BF3-Et2O
BF3-Et2O
BPO-DMAg
AIBN
-78-rte
1650f
1170
870
-78
>99
24
0
96
10
1.0
0.88
0.19
rt
>99
22
17
1050
2560f,h
940i
0
7
60
63
-
a Monomer: 0.4-11.2mmol. b Determined by 1H NMR analysis of the reaction mixture. c THF-sol. part was a mixture of oligomers and unreacted
monomer when the monomer conversion was not quantitative. The yield of this part was calculated excluding the weight of the unreacted monomer.
d Determined by GPC using two TSKgel G1000HHR columns connected in series with oligo(DBF)s as the standard (eluent, THF). e Reaction was
performed first at -78 °C for 24 h and then at rt for an additional 72 h. Monomer conversion during the first stage at -78 °C for 24 h was 2%.
f TSKgel G6000HHR and G3000HHR were used in a serial connection. g BPO ) benzoyl peroxide. DMA ) N,N-dimethylaniline. h Standard polystyrenes
were used for calibration. i Obtained at 28% yield by fractionating CHCl3-sol. part (yield 45%) of the crude mixture by preparative GPC using
JAIGEL 1H and 2H columns connected in series (eluent, CHCl3).
Figure 2. Structures of the model trimer with the definition of the di-
hedral angles (A), the most stable conformer for the trimer (B), a con-
former of the model decamer constructed based on B and minimized by
Figure 1. Absorption (A) and emission (B) spectra of fluorene (a) and
oligo(DBF), 1 (n ) 3-17) (b) in THF at ambient temperature (23 °C).
The solution concentration for a was 1.14 × 10-5 M and that of b was
1.36 × 10-5 M (per monomeric residue). The excitation wavelength in
B was 265 nm (a) and 267 nm (b).
MM calculation (C), and the two conformations with the terminal fluorene
unit being flipped generated in the MD simulation at 600 K (D, E).
fluorenyl groups through an MD simulation at 300 K for 5.2 ns.
On the basis of a simulation at 600 K for 6.6 ns, structures D
and E in Figure 2 were found in addition to structure C, the major
conformation observed in the simulation. Structures D and E had
ca. 10-kcal/mol higher steric energy than structure C. Hence, the
conformation having an all-trans main-chain conformation such
as Figure 2, B and C, is proposed for poly(DBF) as the most
plausible structure with a possible conformational “mistake” at
the chain terminals as in structures D and E.20 Such a conformation
is consistent with the observed photophysical features and is
supported by the 1H NMR spectra that indicates significant upfield
shifts in the aromatic protons (Supporting Information).
In summary, this work revealed the unexpected, high polym-
erization activity of DBF and the unique conformation of the
obtained polymer that may be recognized as a new structural motif
of vinyl polymers. The π-π interaction between the chro-
mophores aligned along the polymer chain could realize some
unprecedented photophysical and photoelectronic characteristics
such as controlled energy and electron transfer, possibly leading
to novel photoconducting and luminescent materials. Further
studies on the detailed structure and photophysical aspects of the
oligo(DBF)s are under way along with the design of novel
π-stacked polymers.
Acknowledgment. We acknowledge Professor A. Tsuchida (Gifu
University) for his advice on the optical measurement and Dr. C.
Yamamoto (Nagoya University) for her advice on the force field
calculations. We are grateful to Professor Y. Kanemitsu (NAIST),
Professor D. Y. Sogah (Cornell University), Professor H. Yao (Himeji
Institute of Technology), Dr. A. Nakasuka, Mr. H. Ishizawa (Sekisui
Chemical), and Mr. H. Inaba (Nippon Paint) for their fruitful discussions.
Supporting Information Available: Experimental procedures, com-
pound characterization, details of molecular mechanics calculation,
Mulliken charges of monomers, and IR spectra (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
emission is red-shifted compared with those of fluorene17 (370
nm in toluene) and poly(2-vinylfluorene)18 (380 nm in THF),
suggesting that the excimer structure for poly(DBF) is somewhat
different from those of fluorene and poly(2-vinylfluorene). These
photophysical features suggest that the fluorene groups in the
oligomers form a regular, stacked structure.
To obtain information on the polymer conformation, molecular
mechanics and dynamics (MM and MD) calculations were
performed with the Dreiding force fiel19 (ver. 2.11) for a model
trimer (1, n ) 2) and a model decamer (1, n ) 9) using the Cerius2
(MSI) software package. For the model trimer, through the
conformer search among the 1296 combinations of the four di-
hedral angles (Figure 2A) with the angle interval of 60°, the one
shown in Figure 2B was found to be the most stable conformer.
The structure of the conformer is characterized by the nearly all-
trans, zigzag main chain and the fully overlapped neighboring
fluorenyl groups. On the basis of this structure, an all-trans
decamer was generated and optimized by the MM calculations
to give the structure shown as Figure 2C. The stability of structure
C was then assessed by the MD simulations. Structure C did not
show any significant change in terms of the stacking of the
(17) Horrocks, D. L.; Brown, W. G. Chem. Phys. Lett. 1970, 5, 117.
(18) Matsuda, M.; Watanabe, A. In Recent AdVances in Anionic Polymer-
ization; Hogen-Esch, T. E., Smid, J., Eds.; Elsevier: New York, 1987; p 73.
(19) (a) Mayo, S. L.; Olafson, B. D.; Goddard, W. A., III. J. Phys. Chem.
1990, 94, 8897. (b) Rappe, A. K.; Goddard, W. A., III. J. Phys. Chem. 1991,
95, 3358.
(20) MD simulations for an isotactic model of poly(2-vinylfluorene)
indicated that π-stacking of the model was much less stable than that of the
DBF decamer model (Supporting Information).
JA0111131