Extension of π-conjugation length via the vacant p-orbital of the boron atom. Synthesis of novel electron deficient π-conjugated systems by hydroboration polymerization and their blue light emission [1]

Full text of DOI:10.1021/ja9741943

5112
J. Am. Chem. Soc. 1998, 120, 5112-5113
Communications to the Editor
Scheme 1
Extension of π-Conjugation Length via the Vacant
p-Orbital of the Boron Atom. Synthesis of Novel
Electron Deficient π-Conjugated Systems by
Hydroboration Polymerization and Their Blue Light
Emission
Noriyoshi Matsumi, Kensuke Naka, and Yoshiki Chujo*
Department of Polymer Chemistry
Graduate School of Engineering
Kyoto UniVersity, Yoshida
Sakyo-ku, Kyoto 606-8501, Japan
ReceiVed December 11, 1997
For the past decades, considerable attention has been focused
on the construction of novel π-conjugated systems for their
remarkable electrochemical and optical properties. Their potential
utility ranges from light emitting diodes (LED), energy storage
systems (batteries), nonlinear optical (NLO) materials, and so on.
A large number of papers have been published about their
preparative methods with electrochemical synthesis,1 chemical
oxidation,2 and polycondensation3 reactions. Here we report a
facile synthesis of novel well-defined π-conjugated systems by
polyaddition using the well-known hydroboration reaction.4
The conjugative interaction between the vinyl group and the
boron atom has been investigated by using the spectral properties
of low molecular weight vinylborane derivatives.5 In the early
studies, it was found that these spectroscopic data such as 11B
NMR and absorption maxima were in accordance with the
calculated results with Hu¨ckel MO theory, which suggested
considerable conjugative overlap of the π-orbitals of the vinyl
groups with the p-orbital of the boron atom.5a
Scheme 2
Table 1. Hydroboration Polymerization of Aromatic Diynes
Using Mesitylboranea
Previously, we have explored various methodologies for the
preparation of organoboron main-chain polymers by hydroboration
polymerization.6 Although most of these polymers obtained were
relatively unstable, several organoboron polymers were found to
have relatively high stability toward air oxidation.7 The air
stability of these polymers might show the possibility of the
application of these polymers as functional materials.
a Reactions were carried out in THF at room temperature. b GPC
(THF), polystyrene standards. c Isolated yields after reprecipitation into
MeOH.
(1) For example: Kovacic, P.; Oziomek, J. J. Am. Chem. Soc. 1963, 85,
454.
Generally, aromatic organoboron compounds such as tri-
phenylborane or trimesitylborane8 are known as strong electron
acceptors. Therefore, the polymeric homologues of these materi-
als are expected to possess unique properties as a novel type of
n-type conjugated polymers. For example, these include high
electron affinity or extened π-conjugation length via the vacant
p-orbital of the boron atom.
The synthesis of a series of conjugated organoboron polymers
was examined by hydroboration polymerization of aromatic diynes
with mesitylborane, utilizing the highly regioselective nature of
the hydroboration reaction of mesitylborane with the acetylene
bond. Aromatic diynes (2)9 and mesitylborane (1)6f,10 were
prepared by the modified procedure shown in Scheme 1 according
to the reported method.
(2) For example: (a) Goldfingger, G. J. Polym. Sci. 1949, 93, 4. (b)
Yamamoto, T.; Hayashi, Y.; Yamamoto, A. Bull. Chem. Soc. Jpn. 1978, 51,
2091. (c) MacDonald, D. N.; Campell, T. W. J. Am. Chem. Soc. 1960, 82,
4669. (d) Murase, I.; Ohnishi, T.; Noguchi, T.; Hirooka, M. Polym. Commun.
1984, 25, 327. (e) Yamamoto, T.; Sanechika, K.; Yamamoto, A. Bull. Chem.
Soc. Jpn. 1983, 56, 1497. (f) Amer, A.; Zimmer, H.; Mulligan, K. J.; Mark,
H. B.; Pons, S., Jr.; MeAleer, J. F. J. Polym. Sci. Part C. Polym. Symp., 1984,
22, 77.
(3) For example: (a) Diaz, A. F.; Logan, J. A. J. Electroanal. Chem. 1980,
111, 111. (b) Sato, M.; Kaneko, K.; Yoshino, K. J. Chem. Soc., Chem.
Commun. 1985, 1629. (c) Dall’Olio, A.; Dascola, Y.; Varacca, V.; Bocchi,
V. C. R. Hebd. Seances Acad. Sci., Ser. C1968, 267, 433. (d) Diaz, A. F.;
Kanazawa, K. K.; Gardini, G. P. J. Chem. Soc., Chem. Commun. 1979, 635.
(4) (a) Brown, H. C. Hydroboration; W. A. Benjamin, Inc.: New York,
1962. (b) Pelter, A.; Smith, K.; Brown, H. C. Borane Reagents; Academic
Press: London, 1988.
(5) For example: (a) Good, C. D.; Ritter, D. M. J. Am. Chem. Soc. 1962,
84, 1162. (b) Good, C. D.; Ritter, D. M. J. Chem. Eng. Data, 1 1962, 416. (c)
Zweifel, G.; Clark, G. M.; Leung, T.; Whitney, C. C. J. Organomet. Chem.
1976, 117, 303.
(6) (a) Chujo, Y.; Tomita, I.; Hashiguchi, Y.; Tanigawa, H.; Ihara, E.;
Saegusa, T. Macromolecules 1991, 24, 345. (b) Chujo, Y.; Tomita, I.; Saegusa,
T. Polym. Bull. 1992, 27, 375. (c) Chujo, Y.; Tomita, I.; Hashiguchi, Y.;
Saegusa, T. Macromolecules 1992, 25, 33. (d) Matsumi, N.; Chujo, Y. Polym.
Bull. 1997, 38, 531.
A typical procedure for the polymerization (Scheme 2) is as
follows. To a freshly distilled tetrahydrofuran solution of diyne
monomer (2) was added dropwise a slightly excess amount of
mesitylborane (1) in THF under nitrogen at room temperature,
(7) Sasaki, Y.; Kinomura, N.; Chujo, Y. Unpublished data.
S0002-7863(97)04194-2 CCC: $15.00 © 1998 American Chemical Society
Published on Web 05/12/1998

Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 20, 1998 5113
Table 2. Optical Properties of Organoboron Polymers 3a-d
polymers UV λmaxa (nm)
ꢀa
PL λmaxa,b (nm)
3a
3b
3c
3d
399
390
350, 390, 407
364, 383, 405
8470
17700
441
440
9670, 10230, 10800 455
3520, 5360, 6120 412, 436, 462
a Absorption and emission spectra were recorded in dilute CHCl3
solutions at room temperature. b Excited at 350 nm.
and the resulting reaction mixture was stirred for several hours.
After the complete addition of mesitylborane, the dark reaction
mixture remained homogeneous and no gelation was observed.
The reaction mixture was directly subjected to gel permeation
chromatographic analysis (THF as an eluent, polystyrene stan-
dards). After removal of the solvent, a dark brown gum was
obtained. The polymer was purified by reprecipitation into
methanol. After freeze-drying, each polymer was obtained as a
yellow powder (3a-3c) or as an orange powder (3d). The
polymers obtained were highly soluble in common organic
solvents such as THF, chloroform, and benzene, even in the case
of 3d having the anthracene unit. The structures of the polymers
were supported by 1H, 11B NMR and IR spectra. In the 1H NMR
spectrum of 3a in CDCl3, the peak corresponding to the acetylene
proton at 3.22 ppm almost disappeared while those of the mesityl
group appeared at 2.18-2.42 (Me) and 6.93 (Ar-H) ppm.
The results of the polymerization with various diyne monomers
are summarized in Table 1. In every case, the corresponding
polymers were obtained in good yields (67-95%). The relatively
low molecular weight of 3d is probably due to the low solubility
of the monomer 2d.
Figure 1. UV-vis spectra of 3a (dot line) and 4 (solid line) in CHCl3.
The polymers obtained were highly fluorescent. The fluores-
cence emission spectra data of 3a-d are also summarized in Table
2. In every case, when a dilute chloroform solution was excited
at 350 nm at room temperature, an intense emission was observed
in a visible blue region. The value of λmax was independent of
the excitation wavelength. In the fluorescence emission spectrum
of 3a (2.00 × 10-4 M in chloroform), the emission maximum
was observed at 441 nm, bathochromic shifted by 38 nm
compared with that of the excitation spectrum of this sample (403
nm). This small Stokes shift indicates that the present polymer
has a relatively rigid structure.
Thermogravimetric analysis (TGA) was recorded for 3a, under
both air and under nitrogen. In both cases, 3a was stable up to
110-120 °C and was slightly more stable under nitrogen.
However, the difference in both weight loss curves was much
smaller than that of other organoboron polymers previously
investigated by us. This result indicates that the thermal
degradation of 3a was not strongly affected by the insertion of
oxygen into the C-B bond.
In conclusion, the extension of the π-conjugation length via
the boron atom was observed in the organoboron main-chain
polymers prepared by hydroboration polymerization. The poly-
mers prepared had high solubility and are expected as novel n-type
conjugated polymers. In addition, blue fluorescence emission was
observed in every case. These results may stimulate interest for
further physical studies about the electrochemical property, EL
activity, conductivity, and so on.
The UV-vis absorption spectra data for the dilute chloroform
solutions of 3a-d are shown in Table 2. Figure 1 represents the
spectrum of 3a, in which the peak due to the π-π* transition of
the polymer backbone is observed at 399 nm. The value was
largely bathochromic shifted in comparison with the model
compound 4. From this result, it was confirmed that the
π-delocalization length was highly extended via the boron atom.
Furthermore, these spectra were independent of the type of
solvents used. This result suggests the red shift should not be
due to a charge transfer. A relatively large band gap might be,
in part, due to a steric distortion of the main-chain by the bulky
mesityl substituent.
(8) (a) Brown, H. C.; Dodson, V. H. J. Am. Chem. Soc. 1957, 79, 2302.
(b) Ramsey, B. G.; Isabelle, L. M. J. Org. Chem. 1981, 46, 179.
(9) (a) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagiwara, N.
Synthesis 1980, 627. (b) Kobayashi, E.; Jiang, J.; Ohta, H.; Furukawa, J. J.
Polym. Sci. Part A: Polym. Chem. 1990, 28, 2641.
(10) Smith, K.; Pelter, A.; Jin, Z. Angew. Chem., Int. Ed. Engl. 1994, 33,
851.
Supporting Information Available: Experimental procedure and 1H
and 11B NMR, IR, UV-vis, and PL spectra of 3a-d (11 pages, print/
PDF). See any current masthead page for ordering information and Web
access instructions.
JA9741943