Macromolecules, Vol. 37, No. 12, 2004
Origin of the Color of π-Conjugated Polymers 4349
Sch em e 6. P ossible Str u ctu r e of Ra d ica l: a
pared using various solvents manifested that the yellow
or orange color of the polymers is ascribed to formation
of the columnar as the π-conjugated self-assembly whose
content can be also decreased with compression associ-
ated with a large red shift of the absorption maximum
in the UV-vis spectrum of the original polymer. Thus,
formation and destruction of the columnar as π-conju-
gated self-assembly is a very important concept for the
color design of π-conjugated polymers because an ab-
sorption maximum in the conjugated polymer is cor-
related with not only the color but also the ionization
potential. Therefore, the columnar formation at the solid
phase becomes a new useful method to control the color
of the conjugated polymer even in the cis-transoid form
without formation of trans polymers; this color element
works as a new concept concerning promising molecular
designs for advanced materials such as non-linear
optical (NLO)34-36 or LED37,38 materials used at the solid
state. At present, the origin of color of other aromatic
polyacetylenes as well as aliphatic polyacetylenes is
being examined more in detail together with the color
of the cast film of the polymers. Experiments regarding
the pressure-induced cis-to-trans isomerization are in
progress in our laboratory, and the results will be
published elsewhere soon.
in model structures, A or A* (see Scheme 6), using the
so-called semiempirical quantum chemical calculation,
AM1 method. The calculation indicated that more than
one-third of the spin density appeared on the each
carbazoyl nitrogen of the main-chain spin density. This
structure will be visualized as case A*, where the
unpaired electron created by rotational scission of the
cis CdC bond delocalized even over the carbazotyl
moiety, i.e., the nitrogen moiety. In other words, this
delocalized structure means that the unpaired electrons
migrate to the side chain moiety, i.e., carbazoyl group
to meet an electron released by the carbazoyl nitrogen
moiety as a strong electron donor group forming the so-
called nitrogen radical cation (see A* in Scheme 6).32
On the other hand, before the compression two cis form
radicals, e.g., a ′ and/or b′, are created as localized main-
chain radicals so that the so-called motional narrowing
in the line width is not expected, but a larger g value
may be observed. Thus, we can reasonably explain the
reason why the g value, 2.0034, in the ESR spectra
observed after the compression is larger than the g )
2.0029 before the compression. Another main-chain
radical, b′, was also examined to determine the spin
density. However, the radical b′ structure did not satisfy
the observed experimental results, unlike the case a ′.
Thus, the calculated large spin density on the carbazoyl
nitrogen clearly explains the reason why the N-CH2
proton peak is broadened after the compression in terms
of the so-called magnetic dipole-dipole interaction
between the unpaired electrons on the nitrogen and the
methylene protons in the chloroform-d1 solution.
This calculation suggests that at least the original
direction of the adjacent two alkyl groups in the carba-
zoyl moieties, R2 and R3 (see Schemes 4 and 5), is
opposite each other to compensate for the dipole moment
of the carbazoyl moiety as such large side groups,
although direction of the other carbazoyl group is
unclear.
Refer en ces a n d Notes
(1) Burroughs, J . H.; Bradley, D. D. C.; Brown, A. R.; Marks, R.
N.; Mackay, K.; Friends, R. H.; Burn, P. L.; Holms, A. B.
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(2) Cf. e.g. reviews on the chemistry of electroluminescent
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2000, 25, 1089.
(3) Cf. e.g. reviews on molecular OLEDs in: (a) Chen, C. H.; Shi,
J .; Tang, W. C. Macromol. Symp. 1998, 125, 1. (b) Zhang, C.;
von Seggern, H.; Pakbaz, K.; Kraabel, B.; Schmidt, H.;
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A. Polym. J . 1991, 23, 1135.
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Macromolecules 2001, 34, 3776. (b) Tang, B. Z.; Poon, W. H.;
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Finally, it should be also noted that the calculated
spin densities on the two carbazoyl moieties are nearly
equal to each other, and the sign of the magnetic spin
in the main-chain carbon is V. On the other hand, the
signs of the two nitrogens are v, as shown in Scheme 6.
Therefore, this polymer obtained after the compression
may be classified as a promising anti and/or ferromag-
netic polymer similar to a model predicted by Ovchin-
nikov et al.33
(13) Dumitsurescu, S.; Percec, V.; Simionescu. C. I. J . Polym. Sci.,
Polym. Chem. Ed. 1977, 15, 2893.
Con clu sion
(14) (a) Tabata, M.; Sone, T.; Sadahiro, Y. Macromol. Chem. Phys.
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(15) Tabata, M.; Takamura, H.; Yokota, K.; Nozaki, Y.; Hoshina,
T.; Minakawa, H.; Kodaira, K. Macromolecules 1994, 27,
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(16) Tabata, M.; Tanaka, Y.; Sadahiro, Y.; Sone, T.; Yokota, K.;
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(17) Tabata, M.; Sone, T.; Sadahiro, Y.; Yokota, K.; Nozaki, Y. J .
Polym. Sci., Part A: Polym. Chem. 1998, 36, 217.
Poly(N-n-octyl-3-carbazoyl)acetylene, p(NOCzA), was
stereoregularly prepared using the Rh complex catalyst,
[Rh(NBD)Cl]2, in the presence of various solvents at
around room temperature to selectively produce the
corresponding cis-transoid polymer in high yields. The
poly(NOCzA)s obtained before and after compression
were characterized in detail using conventional analyti-
cal methods. Consequently, the pristine polymer pre-