Synthesis of conjugated oligomers having aromatic and enediyne units alternately in the backbone that show intense fluorescence emission

Article abstract of DOI:10.1021/ol049277k

(Equation Presented) Synthesis and fluorescence properties of π-conjugated compounds having alternately an aromatic ring such as benzene, pyridine, and thiophene and an enediyne unit in the backbone are described.

Full text of DOI:10.1021/ol049277k

ORGANIC
LETTERS
2004
Vol. 6, No. 14
2373-2376
Synthesis of Conjugated Oligomers
Having Aromatic and Enediyne Units
Alternately in the Backbone that Show
Intense Fluorescence Emission
Yuuki Nakano, Kenichi Ishizuka, Kenji Muraoka, Hiroyuki Ohtani,
Yuuki Takayama,* and Fumie Sato*
Department of Biomolecular Engineering, Tokyo Institute of Technology,
4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8501, Japan
fsato@bio.titech.ac.jp
Received April 20, 2004
ABSTRACT
Synthesis and fluorescence properties of π-conjugated compounds having alternately an aromatic ring such as benzene, pyridine, and thiophene
and an enediyne unit in the backbone are described.
Linearly π-conjugated oligomers and polymers having an
enediyne-scaffold, i.e., oligo- and poly(triacetylene)s, have
attracted much interest as advanced materials for electronic
and photonic applications.1,2 Recently, Diederich and co-
workers have shown that the introduction of aromatic spacers
into oligo- or poly(triacetylene)s results in significant
modulation of their optical properties. They synthesized
highly fluorescent hybrid polymers by inserting biphenyl
moieties into poly(triacetylene)s.2 They also prepared a
comprehensive series of hybrid oligomers by insertion of
different aromatic rings between two enediyne moieties and
investigated their optical and electronic properties compared
to those of the corresponding homotrimer, highlighting the
enhancement of the emission properties.3 Most recently, they
prepared (phenylene enediyne)-monomer, -dimer, -trimer,
and -tetramer of type 1 (n ) 1-4) shown in Figure 1 using
Sonogashira coupling on solid support as a key reaction.
Figure 1.
Interestingly, the dimer and, in particular, the trimer and
tetramer display intense fluorescence emission, with the
highest quantum yield, ΦF ) 0.69, being measured for the
trimer.4,5 We therefore were interested in the fluorescence
property of oligo(arylene enediyne)s that have an aromatic
(1) Martin, R. E.; Diederich, F. Angew. Chem., Int. Ed. 1999, 38, 1350-
1377.
(2) Edelmann, M. J.; Odermatt, S.; Diederich, F. Chimia 2001, 55, 132-
138.
(3) (a) Martin, R. E.; Wytko, J. A.; Diederich, F.; Boudon, C.;
Gisselbrecht, J.-P.; Gross, M. HelV. Chim. Acta 1999, 82, 1470-1485. (b)
Edelmann, M. J.; Raimundo, J.-M.; Utesch, N. F.; Diederich, F.; Boudon,
C.; Gisselbrecht, J.-P.; Gross, M. HelV. Chim. Acta 2002, 85, 2195-2213.
(4) Utesch, N. F.; Diederich, F. Org. Biomol. Chem. 2003, 1, 237-239.
10.1021/ol049277k CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/12/2004

Scheme 1
Scheme 3
group other than a phenyl group. The synthetic method of
Diederich, however, only allowed access to oligo(phenylene
enediyne), and thus, a new synthetic method was needed.
We have now succeeded in developing a general method
for preparing oligo(arylene enediyne)s.
Previously, we reported a synthetic method for preparing
trans-1-iodo-4-(trimethylsilyl)but-1-en-3-yne derivatives start-
ing from 1-trimethylsilyl-1-alkynes and ethynyltrimethylsi-
lane according to the procedure shown in Scheme 1. The
iodoenynes thus prepared were effectively used as building
blocks for synthesizing conjugated oligomers having an
enyne or enediyne unit.6 Applying the protocol shown in
Scheme 1, we have now succeeded in preparing vinyl iodo
Scheme 2a
compounds 2 easily. Thus, as shown in Scheme 2, com-
pounds 2, where Ar is a phenyl (2a), pyridyl (2b), or thienyl
group (2c), were prepared in 32-38% overall yield by
starting with, respectively, diynes 3a-c instead of ethynyl-
trimethylsilane. Compounds 2 thus obtained proved to serve
as building blocks for synthesizing the corresponding oligo-
(arylene enediyne)s (vide infra). Meanwhile, the starting
compounds 3a-c were readily prepared in good overall yield
according to the conventional reaction sequences as shown
in Scheme 3.7 As also shown in Scheme 3, we prepared
a Reagents: (i) For Ar ) Ph, Th; t-BuLi, PrI. For Ar ) Py;
Bu(PhS)CuLi.
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Org. Lett., Vol. 6, No. 14, 2004

preparing conjugated oligomers having an enyne scaffold6
(Scheme 4). Thus, Sonogashira coupling of 2a with 4a
afforded 5 (n ) 1) in 93% yield. Treatment of 5 (n ) 1)
with TBAF and Sonogashira coupling of the resulting
desilylated compound with 2a furnished the dimer 5 (n )
2) in 79% overall yield, which in turn was converted to the
trimer 5 (n ) 3) by desilylation and coupling with 2a.
Similarly, oligomers 6 (n ) 1-3) and 7 (n ) 1-3), which
contain a pyridyl or thienyl group and enediyne units
alternately, were synthesized starting from, respectively, 2b
and 4b and 2c and 4c as shown in Scheme 5.
Scheme 4
The electronic absorption and fluorescence spectra of the
oligo(arylene enediyne)s 5-7 thus prepared are summarized
in Table 1. It can be seen from Table 1 that, in all cases, the
longest-wavelength absorption maxima λmax are bathochro-
mically shifted with increasing conjugation length as ex-
pected.4 While the position of the λmax differs little between
the oligomer having a phenyl or pyridyl group as the arylene
unit, it shifts about 30 nm to lower energy for the thiophene-
containing oligomers 7 in comparison with 5 and 6, presum-
ably due to the electron-rich character of the thiophene ring.
As reported by Diederich, the oligomers having a phenyl
group and an enediyne unit alternately show very intense
fluorescence emission. The fact that the oligomers 5 possess
somewhat higher fluorescence quantum yield compared with
the Diederich oligomer 1 with similar conjugated length
seems to be attributable to a heavy-atom effect of the terminal
iodine group in 1. While the fluorescence intensity and the
emission maxima of the pyridine-containing oligomer 6 are
similar to those of 5, thiophene derivative 7 shows a weaker
but yet intense fluorescence emission with a significantly
larger bathochromic shift in the emission maxima compared
with 5 and 6.
diacetylenes 4a-c, which can be utilized as the terminal unit
for synthesizing oligo(arylene enediyne)s.
Using 2a and 4a thus prepared, we synthesized conjugated
oligomers 5 having alternately a phenyl and an enediyne unit
according to essentially the same synthetic method for
Scheme 5
Org. Lett., Vol. 6, No. 14, 2004
2375

backbone. All the oligo(arylene enediyne)s thus synthesized
show intense fluorescence emission. Investigation on the
utility of these oligomers as organic light-emitting diodes is
now in progress in our group.
Table 1. Electronic Absorption and Emission Data of
Oligo(arylene enediyne)sa
oligo(arylene enediyne)s
emission
λmax [nm]d ΦF
absorption λmax [nm]b
(ꢀ [M-1 cm-1]c)
e
Acknowledgment. Partial financial support of this work
by the Ministry of Education, Culture, Sports, Science and
Technology (Japan) is acknowledged.
n
5
6
7
1
2
3
1
2
3
1
2
3
355 (49 000)
379 (91 600)
390 (122 000)
359 (53 100)
384 (94 200)
394 (158 000)
385 (23 400)
407 (52 000)
427 (58 200)
398
429
442
405
436
447
434
477
494
0.83
0.85
0.76
0.84
0.78
0.81
0.58
0.30
0.34
Supporting Information Available: Experimental pro-
cedures, spectroscopic data, and physical properties of 2-7.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL049277K
a Spectra were recorded at room temperature in CHCl3. b Longest-
wavelength absorption maximum. c Molar extinction coefficient. d Fluo-
rescence emission maximum. e Fluorescence quantum yield, determined by
using Quinine sulfate (ΦF ) 0.577) as reference compound.
(5) For other fluorescent conjugated polymers and oligomers, see
reviews: (a) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem.,
Int. Ed. 1998, 37, 402-428. (b) Bunz, U. H.; Chem. ReV. 2000, 100, 1605-
1644. (c) Mitschke, U.; Ba¨uerle, P. J. Mater. Chem. 2000, 10, 1471-1507.
(6) (a) Takayama, Y.; Delas, C.; Muraoka, K.; Sato, F. Org. Lett. 2003,
5, 365-368. (b) Takayama, Y.; Delas, C.; Muraoka, K.; Uemura, M.; Sato,
F. J. Am. Chem. Soc. 2003, 125, 14163-14167.
(7) (a) Zehner, R. W.; Parsons, F.; Hsung, R. P.; Sita, L. R. Langmuir
1999, 15, 1121-1127. (b) Tilley, J. W.; Zawoiski, S. J. Org. Chem. 1988,
53, 386-390.
In conclusion, we have developed a general and efficient
method for preparing monodisperse π-conjugated oligomers
having aromatic and enediyne units alternately in the
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Org. Lett., Vol. 6, No. 14, 2004