Novel fluorophores: efficient synthesis and photophysical study.

Article abstract of DOI:10.1021/ol0162264

[structure: see text] We have synthesized novel fluorophores by using Sonogashira reactions of 1,4-bis(dibromovinyl)benzene and 2,5-bis(dibromovinyl)thiophene with various aromatic bromides. The emission maxima of these fluorophores vary from the indigo b

Full text of DOI:10.1021/ol0162264

ORGANIC
LETTERS
2001
Vol. 3, No. 16
2469-2471
Novel Fluorophores: Efficient Synthesis
and Photophysical Study
Gil Tae Hwang, Hyung Su Son, Ja Kang Ku, and Byeang Hyean Kim*
Center for Integrated Molecular Systems, Department of Chemistry,
DiVision of Molecular Life Science, Pohang UniVersity of Science and Technology,
Pohang 790-784, Korea
bhkim@postech.ac.kr
Received June 4, 2001
ABSTRACT
We have synthesized novel fluorophores by using Sonogashira reactions of 1,4-bis(dibromovinyl)benzene and 2,5-bis(dibromovinyl)thiophene
with various aromatic bromides. The emission maxima of these fluorophores vary from the indigo blue to the reddish-orange region, depending
on the structures of aromatic nuclei and peripheral moieties.
Organic molecules with high photoluminescence efficiencies
have recently attracted increasing attention in various
research fields as advanced materials for electronic and
photonic applications.1,2 Thus, it is important to synthesize
efficiently novel fluorophores that are amenable to further
chemical functionalization or modification, which in turn is
essential to obtain materials with tunable optoelectronic
properties.
studies on the Sonogashira reaction7 of 1,1-dibromo-1-
alkenes have been reported. Furthermore, to the best our
knowledge, there is no precedent for the synthesis of
fluorophores by using this approach, which may provide
Scheme 1
1,1-Dibromo-1-alkenes 1 are conveniently prepared by the
procedure of Corey and Fuchs.2 They can be converted to
(Z)-1-bromo-1-alkenes 2,3 (Z)-1-aryl(alkenyl)-1-bromo-1-
alkenes 3,4 1,1-diaryl(alkenyl)-1-alkenes 4,5c 1-aryl(alkenyl)-
1-alkynes 5,3,5c and 1,3-diynes 66 (Scheme 1). However, few
(1) (a) Seminario, J. M.; Tour, J. M. In Molecular ElectronicssScience
and Technology; Aviran, A., Ratner, M., Eds.; New York Academy of
Science: New York, 1998. (b) Mu¨llen, K., Wegner, G., Eds. In Electronic
Materials: The Oligomer Approach; Wiley-VCH: New York, 1998.
(2) For recent reviews, see: (a) de Silva, A. P.; Gunaratne, H. Q. N.;
Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.;
Rice, T. E. Chem. ReV. 1997, 97, 1515. (b) Kraft, A.; Grimsdale, A. C.;
Holmes, A. B. Angew. Chem., Int. Ed. 1998, 37, 402. (c) Sheats, J. R.;
Chung. Y. L.; Roitman, D. B.; Stocking, A. Acc. Chem. Res. 1999, 32,
193. (d) Yamaguchi, S.; Endo, T.; Uchida, M.; Izumizawa, T.; Furukawa,
K.; Tamao, K. Chem. Eur. J. 2000, 6, 1683.
(3) Corey, E. J.; Fuchs, P. L. Tetrahedron Lett. 1972, 4831.
10.1021/ol0162264 CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/19/2001

Sonogashira reaction between TMS-protected tetraalkyne 10
(or 11) and aromatic bromide (route b, c).8 This method
consists of in situ liberation of alkyne in the presence of KF
from the corresponding TMS-protected compound and
coupling with aromatic bromide compounds. Because this
approach avoids the TMS deprotection and purification steps,
it is highly suitable for coupling of unstable or volatile
alkynes. Fluorophores 12b-e and 13b-e were synthesized
by this method.
Scheme 2
The structures of fluorophores 10-13 have been confirmed
1
by H and 13C NMR spectroscopy, mass spectrometry, and
elemental analysis data.9 All of the fluorophores 10-13 are
significantly stable toward air and commonly used organic
solvents. The photophysical properties of 10-13 in CHCl3
solutions are summarized in Table 1, and their normalized
emission spectra are shown in Figure 1.
conjugated oligomers and tunable fluorophores through
changes in their conjugation length and substitution. Herein,
we describe the efficient synthesis of fluorophores via
Sonogashira reaction of 1,4-bis(dibromovinyl)benzene 8 and
2,5-bis(dibromovinyl)thiophene 9 (Scheme 2). Treatment of
dialdehydes with CBr4 and PPh3 under the conditions of
Corey and Fuchs3 gave rise to the dibromo alkenes 8 and 9
in excellent yield. Two complementary methods have been
used for the synthesis of fluorophore 12 and 13 (Scheme 3).
One is Sonogashira reaction between dibromo alkene 8 or 9
with phenylacetylene (route a). Fluorophores 12a and 13a
were synthesized by this method. The other is modified
In the UV-visible absorption spectra, the absorption
maxima, ascribed to π-π* transition of fluorophores,
significantly depend on the central unit of the molecules. A
comparison of the absorption maxima of fluorophores 13a-e
containing a thiophene nucleus to that of fluorophores 12a-e
containing a benzene nucleus shows a substantial red shift.
The fluorescence spectra of all fluorophores show two strong
Scheme 3a
a (a) Phenylacetylene, (PPh3)2PdCl2, CuI, Et3N/MeOH, 45-50 °C; (b) trimethylsilylacetylene, (PPh3)2PdCl2, CuX, Et3N/MeOH, 45-50
°C; (c) ArBr, KF, (PPh3)2PdCl2, CuI, Et2NH/MeOH, 45-50 °C for 12b, 13b and Et3N/MeOH for 45-50 °C for the others, as the CuX, CuI
was used for all the fluorophores except 12c and 13c, which used CuCl.
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Org. Lett., Vol. 3, No. 16, 2001

Table 1. Photophysical Data of 10-13
c
d
e
f
compd λabs (nm)a λem (nm)b
ΦF
ΦF
ΦF
τs
10
412
416
438
450
456
420
458
468
486
516
524
522
426, 463
475, 506
498, 532
518, 551
533, 566
472, 502
475, 503
528, 561
550, 587
577, 618
595, 628
587, 612
0.01 0.01 0.03 0.6 ( 0.1
0.19 0.21 0.35 1.2 ( 0.2
0.29 0.32 0.25 1.6 ( 0.2
0.10 0.11 0.16 0.6 ( 0.1
0.08 0.08 0.16 0.8 ( 0.1
12a
12b
12c
12d
12eg
11
13a
13b
13c
13d
13eg
n.d.
n.d.
n.d.
n.d.
0.01 0.01 0.01 0.3 ( 0.1
0.08 0.09 0.08 0.4 ( 0.2
0.03 0.03 0.07 0.7 ( 0.2
0.01 0.01 0.03 0.4 ( 0.1
0.01 0.01 0.03 0.5 ( 0.1
Figure 2. Emissive 12a, 12d, 13a, and 13d.
In summary, we have synthesized novel fluorophores via
Sonogashira reactions of 1,4-bis(dibromovinyl)benzene 8 and
2,5-bis(dibromovinyl)thiophene 9 with different aromatic
bromides. Dramatic absorption and emission spectral changes
show that they can be used as tunable fluorophores. These
unique properties of fluorophores are of interest for electronic
and photonic applications. On the basis of the results obtained
so far, we are currently designing and synthesizing novel
fluorophores with higher quantum yield, and investigation
of the electronic properties of fluorophores is currently under
way in our laboratory.
n.d.
n.d.
n.d.
n.d.
a Only the longest absorption maxima are shown. b Emission maximum
wavelength excited at the absorption maximum. c Quantum efficiencies
using quinine sulfate in 1.0 N H2SO4 as a standard, λex ) 366 nm.
d Quantum efficiencies using 9,10-diphenylanthracene in EtOH as a standard,
λex ) 366 nm. e Quantum efficiencies using fluorescein in 0.1 N NaOH as
a standard, λex ) 436 nm. f Excited state lifetime (ns) at the emission
maximum. g Quantum yields are too low to be measured.
emission bands in the visible region. The emission maxima
also significantly depend on the aromatic nucleus of the
fluorophores and Vary from the indigo blue to the reddish-
orange region. Fluorophores 12 and 13 show a large Stokes
shift of about 50-110 nm when compared to the less
conjugated fluorophores 10 and 11. These dramatic emission
patterns of fluorophores 12a, 12d, 13a, and 13d are visual-
ized in Figure 2. The fluorescence quantum yields (ΦF) of
the fluorophores were determined in CHCl3 using quinine
sulfate, 9,10-diphenylanthracene, and fluorescein as stan-
dards.10 Fluorophores 12a and 12b have a moderate quantum
yield and the longest fluorescence lifetime (τs) among all
the fluorophores, which provides information about the
influence of the nature of the substituent and the photo-
physical property of the fluorophores.
Acknowledgment. We are grateful to KOSEF for the
financial support through CIMS and BK21 program for the
student scholarship. We thank Dr. N. Venkatesan for the
helpful discussion and Prof. Dennis P. Curran for his critical
review.
Supporting Information Available: Synthetic details and
characterization data of fluorophores 10-13. This material
is available free of charge via the Internet at http://pubs.acs.org.
OL0162264
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Figure 1. Normalized fluorescence emission spectra of (a) 12 and
(b) 13 in CHCl3 at room temperature. Emission spectra were
obtained upon excitation at the absorption maximum.
Org. Lett., Vol. 3, No. 16, 2001
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