Synthesis and characterization of unsymmetrical phenylene-ethynylene trimers and tetramers with different chromophores as blue-light-emitters

Article abstract of DOI:10.1016/j.tetlet.2013.01.076

A series of highly fluorescent p-phenylene-ethynylene π-conjugated oligomers (trimers and tetramers) with different chromophore substituents have been synthesized stepwise under oxygen-free Sonogashira conditions. The emission wavelengths range from 400 to 455 nm for the series. All four trimers and tetramers have visible blue emission under sunlight especially for the case of the pyrene-substituted trimer and tetramer. All oligomers also exhibit very high quantum yields (0.9-1.1), which suggest that these types of compounds could be used as blue-light-emitters.

Full text of DOI:10.1016/j.tetlet.2013.01.076

Tetrahedron Letters 54 (2013) 1711–1713
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and characterization of unsymmetrical phenylene–ethynylene
trimers and tetramers with different chromophores as blue-light-emitters
⇑
Xinyan Bai, Xueyi Chen, Jerry R. Dias , T. C. Sandreczki
Department of Chemistry, University of Missouri-Kansas City, 5100 Rockhill Road, Kansas City, MO 64110, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of highly fluorescent p-phenylene–ethynylene
p-conjugated oligomers (trimers and tetramers)
Received 6 December 2012
Revised 12 January 2013
Accepted 16 January 2013
Available online 8 February 2013
with different chromophore substituents have been synthesized stepwise under oxygen-free Sonogashira
conditions. The emission wavelengths range from 400 to 455 nm for the series. All four trimers and tet-
ramers have visible blue emission under sunlight especially for the case of the pyrene-substituted trimer
and tetramer. All oligomers also exhibit very high quantum yields (0.9–1.1), which suggest that these
types of compounds could be used as blue-light-emitters.
Keywords:
Ó 2013 Elsevier Ltd. All rights reserved.
Phenylene–ethynylene
Blue-light-emitters
Fluorescent materials
Sonogashira reaction
Carbon–carbon bond formation
Phenylene–ethynylene-conjugated linear materials have at-
tracted much interest in recent years, for example as potential
molecular wires. Also significant progress has been made in the
application of this type of organic compound for fabricating organ-
ic light-emitting diodes (OLEDs), organic field-effect transistors
(OFETs), solar cells, and chemosensors.1 The study of conjugated
organic polymers in general is popular since polymers often pos-
sess good mechanical properties and are easily processable;2 how-
ever, the investigation of phenylene–ethynylene conjugated
oligomers is specifically attractive due to their well-defined
structures, high purities, and freedom from structural defects.3 In
contrast, phenylene–ethynylene polymers usually do not have
well-defined structures; they are often polydisperse and contain
some diyne units,4 which disturb the conjugation and adversely
affect the polymers’ solubilities and optical properties. In addition,
for oligomers the fine-tuning of emission wavelengths and the
study of structure–optical property relationships can be achieved
by adding additional repeat units or by introducing different chro-
mophores as the end units of the conjugated system.5 For example,
conjugated oligomers with defined lengths containing various fluo-
rescent chrompohores have been synthesized and used as func-
tional components of organic electronic devices.6
In order to synthesize linear trimers and tetramers with differ-
ent chromophores as end units, it is first necessary to synthesize
unsymmetrical mono-iodo-substituted key compounds. The syn-
thesis of phenylene–ethynylene trimers 3a–d is shown in
Scheme 1. Ethynylbenzene was coupled with an excess of 1, 4-dib-
utoxy-2, 5-diiodobenzene to give mono-iodo-substituted com-
pound 2, followed by coupling with various terminal alkynes a
through d to afford unsymmetrical trimers 3a–d. For the synthesis
of tetramers as shown in Scheme 2 the previously synthesized
terminal alkyne, 1, 4-dibutoxy-2-ethynyl-5-(phenylethynyl) ben-
zene, was coupled with an excess of 1, 4-dibutoxy-2, 5-diiodoben-
zene to make mono-iodo oligomer 5, which was then reacted with
various terminal alkynes a through d to yield tetramers 6a–d. The
yields for products 3a–d and 6a–d decrease with increasing size of
the aromatic group on the terminal alkyne, despite the use of
oxygen-free Sonogashira coupling conditions. Thus, it is observed
that terminal alkynes with larger substituent groups contribute to
higher proportions of homo-coupling products. In addition, even
when n-butyl groups are introduced into the conjugated system,
the resulting oligomers’ solubilities still decrease with increasing
conjugation length. For example, the solubility of tetramers with
four n-butyl groups is lower than for the corresponding trimers
with two n-butyl groups. This means that any increase in solubil-
ity due to the butyl groups cannot offset the decrease in solubility
due to the increased rigid-rod conjugation length. It is concluded
that even with butoxy chains on all central units, this series of
phenylene–ethynylene oligomers will become completely
insoluble when the conjugated system reaches some critical
length.
In thisLetter, wedescribe the synthesisof phenylene–ethynylene
linear trimers and tetramers with differentfluorescent chromphores
as end groups. Chemical structure–optical property relationships
(emission maxima and efficiencies) are presented in this work.
⇑
Corresponding author. Tel.: +1 816 235 2284; fax: +1 816 235 5502.
E-mail address: diasj@umkc.edu (J.R. Dias).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.01.076

1712
X. Bai et al. / Tetrahedron Letters 54 (2013) 1711–1713
BuO
BuO
CuI 0.4 mol%, Pd(PPh3)2Cl2
I
I
I
+
0.4 mol%, TEA, THF
OBu
OBu
2
1
BuO
1)
R
R
2) CuI 0.4 mol%, Pd(PPh3)2Cl2
0.4 mol%, TEA, THF
OBu
3a-d
Scheme 1. Synthesis of phenylene–acetylene trimers with various fluorescent chromophores.
BuO
BuO
Entry
Product 3a-d
Yield
97%
1. TMSA, CuI 0.4 mol%, Pd(PPh3)2 Cl2
0.4 mol%, TEA, THF
BuO
I
3a
2. TBAF, THF
OBu
OBu
OBu
OBu
3b
3c
3d
92%
85%
70%
2
4
H3CO
BuO
I
BuO
OBu
1)
I
BuO
BuO
OBu
BuO
I
OBu
2) CuI 0.4 mol%, Pd(PPh3)2Cl2
0.4 mol%, TEA, THF
OBu
OBu
5
BuO
BuO
BuO
1)
R
R
Entry
6a
Product 6a-d
Yield
OBu
OBu
2) CuI 0.4 mol%, Pd(PPh3)2Cl2
0.4 mol%, TEA, THF
OBu
95%
OBu
BuO
OBu
BuO
6a-d
OBu
90%
86%
6b
6c
H3CO
Scheme 2. Synthesis of phenylene–acetylene tetramers with various fluorescent
BuO
OBu
BuO
OBu
chromophores.
BuO
BuO
BuO
OBu
OBu
68%
6d
BuO
Absorption and emission spectra of the oligomers 3a–d shown in
Figure 1 have been measured in dichloromethane. The oligomers
exhibit significant conjugation-dependent UV/vis bathochromic
shifts in their absorption spectra. For example, the absorption max-
ima of trimers 3a–d are 366, 374, 384, and 418 nm, respectively.
This series of oligomers exhibits the same red shift trend in their
emission maxima, with 3a (benzene end group) emitting at
400 nm, and 3d (pyrene end group) emitting at 438 nm, as shown
in Figure 1. A shoulder on right of the emission peaks for all four
oligomers 3a–d indicates that there could be a relaxation of these
series of compounds from the high energy excited state to a lower
energy excited state preceding emission; this relaxation appears
more subdued in the spectra of 6a–d.
The absorption spectra for 6a–d are shown in Figure 2 (dashed
line). For tetramers 6a–d (as chromophores vary from benzene to
pyrene), the absorption peaks shift gradually from 390 nm to
Figure 1. UV–vis and fluorescence spectra of trimers 3a–d in CH2Cl2 solution. The
dashed line represents the UV–vis spectrum and the solid line represents the
fluorescence spectrum.

X. Bai et al. / Tetrahedron Letters 54 (2013) 1711–1713
1713
In summary, a series of sp–sp2-conjugated oligomers including
more than 10 new compounds with phenylene–ethynylene back-
bones were synthesized. Their UV–vis absorption and fluorescence
emission spectra show red shift trends consistent with the theory.
Optical properties of these oligomers can be tuned by introducing a
variety of fluorescent chormophores and by adding repeat units.
Their high quantum yields for emission of blue light under ambient
conditions make these oligomers interesting technologically. The
oligomer with the pyrene chromophore and more repeating units
has the best emission efficiency as a blue-light-emitter.
Acknowledgements
This work was supported in part by grants from the Office of the
Vice Provost for Research (UMKC Research Incentive Fund Grant
K0710) and by the UM Board of Curators (K0906077) (to J.R.D.)
and by the Army Research Office (W911NF-10-1-0476) (to T.C.S.).
We thank Dr. Todd D. Williams, Mr. Robert Drake, and Mr. Larry
Seib of the KU Mass Spectrometry Laboratory for acquiring the
ESI spectra. LCT premier was purchased with support from NIH
SIG S10 RR019398 (T.D.W.).
Figure 2. UV–vis and fluorescence spectra of tetramers 6a–d in CH2Cl2 solution.
The dashed line represents the UV–vis spectrum and the solid line represents the
fluorescence spectrum.
Table 1
Supplementary data
Spectroscopic data for oligomers 3a–d and 6a–d in CH2Cl2 solution
Oligomer
kmax (nm)
kem (nm)
Stokes shift (nm)
Uf
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.01.
076.
3a
3b
3c
3d
6a
6b
6c
6d
366
374
384
418
390
396
402
416
400
406
415
438
436
435
439
455
34
32
31
20
46
39
37
39
0.97
0.93
1.08
0.97
0.91
1.15
1.04
1.00
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the emission maximum for 6d is at
a longer wavelength
(455 nm) than that of 3d (438 nm), probably because of the more
subdued influence of relaxation from the high energy excited state
to a lower energy excited state preceding emission.