Synthesis and characterization of X-shaped oligo(para-phenylene) derivatives functionalized with fluorene ethynylene

Article abstract of DOI:10.1016/j.cclet.2010.06.031

A series of linear and X-shaped oligo(para-phenylene) derivatives functionalized with fluorene ethynylenes 1, 3 and 4 were synthesized through sequent Sonogashira coupling and Suzuki-Miyaura reaction in high yield. The electron-donating group -OCH₃

Full text of DOI:10.1016/j.cclet.2010.06.031

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Chinese Chemical Letters 21 (2010) 1374–1377
www.elsevier.com/locate/cclet
Synthesis and characterization of X-shaped oligo( para-phenylene)
derivatives functionalized with fluorene ethynylene
Ke Hong Ding a, Xing Ge b, Ming Zhang a,*, Si Chun Yuan b,*
a College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225009, China
b Department of Fundamental Science, Beijing University of Agriculture, Beijing 102206, China
Received 29 March 2010
Abstract
A series of linear and X-shaped oligo( para-phenylene) derivatives functionalized with fluorene ethynylenes 1, 3 and 4 were
synthesized through sequent Sonogashira coupling and Suzuki–Miyaura reaction in high yield. The electron-donating group –
OCH3 and electron-withdrawing counterparts –CF3 were introduced to tune the spectra properties of compounds 3 and 4. The detail
investigation of their photophysical properties in solution and film indicated that the introduction of both –OCH3 and –CF3 makes
maximum emission distinct red-shift in comparison with parent compound 1, but the latter more prominently.
# 2010 Ming Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Sonogashira coupling; Suzuki coupling; Oligo(fluorene ethynylene); Electronic effect; Photophysical property
Monodisperse and well-defined p-conjugated oligomers have recently become a subject of intensive research in
material science due to their potential applications in electrical circuits [1], switches and sensors [2], organic light-
emitting diodes (OLEDs) [3–5], solar cells [5], organic field-effect transistors (OFETs) [6], multiphoton absorbing
materials [7] and models to understand the fundamental properties of their analogous polydisperse polymers [8].
Poly(arylene ethynylene)s (PAEs) can be viewed as the dehydro analogues of poly(arylene vinylene) (PAVs) in
which the ethyne groups impart additional rigidity to the conjugated framework. The polymer and oligomer based on
arylene ethylene have shown better photostability [9,10], higher PL efficiency and more facile to synthesis than PAVs
[11]. Oligofluorenes are important model compounds for polyfluorenes, which are the promising blue light-emitting
materials with high photoluminescence quantum yields, and thermal and oxidative stability [12]. Development of
various synthetic methodologies makes it possible to design and synthesize a variety of soluble monodisperse
oligo(fluorene ethynylene) derivatives, which permit color and charge injection tuning through their conjugation
length control, as well as the introduction of electron-donating or -withdrawing groups to the parent p-conjugated
systems [13–16].
Herein, we primarily report the synthesis of a series of linear and X-shaped compounds 1, 3 and 4 and the
investigation of electronic effects on the photophysical properties. Compounds 1, 3 and 4 were synthesized through
sequent Sonogashira coupling and Suzuki–Miyaura reaction in high yield (Scheme 1). All three compounds were
* Corresponding authors.
E-mail addresses: lxyzhangm@yzu.edu.cn (M. Zhang), ysc2007@sina.com (S.C. Yuan).
1001-8417/$ – see front matter # 2010 Ming Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2010.06.031

[(Scheme_1)TD$FIG]
K.H. Ding et al. / Chinese Chemical Letters 21 (2010) 1374–1377
1375
Scheme 1. Synthesis of X-shaped oligo( para-phenylene) derivatives 1, 3 and 4.
readily soluble in common solvents, such as hexane, CH2Cl2, CHCl3, and THF, which allow us to conveniently obtain
1
their H and 13C NMR spectra, and MALDI-TOF MS characterization data to verify their structure and purity.
1. Experimental
All chemicals, reagents, and solvents were used as received from commercial sources without further purification
except tetrahydrofuran (THF), triethylamine (Et3N), and toluene that had been distilled over sodium/benzophenone,
CaH2, and sodium, respectively. All nonaqueous operations were carried out under a dry, oxygen-free, nitrogen
atmosphere. Pd-catalyzed Sonogashira and Suzuki–Miyaura couplings of aryl halides with terminal alkynes and aryl
boronic acid respectively were conducted according to reported procedures [13,17]. All reactions were monitored by
TLC with silica gel. Column chromatography was carried on silica gel (160–200 m). 1H and 13C NMR spectra were
recorded on a Mercury plus 300 MHz or Bruker 400 MHz using CDCl3 as solvent and tetramethylsilane (TMS) as
internal standard in all cases. Mass spectra were obtained with a JEOL JMS-70 spectrometer in EI mode and a
MALDI-TOF (Matrix assisted laser desorption ionization/time-of-flight)-MS mass spectrometer Bruker BIFLEX III.
UV–vis spectra were recorded on PerkinElmer Lambda 35 UV–vis spectrometer. PL spectra were carried out on
PerkinElmer LS 55 Luminescence Spectrometer.
2. Result and discussion
Compound 1 was obtained from 2-ethynyl-9,9-dihexyl-2-fluorene [15] and commercially available 1,4-
dibromobenzene under standard Sonogashira condition. Compounds 3 and 4 were prepared through sequent
Sonogashira coupling and Suzuki–Miyaura reaction. Firstly, 2-ethynyl-9,9-dihexyl-2-fluorene was coupled to 1,4-
dibromo-2,5-diidobenzene at room temperature utilizing the different reactivity between aryl bromide and aryl iodide,
which provided the important intermediate 2 in 76% yield. Then 3 and 4 were synthesized via Suzuki–Miyaura
coupling reaction between 2 and 4-methoxyphenylboronic acid, 3,5-bis(trifluoromethyl)phenyl boronic acid in 83%
and 79% yield respectively (Scheme 1). Their structures and purity were fully characterized and verified by 1H and 13
NMR, elemental analysis, and MALDI-TOF MS [18].
C
The normalized absorption and fluorescence spectra of compounds 1, 3 and 4 in THF solution and in neat film are
shown in Fig. 1a and c respectively. The absorption features of 1, 3 and 4 with two major absorption bands were all
similar to the previously reported oligo(fluorene ethynylene) [13–15]. Their absorption maximum lmax in solution
peaked at 355 nm for 1, 371 nm for 3, and 375 nm for 4 respectively, displaying a distinct red-shift 16 nm for 3 and

[(Fig._1)TD$FIG]
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K.H. Ding et al. / Chinese Chemical Letters 21 (2010) 1374–1377
Fig. 1. Normalized absorption and fluorescence spectra of compounds 1, 3 and 4 in THF solution (a) and (b); solid film (c) and (d).
20 nm for 4 in comparison with the parent compound 1. Meanwhile, the shoulder peak corresponding to maximum
effective conjugation length was measured at 372, 392 and 393 nm, respectively for compounds 1, 3, and 4. It
suggested that the extended conjugation lengths were obtained by introducing electron-donating or -withdrawing
groups in the middle benzene ring. The absorption behaviors in film were very similar to that in solution except for
small red-shifts (Table 1), indicating unobvious aggregation formed which was attributed to their X-shaped skeleton.
To investigate the substitution effect in excited state, the photoluminescence (PL) spectra of compounds 1, 3 and 4
were also studied (Fig. 1b and d). All PL spectra were obtained upon excitation at the corresponding absorption
maximum. The PL spectra of all compounds are also strongly influenced by the substitution patterns of the central
phenyl rings. First, in diluted solution (1.0 Â 10À6 mol/L), the emission peaks of compounds 3 and 4 have obvious red-
shift relative to that of 1, indicating extended conjugation lengths by substitution. The emission peaks at 426 and
423 nm for 3 and 4 respectively showed similar substitution effect by electron donating and withdrawing groups.
Furthermore, the well-defined vibronic structure of 3 that was modified by methoxy phenyl groups means higher
rigidity than that of 4 in excited state. However, in film state, long wavelength emission band assigned to emission
from excimer occurred at 472, 514 and 519 nm for compounds 1, 3, and 4, respectively. Obviously, the formation of
excimers was attributed to strong p–p stacking interaction of planar and shape-persistent p-conjugated system.
In conclusion, we have synthesized a series of linear and X-shaped oligo( para-phenylene) derivatives
functionalized with fluorone ethynylene by sequent Sonogashira and Suzuki–Miyaura coupling reaction.
Spectroscopy investigation indicated that the substitution manner has a distinct influence on their photophysical
Table 1
Photophysical properties of compounds 1, 3 and 4.
Compounds
labs [nm] (THF)
labs [nm] (film)
lPL [nm] (THF)
lPL [nm] (film)
1
3
4
355, 378 (sh)
371, 392 (sh)
375, 393 (sh)
359, 386 (sh)
378, 399 (sh)
382, 401 (sh)
389, 408 (sh)
406, 426 (sh)
423
450, 472 (sh)
434, 514 (sh)
462 (sh), 519

K.H. Ding et al. / Chinese Chemical Letters 21 (2010) 1374–1377
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properties in both dilute solution and film state. In addition, the electron-donating/withdrawing groups prove an
effective way to tune their absorption and emission behaviors.
Acknowledgments
We thank Beijing Natural Science Foundation (No. 2093033) and Scientific Research Project of Beijing
Educational Committee (Nos. KM 200910020012 and KM 201010020002) for financial support.
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[18] Selected data: 1: 1H NMR (300 MHz, CDCl3): d 7.71–7.66 (m, 4H, Ar–H), 7.56 (s, 4H, Ar–H), 7.52–7.50 (m, 4H, Ar–H), 7.35–7.30 (m, 6H,
Ar–H), 2.00–1.95 (t, 8H, J = 8.1 Hz, –CH2–), 1.15–1.08 (m, 24H, –CH2–), 0.79–0.74 (t, 12H, J = 7.2 Hz, –CH3), 0.70–0.60 (m, 8H, –CH2–).
13C NMR (75 MHz, CDCl3,): d 151.04, 150.80, 141.64, 140.35, 131.50, 130.63, 127.57, 126.88, 125.96, 123.15, 122.88, 121.13, 120.02,
119.65, 92.49, 89.17, 55.20, 40.39, 31.51, 29.71, 23.71, 22.59, 13.99. EI MS (m/z): 790 (M+, 100%). 2: 1H NMR (300 MHz, CDCl3): d 7.84 (s,
2H, Ar–H), 7.73–7.68 (m, 4H, Ar–H), 7.58–7.53 (m, 4H, Ar–H), 7.36–7.34 (m, 6H, Ar–H), 2.01–1.96 (t, 8H, J = 8.1 Hz, –CH2–), 1.15–1.06
(m, 24H, –CH2–), 0.79–0.74 (t, 12H, J = 6.9 Hz, –CH3), 0.63–0.60 (m, 8H, –CH2–). 13C NMR (75 MHz, CDCl3,): d 151.12, 150.88, 142.32,
140.19, 136.00, 130.95, 127.77, 126.92, 126.43, 1226.01, 123.57, 122.91, 120.38, 120.14, 119.72, 98.00, 86.96, 55.18, 40.32, 31.49, 29.66,
23.69, 22.56, 13.99. MALDI-TOF MS (m/z): 946.4. 3: 1H NMR (300 MHz, CDCl3): d 7.77–7.74 (s + d, 6H, J = 9.0 Hz, Ar–H), 7.69–7.62 (m,
4H, Ar–H), 7.36–7.31 (m, 10H, Ar–H), 7.08–7.05 (d, 4H, J = 9.0 Hz, Ar–H), 3.92 (s, 6H, –CH3), 1.97–1.91 (t, 8H, J = 8.1 Hz, –CH2–), 1.14–
1.04 (m, 24H, –CH2–), 0.78–0.74 (t, 12H, J = 6.9 Hz, –CH3), 0.60 (s, br, 8H, –CH2–). 13C NMR (75 MHz, CDCl3): d 159.36, 150.99, 150.68,
141.48, 140.34, 133.58, 132.03, 130.56, 130.37, 127.49, 126.84, 125.74, 122.85, 121.54, 121.44, 119.96, 119.56, 113.39, 94.93, 89.63, 55.32,
55.02, 40.33, 31.47, 29.68, 23.67, 22.57, 13.98. MALDI–TOF MS (m/z): 1002.6. 4: 1H NMR (300 MHz, CDCl3): d 8.32 (s, 4H, Ar–H), 8.02 (s,
2H, Ar–H), 7.86 (s, 2H, Ar–H), 7.70–7.64 (m, 4H, Ar–H), 7.39–7.31 (m, 10H, Ar–H), 1.98–1.93 (t, 8H, J = 6.6 Hz, –CH2–), 1.09–1.03 (m,
24H, –CH2–), 0.78–0.73 (t, 12H, J = 7.2 Hz, –CH3), 0.59–0.55 (m, 8H, –CH2–). 13C NMR (75 MHz, CDCl3): d 151.13, 150.92, 142.37, 141.05,
140.16, 140.05, 134.13, 131.92, 131.49, 130.68, 129.53, 127.81, 126.92, 125.71, 125.22, 122.90, 122.32, 121.85, 121.60, 120.14, 120.08,
119.74, 97.31, 87.04, 55.20, 40.30, 31.49, 29.64, 23.66, 22.57, 13.95. MALDI–TOF MS (m/z): 1214.6.