Synthesis of linear and V-shaped oligo(phenylene ethynylene) derivatives: Geometric effects on photophysical properties

Article abstract of DOI:10.1016/j.cclet.2009.08.013

A series of linear and V-shaped oligo(phenylene ethynylene) derivatives 1-3 were synthesized through sequent Sonogashira coupling and propargyl alcohol deprotection reaction in high yields. The alkoxy chains (i.e., n-hexyloxy groups) were introduced to as

Full text of DOI:10.1016/j.cclet.2009.08.013

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Chinese Chemical Letters 21 (2010) 97–100
www.elsevier.com/locate/cclet
Synthesis of linear and V-shaped oligo(phenylene ethynylene)
derivatives: Geometric effects on photophysical properties
b
a
a,
Si Chun Yuan a,b, , Shu Liang Han , Xing Ge , Hui Chuan Wang
*
*
a Fundamental Science Department, Beijing University of Agriculture, Beijing 102206, China
b The Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education,
College of Chemistry, Peking University, Beijing 100871, China
Received 18 May 2009
Abstract
A series of linear and V-shaped oligo(phenylene ethynylene) derivatives 1–3 were synthesized through sequent Sonogashira
coupling and propargyl alcohol deprotection reaction in high yields. The alkoxy chains (i.e., n-hexyloxy groups) were introduced to
assure good solubility of compounds 1–3 in common solvents. The photophysical properties of 1–3 in solution depend strongly on
the geometries of these compounds.
# 2009 Si Chun Yuan. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Sonogashira coupling; Oligo(phenylene ethynylene) derivatives; Geometric effect; Photophysical property
Organic semiconductors with an extended p-conjugation system have been intensively pursued for photonics and
electronics, such as organic light emitting diodes (OLEDs) [1–3], organic field effect transistors (OFETs) [4],
photovoltaic cells [5], and lasers [6,7]. In contrast to polymers, small organic molecules with various topological
characteristic possess well-defined and uniform molecular structure as well as superior chemical purity, which still
retain the solution processable and precisely designable advantage of polymers in device fabrication [8–11]. These
attracting features not only facilitate us to investigate their potential application in various fields, but also to understand
unique properties and direct us to design novel functional materials for optoelectronic applications.
Herein, we primarily report the synthesis of a series of linear and V-shaped compounds 1–3 and the investigation of
geometric effects on the photophysical properties of oligo(phenylene ethynylene) derivatives. Compounds 1–3 were
synthesized through a convergent synthetic strategy with oligo(phenylene ethynylene) branch in high yields (Scheme 1).
The alkoxy chains (i.e., n-hexyloxy groups) were introduced to assure good solubility of compounds 1–3 in common
solvents.
1. Experimental
All chemicals, reagents, and solvents were used as received from commercial sources without further purification
except tetrahydrofuran (THF), triethylamine (Et3N), and toluene had been distilled over sodium/benzophenone, CaH2,
* Corresponding authors at: Fundamental Science Department, Beijing University of Agriculture, Beijing 102206, China.
E-mail addresses: ysc2007@sina.com (S.C. Yuan), whch56221@sohu.com (H.C. Wang).
1001-8417/$ – see front matter # 2009 Si Chun Yuan. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2009.08.013

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S.C. Yuan et al. / Chinese Chemical Letters 21 (2010) 97–100
Scheme 1. Synthesis of linear and V-shaped oligo(phenylene ethylene) derivatives.
and sodium, respectively. All nonaqueous operations were carried out under a dry, oxygen-free, nitrogen atmosphere.
Pd/Cu-catalyzed couplings of aryl halides and terminal alkynes were conducted according to reported procedures [12].
All reactions were monitored by thin layer chromatography with silica gel. Column chromatography was carried on
silica gel (160–200 mesh). 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 case. 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
Our preparation of target compounds was started from hydroquinone. Firstly, hydroquinone was conducted
hexylation with n-bromohexane and then bromonation with Br2 which resulted to 1,4-dibromo-2,5-bis(hexyloxy)-
benzene. The sequenced coupling reaction of 1,4-dibromo-2,5-bis(hexyloxy)benzene with ethylbenzene and 2-
methyl-3-buty-2-ol under standard Sonogashira reaction condition and followed by a deprotection of the propargyl
alcohol group in the presence of NaOH produced 1-ethynyl-2,5-bis(hexyloxy)-4-(2-phenylethynyl)benzene.
Compounds 1–3 were constructed by Sonagashria coupling reaction between 1-ethynyl-2,5-bis(hexyloxy)-4-(2-
phenylethynyl)benzene and o-, m-, and p-diiodobenzene in 43, 82, and 85% yields, respectively (Scheme 1). In
comparison with 2 and 3, the yield of 1 was reduced distinctly which was attributed to strong steric hindrance in ortho-
substituted benzene. All three compounds were readily soluble in common organic solvents, such as n-hexane,
CH2Cl2, CHCl3, THF, which allowed us to conveniently obtain their 1H and 13C NMR spectra, and MALDI-TOF MS
characterization data to verify their structure and purity [13].
The photophysical properties of compounds 1–3 were investigated by absorption and photoluminescence (PL)
spectroscopy in THF solution (Fig. 1). As illustrated in Fig. 1 (left), the absorption spectra of 1, 2 and 3 in dilute THF
solutions are in general characterized by two intense bands 305–327 and 363–391 nm. The two bands have been
assigned to the characteristic p ! p* transitions based on phenyl ethynylene and the overall p-conjugated framework
respectively. As be expected, because of the 1,4-substitution and the linear alignment of the overall molecule, 3
exhibits the most pronounced red shift (labs,max = 391 nm), indicating the highest effective conjugation length within
the series investigated. Although the p-conjugated system is also extended for the ortho-, and meta-angular 1 and 2,
this is to a minor degree in relation to 3, due to the strong steric hindrance of the two adjacent chromophores. The
absorption peaks were located at 305, 363 nm for 1 and 310, 372 nm for 2, respectively, which exhibited blue shift 18–
29 nm relative to 3. In comparison with 1, 2 red shifted 5–9 nm for the two absorption peaks which indicated that the
conjugated length of meta-substituted arrangement for oligo(phenylene ethynylene) derivatives was longer than that of
the ortho-substituted patterns in ground states.

S.C. Yuan et al. / Chinese Chemical Letters 21 (2010) 97–100
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Fig. 1. Normalized absorption (left) and fluorescence spectra (right) of compounds 1, 2 and 3 in THF solution at room temperature.
The photoluminescence spectra of compounds 1, 2 and 3 were also studied (Fig. 1, right). All PL spectra were
obtained upon excitation at the corresponding absorption maximum. The PL spectra of oligo(phenylene ethynylene)
derivatives are also strongly influenced by the substitution patterns of the central phenyl rings. Because of the meta
substitution in V-shaped 2, the conjugated side groups have to be treated as isolated chromophores rather than as
uniform chromophores [14]. The maximum of the emission band of 2 are located at about 410 nm, the shortest
wavelength among the three compounds. As consequence of its longer conjugated length, the absorption lmax of 3 was
exhibited at 433 nm, a red shift of the emission maximum of 23 nm relative to 2. For the ortho-substituted 1, its
fluorescent peak was located at 429 nm which only blue shifted 4 nm relative to that of para-substituted 3, but red
shifted 19 nm in comparison with that of 2. As can be seen from the absorption behavior, a distinct tilt of the
conjugated side groups, due to strong steric hindrance, would be expected for the ground state (S0). We assumed that
the molecular geometry is changed in the excited state (S1) and the overall p-conjugated system could consequently be
enlarged significantly as previously reported literature [15]. The same effects were also observed for the p-conjugated
systems with C C double bond linkages [16].
In conclusion, we have synthesized a series of linear and V-shaped oligo(phenylene ethynylene) derivatives by
Sonogashira coupling reaction. Spectroscopy investigation indicated that the geometries of the molecules have a
distinct influence on their photophysical properties in dilute solution. For the ortho-substituted derivatives, significant
changes in the molecular structure upon excitation have been deduced from the photophysical investigations.
Acknowledgments
We thank Beijing Natural Science Foundation (No. 2093033) and Scientific Research Project of Beijing
Educational Committee (No. KM200910020012) for financial support.
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[13] Selected data: 1: 1H NMR (CDCl3, 300 MHz): d 7.57–7.51 (m, 6H, Ar-H), 7.36–7.29 (m, 8H, Ar-H), 7.03 (s, 1H, Ar-H), 7.01 (s, 4H, Ar-H),
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), 1.41–1.34 (m, 16H, -CH2-), 0.92–0.86 (m, 12H, -CH3). 13C NMR (CDCl3, 75 MHz): d 153.67, 153.58, 134.35, 131.55, 131.26, 128.37,
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(m/z): 878 (M+). 3: 1H NMR (CDCl3, 300 MHz): d 7.56–7.53 (m, 4H, Ar-H), 7.51 (s, 4H, Ar-H), 7.38–7.33 (m, 6H, Ar-H), 7.03 (s, 2H, Ar-H),
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94.99, 94.60, 87.97, 8/5.91, 69.60, 69.57, 31.59, 29.29, 25.74, 22.63, 14.03. MALDI-TOF MS (m/z): 878 (M+).
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