Regiocontrolled synthesis of ethene-bridged para-phenylene oligomers based on PtII- And RuII-catalyzed aromatization

Article abstract of DOI:10.1002/chem.200902231

We report the regiocontrolled syntheses of ethene-bridged paraphenylene oligomers in three distinct classes by using Pt^II- and Ru ^II-catalyzed aromatization. This synthetic approach has been developed based on twofold aromatization of the 1-aryl-2-alkynylbenzene functionality, which proceeds by distinct regioselectivity for platinum and ruthenium catalysts. Variable-temperature NMR spectra provide evi-dence that large arrays of these oligomers are prone to twist from planarity. The UV/Vis and photoluminescence (PL) spectra as well as the band gaps of these regularly growing arrays show a pattern of extensive π conjugation with increasing array sizes, except for in one instance.

Full text of DOI:10.1002/chem.200902231

DOI: 10.1002/chem.200902231
Regiocontrolled Synthesis of Ethene-Bridged para-Phenylene Oligomers
Based on PtII- and RuII-Catalyzed Aromatization
Tse-An Chen,[a] Te-Ju Lee,[b] Ming-Yuan Lin,[a] Shariar M. A. Sohel,[a]
Eric Wei-Guang Diau,[b] Shie-Fu Lush,[c] and Rai-Shung Liu*[a]
Abstract: We report the regiocontrol-
led syntheses of ethene-bridged para-
phenylene oligomers in three distinct
classes by using PtII- and RuII-catalyzed
aromatization. This synthetic approach
has been developed based on twofold
aromatization of the 1-aryl-2-alkynyl-
benzene functionality, which proceeds
by distinct regioselectivity for platinum
and ruthenium catalysts. Variable-tem-
perature NMR spectra provide evi-
dence that large arrays of these oligo-
mers are prone to twist from planarity.
The UV/Vis and photoluminescence
(PL) spectra as well as the band gaps
of these regularly growing arrays show
a pattern of extensive p conjugation
with increasing array sizes, except for
in one instance.
Keywords: catalysis
· platinum ·
polyaromatic oligomers · regiocon-
trol · ruthenium
Introduction
Benzenoid polycyclic aromatic hydrocarbons (BPAHs) have
found widespread applications in various photoelectronic
devices.[1] Progress in the synthesis of discotic molecules[1,2]
of BPAHs has been more successful than ribbon-type fused
benzenes that are best represented by heptacene[3] 1, as de-
picted in Scheme 1. Despite their outstanding performance
in organic thin-film transistors (OTFT), large polyacenes are
strictly restricted to heptacene because of their high instabil-
ity and insolubility.[4]
We sought to prepare new ribbon-type BPAHs[5,6] based
on ethene-bridged para-phenylene frameworks,[7] which
have structures with two central benzene units (A–E) linked
by a HC=CH unit as shown in 2 in Scheme 1. Synthesis of
[a] Dipl.-Chem. T.-A. Chen, Dr. M.-Y. Lin, Dr. S. M. A. Sohel,
Prof. Dr. R.-S. Liu
Department of Chemistry, National Tsing-Hua University
Hsinchu, 30013, Taiwan (R.O. China)
Fax : (+886)3-5711082
E-mail: rsliu@mx.nthu.edu.tw
[b] Dr. T.-J. Lee, Prof. Dr. E. W.-G. Diau
Department of Applied Chemistry
National Chiao-Tung University
Scheme 1. Polyacenes 1 and ethene-bridged para-phenylene oligomers
2–6.
Hsinchu, 30010, Taiwan (R.O. China)
[c] Prof. Dr. S.-F. Lush
such benzene arrays is challenging because possible stereo-
isomers rapidly grow with the increasing number of benzene
units. The important application of such oligomers is demon-
strated by the remarkable OTFT performance of picene (3),
Department of Medical Technology, Yuanpei University
Hsinchu, 30015, Taiwan (R.O. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200902231.
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FULL PAPER
even in air at room temperature.[7] Before the present work
was conducted, the syntheses of such oligomers was only re-
ported to be the best catalyst for such an aromatization
through a p-alkyne intermediate. We found that the plati-
num catalyst provided excellent regioselectivity and satisfac-
tory efficiency through attack of the more hindered (peri)
ported for 3, dibenzoACTHNUTRGNEUNG[a,h]anthracene (4), highly twisted phe-
nacene (5),[7] and [11]phenacene (6).[6c–e] Here, we report the
first regiocontrolled synthesis of such new polyaromatic
oligomers based on platinum- and ruthenium-catalyzed aro-
matization reactions.
ꢀ
C H carbon of the naphthalene and other fused aromatic
arrays at the propyne groups. The large HOMO orbital co-
ꢀ
efficients of these C H carbons control the observed regio-
selectivity.[9] The structures of these air-stable yellowish
1
products A3–A7 were elucidated by H-NOE spectroscopy
Results and Discussion
and MALDI-mass spectra (see the Supporting Information).
The H NMR signals of species A7 are concentration depen-
1
Scheme 2 shows our targeted oligomers, represented by A3,
A5 and A7, with the bridged ethene groups on opposite
dent, presumably caused by molecular aggregation in solu-
tion. The proton signals of A7 are shifted upfield in highly
concentrated solutions, with a Dd up to 0.2–0.3 ppm in the
range of 2.5ꢁ10ꢀ3–1.0ꢁ10ꢀ2 m.[10]
Scheme 4 shows the preparation of additional ethene-
bridged para-phenylenes in new classes, represented by B3–
B5 and C3–C5. The former has two adjacent ethene groups
on opposite sides, whereas the latter have the bridged ethyl-
ene groups all on the same side. The preparation of B3–B5
was accomplished by the use of [Ru
ACHTUTGNREN(NUG CH3CN)2CAHTUNGTERN(NUGN PPh3)Tp]-
AHCTUNGTRENNUNG
zation of substrates B3s–B5s. This catalyst was selected be-
cause it activates the aromatization of 3,5-dien-1-ynes via a
ruthenium–vinylidene intermediate.[11,12] We were surprised
to find that the aromatization proceeded through an addi-
ꢀ
tion of the less-hindered C H bond of the central aromatic
arrays to the adjacent terminal alkynes;[6d] this regioselectiv-
ity is completely distinct from platinum chemistry. We
assume that the bulky {RuACTHNUTRGNE(UGN PPh3)Tp} fragment alters this ar-
omatization regioselectivity.[10] Syntheses of the oligomers
C3–C5 were accomplished through the PtCl2-catalyzed aro-
matization of substrates C3s–C5s. The regioselectivity re-
sembles that seen for species A3–A7. Compounds B3–B5
and C3–C5 were characterized by NMR spectroscopy and
Scheme 2. Targeted ethene-bridged para-phenylene oligomers.
sides with respect to the central
C2 axis. We also prepared the
intermediates A4 and A6 to ex-
amine their compatibility with
this family. Detailed procedures
for the synthesis of the starting
substrates A4s–A7s are provid-
ed in the Supporting Informa-
tion. As shown in Scheme 3,
these substrates comprise a cen-
tral benzene tethered by two
propynyl groups for twofold ar-
omatization.
The
tethered
methyl groups are crucial to
prepare the heavier congeners
A6 and A7 because they lead
to the improved solubility of
starting substrates A6s and
A7s. The catalyst PtCl2/CO[8]
was selected because it was re- Scheme 3. Platinum-catalyzed regiocontrolled synthesis of series A.
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R.-S. Liu et al.
isomers as shown by its varia-
1
ble-temperature H NMR spec-
tra (Figure 1). The activation
energy (DG°) was estimated to
be 14.2 kcalmolꢀ1. To under-
stand the size effect on the pla-
narity twist, we prepared
a
medium ribbon A5’ that
1
showed H NMR signals of two
stereoisomers in a 1:1 ratio at
258C, with each isomer having
a C2 or S2 axis as depicted in
Scheme 5. As depicted in
Figure 2, these NMR peaks
became coalesced to signals of
one single species as the tem-
perature was gradually brought
to 1058C, due to a rapid a–b
exchange. The kinetic parame-
ters of this variable-tempera-
ture NMR investigation gave a
large vaule of DG° =19.6 kcal
molꢀ1.
Figure 3a shows the steady-
state UV/Vis absorption spectra
of BPAHs A3–A7 and their
corresponding
properties are summarized in
Table 1. The absorption
photophysical
maxima gradually move to
large wavelengths with an in-
crease of the molecule length
Scheme 4. Regiocontrolled synthesis of oligomers of B and C series with ruthenium and platinum catalysts.
MALDI mass spectrometry
(see the Supporting Informa-
tion). For oligomers of the
series B and C, attempts to syn-
thesize their heavier congeners
were unsuccessful because of
the poor solubility of the start-
ing substrates.
For species A7, we observed
1H NMR signals of only one co-
alesced species at 258C al-
though the two adjacent methyl
pairs create two axial chiralities.
The interconversion of the two
stereoisomers, through a twist
of planarity,[13] is probably very
fast in such a large array. This
hypothesis was verified by cool-
ing the NMR temperature to
ꢀ258C, at which the coalesced
Figure 1. Variable-temperature 1H NMR spectra of compound A7 in 1:1 CS2/CD2Cl2 (500 MHz) from 25 to
peaks of A7 were split into sep-
arate signals of the two stereo- ꢀ358C. The NMR signals in the d=2.7–3.5 ppm correspond to six methyl signals.
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Regiocontrolled Synthesis of para-Phenylene Oligomers
FULL PAPER
Scheme 5. Planarity twist of oligomer A5’.
Figure 3. a) UV/Vis absorption spectra of A3–A7 in CH2Cl2. b) Unnor-
malized fluorescence spectra obtained at excitation of 375 nm.
oligomers that comprise successive vibronic structures even
for the largest species A7, indicating the rigidity of the
ribbon structures. Species A3 shows a maximum at 403 nm,
which is consistently shifted to larger wavelengths with in-
Figure 2. Variable-temperature 1H NMR spectra of compound A5’ in
CDCl3 from 25 to 1058C.
creasing array size until a maximum is reached at lem
=
(d) from 1.16 to 2.88 nm from A3 to A7. The large arrays
A6 and A7 have low extinction coefficients in their UV/Vis
absorptions, which presumably arise from molecular aggre-
gation because of their low solubility. Figure 3b shows the
un-normalized emission spectra of these para-phenylene
444 nm for A7. Compounds A3–A7 are very fluorescent
with FF =0.17–0.38. These quantum yields are well correlat-
ed with their radiative rate coefficients expressed by tr. The
long excited-state lifetimes (8.13–15.8 ns) are characteristic
of BPAHs. We estimated the HOMO energy levels of spe-
ox
1=2
cies A3–A6 from their oxidation potentials (E ), and ob-
tained the LUMO energy levels from the measured energy
gaps provided by the UV/Vis spectra. The poor solubility of
species A7 meant that its HOMO level could not be deter-
mined.
The lowest electronic states (S1–S0) of A3–A7 could be
recognized from their weak absorption coefficients (loge=
4.14–5.16). We determined the band gap (DE) of each com-
pound from the middle position between the S1–S0 absorp-
tion and the emission peaks and the results are summarized
in Figure 4, which shows a plot of DE versus 1/n (n=the
number of central benzene moieties).[14] A nearly straight
line was established for A3–A7, confirming the compatibili-
ty of such a family.
Table 1. Photophysical properties of compounds A3–A7.
A3
A4
A5
A6
A7
d[nm]
lmax (loge)
lS1 ACHTUNGTRENNUNG
(loge)[a]
1.16
1.59
2.02
2.45
2.88
G
308 (5.16) 331 (4.99) 346 (5.14) 353 (4.14) 364 (4.51)
402 (3.29) 417 (3.12) 430 (3.45) 436 (3.00) 443 (3.32)
403
lem [nm][b]
418
430
438
444
2.80
–
DE[eV][c]
HOMO[eV] 5.57
3.07
2.97
5.45
2.48
0.17
15.80
92.94
2.88
5.42
2.54
0.19
11.29
58.80
2.84
5.36
2.52
0.38
9.01
23.71
LUMO[eV] 2.50
–
[d]
FF
ts [ns][e]
0.21
10.56
51.26
0.23
8.13
35.34
tr [ns][f]
[a] The absorption peaks of the lowest electronic states (S1). [b] Emission
spectra excited at 375 nm. [c] DE=energy gap. [d] Fluorescent quatum
yield measured relative to 9,10-diphenylanthracene. [e] Exicted-state life-
time. [f] Radiative rate coefficients.
We also examined the photophysical properties of B3–B5
and C3–C5 with their UV/Vis and PL spectra, and key data
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R.-S. Liu et al.
Figure 4. Plot of the energy gaps (eV) versus 1/n (n=number of central
benzenes) for A3–A7 and B3–B5.
given in Figures 5 and 6 and Table 2. As we expected, most
of these compounds show a pattern of extensive p conjuga-
tion with increasing array size except for species C3, which
has a larger emission wavelength (lem =428 nm) than those
of C4 (lem =416 nm), A3 (lem =403 nm), and B3 (lem
=
403 nm), all of which have the same array size. Furthermore,
the UV/Vis absorption of C3 has a very small absorption co-
efficient (loge=4.79) although the absorption wavelength
(lmax =302 nm) is smaller than that of C4 (lmax =317 nm).
Figure 6. a) UV/Vis absorption and spectra of C3–C5 in CH2Cl2. b) Un-
normalized fluorescence spectra obtained at excitation of 356 nm.
Table 2. Photophysical properties of compounds B3–B5 and C3–C5.
B3
B4
B5
C3
C4
C5
d[nm]
lmax (loge)
1.16
308
(5.16)
402
1.59
329
(5.02)
425
2.02
353
(5.11)
434
1.14
302
(4.79)
–
1.59
317
(5.22)
–
2.01
330
(5.20)
–
G
lS1 ACHTNUTGRNEUNG
(loge)[a]
(3.29)
403
3.07
(2.75)
425
2.92
(2.63)
436
2.84
lem [nm][b]
DE[eV][c]
428
–
416
–
435
–
HOMO[eV] 5.70
5.47
2.55
0.11
31.80
289.09
5.67
2.83
0.09
27.95
310.55
5.86
–
0.10
13.57
135.16
5.76
–
0.086
17.31
201.27
5.53
–
0.082
13.04
159.48
LUMO[eV] 2.50
[d]
FF
ts [ns][e]
0.21
10.56
51.26
tr [ns][f]
[a] The absorption peaks of the lowest electronic states (S1). [b] Emission
spectra excited at 356 and 375 nm. [c] DE=band gap. [d] Fluorescence
quantum yield measured relative to 9,10-diphenylanthracene (B3–B5)
and anthracene (C3–C5). [e] Exicted-state lifetime. [f] Radiative rate co-
efficients.
Species C3–C5 are particularly notable for their small quan-
tum yields (0.082–0.10), large radiative rate coefficients (tr
=135–202 ns), and the loss of their vibronic structures in PL
emission. We were not able to obtain accurate energy gaps
for species C3, C4, and C5 because of their unrecognizable
S1–S0 band. A plot of band gaps versus 1/n (Figure 4), never-
theless, indicated a well-matched convergence of these olig-
omers of series B (B3, B4, and B5). We envisaged that the
Figure 5. a) UV/Vis absorption and spectra of B3–B5 in CH2Cl2. b) Un-
normalized fluorescence spectra obtained at excitation of 375 nm.
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Regiocontrolled Synthesis of para-Phenylene Oligomers
FULL PAPER
the excitation laser pulse by using a polarizer. In all experiments reported
here, the polarization was fixed at the magic-angle condition (54.78). The
full width at half maximum (FWHM) of the instrument response function
(IRF) was determined to be approximately 80 ps.
methyl groups of the oligomers of series C may impede the
p conjugation, resulting in larger energy gaps than those of
the oligomers of series B. The band gaps of A3–A7 and B3–
B5 are considerably smaller than those of spiro-bridged
ladder-type para-phenylene oligomers of the same size.[15a]
Steady-state absorption, fluorescence, and quantum yield: The UV/Vis
absorption and fluorescence spectra were routinely recorded by using
Cary 50 (Varian) and FluoroLog Tau-3 (Jobin Yvon) spectrometers, re-
spectively. The fluorescence quantum yields (FF) of all of the samples in
dichloromethane were determined by using a comparative method de-
scribed in [Eq. (1)], in which the subscripts S and R refer to the sample
and reference solutions, respectively.[15–16]
Conclusion
To the best of our knowledge, prior to this work there have
been no examples of the regioselective synthesis of a family
of ethene-bridged phenylene oligomers.[14] Herein, we have
reported the regiocontrolled syntheses of such oligomers in
three distinct classes by using PtII- and RuII-catalyzed aro-
matization. The UV/Vis and PL spectra and the band gaps
of these regularly growing arrays showed a pattern of exten-
sive p conjugation with increasing array size. Variable-tem-
perature NMR spectra provided evidence that large arrays
of these oligomers are prone to twist from planarity, which
hampers their p conjugation. Future use of these ethene-
bridged para-phenylene oligomers in optoelectronic or
nanomaterial devices is under current investigation.
FF
FR
nS2
GradS
Þ
GradR
ð1Þ
¼
ꢂ ð
nR2
The refractive indexes (nS and nR) were used to correct the collection ef-
ficiencies of the emissions of different solvents. Grad represents the gra-
dient of the plot of the integrated fluorescence intensity versus absorb-
ance. Anthracene and DPA (9,10-diphenylanthracene) were chosen as
the fluorescence standard and its absolute quantum yield was known to
be 0.27
G
G
temperature.[17] Note that the fluorescence spectra of the sample and the
reference standard must be recorded at identical experimental conditions
so that the relationship shown in [Eq. (1)] holds true. The results ob-
tained according to [Eq. (1)] are summarized in Table 1.
Cyclic voltammetry measurements: The HOMO energy levels of the
studied compounds were calculated from the oxidation potential (E1/2
)
obtained from the cyclic voltammetry (CV) measurement with Pt wire as
the counter electrode and a glassy carbon electrode as the working elec-
trode. The potentials were measured against an Ag/Ag+ (0.01m AgNO3)
reference electrode. The final results were calibrated with the ferrocene/
ferrocenium (Fc/Fc+) couple. Under the assumption that the energy level
of ferrocene/ferrocenium is 4.8 eV below vacuum, the HOMO energy
levels were determined from the equation 4.8 eV+E1/2 (versus Fc/Fc+).
Experimental Section
Solvents and reagents: All experimental operations were performed
under nitrogen and the equipment was dried in an oven at 1508C for sev-
eral hours. THF was distilled over sodium. DCM and toluene were dis-
tilled and dried over calcium hydride. All of the other specified chemicals
were commercially purchased (Aldrich and Strem) and used without fur-
ther purification.
NMR spectroscopic and mass-spectrometric analysis: The 1H and
13C NMR spectra were determined on a Bruker AV 400 NMR spectrome-
ter and Bruker AVANCE 600 NMR spectrometer in a solution of CDCl3
or C2D2Cl4, unless indicated otherwise. Chemical shifts are reported in
ppm downfield of the solvent peak (CDCl3: d=7.24 ppm for 1H NMR,
d=77.00 ppm for 13C NMR) as an internal standard. HRMS was per-
formed on a Finnigan Mat95 mass spectrometer. MALDI mass spectrom-
etry was performed on an AutoflexIII MALDI-TOF mass spectrometer
at the National Chung-San University.
Experimental procedures for the synthesis of A3 (Scheme 6)
Synthesis of compound s1: A mixture of 1,4-dibromo-2,5-diiodobenzene
(4.5 g, 9.2 mmol) in DME (1,2-dimethoxyethane, 50 mL) and aqueous
K2CO3 (46 mL, 20% ) was stirred under nitrogen for 10 min. Tetrakis(tri-
phenylphosphine)palladium(0) (531 mg, 0.46 mmol) was added followed
by a solution of 4-(2-ethylhexyloxy)phenylboronic acid (5.0 g, 20.2 mmol)
in DME (10 mL). The resulting mixture was heated under reflux for 12 h
and allowed to cool to room temperature. The organic phase was separat-
ed and the aqueous phase was washed with diethyl ether (2ꢁ100 mL).
The combined organic layers were washed with water, brine, and dried
with anhydrous MgSO4. The solvent was removed in vacuo and the crude
product was purified by a silica column with hexanes, affording com-
pound s1 (3.86 g, 65%) as a colorless oil.
Measurement of activation energy: The 1H variable-temperature NMR
spectra of species A5ꢀ is shown in Figure 2. On increasing the tempera-
ture from 25 to 1058C, the diastereotopic protons of the two singlets
(7.772 and 7.739 ppm at 258C) of species C5 coalesced at 1058C (Tc).
The activation energy (DG°) was calculated by using the equations Kc =
2.22Dv and DG° =4.58Tc(10.32+log Tc/Kc) calmolꢀ1 (in which Tc =378 K
Synthesis of compound s2: Nitrogen was bubbled through a solution of
piperidine (10 mL) and triethylamine (30 mL) for 30 min. Compound s1
(3.8 g, 5.9 mmol), CuI (112 mg, 0.59 mmol), PPh3 (155 mg, 0.59 mmol),
PdACHTNUGRTNEUNG(PPh3)4 (341 mg, 0.29 mmol), and ethynyltrimethylsilane (1.85 mL,
and Dv=16.6 Hz), which gave
a
large value of DG° =19.6ꢁ
13 mmol) were added to this solution. The resulting solution was stirred
at 808C for 8 h. After cooling to ambient temperature, the solvent was
removed in vacuo and the organic layer was extracted with CH2Cl2.
After concentration in vacuo, the crude material was purified by flash
column chromatography on silica gel with ethyl acetate/hexanes (5/95) as
the eluent to afford compound s2 (3.65 g, 91%) as a yellow oil.
0.2 kcalmolꢀ1
.
Time-resolved fluorescence measurements: Picosecond time-resolved ex-
periments were performed with a time-correlated single-photon-counting
system (TCSPC; PicoQuant Fluotime 200). The excitation pulses at
375 nm were generated from a picosecond laser system (PicoQuant
LDH-P-C-375) controlled by a diode laser driver (PicoQuant PDL-
800B). The excitation laser was focused onto a cuvette (thickness=1 cm)
containing a sample solution. The fluorescence emitted at a right angle
was collected with a lens pair. The wavelength of fluorescence was select-
ed by using a double monochromator (8 nm per mm dispersion, of the
subtractive type to correct for distortion of the group velocity disper-
sion). A multi-channel plate photomultiplier (R3809U-50, Hamamatsu)
served as a photon-counting detector from which the signal was fed into
a computer with a TCSPC-module (SPC-630, Becker and Hickl) for data
acquisition. We selected the polarization of the emission with respect to
Synthesis of compound A3s: n-Tetrabutylammonium fluoride (1.0m) in
THF (1.64 mL, 1.64 mmol) was added to a solution of compound s2
(1.70 g, 1.49 mmol) in THF (20 mL) at 08C, and the resulting solution
was stirred at 08C for 2 h. Column chromatography of the crude material
on silica gel with ethyl acetate/hexanes (5/95) as the eluent afforded com-
pound A3s (1.46 g, 98%) as a yellow oil.
Synthesis of compound A3:
A dry reaction tube containing [Ru-
A
N
ACHTUNGTRENNUNG
2 h before it was charged with compound A3s (279 mg, 0.28 mmol) and
1,2-dichloroethane (28 mL). The mixture was heated at 808C for 24 h.
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R.-S. Liu et al.
Scheme 6.
After concentration in vacuo, the crude material was purified by flash
column chromatography on silica gel by using dichloromethane/hexanes
(1/10) as the eluent to afford compound A3 (162 mg, 58%) as a white
solid.
Spectral data for compound A3s: 1H NMR (400 MHz, CDCl3): d=7.60
(s, 2H), 7.56 (d, J=8.8 Hz, 4H), 6.97 (d, J=8.8 Hz, 4H), 3.89 (d, J=
6.0 Hz, 4H), 3.14 (s, 2H), 1.79–1.73 (m, 2H), 1.57–1.40 (m, 8H), 1.38–
1.32 (m, 8H), 0.97–0.91 ppm (m, 12H); 13C NMR (100 MHz, CDCl3): d=
159.2, 142.1, 134.8, 131.0, 130.2, 120.8, 114.0 (2ꢁCH), 82.9, 81.5, 70.4,
39.4, 30.5, 29.1, 23.9, 23.0, 14.1, 11.1 ppm; HRMS calcd for
C38H46O2:534.3498; found: 534.3491.
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Spectral data for compound A3: 1H NMR (400 MHz, CDCl3): d=8.97 (s,
2H), 8.71 (d, J=8.8 Hz, 2H), 7.89 (d, J=9.2 Hz, 2H), 7.64 (d, J=8.8 Hz,
2H), 7.31 (d, J=8.8 Hz, 2H), 7.28 (s, 2H) 4.02 (d, J=0.8 Hz, 4H), 1.83–
1.80 (m, 2H), 1.60–1.44 (m, 8H), 1.41–1.35 (m, 8H), 0.98 (t, J=7.2 Hz,
6H), 0.93 ppm (t, J=7.2 Hz, 6H); 13C NMR (100 MHz, CDCl3): d=
158.3, 133.3, 130.2, 128.5, 128.0, 126.8, 124.2, 124.2, 121.3, 117.0, 110.1,
70.7, 39.5, 30.6, 29.1, 23.9, 23.1, 14.1, 11.2 ppm; HRMS calcd for
C38H46O2:534.3498; found: 534.3491.
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Acknowledgements
The authors wish to thank the National Science Council, Taiwan for sup-
porting this work.
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Chem. Eur. J. 2010, 16, 1826 – 1833

Regiocontrolled Synthesis of para-Phenylene Oligomers
FULL PAPER
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Received: August 11, 2009
Published online: December 22, 2009
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www.chemeurj.org
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