Syntheses, optical and electrochemical properties of 4,4′-bis-[2-(3, 4-dibutyl-2-thienylethynyl)]biphenyl and its oligomers

Article abstract of DOI:10.1016/j.tet.2005.01.133

New π-conjugated oligomers (1-3) combined of thiophene rings, CC units, and phenylene rings were synthesized. The monomer (1) was synthesized by Sonogashira cross-coupling reaction. Lithiation and oxidation of the monomer led to the formation of dimer (2) and trimer (3). Optical and electrochemical properties of compounds 1-3 were studied. The oligomers with thioacetyl moieties were synthesized for self-assembly.

Full text of DOI:10.1016/j.tet.2005.01.133

Tetrahedron 61 (2005) 3813–3817
Syntheses, optical and electrochemical properties
of 4,40-bis-[2-(3,4-dibutyl-2-thienylethynyl)]biphenyl
and its oligomers
* *
Lei Liu, Zhanxiang Liu, Wei Xu, Hai Xu, Deqing Zhang and Daoben Zhu
*
Organic Solids Laboratory, Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China
Received 15 October 2004; revised 28 January 2005; accepted 31 January 2005
Abstract—New p-conjugated oligomers (1–3) combined of thiophene rings, C^C units, and phenylene rings were synthesized. The
monomer (1) was synthesized by Sonogashira cross-coupling reaction. Lithiation and oxidation of the monomer led to the formation of dimer
(2) and trimer (3). Optical and electrochemical properties of compounds 1–3 were studied. The oligomers with thioacetyl moieties were
synthesized for self-assembly.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
molecular scale electronics: (1) these compounds are rigid
in their frameworks so as to minimize conformational
flexibility yet containing butyl groups for maintaining
solubility and processability of the final products. The
butyl groups have been preferred over long paraffinic chains
because they afford better yields during the syntheses of the
precursors;7 (2) alkynyl units separate the aryl units, so
ground state contiguous p-overlap will be minimally
affected by rotational variations; (3) thiol end groups can
be attached to the ends of the compounds; (4) they are stable
to light and oxygen, so that they can be manipulated in the
air.
In 1975, Moore stated that chip densities would double
every 2 years.1 According to the statement, the reduction in
size of circuits and increase in speed can only continue for
some finite time. This forces us to consider other options.
Many researches are focused towards molecular electronics.
We seek to use the electronic properties intrinsic in the
molecules to fabricate electric devices. p-Conjugated
oligomers of defined length and constitution arise from
new potential to act as molecular wires in molecular scale
electronics. A variety of spectacular molecular architectures
are resulted from the efforts aimed at the construction of
such wires. In addition, modern synthetic organic and
organometallic methodologies are providing powerful tools
for the direct acquisition of the molecules and materials
capable of fulfilling a huge variety of requirements in terms
of mechanical, physical, and chemical properties.2
Oligoynes,3 oligothiophenes,4 and oligophenylenes5 have
been intensively studied. Tour and co-workers have
reported the syntheses of oligo(a-thiophene ethynylene)s
and oligo(p-phenylene ethynylene)s.6 But linear conjugated
oligomers involving combination of three different sub-
structures are seldom discussed. Here, we report the
syntheses of p-conjugated molecules 1–3 consisting of
thiophene rings, C^C units, and phenylene rings as
potential molecular wires. These compounds possess the
following features that make them suitable for studies of
Additionally, the syntheses of compounds (4, 5) bearing one
or two thioacetyl moieties at terminal positions were
presented. On one hand, the thiols resulted from the
hydrolyzation of thioacetyl moieties can serve as molecular
scale alligator clips for adhesion of the molecular scale
wires to the gold probes.8,9 On the other hand, the molecule
with one end-thiol group can be used as a ligand for the
functionalizaion of noble metal nanoparticles. And
molecules with end-thiol groups at both terminal positions
can be used as rigid linkers for the assembly of noble metal
nanoparitcles to form 2D structures.
Keywords: Synthesis; p-Conjugated oligomers; Electrochemistry.
*
Corresponding authors. Tel.: C86 10 6263 9355; fax: C86 10 62559373;
e-mail: wxu@iccas.ac.cn
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.01.133

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L. Liu et al. / Tetrahedron 61 (2005) 3813–3817
2. Results and discussions
and 30%, respectively.13 Compounds 1–3 were stable in the
air. They showed good solubilities in dichloromethane,
chloroform, THF and benzene, yielding excellent process-
ability for practical application.
2.1. Molecular design and synthetic strategy
The synthetic strategy employed two reaction types to grow
the conjugated chains rapidly. As a monomer unit,
compound 1 was synthesized by Pd-mediated Sonogashira
cross-coupling reaction. Oligomers 2 and 3 containing two
or three monomer units were synthesized by lithiation of 1
and followed with oxidation coupling reactions, using
CuCl2 as the oxidant, which were depicted in Scheme 1.
The starting material, 3,4-dibutylthiophene (8) was prepared
according to the procedure described by Tour.10 Then, it
was subjected to react with iodine and mercury oxide,
leading to 9 by an adopted procedure reported by Minnis.11
Diiodide was obtained simultaneously, but it was no need to
separate the iodide and diiodide. The iodide 9 intended to
decompose in the air and should be used as0soon as possible
after workup. Commercially available 4,4 -diiodobiphenyl
was coupled with (trimethylsilyl) acetylene via a
Sonogashira–Hagihara reaction to afford the (trimethyl-
silyl)ethynyl derivative 10,12 which was then desilylated
with potassium carbonate to afford the ethynyl derivative
11. The aryl iodide 9 was coupled with the terminal alkyne
11 to afford compound 1 in a yield of 67% (Scheme 1). Pd/
Cu coupling reactions required the strict exclusion of
oxygen; degassing and use of a dry box was required to
attain reasonable yields. Lithiation of 1 and followed with
an oxidative coupling reaction in the presence of CuCl2 led
to the formation of dimer (2) and trimer (3) in yields of 33
Complementary thiol end groups, protected as thioester
moieties, were attached to one or both ends of molecule 1
via a one-pot lithiation–sulfide-acetylation protocol.14 The
monothiol-terminated system could be used for self-
assembly on gold surfaces. And a, u-difunctionalized
system could serve as a bridge between two gold probes.
Thiol-terminated molecules could also be used for the
functionalization of noble metal nanoparticles.
2.2. UV–vis and fluorescence spectral studies
The UV–vis absorption and fluorescence spectra of
compounds 1–3 in dichloromethane were shown in Figures 1
and 2. In the UV–vis absorption spectra, broad bands with
little vibronic structure were present. The shapes of the
spectra were almost identical. There were two absorption
bands for every compound. A weak band appeared around
270–280 nm. A strong band appeared at 352, 366 and
368 nm for 1, 2 and 3, respectively. Elongation of the
conjugation length from 1 to 2 resulted in a red shift of
14 nm. However, only little red shift was observed from 2 to
3. Similar chain length dependence was observed for their
fluorescence spectra (Fig. 2). The spectroscopic measure-
ments provided definite information on the effective
conjugation length, that was, they exhibited a certain
Scheme 1. Preparation of compounds 1–5. Reagents and conditions: (i) Br2, 84%. (ii) n-BuLi, H2O, 70%. (iii) C4H9MgBr, Ni(dppp)Cl2, Et2O, 61%. (iv) HgO,
I2, 94%. (v) Me3SiC^CH, Pd(PPh3)4, CuI, Et3N, rt, 63%. (vi) K2CO3, MeOH, Et2O, rt, 100%. (vii) Pd(PPh3)4, CuI, Et3N, rt, 67%. (viii) LDA, CuCl2, 33% for
2 and 30% for 3. (ix) 1 equiv n-BuLi, S8, AcCl, 23%. (x) 2 equiv n-BuLi, S8, AcCl, 21%.

L. Liu et al. / Tetrahedron 61 (2005) 3813–3817
3815
Table 1. The absorption lmax (in CH2Cl2), emission lmax (in CH2Cl2), and
fluorescent quantum yields of oligomers 1–3
a
FF
Oligomer
UV lmax (nm)
Em lmax (nm)
1
2
3
352
366
368
398
471
474
0.62
0.13
0.11
Excited wavelengths are 352, 366 and 368 nm for molecules 1, 2 and 3,
respectively.
a Use 9,10-diphenylanthracene (FFZ1.0) to compare with in cyclohexane.
2.3. Electrochemical studies
The electrochemical behaviors of compounds 1–3 were
studied by cyclic voltammetry. The oxidation wave was
observed around 1.22 V for compound 1, which showed no
tendency for reduction under CV conditions but rather
underwent irreversible oxidations in CH2Cl2, due to the fact
that the corresponding radical cations were not stable.
Electrochemical polymerization and film deposition for
compound 1 was carried out using multiple scan cyclic
voltammetry. Upon multiple cycling of 1 in CH2Cl2
solution, a new redox process developed at a lower potential
(w0.4 V) due to the reversible redox process of the as-
deposited polymer (Fig. 3). The current response of this new
redox process continued to increase in intensity upon
additional cycling consistent with the deposition of an
electroactive polymer onto the surface of the working
electrode. As for products 2 and 3, quasi-reversible redox
behaviors were observed with oxidation peaks at 1.18 and
1.16 V, respectively. And no evidence for dimerization or
polymer growth could be observed with continued cyclic
sweeping through oxidizing potentials (Fig. 4). The
electrochemical behavior of compound 1 was similar with
that of 3,4-dibutyl thiophene, which displayed a typical
electrochemical polymerization on the surface of anode.
The difference between compound 1 and its dimer and
trimer was due to stabilizing of radical cations with the
increasing of conjugating length. Oligo(thiophene)s also
displayed different electrochemical behavior with the
different conjugating length. Only oxidation waves could
Figure 1. UV–vis absorption spectra of compounds 1–3 in CH2Cl2.
Figure 2. Fluorescence emission spectra of compounds 1–3 in CH2Cl2.
Excited wavelengths are 352, 366 and 368 nm for molecules 1, 2 and 3,
respectively.
limitation for the extensive conjugation. The saturation in
lmax was observed previously and arose because of the
limitations to electron delocalization in the longer
oligomers. There were several competing factors that must
be considered. It has been known that with increasing of
p-conjugation chain and more extensive electron
delocalization, the absorption maxima should show batho-
chromic shifts. To enhance the solubility of the systems, the
thiophene rings were substituted with n-butyl groups.
The pendant butyl groups had two contrary effects on the
p-systems. The butyl groups could cause distortions of the
conjugated systems from planarity if steric interactions were
severe enough. On the other hand, if steric interactions were
not particularly severe, butyl groups could induce batho-
chromic shifts, which had been explained by inductive and
hyperconjugative effects.6 These two contradictory factors
could take effect concurrently. With increase of chain
length, the steric hindrance due to the butyl groups became
overwhelmingly dominant, the effective conjugation
reached saturation. The fluorescence quantum yields of
1–3 in dichloromethane were 62, 13 and 11%, respectively
(Table 1). The quantum yields decreased as the conjugation
length increased.
Figure 3. Cyclic voltammetric scanning electropolymerization of 1 (10K4
M
in 0.1 M Bu4NCPF6K/CH2Cl2, 5 cycles. Pt working and counter electrodes,
scan rate 100 mV sK1).

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L. Liu et al. / Tetrahedron 61 (2005) 3813–3817
Bu4NPF6 as electrolyte, platinum electrodes as counter
and work electrodes and SCE as reference electrode.
n-Butyllithium (2.5 M solution in hexanes) and LDA
(1.5 M in cyclohexane) were purchased from Acros or
Aldrich Chemical Co., Inc. 4,40-Diiodobiphenyl and
(trimethylsilyl) acetylene were from Alfa Aesar.
Ni(dppp)Cl2, Pd(PPh3)4 from Aldrich Chemical Co., Inc.
were used as received. Tetrahydrofuran (THF) and diethyl
ether (Et2O) were distilled from sodium/benzophenone
immediately prior to use. All other reagents and solvents
(standardgrade)wereusedasreceived unlessotherwisenoted.
All reactions involving n-BuLi or LDA were carried out under
an atmosphere of dry N2. Sonogashira cross-coupling
reactions required the strict exclusion of oxygen and water.
4.1.1. 4,40-Bis(trimethylsilyl)biphenyl 10. To a solution of
4,40-diiodobiphenyl (2.00 g, 4.9 mmol), Pd(PPh3)4 (0.113 g,
0.098 mmol), CuI (0.037 g, 0.196 mmol) in THF (10 mL)
was added 0.744 g (1 mL, 7.35 mmol) of triethylamine. A
solution of 1.01 g (1.045 mL, 10.29 mmol) of (trimethyl-
silyl)acetylene in 5 mL of THF was then added slowly.
The solution was stirred overnight at room temperature
and white precipitate appeared, which was removed by
filtration. The filtrate was evaporated, and the residue was
purified by silica gel column chromatography (hexane) to
give 10 (1.07 g, 63%) as a white solid. Mp 169 8C. IR (KBr,
cmK1): 2959, 2156, 1488, 1249, 861, 847, 824, 759. MS
Figure 4. Cyclic voltammetric scanning electropolymerization of com-
pounds 2 and 3, 10K4 M in 0.1 M Bu4NCPF6K/CH2Cl2. Pt working and
counter electrodes, scan rate 100 mV sK1
.
be observed for shorter oligomers, and reversible redox
wave could be observed for longer oligomers.15
CVs of compounds 4 and 5 were also conducted for
comparison. With masked thiol end-groups at the terminal
positions, 4 and 5 presented peak oxidation potentials at
1.30 V, which were slightly higher than that of the parent
molecule 1. This should be due to the electron withdrawing
effect of the thioacetyl groups.
1
(EI): 346 (MC). H NMR: 7.53 (s, 8H), 0.27 (s, 18H).
4.1.2. 4,40-Diethynylbiphenyl 11. To a solution of 10
(4.66 g, 13.47 mmol) in methanol (30 mL) and diethyl ether
(30 mL) was added potassium carbonate (18.61 g,
134.68 mmol). The solution was allowed to stir for 6 h
before being poured into water. The aqueous layer was
extracted with dichloromethane, and the organic extracts
were washed with brine. The combined organic layers were
dried over magnesium sulfate. The solvent was removed by
rotary evaporation. No further purification was necessary to
afford 2.72 g (100%) of 11 as a white solid. Mp 167 8C.
(lit.16 Mp 165.5–166.5 8C). IR (KBr, cmK1): 3277, 2104,
1488, 857, 825. MS (EI): 202 (MC). 1H NMR: 3.16 (s, 2H),
7.54–7.60 (m, 8H).
3. Conclusions
We reported the synthesis of p-conjugated molecule 1 from
the easy accessible materials with high efficiency. This
molecule had a linear p-conjugated system with a length
scale about 2 nm, and this p-conjugated system could be
easily elongated by one-pot lithiation and oxidation
coupling reaction. Dimer (2) and trimer (3) of compound
1 were obtained in moderate yields. These compounds could
serve as building blocks for other functional molecular
wires with precise conjugation lengths.
4.1.3. 4,40-Bis-[2-(3,4-dibutyl-2-thienylethynyl)]biphenyl
1. Following the method used for the synthesis of 10. Used
4. Experimental
4.1. General
were
9 (6.80 g, 21.6 mmol), Pd(PPh3)4 (0.499 g,
0.432 mmol), CuI (0.165 g, 0.864 mmol), THF (30 mL),
triethylamine (4.5 mL), 11 (2.182 g, 10.8 mmol),THF
(50 mL). The solvent was evaporated and the residue was
purified by silica gel column chromatography (silica gel,
petroleum/dichloromethane 4:1) to give compound
1(4.27 g, 67%) as a pale-yellow solid. Mp 67 8C. IR (KBr,
cmK1): 2956, 2928, 2858, 2199, 1635, 1493, 1406, 868,
Melting points were measured with a Buchi Melting Point
B-540 instrument and uncorrected. 1H NMR and 13C NMR
spectra were recorded with Bruker 300 MHz or Varian
200 MHz spectrometers. All chemical shifts were quoted in
ppm relative to TMS. Infrared spectra were obtained on a
Perkin–Elmer System 2000 FT-IR spectrometer. Mass
spectra were determined with AEI-MS50-MS or MALDI-
TOF-MS. Elemental analysis was performed on Carlo-
Erba-1106 instrument. HRMS was performed on UK GCT-
Micromass. Absorption spectra were measured with Hitachi
(model U-3010) UV–vis spectrophotometer. Fluorescence
measurements were carried out with a Hitachi (model
F-4500) Spectrophotometer in a 1-cm quartz cell. Cyclic
voltammetric experiments were performed on an EGDG
PAR 370 system at a scan rate of 100 mV in CH2Cl2 using
1
819, 743. H NMR (300 MHz, CDCl3): d 0.94–2.76 (m,
36H), 6.87 (s, 2H), 7.55–7.60 (m, 8H). 13C NMR (300 MHz,
CDCl3): d 14.0, 22.6, 22.7, 28.0, 28.8, 31.9, 32.2, 84.3, 94.6,
118.5, 121.5, 122.8, 126.9, 131.7, 139.8, 142.1, 147.1. MS
(EI): 590 (MC). Anal. Calcd for C40H46S2: C, 81.30; H,
7.85; S, 10.85. Found: C, 81.51; H, 8.00; S, 11.06.
4.1.4. 5,50-Bis[40-(3,4-dibutyl-thien-2-ylethynyl)-bi-
phenyl-4-ylethynyl]-3,4,30,40-tetrabutyl-[2,20]-bithio-
phenyl 2. A solution of 1 (0.130 g, 0.22 mmol) in anhydrous

L. Liu et al. / Tetrahedron 61 (2005) 3813–3817
3817
THF (10 mL) was added to lithium diisopropylamine
(0.18 mL, 0.26 mmol, 1.5 M in cyclohexane) in 10 mL of
anhydrous THF at K78 8C under nitrogen. The reaction
mixture were stirred at K78 8C for 30 min, then anhydrous
powered CuCl2 (0.0296 g, 0.22 mmol) was added in one
portion, upon which the color of the solution changed to
black. The solution was stirred until it returned to rt then for
an additional 30 min at 30 8C. The mixture was poured into
20 mL of water, containing 10 mL of 1 M hydrochloric
acid. Then it was extracted twice with 100 mL of diethyl
ether. The combined organic layers were washed with
water, dried over sodium sulfate and concentrated in vacuo.
The crude product was purified by silica gel column
chromatography (petroleum/dichloromethane 2:1) to give
0.043 g (33%) of 2 as a yellow solid. Mp 97 8C. IR (KBr,
cmK1): 2956, 2929, 2862, 2197, 1687, 1602, 1491, 1460,
821, 740. MS (MALDI-TOF): 1177 (MC). 1H NMR
(300 MHz, CDCl3): d 0.85–2.76 (m, 72H), 6.88 (s, 2H),
7.56–7.62 (m, 16H). 13C NMR (300 MHz, CDCl3): d 83.78,
84.34, 94.58, 95.88, 118.43, 118.92, 121.48, 122.60, 122.78,
126.84, 126.87, 130.12, 131.66, 139.73, 139.91, 141.86,
142.09, 147.05, 147.57. HRMS calcd for C80H90S4:
1178.5925. Found: 1178.5919.
126.93, 127.38, 131.50, 131.71, 131.81, 139.71, 140.22,
142.13, 147.10, 147.37, 148.23, 194.23. HRMS calcd for
C42H48OS3: 664.2867. Found: 664.2870.
4.1.7. {4,40-Bis-[2-thioacetic acid-S-(3,4-dibutyl-2- thienyl-
ethynyl)]ester}biphenyl 5. It was prepared similarly as
compound 4. Used were 1 (0.296 g, 0.502 mmol), THF
(10 mL), n-butyllithium (0.8 mL, 2.007 mmol), sulfur powder
(0.048 g, 1.505 mmol), acetyl chloride (0.107 mL,
1.505 mmol). The residue was purified by silica gel
chromatographybyfirst using4:1petroleum/dichloromethane
and then increasing to 2:1 petroleum/dichloromethane to
provide 0.078 g (21%) of 5 as a yellow solid. Mp 93.7–
95.0 8C. MS (EI): 738 (MC), 696 (MKCOCH3C1), 654
(MKCOCH3–COCH3C2), 622 (MK COCH3 –COCH3 –S
C2). IR (KBr, cmK1): 2956, 2862, 2197, 1911, 1726, 1492,
1
1461, 1113, 945, 821, 738. H NMR (300 MHz, CDCl3): d
2.44 (s, 6H), 0.09–2.78 (m, 36H), 7.57–7.63 (m, 8H). 13C
NMR (300 MHz, CDCl3): d 13.84, 13.96, 22.72, 22.77, 27.83,
28.76, 29.56, 32.38, 32.47, 83.28, 96.74, 120.56, 122.42,
123.76, 126.92, 131.78, 140.11, 147.34, 148.17, 194.19.
HRMS calcd for C44H50O2S4: 738.2694. Found: 738.2688.
4.1.5. 4,40-Bis-{5-[40-(3,4-dibutyl-thien-2-ylethynyl)-
biphenyl-4-yl-ethynyl]-[2,20]-bithiophenyl-50-ylethynyl}-
biphenyl 3. 0.039 g (30%) of 3 was also isolated as a yellow
solid. Mp 153 8C. IR (KBr, cmK1): 2956, 2927, 2858, 2197,
1688, 1603, 1492, 1460, 821, 743. MS (MALDI-TOF):
References and notes
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2. Tour, J. M. Chem. Rev. 1996, 96, 537–553.
1
1767 (MC). H NMR (300 MHz, CDCl3): d 0.90–2.82 (m,
108H), 6.93 (s, 2H), 7.64–7.65 (m, 24H). 13C NMR
(300 MHz, CDCl3): d 13.80, 13.97, 14.01, 22.56, 22.67,
22.74, 22.79, 27.54, 27.96, 28.56, 28.71, 29.68, 31.87,
32.19, 32.46, 32.85, 83.76, 83.80, 84.32, 94.56, 95.86,
118.41, 118.90, 121.47, 122.59, 122.63, 122.77, 126.83,
126.86, 130.11, 131.66, 139.72, 139.86, 139.90, 141.85,
142.08, 147.04, 147.56. Anal. Calcd for C120H134S6: C,
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4.1.6. [4-[2-(3,4-Dibutyl-2-thienylethynyl)-40-[thioacetic
acid-S-[2-(3,4-dibutyl-2-thienylethynyl)]ester]biphenyl
4. To a solution of 1 (0.198 g, 0.336 mmol) in THF (10 mL)
at K78 8C was added dropwise n-butyllithium (0.201 mL,
0.503 mmol). The solution was stirred at K78 8C for 30 min
then warmed to 0 8C and sulfur powder (0.011 g,
0.336 mmol) was added. The reaction remained at 0 8C
for 30 min. The solution was recooled to K78 8C, and
acetyl chloride (0.029 mL, 0.403 mmol) was added in one
portion. The solution was allowed to warm to room
temperature overnight. The mixture was extracted with
dichloromethane and dried over magnesium sulfate. The
solvent was removed by rotary evaporation, and the residue
was purified by silica gel chromatography using 4:1
petroleum/dichloromethane as eluent to provide 0.051 g
(23%) of 4 as a yellow solid. Mp 82.3–83.0 8C. MS (EI):
664 (MC), 622 (MKCOCH3C1). IR (KBr, cmK1): 2956,
2861, 2198, 1910, 1725, 1492, 1460, 1112, 821, 741, 605.
1H NMR (300 MHz, CDCl3): d 2.44 (s, 3H), 0.89–2.79 (m,
36H), 6.89 (s, 1H), 7.57–7.63 (m, 8H). 13C NMR (300 MHz,
CDCl3): d 13.92, 14.04, 14.08, 22.63, 22.75, 22.78, 22.82,
22.94, 27.88, 28.03, 28.79, 29.63, 29.76, 31.94, 32.27,
32.43, 32.53, 32.79, 53.47, 83.26, 84.45, 91.65, 96.83,
118.50, 120.53, 121.56, 122.32, 122.90, 123.82, 126.90,
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