Synthesis, characterization, and physical properties of monodisperse oligo(p-phenyleneethynylene)s

Article abstract of DOI:10.1021/ma021656a

A new stepwise synthesis method for the preparation of precisely defined oligo(p-phenyleneethynylene)s (OPEs) is described. High-purity monomer, trimer, pentamer, heptamer, and nonamer of OPEs with dihexyloxy side groups and trimethylsilyl (TMS) or thioac

Full text of DOI:10.1021/ma021656a

Macromolecules 2003, 36, 1457-1464
1457
Synthesis, Characterization, and Physical Properties of Monodisperse
Oligo(p-phenyleneethynylene)s
Ch u a n -Zh en Zh ou , Tia n xi Liu , J in g-Mei Xu , a n d Zh i-Ku a n Ch en *
Institute of Materials Research and Engineering (IMRE), 3 Research Link,
Singapore 117602, Republic of Singapore
Received November 4, 2002; Revised Manuscript Received December 10, 2002
ABSTRACT: A new stepwise synthesis method for the preparation of precisely defined oligo(p-
phenyleneethynylene)s (OPEs) is described. High-purity monomer, trimer, pentamer, heptamer, and
nonamer of OPEs with dihexyloxy side groups and trimethylsilyl (TMS) or thioacetyl (SAc) terminals
have been synthesized. The structures of the oligomers were characterized by 1H NMR, 13C NMR, and
elemental analysis. DSC studies show narrow and well-developed melting and crystallization peaks,
indicating the high perfection of the crystals formed in these oligomers. Optical microscopy was used for
investigating the supramolecular morphology of solid-state films of pentamer with TMS end groups, as
an example. Numerous dendrites with a spherulitic size of ca. 50 µm were observed. UV-vis absorption
and photoluminescence (PL) spectra in both dilute solution and films show a gradual red shift of the λmax
from trimer to nonamer. At the nonamer stage, the optical maximum is approaching a limit of convergence.
In tr od u ction
approach.17 The reaction procedures involved are quite
simplified because of the omission of time-consuming
chromatographic separations of the intermediates as
compared to the solution-phase methods. However,
purification of the final product is not easy because the
incomplete coupling residues on the resins at each step
make the final compounds more complicated. Addition-
ally, solid-phase synthesis is limited to preparation in
small scale. The second method is the solution-phase
iterative divergent/convergent approach. This approach
allows doubling the number of repeating units in every
three steps, independent of the length of the
molecules.18-20 Nevertheless, because of the mechanism
limitation of Pd-catalyzed coupling, the oligomers syn-
thesized through this strategy always contain some
diyne byproducts which have the same chain length as
the target compounds.21 The third method is a stepwise
approach. This features continuous addition of mono-
functional compounds to both ends of a growing chain
based on the difference of reactivity of iodine and
bromine to alkyne.22,23 This approach has been proven
to be a simple and fast route to short length OPEs using
in situ deprotection and coupling reactions. Some in-
soluble byproducts, however, were obtained because of
the undesired but unavoidable oligomerization through
alkyne coupling with both iodide and bromide, and the
precipitation of the insoluble materials increased with
the increase of oligomer chain length.22
During the past decade, extensive efforts have been
paid to conjugated polymers and their oligomers due to
their widely potential applications in electronics, optics,
and optoelectronics.1-3 Recently, research on monodis-
perse conjugated oligomers has dramatically grown and
expanded.4,5 Monodisperse conjugated oligomers, on one
hand, are served as model systems to directly generate
useful and predictive structure-property relationships
to rationalize the properties of the parent polydisperse
polymeric materials.5,6 On the other hand, oligomer
materials with defined length and constitution have
become a subject of its own.7,8 One of the most chal-
lenging areas for monodisperse conjugated oligomers is
molecular scale electronics, in which single or small
packets of molecules will function as the active channels
for information transportation, processing, and
storage.9-11 The driving force behind this research is
clearly the need for suitable alternative technology to
Si-based semiconductors, which is expected to reach its
roadblocks in 10-20 years. Utilizing organic molecules
to construct electronic devices can surmount the fun-
damental physical constraints in Si industry and can
also result in a rapid and low-cost bottom-up manufac-
turing process for electronic device fabrication.
Among many molecule-based devices, molecular wire
is one of the basic components, which is normally a
rigid-rod-like molecular chain with extended electronic
conjugation to convey either charge or excited energy
from one active unit to another. There have been
significant advances in the fabrication and demonstra-
tion of molecular electronic wires.12-16 Oligo(p-phenyle-
neethynylene)s (OPEs), a type of monodisperse and
shape-persistent oligomer, is one of the main classes of
molecules that have been proposed and demonstrated
for use as the wires and other potential backbones of
molecular electronic devices.10 There are three main
methods to the syntheses of monodisperse oligo(p-
phenyleneethynylene)s. One is the solid-phase synthesis
On the basis of the above knowledge, we designed a
new alternative stepwise approach to achieve high-
purity OPEs. This method involves only two sets of
reaction conditions: desilylation and Pd/Cu-catalyzed
coupling. In this paper, we report the synthesis and
characterization of a series of hexyloxy-functionalized
oligo(p-phenyleneethynylene)s with two kinds of differ-
ent terminal groups. In every two steps, 2,5-dihexyloxy-
4-[(trimethylsilyl)ethynyl]iodobenzene or 4-thioacetyl-
iodobenzene was coupled with the diethynyl compounds
which were desilylated from the di(trimethylsilyl)-
ethynyl compounds. Using this approach, undesired
oligomerization is avoided and trimer, pentamer, hep-
tamer, and nonamer with trimethylsilyl or thioacetoxy
* Corresponding author: e-mail zk-chen@imre.a-star.edu.sg;
phone +65-6874 4331; fax +65-6872 0785.
10.1021/ma021656a CCC: $25.00 © 2003 American Chemical Society
Published on Web 01/30/2003

1458 Zhou et al.
Macromolecules, Vol. 36, No. 5, 2003
1
46.3 °C. MS m/z: 278. H NMR (400 MHz, CDCl3): δ 6.82 (s,
4H), 3.90 (t, J ) 6.6 Hz, 4H), 1.80-1.70 (m, 4H), 1.46-1.30
(m, 12H), 0.90 (t, J ) 6.6 Hz, 6H).
1,4-Dih exyloxy-2,5-d iiod oben zen e.24 To a solution of 1,4-
dihexyloxybenzene (11.1 g, 0.04 mol), 90 mL of acetic acid, 7
mL of water, and 3 mL of concentrated H2SO4 were added KIO3
(10.3 g, 0.048 mol) and I2 (13.1 g, 0.048 mol). The reaction
mixture was stirred at 80 °C for 24 h and then cooled to room
temperature. After most of the acetic acid was evaporated
under reduced pressure, aqueous Na2SO3 (20%) was added
until the brown color of iodine had disappeared. The mixture
was poured into ice water with Na2CO3 (500 mL) and extracted
with hexane (3 × 200 mL). The combined organic layer was
washed with water and brine and dried over MgSO4. The
solvent was evaporated under reduced pressure to give a
yellow solid. The white crystals (12.7 g, 60%) were obtained
by recrystallization from ethanol; mp 59.9-60.7 °C. MS m/z:
F igu r e 1. Chemical structure of the OPEs.
terminal groups have been obtained readily. Incorpora-
tion of thioester groups to the oligomers makes them
able to connect with gold surface or electrodes through
self-assembly. The chemical structures of the synthe-
sized oligomers are shown in Figure 1. These compounds
were fully characterized by 1H NMR, 13C NMR, and
elemental analysis. The effect of the chain length on the
physical properties was investigated by thermal, ab-
sorption, and emission analysis. In addition, the su-
pramolecular morphology of solid-state films of a rep-
resentative oligomer was studied using optical
microscopy.
1
530. H NMR (400 MHz, CDCl3): δ 7.17 (s, 2H), 3.92 (t, J )
6.6 Hz, 4H), 1.84-1.75 (m, 4H), 1.55-1.34 (m, 12H), 0.91 (t, J
) 6.6 Hz, 6H). 13C NMR (100 MHz, CDCl3): δ 152.85, 122.80,
86.29, 70.34, 31.43, 29.09, 25.68, 22.55, 13.99.
2,5-Dih e xyloxy-4-[(t r im e t h ylsilyl)e t h yn yl]iod ob e n -
zen e (1a ).25 To a solution of 1,4-dihexyloxy-2,5-diiodobenzene
(7.95 g, 0.015 mol), CuI (0.14 g, 0.75 mmol), and Pd(PPh3)2Cl2
(0.53 g, 0.75 mmol) in 100 mL of diisopropylamine was added
(trimethylsilyl)acetylene (1.47 g, 0.015 mol). The mixture was
stirred at room temperature for 15 h. After removal of the
solvent under reduced pressure, a light yellow oil (1a ; 3.30 g,
44%) was separated from starting material and byproduct by
column chromatography using silica gel with hexane/CH2Cl2
Exp er im en ta l Section
Ma ter ia ls. All reagents and solvents were purchased from
Aldrich Chemical Co. and Merck. Tetrahydrofuran (THF) was
dried over sodium benzophenone. Diisopropylamine was dried
over KOH. Both solvents were then distilled under an argon
atmosphere and deoxygenated by purging with argon for 30
min before use in oligomer synthesis. All other chemicals were
used as received without further purification.
Meth od s. Melting points were determined on a Bu¨chi B-540
capillary melting point apparatus. 1H NMR and 13C NMR
spectral data were obtained on a 400 MHz Bruker DPX FT-
NMR spectrometer with chloroform-d (CDCl3) as solvent and
tetramethylsilane (TMS) as the internal standard. EIMS
spectra were obtained using a micromass 7034E mass spec-
trometer. Elemental analyses of all the synthesized compounds
were conducted on a Perkin-Elmer 2400 elemental analyzer
for C, H, and S determination. The absorption and emission
spectra were obtained using a Shimadzu UV-3101 PC UV-
vis-NIR spectrophotometer and a Perkin-Elmer LS-50B lu-
minescence spectrophotometer with a xenon lamp as light
source, respectively.
1
(20:1) as eluent. MS m/z: 500. H NMR (400 MHz, CDCl3): δ
7.04 (s, 1H), 6.94 (s, 1H), 3.96-3.91 (m, 4H), 1.84-1.72 (m,
4H), 1.56-1.33 (m, 12H), 0.91 (t, J ) 6.6 Hz, 6H), 0.25 (s, 9H).
1,4-Bis[(tr im eth ylsilyl)eth yn yl]-2,5-bis(h exyloxy)ben -
zen e (1b).24 To a solution of 1,4-dihexyloxy-2,5-diiodobenzene
(7.95 g, 0.015 mol), CuI (0.14 g, 0.75 mmol), and Pd(PPh3)2Cl2
(0.53 g, 0.75 mmol) in 100 mL of diisopropylamine was added
(trimethylsilyl)acetylene (2.94 g, 0.03 mol). The mixture was
stirred at reflux for 1 h. After cooling, dichloromethane (100
mL) was added, and the white ammonium iodide precipitate
was filtered off. The solution was passed through a short silica
gel column using toluene as eluent. After the solvent was
evaporated under reduced pressure, the white crystals 1b (6.3
g, 89%) were obtained by recrystallization from ethanol; mp
91.0-91.5 °C. MS m/z: 470. 1H NMR (400 MHz, CDCl3): δ
6.88 (s, 2H), 3.93 (t, J ) 6.6 Hz, 4H), 1.81-1.76 (m, 4H), 1.53-
1.33 (m, 12H), 0.88 (t, J ) 6.6 Hz, 6H), 0.25 (s, 18H). 13C NMR
(100 MHz, CDCl3): δ 154.45, 117.75, 114.45, 101.50, 100.43,
69.92, 31.99, 29.70, 26.07, 23.01, 14.43, 0.33.
The thermal behavior of the oligomers was studied using a
differential scanning calorimeter (DSC)-2920 from TA Instru-
ments coupled with a TA-2000 control system. The tempera-
ture was accurately calibrated with tin, gallium, and indium
using standard procedure. The weights of all the samples were
in the range 4-6 mg. All the powdery samples were heated
and cooled with a scanning rate of 10 °C/min under a nitrogen
atmosphere in order to diminish oxidation.
The powdery sample of oligomer 5b was dissolved in xylene
(at ambient) with a concentration of ca. 1 wt %. The oligomer
films were prepared by casting a few drops of xylene solution
on glass slides. After evaporation of the solvent, the solid films
were used for optical microscopy observation.
1,4-Dih exyloxyben zen e. A suspension of powdered KOH
(50.0 g, 0.9 mol) and anhydrous ethanol (400 mL) was stirred
and degassed at room temperature for 30 min. Hydroquinone
(38.5 g, 0.35 mol) in anhydrous ethanol (150 mL) was added
dropwise. To the stirred mixture, bromohexane (148.0 g, 0.9
mol) in anhydrous ethanol (50 mL) was added. After stirring
for 24 h with heating at reflux, the ethanol was evaporated at
reduced pressure. The brownish residue was poured into water
(500 mL) and extracted with ethyl acetate twice. The combined
ethyl acetate layer was washed with water, brine, and dried
over anhydrous magnesium sulfate. The white product (74.0
g, 76%) was obtained by recrystallization from ethanol after
ethyl acetate was removed under reduced pressure; mp 45.2-
Gen er a l P r oced u r e for th e P r ep a r a tion of 1c, 3c, 5c,
a n d 7c.24 Methanol and NaOH (5 N) were added at room
temperature to a stirred THF solution of 1b, 3b, 5b, and 7b.
The reaction mixture was stirred for 2 h. After removal of the
solvent under reduced pressure, 1c, 3c, 5c, and 7c were
separated by column chromatography, respectively.
1,4-Bis(et h yn yl)-2,5-b is(h exyloxy)b en zen e (1c). The
above general procedure was applied on methanol (30 mL) and
NaOH (2 mL, 5 N) in a stirred solution of 1b (2.82 g, 0.006
mol) in THF (20 mL). The solvent was evaporated, and the
residue was poured into 100 mL of water and extracted with
hexane twice. The combined hexane layer was washed with
water and brine and dried over anhydrous magnesium sulfate.
The pale yellow solid 1c (1.82 g, 93%) was obtained after the
solvent was removed; mp 71.1-72.0 °C. MS m/z: 326. 1H NMR
(400 MHz, CDCl3): δ 6.95 (s, 2H), 3.97 (t, J ) 6.6 Hz, 4H),
3.33 (s, 2H), 1.84-1.75 (m, 4H), 1.50-1.26 (m, 12H), 0.90 (t, J
) 6.6 Hz, 6H). 13C NMR (100 MHz, CDCl3): δ 154.43, 118.26,
113.75, 82.75, 80.19, 70.13, 31.90, 29.50, 25.97, 22.96, 14.37.
The data of deprotected trimer (3c), pentamer (5c), and
heptamer (7c) could be found in the Supporting Information.
Gen er a l P r oced u r e for P r ep a r a tion of 3b, 5b, 7b, a n d
9b. A mixture of diethynyl compound (1c, 3c, 5c, and 7c), 2.5
mole ratio of 2,5-dihexyloxy-4-[(trimethylsilyl)ethynyl]iodo-
benzene, 10 mol % CuI, 10 mol % Pd(PPh3)2Cl2, and mixed
solvents of diisopropylamine/THF was stirred at room tem-

Macromolecules, Vol. 36, No. 5, 2003
Oligo(p-phenyleneethynylene)s 1459
Sch em e 1. Syn th etic Rou tes to Tr im eth ylsilyl- or Th ioester -Ter m in a ted OP Esa
a
Reagents and conditions: (i) bromohexane, KOH/EtOH, reflux, 24 h; (ii) KIO3, I2, AcOH/H2SO4/H2O, 80 °C, 24 h; (iii)
i
Pd(PPh3)2Cl2 (10 mol %), CuI (10 mol %), Pr2NH, trimethylsilylacetylene, reflux, 1 h; (iv) MeOH/THF, 5 N NaOH, rt, 2 h; (v)
1-iodo-4-thioacetylbenzene, Pd(PPh3)2Cl2 (10 mol %), CuI (10 mol %), Pr2NH/THF, rt, 15 h; (vi) 1a , Pd(PPh3)2Cl2 (10 mol %), CuI
(10 mol %), Pr2NH/THF, rt, 15 h.
i
i
perature for 15 h. After removal of the solvent under reduced
pressure, 3b, 5b, 7b, and 9b were separated by column
chromatography, respectively.
The data of thioester-terminated pentamer (5a ), heptamer
(7a ), and nonamer (9a ) could be found in the Supporting
Information.
TMS-Ter m in a ted Tr im er (3b). The general procedure
mentioned was applied on 1c (1.63 g, 5 mmol), 2,5-dihexyloxy-
4-[(trimethylsilyl)ethynyl]iodobenzene (1a ) (6.25 g, 12.5 mmol),
CuI (0.095 g, 0.5 mmol), and Pd(PPh3)2Cl2 (0.351 g, 0.5 mmol)
in 60 mL of diisopropylamine and 30 mL of THF. The bright
yellow solid 3b (4.82 g, 90%) was obtained after chromatog-
raphy using silica gel with hexane/CH2Cl2 (4:1) as eluent; mp
Resu lts a n d Discu ssion
Since many applications require only short but highly
pure and easily processable oligomeric systems, we
designed a novel stepwise approach, which allows
purification of each intermediate and then ensures the
high purity of the resulting oligomers. The synthetic
procedure for the dihexyloxy-substituted OPEs is de-
picted in Scheme 1. The procedure involves only two
types of reaction conditions: desilylation and Pd/Cu-
catalyzed coupling reaction. The initial 2,5-dihexyloxy-
1,4-diiodobenzene was obtained by iodination of the 1,4-
dihexyloxybenzene. Treatment of 2,5-dihexyloxy-1,4-
diiodobenzene with one and two molar amount of
(trimethylsilyl)acetylene gave 2,5-dihexyloxy-4-[(trim-
ethylsilyl)ethynyl]-iodobenzene (1a ) and 2,5-dihexyloxy-
1,4-bis[(trimethylsilyl)ethynyl] benzene (1b), respec-
tively. Removal of the protection group of 1b generated
the desilylated 2,5-dihexyloxy-1,4-bis(ethynyl)benzene
(1c). Pd/Cu-catalyzed coupling of 1c with 1-iodo-4-
thioacetylbenzene or 1a gave trimer 3a with thioester
end groups or trimer 3b with trimethylsilyl end groups.
Since 3b has the same TMS protected alkyne end groups
as 1b, the desilylation and coupling reactions can be
iterated. By repeating the reaction sequence, oligomers
up to nine repeating units with thioester (3a , 5a , 7a ,
and 9a ) and -TMS (3b, 5b, 7b, and 9b) terminals have
been obtained readily using the same reaction condi-
tions. A 2.5 molar ratio of compound 1-iodo-4-thioacetyl-
benzene or 1a was added to the Pd/Cu-catalyzed cou-
pling reaction to ensure the completion of the starting
materials and help to ease the isolation of the products.
All the oligomers possess very good solubility in common
organic solvents. The thioester terminals attached to the
oligomers can serve as alligator clips for adhesion to gold
surfaces or gold nanoelectrodes so that the wiring
1
109.5-110.6 °C. H NMR (400 MHz, CDCl3): δ 7.01 (s, 2H),
6.98 (s, 2H), 6.96 (s, 2H), 4.05-3.97 (m, 12H), 1.87-1.84 (m,
12H), 1.59 - 1.53 (m, 12H), 1.36-1.28 (m, 24H), 0.93-0.88
(m, 18H), 0.28 (s, 18H). 13C NMR (100 MHz, CDCl3): δ 154.60,
153.92, 153.79, 117.92, 117.73, 117.53, 115.03, 114.74, 114.15,
101.62, 100.47, 91.93, 91.83, 70.12, 69.91, 32.01, 29.70, 26.07,
23.02, 14.41, 0.36. Anal. Calcd for C68H102O6Si2: C, 76.21; H,
9.59. Found: C, 76.34; H, 9.75.
The data of TMS-terminated pentamer (5b), heptamer (7b),
and nonamer (9b) could be found in the Supporting Informa-
tion.
Gen er a l P r oced u r e for th e P r ep a r a tion of 3a , 5a , 7a ,
a n d 9a . A mixture of diethynyl compounds (1c, 3c, 5c, and
7c), 2.5 mole ratio of 1-iodo-4-thioacetylbenzene, 10 mol % CuI,
10 mol % Pd(PPh3)2Cl2, and mixed solvents of diisopropy-
lamine/THF was stirred at room temperature for 15 h. After
removal of the solvent under reduced pressure, 3a , 5a , 7a , and
9a were separated by column chromatography, respectively.
Th ioester -Ter m in a ted Tr im er (3a ). The general proce-
dure mentioned was applied on 1c (0.39 g, 1.19 mmol), 1-iodo-
4-thioacetylbenzene (0.69 g, 2.5 mmol), CuI (0.023 g, 0.119
mmol), and Pd(PPh3)2Cl2 (0.084 g, 0.119 mmol) in 60 mL of
diisopropylamine. The bright yellow solid 3a (0.50 g, 67%) was
obtained after chromatography using silica gel with hexane/
CH2Cl2 (2:1) as eluent; mp 112.5-113.7 °C. 1H NMR (400 MHz,
CDCl3): δ 7.56 (d, J ) 8.0 Hz, 4H), 7.40 (d, J ) 8.0 Hz, 4H),
7.01 (s, 2H), 4.04 (t, J ) 6.6 Hz, 4H), 2.44 (s, 6H), 1.87-1.83
(m, 4H), 1.58-1.53 (m, 4H), 1.36-1.26 (m, 8H), 0.90 (t, J )
6.6 Hz, 6H). 13C NMR (100 MHz, CDCl3): δ 193.78, 154.20,
134.55, 132.50, 128.48, 125.14, 117.45, 114.42, 94.55, 88.09,
70.11, 31.97, 29.71, 26.12, 23.01, 14.40. Anal. Calcd for
C
38H42O4S2: C, 72.81; H, 6.75; S, 10.23. Found: C, 73.21; H,
7.26; S, 9.74.

1460 Zhou et al.
Macromolecules, Vol. 36, No. 5, 2003
F igu r e 2. Representative 1H NMR spectra of 5a and 5b in CDCl3.
function can be realized, and their transportation of
signals can be measured.
The structure and purity of the obtained OPEs were
characterized by 1H NMR, 13C NMR, and elemental
1
analysis. The representative H NMR spectra of 5a and
5b are depicted in Figure 2. It can be seen in Figure 2A
(1H NMR of 5a ) that there are two doublets at around
7.55 and 7.40 ppm, which are assigned to the protons
on the thioester-substituted benzene rings. The peak at
7.01 ppm is assigned to the protons on the middle
hexyloxy-substituted benzene rings. The ratio of the
integration of the three distinct aromatic signals at 7.55,
7.44, and 7.01 is 2:2:3, indicating the length and
structure of the expected pentamer 5a . Figure 2B shows
1
the H NMR spectrum of 5b. The peaks at around 7.00
ppm are attributed to the protons of the five symmetric
aromatic rings. The signal at 0.26 ppm is the charac-
teristic peak of -TMS. The ratio of the intensities of
-Si(CH3)3 and the aromatic protons is 9:5, which agrees
with the calculated ratio of the oligomer 5b.
The DSC heating (A) and cooling (B) curves of
oligomers 3b, 5b, 7b, and 9b are shown in Figure 3.
For the purpose of comparison, all the DSC curves are
normalized by weight and shifted vertically for clarifica-
tion. It can be seen that the endothermic melting peaks
(in Figure 3A) systematically shift toward higher tem-
peratures (from 116 °C to 147, 168, and 185 °C for 3b,
5b, 7b, and 9b, respectively) with increasing repeating
units of oligomers, whereas the magnitudes of the
melting peaks gradually decrease as repeating units
increases, indicating a systematic reduction in perfec-
tion of the crystals formed. After the heating scan
(Figure 3A), the samples were kept at their respective
melts (200 or 250 °C) for 5 min under nitrogen to
eliminate thermal history and then cooled at the same
rate. For the cooling scan (Figure 3B), the exothermic
crystallization peaks shift toward higher temperatures
(from 94 °C to 122, 143, and 168 °C for 3b, 5b, 7b, and
9b, respectively). Meanwhile, the same trend of decreas-
ing peak magnitude was observed with increasing
repeating units. Compared with PPEs, which have
shown liquid crystalline behavior,26 the narrow and
F igu r e 3. DSC heating (A) and cooling (B) curves of TMS-
terminated OPEs.
well-developed melting and crystallization peaks indi-
cate the high perfection of the crystals formed and the
well-defined structures of these oligomers.

Macromolecules, Vol. 36, No. 5, 2003
Oligo(p-phenyleneethynylene)s 1461
F igu r e 4. Optical micrographs of self-organized supramolecular morphology of 5b on glass: (A) dendrite crystals and (B) lathlike
crystals.
Organic molecular crystals are promising candidates
for molecular electronic devices. Before such devices can
be realized, however, a better understanding of the
structural factors that control the electronic and optical
properties of organic solids is needed.27 The supramo-
lecular morphology of solid-state films of oligomer 5b,
as an example, was studied using optical microscopy.
The self-organized crystalline morphology of 5b on glass
is shown in Figure 4. Numerous dendrites with a
spherulitic size of ca. 50 µm are observed (Figure 4A).
Nevertheless, because of high nucleation density, the
further growth of the dendritic crystals is restricted, and
consequently the morphological details are somewhat
concealed. In some areas with low nucleation density,
large axialites grown from isolated nuclei are occasion-
ally observed, consisting of several to tens of lathlike
crystals with smooth surfaces which fan out in typical
axialitic fashion to form a full spherulite (Figure 4B).
These laths are usually 30-50 µm long and 5-10 µm
wide. It can also be seen that the lathlike crystal sheets
have rather regular (lateral) edges but irregular growth
tips probably due to the uneven accumulation of rejected
species during crystal growth. Occasionally, some mul-
tilayer overgrowths on the laths are observed upon
closer inspection. In addition, as indicated by arrows,
some minute needlelike crystals always appear oriented

1462 Zhou et al.
Macromolecules, Vol. 36, No. 5, 2003
Ta ble 1. Op tica l P r op er ties of TMS- a n d Th ioester -Ter m in a ted OP Es
λmax (UV nm)
λmax (PL nm)
oligomer
solution
film
solutiona
filma
∆λmax (nm)b
trimer-SAc (3a )
378
410
421
431
397
413
425
430
391, 411
415 (433)
450 (477)
463 (492)
467 (500)
434 (457)
457 (486)
466 (494)
468 (496)
466
pentamer-SAc (5a )
heptamer-SAc (7a )
nonamer-SAc (9a )
trimer-TMS (3b)
pentamer-TMS (5b)
heptamer-TMS (7b)
nonamer-TMS (9b)
423, 436, 462
435, 447, 472
440, 453, 480
401, 413, 435
425, 437, 461
434, 448, 471
438, 452, 476
(480) 504
(497) 521
(504) 528
480 (517)
(494) 508 (532)
523 (544)
(500) 530
35
13
4
23
9
2
a
b
The values in parentheses refer emission shoulders. ∆λmax is the difference of the emission maximum from n to n + 2.
transversely to the outer parts of the edges of the
underlying laths where the distance between the neigh-
boring laths is widely spaced. That is, the lateral growth
of these tiny crystals is normal to the growth direction
of the underlying lath crystals, resembling a homoepi-
taxial growth habit. Similar axialitic and lathlike crystal
morphologies were also observed in syndiotactic polypro-
pylene (sPP) using transmission electron microscopy
(TEM) by Lovinger et al.28 and using atomic force
microscopy (AFM) by Tsukruk et al.29 Under lower
solution concentration (such as 0.1-0.5 wt %), our
preliminary TEM and AFM results have shown that
more perfect and larger single crystals can be obtained
from dilute xylene solutions with lath length exceeding
several hundred micrometers and width of ca. tens of
micrometers.30 Currently, the preparation of single-
crystal thin films and the structural determination of
single crystals of the synthesized oligomers are still
under way.
UV-vis absorption and photoluminescence (PL) spec-
tra of the dilute solution of THF and thin films of OPEs
were measured. The wavelengths of the peaks are
summarized in Table 1. The demonstrative UV and PL
spectra of the oligomers with -TMS terminal groups
in solutions and solid states are shown in parts A and
B of Figure 5, respectively. As expected, an increase in
the absorption and emission maxima from shorter
wavelength 397 and 434 nm for the trimer 3b to the
longer wavelength 430 and 468 nm for the nonamer 9b
in dilute THF solution was observed. The bathochromic
shift of emission from trimer 3b to pentamer 5b is 23
nm, while that of emission from heptamer 7b to non-
amer 9b, however, is only 2 nm. These results indicate
that the λmax values of both UV absorption and PL
emission are approaching a limit of convergence at the
nonamer stage, and this tendency is illustrated in
Figure 5A,B. Indeed, the wavelengths of maximal
absorption and emission of 9b in THF are even higher
than its polymer counterparts with didecyloxy or di-
dodecyloxy side chains measured at the same condi-
tions.21,31 This is in agreement with the observation of
dipropoxy-substituted OPEs, which showed the satu-
rated absorption and emission at the decamer stage.32,33
From Figure 5A, it can be found that each of the
absorptions of these -TMS-terminated OPEs in dilute
THF solution appear as a broad peak with one maxi-
mum. The PL spectra in solution exhibit one maximum
peak and one shoulder, which is red-shifted about 28
nm to their corresponding maximum. It is also noted
that the oligomer emission bands are much narrower
than the absorption bands, which is consistent with
emission from localized excited states.34,35
F igu r e 5. UV-vis absorption and photoluminescence spectra
of TMS-terminated OPEs in (A) THF solution and (B) solid
state.
plates from their solutions in THF (2 wt %). All the films
prepared are visibly uniform and emit blue-green to
green fluorescence. It is noted that the absorption
spectra of the oligomers in solid states exhibit signifi-
cant changes compared with those in dilute solutions.
There are three highly structured absorption peaks in
the UV spectra. For example, the three peaks for 5b
center at 425, 437, and 461 nm in wavelength or 2.918,
2.838, and 2.690 eV in energy. The well-resolved struc-
ture of the absorption spectra is ascribed to the highly
ordered organization of the oligomers in solid state,
which has been illustrated in optical micrographs and
DSC thermograms. The high ordering in thin films
should be attributed to the π-π stacking of the oligomer
backbones and also the hydrophobic interactions be-
tween alkoxy side chains. Among the three well-resolved
peaks, the right dominant peak is assigned to the lowest
singlet (1Bu) exciton transition (0-0), i.e., the transition
from the ground state to the relaxed excitation state in
conjugated oligomers. Others are its vibronic
sidebands.36-39 The energy space between the 0-0 (right
peak) and 0-1 (middle peak) transitions, corresponding
Figure 5B shows the UV-vis absorption and photo-
luminescence (PL) spectra of the four -TMS-terminated
OPEs in thin films. The films were spin-cast onto quartz

Macromolecules, Vol. 36, No. 5, 2003
Oligo(p-phenyleneethynylene)s 1463
photoluminescence spectroscopy investigation of the
oligomers indicates that the bathochromic shift of
optical maximum is approaching a limit of convergence
when the oligomer chain length increases from trimer
to nonamer. DSC and optical microscopy studies reveal
that oligomers with trimethylsilyl end groups form
dendrite crystals with high perfection. Lathlike crystal
sheets with length of several hundred micrometers and
width of tens of micrometers have been observed, which
may be applicable for high mobility field effect transis-
tors through a simple and cheap solution process. The
application of oligomers with thioester end groups as
molecular wires is still under investigation.
Su p p or tin g In for m a tion Ava ila ble: Synthesis and ana-
lytical data of intermediate compounds of 3c, 5c, and 7c, the
TMS-terminated oligomers of 5b, 7b, and 9b, and also the
thioester-terminated oligomers of 5a , 7a , and 9a . This material
is available free of charge via the Internet at http://pubs.ac-
s.org.
Refer en ces a n d Notes
(1) Skotheim, T. A.; Elsenbaumer, R. L.; Reynolds, J . R. Hand-
book of Conducting Polymers, 2nd ed.; Marcel Dekker: New
York, 1998.
(2) Scherf, U. Top. Curr. Chem. 1999, 201, 163.
(3) Segura, J . L.; Martin, N. J . Mater. Chem. 2000, 10, 2403.
(4) Mu¨llen, K.; Wegner, G. Electronic Materials: the Oligomer
Approach; Wiley-VCH: Weinheim, 1997.
(5) Martin, R. E.; Diederich, F. Angew. Chem., Int. Ed. 1999, 38,
1350.
(6) Van Hutten, P. F.; Krasnikov, V. V.; Hadziioannou, G. Acc.
Chem. Res. 1999, 32, 257.
(7) Katz, H. E.; Bao, Z.; Gilat, S. L. Acc. Chem. Res. 2001, 34,
359.
F igu r e 6. UV-vis absorption and photoluminescence spectra
of thioester-terminated OPEs in (A) THF solution and (B) solid
state.
(8) Fichou, D. J . Mater. Chem. 2000, 10, 571.
(9) Tour, J . M.; Reinerth, W. A.; J ones, L.; Burgin, T. P.; Zhou,
C. W.; Muller, C. J .; Deshpande, M. R.; Reed, M. A. Ann. N.Y.
Acad. Sci. 1998, 852, 197.
(10) Tour, J . M. Acc. Chem. Res. 2000, 33, 791.
(11) Reed, M. A.; Tour, J . M. Sci. Am. 2000, 282, 86.
(12) Bumm, L. A.; Arnold, J . J .; Cygan, M. T.; Dunbar, T. D.;
Burgin, T. P.; J ones, L., II; Allara, D. L.; Tour, J . M.; Weiss,
P. S. Science 1996, 271, 1705.
to CdC stretching vibration, is around 150 meV, which
is a typical value and had been reported in other
conjugated polymer systems.40-42 The PL spectra of the
-TMS-terminated OPEs in thin films show less struc-
tured feature and peak at 480, 508, 523, and 530 nm
for the four oligomers. Comparing the maximal emission
and absorption in film states to their solutions, it can
be noted that the red shift for the both cases is about
40-60 nm. These results indicate strong intermolecular
interaction in the solid state.43
The oligomers with thioacetyl terminal groups have
similar optical properties as those with -TMS terminal
groups. The detailed absorption and emission wave-
lengths of the peaks are summarized in Table 1. The
absorption maxima of the -SAc-terminated OPEs in
THF solution are 378, 410, 421, and 431 nm. Except
for 3a , there are no obvious differences in the absorption
maxima compared with those of -TMS-terminated
OPEs, which show maxima at 397, 413, 425, and 430
nm. Similar to the absorption spectra of 3b-9b in solid
thin films, those of 3a -9a also show three highly
structured peaks. Both of the UV absorption and PL
emission spectra have similar trends as -TMS-termi-
nated OPEs: the λmax values of both absorption and
emission are approaching a limit of convergence at the
nonamer stage, which is illustrated in Figure 6A,B.
(13) Reed, M. A.; Zhou, C.; Muller, C. J .; Burgin, T. P.; Tour, J .
M. Science 1997, 278, 252.
(14) Cygan, M. T.; Dunbar, T. D.; Arnold, J . J .; Bumm, L. A.;
Shedlock, N. F.; Burgin, T. P.; J ones, L., II; Allara, D. L.;
Tour, J . M.; Weiss, P. S. J . Am. Chem. Soc. 1998, 120, 2721.
(15) Creager, S.; Yu, C. J .; Bamdad, C.; O’Connor, S.; MacLean,
T.; Lam, E.; Chong, Y.; Olsen, G. T.; Luo, J .; Gozin, M.;
Kayyem, J . F. J . Am. Chem. Soc. 1999, 121, 1059.
(16) Fan, F. F.; Yang, J .; Dirk, S. M.; Price, D. W.; Kosynkin, D.;
Tour, J . M.; Bard, A. J . J . Am. Chem. Soc. 2001, 123, 2454.
(17) Huang, S.; Tour, J . M. J . Org. Chem. 1999, 64, 8898.
(18) J ones, L., II; Schumm, J . S.; Tour, J . M. J . Org. Chem. 1997,
62, 1388.
(19) Ziener, U.; Godt, A. J . Org. Chem. 1997, 62, 6137.
(20) Kukula, H.; Veit, S.; Godt, A. Eur. J . Org. Chem. 1999, 277.
(21) Bunz, U. H. F. Chem. Rev. 2000, 100, 1605.
(22) Huang, S.; Tour, J . M. Tetrahedron Lett. 1999, 40, 3347.
(23) Tour, J . M. Chem. Rev. 1996, 96, 537.
(24) Weder, C.; Wrighton, M. S. Macromolecules 1996, 29, 5157.
(25) Francke, V.; Mangel, T.; Mu¨llen, K. Macromolecules 1998,
31, 2447.
(26) Kloppenburg, L.; J ones, D.; Claridge, J . B.; Loye, H.-C.; Bunz,
U. H. F. Macromolecules 1999, 32, 4460.
(27) Liu, C. Y.; Bard, A. Acc. Chem. Res. 1999, 32, 235.
(28) Lovinger, A. J .; Lotz, B.; Davis, D. D.; Schumacher, M.
Macromolecules 1994, 27, 6603.
Con clu sion s
(29) Tsukruk, V. V.; Reneker, D. H. Macromolecules 1995, 28,
1370.
(30) Liu, T. X.; Zhou, C. Z.; Tjiu, W. C.; Chen, Z. K. Unpublished
results.
(31) Moroni, M.; LeMoigne, J .; Luzzati, S. Macromolecules 1994,
27, 562.
Two series of oligo(p-phenyleneethynylene)s (OPEs)
with either trimethylsilyl end groups or thioacetyl
terminals have been prepared through a new stepwise
synthetic method. Pure oligomers with chain lengths up
to nonamer were easily synthesized. Absorption and

1464 Zhou et al.
Macromolecules, Vol. 36, No. 5, 2003
(32) Meier, H.; Ickenroth, D.; Stalmach, U.; Koynov, K.; Bahtiar,
A.; Bubeck, C. Eur. J . Org. Chem. 2001, 23, 4431.
(33) Ickenroth, D.; Weissmann, S.; Rumpf, N.; Meier, H. Eur. J .
Org. Chem. 2002, 16, 2808.
(34) Li, H.; Powell, D. R.; Hayashi, R. K.; West, R. Macromolecules
1998, 31, 52.
(35) Sluch, M. I.; Godt, A.; Bunz, U. H. F.; Berg, M. A. J . Am.
Chem. Soc. 2001, 123, 6447.
(36) Bao, Z.; Lovinger, A. J . Chem. Mater. 1999, 11, 2607.
(37) Hosaka, N.; Tachibaba, H.; Shiga, N.; Matsumoto, M.; Tokura,
Y. Phys. Rev. Lett. 1999, 82, 1672.
(39) Trznadel, M.; Pron, A.; Zagorska, M.; Chrzaszcz, R.; Pieli-
chowski, J . Macromolecules 1998, 31, 5051.
(40) Hernandez, V.; Castiglioni, C.; Del Zoppo, M.; Zerbi, G. Phys.
Rev. B 1994, 50, 9815.
(41) Bredas, J . L.; Cornil, J .; Heeger, A. J . Adv. Mater. 1996, 8,
447.
(42) Meng, H.; Chen, Z.-K.; Huang, W. J . Phys. Chem. B 1999,
103, 6429.
(43) Halkyard, C. E.; Rampey, M. E.; Kloppenburg, L.; Studer-
Martinez, S. L.; Bunz, U. H. F. Macromolecules 1998, 31,
8655.
(38) Sakurai, K.; Tachibana, H.; Shiga, N.; Terakura, C.; Matsu-
moto, M.; Tokura, Y. Phys. Rev. B 1997, 56, 9552.
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