Pyrazine-containing acene-type molecular ribbons with up to 16 rectilinearly arranged fused aromatic rings

Article abstract of DOI:10.1021/ja800311a

A series of acene-type conjugated molecules (1-5) containing 2-6 pyrazine units and up to 16 rectilinearly arranged fused aromatic rings were synthesized by condensation coupling of 1,2-diamines and 1,2-diketones. The energy gap of the molecules estimated from absorption edge decreases with an increase in molecular length, indicating the well-delocalized nature of the molecules. The cyclic voltammetry measurements suggest that the n-type properties of these ribbonlike pyrazine derivatives are dependent on the molecular length and the number of the pyrazine units. As the number of pyrazine units and the molecular length increase, the first reduction wave onset is shifted from -1.16 to -0.62 V (vs Ag/AgCl), corresponding to the LUMO energy levels of -3.24 and -3.78 eV, respectively. These molecules tend to aggregate in solution more readily with an increase in molecular length, as evident by ¹H NMR and UV-vis absorption spectra. Introduction of t-butyl groups in pyrene units can noticeably suppress the aggregation of these molecules in solution. High electron affinity, high environmental stability, and ease of structural modification make these compounds excellent candidates as a new class of n-type semiconductors.

Full text of DOI:10.1021/ja800311a

Published on Web 06/07/2008
Pyrazine-Containing Acene-Type Molecular Ribbons with up
to 16 Rectilinearly Arranged Fused Aromatic Rings
Baoxiang Gao,†,‡ Ming Wang,† Yanxiang Cheng,† Lixiang Wang,†,* Xiabin Jing,†
and Fosong Wang†
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied
Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China, and College of
Chemistry and EnVironment Science, Hebei UniVersity, Baoding 071002, P. R. China
Received January 14, 2008; E-mail: lixiang@ciac.jl.cn
Abstract: A series of acene-type conjugated molecules (1-5) containing 2-6 pyrazine units and up to 16
rectilinearly arranged fused aromatic rings were synthesized by condensation coupling of 1,2-diamines
and 1,2-diketones. The energy gap of the molecules estimated from absorption edge decreases with an
increase in molecular length, indicating the well-delocalized nature of the molecules. The cyclic voltammetry
measurements suggest that the n-type properties of these ribbonlike pyrazine derivatives are dependent
on the molecular length and the number of the pyrazine units. As the number of pyrazine units and the
molecular length increase, the first reduction wave onset is shifted from -1.16 to -0.62 V (vs Ag/AgCl),
corresponding to the LUMO energy levels of -3.24 and -3.78 eV, respectively. These molecules tend to
aggregate in solution more readily with an increase in molecular length, as evident by 1H NMR and UV-vis
absorption spectra. Introduction of t-butyl groups in pyrene units can noticeably suppress the aggregation
of these molecules in solution. High electron affinity, high environmental stability, and ease of structural
modification make these compounds excellent candidates as a new class of n-type semiconductors.
dimer than that for triphenylene dimer.13 Furthermore, one can
Introduction
also expect higher stability of dNs containing heteroacenes
Organic conjugated compounds are of current interest because
of their potential applications in electronics and optoelectronics
(e.g., organic light-emitting diodes, organic thin film transistors,
and organic photovoltaic cells). p-Type and n-type organic
materials, which are capable of transporting holes and electrons,
respectively, are equally important in these applications. How-
ever, the vast majority of synthetic effort and structure-property
studies in the field has been devoted to p-type organic molecules
(electron donor or hole transport),1–4 and only a limited number
of n-type organic molecules have been synthesized, such as
fluorinated aromatic compounds,5–7 heteroaromatic compounds,8,9
fullerene derivatives,10 naphthalene, and perylene diimides.11
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Heterocyclic aromatic compounds containing imine nitrogen
atoms (dNs) generally have a less negative reduction potential
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promising candidates for n-type semiconductors.12 In addition
to strongly enhanced electron affinity, the successive replace-
ment of CH moieties by nitrogen atoms in polycyclic aromatic
hydrocarbons offers opportunities to manipulate the intermo-
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† Chinese Academy of Sciences.
‡ Hebei University.
9
10.1021/ja800311a CCC: $40.75
2008 American Chemical Society
J. AM. CHEM. SOC. 2008, 130, 8297–8306 8297

A R T I C L E S
Gao et al.
than hydrocarbon analogues toward photooxidation or Diels-
Alder dimerization (two major degradation pathways in
acenes14).
patterns. In particular, some of these compounds exhibit high
charge carrier mobility16a,18a–c and intriguing self-assembling
properties.17c–g,18d–h,19 It is well known that many properties
of conjugated molecules are molecular size-dependent.20 Thus,
one can expect that one-dimensional multipyrazine-containing
acene-type conjugated molecules exhibit some intriguing n-type
semiconducting and self-assembling properties that depend on
the molecular length and the number of pyrazine units. We
herein report the synthesis and characterization of a series of
pyrazine-containing acene-type conjugated molecules with up
to six pyrazine units and 16 rectilinearly arranged fused aromatic
rings and some unique structure-property relationships of this
class of conjugated molecules.
As a typical nitrogen-containing heterocyclic compound,
pyrazine has been proven as a valuable building block for
thermally stable polymers and electron-accepting π-conjugated
polymers.15 Early studies on thermally stable pyrazine-contain-
ing polymers were reviewed by Yu et al.15c Recently, pyrazine
is frequently used in the design and synthesis of n-type organic
semiconductors, such as pyrazinoquinoxaline derivatives,16
hexaazatriphenylenes,17 diquinoxalino[2,3-a:2′,3′-c] phena-
zines,18 and quinoxalino[2′,3′,9,10] phenanthro[4,5-abc] phena-
zines,19 owing to its electron-deficient characteristic. The
electron affinity of the resultant compounds is noticeably higher
than that of analogous polycyclic aromatic hydrocarbons and
can be adjusted by altering molecular structures or substitution
Results and Discussion
All pyrazine-containing acene-type molecules (PAMs) pre-
pared in this work are depicted in Chart 1. According to the
distribution of pyrazine units, these compounds can be divided
into three classes: (I) compound 1, in which two pyrazine units
are separated by a pyrene unit; (II) compounds 2 and 4, in which
two pyrazinoquinoxaline units are separated by a pyrene unit;
and (III) compounds 3 and 5, which have both structural features
of types I and II. Four terminal phenyl groups with alkoxy chains
were introduced to improve the solubility of the intermediates
and the final products. Introduction of t-butyl substituents in
pyrene units was proposed to further improve the solubility and
depress the aggregation of the compounds for ease of purifica-
tion and characterization.
The synthesis of compounds 1-5 is outlined in Schemes 1
and 2. Suzuki coupling reactions between 4,5-dibromo-1,2-
phenylenediamine (6)21 and phenyl boronic acids 7a and 7b
gave diamines 8a (83%) and 8b (78%), respectively. Following
a typical condensation-coupling reaction between 1,2-diamines
and 1,2-diketones,15a a twofold condensation between 8 and
pyrene-4,5,9,10-teteranes 9a and 9b22 in refluxing acetic acid
afforded compounds 1a-c in high yields (75-92%).
The synthesis of monocondensation intermediates 10a and
10b is a key step to longer PAMs 3 and 5. The monocon-
densation was successful by dropwise addition of 8b to an
excess of 9 in acetic acid at 50 °C, yielding 10a (58%) and
10b (52%) as precipitates from the reaction medium. An
attempt to further condense compounds 10 and 11 in refluxing
acetic acid was not successful in yielding 3. By using high
boiling point m-cresol, which is often used in the synthesis
of high molecular weight polyquinoxalines,15b compounds
3a and 3b were obtained in yields of 46 and 51%,
respectively. However, condensation of 10a and 6,7,15,16-
teteraminoquinoxalino[2,3,9,10]phenanthro[4,5-abc]phena-
zine23 in m-cresol did not produce the target compound
because of extremely low solubility of the monocondensation
intermediate. Then one pair of t-butyl groups in each pyrene
unit was introduced to improve the solubility, and as a result
compound 5 was successfully prepared in 46% yield. To
synthesize compounds 2 and 4, 4,4′-didecyloxybenzil (13)24
was first condensed with 1,2-dinitro-4,5-diaminobenzene (14)
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Pyrazine-Containing Acene-Type Molecular Ribbons
A R T I C L E S
Chart 1. Chemical Structures of Compounds 1-5
to give 15, which was reduced with hydrazine in the presence
of Pd/C to afford diamine 16 in a yield of 80%. The diamines
quickly changed from yellow to pink and then brown if
exposed to air. Therefore, the freshly prepared diamine 16
was subsequently condensed with pyrene-4,5,9,10-teteranes
in refluxing acetic acid to give compounds 2a and 2b. The
intermediates 17a and 17b were obtained by dropwise
addition of the 1,2-diamine 16 to an excess of pyrene-
4,5,9,10-teteranes 9a and 9b at 50 °C, respectively, in the
same way as for the preparation of 10. Then the twofold
condensation reactions of 17 with the teteramine 11 in
m-cresol gave the PAMs 4a and 4b in yields of 40 and 42%,
respectively. Very low solubility of 4a in common organic
solvents makes it practically impossible for chromatographic
purification. In comparison, the solubility of 4b is high
enough for purification by column chromatography. Unlike
hydrocarbon acenes,25 PAMs 1-5 show very good environ-
mental stability, as evidenced by the unchanged absorption
(25) (a) Fang, T. Heptacene, Octacene, Nonacene, Supercene and Related
Polymers. Ph.D. Thesis, University of California, Los Angeles, CA,
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A R T I C L E S
Gao et al.
Scheme 1. Synthesis of Compounds 1, 3, and 5a
a (i) Pd(PPh3)4, toluene, K2CO3 (aq), 85 °C; (ii) AcOH, reflux; (iii) AcOH, 50 °C; (iv) m-cresol, 180 °C.
Scheme 2. Synthesis of Compounds 2 and 4a
a (i) AcOH, 80°C; (ii) Pd/C, NH2NH2 ·H2O, EtOH, reflux; (iii) AcOH, reflux; (iv) AcOH, 50°C; (v) m-cresol, 180 °C.
spectra of the samples in solution left in air and under the
laboratory lighting for up to one month. Furthermore, PAMs
are thermally stable in nitrogen without any weight loss up
to 400 °C as revealed by thermogravimetric analysis.
Chemical structures of all the PAMs were confirmed by
spectroscopic means and elemental analysis (Supporting
Information).
Single crystals of 1a suitable to X-ray crystallographic
analysis were successfully obtained by slow evaporation of its
dichloromethane solution at room temperature. As shown in
Figure 1, the central fused aromatic segment deviates from
planar geometry. The molecules adopt a 1-D face-to-face π-π
stacking with a distance of 3.48 Å. The neighboring molecules
are slipped with respect to each other along the molecular long
axis by 4.77 Å. The electron-deficient pyrazine rings are
overlapped with the electron-rich pyrene rings. This packing
motif suggests the presence of strong intermolecular interaction,
ideal for easy intermolecular charge carrier transport.26
The absorption and photoluminescence (PL) spectra of
compounds 1-5 were recorded in chloroform (2 × 10-6 mol/
L), and the data are listed in Table 1. The introduction of
four phenyl groups in 1a resulted in a red-shift of 24 nm in
absorption, while comparing 1a with the previously reported
unsubstituted analogues.15a It was found that the photophysi-
cal properties of PAMs were almost independent of solubi-
(26) (a) Naraso; Nishida, J.-i.; Ando, S.; Yamaguchi, J.; Itaka, K.; Koinuma,
H.; Tada, H.; Tokito, S.; Yamashita, Y. J. Am. Chem. Soc. 2005, 127,
10142–10143. (b) Bre´das, J. L.; Calbert, J. P.; Da Silva Filho, D. A.;
Cornil, J. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5804–5809. (c) Li,
X.-C.; Sirringhaus, H.; Garnier, F.; Holmes, A. B.; Moratti, S. C.;
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Pyrazine-Containing Acene-Type Molecular Ribbons
A R T I C L E S
Figure 1. Crystal packing of compound 1a.
Table 1. UV-Vis Absorption Maxima (λabs), Photoluminescence Maxima (λem), Photoluminescence Quantum Yields (Φ), Energy Gaps
(Egap), First Reduction Potentials (Ered1/2) and Onsets (Eredonset), and LUMO Energy Levels of Compounds 1-5
c
d
e
compd
λabs (ε/10-5 [mol/L·cm])a
λem
(nm)
Φb
(%)
Egap
(eV)
Ered
Ered
LUMOf
(eV)e
1/2
onset
0-1
0-0
1
E1/2 (V)
2
E1/2 (V)
3
E1/2 (V)
λabs
(nm)
λabs
(nm)
(V)
1a
1b
1c
2a
2b
3a
3b
4a
4b
5
408 (0.59)
418 (0.86)
418 (0.59)
471 (0.97)
471 (1.04)
500 (0.48)
494 (0.50)
500 (0.79)
500 (1.36)
508 (0.47)
433 (0.96)
443 (0.53)
443 (0.88)
500 (1.57)
500 (1.64)
538 (0.25)
525 (0.59)
531 (0.40)
529 (0.83)
538 (0.34)
452
490
480
553
533
658
652
73
86
91
65
82
6
2.78
2.66
2.65
2.36
2.34
2.22
2.25
2.25
2.22
2.18
-1.20
-1.20
-1.22
-0.80
-0.78
-0.80
-0.78
-1.38
-1.37
-1.40
-1.43
-1.37
-1.37
-1.38
-1.15
-1.13
-1.16
-0.71
-0.70
-0.69
-0.67
-3.25
-3.27
-3.24
-3.69
-3.70
-3.71
-3.73
4
-0.73
-0.71
-1.23
-1.22
-1.47
-1.42
-0.64
-0.62
-3.76
-3.78
a Measured in chloroform with a concentration of 2.0 × 10-6 mol/L. b Measured in chloroform with Nile red in 1,4-dioxane (Φ ) 68%) as a
reference. c Estimated from absorption onset. d Obtained from differential pulse voltammetry experiments. e Calculated from cyclic voltammograms.
f Estimated from Ered
.
onset
Figure 2. UV-vis absorption spectra of compounds 1c, 2b, 3b, 4b, and
5 in chloroform with a concentration of 2 × 10-6 mol/L.
Figure 3. Photoluminescence spectra of compounds 1c, 2b, and 3b in
chloroform with a concentration of 2 × 10-6 mol/L.
lizing substituents, and the absorptions were red-shifted with
increase of the molecular length. Displayed in Figure 2 are
the absorption spectra of 1c, 2b, 3b, 4b, and 5. All spectra
are well-resolved with clear vibronic structures, which is
consistent with the rigid structural feature of the compounds.
Compound 1c exhibits an absorption band at 443 nm,
corresponding to the 0-0 transition. The 0-0 absorption is
red-shifted to 500, 525, 529, and 538 nm for 2b, 3b, 4b, and
5, respectively, indicative of the lowered energy gap and the
increased conjugation length with increase of the molecular
length. However, it should be pointed out that the energy
gaps of compounds 1-5 are saturated rapidly as the molecular
length increases, in contrast to those of previously reported
hydrocarbon acenes.14c One of the reasons may be the cross-
conjugation in compounds 1-5 due to the incorporation of
pyrene units in the molecular backbone. The normalized PL
spectra of compounds 1c, 2b, and 3b are presented in Figure
3. Relative to the molecular length, the emission maximum
is red-shifted from 480 nm for 1c to 533 nm for 2b, and
then to 652 nm for 3b. Meanwhile, the PL quantum yields
(ΦPL) dramatically decrease from 91 and 82% for 1c and
2b, respectively, to 4% for 3b. Particularly, PL was not
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A R T I C L E S
Gao et al.
Figure 4. 1H NMR spectra of 1c, 2b, 3b, 4b, and 5 in 1,2-dichlorobenzene-d4 at 120 °C.
detectable for the longer molecular ribbons 4b and 5. The
corresponding compounds without t-butyl substituents follow
the same trend for their PL properties, as shown in Table 1.
One of the main reasons for low ΦPL should be attributed to
the increased π-aggregation tendency for long, flat molecules.
It was reported by Mu¨llen et al. that strong aggregation led
to a complete quenching of fluorescence for large discotic
polycyclic aromatic hydrocarbons.27
systems.28 In general, the π-π interactions of aromatic
molecules can lead to additional ring-current effects. As a
result, the 1H NMR spectra of strongly aggregated molecules
are different from those of isolated monomeric species and
are sensitive to solution concentration and temperature.29
Shown in Figure 5 are concentration-dependent 1H NMR
spectra of compounds 1b, 2a, 1c, and 2b in d-chloroform.
As the concentration increases from 3 × 10-4 to 6 × 10-3
mol/L, the chemical shifts of aromatic protons H-a, H-b, H-c,
H-d, and H-e in compound 1b are shifted upfield by 0.28,
0.41, 0.14, 0.10, and 0.05 ppm, respectively. The chemical
shift of the aromatic proton H-b is most sensitive to the
concentration, suggesting that the proton H-b is the closest
to the centroid of the neighboring molecules in π-aggregation.
The 1H NMR spectra of compound 2a are featured by upfield
The aggregation tendency of PAMs in dilute solution was
studied by means of 1H NMR and UV-vis spectroscopic
1
methods. Figure 4 shows the H NMR spectra of 1c, 2-4b,
and 5 in 1,2-dichlorobenzene-d4 at 120 °C. For compounds
1c and 2b, the spectra are well-resolved. With an increase
in molecular length, the spectra are characterized by signifi-
cant line broadening together with upfield shift. Particularly,
only resonances from the alkoxy side chains and t-butyl
groups can be well observed for compound 5. The NMR
studies indicate the correlation between the aggregation or
π-π interaction tendency and the molecular length, identical
to the phenomena observed in other polycyclic aromatic
(28) (a) Tomovic, Z.; Watson, M. D.; Mu¨llen, K. Angew. Chem., Int. Ed.
2004, 43, 755–758. (b) Zhi, L.; Wu, J.; Mu¨llen, K. Org. Lett. 2005, 7,
5761–5764. (c) Wu, J.; Watson, M. D.; Mu¨llen, K. Angew. Chem.,
Int. Ed. 2003, 42, 5329–5333.
(29) (a) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112,
5525–5534. (b) Wu, J.; Fechtenkotter, A.; Gauss, J.; Watson, M. D.;
Kastler, M.; Fechtenkotter, C.; Wagner, M.; Mullen, K. J. Am. Chem.
Soc. 2004, 126, 11311–11321.
(27) Wasserfallen, D.; Kastler, M.; Pisula, W.; Hofer, W. A.; Fogel, Y.;
Wang, Z.; Mu¨llen, K. J. Am. Chem. Soc. 2006, 128, 1334–1339.
9
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Pyrazine-Containing Acene-Type Molecular Ribbons
A R T I C L E S
1
Figure 5. Concentration-dependent H NMR spectra of 1b, 2a, 1c, and 2b in CDCl3 at 20 °C.
1
shift of 0.88 ppm from 3 × 10-4 to 6 × 10-3 mol/L, and
broad line shape even at 3 × 10-4 mol/L, indicating a
dramatically increased aggregation tendency. Introduction of
t-butyl groups in pyrene unit can noticeably suppress the
aggregation of the compounds. The resonances of H-b in
compounds 1c and 2b only shift to the upfield by 0.05 and
0.31 ppm, respectively.
Displayed in Figure 6 are temperature-dependent H NMR
spectra of compounds 1b, 2a, 1c, and 2b at a concentration of
6.0 × 10-3 mol/L in 1,1,2,2-tetrachloroethane-d2. Similar to
the concentration dependence, compound 2a exhibits the
strongest aggregation tendency. At 20 °C, the 1H NMR spectrum
of compound 2a is featured by very weak and broad signals.
By increasing the temperature, the signals become sharp and
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A R T I C L E S
Gao et al.
Figure 6. Temperature-dependent 1H NMR spectra of 1b, 2a, 1c, and 2b in 1,1,2,2-tetrachloroethane-d2 with a concentration of 6 × 10-3 mol/L.
distinct. However, the spectrum at 100 °C is still more poorly
resolved in comparison with that of other three compounds.
self-association.30 Compounds 1b and 2a exhibit a KE value
of 614 ( 61 and 6818 ( 682 M-1, respectively. These values
are much higher than those of phenylacetylene macro-
To give a quantitative depiction of the aggregation
tendency of PAMs, the association constants of 1b, 1c, 2a,
and 2b in d-chloroform at 20 °C (Table 2) were determined
by analyzing 1H NMR chemical shifts of H-b at the different
concentrations with the equal K (EK) model of indefinite
(30) (a) Zhang, J.; Moore, J. S. J. Am. Chem. Soc. 1992, 114, 9701–9702.
(b) Shetty, A. S.; Zhang, J.; Moore, J. S. J. Am. Chem. Soc. 1996,
118, 1019–1027. (c) Martin, R. B. Chem. ReV. 1996, 96, 3043–3064.
(d) Horman, I.; Dreux, B. HelV. Chim. Acta 1984, 67, 754–764.
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Pyrazine-Containing Acene-Type Molecular Ribbons
A R T I C L E S
Table 2. Association Constants of Compounds 1b, 2a, 1c, and 2b
in CDCl3 at Room Temperature As Calculated from Chemical Shift
of the Aromatic Proton H-b
compound
1b
2a
1c
2b
KE (L/mol)
614 ( 61
6818 ( 620
18( 2
326 ( 33
cycles30a,b,31 and perylene tetracarboxylic diimide compounds
in similar solvents.32 In contrast to strong aggregation of 1b
and 2a, compounds 1c and 2b have a lower KE value of 18
( 2 and 326 ( 33 M-1, respectively, indicating that the
aggregation of the compounds is greatly suppressed by bulky
t-butyl groups.
The aggregation of PAMs in solution is also supported by
the changes of the absorption extinction coefficient at different
concentrations. With compounds 1c, 2b, and 3b as examples
(Figure 7), upon increasing the concentration from 2.0 × 10-7
to 2.0 × 10-5 mol/L, the extinction coefficient (0-0 transition)
of 1c, 2b, and 3b decreases by 13% (98 000 to 85 000
M-1cm-1), 25% (187 000 to 141 000 M-1cm-1), and 50%
(84 000 to 42 000 M-1cm-1), respectively. Furthermore, a
noticeable red-shift of absorption for 3b was also observed upon
increasing the concentration gradually. Therefore, the tendency
in molecular aggregation is clearly correlated with the molecular
length for PAMs, as supported by the aforementioned concen-
1
tration- and temperature-dependent H NMR studies.
The electrochemical properties of all the PAMs in dichlo-
romethane, except for 4a because of its poor solubility, were
studied in a three-electrode electrochemical cell with Bu4NClO4
(0.1 M) and Ag/AgCl as electrolyte and reference electrode,
respectively, and the data are collected in Table 1. These
compounds exhibit a reversible or quasi-reversible redox process
in the negative potential region, indicative of the n-type doping
nature. It was found that t-butyl groups had almost no effect
on the redox properties of PAMs. Depicted in Figure 8 are the
cyclic voltammograms of compounds 1c, 2-4b, and 5. Com-
pound 1c exhibits one reversible reduction wave at -1.22 V vs
Ag/AgCl together with the second quasi-reversible reduction
wave at -1.40 V. Compounds 2b and 3b also exhibit two
reduction waves. The first reduction wave of compounds 2b
and 3b is shifted by 440 mV to more positive potential compared
to that of compound 1c, suggesting the enhanced electron affinity
is due to introduction of two more pyrazine units. With addition
of another two fused pyrazine units, compounds 4b and 5 show
three reduction waves along with the more positive first
reduction waves. Compared to that of compounds 2b and 3b,
the first reduction wave of compounds 4b and 5 is shifted by
about 70 mV to -0.73 and -0.71 V, respectively, which are
very close to that of [6,6]-phenyl-C61-butyric acid methyl ester
(PCBM),10d a state-of-the-art n-type semiconductor. This implies
that PAMs are potential n-type semiconductors. It is interesting
to note that PAMs with the same number of pyrazine units have
the identical first reduction potential (Table 1), suggesting that
Figure 7. Concentration-dependent UV-vis spectra of 1c (A), 2b (B),
and 3b (C) in d-chloroform at 20 °C.
the electron affinity of these PAMs largely depends on the
number of pyrazine units.33
The lowest unoccupied molecular orbital (LUMO) energy
levels of 1-5 were estimated from the reduction onset potentials
calibrated by the ferrocene/ferrocenium pair.34 As shown in
Table 1, with an increase in molecular length, the LUMO energy
levels decreased from -3.24 eV for 1c to -3.78 eV for 5.
Meanwhile, introduction of alkoxyphenyl terminal groups and
(31) (a) Hoger, S.; Bonrad, K.; Mourran, A.; Beginn, U.; Moller, M. J. Am.
Chem. Soc. 2001, 123, 5651–5659. (b) Tobe, Y.; Utsumi, N.;
Kawabata, K.; Nagano, A.; Adachi, K.; Araki, S.; Sonoda, M.; Hirose,
K.; Naemura, K. J. Am. Chem. Soc. 2002, 124, 5350–5364. (c)
Nakamura, K.; Okubo, H.; Yamaguchi, M. Org. Lett. 2001, 3, 1097–
1099.
(33) (a) Lee, S. K.; Zu, Y.; Herrmann, A.; Geerts, Y.; Mu¨llen, K.; Bard,
A. J. J. Am. Chem. Soc. 1999, 121, 3513–3520. (b) Fogel, Y.; Kastler,
M.; Wang, Z.; Andrienko, D.; Bodwell, G. J.; Mu¨llen, K. J. Am. Chem.
Soc. 2007, 129, 11743–11749.
(32) (a) Wang, W.; Li, L.-S.; Helms, G.; Zhou, H.-H.; Li, A. D. Q. J. Am.
Chem. Soc. 2003, 125, 1120–1121. (b) Wang, W.; Han, J. J.; Wang,
L.-Q.; Li, L.-S.; Shaw, W. J.; Li, A. D. Q. Nano Lett. 2003, 3, 455–
458. (c) Li, A. D. Q.; Wang, W.; Wang, L.-Q. Chem.-Eur. J. 2003,
9, 4594–4601.
(34) (a) Parker, V. D. J. Am. Chem. Soc. 1976, 98, 98–103. (b) Eckhardt,
H.; Shacklette, L. W.; Jen, K. Y.; Elsenbaumer, R. L. J. Chem. Phys.
1989, 91, 1303–1315.
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A R T I C L E S
Gao et al.
ments reveal that the energy gap of the molecules decreases
with an increase in the molecular length. All these molecules
tend to aggregate in solution, which can be greatly suppressed
by introducing bulky t-butyl groups. The n-type semiconductor
nature of these acene-type compounds has been verified by the
electrochemical data. With the increasing number of pyrazine
rings and the increasing length of the molecules, the electron
affinity of these compounds is significantly enhanced. As a
result, the LUMO energy levels decrease from -3.24 to -3.78
eV, comparable to a well-known n-type PCBM material. High
electron affinity, high environmental stability, and ease of
structural modification make these compounds excellent can-
didates as a new class of n-type semiconductors.
Figure 8. Cyclic voltammograms of 1c, 2b, 3b, 4b, and 5 in dichlo-
romethane at a scan rate of 0.1 V/s.
Acknowledgment. This work was supported by the National
Natural Science Foundation of China (Nos. 20574067 and 50633040),
Science Fund for Creative Research Groups (No. 20621401), and
973 project (2002CB613402). We also thank Professor Zhi Yuan
Wang at Carleton University of Canada for his help with manuscript
revision.
t-butyl substituents showed no effect on the LUMO energy
levels of PAMs.
Conclusions
Under the optimized condensation conditions, 10 acene-type
compounds (1-5) containing 2-6 pyrazine units and up to 16
rectilinearly arranged fused aromatic rings were synthesized.
To the best of our knowledge, compound 5, with 16 rectilinearly
arranged fused aromatic rings and a length about 5 nm, is the
longest acene-type conjugated molecule. Absorption measure-
Supporting Information Available: All experimental details,
including synthesis and characterization of all compounds and
thermogravimetric analysis results (PDF, CIF). This material
is available free of charge via the Internet at http://pubs.acs.org.
JA800311A
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