Large-size linear and star-shaped dihydropyrazine fused pyrazinacenes

Article abstract of DOI:10.1021/ol2030839

Linear and star-shaped pyrazinacenes 1a-b and 2 were synthesized via condensation between a new building block 11 and pyrene tetraones or cyclohexaone. Compound 2 represents the largest star-shaped dihydropyrazine fused pyrazinacene reported so far. These

Full text of DOI:10.1021/ol2030839

ORGANIC
LETTERS
2012
Vol. 14, No. 2
494–497
Large-Size Linear and Star-Shaped
Dihydropyrazine Fused Pyrazinacenes
Chenhua Tong,† Wenguang Zhao,†,§ Jing Luo,†, Hongying Mao,† Wei Chen,†,‡
Hardy Sze On Chan,*,† and Chunyan Chi*,†
Department of Chemistry, National University of Singapore, 3 Science Drive 3,
Singapore, 117543, and Department of Physics, National University of Singapore,
2 Science Drive 3, Singapore, 117542
chmcsoh@nus.edu.sg (Chan); chmcc@nus.edu.sg (Chi)
Received November 17, 2011
ABSTRACT
Linear and star-shaped pyrazinacenes 1aꢀb and 2 were synthesized via condensation between a new building block 11 and pyrene tetraones or
cyclohexaone. Compound 2 represents the largest star-shaped dihydropyrazine fused pyrazinacene reported so far. These largely expanded
pyrazinacenes show good solubility and have a strong tendency to aggregate in both solution and thin films, indicating their potential applications
for organic electronic devices.
Acene derivatives have attracted much attention be-
cause of their applications as semiconducting materials
for organic electronic devices.1 Long acenes are valuable in
view of their large orbital area for stacking, ambipolar
property, and possible diradical property; however, they are
usually unstable due to their high-lying HOMO energy level.2
Peripheral substitution by electron-withdrawing car-
boxylic imide, cyano, or fluorine groups can stabilize
the acenes and also offer n-type properties by lowering the
† Department of Chemistry.
‡ Department of Physics.
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§ Present address: Institute of Chemical and Engineering Sciences, 1 Pesek
Road, Jurong Island, Singapore 627833.
Present address: School of Chemical and Material Engineering, Jiangnan
University, Wuxi 214122, PR China.
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10.1021/ol2030839
Published on Web 01/04/2012
2012 American Chemical Society

Scheme 1. Synthetic Route of Pyrazinacene Derivatives 1a, 1b, and 2
LUMO/HOMO energy level.3 An alternative strategy is
to introduce N-atoms into an acene framework. The
electronegative imine N-atoms can enhance the stability
of acenes and prevent photooxidization and DielsꢀAlder
reaction.4 Moreover, they can also serve as H-bond
acceptors to increase the intermolecular interactions in
the solid state.5
Owing to the high electron deficiency, imine N-atoms
are usually utilized in the design of n-type semiconducting
molecules.6 To achieve n-type behavior, a large number of
imine N-atoms should be incorporated into the acene
framework.5 Thus, pyrazine-containing acenes, namely,
pyrazinacenes, are highly desired. Some linear pyrazin-
acenes have been reported.6,7 However, pyrazinacenes with
high pyrazine unit density but without an electron-donating
group can only be marginally obtained or they are very
sensitive to even weak nucleophiles.8 Moreover, fused
pyrazine derivatives are also readily reduced.9 On the other
hand, a dihydropyrazine (reduced pyrazine) unit has been
applied for the design of p-type semiconductors,10 and they
can also act as H-bond donors. Therefore, some aza-acene
derivatives containing both imine and dihydropyrazine
N-atoms have been prepared to adjust their stability and
electronic properties.11 An ambipolar behavior was ex-
pected for pyrazinacenes composed of both pyrazine and
dihydropyrazine units.8 Besides the linear structures, star-
shaped pyrazinacenes such as hexaazatriphenylene (HAT)
and hexaazatrinaphthalene (HATN) derivatives are also
of interest because they can self-organize into columnar
structures in the solid state and allow one-dimensional
charge transport along the column.12 Larger linear and
star-shaped acenes containing both pyrazine and dihy-
dropyrazine units are interesting because they are sup-
posed to show more extended π-conjugation and a higher
tendency to aggregate, which are essential for material
applications in electronic devices such as organic field-
effect transistors (OFETs). However, synthesis of higher
order (dihydro)pyrazinacenes is challenging because (1)
the large rigid molecules usually show poor solubility if
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Org. Lett., Vol. 14, No. 2, 2012
495

not appropriately substituted by flexible alkyl chains; (2)
there is no efficient way to extend the framework; and (3)
the characterization of the strongly aggregated molecules
is difficult. Herein, we report the successful synthesis and
characterization of large-size linear aza-decacenes 1aꢀb
and star-shaped aza-tripentacene 2 containing both pyr-
azine anddihydropyrazineunits. Their physical properties
and self-assembly in solution and the solid state were also
studied.
1a,1b,and2were nearly insoluble in alcohol and ethyl acetate
but readily soluble in THF, aromatic solvents, chlorinated
alkanes, and even hexane. During the reaction, the exces-
sive diamine 11 formed the imidazole byproduct with
acetic acid, which can be easily removed from the target
products by simply washing with ethanol. Other conden-
sation conditions such as m-cresol and p-toluenesulfonic
acid/toluene were also tested, but the product was compli-
cated. Further dehydrogenation of 1a, 1b, and 2 by
oxidants such as MnO2, PbO2, or DDQ was also tried, but
all of the reactions only gave partially dehydrogenated
products which are difficult to purify and identify.
The chemical structures of 1a, 1b, and 2 were character-
ized by using NMR, MALDI-TOF mass spectrometry,
and elemental analysis (see Supporting Information (SI)).
Although they have very good solubility in chloroform or
dichloromethane, no signal was observed in the aromatic
region of NMR spectra even when the measurement was
conducted at 100 °C in tetrachloroethane-d4. Such a
phenomenon can be attributed to the strong aggregation
effect of the rigid core molecules in solution. TFA-d1 was
hence added to the solution to protonate the N-atoms to
suppress the aggregation. As a result, a well resolved
resonance signal was obtained and the chemical structures
of 1a, 1b, and 2 were confirmed (Figure 1).
The synthesis of aza-acenes 1aꢀb and 2 is shown in
Scheme 1. The key intermediate compound is the alkylated
dihydrotetraazatetracene 1,2-diamine 11, which can un-
dergo a condensation reaction with 1,2-diketones. To
ensure sufficient solubility for the final products, branched
dove-tail chains have to be attached onto the peripheries.
The synthesis started from the etherification of 2-decy-
hetradecan-1-ol 3 with pent-4-ynyl-4-methylbenzenesulfo-
nate 413 to give the alkyne 5 in 61% yield. Hagiharaꢀ
Sonogashira coupling between 5 and 1,2-dibromo-4,
5-dintrobenzene 614 gave compound 7 in 50% yield. It is
worth noting that the spacer between an O-atom and a
triple bond should be at least as long as propyl. Otherwise,
a coupling reaction using alkynes with a shorter spacer
(e.g., methylene) gave the corresponding products in ex-
tremely low yield, presumably due to the coordination of
an O-atom to the Pd catalyst. Hydrogenation15 of both the
nitro and the alkyne groups in compound 7 was then
conducted by using hydrazine over Pd/C to afford the
diamine 8 in 90% yield. The nucleophilic substitution/
coupling reaction between 8 and 916 was highly energetic,
and careful control must be exercised toachievereasonable
yield. The reaction can be conducted in solid form without
solvent, but the purification was difficult because many
byproducts were formed. After careful optimization of
solvent and base, the reaction was found to show the best
result when conducted in DMF without any base and 10
was obtained in 37% yield. Compound 10 was hydroge-
nated by hydrogen over Pd/C in ethanol and afforded the
air-sensitive diamine 11. The freshly prepared diamine 11
then reacted with pyrene tetraone 12a, 12b17 or cyclohex-
aone 13 to form the target products 1a, 1b and 2 in
59ꢀ80% yields. The tert-butyl-substituted pyrene tetraone
12b was chosen as the bulk groups which may partially
suppress molecular aggregation and facilitate NMR char-
acterization in solution. These condensation reactions
were carried out in acetic acid/p-dioxane at 135 °C,
and excessive diamine 11 was used to ensure completion
of multiple condensation reactions. The desired products
1
Figure 1. Aromatic regions of the H NMR spectra of 1a, 1b,
and 2 measured in CDCl3/CF3COOD (1:1). The resonances
were assigned to the corresponding protons shown in Scheme 1.
The UVꢀvis absorption and fluorescence spectra of 1a,
1b, and 2 recorded in dilute chloroform solution and in
a thin film are shown in Figure 2. 1a shows an intense
absorption band with a maximum at 506 nm at 1.0 ꢁ 10ꢀ5
M, and under the same conditions, 1b exhibits two split
bands at 500 and 524 nm (Figure 2a). For 1a, the wave-
length of the absorption maximum (λmax) also shows a
slight hypsochromic shift withanincrease of concentration
(from 511 nm at 5 ꢁ 10ꢀ7 M to 505 nm at 3 ꢁ 10ꢀ5 M,
Figure S1 in SI), which indicates a H-aggregation.18 For
1b, because the aggregation is suppressed by the tert-
butyl on the pyrene, no shift of λmax occurs at various
concentrations and two resolved peaks were observed.
Compound 2 displays a broad absorption band with
ꢀ
€
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496
Org. Lett., Vol. 14, No. 2, 2012

Table 1. Energy Levels and Band Gaps of 1a, 1b, and 2a
HOMO/eV
LUMO/eV
Egopt/eV
1a
1b
2
ꢀ4.88
ꢀ5.04
ꢀ4.90
ꢀ2.79
ꢀ2.86
ꢀ3.63
2.09
2.18
1.27
a HOMO energy level was determined by UPS measurements in thin
Opt
film state; Eg
is the energy band gap derived from the low-energy
opt
absorption edge in thin film state; LUMO = HOMO þ Eg
.
determine their HOMO energy levels in thin films (Table 1,
and Figure S4 and Table S2 in SI). The optical energy band
gap (Egopt) was derived from the low-energy absorption
edge in thin films, and then the LUMO energy level can be
estimated by LUMO = HOMO þ Egopt. Among these
compounds, 2 has the lowest LUMO energy level due to
the high pyrazine unit density.
The decomposition temperatures (Td, corresponding to
the 5% weight loss) for 1a, 1b, and 2 are at 300, 261, and
232 °C respectively (Figure S5 in SI). Differential scanning
calorimetry (DSC) curves show no phase transition from rt
to Td (Figure S6 in SI). XRD patterns of thin films show
one major reflection in the small angle region (with
distances of 4.2, 3.27, and 3.53 nm for 1a, 1b, and 2,
Figure 2. (a) UVꢀvis absorption spectra of 1a, 1b, and 2 in
chloroform at 1 ꢁ 10ꢀ5 M. (b) Normalized UVꢀvis absorption
spectra of 1a, 1b, and 2 in thin film. (c) Fluorescence spectra of
1a, 1b, and 2 in chloroform at 1 ꢁ 10ꢀ6 M.
λmax at 547 nm in solution together with a long tail to the
near-IR region (Figure 2a). The λmax shows a red shift with
increasing concentration (from 543 nm at 1 ꢁ 10ꢀ6 M to
558 nm at 5 ꢁ 10ꢀ5 M, Figure S1 in SI), which is similar to
the HAT molecules and can be explained by a rotated
cofacial aggregation.12cꢀe In thin films, the absorption
spectra of 1a and 1b become slightly broader and the
spectrum of 2 shows an obvious red shift compared to that
in solution (Figure 2b), indicating strong intermolecular
association in film. Compound 1b showed a relatively
sharp fluorescence spectrum compared with 1a, both with
an emission maximum at 531 nm (Figure 2c). The fluo-
rescence quantum yields are relatively low, with 3.5% for
1a and 10.5% for 1b (Rhodamine 6G as reference). Com-
pound 2 exhibited very weak fluorescence (quantum yield
<0.5%) with an emission maximum at 601 nm. The
concentration-dependent fluorescence spectra of 1a, 1b, and
2 (Figure S2 in SI) show a red shift of the emission maxi-
mum and fluorescence quench when the concentration is
>1 ꢁ 10ꢀ5 M. This further confirms the aggregates are
formed in a concentrated solution (>1 ꢁ 10ꢀ5 M).
˚
respectively) and a halo at the wide angle region (4.1ꢀ4.6 A)
(Figure S7 in SI). The intensity of 1a is much higher than the
other two samples, implying that it has a higher packing
order in the solid state.
In summary, large linear and star-shaped aza-acenes
containing fused pyrazine/dihydropyrazine units were
synthesized by using a new building block 11. To the best
of our knowledge, 2 represents the largest star-shaped
dihydropyrazine fused pyrazinacene reported so far.
These new molecules have a strong tendency to aggre-
gate in both solution and thin films, and they might be
used as good semiconductors for thin film OFETs.
Device fabrication and characterization are underway
in our laboratories.
Acknowledgment. This work is supported by the Na-
tional University of Singapore under MOE AcRF FRC
Grant R-143-000-444-112 and Start Up Grant R-143-000-
486-133.
Due to strong aggregation, the cyclic voltammetry
measurements for 1a, 1b, and 2 were conducted in chloro-
benzene at elevated temperature (80 °C), but only weak
redox waves were obtained(Figure S3 in SI). Alternatively,
ultraviolet photoelectron spectroscopy (UPS) was used to
Supporting Information Available. Synthetic proce-
dures and characterization data, additional UVꢀvis
and fluorescence spectra, CV/UPS data, TGA/DSC
curves, and XRD patterns. This material is available
free of charge via the Internet at http://pubs.acs.org.
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Org. Lett., Vol. 14, No. 2, 2012
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