Substituent effect on the electronic properties of pyrazino[2,3-g] quinoxaline molecules

Article abstract of DOI:10.1039/c1jm12347e

We report the synthesis and characterization of pyrazino[2,3-g]quinoxaline derivatives with a systematic change in the substituents at the 2, 3, 7 and 8 positions and with or without 2-thienyl at the 5 and 10 positions to study the substituent effect and quinoid character of such a system. We performed density functional theory calculations using the B3LYP functional and 6-31G* basis set under the geometry optimization condition, together with the resonance effect of a valence bond theory to understand the electronic structures of these molecules. It was found that a combination of conjugation and cross-conjugation effects is responsible for the observed trends in their electronic properties, thus giving insights into designing molecules utilizing such effects.

Full text of DOI:10.1039/c1jm12347e

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2011, 21, 17798
www.rsc.org/materials
PAPER
Substituent effect on the electronic properties of pyrazino[2,3-g] quinoxaline
molecules†
Teck Lip Tam,ab Feng Zhou,ab Hairong Li,ab Jane Chieh Yu Pang,ab Yeng Ming Lam,ab
ab
ab
Subodh G. Mhaisalkar,ab Haibin Su
*
*
and Andrew C. Grimsdale
Received 26th May 2011, Accepted 8th September 2011
DOI: 10.1039/c1jm12347e
We report the synthesis and characterization of pyrazino[2,3-g]quinoxaline derivatives with
a systematic change in the substituents at the 2, 3, 7 and 8 positions and with or without 2-thienyl at the
5 and 10 positions to study the substituent effect and quinoid character of such a system. We performed
density functional theory calculations using the B3LYP functional and 6-31G* basis set under the
geometry optimization condition, together with the resonance effect of a valence bond theory to
understand the electronic structures of these molecules. It was found that a combination of conjugation
and cross-conjugation effects is responsible for the observed trends in their electronic properties, thus
giving insights into designing molecules utilizing such effects.
a systematic change in the substituents at various positions.
Introduction
Together with computational modeling using density functional
theory (DFT)18 and valence bond theory (VBT), a study of
substituent effects on the electronic properties of these complex
systems was carried out.
To date, the donor–acceptor (D–A) approach utilizing intra-
molecular charge-transfer between donor and acceptor units,
and a planarization approach via quinoidization of a conjugated
system are the two main approaches to achieve low band gap
materials.1 The former is in molecular-orbital theory, a conse-
quence of hybridization of energy levels of donor and acceptor
moieties.2,3 The latter is a result of dearomatization of the
conjugated backbone via quinoid formation, by incorporation of
an aromatic system with higher resonance energy.1 The pyrazino
[2,3-g]quinoxaline (PQ) system is thus a good candidate acceptor
unit for low band gap materials due to its strong electron
accepting ability and high quinoid character at 5 and 10 posi-
tions.4–8 The strong p–p interactions between the PQ moieties
also make them highly crystalline in nature which is desirable for
promoting efficient exciton migration and high charge carrier
mobility.4,9–13 These intrinsic properties of PQ thus make it
a good candidate for the development of low band gap and high
mobility materials for1 organic photovoltaics (OPVs).4,8,11,14–16
Varying substituents at the different positions of PQ using
appropriate synthetic routes, allows the study of substituent
effects on the overall electronic properties. Here, we report the
synthesis and characterization of a series of PQ molecules with
Results and discussion
Synthesis
Synthesis of PQ units can be easily carried out by condensation
of 1,2-diketones with 1,2,4,5-tetraaminobenzene tetrahydro-
chloride (TAB). Using different substituted 1,2-diketones, the 2,
3, 7 and 8 positions (arm positions) of PQ can be functionalized
with various groups.9–11,15,19 For PQ substituted at the 5 and 10
positions (quinoidal positions), the required synthetic approach
is more complex. It requires reduction of 5,6-dinitro-2,1,3-ben-
zothiadiazole derivatives using zinc and acetic acid, followed by
addition of 1,2-diketones.14,16 In a previous paper, we have
demonstrated the use of 4,8-disubstituted benzobisthiadiazole
(BBT) as a synthetic equivalent to 4,7-disubstituted-5,6-dinitro-
2,1,3-benzothiadiazole for the synthesis of 5,10-disubstituted PQ
derivatives.17,20 In this paper, we expand this approach and
introduce four new PQ molecules with biphenyl and/or bithienyl
arms, with or without thienyl substitutions at the quinoidal
positions to perform a systematic study of the effects of substi-
tution at these positions (Fig. 1).
aSchool of Materials Science and Engineering, Nanyang Technological
University, 50 Nanyang Avenue, Singapore, 63979, Singapore. E-mail:
hbsu@ntu.edu.sg; Tel: +65 6790 4346
bEnergy Research Institute @NTU (ERI@N), Nanyang Technological
University, 50 Nanyang Drive, Singapore, 637553, Singapore. E-mail:
ACGrimsdale@ntu.edu.sg; Tel: +65 6790 6728
The synthesis of PQ-1A and PQ-3A involves the condensation
of TAB with 1,2-diketones in acetic acid under reflux conditions
(Scheme 1). It was found that addition of a catalytic amount of
2-iodoxybenzoic acid increases the yield for these otherwise low
yielding condensation reactions between TAB and 1,2-diketones.
Synthesis of 5 and 10 substituted PQ derivatives follows our
† Electronic
supplementary
information
(ESI)
available:
Characterization including NMR, MALDI-TOF, TGA and DSC.
Includes additional DFT information. See DOI: 10.1039/c1jm12347e
17798 | J. Mater. Chem., 2011, 21, 17798–17804
This journal is ª The Royal Society of Chemistry 2011

View Article Online
the molecules in series A. The subsequent addition of quinodal
thienyls reduces the band gap further. However, this reduction in
band gap from series A to B decreases from changing biphenyls
to bithienyls (PQ-1A / PQ-1B > PQ-2A / PQ-2B > PQ-3A
/ PQ-3B). We suspect that this decrease in reduction of the
band gap might be a consequence of cross-conjugation between
the arms and the quinoidal positions. To confirm this, the proton
shifts of the quinoidal hydrogens in series A were compared
(Table 2). The PQ with the strongest quinoid character will have
the most deshielded quinoidal hydrogens and thus these will
1
show up at the most down-field position in the H NMR spec-
trum.22 From the proton shift of series A, we can see that the
strength of the quinoid character is PQ-1A > PQ-2A > PQ-3A.
The progressive weakening of the quinoid character according to
the proton shift in the series can be attributed to a cross-conju-
gation effect caused by the addition of aromatic arms with
increasing electron donating nature. Scheme 2 shows the reso-
nance structures depicting conjugation and cross-conjugation
between the arm and quinodal positions within the PQ core.
Decreasing the quinoid character would mean that the proton
shift in series B would follow the trend: PQ-1B > PQ-2B > PQ-
3B, which is in agreement with the observed results.
Fig. 1 Structure of PQ, its IUPAC numbering system and the deriva-
tives discussed in this paper. PQ-1B and PQ-3B were previously
synthesized by us.17
Computational modeling results are tabulated in Table 3. The
theoretical trends of the HOMO match those of the experimental
data. As expected, series A shows a major HOMO contribution
from the arm position while series B shows higher HOMO
electron density along the quinoidal positions and core due to the
high quinoid character at these positions.4–8 For series A, both
theoretical and experimental HOMO values increase due to the
stronger electron-donating nature of bithienyl as compared to
biphenyl. The HOMO (HOMO ꢀ 1) orbitals show two nodes
existing in the PQ core, in PQ-1A (PQ-1B) and PQ-3A (PQ-3B)
at the center carbons bearing the hydrogen and in PQ-2A (PQ-
2B) at the two nitrogens nearer to the biphenyl arms. Cross-
conjugation has been reported to produce destructive quantum
interference (node) in molecular electronics.23–25 In Table 4,
tetramethyl substituted molecules of series A (PQ-0A) and B
(PQ-0B) show the absence of nodes in their HOMO and
HOMO ꢀ 1. Thus the origin of these nodes in PQ-1A / PQ-3A
and PQ-1B / PQ-3B must be a result of cross-conjugation of
the aromatic arm substituents. With reference to the resonance
structure j in Scheme 2, it becomes intuitive that the contribution
from two arm substituents on the same side (quinoid positions as
reference line) will result in cross-conjugation with another two
on the other side.
Scheme 1 Synthesis of the PQ molecules.
From series A to B, the increase in both theoretical and
experimental HOMO levels can be thought of as the formation of
a new set of bonding orbitals from the addition of the thienyl
substitutions at the quinoidal positions, which is localized along
the quinoid–core–quinoid system. Within series B, both theo-
retical and experimental HOMO values are almost constant, with
a slight stabilization from PQ-1B to PQ-3B. This slight stabili-
zation can be thought as the HOMO being slightly more delo-
calized due to the contribution from bithienyls, a case of pseudo-
cross-conjugation as suggested by five resonance structures (d, e,
h, i and j in Scheme 2) that show conjugation between quinoidal
and arm positions.
previously reported methodology starting from 4,8-dibromo-
BBT.17,20 Reduction of the BBT precursor using zinc in acetic
acid, followed by addition of the diketone(s) yielded PQ-1B, PQ-
2B and PQ-3B. Synthesis of PQ-2A starts with reduction of
dinitroditosylaminobenzene 3, which is prepared according to
the literature,21 using tin(II) in 37% HCl, followed by condensa-
tion with the diketone 1. Subsequent deprotection of the tosyl
group via concentrated sulfuric acid, followed by condensation
with diketone 2 yielded PQ-2A.
Comparison of experimental and theoretical results
From Table 1, an increase in conjugation from biphenyl to
bithienyl with the PQ core results in the band gap reduction of
The theoretical trend of LUMO energy levels for series B
agrees nicely with the experimental data. However, the
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 17798–17804 | 17799

View Article Online
Table 1 Characterization of PQ moleculesa
HOMO/eV
LUMO/eV
Eg (CV)/eV
Eg (UV)/eV
lmax/nm solution
Tm/ꢁC
PQ-1A
PQ-1B
PQ-2A
PQ-2B
PQ-3A
PQ-3B
ꢀ5.91
ꢀ5.10
ꢀ5.82
ꢀ5.14
ꢀ5.49
ꢀ5.15
ꢀ3.66
ꢀ3.12
ꢀ3.61
ꢀ3.14
ꢀ3.55
ꢀ3.26
2.25
1.98
2.21
2.00
1.94
1.89
2.48
1.93
2.16
1.96
1.98
1.86
454, 339s, 304
>300
>300
211
132
191
543t, 490, 382, 324
499, 361s, 301
541, 409, 324
573, 550, 387, 331
607, 575s, 447, 339
170
a
HOMO and LUMO were calculated from cyclic voltammetry. s/t denotes position of shoulder/tail in UV-vis absorption spectra.
Table 2 Proton shift of the quinoid position hydrogens
PQ-1A PQ-1B PQ-2A PQ-2B PQ-3A PQ-3B
H shift/ppm 9.07
8.52
8.81
8.44
8.57
8.37
experimental LUMO values within series A and from series A /
B increase but the reverse was observed in the calculations. Only
the experimental and theoretical LUMO trends for series B
match. The discrepancies may arise from the possible agglo-
meration in the cyclic voltammetry experiment as compared to
the single molecule gas phase used in the computational studies.
More detailed studies on the discrepancies are currently
underway. Here we focus on the understanding from theoretical
results. For series A, the LUMO gets more delocalization from
1A to 3A, thus lower energy values. Addition of the thienyls at
the quinoidal positions for series B results in a more delocalized
LUMO as compared to series A. Thus the LUMOs of series B are
lower than those of series A. Within series B, the LUMO also gets
more delocalization from 1B to 3B, thus lower energy values.
However, the decrease is of a smaller magnitude as compared to
series A. This is because the pseudo-cross-conjugation discussed
earlier only induces a little more delocalization of the LUMO in
series B.
Scheme 2 Resonance structures illustrating conjugation and cross-
conjugation effects between the quinoidal and arm positions. Blue
denotes conjugation along the quinoidal positions and red denotes
conjugation along the arm positions.
series (from biphenyl to bithienyl substitutions). The overall
absorption band measured by experiment can be described by the
calculated spectra broadened with a Gaussian function. The first
absorption bands of series A are all attributed to the HOMO /
LUMO transition and their oscillator strengths are quite strong,
ranging from 0.9225 to 1.0321. In contrast, the HOMO /
LUMO transitions for series B have very weak oscillator
strength, ranging from 0.0135 to 0.1033. For series B, only the
HOMO / LUMO transition for PQ-1B is observable. This is
because the next absorption band for PQ-1B, which is attributed
to the HOMO ꢀ 1 / LUMO transition, is much higher in
energy (>1 eV difference) than its HOMO / LUMO transition.
For PQ-2B and PQ-3B, the next absorption bands, which are
also attributed to the HOMO ꢀ 1 / LUMO transition, have
much higher oscillator strength than their HOMO / LUMO
transition and both bands are similar in energy (<0.25 eV
difference). The combination of the two effects causes the
HOMO / LUMO transition for both PQ-2B and PQ-3B to be
not observable after spectra broadening.
Photophysical properties
In the theoretical calculation, the vertical absorptions and UV-
vis spectrum were calculated by the TD-DFT approach with the
6-31G(d) basis set. 20 states were calculated. The calculated
dipole-allowed absorptions associated with the oscillator
strengths and the assignments are listed in Table 5, and the
simulated Gaussian type absorption spectra (the UV-vis peak
half-width at half height is 0.15 eV) are shown in Fig. 2a. The
electron excitation from the HOMO to the LUMO corresponds
to the p–p* type transition between donor and acceptor moie-
ties. Interestingly, the lowest-lying absorptions are red-shifted
from series A to series B, and the red-shifts also exist within each
For the experimental absorption spectra measured in solution,
the lowest-lying absorptions are also red shifted from series A to
series B, and the red-shifts also exist within each series (from
biphenyl to bithienyl substitutions). This is consistent with the
theoretical calculation. The tailing observed in PQ-1B is expec-
ted to be attributed to the weak HOMO / LUMO transition, as
predicted by theoretical calculation.
17800 | J. Mater. Chem., 2011, 21, 17798–17804
This journal is ª The Royal Society of Chemistry 2011

View Article Online
Table 3 Density functional theory modeling of the PQ molecules using the B3LYP functional and 6-31G* basis set under the geometry optimization
condition
Molecule (symm.)
PQ-1A (D2h
)
PQ-1B (C2)
PQ-2A (C2)
PQ-2B (C1)
PQ-3A (D2h
)
PQ-3B (C2)
LUMO + 1/eV/ orbital
symm.
ꢀ1.46/a
ꢀ2.50/b2
ꢀ5.36/b1
ꢀ1.48/a
ꢀ2.65/a
ꢀ4.88/b
ꢀ1.89/a
ꢀ2.60/b
ꢀ5.15/b
ꢀ1.86/a
ꢀ2.69/a
ꢀ4.90/a
ꢀ2.00/a
ꢀ2.68/b2
ꢀ5.01/b1
ꢀ1.98/a
ꢀ2.73/a
ꢀ4.92/b
LUMO/eV/orbital symm.
HOMO/eV/orbital symm.
HOMO ꢀ 1/eV/orbital
symm.
ꢀ5.54/b2
ꢀ5.40/b
ꢀ5.40/a
ꢀ5.15/a
ꢀ5.31/b3
ꢀ5.04/b
Table 4 HOMO and HOMO ꢀ 1 of the PQ-0A and PQ-0B using the
B3LYP functional and 6-31G* basis set under the geometry optimization
condition
Table 5 Singlet excitation energies, absorption wavelength and their
corresponding oscillator strength from TD-B3LYP calculations for the
PQ molecules in gas phase
PQ-0A
PQ-0B
l/nm
Eex/eV
Oscillator
Transition
PQ-1A
PQ-1B
498
351
337
655
523
427
365
558
397
667
587
476
430
611
534
426
676
628
492
456
2.49
3.53
3.68
1.89
2.37
2.90
3.40
2.22
3.12
1.86
2.11
2.61
2.88
2.03
2.31
2.91
1.83
1.97
2.52
2.72
0.9425
0.5875
1.9795
0.1033
0.6464
0.4830
0.7713
0.9225
1.0811
0.0431
0.5759
0.6721
0.5311
1.0321
0.1698
1.3236
0.0135
0.7467
0.8852
0.8772
HOMO / LUMO
HOMO / LUMO + 1
HOMO ꢀ 1 / LUMO + 1
HOMO / LUMO
HOMO ꢀ 1 / LUMO
HOMO / LUMO + 1
HOMO ꢀ 1 / LUMO + 1
HOMO / LUMO
PQ-2A
PQ-2B
HOMO ꢀ 1 / LUMO + 1
HOMO
HOMO / LUMO
HOMO ꢀ 1 / LUMO
HOMO / LUMO + 1
HOMO ꢀ 1 / LUMO + 1
HOMO / LUMO
HOMO ꢀ 1 / LUMO
HOMO ꢀ 1 / LUMO + 1
HOMO / LUMO
HOMO ꢀ 1 / LUMO
HOMO / LUMO + 1
HOMO ꢀ 1 / LUMO + 1
PQ-3A
PQ-3B
HOMO ꢀ 1
From both theoretical calculation and experimental results,
the lowest-lying intense absorptions for both series A and B
correspond very well with each other. As mentioned previously,
this absorption band is attributed to the HOMO / LUMO
transition for series A and HOMO ꢀ 1 / LUMO transition for
series B. From the molecular orbitals in Table 3, we can see that
the HOMO ꢀ 1 and LUMO of series B are very similar in shapes
and energies to the HOMO and LUMO of series A, respectively.
This striking resemblance thus accounts for the great similarity in
the lowest-lying intense absorptions between series A and B.
Using such a strategy to introduce different chromophores in
PQ molecules enables broader light absorption. This is proven by
the thin film UV-vis absorption shown in Fig. 2c, where the
molecules in series B can show strong light absorption up to
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 17798–17804 | 17801

View Article Online
Fig. 2 The (a) simulated Gaussian type absorption spectra for the PQ molecules in gas phase and experiment in (b) solution and (c) thin film.
ꢂ700 nm. Preliminary results suggest that polymerization
through the quinoid positions could produce efficient light
absorption up to 900 nm. Such broad absorption spectra is one
of the criteria for efficient harvesting of solar energy,26 suggesting
such polymers would be very promising materials for organic
solar cell applications. Investigations into such polymers are
underway and will be published separately.
2000 Series CHNS/O Analyzer. Differential scanning calori-
metry (DSC) was carried out under nitrogen on a DSC Q100
instrument from TA Instruments at a scanning rate of 10 ꢁC
minꢀ1. 2 cycles were performed, with the 1st cycle from 40 ꢁC to
300 ꢁC to 0 ꢁC and 2nd cycle from 0 ꢁC to 300 ꢁC to 0 ꢁC.
Thermogravimetric analysis (TGA) was carried out on a TA
Instrument Q500 Thermogravimetric Analyzer at a heating rate
of 20 ꢁC minꢀ1 up to 900 ꢁC. UV-vis absorption spectra were
recorded on a Lambda 900 Spectrometer from Perkin Elmer.
Cyclic voltammetry experiments were performed using a multi-
channel potentiostat (Model 1470E) from Solartron Analytical.
All CV measurements were recorded in CH2Cl2 with 0.1 M
Conclusions
In conclusion, we have synthesized and characterized a series of
PQ molecules with a systematic change in the substituents. We
have observed a series of complex conjugation effects from the
arms and substituents at the quinoidal positions, which explains
the electronic properties of the PQ molecules. By changing the
substituents on the arms, we can achieve strong light absorption
up to 700 nm for the small molecules and we believe absorption
at even longer wavelengths will be possible for conjugated
polymers containing these units. Computational modeling shows
agreement with the experimental results in the HOMO trend but
fails at times to predict the LUMO, possibly due to agglomera-
tion in the cyclic voltammetry experiment. Nonetheless, this
fundamental work allows better understanding of the electronic
structures of these systems, which is crucial in designing new
materials for organic electronics.
tetrabutylammonium hexafluorophosphate as
a supporting
electrolyte (scan rate of 100 mV sꢀ1). The experiments were
performed at room temperature with a conventional three elec-
trode configuration consisting of a platinum wire working elec-
trode, a gold counter electrode, and an Ag/AgCl in 3 M KCl
reference electrode. The measured potentials were converted to
the SCE (saturated calomel electrode with reduction potential of
ꢀ4.4 eV relative to vacuum).
Computational methods
Atomistic simulation, using density function theory (DFT) at
B3LYP27,28 (which includes the gradient corrected exchange and
correlation functionals along with the exact exchange) method
with double-z quality basis functions 6-31G* (augmented with
polarized function for all non-hydrogen atoms), was used to
optimize the geometry of the pyrazino quinoxaline molecules.
Geometry was fully relaxed and no symmetry constraints were
imposed during optimization using Gaussian 09 code29 with
a convergence criterion of 10ꢀ3 a.u. on the gradient and
displacement and 10ꢀ6 a.u. on energy and electron density.
Harmonic vibrational analyses showed no imaginary frequency,
indicating these structures are a local minimum.
Experimental section
Materials
All reagents were purchased from Sigma Aldrich and used
without further purification, unless otherwise stated. Column
chromatography was carried out with Merck silica-gel (230–
400 mesh) while thin layer chromatography (TLC) was per-
formed on Merck silica-gel 60 Al-backed plates (20 cm ꢃ 20 cm).
The theoretical UV-vis spectrum was calculated by the time-
dependent density functional theory (TD-DFT) approach. All of
the calculations were based on 6-31G(d) with Gaussian 09 soft-
ware package. For each molecule, 20 states are calculated. The
calculations are in the gas phase. The calculated dipole-allowed
absorptions associated with the oscillator strengths and their
assignments are given in Table 5. The simulated Gaussian type
Characterization
1H data were obtained on a Bruker DPX 400 MHz spectrometer
with chemical shifts referenced to CDCl3. Matrix assisted laser
desorption/ionization time-of-flight (MALDI-TOF) mass
spectra were obtained on a Shimadzu Biotech AXIMA-TOF.2
Elemental analysis was obtained via a Thermo Scientific Flash
17802 | J. Mater. Chem., 2011, 21, 17798–17804
This journal is ª The Royal Society of Chemistry 2011

View Article Online
absorption spectra (Fig. 2a) are plotted using half-width at half
height of 0.15 eV.
reaction mixture was cooled down to room temperature, poured
into water, and extracted with DCM. The organic layer was
washed with saturated NaHCO3 solution, dried over MgSO4 and
concentrated in vacuo. The crude was purified by column chro-
matography (hexane/DCM ¼ 1/1) to afford deep red solid with
Synthesis
1
2,3,7,8-Tetra(40-butoxy-10,4-biphen-1-yl)pyrazino[2,3-g]quinoxa-
line (PQ-1A). 120 mg of TAB (0.42 mmol), 471 mg of 1
(0.93 mmol) and 5 mg of IBX (2 mol%) were added to a round
bottom flask containing 20 mL of acetic acid. The reaction was
refluxed for 2 days. The reaction mixture was poured into water,
filtered and the solids were washed with water and ethanol.
Column chromatography was carried out using dichloromethane.
The resultant yellowish-orange solids were recrystallized using
toluene/ethanol to yield yellowish-orange crystals (278 mg, 61%
yield). 1H NMR (CDCl3): d [ppm] 1.00 (t, 12H, J ¼ 7.2 Hz), 1.52
(m, 8H), 1.80 (quintet, 8H, J ¼ 8.0 Hz), 4.02 (t, 8H, J ¼ 6.8), 6.99
(d, 8H, J ¼ 8.8 Hz), 7.58 (d, 8H, J ¼ 8.8 Hz), 7.61 (d, 8H, J ¼ 8.4
Hz), 7.75 (d, 8H, J ¼ 8.4 Hz), 9.07 (s, 2H). 13C NMR (CDCl3):
d [ppm] 14.0, 19.4, 31.5, 68.0, 115.1, 126.7, 128.3, 128.7, 130.6,
132.6137.2, 142.0, 155.0, 159.3. Anal. Calcd for C74H70N4O4: C,
82.34; H, 6.54; N, 5.19; O, 5.93. Found: C, 81.84; H, 6.50; N, 5.18.
MALDI-TOF-MS m/z: 1079.44 (M+); calcd. for C74H70N4O4 ¼
1079.37.
overall 46% yield (0.35 g). H NMR (CDCl3): d [ppm] 0.95 (m,
12H), 1.35 (m, 12H), 1.52 (sextet, 4H, J ¼ 7.6 Hz), 1.77 (m, 8H),
2.83 (t, 4H, J ¼ 7.6 Hz), 4.01 (t, 4H, J ¼ 6.4), 6.75 (d, 2H, J ¼ 3.6
Hz), 6.97 (d, 4H, J ¼ 7.6 Hz), 7.04 (d, 2H, J ¼ 4.0 Hz), 7.17 (d,
2H, J ¼ 3.2 Hz), 7.45 (d, 2H, J ¼ 3.6 Hz), 7.57 (m, 8H), 7.69 (d,
4H, J ¼ 7.2 Hz), 8.81 (s, 2H). 13C NMR (CDCl3): d [ppm] 14.5,
14.7, 19.9, 23.2, 29.4, 30.3, 30.9, 32.0, 32.2, 68.4, 115.5, 123.9,
125.4, 125.8, 127.1, 128.2, 128.8, 129.4, 131.1, 131.5, 132.1, 133.1,
134.8,137.5, 140.1, 140.3, 142.4, 143.4, 147.5, 148.2, 159.8. Anal.
Calcd for C70H70N4O2S4: C, 74.56; H, 6.26; N, 4.97; S, 11.37.
Found: C, 74.40; H, 6.44; N, 5.11; S, 11.15. MALDI-TOF-MS
m/z: 1128.99 (M+); calcd. for C70H70N4O2S4 ¼ 1127.59.
2,3-Bis(40-butoxy-10,4-biphen-1-yl)-7,8-bis(50-hexyl-20,5-bithien-
2-yl)-4,9-bis(thien-2-yl)pyrazino[2,3-g]quinoxaline (PQ-2B). The
reduced intermediate (0.2 g, 0.56 mmol) in 20 mL acetic acid
prepared from 8 according to our previous studies was trans-
ferred without purification via syringe to a round bottom flask
with a mixture of 1 (0.25 g, 0.50 mmol) and 2 (0.27 g, 0.50 mmol)
in 15 mL chloroform under N2. The reaction was stirred at 60 ꢁC
for 3 days, cooled down to room temperature and poured into
water. Extraction was carried out using dichloromethane. The
organic layer was washed with saturated NaHCO3 solution,
dried over MgSO4 and concentrated in vacuo. The crude was
purified by column chromatography (hexane/DCM ¼ 1/1) to
afford dark purple solid with overall 15% yield (0.11 g). 1H NMR
(CDCl3): d [ppm] 0.91 (t, 6H, J ¼ 7.2 Hz), 1.00 (t, 6H, J ¼ 7.6
Hz), 1.35 (m, 12H), 1.52 (m, 4H), 1.77 (m, 8H), 2.84 (t, 4H, J ¼
7.6 Hz), 4.02 (t, 4H, J ¼ 6.4), 6.76 (d, 2H, J ¼ 3.6 Hz), 6.98 (d,
4H, J ¼ 8.8 Hz), 7.05 (d, 2H, J ¼ 4.0 Hz), 7.17 (d, 2H, J ¼ 3.6
Hz), 7.42 (dd, 2H, J ¼ 4.0 & 5.0 Hz), 7.60 (m, 8H), 7.78 (dd, 2H,
J ¼ 5.2 & 0.8 Hz), 7.93 (d, 4H, J ¼ 8.4 Hz), 8.45 (dd, 2H, J ¼ 4.0
& 1.2 Hz). 13C NMR (CDCl3): d [ppm] 14.6, 14.8, 20.0, 23.3, 29.5,
31.0, 32.0, 32.3, 68.5, 115.6, 124.0, 125.4, 125.9, 126.9, 127.1,
127.9, 128.8, 130.9, 131.6, 132.3, 133.2, 135.0, 135.1, 136.6, 137.2,
137.5, 140.9, 142.4, 144.0, 145.4, 147.6, 152.5, 159.8. Anal. Calcd
for C78H74N4O2S6: C, 72.52; H, 5.77; N, 4.34; S, 14.89. Found:
C, 72.42; H, 5.89; N, 4.51; S, 14.78. MALDI-TOF-MS m/z:
1293.68 (M+ + H); calcd. for C78H74N4O2S6 ¼ 1291.84.
2,3-Bis(40-butoxy-10,4-biphen-1-yl)-7,8-bis(p-toluenesulfonamido)-
phenazine (4). A mixture of compound 3 (0.5 g, 0.99 mmol) and
SnCl2 (1.1 g, 5.9 mmol) suspended in a mixture of EtOH (15 mL)
ꢁ
and 37% HCl (10 mL) was heated at 85 C for 3 h. The reaction
mixture was cooled down to room temperature and poured into
water, neutralized with saturated NaHCO3 solution and extracted
with ethyl acetate. The organic layer was collected, dried over
anhydrous MgSO4 and filtered. The filtrate was stripped from
solvent to obtain yellow solid intermediate which was immediately
mixed with 1 (0.45 g, 0.90 mmol) in 20 mL acetic acid and heated to
85 ꢁC for 1 day. Then the reaction mixture was poured into water,
and extracted with dichloromethane. The organic layer was washed
with saturated NaHCO3 solution, dried over MgSO4 and concen-
trated in vacuo. The crude was purified by column chromatography
(hexane/EA ¼ 1/1) to afford orange red solid with overall 70% yield
(0.63 g). 1H NMR (CDCl3): d [ppm] 0.99 (t, 6H, J ¼ 7.2 Hz), 1.51
(sextet, 4H, J ¼ 7.6), 1.79 (quintet, 4H, J ¼ 8.0 Hz), 2.35 (s, 6H),
3.99 (t, 4H, J ¼ 6.4 Hz), 6.95 (d, 4H, J ¼ 8.4 Hz), 7.23 (d, 4H, J ¼
7.6 Hz), 7.50 (m, 12H), 7.73 (m, 4H), 7.96 (s, 2H). 13C NMR
(CDCl3): d [ppm] 14.5, 19.9, 22.3, 32.0, 68.4, 115.5, 123.6, 127.1,
128.4, 130.5, 130.9, 132.9, 133.3, 135.8, 137.2, 139.7, 142.1, 145.2,
154.0, 159.8. Anal. Calcd for C54H52N4O6S2: C, 70.72; H, 5.71; N,
6.11; S, 6.99. Found: C, 70.53; H, 5.76; N, 6.29; S, 6.86. MALDI-
2,3,7,8-Tetra(50-hexyl-20,5-bithien-2-yl)pyrazino[2,3-g]quinoxa-
line (PQ-3A). 120 mg of TAB (0.43 mmol), 516 mg of 2 (0.93
mmol) and 5 mg of IBX (2 mol%) were added to a round bottom
flask with 20 mL of acetic acid. The reaction was refluxed for 2
days. The reaction mixture was poured into water, filtered and
the solids were washed with water and ethanol. Column chro-
matography was carried out using hexane/dichloromethane. The
resultant dark purple solids were recrystallized using toluene/
TOF-MS m/z: 914.09 (Mꢀ ꢀ H); calcd. for C54H52N4O6S2
¼
917.14.
2,3-Bis(40-butoxy-10,4-biphen-1-yl)-7,8-bis(50-hexyl-20,5-bithien-
2-yl)pyrazino[2,3-g]quinoxaline (PQ-2A). Compound (4) (0.5 g,
0.55 mmol) was added to 10 mL conc. H2SO4 and heated to 80 ꢁC
for 2 h. The reaction mixture was cooled down to room
temperature, poured into ice water and filtered. The solid was
washed with saturated NaHCO3 solution thoroughly followed
by deionized H2O, EtOH and hexane to afford purple red solid
intermediate which was immediately mixed with 2 (0.27 g, 0.49
mmol) in 20 mL acetic acid and heated at 85 ꢁC for 3 days. The
1
ethanol to yield dark purple crystals (298 mg, 60% yield). H
NMR (CDCl3): d [ppm] 0.91 (t, 12H, J ¼ 6.8 Hz), 1.37 (m, 24H),
1.71 (quintet, 8H, J ¼ 8.0 Hz), 2.82 (t, 8H, J ¼ 7.6), 6.73 (d, 4H,
J ¼ 3.6 Hz), 7.02 (d, 4H, J ¼ 4.0 Hz), 7.15 (d, 4H, J ¼ 3.6 Hz),
7.41 (d, 4H, J ¼ 3.6 Hz), 8.57 (s, 2H). 13C NMR (CDCl3): d [ppm]
14.3, 22.8, 28.9, 30.4, 31.7, 123.4, 124.9, 125.3, 127.0, 131.5,
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 17798–17804 | 17803

View Article Online
17 H. Li, T. L. Tam, Y. M. Lam, S. G. Mhaisalkar and A. C. Grimsdale,
Org. Lett., 2011, 13, 46–49.
18 F. De Proft and P. Geerlings, Chem. Rev., 2001, 101, 1451–1464.
19 H. Wang, Y. Wen, X. Yang, Y. Wang, W. Zhou, S. Zhang, X. Zhan,
Y. Liu, Z. Shuai and D. Zhu, ACS Appl. Mater. Interfaces, 2009, 1,
1122–1129.
134.4139.8, 140.1, 142.8, 147.0, 147.5. Anal. Calcd for
C66H70N4S8: C, 67.42; H, 6.00; N, 4.76; S, 21.82. Found: C,
69.63; H, 6.05; N, 4.72; S, 21.89. MALDI-TOF-MS m/z: 1175.55
(M+); calcd. for C66H70N4S8 ¼ 1175.81.
20 T. L. Tam, H. Li, F. Wei, K. J. Tan, C. Kloc, Y. M. Lam,
S. G. Mhaisalkar and A. C. Grimsdale, Org. Lett., 2010, 12, 3340–
3343.
Acknowledgements
This work was supported by Robert Bosch (SEA) Pte Ltd and by
A*STAR (SERC grant M 47070020).
21 G. W. H. Cheeseman, J. Chem. Soc., 1962, 1170–1176.
22 M. Kikuchi, H. Kurosu and I. Ando, J. Mol. Struct., 1992, 269, 183–
195.
23 D. Q. Andrews, G. C. Solomon, R. H. Goldsmith, T. Hansen,
M. R. Wasielewski, R. P. V. Duyne and M. A. Ratner, J. Phys.
Chem. C, 2008, 112, 16991–16998.
References
1 J. Roncali, Macromol. Rapid Commun., 2007, 28, 1761–1775.
2 G. Brocks and A. Tol, J. Phys. Chem., 1996, 100, 1838–1846.
3 A. Ajayaghosh, Chem. Soc. Rev., 2003, 32, 181–191.
4 C. Kitamura, S. Tanaka and Y. Yamashita, Chem. Mater., 1996, 8,
570–578.
24 T. Markussen, R. Stadler and K. S. Thygesen, Nano Lett., 2010, 10,
4260–4265.
25 G. C. Solomon, D. Q. Andrews, R. H. Goldsmith, T. Hansen,
M. R. Wasielewski, R. P. V. Duyne and M. A. Ratner, J. Am.
Chem. Soc., 2008, 130, 17301–17308.
5 C.-L. Pai, C.-L. Liu, W.-C. Chen and S. A. Jenekhe, Polymer, 2006,
47, 699–708.
26 X. Wang, E. Perzon, J. L. Delgado, P. D. L. Cruz, F. Zhang,
€
F. Langa, M. Andersson and O. Inganas, Appl. Phys. Lett., 2004,
6 W.-C. Wu and W.-C. Chen, J. Polym. Res., 2006, 13, 441–449.
7 Y. Fu, W. Shen and M. Li, Polymer, 2008, 49, 2614–2620.
8 A. P. Zoombelt, M. Fonrodona, M. G. R. Turbiez, M. M. Wienk and
R. A. J. Janssen, J. Mater. Chem., 2009, 19, 5336–5342.
9 Y.-Y. Fang, Y.-M. Guo, M. Du, H. Liu, X.-B. Leng and
R.-H. Zhang, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2001,
57, o380–0382.
10 M. Bellizzi, G. Crundwell, M. Zeller, A. D. Hunter and B. McBurney,
Acta Crystallogr., Sect. E: Struct. Rep. Online, 2006, 62, o5249–o5251.
11 X. Wang, Y. Zhou, T. Lei, N. Hu, E.-Q. Chen and J. Pei, Chem.
Mater., 2010, 22, 3735–3745.
12 S. Miao, M. D. Smith and U. H. F. Bunz, Org. Lett., 2006, 8, 757–760.
13 A. Neels and H. Stoeckli-Evans, Acta Crystallogr., Sect. E: Struct.
Rep. Online, 2006, 62, o4979–o4980.
14 F. Zhang, J. Bijleveld, E. Perzon, K. Tvingstedt, S. Barrau,
O. Inganas and M. R. Andersson, J. Mater. Chem., 2008, 18, 5468–
5474.
85, 5081–5083.
27 A. D. Becke, J. Chem. Phys., 1993, 98, 5648.
28 C. Lee, W. Yang and R. G. Parr, Phys. Rev. B, 1988, 37, 785.
29 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li,
H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng,
J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta,
F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin,
V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari,
A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi,
N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross,
V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts,
R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli,
J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski,
G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich,
15 M. Mastalerz, V. Fischer, C.-Q. Ma, R. A. J. Janssen and P. Bauerle,
Org. Lett., 2009, 11, 4500–4503.
16 E. Wang, L. Hou, Z. Wang, S. Hellstrom, W. Mammo, F. Zhang,
O. Inganas and M. R. Andersson, Org. Lett., 2010, 12, 4470–4473.
€
A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz,
J. Cioslowski and D. J. Fox, Gaussian 09 Revision, Gaussian, Inc.,
Wallingford CT, 2009.
17804 | J. Mater. Chem., 2011, 21, 17798–17804
This journal is ª The Royal Society of Chemistry 2011