Rational design, synthesis, and optical properties of film-forming, near-infrared absorbing, and fluorescent chromophores with multidonors and large heterocyclic acceptors

Article abstract of DOI:10.1002/chem.200900891

A new series of film-forming, low-bandgap chromophores (1a,b and 2a,b) were rationally designed with aid of a computational study, and then synthesized and characterized. To realize absorption and emission above the 1000 nm wavelength, the molecular desig

Full text of DOI:10.1002/chem.200900891

DOI: 10.1002/chem.200900891
Rational Design, Synthesis, and Optical Properties of Film-Forming, Near-
Infrared Absorbing, and Fluorescent Chromophores with Multidonors and
Large Heterocyclic Acceptors
Min Luo,[a, b] Hooman Shadnia,[c] Gang Qian,[a, b] Xiaobo Du,[a, b] Dengbin Yu,[a]
Dongge Ma,[a] James S. Wright,[c] and Zhi Yuan Wang*[a, c]
Abstract: A new series of film-forming,
low-bandgap chromophores (1a,b and
2a,b) were rationally designed with aid
of a computational study, and then syn-
thesized and characterized. To realize
absorption and emission above the
1000 nm wavelength, the molecular
design focuses on lowering the LUMO
level by fusing common heterocyclic
units into a large conjugated core that
acts an electron acceptor and increas-
ing the charge transfer by attaching the
multiple electron-donating groups at
the appropriate positions of the accept-
or core. The chromophores have
bandgap levels of 1.27–0.71 eV, and ac-
cordingly absorb at 746–1003 nm and
emit at 1035–1290 nm in solution. By
design, the relatively high molecular
weight (up to 2400 gmolÀ1) and non-
coplanar structure allow these near-in-
frared (NIR) chromophores to be read-
ily spin-coated as uniform thin films
and doped with other organic semicon-
ductors for potential device applica-
tions. Doping with [6,6]-phenyl-C61 bu-
tyric acid methyl ester leads to a red
shift in the absorption only for 1a and
2a. An interesting NIR electrochrom-
ism was found for 2a, with absorption
being turned on at 1034 nm when elec-
trochemically switched (at 1000 mV)
from its neutral state to a radical
cation state. Furthermore,
a large
Stokes shift (256–318 nm) is also
unique for this multidonor–acceptor
type of chromophore, indicating a sig-
nificant structural difference between
the ground state and the excited state.
Photoluminescence of the film of 2a
was further probed at variable temper-
atures and the results strongly suggest
that the restriction of bond rotations
certainly helps to diminish non-radia-
tive decay and thus enhance the lumi-
nescence of these large chromophores.
Keywords: absorption
·
chromo-
phores · films · heterocyclic recep-
tors · near-infrared · photolumines-
cence
Introduction
A large number of the donor–acceptor (D–A) type of conju-
gated organic compounds, oligomers, and polymers having
bandgap energies between 1.75 and 3.1 eV are known and
typically absorb or emit in the visible spectral region.[1] The
design and realization of low-bandgap organic chromo-
phores (e.g., 1.19–0.56 eV or 800–1600 nm) are of current in-
terest, owing to their potential applications such as fluores-
cent contrast agents for bio-imaging in the near-infrared
(NIR) spectral region,[2] NIR light-emitting diodes for infor-
mation-secured display and background lighting,[3] NIR pho-
todetectors,[4] and NIR photovoltaic devices.[5] Besides NIR
absorption and fluorescence, other stimulus-responsive
properties such as NIR electrochromism have also attracted
a great deal of interest for potential use in devices for mod-
ulating NIR light (e.g., at 1310 and 1550 nm).[6]
[a] M. Luo, G. Qian, X. Du, D. Yu, Prof. D. Ma, Prof. Z. Y. Wang
State Key Laboratory of Polymer Physics and Chemistry
Changchun Institute of Applied Chemistry
Chinese Academy of Sciences
Changchun 130022, P. R. China
Fax : (+86)1-613-520-2316
[b] M. Luo, G. Qian, X. Du
Graduate School of Chinese Academy of Sciences
Changchun 130022 (P. R. China)
[c] H. Shadnia, Prof. J. S. Wright, Prof. Z. Y. Wang
Department of Chemistry, Carleton University
1125 Colonel By Drive, Ottawa, Ontario, K1S 5B6 (Canada)
E-mail: wayne_wang@carleton.ca
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900891.
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FULL PAPER
Among various types of NIR chromophores, much effort
has been directed toward the design and synthesis of chro-
mophores composed of powerful electron donor (D) and ac-
ceptor (A) units for use as either active components in devi-
ces or building blocks for the synthesis of oligomers and
polymers with a wide range of optoelectronic properties.
Andersson et al. systematically studied the D–A and D-A-D
types of low bandgap polymers.[7] Their work has clearly
demonstrated that linear polymers can be designed to
absorb in the visible and NIR spectral region or over the
solar spectrum and are promis-
other way to further narrow the bandgap of the D–A molec-
ular systems is to extend the p-conjugation of the known ac-
ceptors and thus to induce a more efficient intramolecular
charge transfer (CT).[9] There is a group of well-known elec-
tron-deficient heterocyclics, such as hexaazatriphenylene
(HAT),[10]
quinoxalino[2’,3’,9,10]phenanthroACHTUNGTRENNNUG
benzene-fused
HAT
[4,5-abc]phenazines
derivatives,[11]
and
(PPP);[12] however, none of them are able to yield NIR-ab-
sorbing chromophores. For example, PPP-Ph and HAT-Ph
(Scheme 1) were reported to have the bandgaps of 2.78 and
ing candidates for use in single
or tandem solar cells. With re-
spect to small low-bandgap
molecules, we recently reported
a series of NIR photolumines-
cent and electroluminescent
chromophores containing ben-
zobis(thiadiazole) (BBT) as a
stronger electron acceptor,
rather than the commonly used
benzothiadiazole (BT).[8] As the
LUMO level of these chromo-
phores is lowered due to the
presence of a strong acceptor of
BBT, the absorption and emis-
sion are typically pushed into
the NIR region (>800 nm).
However, any strong elec-
Scheme 1. Computation-aided rational design of low-bandgap chromophores with large heterocyclic cores as
an acceptor with low LUMO levels.
tronic interaction in the D–A
type of conjugated molecular
systems does not render them
either strong electron donors or acceptors. In practice, D
and A components are often covalently linked by a conju-
gated spacer (D-p-A), thus favoring stronger electronic cou-
pling and lowering the bandgap. Nevertheless, there is a re-
alistic limitation to the availability of very strong acceptors
and donors. Alternative approaches to the design of low-
bandgap molecular systems are urgently needed. In this
work, we intend to demonstrate another approach to low-
bandgap chromophores with a low LUMO level by using a
large heterocyclic acceptor with extended conjugation and
as well by attaching multiple electron-donating groups at
the appropriate positions of the acceptor core. Herein, we
report the design, synthesis, and characterization of a new
series of multi-armed disk-like D–A type of NIR chromo-
phores having a large heterocyclic acceptor as a core. In ad-
dition, the HOMO and LUMO levels of the new chromo-
phores (1a,b and 2a,b) and the chromophores having struc-
turally similar but less conjugated heterocyclic cores are cal-
culated and compared.
2.85 eV, or HOMO/LUMO energies of 5.92/3.25 and 6.37/
3.70 eV, respectively.[12] The calculations indicate that PPP-
Ph and HAT-Ph have a similar bandgap, 3.30 and 3.20 eV,
respectively (Scheme 1). However, by fusing a thiadiazole
unit onto PPP and HAT cores, our calculations show a large
decrease in the LUMO levels (3.50 and 3.80 eV from 2.70
and 3.10 eV, respectively), resulting in a significant decrease
in the bandgap for hypothetic PPP-BT (2.30 eV) and HAT-
BT (2.20 eV).
Encouraged by computation-aided design, we intend to
fuse some common heterocyclics together as a route to new
electron acceptors. In particular, we can combine benzothia-
diazole and the PPP and HAT units to form a new p-conju-
gated heterocyclic as an acceptor with a lower LUMO level.
At the same time, the donor groups are placed in a less con-
gested position, unlike the twisted vicinal phenyl groups in
HAT-Ph, to ensure a better conjugation and thus a more ef-
ficient D–A transfer. Therefore, new chromophores would
have a lower bandgap and become NIR-absorbing and fluo-
rescent. In this work, we report a new series of multidonor–
acceptor type NIR chromophores having large HAT-BT and
PPP-BT cores as the new acceptor (Scheme 1). The triangu-
lar HAT-BT core has extended p-conjugation and can be at-
tached to up to six electron donors. The length of the
planar, rod-like PPP-BT core is longer than that of the
Results and Discussion
Design and synthesis: Considering the limited availability of
the known electron-withdrawing groups as an acceptor, an-
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8903

Z. Y. Wang et al.
HAT-BT core, but its overall
electron delocalization may be
interrupted by the tetrahydro-
pyrene resonance structure. The
selection of the donors is quite
flexible because many electron-
donating groups are known. In
addition, the donor groups are
expected to impart good solu-
bility and film-forming proper-
ties to the designed chromo-
phores owing to their twisted
and branched structures (TPA
and long alkyl groups on fluo-
rene), the large molecular size,
and the relatively high molecu-
lar weights.
Scheme 2. Synthesis of NIR chromophores 1a,b and 2a,b.
Table 1. Optical and electrochemical properties and bandgap data of chromophores.
Scheme 2 outlines the syn-
thetic routes to four representa-
tive chromophores (1a,b and
2a,b). The starting materials
(3a,b) were obtained by a Stille
coupling reaction between 4,7-
dibromo-5,6-dinitro-2,1,3-ben-
zothiadiazole and tributyltin de-
rivatives of the corresponding
donors (a and b, Scheme 2).[8]
Reduction of the nitro groups
Stokes shift HOMO/LUMO[eV] Bandgap[eV]
abs
max
abs
max
abs
max
PL
max
l
[nm][a]
l
[nm][b]
l
[nm][c]
l
[nm][a]
[e]
[nm][a]
exptl[d]
calcd
E[d] Eop
Ecalcd
1a 932, 665
1b 1003, 680 1040, 702
2a 746
2b 844
971, 685
1035, 713 1250
1054, 711 1290
798
856
318
287
279
256
4.97/4.00 4.30/3.30 0.97 1.01 1.00
5.15/4.44 5.00/3.90 0.71 0.97 1.10
5.00/3.73 4.80/3.30 1.27 1.40 1.50
5.08/3.86 5.60/3.60 1.22 1.24 2.00
772
864
1035
1100
[a] Measured in toluene with a concentration of 1ꢁ10À5 m. [b] Measured as a film. [c] Measured with PCBM-
doped films (1:1 w/w). [d] From electrochemical data,
E
(HOMO)=ÀACHTNUGTREN(NUGN Eox+4.44)[eV], EACHTUNTGREG(NNUN LUMO)=
À(Ered+4.44)[eV]. [e] Optical bandgap (op)=1240/loabnsset [eV]
in 3a,b was best carried out in acetic acid at 808C using iron
powders to afford diamines 4a and 4b in good yields. Subse-
quent condensation reactions with hexaketocyclohexane and
pyrene-4,5,9,10-tetraones afforded chromophores 1a,b and
2a,b in 40–60% yields, respectively.
Bandgap energy and computational study: The HOMO,
LUMO, and bandgap levels of these compounds, as deter-
mined from their electrochemical data, coincide well with
the values calculated from their absorption data and from
theoretical calculations (Table 1). Although the HOMO
levels of all the compounds are ꢀ5.00 eV, their LUMO
levels are quite different. This difference is clearly related to
the delocalization of p-electrons in the HAT-BT and PPP-
BT cores. As a result, the HAT-BT series have a lower
bandgap (0.71 and 0.97 eV) than the PPP-BT series (1.22
and 1.27 eV). Compound 1b has the lowest bandgap
(0.71 eV) among all, which is believed to be mainly due to
better conjugation and an efficient CT process between the
fluorene donor and the HAT-BT acceptor via the thiophene
bridging unit. In comparison, 2a has a bandgap of 1.27 eV,
owing to its relatively higher LUMO (3.73 eV).
To compare the delocalizing effects, Figure 1 shows the
calculated LUMOs of the reference compounds along with
the novel compounds. The structures and orbitals in these
drawings were rendered using the same scale and isovalues
(0.01 a.u) to allow a visual comparison of the MOs. When
comparing PPP-BT vs. PPP-Ph, it is clear that the benzo-
thiadiazole moiety of the former compound gives an addi-
tional delocalization advantage. Furthermore, steric interfer-
ence between the two neighboring phenyl groups in PPP-Ph
forces mutual twisting, which diminishes conjugation to the
central unit. In PPP-BT, however, the phenyl group is less
twisted relative to the core. Accordingly, this design feature
allows better delocalization or more efficient charge trans-
Characterization: The final products were characterized by
spectroscopic means, and their structures are consistent with
the spectral data (see the Supporting Information). Owing
to the long alkyl groups and the twisted donor moieties rela-
tive to the core, these chromophores are quite soluble in
common organic solvents such as chloroform, tetrahydrofur-
an, toluene, and xylene. Furthermore, owing to their high
molecular weights (up to 2400 gmolÀ1) and non-coplanar
structures, the uniform thin films can be readily obtained by
spin-coating. The onset temperatures for 5% weight loss in
nitrogen, as assessed by thermogravimetry, are 3998C and
3658C for 1a and 1b, and 3958C and 3888C for 2a and 2b,
respectively (Figure S1).
Electrochemical properties: The electrochemical properties
of the chromophores were investigated by cyclic voltamme-
try (CV) in dichloromethane (DCM) with a concentration
of 10À3 m (Figure S2). The chromophores are all electro-
chemically active and have two or three quasi-reversible oxi-
dation and reduction waves (Table 1). The reduction occurs
at the heterocyclic cores of these chromophores, while the
two reversible oxidation waves of 1a and 2a are attributed
to the oxidation of the triphenylamine (TPA) moiety.
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Design, Synthesis and Optical Properties of Film-Forming Chromophores
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Figure 2. Normalized absorption and PL spectra of 1a,b (top) and 2a,b
(bottom) in toluene (10À5 m).
ing a greater electron-accepting ability of the HAT-BT core
in 1a,b than the PPP-BT core in 2a,b. The absorption at rel-
atively short wavelengths for 2a,b could be related to the
less well-delocalized PPP-BT core due to the resonance
structure of tetrahydropyrene. There are two low-energy
bands observed for the HAT-BT based chromophores 1a,b
(Figure 2). On changing the donor from TPA to the thio-
phene-bridged fluorene (1a vs 1b), only the band at the lon-
gest wavelength showed a significant red shift (D71 nm),
while the absorption at 665 nm for 1a was only slightly red-
shifted in 1b (680 nm). Thus, the bands at 665 and 680 nm
for 1a,b seem to arise from the same or a quite similar CT
transition between the HAT-BT core and an electron-donat-
ing moiety, although the exact structural origin could not be
unambiguously assigned.
Figure 1. Comparison of LUMO spatial distribution (isovalue=0.01 a.u.).
fer, as evidenced by extended delocalization to the electron-
donating moieties of 2a and 2b, in particular even to the
fluorene moiety of 2b. A comparison of the hexaazatriphe-
nylene series shows again that steric interference of vicinal
phenyl groups in HAT-Ph does not allow perfect delocaliza-
tion. The delocalization in compounds 1a and 1b extends
further into the side chains, creating large chromophores
with the smallest bandgaps (Table 1).
Absorption and photoluminescence in solution: The absorp-
tion and photoluminescence of all chromophores were re-
corded in toluene (Figure 2). The absorption bands in the
UV/Vis region below 525 nm are attributed to p–p* and n–
p* transitions of the conjugated aromatic segments
(Table 1). The absorption bands above 600 nm are attributed
to intramolecular CT transitions between the donor groups
and the central acceptor cores of these chromophores. The
CT bands (932 and 1003 nm) of 1a,b appear at longer wave-
lengths than those of 2a,b (746 and 844 nm), further indicat-
The NIR electrochromic properties of these chromo-
phores were further investigated using an OTTLE cell with
a concentration of 10À5 m in DCM containing 0.1m Bu4NPF6.
Noticeably, compound 2a is NIR-electrochromic because a
new absorption appears at 1034 nm when being electro-
chemically switched (at 1.0 and 1.5 V) from its neutral state
to the radical cation state (Figure S3). This “turn-on” NIR
absorption is probably attributable to the mixed-valence
state or charge transfer within the TPA-phenylene-TPA
moiety, in which one of the nitrogen centers of TPA is oxi-
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Z. Y. Wang et al.
dized. On further oxidation at a higher potential (e.g.,
2.5 V), the bandgap peak at 750 nm and the NIR peak at
1034 nm almost disappeared (Figure S3).
The photoluminescence of these chromophores in toluene
are all above 1000 nm and vary according to their bandgap
energies (Figure 2). Chromophores 1a and 1b emit at 1250
and 1290 nm on excitation at 932 and 900 nm, respectively.
Compounds 2a and 2b also emit above 1000 nm (1035 and
1100 nm) on excitation at 746 and 844 nm, respectively.
Fluorescence quantum yields (F) of these chromophores in
toluene were measured relative to IR-125 (F=0.13 in
DMSO) and found to be lower (0.83–1.97%, see the Sup-
porting Information) than those of the D-p-A-p-D type of
NIR chromophores we previously prepared.[8]
It is interesting to note that the identical PL spectra with
a maximum peak at 1250 nm were obtained for 1a, regard-
less of whether excitation was at 665, 694, or 932 nm. A very
large Stokes shift (256–318 nm) was also observed for these
chromophores, indicating a significant structural difference
between the ground state and the excited state. The large
Stokes shift observed for these new NIR chromophores im-
plies a potential application in bio-imaging because the
large separation of absorption and NIR emission maxima of
NIR dyes would result in a far greater signal-to-noise ratio
in biomolecular imaging and would allow greater depth pen-
etration in tissues.
The absorption and emission spectra of the four com-
pounds were also recorded at concentrations varying from
10À6 to 10À4 mol/L. They did not show any changes in the
peak positions (e.g., for 2a in Figure S4). The results indi-
cate that these chromophores, despite having a large p-con-
jugated flat core, do not readily form excimers or strong in-
termolecular CT in solution and have a rather weak dipole
effect.
Figure 3. Normalized absorption and PL spectra of 2a in toluene (10À5 m),
in a film and in a film doped with PCBM (1:1 by weight) (top). Variable-
temperature PL spectra of the as-prepared film of 2a (from 77.2 to
487.2 K at a heating rate of 28CminÀ1) (bottom).
has a crystalline nature. Only compound 2a shows one non-
reversible solid-to-solid transition at 385 K during the first
DSC heating scan up to 523 K (Figure S5) and no other
transitions during the subsequent cooling and the second
heating–cooling process, which implies packing or ordering
in the original as-prepared solids due to its fairly large twist-
ed structure (TPA groups relative to the planar PPP-BT
core). Because the twisted intramolecular charge transfer
may greatly affect the emission properties, the photolumi-
nescence of the 2a film was further probed at variable tem-
peratures. The PL measurement of the as-prepared spin-
coated film was carried out from 77.2 to 487.2 K (Figure 3
Photoluminescence in solid films: The optical properties of
these chromophores were further characterized in solid
films. The films were spin-coated onto a glass substrate from
a chlorobenzene solution (concentration 10 mgmLÀ1) at a
speed of 1000 rpm. Typically, a 20–30 nm bathochromic shift
in absorption was observed for the films compared to those
in toluene (Table 1). Considering the potential photovoltaic
applications of these chromophores, the films doped with
[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) (1:1 by
weight) were also studied. The chromophores containing the
TPA donor showed a noticeable red-shift in absorption:
64 nm for 1a and 26 nm for 2a (Table 1 and Figure 3
(top)).
ACHTUNGTREN(NUGN bottom)), and the PL intensity was found to steadily de-
This red shift can be attributed to intermolecular interac-
tions between PCBM and the TPA moieties. Because the
calculation also indicates that delocalization extends more
to the fluorene donor than to the TPA donors, the TPA
donor is more available for intermolecular CT with the
dopant. As expected, the films of 2a with or without the
crease at a rate of 0.12%KÀ1 (see the Supporting Informa-
tion) as the temperature rose from 77.2 to 407.2 K. Howev-
er, when the solid-to-solid transition temperature of 417.2 K
was reached, the PL intensity suddenly increased nearly
two-fold and then continued to decrease at the same rate of
0.12%KÀ1 (Figure 3
ACTHNUTRGNEU(GN bottom) on the right). After cooling
PCBM dopant (Figure 3
spectra.
As anticipated from their chemical structures, the DSC
study indicates that none of these chromophore compounds
(top)) show nearly the same PL
rapidly at the end of the first heating process, the PL inten-
sity was about twice that of the as-prepared sample at
77.2 K (Figure S6). During subsequent heating from 77.2 to
427.2 K, the PL intensity decreased twice as fast compared
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Design, Synthesis and Optical Properties of Film-Forming Chromophores
FULL PAPER
to the first heating process at the rate of 0.24%KÀ1 (Fig-
ure S6). Although the exact packing or ordering of these
large molecules in the solid state is unclear, the variable-
temperature PL results suggest strongly that the solid-state
morphology can play an important role in the PL properties
of these large chromophores.
3.82–3.80 (m, 24H), 1.74 (m, 24H), 1.56 (m, 24H), 1.40–1.20 (m, 96H),
0.88 ppm (t, 24H, J=7.20 Hz); 13C NMR (100 MHz, 1008C, C2D2Cl4): d=
157.72, 155.86, 150.62, 144.72, 142.09, 139.97, 136.44, 131.21, 128.98,
128.59, 120.23, 117.51, 117.18, 70.38, 33.24, 31.01, 30.81, 30.60, 27.60,
˜
23.99, 15.30 ppm; IR: n=3039, 2922, 2852, 1594, 1505, 1468, 1448, 1377,
1318, 1281, 1238, 1194, 1153, 1107, 1029, 978, 918, 825, 721, 697, 605,
538 cmÀ1; elemental analysis calcd for C228H276N18O12S3: C 76.99, H 7.82,
N 7.09; found: C 77.05, H 7.99, N 7.10.
Synthesis of chromophore 1b: This was synthesized according to the pro-
cedure used to prepare 1a. The crude product was purified by column
chromatography (silica gel, dichloromethane/petroleum ether 1:4 v/v) to
afford the product as a dark-green solid (40%). 1H NMR (400 MHz,
1008C, C2D2Cl4): d=9.23 (s, 6H), 8.61 (s, 12H), 8.46 (s, 6H), 8.41–8.30
(br, 36H), 2.00 (s, 24H), 1.15–0.98 (br, 144H), 0.78 (d, 36H) ppm;
13C NMR (100 MHz, 1008C, C2D2Cl4): d=154.29, 153.74, 153.19, 152.81,
145.24, 142.71, 142.25, 139.44, 138.61, 137.97, 134.96, 128.56, 128.30,
126.91, 126.07, 124.66, 124.07, 121.97, 121.49, 121.29, 56.74, 41.76, 33.17,
Conclusions
In conclusion, a new series of NIR-absorbing and NIR-fluo-
rescent chromophores have been successfully designed and
synthesized. The computer-aided molecular design focused
on lowering the LUMO level of the acceptor by the use of a
large heterocyclic with extended conjugation and by an in-
crease in the donor–acceptor charge transfer by attaching
multiple electron-donating groups at the appropriate posi-
tions of the acceptor core. Computer models were used to
calculate the energies and spatial delocalization of the fron-
tier molecular orbitals to aid and support the rational
design. Theoretical and experimental data confirm that the
small bandgaps of these chromophores are mainly attributed
to the low LUMO levels of the acceptors and efficient mul-
tidonor–acceptor charge transfer. The NIR-absorbing and
fluorescent properties, along with a very large Stokes shift
and film-forming ability, make this type of chromophore po-
tentially useful for applications in NIR photovoltaics, light-
emitting diodes and bio-imaging. Our ongoing work with
these new materials is focused on further in-depth studies of
NIR photovoltaic devices.
˜
31.48, 30.63, 25.54, 23.92, 15.28 ppm; IR: n=3061, 2924, 2851, 1605, 1522,
1502, 1465, 1407, 1377, 1268, 1240, 1175, 1134, 1100, 1065, 1005, 915, 825,
802, 778, 737, 630, 538, 443 cmÀ1
; elemental analysis calcd for
C222H258N12S9: C 78.81, H 7.69, N 4.97; found: C 78.90, H 7.79, N 5.15.
Synthesis of chromophore 2a: Pyrene-4,5,9,10-tetraones (0.262 g,
1.0 mmol), 4a (2.680 g, 2.3 mmol), and AcOH (50 mL) were added to a
250 mL round-bottomed flask equipped with a magnetic stirrer. The reac-
tion mixture was heated at reflux for 48 h; it was initially orange in color,
then turned red. After 12 h, the color was still red and there was some
solids in the flask. Column chromatography (DCM/PE=1:1 v/v) gave the
pure products as
(300 MHz, CDCl3):
a
d
dark-green powder (1.523 g, 60%). 1H NMR
=9.26 (d, 4H, J=7.77 Hz), 8.01 (d, 8H, J=
8.67 Hz), 7.95 (t, 2H, J=7.89 Hz); 7.28 (d, 16H, J=9.00 Hz), 7.21 (d,
8H, J=7.92 Hz), 6.92 (d, 16H, J=6.27 Hz), 3.97 (t, 16H, J=6.45 Hz),
1.79 (m,16H), 1.44 (m, 16H), 1.34–1.30 (m, 64H), 0.87 ppm (t, 24H, J=
6.60 Hz); 13C NMR (100 MHz, 1008C, C2D2Cl4): d=157.80, 154.76,
150.70, 144.65, 142.35, 139.31, 135.77, 132.05, 130.73, 130.63, 130.53,
128.79, 120.28, 117.64, 70.49, 33.23, 31.03, 30.81, 30.60, 27.61, 23.98,
15.30 ppm; IR: n˜ =3039, 2924, 2854, 1597, 1505, 1457, 1419, 1365, 1319,
1262, 1238, 1194, 1166, 1145, 1110, 1029, 990, 910, 866, 827, 810, 722, 699,
645, 624, 605, 586, 539, 527, 499, 448, 411 cmÀ1; elemental analysis calcd
for C164H190N12O8S2: C 78.12, H 7.60, N 6.67; found: C 77.94, H 7.71, N
6.77.
Experimental Section
Synthesis of chromophore 2b: This was synthesized according to the pro-
cedure used to prepare 2a. The crude product was purified by column
chromatography (silica gel, dichloromethane: petroleum ether 1:4 v/v) to
afford the product as a brown powder (40%). 1H NMR (400 MHz,
1008C, C2D2Cl4): d=9.83 (d, 2H, J=6.00 Hz), 9.07 (s, 4H), 8.24 (t, 2H,
J=7.2 Hz), 7.85 (s, 8H), 7.77–7.70 (m, 8H), 7.54 (s, 4H), 7.38 (s, 4H),
7.33 (s, 8H), 2.14 (s,16H), 1.16–1.14 (br, 96H), 0.77 (s, 24H) ppm;
13C NMR (100 MHz, 1008C, C2D2Cl4): d=153.43, 153.29, 152.80, 151.76,
144.23, 142.85, 142.26, 138.04, 137.62, 136.92, 134.95, 131.61, 128.68,
128.43, 126.66, 124.74, 122.43, 121.79, 121.65, 121.39, 56.86, 41.82, 33.19,
Computational method: The structures were drawn with MOE2007.[13]
The geometries were initially optimized using the AM1 method in the
gas phase, followed by gas-phase B3LYP/6-31G+(d) optimization using
Gaussian03[15] running on HPCVL supercomputers.[16] The optimized ge-
ometries were checked for negative vibrations. The HOMO and LUMO
energies and coordinates were taken from the Gaussian output and the
graphical orbital plots were generated with GABEDIT2.18.[17] The calcu-
lation time was reduced by replacing the long alkyl chains of 1a and 1b
with a methyl group.
˜
31.70, 30.74, 30.61, 25.71, 13.93, 15.26 ppm; IR: n=3064, 2954, 2924,
Materials and general methods: All the chemicals and reagents were
used as received from commercial sources without purification. The sol-
vents for chemical reactions were carefully dried and purified under a ni-
trogen flow. All the reactions were carried out in an argon atmosphere in
flame-dried glassware. Syringes were used to transfer anhydrous solvents
or reagents and were purged with argon prior to use. The [6,6]-phenyl
C61 butyric acid methyl ester (PCBM) was obtained from FEM Technolo-
gy Co. 4,7-Diaryl-5,6-dinitro-2,1,3-benzothiadiazoles (3a,b),[8] hexaketo-
cyclohexane,[18] and pyrene-4,5,9,10-tetraones[19] were prepared according
to the procedures reported in the literature. Details and purification
methods used for 4a and 4b are given in the Supporting Information.
2851, 1605, 1518, 1491, 1456, 1420, 1375, 1329, 1308, 1260, 1210, 1154,
1066, 1020, 912, 877, 800, 776, 735, 715, 576, 536, 497, 442 cmÀ1; elemental
analysis calcd for C160H178N8S6: C 79.89, H 7.46, N 4.66; found: C 79.94,
H 7.73, N 4.63.
Acknowledgements
Synthesis of chromophore 1a: Hexaketocyclohexane (0.312g, 1.0mmol),
4a (4.660g, 4.0mmol), and AcOH (50mL) were added to a 250mL,
round-bottomed flask equipped with a magnetic stirrer. The reaction
mixture was heated at reflux for 48h under an argon atmosphere.
Column chromatography (DCM/PE 1:1 v/v) gave the pure product as a
dark-green powder (1.423g, 40%). 1H NMR (300 MHz, CDCl3): d=8.00
(d, 12H, J=8.34 Hz), 7.15–7.00 (br, 24H), 6.71 (d, 36H, J=8.55 Hz);
This work was supported by the Senior Researcher Program and Explor-
atory Basic Research Project (CX07QZJC-20) of the Changchun Insti-
tute of Applied Chemistry, the National Natural Science Foundation of
China (20834001), the Science Fund (20621401) for Creative Research
Groups of NSFC, and the Natural Sciences and Engineering Research
Council of Canada. Some calculations were run using the high-perfor-
mance computing virtual lab facility (HPCVL: http://www.hpcvl.org).
Chem. Eur. J. 2009, 15, 8902 – 8908
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8907

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Received: April 3, 2009
Published online: July 27, 2009
8908
www.chemeurj.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 8902 – 8908