Synthesis of a polymeric electron acceptor based on perylenediimide-bridged ladder polysiloxane

Article abstract of DOI:10.1021/ma1025797

A soluble perylenediimide (PDI) bridged ladder polysiloxane (PDI-LPS) was prepared using ladder superstructure (LS) directed dehydration polycondensation. LS was first assembled via synergistic interactions of hydrogen-bonding and π - π stacking of tetras

Full text of DOI:10.1021/ma1025797

Macromolecules 2011, 44, 203–207 203
DOI: 10.1021/ma1025797
Synthesis of a Polymeric Electron Acceptor Based on Perylenediimide-
Bridged Ladder Polysiloxane
Wenxin Fu,† Chang He,† Shidong Jiang,‡ Zhize Chen,† Jintao Zhang,† Zhibo Li,*,†
Shouke Yan,*,‡ and Rongben Zhang*,†
†Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy
of Sciences, Beijing 100190, China, and ‡State Key laboratory of Chemical Resource Engineering,
Beijing University of Chemical Technology, Beijing 100029, China
Received November 14, 2010; Revised Manuscript Received December 8, 2010
ABSTRACT: A soluble perylenediimide (PDI) bridged ladder polysiloxane (PDI-LPS) was prepared using
ladder superstructure (LS) directed dehydration polycondensation. LS was first assembled via synergistic
interactions of hydrogen-bonding and π-π stacking of tetrasilanol monomers (M2s) as verified from XRD, 29Si
NMR, and VPO characterizations. The ladder regularity of PDI-LPS was confirmed by a narrow half-peak
width (Δ) of <1 ppm for SiO2/2 unit in the 29Si NMR spectra. The alignment of ladder chains was also
demonstrated from high-resolution transmission electronic microscopy (HR-TEM) observations. We
successfully used ladderlike structure to chemically confine PDI cores within one-dimensional polymeric
semiconductor. The resulting PDI-LPS not only retained PDI’s optoelectronic properties but also gained
enhanced solubility in common organic solvents and improved thermal stability.
€
Introduction
semiconductors. Toward this goal, Wurthner took the advantage
of the supramolecular forces to synthesize polymer containing
PDIs.17 Herein, we adopted a different approach based on the
“supramolecular architecture-directed stepwise coupling and
polymerization” (SCP) strategy.16 Inthis method, tetrafunctional
precursor first self-assembled into ladder superstructure (LS) via
H-bonding and π-π stacking. The supramolecular LS acted as
both polymerization precursor and template to form LPS.18
Considering the encountered insolubility issue of PDI, we used
tetra-tert-butylphenoxy-substituted PDI as the bridges for LPS
shown in Scheme 1.
Polymer solar cells (PSCs) are being extensively studied because
of their great potential application. Compared with inorganic
materials based solar cells, PSCs can be easily fabricated via spin-
coating, enabling the manufacture of large area, flexible, light-
weight, inexpensive, and renewable devices.1-3 Generally, all of
the solar cell devices require both hole-transporting (p-type) and
electron-transporting (n-type) materials, which are desirable to
have a good combination of physical, chemical, electrical, and
photochemical properties.4-7 In particular, fullerene derivatives,
e.g. [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), were
extensively used as acceptor material. However, the absorption of
PCBM in the visible region is relatively weak. Thus, development
of new n-type substitutes for PCBM has attracted intensive
research activities.8-10 Perylenediimide (PDI) and its derivatives
were well-known n-type organic semiconductors with high elec-
tron mobility.11,12 However, their poor solubility made them
difficult to form flexible films in common organic solvents. Herein,
to solve the above-mentioned problems, we proposed an alter-
native strategy of using a PDI derivative as the central bridge
to form ladder polymer. Through chemically incorporating PDI
into ladder polymer backbone, we hope to minimize the con-
glomeration of PDI and improve materials’ thermostability and
solubility while retaining its optoelectronic properties.
Experimental Section
Materials and Characterizations. 1,6,7,12-Tetra-tert-butyl-
phenoxyperylene-3,4,9,10-tetracarboxylic dianhydride (1) was
purchased from Beijing Wenhaiyang Industry and Trading Co.
Ltd. 3-Aminopropylmethyldiethoxysilane (2) was purchased
from Alfa Aesar. Tetrahydrofuran (THF) and 1,4-dioxane were
distilled over sodium benzophenone complex. Number-averaged
molecular weight (Mn) was determined from the vapor pressure
osmometry (VPO) method using a Knauer VPO instrument
(Germany) in THF or dioxane at 40-50 °C. FT-IR measure-
ment was performed using a Perkin-Elmer 80 spectrometer, and
the samples were prepared by casting solution sample on KBr
flake. UV-vis spectra were obtained on a Shimadzu UV-
vis spectrometer (UV-1601PC). Fluorescence spectra were
recorded on a Hitachi F-4500. 1H and 29Si NMR spectra were
obtained on a Bruker Advance DPS-400 (400 MHz) spectro-
meter. For HR-TEM observation, 1 mg/mL solution of PDI-
LPS in ethanol was dropped onto the carbon-coated mica surface.
After evaporation of the solvent, the carbon film together with
the sample was then floated onto the surface of distilled water
and subsequently transferred onto 400 mesh copper grids for
TEM observation. JEOL JEM-2100F TEM operated at 200 kV
was used. X-ray diffraction (XRD) analysis was performed on a
Rigaku D/MAX 2400 diffractometer. Cyclic voltammetry was
performed on CHI660D voltammetric analyzer (CH Instruments)
It was known that polysiloxane-based materials generally had
good solubility in common organic solvents and good film-
forming ability.13,14 Compared to the single-chain counterpart,
the ladder polysiloxanes (LPSs) were found to have greater
resistance to irradiation as well as thermal and chemical
degradation.15,16 These features inspired us to think of combining
organic semiconductor compounds such as PDIs and ladder
architecture together to create intramolecularly ordered polymeric
*To whom all correspondence should be addressed: e-mail zbli@
iccas.ac.cn (Z.L.), skyan@mail.buct.edu.cn (S.Y.), zhangrb@iccas.ac.
cn (R.Z.); Tel þ86-10-62565612; Fax þ86-10-62565612.
r 2010 American Chemical Society
Published on Web 12/28/2010
pubs.acs.org/Macromolecules

204 Macromolecules, Vol. 44, No. 2, 2011
Fu et al.
Scheme 1. Synthetic Route to Target PDI-LPS
3.01-3.05 (br d, 4H, Si-OH), 4.11-4.14 (t, 4H, J = 6.8 Hz,
Si-CH2CH2CH2), 6.80-6.82 (d, 8H, J = 8.28 Hz, H-3 of tert-
butylphenoxy), 7.22-7.24 (d, 8H, J = 8.4 Hz, H-2 of tert-
butylphenoxy), 8.20 (s, 4H, H-2 of perylenediimide). 29Si NMR
(59.6 MHz, δ, ppm): -8.51 with half-peak width Δ of <1 ppm.
MALDI-TOF MS (m/z) Calcd: for C72H78N2O12Si2, 1218.5.
Found: 1218.9 Mþ.
Preparation of the Target PDI-LPS. 1.00 g of M2 (0.82 mmol)
was dissolved in 50 mL of dioxane, and then two drops of
concentrated H2SO4 were added to the mixture to catalyze the
condensation reaction. After 72 h, the reaction mixture was
concentrated to about 20 mL under reduced pressure at 45 °C,
and the concentrated mixture was stirred for another 72 h to
complete the condensation reaction. To end-cap the terminal
silanol groups of the polymer, 0.83 g of triethylamine (8 mmol)
and 0.88 g of trimethylchlorosilane (8 mmol) dissolved in 5 mL
of dioxane were added drop by drop into the flask under stirring.
The mixture was stirred at rt for another 24 h. Filtration was
performed to remove the small amount of gel generated during
the condensation process. Adding 50 mL of methanol to the
filtrate caused instantaneous precipitation of a dark red solid,
which was further purified by Soxhlet extraction with methanol,
n-hexane, and THF in turn and then eluted by THF and CH2Cl2
on silica gel columns. Finally, 0.87 g of PDI-LPS was obtained in
89.6% yield after being dried in a vacuum oven at 100 °C for
24 h. FT-IR (KBr, thin film, cm-1): 1259 (Si-CH3), 1012-1078
(Si-O-Si). 1H NMR (CDCl3, 400 MHz, δ, ppm): 0.07 (br s, 6H,
Si-CH3), 0.23-0.61 (br s, 4H, Si-CH2), 0.83-0.88 (br s, 9H,
Si-(CH3)3), 1.25 (br s, 36H, -C(CH3)3), 1.56-1.63 (br s, 4H,
Si-CH2CH2), 3.87-4.07 (br s, 4H, Si-CH2CH2CH2), 6.36-
6.48, 6.77-6.79, 7.05-7.14 (br m, 16H, phenyl-H), 8.0-8.06 (br s,
4H, perylenediimide-H). 29SiNMR (59.6 MHz, δ, ppm):-22.6with
with a standard three-electrode electrochemical cell in a 0.1 M
tetrabutylammonium tetrafluoroborate solution in CH2Cl2 at
room temperature (rt) under a nitrogen atmosphere. The scan-
ning rate is 0.1 V/s. A glassy carbon working electrode, a Pt wire
counter electrode, and a silver wire as a quasi-reference electrode
were used. The experiments were calibrated with the internal
standard ferrocene/ferrocenium (Fc) redox system.
Synthesis. Preparation of 1,6,7,12-Tetra-tert-butylphen-
oxyperylene-N,N0-bis(3-aminopropylmethyldiethoxysilane)-3,4,9,10-
tetracarboxylic Dianhydride (M1). 2.44 g of 1 (2.48 mmol) was
added into a three-necked flask with magnetic stirring bar. The
flask was then evacuated and purged with argon three times.
200 mL of dry ethanol was added under an argon atmosphere.
The reaction mixture was heated to reflux prior to addition of
4.75 g (24.8 mmol) of 2 as one portion. The reaction mixture was
then refluxed for 10 h under argon before cooled to rt. Then the
reaction mixture was filtrated to give a fluffy cake, which was
subsequently washed with cold ethanol. 1.95 g of dark red
product was obtained using flash chromatography on a silica
gel column (CH2Cl2/petroleum ether = 2/1) in 60% yield; Rf =
0.4 (CH2Cl2/petroleum ether = 2/1).19 1H NMR (CDCl3,
400 MHz, δ, ppm): 0.094 (s, 6H, Si-CH3), 0.65-0.7 (t, J =
8 Hz, 4H, Si-CH2), 1.15-1.19 (t, 12H, J = 6.94 Hz, Si-
OCHH2CH3), 1.29 (s, 36H, -C(CH3)3), 1.74 (s, 4H, Si-
CH2CH2), 3.69-3.75 (q, 8H, J = 6.96 Hz, Si-OCHH2CH3),
4.07-4.11 (t, 4H, J = 7.28 Hz, Si-CH2CH2CH2), 6.81-6.83 (d,
8H, J = 8.32 Hz, H-3 of tert-butylphenoxy), 7.22-7.24 (d, 8H,
J = 8.44 Hz, H-2 of tert-butylphenoxy), 8.22 (s, 4H, H-2 of
perylenediimide). 29Si NMR (59.6 MHz, δ, ppm): -6.97 with
half-peak width Δ of <1 ppm. MALDI-TOF MS (m/z) Calcd:
for C80H94N2O12Si2, 1330.63. Found: 1330.8 Mþ.
half-peak width Δ of <1 ppm for the repeat unit SiO2/2
.
Results and Discussion
Syntheses and Characterizations of M1 and M2 Precursors.
M1 was synthesized according to literature procedure19 and
purified by flash column chromatography with yield of
about 60%. Tetrafunctional M2 was obtained after hydro-
lyzing M1 in dilute THF solution (∼5 mg/mL in THF). Also,
HCl solution (1 M) used as limiting catalyst and low tem-
perature were strictly controlled to minimize undesired ran-
dom condensation between the newly formed silanol groups
in M2 (Scheme 1). The structures of M1 and M2 were unam-
biguouslyverified using FT-IR and 1H NMR (Figures S2 and
S3). For the FT-IR spectrum of M1, the doublet peaks at
1106 and 1077 cm-1 represent the asymmetric Si-O-C
stretching bands, and the peaks at the 1178 and 966 cm-1
are characteristics of ethoxy group (Figure S2). For M2, the
wide hydroxyl peak around 3400 cm-1 suggests the success-
1
ful formation of silanol groups after hydrolysis of M1. H
NMR spectra shown in Figure S3 indicated the complete
disappearance of protons from Si-OCH2-CH3 (e and h)
[δ = 3.69-3.75 (q, 8H) and 1.15-1.19 (t, 12H) ppm] when
M1 is hydrolyzed to M2. A new resonance attributed to
proton of Si-OH [δ 3.01-3.05 (br d, 4H) ppm] was easily
identified. The chemical shift changes of Si atom between M1
and M2 are also confirmed from 29Si NMR shown in Figure 1.
Synthesis and Characterization of LS. Since M2 consists of
polar silanol groups and nonpolar perylene derivatives, M2
displays a certain degree of amphiphilicity. We found that
M2 might spontaneously self-assemble into ordered supra-
molecular structures in several solvents. Considering the
planar PDI bridge and H-bonding capability of Si-OH
groups, we assumed that M2 would stack up perpendicular
to the PDI molecular plane to form a ladder superstructure
(LS) via the synergistic interactions of silanols’ square-planar
H-bonding20 and the π-π stacking of perylenediimide bridges
Preparation of 1,6,7,12-Tetra-tert-butylphenoxyperylene-N,
N0-bis(3-aminopropylmethyldihydroxylsilane)-3,4,9,10-tetracar-
boxylic Dianhydride (M2). 1 g of M1 (0.75 mmol) was first
dissolved in a mixture of 300 mL of THF and 4 mL of water. To
this solution, three drops of 1 M HCl were added with stirring at
0 °C. The reaction was monitored by TLC (methanol/CH2Cl2 =
1/20). After 6 h, 100 mL of diethyl ether and 80 mL of water were
added. The ether layer was separated and washed with 40 mL of
water three times. The organic layer was then dried over sodium
sulfate and filtered. The filtrate was condensed to ∼40 mL and
precipitated using 100 mL of hexane, followed by filtration to
give a cakelike dark red product. The product was dried under
vacuum at rt, and 800 mg of dark red powder was obtained in
1
87.5% yield. H NMR (CDCl3, 400 MHz, δ, ppm): 0.094 (s,
6H, Si-CH3), 0.66-0.7 (t, J = 7.17 Hz, 4H, Si-CH2), 1.29 (s,
36H, -C(CH3)3), 1.82-1.85 (t, 4H, J = 6.92 Hz, Si-CH2CH2),

Article
Macromolecules, Vol. 44, No. 2, 2011 205
Table 1. VPO Results of Ladder Superstructure LS in Different
Conditions
T (°C)
solvent/DNa
Mn
DPb
43
50
45
dioxane/14.8
dioxane/14.8
THF/20.0
7655
1428
2274
6.3
1.2
1.9
a The donor numbers (DN) were given in the literature.22 b Number-
averaged degree of polymerization.
formed at this temperature. Solvent also had a similar effect.
If we changed the solvent to THF, LS only had repeating
number of about two. The effects of temperature and solvent
polarity toward ladder structure of LS were also compared
by XRD measurements shown in Figure S1. LS prepared in
dioxane at 43 °C still kept the ladder structure with char-
acteristic LS width and thickness around 1.83 and 1.54 nm,
respectively, consistent with molecular dimension of M2.
Meanwhile, when the LS was prepared at 50 °C, the resulting
samples basically lost its supramolecular ordering. Changing
solvent from dioxane to THF had similar effects that M2
could not form well-ordered ladder structures in such medium.
In addition, the regularity of LS derived from M2 was also
supported by 29Si NMR measurement shown in Figure 1. It
was known that the more narrow the NMR resonance peak,
the more regular the ladder structure.23 Figure 1 shows that
the half-peak width (Δ) of LS’s 29Si NMR spectrum is less
than 1 ppm, which indicates all Si atoms almost have the same
chemical environment, suggesting a great regularity for LS.
Otherwise, the corresponding 29Si NMR will display a broach
resonance peak. Combining the results from XRD, VPO,
and 29Si NMR, we believed that M2 predominately self-
assembled into ordered ladder superstructure (LS) via pre-
sumably synergistic hydrogen-bonding and π-π stacking
interactions in dioxane below 50 °C, instead of randomly
conglomerated or branched structures. We then used the LS
as precursor template for subsequent polycondensation to
make PDI-LPS.
Figure 1. 29Si NMR spectra of M1, M2, and PDI-LPS in dioxane with
half-peak width (Δ) <1 ppm.
Figure 2. XRD of LS and PDI-LPS.
(Scheme 1), similar to literature reported systems.18,21 In this
way, LS can maximize the H-bonding and π-π interactions,
while minimizing the formation of random branched struc-
tures. The regularity of LS was probably the most critical
factor to prepare well-defined LPS. In addition, we found
that particular experimental conditions, such as temperature,
electrodonating ability, and polarity of solvent, can greatly
affect the regularity of LS.21,22 After exploring several reac-
tion systems, we found that dioxane at relatively low tem-
perature can offer LS with good ladder superstructures.
Figure 2 shows XRD spectra of freeze-dried LS sample.
Note that using freeze-dry method can preserve the pristine
ladder structure of LS in solution to a great extent. In parti-
cular, the XRD profile of freeze-dried LS sample prepared
from dioxane solution shows two primary peaks with
d = 1.83 and 1.54 nm. The former represents the width of
LS, while the latter corresponds to the thickness of LS.
Moreover, there is a diffusing peak ranging from 20° to
30°, within which there were small spikes with d around
0.44 nm presumably arising from the π-π of PDI groups.
The supramolecular nature of LSs can be inferred from their
aggregation number, which can be calculated from their
number-averaged molecular weight determined using VPO.
The corresponding results are summarized in Table 1.
As mentioned above, the ladder structure of LS strongly
depended on solvent polarity, solvent electrodonating ability,
and solution temperature. For example, LS assembled in
dioxane at 43 °C had about six repeat units (M2). Mean-
while, when the temperature increased to 50 °C, the average
repeating number decreased to about one, indicating no LS
Synthesis and Characterization of PDI-LPS. As Williams
pointed out, the acidic environment is favorable to the
formation of H-bonding between the silanols, and acidic cata-
lysts usually have stronger condensing ability than basic ones
for silanols with electrodonating groups.24 We thus used
concentrated sulfuric acid to polymerize M2 into the target
PDI-LPS. We also found that extension of reaction time
helped increasing conversion and molecular weight of LPS.
After polycondenstion, we used Me3SiCl to end-cap the
Si-OH end groups to give final product PDI-LPS, which
was found soluble in THF, CH2Cl2, and toluene, etc. In
particular, PDI-LPS had solubility larger than 100 mg/mL
in THF, while its low molecular weight counterpart
(N,N0-bis(3-triethoxysilylpropyl)perylene-3,4:9,10-tetra-
carboxydiimide) has solubility lower than 1 mg/mL in
THF.19 The Mn of PDI-LPS was about 40 470 Da determined
by VPO measurement, which gave the degree of polymeri-
zation being about 34. Furthermore, the expected chemical
1
structure of PDI-LPS was verified using FT-IR, H NMR,
and MALDI-TOF mass spectra available in the Supporting
Information. For the FT-IR spectra of PDI-LPS (Figure S2),
the characteristic absorptions of the Si-OH group disap-
peared after polycondensation, and the two strong peaks in
the region of 1000-1100 cm-1 emerged, which were attrib-
uted to the asymmetric Si-O-Si stretching vibration. The
29Si NMR spectrum of PDI-LPS is shown in Figure 1, in
which the half-peak width (Δ) was less than 1 ppm, indicating
the high ladder regularity of PDI-LPS. In contrast, literature
Δ values of oligomeric polyphenylsilsesquioxane were about
4-5 ppm.25 Figure 2 shows XRD data of PDI-LPS. Compared

206 Macromolecules, Vol. 44, No. 2, 2011
Fu et al.
Scheme 2. (a) Molecular model of M2; (b) 3D Mode of Molecular
Simulated PDI-LPS with 10 Repeat Units (Hyper Chem7.0 Geometry
Optimization with RMS Gradient of 0.1 kcal/mol); (c), (d), and (e) Are
the Corresponding Molecular Dimensions of PDI-LPS along a, b, and c
Axis, Respectively
Figure 3. HR-TEM image of PDI-LPS.
to LS, PDI-LPS did not display distinct diffraction of ladder
structure width and thickness due possibly to variation of
their structure. LS was assembly via oriented π-π stacking
and hydrogen bonding, while PDI-LPS was linked
by -O-Si-O- with more freedom. The chain rigid of chemi-
cally linked PDI-LPS was actually decreased after polycon-
densation from hydrogen-bonding-linked LS. As a result, a
smeared XRD diffraction pattern was obtained, whereas
primary diffraction gives spacing about 1.97 nm, which agrees
well with theoretical width of PDI-LPS (Scheme 2). Note that
the π-π stacking of PDI-LPS was apparently less ordered
compared to LS from their XRD diffraction. We presume
the main reason is also due to decrease of chain rigid
surface as illustrated in Scheme 2e, and the Si-O-Si double-
stranded backbones are aligned perpendicular to substrate
with column width close to ladder thickness (1.7 nm). For the
latter, whole PDI-LPS would lie down on substrate, which
will give layered structures as illustrated in Scheme 2c and 2d.
The periodicity of these layers should be close to π-π stacking
distance. Careful examination of TEM results revealed that the
spacing was around 0.3 nm, which was very close to π-π
stacking distance estimated by XRD. Unfortunately, we could
not clearly identify the width of PDI-LPS because of the neg-
ligible contrast between adjacent samples. Also, additional TEM
images at different magnifications are shown in Figure S6.
Photophysical Properties of PDI-LPS. Figure S7 compares
the UV-vis spectra of M1, M2, and PDI-LPS. Apparently,
M1 has nearly identical absorption spectra to M2. Both have
the characteristic absorption of PDI. The absorption max-
imum at 570 nm belongs to the electronic S0-S1 transition,
with a transition dipole moment along the long molecular
axis,26 and the second absorption band in the region of
400-460 nm is attributed to the electronic S0-S2 transition,
with a transition dipole moment perpendicular to the long
molecular axis.27 The effects of π-π stacking interactions
between M2s intensified with increase of concentration. M2
showed an obvious red shift with increase of sample concentra-
tion THF and dioxane, which suggested a dynamic assembly
among M2 in THF and dioxane (Figure S9).26 In contrast,
PDI-LPS had a 12 nm red shift compared to M2 after poly-
merization. We believed that such a red shift was presumably
induced by delocalization of π-electrons after the aromatic
PDIs were confined into one-dimensional ladder space. For
PDI-LPS, variation of polymer concentration did not cause
noticeable change of UV/vis spectra except the increase of
absorption intensity, which indicated that there was no
serious aggregation when concentrations were lower than
10-4 mg/mL.
converting hydrogen bonded -Si-OH O(H)-Si- into cova-
3 3 3
lent -Si-O-Si- bond.
HR-TEM Observation of PDI-LPS. From the chemical
structure of precursor (Scheme 2a), we can estimate the
dimension of M2 as well as PDI-LPS illustrated in Scheme 2b.
The PDI core molecule has length about 1.2 nm along the
imide bonds and width being 0.7 nm along PDI bay region.
Considering the substitute groups, the spatial dimension of
M2 precursor is 1.2 nm long and 1.5 nm wide. After
polymerization, the M2 precursors were chemically confined
within one-dimensional space (Scheme 2b). We thus tried
to visualize the ladder structure of PDI-LPS assuming the
π-conjugated PDI system might provide sufficient contrast
for HR-TEM characterization. The solid PDI-LPS was
dispersed in ethanol before casting onto freshly prepared
carbon film, which was deposited on mica surface. The sample
and carbon film were subsequently transferred to bare TEM
grid for characterization. Figure 3 shows a representative
HR-TEM image of PDI-LPS sample.
Interestingly, PDI-LPS displayed laminated structures with
random orientation. Apparently, rodlike PDI-LPS has three
possible orientations when deposited on carbon surface as
depicted in Scheme 2. There are two kinds of structures, i.e.,
the column structure and the layered structure. PDI-LPS
contains two types of substituted groups, namely, the aro-
matic PDI with four steorically hindered tert-butoxy groups
and polar Si-O-Si double-stranded backbone. The flat
aromatic PDI core could choose face-up or edge-on orienta-
tion with respect to substrate surface. For the former, PDI-
LPS would form columnlike structures standing on carbon
Cyclic Voltammetry Study. To test the influence of ladder
structure on PDI optoelectronic properties, we found the
blends of PDI-LPS and P3HT (1:1) showed enhanced ab-
sorption areas spectrum compared to their precursors. Fluores-
cence measurements showed the blended thin film had strong
fluorescence quenching indicating efficient energy transfer
from P3HT to PDI-LPS (Figure S8). The HOMO and LUMO

Article
Macromolecules, Vol. 44, No. 2, 2011 207
Acknowledgment. The financial support of NSFC [No.
50821062, 50521302, and 50973008] is gratefully acknowledged.
Supporting Information Available: Text giving the experi-
mental information and figures showing XRD of LS, FT-IR
1
spectra, H NMR spectra, MALDI-TOF mass spectra, TGA
and DSC traces, HR-TEM images, UV/vis, and fluorescence
spectra of PDI-LPS. This material is available free of charge via
the Internet at http://pubs.acs.org.
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þ
onset
4.39) = -e(-0.67 þ 4.39) = -3.72 eV. The absorption edge
of the film extends to 660 nm, from which the corre-
sponding optical energy gap (Eogpt) is calculated as 1.88 eV.
So Egopt = -e(EHOMO - ELUMO) and EHOMO = -5.60 eV.
As shown in Figure 4b, PDI-LPS has the proper HOMO and
LUMO level with P3HT. Compared with the low molar mass
PDI derivatives (e.g., PDI-FCN229), PDI-LPS has higher
LUMO level, which could help the solar cell device to own
higher open-circuit voltage.
Thermal Properties of PDI-LPS. We also investigated the
thermal properties of PDI-LPS (Figure S5). The PDI-LPS
began to decompose at 442 °C and lost about 5 wt % at ca.
430 °C. The DSC curve shows that the PDI-LPS does not
have any melting peaks, and its glass transition temperature
(Tg) locates at ∼310 °C, which is much higher than that of the
low molar mass PDI derivatives, for example, 135 °C of
N,N0-bis(cyclohexyl)-(1,7 and 1,6)-dicyanoperylene-3,4,9,
10-bis(dicarboximide) (PDI-CN).30 We consider the surpris-
ingly higher Tg is due to the excellent regular ladder structure
of the PDI-LPS. The double-strained structure could pro-
vide this material with excellent thermal property and di-
mensional stability due to the minimized movements of chain
segments.
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In summary, we synthesized a novel polymeric electron accep-
tor based on PDI-bridged ladder polysiloxane (PDI-LPS) via
self-assembled ladder superstructure (LS) of the monomer M2
for the first time. XRD, 29Si NMR, and VPO characterizations
supported that supramolecular LS directed the polymerization
pathway. Moreover, the target polymer PDI-LPS was confirmed
to possess high ladder regularity by an extremely narrow baseline
width (Δ < 1 ppm) for SiO2/2 unit in 29Si NMR spectra. HR-
TEM measurements revealed that the PDI-LPS had layered
structures in bulky state. In particular, PDI-LPS exhibited out-
standing physical properties such as good solubility, higher
thermal stability, a wide absorption region, and proper LUMO
energy level in contrast to PDI precursor. We thus think pre-
paration of polymeric n-type semiconductor might be an alter-
native electron acceptor material for polymer solar cells. More-
over, the methodology used is a universal route to construct
ladder electron acceptor materials with other organic semicon-
ductor bridges.
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