Responsive gels with the polymer containing alternating naphthalene diimide and fluorinated alkyl chains: Gel formation and responsiveness as well as electrical conductivity of polymer thin films

Article abstract of DOI:10.1002/cjoc.201200029

A new electroactive polymer 1 with alternating NDI (naphthalene diimide) moieties and fluorinated alkyl chains was prepared and characterized. Gels of polymer 1 were formed in several solvents. Interestingly, gels of polymer 1 exhibited responsiveness toward N₂H₄, F^- and CN^-. Absorption and ESR spectroscopic studies revealed that such responsiveness is owing to the reduction of NDI moieties into the respective NDI^.-. In addition, thin films of polymer 1 were easily prepared with spin-coating technique and the electrical conductivity of thin films reached 52.4 S/m after exposure to N₂H₄ vapor.

Full text of DOI:10.1002/cjoc.201200029

FULL PAPER
DOI: 10.1002/cjoc.201200029
Responsive Gels with the Polymer Containing Alternating
Naphthalene Diimide and Fluorinated Alkyl Chains: Gel
Formation and Responsiveness as Well as Electrical
Conductivity of Polymer Thin Films
Luo, Hewei(罗河伟)
Liu, Zitong*(刘子桐)
Cai, Zhengxu(蔡政旭)
Wu, Liusuo(吴刘锁)
Zhang, Guanxin(张关心)
Liu, Chenyang(刘琛阳)
Zhang, Deqing*(张德清)
Beijing National Laboratory for Molecular Sciences, Organic Solids Laboratory, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, China
A new electroactive polymer 1 with alternating NDI (naphthalene diimide) moieties and fluorinated alkyl chains
was prepared and characterized. Gels of polymer 1 were formed in several solvents. Interestingly, gels of polymer 1
exhibited responsiveness toward N2H4, F－ and CN－. Absorption and ESR spectroscopic studies revealed that such
responsiveness is owing to the reduction of NDI moieties into the respective NDI·－. In addition, thin films of po-
lymer 1 were easily prepared with spin-coating technique and the electrical conductivity of thin films reached 52.4
S/m after exposure to N2H4 vapor.
Keywords responsive gels, naphthalene diimide, radical anion of naphthalene diimide, electroactive polymer,
electrical conductivity
Introduction
materials, such as sensors, actuators, etc.[4] The molecu-
lar design rationale for these stimuli-responsive gels is
to incorporate photoresponsive/electroactive/chemical
reactive segments into the respective LMWGs derived
from cholesterol, urea and other gelating moieties.[2,5-8]
For instance, organogels which respond to light irra-
diations have been achieved by incorporating photore-
sponsive moieties (e.g. azobenzene and stilbene)[1d,2b]
into the corresponding LMWGs. By designing LMWGs
with electroactive moieties, organogels showing respon-
siveness to redox reactions have been described.[5e,7]
In this paper we report physical gels based on poly-
mer 1 (Scheme 1) which contains alternating naphtha-
lene diimide (NDI) and fluorinated alkyl chains. The
molecular design is based on the following considera-
tions: (1) NDI is a planar conjugated moiety and thus
tends to form stacks in the solid state via π-π interac-
tions. In fact, NDI has been recently intensively inves-
tigated for developing organic semiconductors.[9] More-
over, NDI derived gelators and the resulting gels have
been reported recently;[10] (2) the polymer with multiply
NDI moieties would enhance the intermolecular π-π
interactions. The intra-chain π-π interactions of NDI
moieties may also exist as schematically depicted in
Scheme 1; (3) NDI is an electron acceptor moiety which
Self-assembly of certain molecules, referred to as
low-molecular weight gelators (LMWGs), through weak
intermolecular interactions such as H-bonding, π-π
stacking and van der Waals interactions, can lead to
solvent gelation and formation of physical gels.[1,2] Be-
cause of the weak intermolecular interactions such
physical gels are thermally reversible, namely, the
gel-sol (solution) transition can be reversibly tuned by
alternating heating and cooling. In comparison, chemi-
cal gels are usually formed through the covalent-linkage
of polymer chains, and as a result the chemical gels
cannot be redissolved. Note that polymer-based ther-
mally reversible hydrogels are known by manipulating
the interactions of polymer chains through supra-
molecular means.[3]
Physical gels have received increasing attentions re-
cently as they may have potential applications in a
number of areas including nanomaterials and delivery or
modification agents for paints, inks, cleaning agents,
drugs, etc.[1e-1g] Apart from studies on the gel formation
and structures, extensive efforts have been made to in-
vestigate functional and stimuli responsive gels. Stim-
uli-responsive physical gels offer us promising opportu-
nities for designing and constructing new functional
*
E-mail: dqzhang@iccas.ac.cn; Tel.: 0086-10-62639355; Fax: 0086-10-62569349
Received January 11, 2012; accepted April 16, 2012; published online XXXX, 2012.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201200029 or from the author.
Chin. J. Chem. 2012, XX, 1—6
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Luo et al.
FULL PAPER
Scheme 1 Chemical structure of polymer 1 and schematic illustration intermolecular interactions and responsiveness toward N2H4, F－
and CN－
Scheme 2 Synthetic approach for polymer 1
O
N
O
O
N
O
O
O
O
O
O
O
H
N
N
N
H2N
THF
Reflux
30%
+
H
H
2
O
O
Cl
HO
C
OH
8
+
O
C
O
8
r.t.
49%
F2
F2
O
3
O
N
O
O
N
O
O
O
N
N
O
C
O
8
2
+
3
F2
50 oC, 5 d
86%
polymer 1
can be easily transformed into NDI•－ by reduction.[11]
The results reveal that polymer 1 can gel several sol-
vents and the resulting gels show responsiveness toward
either hydrazine or CN－ or F－. Moreover, polymer can
form thin films which exhibit semiconducting behavior
upon exposure to hydrazine.
dures.[12]
Characterization techniques
1
The H NMR and 13C NMR spectra were recorded
on a Bruker DMX-400 NMR spectrometer using
tetramethylsilane as an internal standard. MS spectra
were determined with BEFLEX III for TOF-MS. The
gel permeation chromatography (GPC) measurements
were performed on Waters 410 system against polysty-
rene standards with THF as an eluent. Infrared spectra
were obtained on a Perkin-Elmer System 2000 FT-IR
spectrometer. Electronic absorption spectra were ob-
tained on a Shimadzu UV-vis spectrometer model
UV-1601PC. TGA (NETZSCH STA 409C), and DSC
(Perkin-Elmer 7 series thermal analyzers) measurements
were performed under nitrogen atmosphere at a heating
rate of 10 ℃/min. ESR (Electron Paramagnetic Reso-
Experimental
Materials
1,4,5,8-Naphthalenetetracarboxylic
dianhydride,
N-methylpropane-1,3-diamine, 1H,1H,10H,10H-per-
fluoro-1,10-decanediol, acryloyl chloride were pur-
chased commercially from Alfa Aesar/Acros/Aldrich
and used as received without further purification unless
otherwise indicated. Compound 2 (Scheme 2) was syn-
thesized according to the literature-reported proce-
2
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Chin. J. Chem. 2012, XX, 1—6

Gels with the Polymer Containing Alternating Naphthalene Diimide and Fluorinated Alkyl Chains
Hz, 4H), 2.70 (t, J ＝6.8 Hz, 4H), 2.45 (s, 6H)
nance) spectra were obtained by a Bruker-E500 spec-
trometer. Cyclic voltammetric measurements were car-
ried out in a conventional three-electrode cell using Pt
wires of 2 mm diameter as working and counter elec-
trodes, and an Ag/AgCl reference electrode on a com-
puter-controlled CHI660C instruments at room tem-
perature. For all scanning electron microscopy (SEM)
experiments, a JOEL JSM 6700F field emission scan-
ning electron microscope was used with a sample,
which was sputtered with platinum; suspensions of xe-
rogels were directly put onto the clean Si substrate for
SEM measurements. X-ray diffraction data were col-
lected with Rigaku D/max-γA 12 kW with Cu Kα radia-
tion. Rheological experiments were performed with an
AR 2000 advanced rheometer (TA Instruments) by cone
and plate geometry in a peltier plate. The electrical
conductivity of polymer 1 was measured with a two-
electrode device; the polymer thin film of ca. 52 nm in
thickness was at the bottom; the silver electrodes were 9
mm in length and 2 mm in width.
1.92—2.00 (m, 4H) 1.39 (s, 2H); 13C NMR (CDCl3, 75
MHz) δ: 162.9, 131.0, 126.7, 126.6, 49.3, 39.0, 36.4,
28.2; MS (MALDI-TOF) m/z (M ＋1) ＋ : calcd for
C22H24N4O4 409.2, found 409.1.
Synthesis of compound 3
1H,1H,10H,10H-Perfluoro-1,10-decanediol (2.0 g,
4.33 mmol) and triethylamine (7.0 mL, 50.4 mmol)
were dissolved in 150 mL of THF. Acryloyl chloride
(4.0 mL, 49.48 mmol) was added dropwise into the so-
lution at 0 ℃. The solution was then stirred at room
temperature overnight. The precipitate was filtered and
the solvent was evaporated. The product was obtained
by column chromatography on silica gel using petro-
leum ether∶CH2Cl2 （4∶1）as eluent to afford com-
1
pound 3 as colorless oil with a 49% yield (1.2 g). H
NMR (CDCl3, 400 MHz) δ: 6.52 (d, J＝17.3 Hz, 2H),
6.18 (dd, J＝17.3, 10.5 Hz, 2H), 5.98 (d, J＝10.5 Hz,
2H), 4.66 (t, J＝13.5 Hz, 4H); 13C NMR (CDCl3, 75
MHz) δ: 164.3, 133.2, 126.6, 60.0, 59.6, 59.2; 19F NMR
(CDCl3, 376 MHz) δ: －120.1, －122.5, －124.0;
HRMS (High resolution mass spectrometry) calcd for
C16H10F16O4 570.0324, found 570.0328.
Gel formation
In a typical gelation experiment, a weighed amount
of the polymer 1 and 1.0 mL of the solvent were placed
in a test tube, which was sealed and then heated until the
sample was dissolved. If the compound was unable to
dissolve, it was noted as insoluble (I). After cooling
down to 25 ℃ with water bath, if a stable and trans-
parent gel was formed when inverting the vial, it was
noted as gelation (Gt); if a stable and opaque gel was
formed, it was noted as gelation (Go); if solution was
formed, it was noted as soluble (S). Repeated heating
and cooling confirmed the thermo-reversibility of the
gelation process. The critical gelator concentration
(CGC) of the organogelator was determined by measur-
ing the minimum amount of gelator required for the
formation of a stable gel at room temperature (25 ℃).
For the determination of gel-sol phase-transition tem-
perature, DSC (differential scanning calorimetry) analy-
sis of the gel was carried out. 6.0 mg of the gelled sam-
ple in THF (25 mg/mL) was carefully added to the alu-
minum crucible, sealed and heated from －10 to 50 ℃;
the heating rate was 5.0 ℃/min. During the measure-
ment, the oven was flushed with 20 mL/min nitrogen as
protective gas and 50 mL/min nitrogen as purge gas.
Synthesis of polymer 1
Compound 2 (0.60 g, 1.47 mmol) and compound 3
(0.84 g, 1.47 mmol) were charged in a dry flask con-
taining 15.0 mL of chloroform at room temperature, and
then the reaction mixture was stirred at 50 ℃ for 5 d.
The mixture was poured into hexane to precipitate the
solid product. The product was collected by filtration.
Then, the crude polymers were extracted by acetone in
Soxhlet for 24 h to afford the desired polymer 1 (1.2 g,
86% yield). 1H NMR (CDCl3, 400 MHz) δ: 8.73 (s, 4H),
4.59 (t, J＝13.6 Hz, 4H), 4.22 (t, J＝6.7 Hz, 4H), 2.74
—2.76 (m, 4H), 2.61—2.54 (br, 8H), 2.27 (s, 6H), 1.90
—1.92 (m, 4H); 13C NMR (CDCl3, 75 MHz) δ: 171.0,
162.8, 130.9, 126.6, 126.6, 59.8, 59.5, 59.1, 55.0, 52.6,
41.5, 39.2, 32.1, 25.6; 19F NMR (CDCl3, 376 MHz) δ:
－120.1, －122.4, －123.9; IR (KBr) ν: 3081, 2949,
2870, 2803, 1734, 1705, 1661, 1455, 1342, 1244, 769
－
1
cm ; GPC (THF, PS standard) Mn＝7000, Mw＝10000,
PDI＝1.47.
Results and Discussion
Synthesis of compound 2
Synthesis and characterization
1,4,5,8-Naphthalene-tetracarboxylic dianhydride (3.0
g, 11.2 mmol) and N-methyl-1,3-propane diamine (30.0
mL, 0.24 mol) in THF (40.0 mL) were stirred and re-
fluxed for 8.0 h. After being cooled down to room tem-
perature, the reaction mixture was left overnight. The
precipitate formed was filtered and taken up in CHCl3.
The undissolved material was filtered off, and the sol-
vents were evaporated under reduced pressure. The de-
sired material was obtained as a deep brown needle after
The synthesis of polymer 1 is shown in Scheme 2.
Condensation of 1,4,5,8-naphthalene-tetracarboxylic
dianhydride with N-methyl-1,3-propane diamine in THF
at 60 ℃ led to compound 2 in 30% yield after separa-
tion. Reaction of 1H,1H,10H,10H-perfluoro-1,10-
decanediol with acryloyl chloride yielded compound 3
in 49% yield after separation. Polymer 1 was obtained
by the Michael reaction between compounds 2 and 3 (in
1∶1 molar ratio). After workup, polymer 1 was charac-
1
recrystallization from ethanol (1.34 g, yield 30%). H
NMR (CDCl3, 400 MHz) δ: 8.76 (s, 4H), 4.29 (t, J＝7.1
1
terized with H NMR, 13C NMR and 19F NMR spec-
Chin. J. Chem. 2012, XX, 1—X
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Luo et al.
FULL PAPER
troscopy. These NMR data are in good agreement with
the repeating-moieties of polymer 1. Polymer 1 exhibits
two reversible reduction waves with E1/2red1＝－0.58 V
and E1/2red2＝－0.99 V as expected for the NDI moieties
(see Figure S1). The resulting polymer was found to be
soluble in common organic solvents, such as toluene,
dichloromethane, chloroform, and THF. Based on the
GPC (gel permeation chromatography) analysis with
THF as the eluent and polystyrene as the calibration
standard, the weight-average molecular weight (Mw) of
polymer 1 was determined to be ca. 10000 with a nar-
row molecular weight distribution (1.47). Polymer 1
melts above 172 ℃ (Tm) based on the DSC data, and
starts to decompose at 230 ℃ (Td) on the basis of TGA
analysis (Figure S2).
Figure 1 Illustration of gel formation with polymer 1 (30
mg/mL) in THF and disappearance of the gel phase after addition
of hydrazine hydrate.
Gel formation and responsiveness
The gelation ability of polymer 1 was examined in
different solvents at 25 ℃, and the results were listed in
Table 1. Among the solvents tested, polymer 1 can gel
several organic solvents including THF, dioxane, tolu-
ene and chlorobenzene. The corresponding critical gela-
tion concentrations (CGCs) at 25 ℃ were also meas-
ured (see Table 1). As demonstrated in Figure 1, a
transparent yellow gel was formed by cooling a hot THF
solution of polymer 1 (30 mg/mL) to 25 ℃. The gel-sol
transition was also investigated with differential scan-
ning calorimetry (DSC). As depicted in Figure S2, the
gel-sol transition occurred at ca. 20 ℃ for the gel of
polymer 1 (25 mg/mL in THF) and the transition took
place gradually. Of note is the fact that gels of polymer
1 are thermally reversible and the sol-gel interconver-
sion can be repeated by alternating heating and cooling
as for typical physical gels. Figure 2 shows the SEM
images of xerogel of polymer 1 formed in THF. Fibers
Figure 2 SEM image of the xerogel of polymer 1 from THF.
and sheets of various sizes were interconnected to form
porous structures. The intermolecular π-π interactions
due to NDI moieties and those among fluorinated alkyl
chains as well may be the driving-forces for the gelation
process. Figure 3 shows the 1H NMR spectra of polymer
1 at different concentrations in d8-THF. When the con-
centration of polymer 1 increased, the corresponding
signal at δ 8.66 owing to NDI moieties was gradually
up-shifted, as shown in Figure 3. For instance, the signal
at δ 8.66 was shifted to δ 8.62 by increasing the concen-
1
tration of polymer 1 from 5.0 to 60 mg/mL. Such H
NMR spectral shifts manifest the π-π interactions among
NDI moieties which can be operated in either intra-
chain or inter-chain ways as illustrated in Scheme 1. For
comparison, the polymer (Mw＝11000) with alternating
naphthalene diimide and alkyl chains was prepared (see
page S4 of Supporting Information). But, gelation was
not observed with this polymer under the same condi-
tions as for polymer 1.
Table 1 The gelation experimental results with polymer 1 in
different solvents at 25 ℃a
Solvent
Gelation examination
THF
Gt (25 mg/mL)
Dioxane
Gt (50 mg/mL)
Chlorobenzene
Toluene
Go (20 mg/mL)
Go (50 mg/mL)
DMF
P
I
Methanol
Ethanol
I
Hexane
I
Cyclohexane
n-Heptane
Dichloromethane
Chloroform
a Gt: transparent gel; Go: opaque gel; I: insoluble; P: precipitate; S:
solution. The CGCs of the corresponding gels were measured at
room temperature (25 ℃) and listed in the parentheses. The Tgel
for the corresponding organogels under CGC condition were 25
℃.
I
I
S
Figure 3 Partial 1H NMR spectra for the polymer 1 of different
concentrations (5.0—100 mg/mL) in d8-THF.
S
Notably, gels of polymer 1 can be destroyed after the
addition of hydrazine hydrate. For example, in the pres-
ence of hydrazine hydrate, the gel at the contact surface
4
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Chin. J. Chem. 2012, XX, 1—6

Gels with the Polymer Containing Alternating Naphthalene Diimide and Fluorinated Alkyl Chains
NDI·－ in the presence of N2H4. Accordingly, the
transformation of the gel of polymer 1 in the presence of
hydrazine hydrate can be attributed to the generation of
NDI·－ as discussed above. Similarly, new absorptions
around 474, 608, 678, 754 nm were observed for poly-
mer 1 after addition of either F－or CN－ (see Figures S4
and S5). ESR signals were also detected (see the insets
of Figures S4 and S5). These spectral data also indicate
the formation of NDI·－ for the solution of polymer 1 in
the presence of either F－ or CN－. Again, this is under-
standable for the responsiveness of gels of polymer 1
toward either F－ or CN－.
Thin-film formation and electrical conductivity
Thin films of polymer 1 can be prepared with
spin-coating technique. Figure S6 shows the XRD pat-
tern and AFM image for the thin film of polymer 1 on
the modified SiO2 substrate. The thin film was found to
be smooth based on AFM image, but no XRD diffrac-
tion peaks were detected, indicating that polymer chains
of 1 were not orderly arranged. However, diffraction
peaks at 21.2° and 23.5° appeared after the thin film was
annealed at 180 ℃ under vacuum. The diffraction
peaks correspond to d-spacing of 3.8 and 4.2 Å, which
may indicate the π-π interactions of NDI moieties. Si-
multaneously, morphological changes occurred for thin
films of polymer 1 after annealing; non-continuous do-
mains were formed after annealing (see Figure S6).
The conductivity of the as-prepared thin film of
polymer 1 was measured to be about 1.7—1.8 S/m in air,
and it kept almost unaltered in the temperature range of
－30—50 ℃ (Figure 5). Interestingly, the conductivity
increased after the thin-film was exposed to the satu-
rated hydrazine vapor; moreover the conductivity fur-
ther increased by increasing the temperature as shown in
Figure 5. After treatment with saturated hydrazine vapor,
the conductivity of the thin film of polymer 1 reached
19.7 S/m at 25 ℃ and 52.4 S/m at 50 ℃. Such con-
ductivity enhancement can be attributed to reduction of
NDI moieties into the respective NDI·－ and as a result
Figure 4 UV-vis absorption spectra of polymer 1 (3.0×10－
5
mol/L in DMF) in the presence of different amounts of N2H4 (0—
20400 equiv. vs. NDI moieties); the inset shows the ESR spec-
trum of polymer 1 (3×10－ mol/L in DMF) in the presence of
4
N2H4 (20400 equiv. vs. NDI moieties, g＝2.0058).
collapsed immediately. But, it took about 1.0 h or more
for the transformation of the whole gel due to the slow
diffusion of hydrazine into the gels. As illustrated in
Figure 1 the gel of polymer 1 in THF was transformed
into fuscous precipitates suspended in the solution after
the addition of hydrazine hydrate. Similar phenomena
were also observed for gels of polymer 1 after the addi-
tion of either F－ or CN－. Rheological studies also
demonstrate such responsiveness for polymer 1. For
instance, the viscosity of polymer 1 (200 mg/mL) in
DMF/dioxane (V/V, 1/4) at 50 ℃ was measured to be
47 Pa•s; but, the viscosity decreased to 4.0 Pa•s after
the addition of hydrazine hydrate (0.5 equiv. vs. NDI
moieties) within 10 s as shown in Figure S3. However,
the viscosity could not be restored (reaching 17 Pa•s)
after further addition of excess NOPF6 (see Figure S3).
This is indeed in agreement with the observation that
gels of polymer 1 that have been treated by either hy-
drazine hydrate or F－ or CN－ cannot be recovered by
further addition of excess oxidants such as NOPF6.[13]
The responsiveness of gels of polymer 1 is ascribed to
the reduction of NDI moieties into the respective radical
anions in the presence of either hydrazine hydrate or F－
or CN－, and as a result the π-π interactions among NDI
moieties are destabilized due to the electrostatic repul-
sions.
Figure 4 shows the absorption spectrum of polymer 1
and those after addition of hydrazine hydrate. Polymer 1
exhibits typical absorptions of NDI moieties at 344 nm,
360 and 380 nm. New absorptions around 474, 608, 678,
754 nm emerged gradually after introducing hydrazine
hydrate to the solution (Figure 4). The appearance of
these new absorptions is consistent with the formation
of NDI·－ according to previous reports.[11] The solution
of polymer 1 was ESR inactive, but, nine ESR signals
were observed after addition of hydrazine hydrate as
depicted in the inset of Figure 4. These spectral data
reveal the reduction of NDI moieties into the respective
Figure 5 Variation of the conductivity vs. temperature for the
thin film of polymer 1 (50 nm in thickness) in air or in saturated
hydrazine vapor; the inset shows the schematic illustration of the
two-electrode device fabricated on a SiO2 substrate.
Chin. J. Chem. 2012, XX, 1—X
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Luo et al.
FULL PAPER
the carrier concentration increases.[14] Therefore, the
thin film of polymer 1 is potentially useful for sensing
N2H4 and other reducing agents.
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Conclusions
In summary, a new electroactive polymer 1 with al-
ternating NDI moieties and fluorinated alkyl chains was
prepared and characterized. Polymer 1 can gel several
organic solvents and the resulting gels show respon-
siveness toward N2H4, F－ or CN－. Absorption and ESR
studies reveal that such responsiveness is owing to re-
duction of NDI moieties in polymer 1 into the respective
NDI·－ in the presence of either N2H4 or F－ or CN－,
and as a result the π-π interactions among NDI moieties
may be altered. Thin films of polymer 1 can be prepared
with the spin-coating technique. Moreover, the conduc-
tivity of the thin films of polymer 1 increases upon ex-
posure to N2H4 vapor reaching 19.7 S/m at 25 ℃. The
results of current research demonstrate the potential ap-
plication of such electroactive polymer in developing
new responsive materials.
Acknowledgement
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The present research was financially supported by
the National Natural Science Foundation of China (Nos.
50973120, 21021091, 20902092), Chinese Academy of
Science, and State Key Basic Research Program.
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(Lu, Y.)
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