Polyfluorene-based push-pull type functional materials for write-once-read-many-times memory devices

Article abstract of DOI:10.1021/cm1012872

A highly soluble polyfluorene-based copolymer containing electron-rich triphenylamine (TPA) and electron-poor 9,9-bis(3,4-bis(3,4-dicyanophenoxy)phenyl side chains in the C-9 position of the fluorene unit was synthesized under Yamamoto conditions. By appl

Full text of DOI:10.1021/cm1012872

Chem. Mater. 2010, 22, 4455–4461 4455
DOI:10.1021/cm1012872
Polyfluorene-Based Push-Pull Type Functional Materials for
Write-Once-Read-Many-Times Memory Devices
Xiao-Dong Zhuang,† Yu Chen,*,† Bi-Xin Li,‡ Dong-Ge Ma,*,‡ Bin Zhang,† and Yongxi Li†
†Key Laboratory for Advanced Materials, Institute of Applied Chemistry, East China University of Science and
Technology, 130 Meilong Road, Shanghai 200237, China, and State Key Laboratory of Polymer Physics and
Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China
‡
Received May 7, 2010. Revised Manuscript Received June 29, 2010
A highly soluble polyfluorene-based copolymer containing electron-rich triphenylamine (TPA) and
electron-poor 9,9-bis(3,4-bis(3,4-dicyanophenoxy)phenyl side chains in the C-9 position of the fluorene unit
was synthesized under Yamamoto conditions. By applying 306 nm as excitation wavelength, the resultant
polymer exhibits strong photoluminescence with maximum emission peaks centered at 413 and 433(sh) nm
in chloroform. The calculated highest occupied molecular orbital (HOMO), lowest unoccupied molecular
orbital (LUMO), energy bandgap, ionization potential, and electron affinity are -5.66, -3.44, 2.22, 5.92,
and 3.70 eV, respectively. The as-fabricated polymer film exhibited typical stable write-once-read-many-
times (WORM) memory characteristics, which are desirable for ultralow-cost permanent storage of digital
images. The currents in both ON and OFF states did not show any degradation, suggesting good device
stability. The ON/OFF current ratio observed in the sweep I-V characteristics at þ1.0 V is 6.1 ꢀ 103. The
conduction mechanism through ITO/polymer/Al device is discussed.
Introduction
optoelectronic and photonic devices. In our previous work,
we designed a series of pull-push type functional polymers,
including C60-PDPAF-C60,10 CNF-TPA,9 DR1-PDPAF-
DR1,8,13 TPAPAM-GO (GO: graphene oxide),14 and GO-
PVK.15 Memory devices based on the last three materials
exhibit typical bistable electrical switching and nonvolatile
rewritable memory effect, with an ON/OFF state current
ratio of more than 103. Both the ON and OFF states are
stable under a constant voltage stress and survived up to 108
read cycles at a small read voltage.
As a type of nonvolatile memory that is capable of
holding data permanently and being read from repeat-
edly, the write-once-read-many-times (WORM) memory
is very desirable for ultralow-cost permanent storage of
digital images, eliminating the need for slow, bulky, and
expensive mechanical drives used in conventional mag-
netic and optical memories.16-20 Basically the WORM
One of the aims of the investigations into the donor-
acceptor (D-A) type polymers, in which the hybridization
of the energy levels of the donor and acceptor moieties
result in D-A systems with unusually low highest occu-
pied molecular orbital (HOMO) to lowest unoccupied
molecular orbital (LUMO) separation,1 is to construct
artificial systems that mimic the electron transfer lying at
the heart of photosynthetic solar energy conversion and
in developing molecular electronics.1-12 Some known
electron withdrawing groups, for example, cyano, nitro,
quinoxalines, pyrazines, thiadiazoles, TCNQ, AIDCN,
F16PcCu, C60 and others, have been widely used in the
*To whom correspondence should be addressed. E-mail: chentangyu@
yahoo.com (Y.C.), mdg1014@ciac.jl.cn (D.-G.M.).
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2010 American Chemical Society
Published on Web 07/16/2010
pubs.acs.org/cm

4456 Chem. Mater., Vol. 22, No. 15, 2010
Zhuang et al.
memory functions as conventional CD-R, DVD-R, or pro-
grammable read-only memories(PROMs). Choi et al.16
reported a new cyano-rich hyperbranched copper phthalo-
cyanine polymer (HCuPc) which is thermally stable up to
450 °C. The device based on the HCuPc polymer exhibited
excellent WORM memory characteristics and reliability,
even in air ambient conditions. However, it should be worth
noting that, from the molecular structure of HCuPc the
authors have given, this material might have the relatively
low solubility in common organic solvents because of the
strong π-π stacking effect between the coplanar phthalo-
cyanine rings, giving rise to a technical difficulty in the
fabrication of large-area thin film devices by spin- or
blade-coating a solution in organic solvent. On the other
hand, it would also be difficult to control and/or adjust the
content of the electron-withdrawing cyano groups in the
polymer structure, which may have an important influence
on the memory performance of the devices, especially in such
a device based on the D-A type polymers. Recently we
reported a novel WORM device by spin-coating a pre-
assembled solution of D-A type Dispersed Red-1(DR1)-
grafted poly(N-vinylcarbazole) (PVDR) as the active layer
sandwiched between Al and ITO electrodes.21 Upon self-
assembly, the nanoaggregated PVDR film displays helical
columnar stacks with large grain sizes, whereas a non-
nanoaggregated PVDR film exhibits an amorphous morpho-
logy with smaller grain size. This device shows very good
memory performance, with an ON/OFF current ratio of
more than 105 and a low misreading rate through the precise
control of the ON and OFF states. The stability of the
nanoaggregated PVDR device is much higher than that of
the non-nanoaggregated PVDR device. This difference in
device stability under constant voltage stress can be mainly
attributed to the difference in the film crystallinity and surface
morphology. In this study, we designed and synthesized a
highly soluble polyfluorene-based polymer 6 containing
electron-rich triphenylamine and electron-poor 9,9-bis(3,4-
bis(3,4-dicyanophenoxy)phenyl side chains in the C-9 posi-
tion of fluorene unit, as shown in Scheme 1. This material
exhibits typical WORM memory characteristics, with long
retention time of more than 3000 s without any degradation
and high ON/OFF ratio of 6 ꢀ 103. In contrast to the
hyperbranched HCuPc polymer,16 the content of the cyano
groups in the polymer structure can be easily controlled by
adjusting the monomer levels used for polymerization.
Scheme 1. Synthesis of 3, 5, and 6
were measured using a Perkin-Elmer Pyris 1 thermogravi-
metric analyzer in flowing (100 mL.min-1) nitrogen atmo-
sphere. Steady-state fluorescence spectra were measured on a
Shimadzu RF-5300 PC spectrofluoro-photometer equipped
with a photomultiplier tube having high sensitivity in the
700-800 nm region. The sample for the fluorescence measure-
ment was dissolved in the dry chloroform, filtered, transferred to
a long quartz cell, and then capped and bubbled with high pure
argon(without O2 and moisture) for 10 min before measure-
ment. Gel permeation chromatography trace of the samples
were recorded on a Viscotek T60A/LR40 GPC system. Tetra-
hydrofuran (THF) was used as the eluent, and linear polystyrene
was used as the standard.
Cyclic voltammetry was performed on an ALS630B electro-
chemical analyzer in deaerated benzonitrile containing recrys-
tallized tetrabutylammonium perchlorate (Bu4NClO4, 0.1 M) as
the supporting electrolyte at 298 K. A conventional three-
electrode cell was used with a platinum working electrode
(surface area of 0.3 mm2) and a platinum wire as the counter
electrode. The Pt working electrode (Bioanalytical System-
(BAS), Inc.) was routinely polished with BAS polishing alumina
suspension and rinsed with acetone before use. The measured
potentials were recorded with respect to the Ag/AgNO3 (0.01 M)
reference electrode. All electrochemical measurements were
carried out under an atmospheric pressure of argon.
Experimental Section
General Procedures. The operations for synthesis prior to the
termination reaction were carried out under purified argon. All
chemicals were purchased from Aldrich and used without
further purification. Organic solvents were purified, dried, and
distilled under dry nitrogen.
The UV/vis absorption spectral measurements were carried
out with a Shimadzu UV-2450 spectrophotometer. FTIR spec-
tra were recorded on a Nicolet Nagma-IR 550 spectrophoto-
meter using KBr pellets. Thermal properties of the samples
Device Fabrication. The Indium tin oxide (ITO) glass sub-
strate was carefully precleaned sequentially with deionized
water, acetone, and 2-propanol in an untrasonic bath for
15 min, and then treated with oxygen plasma. A dimethylform-
amide (DMF, 6 mL)/toluene (2 mL) solution of polymer 6
(6.4 mg) was spin-coated (1000 rpm, 40 s) onto the ITO
substrate, followed by the removal of the solvent in a vacuum
(21) Zhuang, X.D.; Chen,Y.; Liu,G.; Zhang,B.; Neoh, K. G.; Kang,
E. T.; Zhu, C. X.; Li,Y. X.; Niu, L. J. Adv. Funct. Mater. 2010,
in press, DOI: 10.1002/adfm.201000258.

Article
Chem. Mater., Vol. 22, No. 15, 2010 4457
chamber at 10-5 Torr at 100 °C for 5 h. The thickness of the
polymer layer was about 50 nm. Finally, a 300 nm-thick Al top
electrode was thermally evaporated at a pressure around 10-7
Torr through a shadow mask. The measurements were carried
out on devices of four 3 ꢀ 3 mm2 in size. The current-voltage
(I-V) characteristics were performed using a Keithley 2400
sourcemeter controlled by a computer, and the positive voltage
was applied to the top Al electrode. All the electrical measure-
ments were carried out in ambient condition without any device
encapsulation.
After heating at 80 °C for 24 h, an excess of bromobenzene
(2 g,12.74 mmol) was added in the above reaction system, followed
by additional reaction at the same temperature for 24 h. The
reaction mixture was poured into a methanol/HCl(2:1) solution.
The collected crude product was dissolved in CHCl3, reprecipitated
from methanol/acetone(4:1), and then subject to a Soxhlet extrac-
tion with acetone to give the target product 6 (300 mg, 47%). GPC:
Mn = 1.1 ꢀ 104, Mw/Mn = 4.4; UV/vis (in CHCl3): λmax = 236,
306 nm; PL (in toluene, λex = 306 nm): λmax= 411 nm.
Synthesis of 4,40-(2,7-Dibromo-9H-fluorene-9,9-diyl)dibenzene-
1,2-diol (3). A stirred mixture of 2,7-dibromofluorenone (3.1 g,
9 mmol), pyrocatechol (5.0 g, 45 mmol), and methanesulfonic acid
(2.7 mL, 40 mmol) in CCl4 (20 mL) was heated at 80 °C for 40 h.
After cooling to room temperature, the reaction mixture was
filtered, washed with a large amount of CH2Cl2, and then repreci-
pitated from acetone to produce 2.56 g of gray powder (yield
52.7%). 1H NMR (DMSO-d6, 500 MHz): δ/ppm = 6.32 (dd, 2H),
6.49 (d, 2H), 6.60 (d, 2H), 7.42 (s, 2H), 7.57 (dd, 2H), 7.88(d,2H),
8.85(s,2H), 8.89(s,2H); Elemental Analysis (EA): calcd for C25H16-
Br2O4(537.94 g/mol) C55.58; H2.99, found C55.65; H3.38; MS
(EI): m/z 538.00 (Mþ).
Results and Discussion
Fluorene-based polymers and oligomers have emerged as
one of the most promising materials owing to their high
photoluminescence quantum yields, good charge transport
properties, and better thermal and chemical stability.22 The
classical poly(alkylfluorene) homopolymers suffer from
poor spectral stability associated with keto-defects in the
polymer backbone or excimer emission.23 The common
strategies, to address this issue, are the introduction of bulky
groups to the C-9 position of the fluorene unit, for example,
spirobifluorene, or copolymerization with suitable co-
monomers.24 A large number of theoretical and experimen-
tal results have demonstrated that the facile functionaliza-
tion at the C-9 position of the fluorene unit may offer an
opportunity to reduce the interchain interactions thereby
improving the electric, optoelectronic, and photonic proper-
ties of the resulting polymers. In our previous work,25 we
prepared a novel conjugated PFO/PPV copolymer contain-
ing the pendant bis(4-alkoxyphenyl) groups in the C-9
position of every fluorene unit through a typical Heck-
couping reaction. The resulting polymer exhibits very strong
photoluminescence with maximum emission peaks centered
at 474 nm in dilute benzonitrile, and a single glass-transition
temperature at about 95 °C. The excited triplet-state
Synthesis of 9,9-Bis[3,4-bis(3,4-dicyanophenoxy)phenyl]-2,7-
dibromo-9H-fluor-ene (4). A mixture of 3 (1.61 g, 3.0 mmol),
4-nitrophthalonitrile (2.08 g, 12.0 mmol), and K2CO3 (1.66 g,
12.0 mmol) in anhydrous dimethyl sulfoxide (DMSO, 30 mL)
was stirred at 25 °C for 20 h. With completion of the reaction,
the mixture was poured into 200 mL of cold dilute HCl. The
collected crude product was washed with water until neutral,
and then redissolved in CH2Cl2 (100 mL), washed with
5% NaOH solution (400 mL) and water (100 mL), respectively.
The organic layers were dried over MgSO4 and filtered.
Evaporation of the solvent was followed by column chromato-
graphy (SiO2, ethyl acetate/petroleum ether(v/v 1:4) as eluant,
Rf = 0.22). A pale yellow solid obtained was thoroughly vacuum-
dried at 40 °C overnight. Yield 1.28 g (41%). 1H NMR (DMSO-
d6): δ/ppm = 6.91(dd, 2H), 7.29 (d, 2H), 7.32 (dd, 2H), 7.35 (dd,
2H), 7.66 (d, 2H), 7.68-7.73(m,6H), 7.73(d, 2H), 7.89 (d, 2H), 8.00
(d, 4H); 13C NMR (DMSO-d6): δ 64.5, 108.9, 109.3, 115.4,
115.7,116.0, 116.2, 116.8, 117.0, 121.3, 121.8, 122.1, 123.6, 123.7,
124.6, 126.7, 129.2, 132.1, 136.3, 136.5, 138.3, 144.2, 144.4, 144.7,
152.1, 160.3, 161.0; EA calcd for C57H24N8O4Br2: C65.53, N10.73,
H2.32; found C63.98, N10.28, H2.02.
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Synthesis of 9,9-Bis(4-diphenylaminophenyl)-2,7-dibromo-
fluorene (5). To a stirred mixture of 2,7-dibromofluorenone
(1.72 g, 5.10 mmol) and triphenylamine (17.5 g, 0.071 mol)
was added methane sulfonic acid (0.49 g, 5.10 mmol) under a
purifed argon atmosphere, and then reacted at 140 °C for 6 h.
After cooling to the room temperature, the dichloromethane
extract was washed with Na2CO3 solution until the aqueous
layer reached neutral, and then dried over anhydrous MgSO4
and filtered. Evaporation of the solvent was followed by column
chromatography (SiO2/ hexane-CH2Cl2). The obtained product
was recrystallized from acetone to give the TPA-substituted 2,7-
dibromofluorene (2.95 g,70%). FDMS: m/z = 811[Mþ]; UV/
vis(in CHCl3): λ/nm= 256, 310, 322; 1H NMR (in CDCl3):
δ/ppm= 6.99(m, 20H, aryl H), 7.22(m,8H, aryl H), 7.46(dd,2H),
7.51(d,2H),7.56(d,2H).
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Synthesis of the Copolymer 6. A mixture of Ni(COD)2
(620 mg, 2.2 mmol), 2,2-bipyridine (348 mg, 2.2 mmol), and
1,5-cyclooctadienyl(0.3 mL,2.2 mmol) in dry DMF (12 mL)
was stirred at 75 °C for 30 min under an argon atmosphere.
To the above solution was added a solution of 5 (1.57 g, 1.93 mmol)
and 4 (72 mg, 0.07 mmol) in dry toluene (15 mL)/DMF(5 mL).
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4458 Chem. Mater., Vol. 22, No. 15, 2010
Zhuang et al.
maximum of the polymer occurs in the region of 460-540
nm with a lifetime of 65.8 μs. In this study, as shown in
Scheme 1, the acid-catalyzed condensation reaction of 2,7-
dibromo-9-fluorenone (1) with excess pyrocatechol gives
4,40-(2,7-dibromo-9H-fluorene-9,9-diyl)dibenzene-1,2-diol
(3, 52.7%), which was used to react with 4-nitrophthalo-
nitrile to produce compound 4(41%) in the presence of anhy-
drous K2CO3. Condensation of 2,7-dibromofluorenone with
a large excess of triphenylamine (TPA) in methanesulfonic
acid gives 9,9-bis(4- diphenylamino- phenyl)-2,7-dibromo-
fluorene(5) in a yield of 70%. As one of the useful hole
transporting materials, TPA and its organic and/or poly-
meric derivatives have been widely used in optoelectronic
devices because of their low ionization potentials, tridimen-
sional steric, and good UV-light harvesting properties.26 The
polymerization of monomers 4 and 5 was carried out under
Yamamoto-coupling conditions to give a soluble D-A type
polymer 6 in good yield. It should be noted that the synthesis
employs an excess of bromobenzene to remove any residual
bromine functionalities. The resultant polymer has a number
average molecular weight (Mn) of 1.1 ꢀ 104, with a poly-
dispersity of 4.4.
Figure 1. 1H NMR spectrum of the sample in DMSO-d6/CDCl3 (v/v 2:1).
To dermine the ratio of x to y in the polymer structure
1
by the H NMR spectroscopy, the cyano groups in the
polymer needs to be converted to the carboxyl groups first
by hydrolysis of 6 accomplished with hydrochloric acid
alone.27 In a typical procedure, 6 was added into a con-
centrated HCl solution. After stirring at room tempera-
ture for 10 h, the reaction mixture was poured into a
MeOH/H2O (v/v 90:10) solution. The collected crude pro-
duct was washed with a large amount of water until the
aqueous layer reached neutral, followed by the vacuum-dry
at 40 °C overnight. The obtained resulting polymer with
carboxyl groups has good solubility in chloroform, but a
little bit dissolves in DMSO. In the 1H NMR spectrum of 6
the proton signal of CDCl3 at δ =7.24 ppm in general
overlaps with the signals for the aromatic protons. To avoid
the influence of its proton signal on the integral measure-
ments, the 1H NMR spectrum of 6 was done in a mixture of
CDCl3 and DMSO-d6 (v/v 1:2). In this case, the signal of
CDCl3 appears at δ = 8.10 ppm (in the literature28 the
proton signal of chloroform is located at δ = 8.32 ppm in
the presence of deuterated DMSO), as shown in Figure 1.
The integral ratio of the signals of the total carboxyl protons
to the total aromatic protons is about 2:71, from which the
ratio of x to y can be roughly estimated as 9:91.
Figure 2. UV/vis absorption and photoluminescence (λex = 306 nm)
spectra of the polymer 6 in dilute CHCl3.
As shown in Figure 2, the main absorption peak in the
UV/vis spectrum of 6 is located at 306 nm because of
the n-π* transition of the triphenylamine moiety.29
By applying 306 nm as excitation wavelength, the polymer 6
exhibits strong photoluminescence with maximum emission
peaks centered at 413 and 433(sh) nm in chloroform. The
observed larger Stokes shift for the sample (8408 cm-1) is
associated with a substantial change of the geometric
structure from the ground state (S0) to the first excited state
(S1)30 as a result of an intramolecular charge transfer (ICT)
or excited-state proton transfer.31
The electrochemical measurement was carried out
using cyclic voltammetry (CV) in deaerated acetonitrile
containing recrystallized Bu4NClO4(0.1M) at room tem-
perature (Figure 3). The potentials were measured against
Ag/AgCl as reference electrode. All potentials (vs Ag/
Agþ) were converted to values versus SCE by adding
0.29 V.32 The first oxidation and reduction potentials
for 6 were found to be as þ1.24 versus Ag/Agþ that corres-
ponds to þ1.53 V versus SCE, and -0.98 V versus Ag/Agþ
that corresponds to -0.69 V versus SCE, respectively.
The HOMO and LUMO values of 6 were experimentally
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Chem. Mater., Vol. 22, No. 15, 2010 4459
Figure 3. Cyclic voltammogram of the polymer film coated on platinum
plateelectrode indeaerated CH2Cl2 containing Bu4NClO4 (0.1M). Sweep
rate: 100 mV s-1
.
vacuum level, according to the eq 1:11,33
HOMO=LUMO ¼ - ½Eonset - Eox:ðferroceneÞꢁ - 4:8 eV ð1Þ
In our electrochemical experiments ferrocene exhibits
an oxidation peak with an onset of 0.38 V versus Ag/AgCl.
The calculated HOMO, LUMO, and bandgap values were
-5.66, -3.44, and 2.22 eV, respectively. The values of the
ionization potential (IP) and the electron affinity (EA) can
also be estimated from these onset potentials (vs SCE) using
eqs 2,3:34
Figure 4. Typical I-V characteristics of the ITO/polymer 6/Al device.
IP ¼ Efirst oxidation þ 4:39 eV
ð2Þ
EA ¼ Efirst reduction þ 4:39 eV
ð3Þ
Where the constant 4.39 eV is the relationship between IP,
EA, and the electrochemical potentials.35 The calculated IP
and EA values are 5.92 and 3.70 eV, respectively.
For the memory device, the I-Vcharacteristics (Figure 4)
were measured by scanning the voltage from 0 to (5 V and
back to 0 V, with grounded ITO. Starting with the high
conductivity state where one can refer to it as the ON state in
the as-fabricated device, the current increases slowly with
the increase of the applied positive voltage, and then
decreases abruptly at a critical bias (about 2.2 V). The device
remains in the low current state, for example, OFF state, as
the voltage is swept to higher value or back to -5 V. In other
words, the low OFF state is retained permanently in the
subsequent voltage sweeps. This irreversible nature of the
transition from a high conduction to a low conduction
Figure 5. Currents of ON and OFF states as a function of time for ITO/
polymer 6 (50 nm)/Al under a constant bias of þ1 V. The ON/OFF current
ratio as a function of the positive sweeping voltages is shown in the inset.
makes it suitable for WORM memory applications. Some
functional materials16,36-38 also exhibited similar WORM
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4460 Chem. Mater., Vol. 22, No. 15, 2010
Zhuang et al.
To further explain the conduction mechanism through
ITO/polymer 6/Al device, we analyzed the I-V curves
in the ON and OFF states respectively. As depicted in
Figure 7, a straight line with slope of 1 is observed in the
ON state, indicating that the current is Ohmic conduc-
tion.39 On the other hand, the I-V characteristic shows a
slope of 2 in the OFF state, implying that the Space
Charge Limited Currents (SCLC) is probably the main
conduction mechanism.40 Under a low positive bias
voltage (0f2.2 V), the continuous hole injection makes
the hole transport and linear conductivity more effective.
While this hole injection process goes ahead, the positive
charges on the TPA moieties will rapidly be consumed by
cyano groups as a result of the switching operation
(ONfOFF). Hence, the device maintains the OFF state
during the subsequent operation. In a typical low con-
duction state (5f0 V), the SCLC model was found to
suitably fit the logarithmic plot of the I-V data of the
OFF state. These results may suggest that the generated
carriers lead to an accumulation of space charges and a
redistribution of electric field after the ON state to OFF
state transit.41 The stable charge transfer complex of the
polymer 6 was formed42 after these operation. As the
reverse bias voltage is applied (ITO as anode), the low
conduction state was thus holding throughout the pro-
cess, and the same phenomena occurred in the next
sweeping cycles.
Figure 6. HOMO and LUMO energy levels ofpolymer 6, work functions
of Al, ITO (before and after plasma treatments), and possible electric field
induced hole migration process between polymer and electrodes.
As we have noted previously, in the work of Williams
and his co-workers at Hewlett-Packard,43-49 they defini-
tively established that such devices involve metal fila-
ments that can be formed reversibly.43,44 This seems to
imply that it is doubtful that anyone is going to follow this
work to make a real memory device. Indeed, nobody can
completely avoid the possibility of metal filaments. But
from our results one can clearly see that in our devices
metal filaments should not be the main mechanism, as
described in our text. Although 50-nm-thick polymer film
Figure 7. I-V characteristics of the ITO/polymer 6/Al device in the ON
and OFF states.
memory effects. The data retention ability tests were carried
out on the devices in the ON- and OFF-states by applying a
reading voltage of 1 V. As shown in Figure 5, the currents in
both ON and OFF states did not show any degradation, sug-
gesting good device stability. The ON/OFF current ratio ob-
served in the sweep I-V characteristics at þ1.0 V is 6.1 ꢀ 103.
From Figure 6, one can clearly see that, for the positive
sweep (ITO was used as cathode and Al as anode), the
energy barriers of Al/polymer 6 (LUMO) and polymer 6
(HOMO)/ITO interfaces are 0.86 and 0.84 eV, respec-
tively. While for the negative sweep (in this case the ITO
was used as anode and Al as cathode), the energy barriers
of ITO/polymer 6 (LUMO) and polymer 6 (HOMO)/Al
interfaces are 1.38 and 1.36 eV, respectively. The high
energy barriers prevent switching from the OFF state to
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positive sweep because of the electron-withdrawing effect
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resulting in the formation of the high current state.
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tion is rather difficult, giving rise to an abrupt current
decrease. This phenomenon is irreversible, which implies
that the low OFF state will be retained permanently in the
subsequent voltage sweeps.
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Chem. Mater., Vol. 22, No. 15, 2010 4461
may have pinholes, but if so, the device would not be able
to work normally. In our experiment, the polymer can
form uniform good film without pinholes, which means
that the short circuits seem impossible after evaporating
Al. As a result, we did not observe the short circuits
during the measurements. We tested many devices to test
this. As expected, all the results were similar.
(Mn) of 1.1 ꢀ 104, with a polydispersity of 4.4, and exhibits
strong photoluminescence with maximum emission peaks
centered at 413 and 433(sh) nm in chloroform. The ob-
servedlargerStokes shift for the sample isassociatedwith a
substantial change of the geometric structure from the
ground state (S0) to the first excited state (S1) as a result of
an ICT or excited-state proton transfer. The estimated
HOMO, LUMO, energy bandgap, ionization potential,
and electron affinity were -5.66, -3.44, 2.22, 5.92, and
3.70 eV, respectively. The as-fabricated polymer film ex-
hibited typical stable WORM memory characteristics,
with long retention time of more than 3000 s without any
degradation and high ON/OFF ratio of 6 ꢀ 103. The
conduction mechanism through ITO/polymer/Al device
was discussed in detail.
Conclusions
The facile functionalization at the C-9 position of the
fluorene unit may offer an opportunity to reduce the
interchain interactions thereby improving the electric, opto-
electronic, and photonic properties of the resulting poly-
mers. In this study, a highly soluble polyfluorene-based
copolymer was prepared by reaction of 9,9-bis[3,4-bis(3,4-
dicyanophenoxy)phenyl]-2,7-dibromo-9H-fluorene with 9,9-
bis(4- diphenylaminophenyl)-2,7-dibromofluorene under
Yamamoto-coupling conditions. The content of the cyano
groups in the polymer structure can be easily controlled by
adjusting the monomer levels used for polymerization.
This material has a number average molecular weight
Acknowledgment. The authors are grateful for the financial
support of the National Natural Science Foundation of China
(20876046), the Ministry of Education of China (309013), the
Fundamental Research Funds for the Central Universities, the
Shanghai Municipal Educational Commission for the Shuguang
fellowship (08GG10) and the Shanghai Eastern Scholarship.