Electrochemical fluorescence switching properties of conjugated polymers composed of triphenylamine, fluorene, and cyclic urea moieties

Article abstract of DOI:10.1002/pola.26354

The study of the electrochemical fluorescence switching properties of the conjugated copolymers containing fluorene, triphenylamine, and 1,3-diphenylimidazolidin-2-one moieties is reported. The polymers show high fluorescence quantum yields, excellent thermal stability, and good solubility in polar organic solvents. While the polymer emits blue light under UV irradiation, the fluorescence intensity is quenched upon electrochemical oxidation. The fluorescent behavior can be reversibly switched between nonfluorescent (oxidized) state and strong fluorescence (neutral) state with a high contrast ratio (I_f/I_f0) of 16.3. The role of the electrochemical oxidation of the triphenylamine moieties is to generate the corresponding radical cations that lead to fluorescence quenching in the solid matrix.

Full text of DOI:10.1002/pola.26354

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Electrochemical Fluorescence Switching Properties of Conjugated Polymers
Composed of Triphenylamine, Fluorene, and Cyclic Urea Moieties
Cheng-Po Kuo,1,2 You-Shiang Lin,1,2 Man-kit Leung1,2
1Department of Chemistry, National Taiwan University, Taipei, Taiwan, Republic of China
2Institute of Polymer Science and Engineering, National Taiwan University, Taipei, Taiwan, Republic of China
Correspondence to: M.-k. Leung (E-mail: mkleung@ntu.edu.tw)
Received 27 June 2012; accepted 18 August 2012; published online
DOI: 10.1002/pola.26354
ABSTRACT: The study of the electrochemical fluorescence
switching properties of the conjugated copolymers containing
fluorene, triphenylamine, and 1,3-diphenylimidazolidin-2-one
moieties is reported. The polymers show high fluorescence
quantum yields, excellent thermal stability, and good solubility
in polar organic solvents. While the polymer emits blue light
under UV irradiation, the fluorescence intensity is quenched
upon electrochemical oxidation. The fluorescent behavior can
be reversibly switched between nonfluorescent (oxidized) state
and strong fluorescence (neutral) state with a high contrast ra-
tio (If/If0) of 16.3. The role of the electrochemical oxidation of
the triphenylamine moieties is to generate the corresponding
radical cations that lead to fluorescence quenching in the solid
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matrix.
2012 Wiley Periodicals, Inc. J. Polym. Sci. Part A:
Polym Chem 000: 000–000, 2012
KEYWORDS: conjugated polymers; copolymerization; electro-
chemistry; fluorescence
INTRODUCTION Organic p-conjugated polymers are increas-
ingly adaptable to many fluorescence-based chemical and bi-
ological applications.1,2 The ability of these conjugated poly-
mers to amplify their sensitivity on fluorescence responses
is attributed to the efficient electronic delocalization and
rapid transport of excitons along the p-conjugated backbone.
Much research has been done on polyureas due to their out-
standing thermal and mechanical properties.3–9 However, to
our knowledge, fabrication of solid-state electrofluorescent
devices (EFD) using PU based materials has not been exam-
ined and reported. Electrofluorescent (EF) materials display
a reversible optical change in the fluorescence resulting from
electrochemical oxidation or reduction. Materials such as
organic/inorganic hybrid thin films,10 organic metal com-
plexes,11–14 organic molecules,15,16 and fluorescent poly-
mers17,18 have been explored. In addition, fluorescence emis-
sion shows high sensitivity on response to environmental
conditions. Because of these interesting properties, the redox
fluorescence switching has been subject of research for
potential applications such as fluorescent imaging, sensors,
displays, spectroscopy, and many other optoelectronic appli-
cations.10–26
in a polymer electrolyte/photocured polymer electrolyte/ITO
electrode. Upon reduction, the highly fluorescent tetrazine
molecules were reduced to their nonemissive anion-radical
state; the fluorescent emission from EFD was then quenched
in this state. The EF process was reversible so that the de-
vice on/off could be controlled with the applied electrical
potential. The EFD concept could be extended to fluorescent
polymers such as poly(methylene anthracene)17 and s-tria-
zine-bridged para-phenylenevinylene,18 so that the fluores-
cence properties could be electrochemically tuned. However,
due to the poor miscibility of many fluorescent polymers
with polymeric electrolytes, the EFD applications are limited.
Recently, a novel n-type doping poly(oxadiazole) (POD)26
that enables easy fabrication of EFD has been reported. The
POD exhibits a reversible change in the fluorescence proper-
ties by repetitive transformation between its reduced state
(nonfluorescent) and the neutral state (fluorescent) with a
maximum on/off ratio of 2.5.
Complementary to the electron-deficient properties of POD,
triarylamines (TAAs) are considered as a family of electron-
rich materials that play an important role in optoelectronics.
First, TAAs can be easily doped by oxidation to form stable
radical cations. In addition, TAAs usually show good hole-
transporting properties so that they are frequently adopted
in the hole-transporting layers in organic electroluminescent
devices. Furthermore, TAAs usually demonstrate a noticeable
change of coloration upon electrochemical redox processes
A pioneer example of the solid-state EFD that switches the
fluorescence by changing the redox states of tetrazine mole-
cule, between its neutral state and an anion-radical state,
was reported by Kim et al.15 The structure of the EFD was
composed of ITO electrode/a blend of electroactive tetrazine
Additional Supporting Information may be found in the online version of this article.
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so that they can be applied in electrochromic devices.27–39 In
addition, most of these polymers show good solubility in or-
ganic solvents due to their nonplanar propeller-shaped triar-
ylamine units that disrupt the packing of aromatic rings in
solid state; this leads to a relatively low crystallinity of the
materials so that they can form homogeneous amorphous
films in large-area thin-film devices, either by thermal vac-
uum deposition or by spin-coating methods.
The family of fluorene-based conjugated polymers (PFs) has
generated interest as blue-emitting materials because of
their excellent thermal stability and high fluorescence quan-
tum yields in solution and in solid state. However, it is diffi-
cult to maintain long-term morphological stability of thin
films due to crystallization or phase separation. Several
methods have been used to overcome this drawback, such as
introduction of bulky groups at the C-9 position of the fluo-
rene, copolymerization with other electroactive monomers,
and optimization of EL device configurations.40–51 We expect
that modification of the PFs by introduction of cyclic urea
and propeller-shaped triphenylamine units into the polymer
backbone can increase morphological stability.
Polyurethanes and polyureas (PUs) form an important family
of industrial polymers that have been widely used due to
their good properties such as elasticity, flexibility, thermal
stability, and excellent chemical resistance.52–58 In addition,
PU conducting polymers have recently been developed59–61
and applications of PUs on PLED have been reported. For
example, PUs can be used as effective hole-transport ma-
trix62–65 as well as applied for light emitting layers.66–68 Tra-
ditional methods for PU synthesis usually involve diamine
and diisocyanate moieties, which are toxic reagents. In this
study, we report the synthesis of our target PUs of 1 and 2
(Scheme 1) through a two-step sequence from a combination
of 3–6. The use of toxic diisocyanate reagent can be avoided.
In addition, we successfully fabricated an electrochemical
fluorescence switching device of 2, containing triphenyl-
amine, fluorene, and cyclic urea moieties.
SCHEME 1 Synthetic routes of 1, 2, and 7.
EXPERIMENTAL
1,3-Bis(4-bromophenyl)imidazolidin-2-one (3)
To a two-necked flask were charged imidazolidin-2-one (0.75
g, 8.7 mmol), 1,4-dibromobenzene (10.2 g, 43.5 mmol),
K2CO3 (3.0 g, 22 mmol), DMCHDA (0.49 g, 3.5 mmol), and
CuI (0.33 g, 1.7 mmol) under argon. The mixture was
refluxed at 120 ꢀC for 24 h. After reaction, the mixture was
cooled to room-temperature and extracted with methylene
chloride and water. The separated organic layer was dried
over anhydrous MgSO4, and the solvent was evaporated. The
residue was stirred for 2 h in hexane (200 mL). The result-
ing precipitate was collected by filtration, washed with hex-
ane, and then dried. The crude product was purified by col-
umn chromatography, using hexane/CH2Cl2 (1:1) as the
eluent, followed by recrystallization from CHCl3 to afford 3
(2.1 g, 61%) as white crystals. 1H NMR (400 MHz, CDCl3, d,
ppm): 7.46 (s, 8H), 3.94 (s, 4H). 13C NMR (100 MHz, CDCl3,
d, ppm): 131.10, 118.86, 41.64. HRMS (FAB): calcd for
Materials
All reagents, including poly(9,9-di-n-octylfluorene-2,7-diyl)
(PDOF), imidazolidin-2-one, 1,4-dibromobenzene, diphenyl-
amine, 4-bromotoluene, Cu powder, CuI, N-bromosuccini-
mide, Aliquat 336, and K2CO3 were commercially available.
trans-N, N0-Dimethylcyclohexane-1,2-diamine (DMCHDA)69
was synthesized according to literature procedures.
Monomer Synthesis
2,7-Dibromo-9,9-dibutylfluorene (4)70 and 2,7-bis(4,4,5,5-tet-
ramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dibutylfluorene (6)71
were synthesized according to literature procedures.
4-Bromo-N-(4-bromophenyl)-N-p-tolylaniline (5) was synthe-
sized according to the Supporting Information. All of the
monomers were carefully purified prior to use in the poly-
merization reaction.
C
15H12Br2N2O, 393.9316 (Mþ); found, 393.9307. Anal. calcd
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for C15H12Br2N2O: C, 45.49; H, 3.05; N, 7.07. Found: C, 45.49;
H, 3.38; N, 7.13.
let apparatus, and dried under vacuum to give 7 (0.52 g,
50%). H NMR (400 MHz, CDCl3, d, ppm): 7.75–7.47 (m, 6H),
1
7.23–7.13 (m, 12H), 2.36 (br, 3H), 2.03 (br, 4H), 1.12–1.08
(br, 4H), and 0.71–0.59 (br, 10H).
Synthesis of Copolymer 1
To a degassed solution of 3 (0.76 g, 1.9 mmol), 4 (0.83 g,
1.9 mmol), and 6 (2.0 g, 3.8 mmol) in toluene (30 mL),
K2CO3 (aq) (2.0 M, 15 mL), Aliquat 336 (30 mg), and tetra-
kis(triphenylphosphine)palladium (65.4 mg, 1.47 mol %)
were added under nitrogen atmosphere. The solution was
Fabrication of Electrofluorescent Switching Devices
An EF polymer film was prepared by spin coating a solution
of the 2 (10 mg mLꢁ1 in CHCl3) onto an ITO-coated glass
substrate and then dried in vacuum.72 In our study, this
layer also possessed a switching mechanism as well as pho-
toluminescence. On the other layer, a gel electrolyte based
on PMMA (Mw: 102,600) and LiClO4 was plasticized with
propylene carbonate to form a highly transparent and con-
ductive gel. PMMA (1 g) was dissolved in dry acetonitrile
(MeCN) (5 g), and LiClO4 (0.1 g) was added to the polymer
solution as supporting electrolyte. Then, after propylene car-
bonate (1.5 g) was added as plasticizer, the mixture was
slowly heated until gelation. The gel electrolyte was spread
on the polymer-coated side of the ITO electrode, and the
electrodes were sandwiched. Finally, the device was sealed
with epoxy resin.
ꢀ
then refluxed at 90 C for 3 days. At the end of the conden-
sation copolymerization, the mixture was cooled and poured
into a mixture of MeOH and water (300 mL, 7:3 v/v). The
crude polymer was collected, washed with excess MeOH, dis-
solved in CH2Cl2, and then reprecipitated into MeOH. Finally,
the polymer was washed with acetone for 24 h using a Soxh-
let apparatus, and dried under vacuum to give 1 (1.23 g,
60%). 1H NMR (400 MHz, CDCl3, d, ppm): 7.82–7.58 (m,
26H), 4.08 (br, 4H), 2.12 (br, 12H), 1.17–1.15 (br, 12H),
0.77–0.70 (br, 30H). 13C NMR (100 MHz, CDCl3, d, ppm):
154.94, 151.82, 140.43, 140.04, 139.86, 139.35, 139.20,
136.36, 128.78, 127.54, 127.18, 126.10, 125.71, 121.46,
121.19, 119.99, 118.40, 74.38, 55.27, 55.23, 44.77, 42.05,
40.24, 26.13, 23.10, 13.83. Anal. Calcd for C87H95N2O: C,
88.20; H, 8.08; N, 2.36. Found: C, 85.81; H, 7.82; N, 2.23.
Measurement
1H and 13C NMR spectra were recorded on a Varian Unity
Plus 400 MHz spectrometer. Molecular weights and polydis-
persities of polymers were determined by gel permeation
chromatography (GPC) analysis relative to polystyrene cali-
bration (Waters Styragel columns of HR3 and HR4E) using
THF as eluent at a flow rate of 1.0 mL minꢁ1. Differential
scanning calorimetry (DSC) analysis was performed under
Synthesis of Copolymer 2
To a degassed solution of 3 (0.76 g, 1.9 mmol), 5 (0.80 g,
1.9 mmol), and 6 (2.0 g, 3.8 mmol) in toluene (30 mL),
K2CO3 (aq) (2.0 M, 15 mL), Aliquat 336 (30 mg), and tetra-
kis(triphenylphosphine)palladium (65.4 mg, 1.47 mol %)
were added under nitrogen atmosphere. The reaction mix-
nitrogen at a heating rate of 10 C minꢁ1. Thermogravimetric
ꢀ
analysis (TGA) was determined under nitrogen by measuring
ꢀ
ture was then refluxed at 90 C for 3 days. At the end of the
weight loss while heating at a rate of 10 C minꢁ1. Photolu-
ꢀ
condensation polymerization, the mixture was cooled and
poured into a mixture of MeOH and water (300 mL, 7:3 v/
v). The crude polymer was collected, washed with excess
MeOH, dissolved in CH2Cl2, and then reprecipitated into
MeOH. Finally, the polymer was washed with acetone for 24
h using a Soxhlet apparatus, and dried under vacuum to give
2 (1.25 g, 54%). 1H NMR (400 MHz, CDCl3, d, ppm): 7.77–
7.37 (m, 20H), 7.26–7.16 (m, 12H), 4.03 (br, 4H), 2.38–2.36
(br, 3H), 2.09 (br, 8H), 1.14 (br, 8H), 0.74–0.71(br, 20H). 13C
NMR (100 MHz, CDCl3, d, ppm): 154.26, 151.02, 146.48,
144.37, 139.23, 138.85, 135.77, 134.94, 132.77, 129.66,
128.37, 127.34, 127.09, 126.76, 125.29, 125.12, 124.82,
123.36, 120.70, 120.50, 119.60, 117.99, 55.29, 42.20, 40.56,
26.40, 23.47, 21.30, 14.30. Anal. Calcd for C110H108N4O: C,
88.08; H, 7.27; N, 3.59. Found: C, 86.93; H, 7.29; N, 3.24.
minescence spectra were obtained on a Hitachi F-4500 lumi-
nescence spectrometer.
RESULTS AND DISCUSSION
Synthesis and Characterization of 1, 2, and 7
The synthetic routes for 1, 2, and 7 are shown in Scheme 1.
1,3-Bis(4-bromophenyl)imidazolidin-2-one (3), which was
synthesized using copper (I) iodide catalyzed Goldberg–
Buchwald–Nandakumar CAN coupling reactions,73 was used
as a starting material for the preparation of 1 and 2. The
reaction proceeded smoothly under the modified conditions
of using larger amounts of CuI (20 mol %) under solvent-
free condition, which gave 3 in an acceptable yield (61%). In
addition, excessive 1,4-dibromobenzene (5 equiv) was used
to suppress the linear polymerization between 1,4-dibromo-
benzene and imidazolidin-2-one.
Synthesis of Copolymer 7
To a degassed solution of 5 (0.8 g, 1.9 mmol) and 6 (1.0 g,
1.9 mmol) in toluene (15 mL), K2CO3(aq) (2.0 M, 7.5 mL), Ali-
quat 336 (15 mg), and tetrakis(triphenylphosphine)palla-
dium (32.7 mg, 1.47 mol %) were added under nitrogen
atmosphere. The reaction mixture was refluxed at 90 ꢀC for
3 days. At the end of the polymerization, the mixture was
cooled and poured into MeOH in water (150 mL, 7:3 v/v).
The crude polymer was collected, washed with excess MeOH,
dissolved in CH2Cl2, and reprecipitated into MeOH. Finally,
the polymer was washed with acetone for 24 h using a Soxh-
Polymers 1 and 2 were synthesized by the Suzuki coupling
reaction.74 The copolymerization was carried out using
Pd(PPh3)4 as the catalyst in a mixture of PhMe and aqueous
K2CO3 (2.0 M) with Aliquat 336 as a phase transfer reagent.
In the synthesis of 1, a feeding ratio of 1:1:2 for 3, 4, and 6
was respectively adopted. The same feeding ratio for 3, 5,
and 6 was adopted in the preparation of 2.
Polymers 1 and 2 were characterized by 1H, 13C NMR, ele-
mental analysis, and IR spectroscopy (see Supporting
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(Mn) of 1.54 ꢂ 104, with the PDI of 5.53, was recorded. The
TABLE 1 Polymerization Results and Characterization of the
Copolymers
bimodal distribution pattern is unexpected and has not been
1
observed in the preparation of 1 and 2. Nevertheless, the H
a
a
Mn
Mw
NMR integration suggests that a ratio of 1:1 for 5 and 6 was
incorporated in 7. The TGA analysis of 7 shows a high onset
of the weight-loss temperature at 440 ꢀC. In addition, DSC
curve of 7 ꢀalso displays high glass transition temperature
(Tg) of 246 C (Fig. 1).
b
Td (ꢀC)
Polymer
(ꢂ104)
(ꢂ104)
PDIa
Tg (ꢀC)
1
2
7
a
0.77
1.32
1.54
1.42
2.80
8.53
1.84
2.12
5.53
178
270
246
453
395
440
Mn, Mw, and PDI of the polymers were determined by gel permeation
chromatography using polystyrene standards in THF.
Optical Properties
b
Temperature resulting in a 5% weight loss based on the initial weight.
UV–vis and photoluminescence spectra of 1 and 2 in CHCl3
or in thin film on quartz plates are shown in Figure 4. The
data are summarized in Table 2. The UV–vis spectrum of 1
in CHCl3 shows an absorption band peaking at about 366
nm, which is corresponding to the p-p* electronic transitions
in the conjugated polyfluorene chromophore [Fig. 4(a)].
However, in comparison to that of PDOF, the absorption
spectrum of 1 is blue-shifted by 20 nm. This result suggests
that inserting cyclic urea units onto the polyfluorene back-
bone interrupts the linear p-conjugation. However, the solu-
tion of 2 in CHCl3 displays an absorption band peaking at
about 360 nm, which is corresponding to the p-p* transitions
of the fluorene units. In addition, 2 shows a shoulder band
peaking at about 300 nm that is mostly attributed to the
absorption of the triphenylamine moieties.
Information). Both polymers were soluble in common or-
ganic solvents, such as CHCl3, PhMe, and THF at room tem-
perature. Their molecular weight data and thermal proper-
ties are summarized in Table 1. The weight-average
molecular weight (Mw) of 1.42 ꢂ 104 and 2.80 ꢂ 104 for 1
and 2, with the polydispersity index (PDI) of 1.84 and 2.12,
respectively, were found. The TGA analyses of 1 and 2 show
a high onset of the weight-loss temperatures at 453 and 395
ꢀC, respectively, indicating that the polymers are thermally
stable (Fig. 1). In addition, DSC curves of 1 and 2 also dis-
play high glass transition temperatures (Tg) of 178 and 270
ꢀC respectively, which are much higher than that of PDOF
75
ꢀ
(reported to be about 75 C).
Figure 2 shows the 1H NMR of 1 and 2. The composition of
1 was characterized on the basis of the signal intensities at
d 4.08 and 2.12, which were respectively assigned to the eth-
ylene protons of the imidazolidin-2-one and the 1-methylene
protons on the butyl side-chains of the fluorene moieties.
The area ratio of 1:3.40 was obtained in the spectrum, indi-
cating that a ratio of 1:1.2:2.2 for 3:4:6, respectively, was
incorporated in the condensation copolymerization. This
value is consistent with the monomer-feeding ratio of 1:1:2.
The values of the fraction of the basic units of 0.45 and 0.55
for (x) and (1 ꢁ x) respectively, shown in the formula of 1
in Figure 2, were then calculated.
However, the composition of 2 was established on the basis
of the integrations of 4.0:5.6:11.3 at d 4.03, 2.37, and 2.09
respectively. While the signals at d 4.03 and 2.09 were
assigned as before, the sharp singlet at d 2.37 was assigned
to the methyl protons on the tolyl group of the triphenyl-
amine units. The integration ratio suggests that a ratio of
1:1.9:2.9 for 3:5:6, respectively, was incorporated. Even
though the reason is still unclear, the above results sug-
gested that the monomer 5 is more reactive than 3 and pref-
erentially incorporated in the condensation copolymerization
with 6. The fraction of 0.35 and 0.65 for (x) and (1 ꢁ x)
respectively, shown in the formula of 2 in Figure 2, were
then estimated.
To make a comparison, copolymer 7 was included as refer-
ence in our study. Therefore, we prepared copolymer 7 from
5 and 6 (1:1) according to the same Suzuki coupling proto-
col. However, as shown in the GPC (Fig. 3), the Suzuki cou-
pling reaction of 5 and 6 led to a polymeric product with bi-
modal distribution. The number-average molecular weight
FIGURE 1 (a) TGA and (b) DSC thermograms of 1, 2, and 7.
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FIGURE 2 1H NMR spectra of (a) 1 and (b) 2.
Compared with that of 1, the absorption maximum of 2 is
slightly blue-shifted by 6 nm, suggesting that the conjugated
length of fluorene units of 2 is shorter. This is reasonable
because one might expect that the fluorene moieties be sepa-
rated either by the cyclic urea or by the triphenylamine moi-
eties on 2.
Under UV irradiation, 1, 2, and 7 show strong PL in CHCl3.
As shown in Figure 4(a), even though the conjugated length
of 1 is expected to be different from that of PDOF in ground
state, polymer 1 shows a blue emission peaking at 415 nm
that is almost identical to that of PDOF. In addition, both
emission spectra present similar vibronic fine patterns,
which are characteristic for rigid p-conjugated systems. On
the contrary, polymers of 2 and 7 show relatively broad and
red-shifted emission bands peaking at 432 and 434 nm,
respectively. The PL quantum yields (UPL) of 0.87, 0.82, and
0.65 were respectively estimated for 1, 2, and 7, by compar-
ing against Coumarin 1 (UPL ¼ 0.85) as the standard.76
However, 7 shows two absorption bands; one peaking at 386
nm while the other peaking at 300 nm. The absorption band
at 386 nm overlaps with that of PDOF, indicating that this
absorption is arising from the fluorene chromophore. How-
ever, the shoulder band peaking at about 300 nm is attrib-
uted to the absorption of the triphenylamine moieties.
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100 mV/s. Figure 6(a) shows the cyclic voltammograms of 1,
2, and 7 (5 mg) in the supporting electrolyte of TBAP/CH2Cl2
(0.1 M). Polymers 2 and 7 show one reversible oxidation wave
at E1/2(Ox) ¼ 0.88 V, which is attributed to the electrochemical
oxidation of the triphenylamine units to the cation radicals.
The second wave shows up after 1.2 V. In addition, polymer 1
has an onset voltage at 1.15 V. The HOMOs of ꢁ5.48 and
ꢁ5.12 eV for 1 and 2, respectively, were therefore estimated
with regard to the energy level of the ferrocene reference (4.8
eV below the vacuum level). The LUMOs of ꢁ2.46 and ꢁ2.17
eV for 1 and 2, respectively, were then calculated from the
HOMO energy level and the optical band gap.
CV of the polymer films coated on an ITO electrode was per-
formed in an electrolyte of TBAP/acetonitrile (MeCN) (0.1
M) [Fig. 6(b)]. We intentionally picked out MeCN as the sol-
vent for two reasons: (1) the polymer films are insoluble in
MeCN; (2) MeCN is a polar aprotic solvent that benefits the
electrochemical oxidation process. The polymer films were
prepared by spin coating the solution of 1 and 2 (10 mg
mLꢁ1 in CHCl3) on ITO-coated glass substrates and then
dried in vacuum. As shown in Figure 6(b), the oxidation of 1
has an onset at 1.19 V with an anodic wave peaking at 1.48
V. The oxidation is quasi-reversible with the corresponding
cathodic current peaking at 1.25 V. However, 2 and 7 are oxi-
dized at 0.82 V, followed by the second and the third waves
FIGURE 3 GPC of 1, 2, and 7.
Figure 4(b) shows the UV–vis and PL spectra of the poly-
mers in the solid-film state. The absorption spectra of the
polymers in the solid-film state are similar to those in CHCl3.
The absorption maximum of 1, 2, and 7 at 365, 358, and
376 nm respectively, are attributed to the p-p* electronic
transitions in the conjugated fluorene chromophore.
The emission maximum of 1 and 2 were recorded at 423
and 443 nm, respectively. More importantly, the PL spectra
of 1 and 2 in the solid-film state were found to be almost
identical to those in dilute CHCl3, indicating that the inter-
molecular interactions in the solid state are negligible. By
comparing against the fluorescence intensity of PDOF thin-
film sample (UPL ¼ 0.55),77 the PL quantum yields of 0.77
and 0.73 for 1 and 2, respectively, were found. Although the
emission maximum of 7 in the solid-film state is almost
identical with that in dilute CHCl3, vibronic pattern is clearly
observed in the solid-film state spectrum. The PL quantum
yield of 0.43 is much lower than that of 1 and 2.
To examine the thermal effects on the optical behavior, the
ꢀ
thin films of 1, 2, 7, and PDOF were heated at 220 C under
nitrogen for 40 min (Fig. 5). After thermal treatment, the PL
spectra of the thin film of 1, 2, and 7 remained almost
unchanged. These results indicate that the thin films are
morphologically stable. Although we cannot provide any ra-
tionale about this effect in this stage, this observation clearly
suggested that introduction of the cyclic urea and triphenyl-
amine moieties can effectively suppress p-stacking/aggrega-
tion of the conjugated polymers in the solid state. However,
the PL spectrum of the annealed PDOF film is red shifted by
10 nm with the excimer emission peaking at 520 nm being
observed.78,79 This observation again implies that introduc-
tion of cyclic urea and the triphenylamine units can increase
the morphological stability of the film at high temperature.
Electrochemical Properties
The redox behavior of the polymers was investigated by cyclic
voltammetry (CV) in tetrabutylammonium perchlorate
(TBAP)/CH2Cl2 (0.1 M/10 mL), using a Pt electrode as the
work electrode, a Pt wire as the counter electrode, and an Ag/
AgCl couple as the reference. The scan rate being adopted was
FIGURE 4 UV–vis absorption and PL spectra of 1, 2, PDOF, and
7 in (a) CHCl3 solution and (b) solid-film state.
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TABLE 2 Optical Properties of the Copolymers
Solution
Film
Abs
PL
Abs
PL
(nm)
kmax
(nm)
kmax
(nm)
kmax
(nm)
kmax
a
b
Polymer
UPL
UPL
1
366
360
386
386
415
432
434
417
0.87
0.82
0.65
0.78
365
358
376
382
423
443
435
423
0.77
0.73
0.43
0.55
2
7
PDOF
a
Coumarin 1 (UPL ¼ 0.85)76 as the standard in THF solution.
b
Poly(9,9-dioctylfluorene) (UPL ¼ 0.55)77 as the standard in thin-film state.
after 1.2 V. When the CV scan was swept from 0 V to 1 V, 2
and 7 showed a strong oxidation wave with an onset at 0.80
V. However, 1 showed only a very weak residual current,
indicating that the triphenylamine moiety can be preferen-
tially oxidized over the fluorene and the imidazolidin-2-one
moieties under the CV condition.
Electrochemical Redox Switching of Fluorescence
In preliminary study, the polymer films used in the
experiments of fluorescence intensity changes were prepared
by spin coating the solution of copolymers
1 and 2
FIGURE 6 (a) CV of 1, 2, and 7 in TBAP/CH2Cl2 (0.1 M); (b) CV
of the films of 1, 2, and 7 on an ITO-coated glass substrate in
TBAP/MeCN (0.1 M). The scan rate was 100 mV/s.
(10 mg mLꢁ1 in CHCl3) onto an ITO-coated glass substrate.
The films were highly fluorescent in the blue to sky-blue
region. The electrochemical oxidation was performed in
TBAP/MeCN (0.1 M) with the applied electrical potential of
1.0 V that led to fluorescence decrease for 1 and 2 (Fig. 7).
The fluorescence intensity changes were collected every 1
min. The fluorescence intensity of
2 was completely
quenched after 1.0 min of oxidation. However, the fluores-
cence intensity of 1 was reduced to only half of its original
intensity after electrochemical oxidation for 5 min. This
result suggests that fluorescence of 2 can be quenched more
effectively than 1 during oxidation.
To test the possibility of using 1 and 2 as an EF switching
material, we had subjected the polymers to repetitive CV-
sweeps in which the fluorescence intensity of the polymers
were simultaneously followed (Fig. 8). The studies were car-
ried out under similar conditions as described above, with
the polymer coated on ITO glass. The sweep was set
between 0 and 1 V with a scan rate of 100 mV/s. TBAP/
MeCN (0.1 M) was adopted as the supporting electrolyte.
FIGURE 5 Comparison of the PL spectra of (a) 1 and PDOF; (b)
2 and 7, before and after annealing at 220 ꢀC for 40 min under
nitrogen.
Upon electrochemical oxidation, the bright fluorescence of 2
was gradually reduced and completely quenched at 1.0 V.
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tion, as we mentioned in Figure 6(b), even though the anodic
current of 1 below 1 V was small, a residual current was still
observed in the CV, which supports this hypothesis.
EF switching of the film of 7 on an ITO-coated glass sub-
strate in the supporting electrolyte of TBAP/MeCN (0.1 M)
can also be observed by chronoamperometric analysis. How-
ever, for unknown reasons, the EF switching processes pro-
ceeded less effectively and unevenly on the ITO electrode.
Figure 9 shows how the quenching process of the blue-light
emission evolved. After an electrical potential of 0.9 V was
applied, the fluorescence quenching was started to occur at
the right-bottom corner [Fig. 9(b)], gradually extended to the
central part of the plate after 1 s [Fig. 9(c)] and reached the
quenched state after 3 s [Fig. 9(d)].
Proposed mechanisms of the oxidative fluorescence quench-
ing process of 2 are illustrated in Scheme 2. When an electri-
cal potential set below 1.0 V was applied, oxidation selec-
tively occurred on the triphenylamine moieties and gave rise
to the corresponding radical cations. Since the radical cation
is known to have a strong absorption in the blue-light
region,29 it acts as an effective fluorescence quencher so that
the photoluminescence of 2 is efficiently quenched. When a
reverse sweep was applied, the triphenylamine radical cati-
ons in 2 were annihilated and the fluorescence at a neutral
state of 2 was recovered. Perhaps due to the dipolar proper-
ties of the ureyleno groups that could help to stabilized the
formation of the triphenylamine radical cations, the redox
fluorescence switching of 2 is more effective than 7.
FIGURE 7 Fluorescence intensity changes (kexc ¼ 365 nm) of
the film of 1 on an ITO-coated glass substrate were carried out
in TBAP/MeCN (0.1 M) after applying an oxidation potential of
1.0 V (vs. Ag/AgCl) at different times (min). Inset: Comparison
of the fluorescence intensity changes (Normalized) of 1 and 2
at 423 and 443 nm, respectively, versus time (min).
However, the quenching process is reversible so that the flu-
orescence intensity of 2 can be gradually resumed to the
original emission intensity (>95%) in the reverse-sweep.
On the contrary, the intensity of the bright fluorescence of 1
only slightly altered upon electrochemical oxidation. How-
ever, the process is only partially reversible so that about
40% of the original fluorescence loss can be recovered after
the first CV cycle. The maximum intensity gradually drops af-
ter each cycle, indicating that the film is unstable upon elec-
trochemical oxidation. Since polymer 1 may contain oligo-
fluorene segments with various conjugation lengths, we
tentatively attribute the partial quenching phenomenon to
electrochemical oxidation occurring on the long oligofluorene
segments. First, the longer the conjugation length of the seg-
ment, the lower the oxidation potential it possesses. In addi-
The durability of the potential-dependent switchable fluores-
cence properties of 2 was investigated by chronoamperometric
FIGURE 8 Fluorescence intensity changes of the films of 1 and
2 on ITO were carried out by CV for five scanning-cycles. The
fluorescence intensity changes at 0.0 V (reduction) and 1.0 V
(oxidation) were recorded.
FIGURE 9 EF switching of the film of 7 on ITO after an electri-
cal potential of 0.9 V was applied: (a) Before oxidation. (b) Af-
ter oxidation for 0.5 s. (c) After oxidation for 1 s. (d) After
oxidation for 3 s.
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SCHEME 2 Illustration of the EF switching based on a copoly-
mer of 2.
analysis (Fig. 10). Twenty-five redox pulse-cycles were carried
out for the thin-film of 2 in the supporting electrolyte of
TBAP/MeCN (0.1 M) with fixed electrical potential steps of 0.9
V for oxidation and 0.0 V for reduction. The potential step was
flipped every 3 s. Indeed, the EF switching of the thin-film of
2 was found to be responsive within 3 s with a maximum on/
off ratio of 16.3. The fluorescence intensity was recovered after
each step and found to be recovered rapidly and fully on com-
pletion of the step, thus, demonstrating the reversibility of the
switching process.
FIGURE 11 (a) Photographs of a single-layer ITO-coated glass
EF device, using 2 as an active layer. Successive images of
the cell fluorescence recorded at different states under UV
irradiation (365 nm). (b) Schematic diagram of the 2-EFD sand-
wich cell.
sandwich the electroactive layer. To prevent any electrolyte
leakage, an epoxy resin was applied to seal the device. Redox
pulse-cycles were carried out for the EFD of 2. Fixed poten-
tials at 1.0 V (for oxidation) and 0.0 V (for reduction) were
used and flipped every 5 s. Before data collection, the device
was pretreated with the pulse-cycles for 400 s (Fig. 12).
Electrochemical Fluorescence Switching in an
Electrofluorescent Device
We had fabricated a prototype of the single-layer EFD as
shown in Figure 11. The thin-film of 2 was spun onto a pi-
ece of ITO-glass and dried, and a layer of gel electrolyte was
spread on top. The counter electrode was then placed to
The polymer film showed strong fluorescence in the neutral
form. When a voltage of 1.0 V was applied, the fluorescence
disappeared due to electrochemical oxidation (the same
FIGURE 10 Twenty-five redox pulse-cycles of a thin-film of 2
were carried out in TBAP/MeCN (0.1 M) with fixed electrical
potential steps of 0.9 V for oxidation and 0.0 V for reduction.
The potential step was flipped every 3 s. All the bottom dots
were detected after the thin film was oxidized at an applied
potential of 0.9 V, and all the top dots were measured after the
thin film was reduced at an applied potential of 0.0 V. Inset:
Photographs are the image of the fluorescence changes of
a thin-film of 2 at indicated potentials under UV irradiation
(365 nm).
FIGURE 12 Fluorescence intensity changes (kexc ¼ 360 nm) of
the solid-state 2-EFD at 443 nm after 400 s of pretreatment.
Fixed potentials at 1.0 V (for oxidation) and 0.0 V (for reduc-
tion) were used and flipped every 5 s. Inset: Fluorescence in-
tensity changes of the solid-state 2-EFD.
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sive properties of 2. On the one hand, 2 showed modest
solubility in organic solvents and high Tg, which would
benefit device fabrication. On the other hand, reversible
fluorescence switching of 2 from a highly fluorescent neutral
state to a non-fluorescent. When comparing against the
fluorescence switching performance of the thin-film of 7,
one may aware that introduction of the cyclic urea moieties
is particularly important for fabrication of the fast response
EFD. This is probably due to the dipolar properties of the
urea groups that benefit the LiClO4 penetration. In addition,
the presence of the dipolar groups may also help to reduce
the barrier of the triphenylamine radical cation formation.
ACKNOWLEDGMENT
This work was supported by the Ministry of Education and
National Taiwan University, National Science Council of Taiwan
(NSC-98-2119-M-002-006-MY3 and 99-2119-M-002- 007),
and the thematic project, Academia Sinica.
FIGURE 13 Spectroelectrochemistry of the solid-state 2-EFD.
Inset: The absorption intensity changes of the solid-state 2-EFD
at 500 nm versus voltage (V).
result was reported for the thin film in the previous chro-
noamperometric experiments). When the potential was sub-
sequently set back at 0 V, the polymer film was turned back
to the strongly fluorescent neutral state (Fig. 11).
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