Optical and electronic properties of fluorene-based copolymers and their sensory applications

Article abstract of DOI:10.1002/pola.26454

A series of novel, fluorene-based conjugated copolymers, poly[(9,9-bis{propenyl}-9H-fluorene)-co-(9,9-dihexyl-9H-fluorene)] (P1), poly[(9,9-bis{carboxymethylsulfonyl-propyl}fluorenyl-2,7-diyl)-co-(9, 9-dihexyl-9H-fluorene)] (P2) and poly[(9,9-dihexylfluorene)-co-alt-(9,9-bis-(6- azidohexyl)fluorene)] (P3), are synthesized by Suzuki coupling reactions and their electrochemical properties, in the form of films, are investigated using cyclic voltammetry. The results reveal that the polymer films exhibit electrochromic properties with a pseudo-reversible redox behavior; transparent in the neutral state and dark violet in the oxidized state. Among the three polymers, P2 possesses the shortest response time and the highest coloration efficiency value. These polymers emit blue light with a band gap value of around 2.9 eV and have high fluorescent quantum yields. Their metal ion sensory abilities are also investigated by titrating them with a number of different transition metal ions; all of these polymers exhibit a higher selectivity toward Fe³⁺ ions than the other ions tested with Stern-Volmer constants of 4.41 × 10⁶M^-1, 3.28 × 10⁷M ^-1, 1.25 × 10⁶M^-1, and 6.56 × 10⁶M^-1 for P1, P2, water soluble version of P2 (P2S) and P3, respectively.

Full text of DOI:10.1002/pola.26454

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Optical and Electronic Properties of Fluorene-Based Copolymers and
Their Sensory Applications
1
3
1,2
Vu¨ sala Ibrahimova,1,2 Meliha Eda Kocak, Ahmet M. Onal, Donu¨ s Tuncel
¨
€
1Department of Chemistry, Bilkent University, 06800 Ankara, Turkey
2UNAM-National Nanotechnology Research Center, Bilkent University, 06800 Ankara, Turkey
3Department of Chemistry, Middle East Technical University, 06800 Ankara, Turkey
¨
Correspondence to: A. M. Onal (E-mail: aonal@metu.edu.tr) or D. Tuncel (E-mail: dtuncel@fen.bilkent.edu.tr)
Received 16 October 2012; accepted 25 October 2012; published online 26 November 2012
DOI: 10.1002/pola.26454
ABSTRACT: A series of novel, fluorene-based conjugated
copolymers, poly[(9,9-bis{propenyl}-9H-fluorene)-co-(9,9-dihexyl-
9H-fluorene)] (P1), poly[(9,9-bis{carboxymethylsulfonyl-propyl}-
fluorenyl-2,7-diyl)-co-(9,9-dihexyl-9H-fluorene)] (P2) and poly[(9,9-
dihexylfluorene)-co-alt-(9,9-bis-(6-azidohexyl)fluorene)] (P3), are
synthesized by Suzuki coupling reactions and their electrochemi-
cal properties, in the form of films, are investigated using cyclic
voltammetry. The results reveal that the polymer films exhibit
electrochromic properties with a pseudo-reversible redox behav-
ior; transparent in the neutral state and dark violet in the oxidized
state. Among the three polymers, P2 possesses the shortest
response time and the highest coloration efficiency value. These
polymers emit blue light with a band gap value of around 2.9 eV
and have high fluorescent quantum yields. Their metal ion sen-
sory abilities are also investigated by titrating them with a num-
ber of different transition metal ions; all of these polymers exhibit
a higher selectivity toward Fe3þ ions than the other ions tested
with Stern–Volmer constants of 4.41 ꢀ 106
M
ꢁ1, 3.28 ꢀ 107 Mꢁ1
,
1.25 ꢀ 106
M
ꢁ1, and 6.56 ꢀ 106 Mꢁ1 for P1, P2, water soluble ver-
C
V
sion of P2 (P2S) and P3, respectively. 2012 Wiley Periodicals,
Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 815–823
KEYWORDS: fluorene-based polymers; electrochromism; ion
sensing; fluorescence
INTRODUCTION Conjugated polymers with tailored proper-
ties are of still great attention due to their wide range of
applications including organic light emitting diodes,1–3 solar
cells,4,5 and electrochromic devices.6,7 Moreover, the use of
light emitting conjugated polymers as chemosensors to
detect analytes is rather appealing owing to the ability of the
conjugated backbone to amplify the signal through the wir-
ing effect.8–10
rated to the fluorene backbone easily. These features make
them suitable in sensing biologically and environmentally
important metal ions selectively and at low detection lim-
its.26–32 For instance, C-9 position of fluorene was functional-
ized with various groups such as phosphonates, sulfonates,
and dendronized amino acids and subsequently these mono-
mers were polymerized using Suzuki Coupling reactions to
obtain water soluble blue emitting polymers which can be
utilized as chemosensors.29–32 It was shown that the PF with
having phosphonates30 and sulfonates31 groups detects Feþ3
ions in water with the Stern–Volmer constants (Ksv) of 1.50
Among a great number of conjugated polymers with differ-
ent emissive colors, polyfluorenes (PFs) are promising mate-
rials for optoelectronic devices due to their high thermal and
chemical stability together with high photoluminescence effi-
ciency and good photostability.11–13 A further advantage of
utilizing PFs is that their properties can be tuned by struc-
tural design, either via substitution of the C-9 position of flu-
orene by suitable substituents to enhance solubility14–16 or
copolymerization of fluorene with various aryl partners to
tune their electronic properties.17–25
ꢀ 106 Mꢁ1 and 1.98 ꢀ 106
M
ꢁ1, respectively, whereas
dendronized amino acid functionalized polyflurone32 was
found to be selective toward Hg2þ ions with Ksv of 1.33 ꢀ
106
M
ꢁ1. The fluorescent quenching of the aforementioned
polymers by those metal ions have been attributed to a com-
bination of factors such as electron transfer, delocalization of
excitons, and energy migration along the polymer chain.30–32
Furthermore, the use of fluorene-based conjugated polymers
as a fluorescent chemosensor is becoming highly attractive
because of a number of reasons. First, they possess high flor-
escent quantum yields; second, suitable ligands which have
ability to coordinate with the desired ions can be incorpo-
Herein, we report on the synthesis and characterization of a
series of new spray processable fluorescent copolymers;
poly[(9,9-bis{propenyl}-9H-fluorene)-co-(9,9-dihexyl-9H-fluo-
rene)] (P1), poly[(9,9-bis{carboxymethylsulfonyl-propyl}-
fluorenyl-2,7-diyl)-co-(9,9-dihexyl-9H-fluorene) (P2), and
Additional Supporting Information may be found in the online version of this article.
C
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2012 Wiley Periodicals, Inc.
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SCHEME 1 The structures of the polymers (P1–P3 and PF-H) used in this study and the synthesis of the polymers P1–P3. (a) Allyl-
bromide, 50% NaOH aq.solution (w/w), DMSO, TBAB, 2 h, 89%; (b) 9,9-dihexylfluorene-2,7-bis(trimethyleneborate), K2CO3, (Pd
(PPh3)4), THF/H2O/Toluene, TBAB, 80–90 ꢂC, 48 h, 60%; (c) Thioglycolic acid, CHCl3, 25 ꢂC, 24 h, 90%; (d) 1,6-dibromohexane, 50%
NaOH aq.solution (w/w), DMSO, TBAB, 2 h, 60%; (e) 9,9-dihexylfluorene-2,7-bis(trimethyleneborate), K2CO3, (Pd (PPh3)4), THF/H2O/
Toluene, TBAB, 80–90 ꢂC, 48 h, 56%; (f) NaN3, DMF, 70 ꢂC, 24 h, 85%.
poly[(9,9-dihexylfluorene)-co-alt-(9,9-bis-(6-azidohexyl)fluor-
ine)] (P3) (Scheme 1). Electro-optical and electrochromic
properties of these new copolymers were investigated using
cyclic voltammetry (CV) and in situ spectroelectrochemical
techniques. Furthermore, the fluorescence properties of
polymers were investigated in tetrahydrofuran (THF). Their
metal ion sensory abilities were also investigated and the
results were compared with poly(9,9-dihexyl-9H-fluorene)
(PF-H) (Scheme 1) carrying no functional groups to under-
stand the extent of the functional group effect on the fluo-
rescent sensing.
itating into cold methanol. Colorless crystals were collected
and dried under vacuum (3.3 g, 89%).
1H NMR (400 MHz, CDCl3): d 7.52 (m, 6 H, ArAH), 5.21 (m,
2H, ACH¼¼CH2), 4.89 (m, 4H, ACH¼¼CH2), 2.68 (t, 4H,
ACH2A,3J ¼ 4 Hz). 13C-NMR (100 MHz, CDCl3): d 42, 46,
114, 122, 129, 130, 132, 135, 138, and 144.
Synthesis of Poly[(9,9-bis{propenyl}-9H-fluorene)-co-(9,9-
dihexyl-9H-fluorene)] (P1)
2,7-Dibromo-9,9-bis-(propenyl)-9H-fluorene (1.00 g, 3.00
mmol), 9,9-dihexylfluorene-2,7-bis(trimethyleneborate) (0.49
g, 3.00 mmol) and K2CO3 (4.09 g, 3.00 mmol) were dried
under vacuum. First the degassed solvents, THF (10 mL),
water (10 mL), and toluene (10 mL) were added under
Argon gas and then, catalyst tetrakis (triphenylphosphine)
palladium (Pd (PPh3)4) was added quickly. After 3 h stirring
of the mixture under argon at 80–90 ꢂC, the phase transfer
catalyst, tetra-n-butylammonium bromide (TBAB) was added.
The stirring was continued for another 48 h at 80–90 ꢂC to
complete the polymerization reaction. The mixture was
evaporated under vacuum to obtain a solid residue, which
was suspended in water; the water insoluble particles were
collected by suction and dissolved in THF (20 mL) and the
solution was precipitated into cold methanol (200 mL). The
precipitates were collected by suction and dried under vac-
uum for 5 h at rt. (0.900 g, 60%).
EXPERIMENTAL
Materials
All chemicals including 2,7-dibromofluorene, 9,9-dihexyl-
fluorene-2,7-bis(trimethyleneborate), phase transfer catalyst
tetrabutylammoniumbromide and tetrakis(triphenylphosphi-
ne)palladium, were purchased from Aldrich.
Synthesis of 2,7-Dibromo-9,9-bis-(propenyl)-
9H-fluorene (M)
2,7-Dibromofluorene (3.00 g, 9.25 mmol) and tetrabutylam-
monium bromide (TBAB) (0.600 g, 1.85 mmol) were dried
under vacuum for 30 min. Degassed dimethyl sulfoxide
(DMSO) (15 mL), 50% (w/w) NaOH (15 mL), allylbromide
(16 mL, 90 mmol) were added into the mixture, respectively,
and stirred under argon gas for 2 h at room temperature. Af-
ter 2 h, t-butyl methyl ether (125 mL) and deionized water
(50 mL) were added into the mixture and stirred 15 min.
Organic layer was separated and subsequently washed with
deionized water (50 mL), 2N HCl (50 mL), brine solution
(50 mL), and deionized water (50 mL), respectively. After
extraction, t-butyl methyl ether was evaporated by rotary
evaporator to obtain a solid residue which was purified
using a silica-packed column and cyclohexane as an eluent.
The solid was further purified by dissolving in CHCl3, precip-
1
ꢂ
H NMR (400 MHz, CDCl3, 25 C): d 7.85 (m, 8H, ArH), 5.54
(m, 2H, ACH¼¼CH2), 5.02 (t, 4H, ACH¼¼CH2), 2.89 (m, 4H,
ACH2CH¼¼CH2), 1.54 (m, 4H, ACH2A), 1.15 (m, 4H,
ACH2A), and 0.81 (m, 6H, ACH3) (see NMR spectrum in
Supporting Information Fig. S1).
IR [KBr, pellet, tmax(cmꢁ1)]: 3023 (ACH, w), 2928 (ACH, s),
919 (C¼¼CA, w).
Gel permeation chromatography (GPC): Mn ¼ 4.01 ꢀ 103
g molꢁ1, Mw ¼ 9.02 ꢀ 103 g molꢁ1 (THF as a solvent and
polystyrene as standard). Anal. calcd for C239H256Br2 (Exact
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Mass: 3283.8399): C, 87.29; H, 7.85; Br, 4.86. Found: C,
87.37; H, 7.97.
Synthesis of Poly[(9,9-bis{carboxymethylsulfonyl-
propyl}fluorenyl-2,7-diyl)-co-(9,9-dihexyl-9H-fluorene)
(P2)
P2 was synthesized through thiol-ene click chemistry.
Poly[(9,9-bis{propenyl}-9H-fluorene)-co-(9,9-dihexyl-9H-fluo-
rene)] (200 mg, 0.374 mmol) was dissolved in chloroform
(5 mL) and treated with thioglycolic acid (0.163 mL, 1.87
mmol). The mixture was stirred under argon at rt for 24 h.
Chloroform was evaporated off and the remaining residue
was dissolved in THF (5 mL) and the solution was precipi-
tated into water (50 mL). Yellow precipitate was collected by
centrifugation and the precipitation process was repeated
twice to remove the excess thioglycolic acid. The yellow
precipitate was dried under vacuum for 5 h at rt. (180 mg,
90%). IR (KBr, pellet, tmax(cmꢁ1)): 3036 (ACH, w), 2928
(ACH, s), 1458 (C¼¼C-, w), 1705 (C¼¼O, s). GPC: Mn ¼ 9.88
ꢀ 103 g molꢁ1, Mw ¼ 12.9 ꢀ 104 g molꢁ1 (THF as a
solvent and polystyrene as standard). Anal. calcd for
FIGURE 1 FT-IR spectra of the polymers P1, P2, and P3.
cence Spectrophotometer. FT-IR spectra were performed
with Bruker Vertex 70 Spectrophotometer. Molecular
a
weights were determined by Algilent GPC 1200 in THF using
a calibration curve of polystyrene standards. TA Instruments
Q500 thermal gravimetric analysis (TGA) was used to per-
form the thermogravimetric analysis measurements. Cyclic
voltammetric measurements were performed using Gamry
PCI4/300 potentiostat–galvanostat in acetonitrile (ACN)-tet-
rabutylammonium tetrafluoroborate (TBABF) solvent-electro-
lyte couple containing 2% borontrifluoro diethylether (BF3-
Et2O) by volume. For spectroelectrochemical characteriza-
tions polymer films were coated on ITO (Delta Tech. 8–12 X,
0.7 ꢀ 5 cm2) via spray coating. The polymer films were
rinsed and switched between neutral and doped states sev-
eral times to equilibrate its redox behavior in electrolytic so-
lution. In situ spectroelectrochemical studies were performed
using Hewlett-Packard 8453 A diode array spectrometer. A
Pt wire was used as a counter electrode, and an Ag wire as
a pseudo-reference electrode which was calibrated externally
using 5 mM solution of ferrocene/ferrocenium couple in the
electrolytic solution. Also, square wave potential method was
used to investigate the ability of switching of the polymer
film between its neutral and doped state.
C403H464Br2O32S16 (Exact Mass: 6484.8579): C, 74.55; H,
7.20; Br, 2.46; O, 7.89; S, 7.90. Found: C, 74.59; H, 7.27; S,
7.68.
Synthesis of Poly[(9,9-dihexylfluorene)-
co-alt-(9,9-bis-(6-azidohexyl) fluorene)] (P3)
P3 was synthesized following the literature procedure.33–34
IR (KBr, cmꢁ1): 3065(CHA), 2935 (CHA), 2859(CHA), 2100
(AN3), 1613(C¼¼CA), and 1571(C¼¼C).
1H NMR (400 MHz, CDCl3): d 7.78 (m, 12H), 3.17 (m, 4H),
1.95 (m,4H), 1.7 (m, 4H), 1.20(m, 8H), and 0.65 (m, 4H, f).
GPC: Mn ¼ 3.61 ꢀ 103 g molꢁ1, Mw ¼ 2.04 ꢀ 104 g molꢁ1
(THF as a solvent and polystyrene as standard).
Preparation of Polymers and Metal Ion Solutions, and
the Fluorescent Titrations
Polymers P1, P2, and P3 (0.75, 0.92, and 0.97 mg, respectively)
were dissolved in THF (250 mL) to prepare the stock solutions
with 5 mM concentrations (respect to per repeat unit). P2S (9.7
mg) was dissolved in water (250 mL) to prepare its stock solu-
tion with a concentration of 50 mM. Metal ions stock solutions
were prepared by dissolving Fe(NO3)3ꢃ9H2O, Cu(NO3)2ꢃ3H2O,
Ni(NO3)2ꢃ6H2O, Zn(NO3)2ꢃ6H2O, AgNO3, Hg(NO3)2, Pb(NO3)2,
and Cd(NO3)2ꢃ4H2O, Fe(NO3)2ꢃ6H2O (2.02, 1.21, 1.45, 1.49,
0.85, 1.62, 1.66, 1.54, and 1.44 mg, respectively) in 5 mL of
water to get the concentration of 0.001 M. For the fluorescent
titration experiments, 3 mL stock solution of polymers were
taken to the UV cuvette followed by successive additions of 15
mL of metal ion stock solutions. Fluorescence spectrum were
taken after each addition.
RESULT AND DISCUSSIONS
Synthesis and Characterization of Copolymers
Polymer P1 was synthesized by the Suzuki coupling of 2,7-
dibromo-9,9-bis-(propenyl)-9H-fluorene and 9,9-dihexylfluor-
ene-2,7-bis(trimethyleneborate) in 60% yield (Scheme 1).
Copolymer formation was confirmed by 1H NMR (see Sup-
porting Information Fig. S1) and FT-IR measurements. Poly-
mer P2 was synthesized from P1 by reacting P1 with excess
thioglycolic acid (Scheme 1). Its 1H NMR spectra in various
solvents were recorded but in all spectra the signals of pro-
tons were very broad and poorly resolved to be useful. How-
ever, FT-IR spectrum of P2 depicted in Figure 1 together
with the spectrum of P1 for comparison reasons shows the
formation of a strong peak at about 1700 cmꢁ1 due to C¼¼O
stretching suggesting the successful conversion of P1–P2.
Furthermore, from the sulfur content determined by the ele-
mental analysis we were able to estimate the degree of
Instrumentation
NMR spectra were recorded on a Bruker NMR Spectrometer
(DPX-400) in deuterated chloroform and fluorescence meas-
urements were recorded on a Varian Cary Eclipse Fluores-
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spectra in THF and the results are depicted in Figure 2. As
seen from Figure 2, all three polymers exhibit an absorption
band around 385 nm due to p ! p* transition. Their wave-
lengths of maximum fluorescence emission are well sepa-
rated from their absorption edges indicating that self-absorp-
tion is negligible (Fig. 2).33 The fluorescence quantum yields
were calculated as 0.75, 0.48, and 0.86 for P1, P2, and P3,
respectively, using quinine sulfate as the standard. The rela-
tively small Stokes shifts (34 nm for P1, 35 nm for P2 and
26 nm for P3 nm) between the absorption and emission
maximum wavelengths of the polymers, P1, P2, and P3 indi-
cate the rigidity of the conjugated polymer backbone which
hinders deformation in going from the ground to the excited
state.26
FIGURE 2 UV–vis and PL spectra of the polymer P1 (kex ¼ 384
nm), P2 (kex ¼ 386 nm), and P3 (kex ¼ 391 nm).
Thermal Properties
Thermal stability of the polymers P1, P2, and P3 were eval-
uated by TGA under nitrogen gas flow and their TGA curves
are illustrated in Figure 3. Polymer P1 displays a main
decomposition at 455 ꢂC due to the scission of alkly side
chains from the polymer backbone [Fig. 3(a)]. Conversely,
TGA profile of P2 shows that decomposition occurs in sev-
eral steps, in which the first step involves the decomposition
of carboxyl groups of side chains by the release of carbon
dioxide with a weight loss of 12% and the second step is
associated with the scission of alkyl chains [Fig. 3(b)]. TGA
thermograph of P3 clearly indicates that the thermal degra-
dation of P3 takes place in more than one step [Fig. 3(c)]. In
the first step, azide groups of the side chains are decom-
posed at around 223 ꢂC as reported previously33,34 and the
subsequent steps at 354 and 453 ꢂC involve the scission of
alkyl chains from the fluorene backbone. These results indi-
cate that although all three polymers are thermally stable,
P1 exhibits higher thermal stability than the polymers P2
and P3.
conversion of allyl side chains through thiol-en reaction as
higher than 95% (see the proposed structure in the experi-
mental part). Water soluble version of P2 (abbreviated as
P2S) was synthesized treating P2 with aqueous NaOH solu-
tion. P3 was synthesized following the literature prece-
dence,33,34 in which briefly the precursor copolymer
poly[(9,9-dihexylfluorene)-co-alt-(9,9-bis(6-bromohexyl)fluor-
ene)] was treated with sodium azide in DMF (Scheme 1).
The conversion of bromide, in poly[(9,9-dihexylfluorene)-co-
alt-(9,9-bis(6-bromohexyl)fluorene)], to azide was confirmed
1
by H NMR and FT-IR measurements.33,34 The upfield-shifted
peak of methylene protons adjacent to azide from 3.32 to 3.17
1
ppm was observed in the H NMR spectrum upon the conver-
sion of bromide to azide. The appearance of a new peak at
about 2094 cmꢁ1, due to azide stretching, in the FT-IR spec-
trum of P3 further indicates the conversion of bromide to az-
ide group (Fig. 1). The weight average and number average
molecular weights (Mw/Mn) of P1, P2, and P3 were deter-
mined via GPC using polystyrene as standard and were found
to be 9.02 ꢀ 103/4.01 ꢀ 103, 12.9 ꢀ 104/9.88 ꢀ 103, and
2.04 ꢀ 104/3.61 ꢀ 103 g molꢁ1 for P1, P2, and P3, respec-
tively. Furthermore, using elemental analysis results, we esti-
mated the number of repeating units of P1 and P2 as 5 and 8,
respectively. The proposed structures in accordance with the
elemental analysis are shown in the experimental section.
Electrochemical and Electro-Optical Properties
To elucidate the electrochemical properties of the polymers,
polymers were deposited on an ITO electrode via spray coat-
ing from their THF solutions (6.0 mg/1.0 mL THF). Cyclic
voltammograms were recorded in 0.1 M TBABF solution in
ACN containing BF3-Et2O (2% by volume) and the resulting
voltammograms are shown in Figure 4. The polymer films
exhibited a pseudo-reversible redox couple (Ep,a ¼ 1.22 V
and Ep,c ¼ 0.96 V for P1, Ep,a ¼ 1.07 V, and Ep,c ¼ 0.97 V for
P2 and Ep,a ¼ 1.16 V and Ep,c ¼ 0.94 V for P3) due to
Photophysical Properties
Optical characterization of three polymers was accomplished
by recording their absorption and fluorescence emission
FIGURE 3 Thermogravimetric analysis of (a) P1, (b) P2, and (c) P3 under nitrogen atmosphere at a heating rate of 20 ꢂC/min.
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FIGURE 4 Cyclic voltammograms of (a) P1, (b) P2, and (c) P3 spray coated on ITO at 100 mV sꢁ1 in 0.1 M TBABF/ACN.
doping and de-doping with a half peak potential of 1.07,
0.97, and 1.07 V for P1, P2, and P3, respectively. These very
close Ep,a values of the three polymers indicate that different
side groups on the repeating unit have negligible effect on
the doping process.
switching in a short response time,35 these parameters for
the films, P1, P2, and P3 were investigated under a rectan-
gular wave input of 0.0 V (residence time 2 s) to 1.15 V (res-
idence time 3 s) for P1, 0.0 V (residence time 1 s) to 1.25 V
(residence time 2 s) for P2 and P3 by monitoring the visible
transmittance at 560 nm for P1 and P3, at 540 nm for P2
and the results are shown in Figure 6 for P2 and Supporting
Information Figure S3 for P1 and P3. To evaluate the elec-
trochemical stability, cyclic voltammograms after each 100
switch were recorded with the scan rate of 200 mV/s.
Although P1 showed good electrochemical stability (only
12% loss in electroactivity after 600 cycle), P2 and P3
exhibited lower stability. The reason for the lower stability
was thought to be due to the presence of polar pendant
groups which makes them slightly soluble in ACN. Thus, to
make them stable, they were crosꢂslinked via thermal treat-
ment. P2 was administrated to 90 C for 2 h in vacuum oven
Electro-optical properties of the polymer films on ITO were
elaborated by recording the changes in the absorption spec-
tra under a variety of voltage pulses after neutralization at
0.0 V. The electronic absorption spectra of neutral forms of
the films exhibit an absorption band at around 382, 392,
and 388 nm for P1, P2, and P3, respectively. These bands
are due to p–p* transition and they all lose intensity during
oxidation which is accompanied by the appearance of new
intensifying bands. In the case of P2 (Fig. 5), as the valance-
conduction band at 392 nm diminishes a new band starts to
intensify at about 540 nm in the potential range of 0.0–1.20
V. Upon further oxidation up to 1.30 V, a new broad band
beyond 900 nm also starts intensifying. Appearance of these
new bands indicates the formation of charge carriers. All
spectra recorded during potential cycling between 0.0 and
1.30 V passes through a clear isosbestic points at 443 nm,
indicating that polymer film was being interconverted
between its neutral and oxidized states. The changes in the
electronic absorption spectra of P2 film is also accompanied
by a color change, transparent to violet, indicating that P2
film exhibits electrochromic behavior. In the case of P1
[Supporting Information Fig. S2(a)], the new band due to the
formation of charge carriers appears at about 560 nm
accompanied with an electrochromic response (transparent
in the neutral state and violet in the oxidized state). Similar
spectroelectrochemical changes were also observed for P3
during potential scanning from 0.0 to 1.45 V [see Supporting
Information Fig. S2(b)].
ꢂ
while P3 was encountered to 100 C for 5 h prior to poten-
tial pulses and the results are shown in Supporting Informa-
tion Figure S4. Although, P2 lost 10% of its electroactivity
even after 100 switches, after crosslinking of the polymer
film via thermal treatment its stability increased to a great
extent and a 21% loss in the electroactivity was observed af-
ter 500 switches. In addition, almost same situation was
observed for P3 after crosslinking it retains 14% its electro-
activity after 700 switches.
In our previous publications, we discussed in detail the ther-
mal and the light-triggered cross-linking mechanism of azide
The band gap (Eg) values of polymer films deposited on ITO
electrodes via spray coating determined from the commence-
ment of low energy end of p–p* transitions (i.e., 382, 392,
and 388 nm for P1, P2, and P3, respectively), utilizing spec-
troelectrochemical data. It was found that Eg values are very
close to each other being in the range of 2.80–2.90 eV
(Table 1).
Electrochemical and Optical Stability of Polymer Films
As one of the important characteristics of an electrochromic
material is its electrochemical/optical stability upon multiple
FIGURE 5 Electronic absorption spectra of P2 in 0.1 M TBABF/
ACN solution during anodic oxidation of the polymer film.
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TABLE 1 Optical and Electrochemical Properties of Polymers P1, P2, and P3
Switching
Time (s)
kmax (nm)
Eg (eV)
HOMO (eV)
LUMO (eV)
CE
Electro-Chemical Stability
tox
tred
P1
P2
P3
382
392
388
2.93
2.80
2.89
5.33
5.27
5.30
2.40
2.47
2.41
1.1
1.7
1.6
2.0
145
352
119
12% loss after 600 cycle
21% loss after 500 cycle
14% loss after 700 cycle
1.0
0.91
containing polymer P1.33–34 Briefly, azide groups decompose
under light or at certain temperature to generate a reactive
nitrene species which can insert in the side chain or the back-
bone causing intracrosslinking and intercrosslinking of the
polymer chains. Conversely, the cross-linking mechanism of
the polymer P3 is probably taking place through forming a
carboxylic acid anhydride between the polymer side-chains.
Highest occupied molecular orbital/lowest unoccupied mo-
lecular orbital (HOMO/LUMO) energy levels of the polymers
were elucidated utilizing their ionization potentials and elec-
tron affinities obtained from experimental data. First, the
ionization potentials were evaluated using the following em-
pirical relation, eq 1;38
Ip ¼ ðEox þ 4:4ÞeV
(1)
It is found that P2 showed a reversible response between a
potential range from 0.0 V (neutral) to 1.25 V (oxidized)
with a response time of 1.0 s for oxidation and 1.6 s for
reduction at 95% of the maximum transmittance and the op-
tical contrast (D%T) was calculated as 7.01% at 540 nm
(Fig. 6). Optical contrast values of 20.00% (540 nm), and
14.80% (560 nm) were obtained for P1 and P3, respectively
(Supporting Information Fig. S3).
The onset of oxidation potentials of P1 (0.93 V vs. Ag wire),
P2 (0.87 V vs. Ag wire), and P3 (0.90 V vs. Ag wire) were
used as Eox in eq 1. Electron affinities were estimated by
subtracting the band gap energy from Ip and the results are
illustrated in Table 1. From the optical and electrochemical
data given in Table 1, we can conclude that the nature of the
side chains of polymers has negligible influence on the emis-
sion properties and energy levels of the main chain, such as
the energy band gap.
Coloration efficiency (CE) is a useful term for measuring the
power efficiency of the electrochromic devices and can be
calculated via optical density using the following equations
Metal Ion Sensing Properties of the Polymers P1, P2,
P2S, and P3
at a given wavelength36,37
;
The metal ion sensory abilities of the fluorene-based poly-
mers P1, P2, P2S, and P3 having different functional groups
on their side chains (i.e., allyl, carboxylic acid, carboxylate,
and azide) were investigated and the results were compared
with PF-H carrying no functional groups to understand the
extent of the functional group effect on the fluorescent
sensing.
CEðkÞ ¼ DODðkÞ=Qd and DODðkÞ ¼ log½ToxðkÞ=TredðkÞꢄ
(where Qd is the injected/ejected charge during a redox pro-
cess; Tox and Tred are the transmittance in the oxidized and
the neutral states, respectively).
On the basis of these equations, CE values were found as
145, 119, and 352 cm2/C at 540 nm for P1, P3, and P2,
respectively. CE values for all polymer films are also tabu-
lated in Table 1 for comparison reasons. Among the three
polymers, P2 exhibits the highest CE value.
The sensing ability of the polymers P1, P2, P3, and PF-H in
THF and P2S in water was investigated by titrating them
with a number of different aqueous metal ion solutions
including Fe3þ, Fe2þ, Cu2þ, Ni2þ, Zn2þ, Agþ, Pb2þ, Cd2þ, and
FIGURE 6 (a) Chronoabsorbtometry, (b) current density, and (c) chronocoloumetry experiments for P2 at kmax (540 nm) under rec-
tangular voltage signal between 0 and 1.25 V.
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FIGURE 7 (a) The Stern–Volmer plots of P2 in the presence of various metal ions (each 5–40 mM). Metal ion solutions were pre-
pared from Fe(NO3)3ꢃ9H2O, Cu(NO3)2ꢃ3H2O, Ni(NO3)2ꢃ6H2O, Zn(NO3)2ꢃ6H2O, AgNO3, Hg(NO3)2, Pb(NO3)2, Cd(NO3)2ꢃ4H2O in THF, (b)
Ksv values, (c) the fluorescence emission spectra of P2 (5.0 ꢀ 10ꢁ6 mol Lꢁ1 in THF, with respect to the repeat unit) with successive
addition of 5–40 mM Fe3þ aqueous solution, and (d) photographs of P2 solution in THF in the absence (i and iii) and the presence
of Fe3þ ions (ii and iv) in daylight (i and ii) under handheld UV light (iii and iv).
Hg2þ at desired concentrations and subsequently monitoring
the changes in their emission spectra at room temperature
(kexc ¼ 380 nm). Binding affinities of these polymers against
the aforementioned metal ions (Fe2þ not shown here) are
given in the form of Stern–Volmer plot in Figures 7 and 8
for P2 and P2S, respectively (see also Supporting Informa-
tion Fig. S5 for other polymers and Supporting Information
Fig. S8 for the binding affinity of Fe2þ with P2).
solvent effect because the measurements were taken in
aqueous solution. However, its Ksv value is still quite high to
sense Fe3þ ions in aqueous solution and this feature is highly
desirable in the design of biological sensors. Moreover, the
absorption maxima of P1, P2, and P3 at 384, 396, and 391,
respectively, showed around a 8 nm red-shift upon titrating
with Fe3þ (Supporting Information Fig. S7) but no changes
were observed by the addition of other aforementioned metal
ions including Fe2þ (Supporting Information Fig. S8).
All of these polymers appear to have a higher selectivity to-
ward Fe3þ ions than the others, albeit at a different degree.
The effect of water addition into THF solution of polymers
that might cause possible changes in the optical properties
of polymers due to conformational changes was also taken
into account. Similar volumes of water used in the addition
of metal ion solutions were added into THF solution of poly-
mers. Their absorption and emission spectra were recorded
but no changes were observed in their wavelengths and
intensities (Supporting Information Fig. S6 shows the effect
of water addition to P2) indicating that the water effect is
negligible for a given concentration.
Although all polymers possess a similar fluorene backbone,
different groups and functionalities were attached to their
ninth position. The results indicate that even PF-H which
does not contain any functional groups but carrying only
alkyl chains as a side group, exhibits a high fluorescent
quenching efficiency toward Fe3þ ions suggesting that fluo-
rene backbone binds with Fe3þ ions probably forming a fer-
rocene-like complex. In the case of P2, the presence of fluo-
rene backbone as well as carboxylic groups which can bind
with Fe3þ ions causes a very efficient energy transfer and in
turn, this quenches the fluorescent emission.
Their quenching efficiencies against Fe3þ ions were plotted
and illustrated in Figure 9 for comparison. The Stern–Volmer
Although the addition of Agþ, Pb2þ, Cd2þ, Cu2þ, Zn2þ, Ni2þ
,
and Fe2þ ions to the polymer solutions has resulted only a
negligible quenching, as shown in Figure 7(a), P2 displayed a
small affinity for Hgþ2 ion resulting with the possibility of in-
terference by those two ions. To check the reliability of this
possibility, the fluorescence spectrum of P2, after addition of
Feþ3 and further addition of Hgþ2 was recorded. As shown in
the Supporting Information Figure S9, addition of Hgþ2 did
not result in further decrease of fluorescence intensity.
constants of (Ksv) were calculated as 4.41 ꢀ 106
M
ꢁ1, 3.28
ꢀ 107
M
ꢁ1, 1.25 ꢀ 106
M
ꢁ1, 6.56 ꢀ 106
M
ꢁ1, and 5.88 ꢀ
106 Mꢁ1 for P1, P2, P2S, P3, and PF-H, respectively. As can
be seen, Ksv value of P2 is almost fivefolds greater than
those of P1, P3, PF-H, and 25-folds greater than that of P2-
S. Relatively small Ksv value for P2-S can be attributed to its
low fluorescent quantum yield (around 10%) as well as a
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FIGURE 8 (a) The Stern–Volmer plots of P2S in the presence of various metal ions (each 5–50 mM). Metal ion solutions were pre-
pared from Fe(NO3)3ꢃ9H2O, Cu(NO3)2ꢃ3H2O, Ni(NO3)2ꢃ6H2O, Zn(NO3)2ꢃ6H2O, AgNO3 Hg(NO3)2, Pb(NO3)2, Cd(NO3)2ꢃ4H2O in THF, (b)
Ksv values, and (c). the fluorescence emission spectra of P2S (5.0 ꢀ 10ꢁ5 mol Lꢁ1 in water) with successive addition of 5–50 mM
Fe3þ aqueous solution, (d) photographs of aqueous solution of P2-S in the absence (i and iii) and the presence of Fe3þ ions (ii and
iv) in daylight (i and ii) under handheld UV light (iii and iv).
CONCLUSIONS
H which does not contain any functional groups but carrying
only alkyl chains as a side group, exhibits a high fluorescent
In this work, a series of novel, solvent processable, fluorene-
quenching efficiency toward Fe3þ ions suggesting that fluo-
based conjugated copolymers were synthesized, character-
rene backbone binds with Fe3þ ions probably forming a
ized and their thermal, optical, electrochemical, and sensory
ferrocene-like complex. However, among them P2 has the
properties were investigated. The polymers were deposited
on an ITO electrode by spray coating and the electrochemical
properties of the resulting thin films were investigated using
CV. The polymer films exhibited a pseudo-reversible redox
highest Ksv value (3.28 ꢀ 107 Mꢁ1) which is almost fivefolds
greater than those of P1, P3, and PF-H because it contains
thioglycolic acid side chains as well as fluorene backbone.
This value is also higher than the one reported in the litera-
and electrochromic behaviors; transparent in the neutral
ture (2.24 ꢀ 106
M
ꢁ1), in which Fe3þ was detected in the
state and dark violet in the oxidized state. Among three poly-
mers, P2 possesses the shortest response time and the high-
est CE value (352 cm2/C at 540 nm). These polymers emit
blue light (emission wavelength at 440 nm in THF) with
high fluorescent quantum yields and have a band gap value
around 2.9 eV. It was also demonstrated that these polymers
were able to detect Fe3þ ions in THF (P1, P2, and P3) and
in aqueous solution (P2S). The results indicate that even PF-
solution of phosphonate containing PF in CH2Cl2.29 Thus,
Fe3þ ions can form complex with thioglycolic acid groups
through electrostatic interaction and this further causes a
decrease in fluorescent emission. As the results indicate, P2
is a promising candidate as a metal ion sensor with a
remarkably high fluorescent quenching efficiency and an
ability to detect Fe3þ ions selectively in organic solvents as
well as in aqueous solutions when it is converted to its salt.
The later feature is particularly important for biological sen-
sor applications.
ACKNOWLEDGMENTS
This work was supported by TUBITAK-TBAG 210T139.
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