Fluorene-dithienothiophene- S,S -dioxide copolymers. Fine-tuning for OLED applications

Article abstract of DOI:10.1021/ma4016592

Three groups of fluorene-dithieno[3,2-b;2′,3′-d]thiophene-S,S- dioxides (DTT-S,S-dioxide) copolymers, each having four different ratios of DTT-S,S-dioxide (5, 15, 25, and 50%) were successfully synthesized through Suzuki coupling method. While the first group copolymers P1 had direct connection of fluorene to the peripheral thiophenes of DTT-S,S-dioxide, second group copolymers P2 had a thiophene extension between fluorene and DTT-S,S-dioxide, and in the third group, copolymers P3, fluorene had a connection with DTT-S,S-dioxide through the phenyl moiety of DTT. Absorbance and emission measurements of first two groups P1 and P2 displayed a regular bathochromic shift with increasing content of DTT-S,S-dioxide, which was more clearly observed in their solid state fluorescence measurements. Introduction of thiophene to the peripherals of the DTT-S,S-dioxide in copolymers P2 caused even further bathochromic shift in absorbances and emissions. As the absorbance and emission of P1 went up to 447 and 558 nm in solution, respectively, P2 had them at 472 and 592 nm, respectively. In solid state, emissions of P1 and P2 even went further up to 585 and 646 nm, respectively. The bathochromic trend of P1 and P2 became opposite with absorbance and solid state emission of P3, which had a hypsochromic shift with increasing content of DTT-S,S-dioxide. Solid state emission of P3, particularly the copolymers having 5, 15 and 50% DTT-S,S-dioxide, covered a wide region between 400 and 675 nm. A spread of colors from light blue (border of white) to red through green and yellow was obtained with the OLED applications of the copolymers. Their optical and electronic band gaps varied between 2.17 and 2.99 eV and between 2.68 and 3.57 eV, respectively. While the highest quantum yield was obtained with P2 (5%) as 0.66, the lowest was observed with P2 (50%) as 0.03. Almost all of the polymers displayed good thermal stabilities. No weight loss was observed with the copolymers P2 (5-15%) and P3 up to 400 C.

Full text of DOI:10.1021/ma4016592

Article
pubs.acs.org/Macromolecules
Fluorene−Dithienothiophene-S,S-dioxide Copolymers. Fine-Tuning
for OLED Applications⊥
Ipek Osken,† Ali Senol Gundogan,† Emine Tekin,‡ Mehmet S Eroglu,§,‡ and Turan Ozturk†,‡,*
†Department of Chemistry, Faculty of Science, Istanbul Technical University, Maslak, Istanbul 34469, Turkey
‡Chemistry Group Laboratories, TUBITAK UME, PO Box 54, 41470, Gebze-Kocaeli, Turkey
§Department of Chemical Engineering, Marmara University, 34722, Kadikoy, Istanbul, Turkey
S
* Supporting Information
ABSTRACT: Three groups of fluorene−dithieno[3,2-b;2′,3′-d]-
thiophene-S,S-dioxides (DTT-S,S-dioxide) copolymers, each hav-
ing four different ratios of DTT-S,S-dioxide (5, 15, 25, and 50%)
were successfully synthesized through Suzuki coupling method.
While the first group copolymers P1 had direct connection of
fluorene to the peripheral thiophenes of DTT-S,S-dioxide, second
group copolymers P2 had a thiophene extension between fluorene
and DTT-S,S-dioxide, and in the third group, copolymers P3,
fluorene had a connection with DTT-S,S-dioxide through the
phenyl moiety of DTT. Absorbance and emission measurements of first two groups P1 and P2 displayed a regular bathochromic
shift with increasing content of DTT-S,S-dioxide, which was more clearly observed in their solid state fluorescence
measurements. Introduction of thiophene to the peripherals of the DTT-S,S-dioxide in copolymers P2 caused even further
bathochromic shift in absorbances and emissions. As the absorbance and emission of P1 went up to 447 and 558 nm in solution,
respectively, P2 had them at 472 and 592 nm, respectively. In solid state, emissions of P1 and P2 even went further up to 585
and 646 nm, respectively. The bathochromic trend of P1 and P2 became opposite with absorbance and solid state emission of
P3, which had a hypsochromic shift with increasing content of DTT-S,S-dioxide. Solid state emission of P3, particularly the
copolymers having 5, 15 and 50% DTT-S,S-dioxide, covered a wide region between 400 and 675 nm. A spread of colors from
light blue (border of white) to red through green and yellow was obtained with the OLED applications of the copolymers. Their
optical and electronic band gaps varied between 2.17 and 2.99 eV and between 2.68 and 3.57 eV, respectively. While the highest
quantum yield was obtained with P2 (5%) as 0.66, the lowest was observed with P2 (50%) as 0.03. Almost all of the polymers
displayed good thermal stabilities. No weight loss was observed with the copolymers P2 (5−15%) and P3 up to 400 °C.
emitting devices as they have low electron affinities and low
solid-state photoluminescence efficiencies.8−14 On the other
hand, converting oligothiophenes into the corresponding
oligothiophene-S,S-dioxides has been shown to be useful for
increasing both thin film photoluminescence efficiencies and
molecular energy levels.7c,14−16
In recent years, syntheses and investigations of the properties
of dithieno[3,2-b;2′,3′-d]thiophenes (DTT) (1), DTT-S,S-
dioxides (2), and their polymers have been the emerging area
as they show interesting electronics and optoelectronics
properties.17 DTTs comprise three fused thiophene rings
which create an electron rich rigid core. Oxidation of the sulfur
atom of the central thiophene to the S,S-dioxide introduces the
molecule a fluorescent property with, in general, an emission in
the blue region. Our continuing research on the synthesis of
analogues of DTT, applying the method developed by our
group, let us to the syntheses of functionalized DTT analogues,
such as 3,5-diphenylDTT (3), 3,5-di(4-bromophenyl)DTT (4),
INTRODUCTION
■
Organic electronic materials, both small molecules and
conjugated polymers, have been the focus of an increasing
attention of academic and industrial research, particularly in
chemistry and physics.1 This is due to their possible
applications in electronics and optoelectronics such as organic
light-emitting diodes (OLED),2 field-effect transistors (FET),3
lasers,4 photodiodes,5 and solar cells.6 The main attraction of
this field comes from the ability to modify the chemical
structure of organic compounds in a way that the properties of
the materials could directly be affected. Moreover, some
important tunings in structure or composition of an organic
material can markedly alter its bulk properties. Understanding
of their electronic structure is the key to the design of high
performance optical and electronic organic devices. Currently,
modification of the molecular structure of the conjugated
materials to tune their optoelectronic properties is a challenging
topic,1a among which, thiophene based organic materials are
the most promising compounds to be modified by proper
molecular engineering as they have tunable functional proper-
ties.7 For example, thiophenes and their oligomers and
polymers are not proper materials for applications in light
Received: August 7, 2013
Revised: November 5, 2013
Published: November 20, 2013
© 2013 American Chemical Society
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alcohol. Lastly, the substrates were dried under N2 flow. PEDOT:PSS,
as a hole injection layer, was spin-coated at 3000 rpm for 30 s, which
was then dried at 110 °C for 10 min. Subsequently, the polymer films,
as active layers, were produced by spin coating at 1200 rpm and cured
at 120 °C for 5 min. Finally, calcium (Ca, 6 nm) and aluminum (Al,
100 nm) were deposited under vacuum (∼5 × 10−6 bar) by thermal
evaporation technique, as a cathode layer. The active emission area was
9 mm2. Electroluminescence, brightness−voltage, luminous efficiency
and EQE−current density curves of the fabricated OLEDs were
obtained by a Hamamatsu PMA-12 C10027 photonic multichannel
analyzer and a digital multimeter (2427-C 3A Keithley). All the devices
were measured in a dark sample chamber to remove any influence of
3,5-di(4-methoxyphenyl)DTT (5), and 3,5-di(4-nitrophenyl)-
DTT (6),18 oxidation of which yield their fluorescent
derivatives. As copolymerization of thiophenes with various
monomers is an important strategy for fine-tuning the resultant
material,7 in this study, Ph2DTT-S,S-O2 (7), (BrPh)2DTT-S,S-
O2 (8), and Br2Ph2Th2DTT-S,S-O2 (14) were copolymerized
with a commonly used monomer, fluorene, in different ratios of
DTTs, i.e., 5%, 15%, 25%, and 50%, considering that such ratios
could be a good tuning for the synthesized materials for OLED
applications. Their devices were fabricated and the properties of
the polymers in solution, in solid state and the properties of
their devices were investigated.
ambient light. A stylus profiler (KLA Tencor P6) was used to
̅
determine the thickness of the organic layers.
3,5-Diphenyldithieno[3,2-b;2′,3′-d]thiophene-S,S-dioxide (7). To
a solution of 3 (0.2 g, 0.57 mmol), dissolved in DCM (30 mL), was
added dropwise mCPBA (0.51 g, 2.3 mmol) dissolved in DCM (15
mL) at 0 °C. The reaction was then left stirring overnight at room
temperature for 2 days. The solution was extracted with 10% KOH,
10% NaHCO3 and brine. Organic layer was dried over Na2SO4,
filtered and the solvent was evaporated under reduced pressure. The
crude product was purified by column chromatography, eluting with a
mixture of hexane/DCM (3:1) to give the title compound (7) (0.16 g,
EXPERIMENTAL SECTION
■
2-Bromo-9,9-dioctyl-9H-fluorene (F1), 2,7-bis(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (15), 2,7-dibromo-9,9-di-
octyl-9H-fluorene (16), 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)-9,9-dioctylfluorene (17), 4,4,5,5-tetramethyl-2-(thiophen-2-yl)-
[1,3,2]dioxaborolane (13),19 and poly[2,7-(9,9-dioctylfluorene)]
(F)20 were synthesized according to the literature procedures.
Materials. N-Bromosuccinimde (NBS), thiophene, dichlorome-
thane (DCM), and N,N-dimethylformamide (DMF) were purchased
from Merck. m-Chloroperbenzoic acid (mCPBA) and 2-isopropoxy-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane were obtained from Sigma-
Aldrich. All chemicals were used as received unless it is stated.
Quantum yield measurements of the polymers, dissolved in THF, were
performed using coumarin1 (quantum yield = 0.73, excited at 360 nm,
in ethanol), coumarin6 (quantum yield = 0.78, excited at 420 nm, in
ethanol), and quinine sulfate (quantum yield = 0.546, excited at 310
nm, in 0.5 M H2SO4) as standards
1
72%) as a yellow powder, mp 187−190 °C. H NMR (600 MHz,
CDCl3): δ (ppm) 7.83 (dd, J = 7.6 Hz, 1.2 Hz, 4H), 7.47 (t, J = 7.6
Hz, 4H), 7.40 (t, J = 7.6 Hz, 2H), 7.36 (s, 2H). 13C NMR (150 MHz,
CDCl3): δ (ppm) 141.1, 139.2, 137.0, 131.9, 129.2, 128.9, 127.8,
124.2. m/z = 381.00 (M+ + 1).
3,5-Bis(4-bromophenyl)dithieno[3,2-b;2′,3′-d]thiophene-S,S-di-
oxide (8). It was synthesized similar to 7, starting from 4. The crude
product was purified by column chromatography, eluting with a
mixture of hexane/DCM (1:1) to give the title compound (8) (0.07 g,
1
Instrumentation. Cyclic voltammetry (CV) studies were
performed using CH-Instruments Model 400A as a potentiostat.
UV−vis measurements were studied on HITACHI U-0080D. H and
34%) as a yellow powder, mp 273−275 °C. H NMR (600 MHz,
CDCl3): δ (ppm) 7.68 (d, J = 8.2 Hz, 4H), 7.60 (d, J = 8.2 Hz, 4H),
7.35 (s, 2H). 13C NMR (150 MHz, CDCl3): δ (ppm) 140.0, 137.9,
137.1,132.4, 130.1, 129.3, 124.7, 123.4. m/z = 539 (M+ + 1).
1
13C NMR spectra were recorded on Varian model NMR (600 MHz).
Proton and carbon chemical shifts were reported in ppm downfield
from tetramethylsilane (TMS). Mass spectra were recorded on Bruker
MICROTOFQ and Thermo LCQ-Deca ion trap mass instruments.
The molar mass and molar mass distribution of the polymers were
determined by gel permeation chromatography (GPC), equipped with
Perkin-Elmer 200 GPC high pressure pump, injector, THF columns
connected in series (guard column + Styragel HR2 + Styragel HR 3 +
Styragel HR 4E + Styragel HR 5E), Wyatt Optilab differential
refractive index detector (RI) at 654 nm, and Dawn Heleos multi angle
light-scattering (LS) detector. The mobile phase was THF with a flow
rate of 0.7 mL/min. Measurements were conducted at 25 °C. Polymer
concentrations were in the range of 0.5−2.0 mg/mL and all the
samples were filtered through 0.2 μm filter prior to use. Thermal
analyses (TA) of the polymers were performed by a Perkin-Elmer
thermal analyzer system equipped with Pyris 6 model thermogravim-
etry and Jude differential scanning calorimetry (DSC) systems.
Device Fabrication and Characterization. ITO-coated glass
substrates (ITO, thickness 120 nm, 15 ohms/sq) were purchased from
Visiontek Systems. Poly(3,4-ethylenedioxythiophene) poly-
(styrenesulfonate) (PEDOT:PSS) was obtained from Heraeus Clevios
GmbH. All the copolymers were prepared by dissolving them in a
mixture of toluene/dichlorobenzene, having a concentration of 8 mg/
mL, and filtered through a 0.45 μm membrane filter (Millipore,
PTFE). The substrates were etched in aqua regia solution (3:1:3/
HCl:HNO3:H2O) for 2 min, and then cleaned consecutively in
ultrasonic baths containing deionized water, acetone, and isopropyl
2,6-Dibromo-3,5-diphenyldithieno[3,2-b;2′,3′-d]thiophene (9).
To a solution of 3 (0.2 g, 0.57 mmol) dissolved in DMF (20 mL)
and protected from light was added 0.22 g NBS (0.22 g, 1.26 mmol)
portionwise at 0 °C. After 7 h, the reaction mixture was poured into
cold water to precipitate the crude product, which was recrystallized
from toluene to give the title compound (9) (0.24 g, 83%) as a white
powder, mp 241−242 °C. 1H NMR (600 MHz, CDCl3): δ (ppm) 7.67
(d, J = 7.5 Hz, 4H), 7.50 (t, J = 7.5 Hz, 4H), 7.43 (t, J = 7.5 Hz, 2H).
13C NMR (150 MHz, CDCl3): δ (ppm) 139.8, 134.9, 133.2, 129.3,
128.9, 128.7, 128.6, 108.8. m/z = 506 (M+).
2,6-Dibromo-3,5-bis(4-bromophenyl)dithieno[3,2-b;2′,3′-d]-
thiophene (10). It was synthesized similar to 9, starting from 4. The
crude product was recrystallized from toluene to give the title
compound (10) (0.23 g, 87%) as a white powder, mp 309−312 °C. 1H
NMR (600 MHz, CDCl3): δ (ppm) 7.62 (d, J = 8.4 Hz, 4H), 7.53 (d, J
= 8.4 Hz, 4H). 13C NMR (150 MHz, CDCl3): δ (ppm) 139.7, 136.1,
132.5, 132.1, 131.0, 128.7, 124.0, 113.6. m/z = 667 (M+ + 1).
2,6-Dibromo-3,5-diphenyldithieno[3,2-b;2′,3′-d]thiophene-S,S-
dioxide (11). It was synthesized similar to 7, starting from 9. The
crude product was purified by column chromatography, eluting with a
mixture of hexane/DCM (1:1) to give the title compound (11) (0.19
1
g, 90%) as a yellow powder, mp 220−222 °C. H NMR (600 MHz,
CDCl3): δ (ppm) 7.68 (dd, J = 7.8 Hz, 1.2 Hz, 4H), 7.47 (t, J = 7.8
Hz, 4H), 7.44 (t, J = 7.8 Hz, 2H). 13C NMR (150 MHz, CDCl3): δ
(ppm) 140.3, 137.3, 134.1, 129.9, 129.4, 129.3, 128.8, 113.1. m/z =
539 (M+ + 1).
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2,6-Dibromo-3,5-bis(4-bromophenyl)dithieno[3,2-b;2′,3′-d]-
thiophene-S,S-dioxide (12). It was synthesized similar to 8, starting
from 10. The crude product was purified by column chromatography,
eluting with a mixture of hexane/DCM (2:1) to give the title
compound (12) (0.17 g, 83%) as a yellow-orange powder, mp 284−
Table 2. Ratios of the Contents for the Syntheses of the
Polymers P2
entry
P2-5
P2-15
P2-25
P2-50
14
15
16
0.023 mmol
(16.1 mg)
0.063 mmol
(44.3 mg)
0.116 mmol
(81.5 mg)
0.232 mmol
(163 mg)
1
287 °C. H NMR (600 MHz, CDCl3): δ (ppm) 7.53 (d, J = 9 Hz,
4H), 7.48 (d, J = 9 Hz, 4H). 13C NMR (150 MHz, CDCl3): δ (ppm)
140.0, 136.1, 134.2, 132.1, 131.0, 128.7, 124.0, 113.6. m/z = 697 (M+ +
1).
0.232 mmol
(150 mg)
0.232 mmol
(150 mg)
0.232 mmol
(150 mg)
0.232 mmol
(150 mg)
0.209 mmol
(114.4 mg)
0.169 mmol
(92.7 mg)
0.116 mmol
(63.6 mg)
−
2,6-Bis(5-bromothiophen-2-yl)-3,5-diphenyldithieno[3,2-b;2′,3′-
d]thiophene-S,S-dioxide (14). In a Schlenk tube, a mixture of 11 (0.2
g, 0.37 mmol), 13 (0.2 g, 0.93 mmol), Pd(PPh3)4 (0) (0.02 g, 0.02
mmol), and K2CO3 (1 mL, 2 M) in THF (50 mL) was degassed with
N2 for half an hour. Then, the mixture was sealed and stirred at 68 °C
for 2 days. It was filtered through Celite and the crude product was
purified by column chromatography, eluting with a mixture of
hexane:DCM (2:1). The product was subsequently dissolved in
DMF and reacted with NBS. The mixture was poured into cold water
and the precipitate was filtered. It was dissolved with a minimum
amount of DCM, precipitated in diethyl ether and filtered to obtain
the title compound 14 (0.20 g, 76%) as an orange powder, mp 300−
K2CO3
(2 M)
1 mL
1 mL
1 mL
1 mL
Pd(PPh3)4
0.001 mmol
(12 mg)
0.001 mmol
(12 mg)
0.001 mmol
(12 mg)
0.001 mmol
(12 mg)
17
F1
0.068 mmol
(35 mg)
0.068 mmol
(35 mg)
0.068 mmol
(35 mg)
0.068 mmol
(35 mg)
0.074 mmol
(35 mg)
0.074 mmol
(35 mg)
0.074 mmol
(35 mg)
0.074 mmol
(35 mg)
Table 3. Ratios of the Contents for the Syntheses of the
Polymers P3
1
303 °C. H NMR (600 MHz, CDCl3): δ (ppm) 7.52 (d, J = 4 Hz,
entry
P3-5
P3-15
P3-25
P3-50
4H), 7.40 (t, J = 4 Hz, 6H), 6.92 (d, J = 3.6 Hz, 2H), 6.81 (d, J = 3.6
Hz, 2H). 13C NMR (150 MHz, CDCl3): δ (ppm) 143.0, 135.5, 135.4,
134.2, 132.6, 130.3, 130.2, 129.7, 129.3, 129.0, 127.8, 114.6. m/z = 703
(M+ + 1).
8
0.023 mmol
(12.4 mg)
0.063 mmol 0.116 mmol 0.232 mmol
(34 mg) (62.4 mg) (124.9 mg)
15
16
0.232 mmol
(150 mg)
0.232 mmol 0.232 mmol 0.232 mmol
(150 mg) (150 mg) (150 mg)
General Method for the Syntheses of the Polymers P1-5. A
mixture of 11 (0.023 mmol, 12.4 mg), 15 (0.232 mmol, 150 mg), 16
(0.209 mmol, 114.4 mg), K2CO3 (1 mL, 2 M) and Pd(PPh3)4 (0.001
mmol, 12 mg) (Table 1) in a solvent mixture of toluene:dioxane (9:1,
0.209 mmol
(114.4 mg)
0.169 mmol 0.116 mmol
(92.7 mg) (63.6 mg)
−
K2CO3
(2 M)
1 mL
1 mL 1 mL
1 mL
Pd(PPh3)4
0.001 mmol
(12 mg)
0.001 mmol 0.001 mmol 0.001 mmol
(12 mg) (12 mg) (12 mg)
Table 1. Ratios of the Contents for the Syntheses of the
Polymers P1
17
F1
0.068 mmol
(35 mg)
0.068 mmol 0.068 mmol 0.068 mmol
(35 mg) (35 mg) (35 mg)
entry
P1-5
P1-15
P1-25
P1-50
0.074 mmol
(35 mg)
0.074 mmol 0.074 mmol 0.074 mmol
(35 mg) (35 mg) (35 mg)
11
15
16
0.023 mmol
(12.4 mg)
0.063 mmol 0.116 mmol 0.232 mmol
(34 mg) (62.4 mg) (124.9 mg)
0.232 mmol
(150 mg)
0.232 mmol 0.232 mmol 0.232 mmol
(150 mg) (150 mg) (150 mg)
0.209 mmol
(114.4 mg)
0.169 mmol 0.116 mmol
(92.7 mg) (63.6 mg)
−
47 180; Mw/Mn, 1.51; dn/dc, 0.1691 mL/g. P3-25: 53% yield; Mw, 24
710; Mn, 6863; Mw/Mn, 3.60; dn/dc, 0.80 mL/g. P3-50: 36% yield;
Mw, 16 310; Mn, 7694; Mw/Mn, 2.12; dn/dc, 0.26 mL/g.
K2CO3
(2 M)
1 mL
1 mL 1 mL
1 mL
Pd(PPh3)4
0.001 mmol
(12 mg)
0.001 mmol 0.001 mmol 0.001 mmol
(12 mg) (12 mg) (12 mg)
RESULTS AND DISCUSSION
■
17
F1
0.068 mmol
(35 mg)
0.068 mmol 0.068 mmol 0.068 mmol
(35 mg) (35 mg) (35 mg)
Syntheses. DTTs 3 and 4 were prepared following the
literature procedures.18,21,22 While oxidation of the DTTs with
mCPBA produced dithienothiophene-S,S-dioxides (DTT-S,S-
O2) 7 and 8, which were dibrominated with NBS to yield the
dibromodithienothiophene-S,S-dioxides (Br2DTT-S,S-O2) 11
and 12, respectively, bromination of the DTTs with NBS
gave dibromo-DTTs (Br2DTT) 9 and 10, which were also
oxidized to obtain 11 and 12, respectively (Scheme 1). Suzuki
coupling of Br2DTT-S,S-O2 (11) with 4,4,5,5-tetramethyl-2-
thiophen-2-yl-[1,3,2]dioxaborolane (13)19 was followed by
bromination with NBS to yield the monomer 14.
The monomers 11, 14, and 8 were subjected to polymer-
ization with flouorenes 15 and 16 in four ratios of 5, 15, 25, and
50% to obtain three series of DTT-S,S-dioxide-fluorene random
copolymers P1, P2, and P3, respectively. All the polymerization
reactions were performed applying Suzuki coupling technique,
which yielded three groups, each having four polymers and
totaling to 12 polymers (P1-5, P1-15, P1-25, P1-50; P2-5, P2-
15, P2-25, P2-50 and P3-5, P3-15, P3-25, P3-50) (Scheme 2).
The ratios of DTT-S,S-dioxides in the fluorene polymers were
described with the integration ratios of the aromatic peaks to
the alphatic peaks of the carbon chains on the flurenes (see
Supporting Information). Moreover, particularly the solid state
0.074 mmol
(35 mg)
0.074 mmol 0.074 mmol 0.074 mmol
(35 mg) (35 mg) (35 mg)
5 mL) in a Schlenk tube was degassed with N2 and sealed. The mixture
was stirred for 2 days, and 17 (0.068 mmol, 35 mg) was then added.
Stirring was continued for 1 day. Lastly, F1 (0.074 mmol, 35 mg) was
added and the mixture was stirred one more day. The reaction mixture
was filtered through Celite and the solvent was evaporated under
reduced pressure. The crude product was dissolved in a minimum
amount of THF and precipitated in methanol. P1-5: 81% yield; Mw,
313 900; Mn, 83 640; Mw/Mn, 3.75; dn/dc, 0.13 mL/g. P1-15: 76%
yield; Mw, 69 840; Mn, 27 330; Mw/Mn, 2.55; dn/dc, 0.17 mL/g. P1-25:
73% yield; Mw, 40 110; Mn, 23 620; Mw/Mn, 1.70; dn/dc, 0.20 mL/g.
P1-50: 83% yield; Mw, 859 100; Mn, 699 600; Mw/Mn, 1.23; dn/dc,
0.14 mL/g.
The following were prepared similarly.
Synthesis of P2 (Table 2). P2-5: 83% yield; Mw, 37 060; Mn, 27
760; Mw/Mn, 1.33; dn/dc, 0.11 mL/g. P2-15: 87% yield; Mw, 501 200;
Mn, 219 200; Mw/Mn, 2.29; dn/dc, 0.16 mL/g. P2-25: 78% yield; Mw,
102 200; Mn, 76 210; Mw/Mn, 1.34; dn/dc, 0.19 mL/g. P2-50: 81%
yield; Mw, 60 630; Mn, 43 680; Mw/Mn, 1.39; dn/dc, 0.21 mL/g.
Synthesis of P3 (Table 3). P3-5: 48% yield; Mw, 9567; Mn, 8261;
Mw/Mn, 1.16; dn/dc, 0.149 mL/g. P3-15: 44% yield; Mw, 71 200; Mn,
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Scheme 1. Syntheses of the DTTs
Scheme 2. Syntheses of DTT−Fluorene Copolymers (P1, P2, and P3)
fluorescence measurements displayed distinct peaks for each
polymer.
took place, which became gradually stronger at higher contents
of DTT-S,S-dioxide and reached maximum with P1-50,
containing 50% of DTT-S,S-dioxide, as the absorbance of
fluorene reached minimum. These results demonstrated that
two absorbencies varying with the contents of fluorene and
DTT-S,S-dioxide, were obtained at around 380 and 430 nm,
respectively. Fluorescence measurements followed the same
trend. Two emission bands around 420 nm due to fluorene and
between 443 and 558 nm due to DTT-S,S-dioxide were
observed (Figure 1b, Table 4). As the content of DTT-S,S-
Optical and Electrochemical Properties of the
Polymers. Optical properties of the polymers were inves-
tigated in solution (THF) and solid state, spin coated on
glasses. Absorbance spectra of the polymers P1 displayed that
the polymer having 5% of DTT-S,S-dioxide P1-5 (382 nm) had
almost the same absorbance with polyfluorene F (383 nm)
(Figure 1a, Table 4). Upon increasing the content of DTT-S,S-
dioxide in the polymer, appearance of a peak around 430 nm
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Figure 1. UV−vis (a) and fluorescence (b) measurements of P1 in THF and emission in films from THF (c).
Table 4. Electrochemical, Optical, and Electronic Properties of the Polymers
a
b
c
d
h
i
sample
abs. λmax
383
emission λmax
QY
abs. λmax
emission λmax
Egopt
HOMO
LUMO
Egcv
e
F
420, 443sh
421, 444sh
421, 445
0.78
0.38
0.15
0.12
0.08
0.66
0.30
0.12
0.03
0.56
0.17
0.12
0.09
360
387
399
412
426
422sh, 436, 463sh, 528
2.98
2.95
2.48
2.47
2.47
2.95
2.26
2.26
2.17
2.99
2.97
2.97
2.97
−5.74
−5.81
−5.81
−5.74
−5.78
−5.81
−5.65
−5.66
−5.60
−5.72
−5.73
−5.82
−5.89
−2.26
−2.49
−2.27
−2.30
−3.10
−2.24
−2.57
−2.61
−3.22
−2.44
−2.36
−2.34
−2.83
3.48
3.32
3.54
3.44
2.68
3.57
3.08
3.05
2.38
3.28
3.37
3.48
3.06
e
e
e
f
P1-5
382
545
P1-15
P1-25
P1-50
P2-5
380
561
374, 430sh
343, 447
375
419, 436sh, 535
420, 448sh, 558
419, 442sh
418, 440sh, 589
418, 440sh, 589
400, 592, 725sh
418, 442sh
575
585
e
e
e
e
e
e
g
g
372, 485
379, 489
380, 494
364, 475
410
620
P2-15
P2-25
P2-50
P3-5
369, 445
631
307, 360, 449
318, 342sh, 472
636
646
377
376
368
439, 465, 500
420, 440, 465, 498
413, 438, 518
470, 496
P3-15
P3-25
P3-50
423, 446
397
420, 447
383
340
461
364
a
b
c
d
Absorbtion in THF. Emission in THF. Absorption maxima of the polymer films coated on ITO Emission maxima of the polymer films coated
on ITO. Quantum yield calculated using Coumarin1 as a standard in ethanol. Quantum yield calculated using Coumarin6 as a standard in ethanol.
Quantum yield calculated using quinine sulfate as a standard in 0.5 M H2SO4. HOMO = −Eonset (ox) + (4.4) (eV). LUMO = −Eonset (red) + 4.4
e
f
g
h
i
(eV).
Figure 2. UV−vis (a) and fluorescence (b) measurements of P2 in THF and emission in films from THF (c).
dioxide was increased, the second peak became stronger. On
the other hand, in solid state, a different trend was displayed
(Figure 1c). While polyfluorene F gave two emission bands at
436 and 528 nm, the rest of the polymers had one emission
band. Effect of the increasing content of DTT-S,S-dioxide on a
regular red shift emission was clearly observed in solid state
fluorescent measurements. Although absorbance and emission
bands of polyfluorene and the polymer having 5% of DTT-S,S-
dioxide P1-5 in THF were nearly the same, a clear difference
was displayed with a 17 nm red shift of P1-5 in solid state.
As the phenyl groups at C-3 and C-5 of DTT could restrict
conjugation through the polymer chain by blocking free
rotations between two DTT or DTT-florene moieties,
thiophenes were introduced to the peripherals of DTT to
obtain the polymers P2. A substantial red sift and a wide range
of absorbance between 300−600 nm of the polymers P2-15,
25, and 50 were observed, which implied that a better rotation
and a good conjugation have been provided (Figure 2a, Table
4). Fluorescence measurements in THF displayed that as the
polymer P2-5 gave a similar emission with polyfluorene F, the
rest had emission maxima around 600 nm, ranging from 500 to
750 nm (Figure 2b). The polymers P2, like the polymers P1,
displayed quite different behaviors in solid state emissions
(Figure 2c). A similar emission of P2-5 with F in THF did not
take place in solid state, rather a similar emission trend of P1
was observed. This time an emission difference of 92 nm took
place between F and P2-5. While F displayed an emission
maxima at 528 nm, the polymers P2-5, P2-15, P2-25, and P2-
50 had a regular emission maxima of 620, 631, 636, and 646
nm, respectively, in line of increasing DTT-S,S-dioxide
contents.
Lastly, optical properties of the polymers P3, obtained
through the polymerization of DTT-S,S-dioxide from its phenyl
groups with fluorene in 5, 15, 25 and 50% ratios, were
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Figure 3. UV−vis (a) and fluorescence (b) measurements of P3 in THF and emission in films from THF (c).
investigated (Figure 3, Table 4). As the conjugation through
phenyl group is not as effective as through the thiophenes,
absorbances of P3 had a similar behaviors with polyfluorene F
(Figure 3a), and as the content of DTT-S,S-dioxide was
increased, a hypsochromic shift at the absorbances was
observed, which was more distinct with the polymers P3-25
and P3-50, having 25 and 50% of DTT-S,S-dioxide with
absorbance maxima of 368 and 340 nm, respectively. This
proves that increasing content of DTT-S,S-dioxide provides less
conjugation. Investigation of emission behaviors of the
polymers P3 indicated that while the polymers had red shift
(bathochromic) emission in THF with the increasing content
of DTT-S,S-dioxide, a reverse situation, a blue shift
(hypsochromic) from fluorene, generally with decreasing
content of DTT-S,S-dioxide was observed. This could be the
result of polyfluorene tending to emit light around 550 nm and
DTT-S,S-dioxide polymerized through its phenyl groups having
emission behavior around 440 nm in solid state (Figure 3c,
Table 4). The emission of P3-50 almost covered whole region
from 400 extending to 700 nm, which indicates that emission
properties of both fluorene and DTT-S,S-dioxide were
combined under a wide curve.
Thermal Gravity Analyses of the Polymers. Thermal
analyses of the polymers indicated that although all the
polymers had good thermal stability, the best stabilities, i.e. up
to 400 °C, were obtained with the polymers P2-5, 15, 25, and
P3 (Figure 5). While increasing content of DTT-S,S-dioxide
Cyclic voltammetry (CV) behaviors of the polymers P1−P3
were investigated in dry and degassed acetonitrile, using
Bu4NPF6 as an electrolyte, under nitrogen atmosphere, by
drop-casting onto glassy carbon disc electrode from their THF
solutions (Figure 4, Table 4 and see Supporting Information).
Pt wire and Ag/AgCl were counter and reference electrodes,
respectively. Bandgaps of the polymers were calculated to be
between 2.38 and 3.57 eV from the differences between the
onsets of oxidation and reduction peaks, scanning between
(−2.5) and (+2) V. Their optical bandgaps varied between 2.17
and 2.99 eV (Table 4).
Figure 5. Thermal gravity analyses of the polymers P1−P3.
effected thermal stability of the polymers P1 and P2-50,
bringing start of their weight loss to 170 and 280 °C,
respectively, such a loss was not observed with P2-5, 15, 25,
and P3 up to around 400 °C. When the thermal behaviors of
the polymers P1 and P2 were investigated more closely, initial
degradation temperatures of the polymers P1-50 and P2-50,
having fluorene/DTT-S,S-dioxide ratio of 1:1, were observed to
be at lower temperatures. On the other hand, such a weight loss
was not observed with P3-50, which had the same ratio of
fluorene/DTT-S,S-dioxide (1:1). This result could be due to
the polymerization through the phenyl−phenyl linkage in
polymer P3-50.
Device Characterization. OLED devices based on the
device architecture of ITO/PEDOT:PSS (50 nm)/copolymer
(60 nm) (P1, P2, or P3)/Ca (6 nm)/Al (100 nm) structure
were fabricated and their propertiesluminance (brightness),
luminous efficiency, and external quantum efficiencywere
investigated.
Luminance and both luminous and quantum efficiencies of
the polymers P1 were significantly changed with varying
content of DTT-S,S-dioxides, which were improved with
increasing DTT-S,S-dioxides up to 25% (Figure 6, Table 5).
Although the maximum efficiency values of devices P1-5 and
P1-15 were found to be similar (0.100 and 0.104 cd/A), P1-15
reached its maximum at a relatively low current density.
Figure 4. Cyclic voltammograms of the polymers P1 on glassy carbon
disk electrode in acetonitrile, 0.1 M Bu4NPF6, 500 mV/s scan rate.
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Figure 6. (a) Luminance−voltage, (b) luminous efficiency−current density, and( c) external quantum efficiency− (EQE−) current density of P1.
a
a
Table 5. Electroluminescence Data of P1
Table 6. Electroluminescence Data of P2
sample λEl (nm) LE (Cd/A) VONSET CIE coord (x,y) EQE (%)
λEl
efficiency
CIE
EQE
(%)
sample
(nm)
(Cd/A)
VONSET
coord (x,y)
P1-5
530
540
545
565
0.100
0.104
0.129
0.036
6.2
4.0
3.6
3.4
0.34−0.56
0.39−0.56
0.42−0.55
0.47−0.52
0.032
0.033
0.042
0.013
P2-5
594
612
616
630
0.050
0.014
0.016
0.006
6.1
3.6
3.2
4.5
0.54−0.43
0.60−0.39
0.62−0.38
0.61−0.38
0.018
0.011
0.015
0.006
P1-15
P1-25
P1-50
P2-15
P2-25
P2-50
a
λEl, main emission peak of the electroluminescense spectrum; LE,
a
λEl main emission peak of the electroluminescense spectrum, LE
luminous efficiency; VONSET, onset voltage; chromaticity coordinates
according to the CIE 1931 diagram; and EQE, external quantum
efficiency.
luminous efficiency, VONSET onset voltage, chromaticity coordinates
according to the CIE 1931 diagram and EQE external quantum
efficiency.
Moreover, maximum luminance of P1-5 was measured to be
relatively higher compared to that of P1-15 and required
considerably lower voltage to reach its maximum. The most
significant and the systematic changes were found to be related
to the on-set voltages, which decreased regularly with increasing
DTT-S,S-dioxide ratio. It could be explained that DTT-S,S-
dioxide lowers the energy barriers for better charge transport.
On the other hand, P1-50 yielded the poorest performances of
luminance and efficiencies, which could mean that electron−
hole balance in the active layer was negatively affected by
alternating DTT-S,S-dioxide and fluorene units. A regular
bathochromic shift on the electroluminescence spectra of the
devices with the increasing content of DTT-S,S-dioxide of the
copolymers was observed, which demonstrated that the initial
green emission turned into yellow (see Supporting Informa-
tion).
Regarding the device performances of polymers P2, the best
efficiency was observed with polymer P2-5, even though its
maximum luminous efficiency and the maximum luminance
were obtained at relatively higher current densities and voltages
compare with the outputs of the other P2 devices (Figure 7,
Table 6). Although the luminous efficiency of the P2-5 was
more than three times higher than that of P2-15 and P2-25, the
differences between the EQE values were not that significant.
P1 and P3 devices displayed similar trends in luminous and
external quantum efficiencies. The onset voltages of P2-15, 25,
and 50 were considerably lower than that of P2-5. A similar
trend, observed with the polymers P1, a bathochromic shift
with increasing content of DTT-S,S-dioxide took place with the
polymers P2, the colors of which were orange-red (see
Supporting Information).
The devices, fabricated using the copolymers P3, exhibited
relatively lower luminance and efficiencies compared to that of
the devices P1 and P2 (Figure 8, Table 7). Among the
copolymers P3, while the best performance was achieved with
P3-25, copolymer P3-50 had the poorest characteristics, as it
was observed with copolymers P1. As distinct from both P1
and P2 devices, increasing DTT-S,S-dioxide content raises the
on-set voltages of P3 devices. As expected, increasing DTT-S,S-
dioxide content caused shift of electroluminescence spectra to
higher wavelengths. Emission colors obtained from P3 varied
from blue to green and yellow. As happened with the polymers
P1 and P2, a bathocromic shift with the increasing content of
DTT-S,S-dioxide was observed with the polymers P3 (see
Supporting Information).
The colors of the three groups of the polymers P1, P2, and
P3, demonstrated that while the most bathochromic shift was
observed with the polymers P2, which had the longest
extension with the thiophenes at the peripherals of DTT-S,S-
dioxide, the polymers P3 had the most hypsochromic shift,
which had the least extension through the peripheral
thiophenes of DTT-S,S-dioxide (Figure 9). A spread of colors
from light blue to red through green and yellow was obtained.
Figure 7. (a) Luminance−voltage, (b) luminous efficiency−current density, and (c) EQE−current density of P2.
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Figure 8. (a) Luminance−voltage, (b) luminous efficiency−current density, and (c) EQE−current density of P3.
a
P1 and P2 had increasing bethochromic absorption and
emission shifts with increasing contents of DTT-S,S-dioxide
up to 447 and 472 nm (absorptions) and 558 and 592 nm
(emissions) in solution, respectively, the copolymers P3 had
increasing hypsochromic absorptions in solution and in solid
state emissions with increasing contents of DTT-S,S-dioxide.
Solid state emissions of the copolymers P3, having 5, 15, and
50% of DTT-S,S-dioxide displayed wide region of emission
from 400 to 675 nm. OLEDs of the copolymers were
fabricated, which had a spread of colors from light blue
(border of white) to red, including green and yellow colors. In
particular, the copolymers P2 (5−25%) and P3 had higher
thermal stabilities with no weight loss up to 400 °C.
Table 7. Electroluminescence Data of P3
λEl
(nm)
efficiency
(Cd/A)
CIE
EQE
(%)
sample
VONSET
coord (x,y)
P3-5
441
502
556
596
0.006
0.013
0.042
0.032
5.5
6.3
6.5
9.4
0.22−0.25
0.22−0.38
0.40−0.52
0.40−0.46
0.004
0.006
0.015
0.002
P3-15
P3-25
P3-50
a
λEl main emission peak of the electroluminescense spectrum, LE
luminous efficiency, VONSET onset voltage, chromaticity coordinates
according to the CIE 1931 diagram and EQE external quantum
efficiency.
ASSOCIATED CONTENT
■
S
* Supporting Information
1
13C and H NMR, MS, and electroluminescence spectrat and
device photographs. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*(T.O.) Telephone: 90 212 285 6994. Fax: 90 212 285 6386.
E-mail ozturktur@itu.edu.tr.
Author Contributions
The manuscript was written through the contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Figure 9. The x, y chromaticity coordinates of the polymers P1, P2,
and P3, marked in CIE 1931 diagram.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Most interestingly, a light blue color just on the border of white
region was obtained with P3-5.
■
We thank Istanbul Technical University, Turkey, for a grant to
Ipek Osken (PhD) and Unsped Global Lojistik for financial
support.
CONCLUSIONS
■
In this work, copolymers of fluorene and dithienothiophene-
S,S-dioxide (DTT-S,S-dioxide), having different ratios of DTT-
S,S-dioxides (5, 15, 25 and 50%) were synthesized for fine-
tuning the properties of the final materials such as their
absorptions and fluorescence. The copolymers had three
different groups (P1, P2, and P3) varying with the connection
of fluorene to DTT-S,S-dioxide, where the copolymers P1 and
P2 had direct connection of fluorene with DTT and connection
through a thiophene unit, respectively. The connection
between fluorene and DTT-S,S-dioxide in copolymers P3 was
provided through the phenyl moieties of the DTTs. Properties
of the finely tuned copolymers were investigated by absorption
and fluorescence spectroscopies, thermal gravity analyses, CV
measurements and OLED applications. While the copolymers
DEDICATION
■
⊥Dedicated to Prof Olgun Guven of Hacettepe University,
Ankara, Turkey, on his retirement.
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