Supramolecular light-emitting polymers for solution-processed optoelectronic devices

Article abstract of DOI:10.1039/c2jm31773g

Supramolecular light-emitting polymers (SLEPs) based on host-guest interactions were developed for solution processed organic electronic devices. The dibenzo-24-crown-8 functionalized blue-emitting conjugated oligomer 1 and green-emitting conjugated oligomer 3 were used as the host materials, and the dibenzylammonium salt functionalized blue-emitting conjugated oligomer 2 was used as the guest material. The resulting linear SLEPs were obtained from the self-organization of the host and guest oligomers, which were confirmed by the nuclear magnetic resonance, viscosity and differential scanning calorimetry studies. Highly fluorescent SLEP nanofibers can be easily obtained by drawing or electron-spinning from the equimolar solution of the host and guest oligomers. The photophysical and electroluminescence properties of the resulting SLEPs were fully investigated. It was found that the SLEPs' emission colors can be well tuned from blue to green with significantly enhanced photoluminescent efficiencies by using 3 as the dopant, which is due to the efficient energy transfer caused by the exciton trapping on narrow band gap host oligomer 3 in the SLEPs. As a result, the designed SLEPs showed comparable electroluminescence device performances to those analogous traditional conjugated polymers. Considering the precisely defined starting monomers and catalyst-free polymerization process for the designed SLEPs, combining the good device performances, the present study provides a promising alternative route to develop solution processed semiconductors for optoelectronic applications.

Full text of DOI:10.1039/c2jm31773g

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PAPER
Supramolecular light-emitting polymers for solution-processed optoelectronic
devices†
*
Jie Zhang, Kai Zhang, Xuelong Huang, Wanzhu Cai, Cheng Zhou, Shengjian Liu, Fei Huang and Yong Cao
Received 21st March 2012, Accepted 26th April 2012
DOI: 10.1039/c2jm31773g
Supramolecular light-emitting polymers (SLEPs) based on host–guest interactions were developed for
solution processed organic electronic devices. The dibenzo-24-crown-8 functionalized blue-emitting
conjugated oligomer 1 and green-emitting conjugated oligomer 3 were used as the host materials, and
the dibenzylammonium salt functionalized blue-emitting conjugated oligomer 2 was used as the guest
material. The resulting linear SLEPs were obtained from the self-organization of the host and guest
oligomers, which were confirmed by the nuclear magnetic resonance, viscosity and differential scanning
calorimetry studies. Highly fluorescent SLEP nanofibers can be easily obtained by drawing or electron-
spinning from the equimolar solution of the host and guest oligomers. The photophysical and
electroluminescence properties of the resulting SLEPs were fully investigated. It was found that the
SLEPs’ emission colors can be well tuned from blue to green with significantly enhanced
photoluminescent efficiencies by using 3 as the dopant, which is due to the efficient energy transfer
caused by the exciton trapping on narrow band gap host oligomer 3 in the SLEPs. As a result, the
designed SLEPs showed comparable electroluminescence device performances to those analogous
traditional conjugated polymers. Considering the precisely defined starting monomers and catalyst-free
polymerization process for the designed SLEPs, combining the good device performances, the present
study provides a promising alternative route to develop solution processed semiconductors for
optoelectronic applications.
remaining traces of metal catalysts have a detrimental effect on
Introduction
the resulting thin film device performance, time consuming
Conjugated polymers (CPs) have attracted considerable atten-
purification techniques have to be performed to remove the
tion from both academic and industrial communities because of
remaining metal catalyst among the conjugated polymers.4 In
their versatile applications in optoelectronic devices, such as
addition, the molecular weights as well as polydispersity of CPs
polymer light-emitting diodes (PLEDs) and polymer solar cells
that also play an important role on the performance of the
(PSCs).1 Compared to traditional inorganic and small molecule
resulting devices are hard to be defined during the polymeriza-
organic semiconductor based devices, the CP-based devices can
tion process, which usually generate the batch to batch
be fabricated through a low cost solution process and thereby
variation.5
possess unique advantages in the production of low cost, large
To overcome these challenges, a possible solution is to replace
area, flexible optoelectronic devices.2 Significant progress has
the traditional CPs with supramolecular light-emitting polymers
been made in CP-based devices, where high efficiency PLEDs
(SLEPs).6 The supramolecular polymers are generally formed
and PSCs have been successfully demonstrated by all spin-
from the monomeric units by directional and reversible
coating or a roll to roll process.3 Despite promising progress,
secondary interactions such as hydrogen bonding, p–p stacking,
host–guest interactions, hydrophobic interactions, etc., and the
there are still many challenges in developing high performance
and long lifetime CP-based optoelectronic devices. For example,
employment of SLEPs would present several potential advan-
most CPs are synthesized by the transition-metal-catalyzed
tages in device applications: (1) the polymerization starts from
cross-coupling reactions. Since it has been proven that the
the precisely defined monomers without using any metal catalyst;
(2) the comparable viscosity of the supramolecular polymers to
the traditional polymers ensure good solution processability,
State Key Laboratory of Luminescent Materials and Devices, Institute of
which maintains the key advantage of polymer-based electronics;
and (3) the strong interactions among the monomers ensure
supramolecular polymers a good morphology stability compared
to those solution-processed small molecules, which is important
for optoelectronic device applications. Despite all these potential
Polymer Optoelectronic Materials & Devices, South China University of
Technology, Guangzhou, P. R. China. E-mail: msfhuang@scut.edu.cn;
Fax: +86-20-87110606; Tel: +86-20-87114346
† Electronic supplementary information (ESI) available: NMR and
1
MADLI-TOF spectra of the monomers; H NMR spectra of equimolar
solutions of 1 and 2. See DOI: 10.1039/c2jm31773g
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 12759–12766 | 12759

View Online
advantages, most of the research on SLEPs only focus on their
photophysical properties, while the application of supramolec-
ular polymers in optoelectronic devices has been rarely reported.7
Meijer et al. reported the pioneering work of using hydrogen-
bonded supramolecular copolymers for PLEDs, while the
resulting devices exhibited poor performances with low lumi-
dioctylfluorene-2-yl)-2,1,3-benzothiadiazole (10)12 were prepared
according to the reported procedures. All chemicals were
purchased from commercial sources (Aldrich, Acros, and Alfa
Aesar) and used without further purification unless stated
otherwise. All the solvents used were further purified prior to use.
nance efficiencies less than 0.1 cd Aꢀ1 7d
It is challenging to
.
Synthesis of 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-dibenzo-
24-crown-8 (5)
develop high performance supramolecular copolymers for
optoelectronic device applications.
4-Bromo-dibenzo-24-crown-8 (4) (5.27 g, 10.0 mmol), bis(pina-
colato)diboron (3.81 g, 15.0 mmol), and potassium acetate
(4.00 g, 40.0 mmol) were added in a 100 mL two-necked round-
bottomed flask. Anhydrous 1,4-dioxane (60 mL) and [1,10-bis-
Herein we report the first use of host–guest interaction-based
SLEPs for PLEDs. The supramolecular polymers based on host–
guest interactions have advantages of highly directional binding
interactions at well-defined positions and interlocking bonds to
afford rotaxane- and catenane-type mechanically interlocked
structures.6j,8 The dibenzo-24-crown-8 (DB24C8) and dibenzyl-
ammonium salt (DBA) functionalized blue-emitting fluorene-
based conjugated oligomers 1 and 2 were used as the host and
guest of the resulting SLEPs respectively (Scheme 1). The resulting
linear SLEPs can be obtained from the self-organization 1 and 2 in
solution.6j,8 Moreover, another green-emitting host monomer 3
was developed as a dopant unit in SLEPs. It was found that the
resulting SLEPs’ emission colors can be well tuned and their
photoluminescent (PL) efficiencies are significantly enhanced
upon the doping of 3, due to the efficient energy transfer caused by
the exciton trapping effect.9 As a result, the resulting SLEPs’
device performances were greatly improved and are comparable
to those analogous traditional conjugated polymers.
(diphenylphosphino)ferrocene]palladium(II)
dichloride
[Pd(dppf)Cl2] (150 mg) were added to the flask. The mixture was
refluxed for 36 h under argon. After the mixture was cooled to
room temperature, it was poured into brine and extracted twice
with dichloromethane. The combined organic layers were dried
over MgSO4 and the solvent was removed. The crude product
was purified with column chromatography (silica gel, petroleum
ether (60–90 ꢁC)–ethyl acetate–dichloromethane (4/4/1) as
eluent) to yield 4.02 g (70%) of 5 as a white semisolid. 1H NMR
(CDCl3, 300 MHz) d (ppm): 7.39 (dd, J ¼ 1.3, 7.9 Hz, 1H), 7.28
(d, J ¼ 1.4 Hz, 1H), 6.90–6.83 (m, 5H), 4.20–4.12 (m, 8H), 3.92–
3.89 (m, 8H), 3.83 (s, 8H), 1.32 (s, 12H). 13C NMR (CDCl3, 75
MHz) d (ppm): 151.7, 149.0, 148.2, 129.0, 121.4, 119.6, 114.2,
112.8, 83.6, 71.3, 71.2, 69.9, 69.8, 69.5, 69.4, 69.1, 24.9.
Synthesis of 7,700-bis(dibenzo-24-crown-8)-2,20:70,200-ter(9,9-
dioctylfluorene) (1)
Experimental details
7,700-Dibromo-2,20:70,200-ter(9,9-dioctylfluorene) (6) (1.60 g, 1.2
Materials
mmol), compound
5 (1.73 g, 3.0 mmol), and tetrakis-
4-Bromo-dibenzo-24-crown-8 (4),10 7,700-dibromo-2,20:70,200-
(triphenylphosphine)palladium [Pd(PPh3)4] (60 mg) were added
in a 100 mL two-necked round-bottomed flask. Toluene (40 mL),
THF (20 mL) and 20% aqueous tetraethylammonium hydroxide
(6 mL) were added to the flask under argon. The mixture was
refluxed for 48 h under argon atmosphere. After the mixture was
cooled to room temperature, it was poured into brine and
extracted twice with dichloromethane. The combined organic
extracts were washed three times with water, dried over MgSO4,
evaporated, and purified with column chromatography (silica
gel, ethyl acetate–dichloromethane (1/6) as eluent) to yield 1.81 g
ter(9,9-dioctylfluorene)
(6),11
and
4,7-bis(7-bromo-9,9-
1
(73%) of 1 as a light yellow solid. H NMR (CDCl3, 300 MHz)
d (ppm): 7.84–7.81 (m, 6H), 7.69–7.64 (m, 8H), 7.52 (d, J ¼ 8.1
Hz, 4H), 7.22 (d, J ¼ 7.2 Hz, 4H), 6.97 (d, J ¼ 8.6 Hz, 2H), 6.89
(s, 8H), 4.29–4.27 (m, 4H), 4.23–4.21 (m, 4H), 4.18–4.16 (m, 8H),
3.98–3.93 (m, 16H), 3.87 (s, 16H), 2.09–2.05 (m, 12H), 1.26–1.10
(m, 60H), 0.82–0.78 (m, 30H). 13C NMR (CDCl3, 100 MHz)
d (ppm): 151.8, 151.7, 149.2, 149.1, 148.7, 140.5, 140.4, 140.0,
139.7, 135.4, 126.1, 125.7, 121.5, 121.2, 120.3, 119.9, 114.5, 114.3,
113.8, 71.3, 70.0, 69.9, 69.8, 69.7, 69.5, 55.3, 55.2, 40.4, 31.8, 30.0,
29.2, 23.9, 22.6, 14.0. MALDI-TOF (m/z): calcd 2059.34, found
[M]+ 2058.90, [M + Na]+ 2081.88.
Synthesis of 7,700-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
2,20:70,200-ter(9,9-dioctylflourene) (7)
To a solution of compound 6 (6.0 g, 4.5 mmol) in 150 mL THF in
a 250 mL three-necked round-bottomed flask at ꢀ78 ꢁC was
Scheme 1 Schematic representation of the construction of SLEPs from
host 1, guest 2 and host 3.
12760 | J. Mater. Chem., 2012, 22, 12759–12766
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added dropwise 4.5 mL of n-butyllithium (2.5 M in hexane). The
ꢁ
amount of DMAP in dry 50 mL dichloromethane for 12 h at
room temperature. The solution was evaporated and the residue
was purified by column chromatography (silica gel, petroleum
ether–ethyl acetate (50/1) as eluent) to yield 9 as a white semisolid
(3.0 g, 78%). 1H NMR (CDCl3, 300 MHz) d (ppm): 7.84–7.81 (m,
4H), 7.80 (d, J ¼ 2.5 Hz, 2H), 7.70–7.67 (m, 6H), 7.66–7.64 (m,
6H), 7.62–7.59 (m, 4H), 7.38–7.27 (m, 14H), 4.48 (s, 4H), 4.40 (s,
4H), 2.11–2.06 (m, 12H), 1.54 (s, 18H), 1.20–1.10 (m, 60H), 0.85–
0.78 (m, 30H). 13C NMR (CDCl3, 100 MHz) d (ppm): 156.1,
151.8, 140.7, 140.6, 140.5, 140.2, 140.0, 139.9, 139.7, 138.0, 137.0,
128.6, 128.0, 127.5, 127.3, 127.2, 126.2, 126.0, 121.5, 120.0, 80.1,
55.3, 49.0, 40.4, 31.8, 30.0, 29.2, 28.5, 23.9, 22.6, 14.0. MADLI-
TOF (m/z): calcd 1758.27, found [M]+ 1758.07.
mixture was stirred at ꢀ78 C for 1 h and 2-isoproxoy-4,4,5,5,-
tetramethyl-1,3,2-dioxaborolane (5 mL, 24.5 mmol) was added
rapidly to the solution. The resulting mixture was warmed to
room temperature and stirred overnight. The mixture was
poured into brine, extracted with dichloromethane three times
and dried over MgSO4, evaporated, and purified with column
chromatography (silica gel, petroleum ether–ethyl acetate (1/20)
1
as eluent) to yield 4.8 g (75%) of 7 as a white solid. H NMR
(CDCl3, 300 MHz) d (ppm): 7.83 (d, J ¼ 7.7 Hz, 4H), 7.78 (d, J ¼
4.1 Hz, 4H), 7.73 (d, J ¼ 7.6 Hz, 2H), 7.67–7.63 (m, 8H), 2.08–
2.03 (m, 12H), 1.40 (s, 24H), 1.22–1.07 (m, 60H), 0.83–0.70 (m,
30H). 13C NMR (CDCl3, 100 MHz) d (ppm): 152.1, 151.8, 150.2,
143.8, 141.0, 140.5, 140.1, 140.0, 133.8, 128.9, 126.2, 126.0, 121.5,
120.3, 119.9, 119.0, 83.7, 55.3, 55.2, 40.3, 40.2, 31.8, 30.0, 29.2,
24.9, 23.9, 23.8, 22.6, 14.0. MADLI-TOF (m/z): calcd 1419.12,
found [M]+ 1418.92.
Synthesis of N,N0-(((2,20:70,200-ter(9,9-dioctylflourene)-7,700-diyl)
bis(4,1-phenylene))bis(methylene))bis(1-phenylmethanaminium)
hexafluorophosphate(V) (2)
TFA (3.0 mL, 39.0 mmol) was added to a solution of 9 (2.8 g, 1.6
mmol) in dichloromethane (30 mL) and the mixture was stirred
for 12 h at room temperature. A saturated aqueous solution of
NH4PF6 was added to the reaction mixture. On removal of the
volatile solvents, a precipitate was formed which was filtered off
and dissolved in acetonitrile. Afterwards, the acetonitrile solu-
tion was also treated with the aqueous NH4PF6 solution to yield
Synthesis of 4,40-(2,20:70,200-ter(9,9-dioctylflourene))-7,70-
diyldibenzaldehyde (8)
Compound 7 (4.3 g, 3.0 mmol), 4-bromo-benzaldehyde (1.3 g, 7.0
mmol), sodium carbonate (2.6 g, 24.0 mmol), and Pd(PPh3)4 (105
mg) were added in a 150 mL two-necked round-bottomed flask.
Toluene (60 mL) and deionized water (10 mL) were added to the
flask under argon. The mixture was refluxed for 24 h under argon
atmosphere. After the mixture had been cooled to room
temperature, it was poured into brine and extracted twice with
dichloromethane. The combined organic layers were dried over
MgSO4 and the solvent was removed. The crude product was
purified with column chromatography (silica gel, petroleum
ether–ethyl acetate (20/1) as eluent) to yield 4.2 g (82.5%) of 8 as
a light yellow solid. 1H NMR (CDCl3, 300 MHz) d (ppm): 10.09
(s, 2H), 8.00 (d, J ¼ 8.4 Hz, 4H), 7.87–7.82 (m, 10H), 7.71–7.64
(m, 12H), 2.12–2.07 (m, 12H), 1.25–1.10 (m, 60H), 0.81–0.77 (m,
30H). 13C NMR (CDCl3, 100 MHz) d (ppm): 191.8, 152.0, 151.9,
151.8, 147.7, 141.4, 141.0, 140.4, 140.1, 139.6, 138.4, 135.1, 130.3,
127.7, 126.5, 126.3, 126.2, 121.7, 121.5, 120.3, 120.0, 55.4, 55.3,
40.3, 31.8, 30.0, 29.2, 23.9, 22.6, 14.0. MADLI-TOF (m/z): calcd
1376.01, found [M]+ 1375.91.
1
2 (2.67 g, 90%) as a light yellow solid. H NMR (DMSO, 300
MHz) d (ppm): 7.93 (d, J ¼ 7.5 Hz, 6H), 7.85 (d, J ¼ 7.9 Hz, 4H),
7.78–7.70 (m, 12H), 7.61 (d, J ¼ 8.0 Hz, 4H), 7.51–7.47 (m, 10H),
4.27 (s, 4H), 4.22 (s, 4H), 2.08 (m, 12H), 0.99 (m, 60H), 0.67 (m,
30H). 13C NMR (CD3COCD3, 100 MHz) d (ppm): 162.1, 151.9,
151.7, 142.5, 140.9, 140.6, 140.4, 140.2, 140.0, 138.8, 131.1, 130.8,
131.1, 130.8, 130.2, 129.9, 129.6, 129.1, 127.4, 126.2, 126.0, 121.3,
120.3, 120.2, 55.4, 55.3, 51.5, 51.3, 39.9, 35.4, 31.5, 30.2, 29.6,
ꢀ +
23.7, 22.3, 13.4. MADLI-TOF (m/z): calcd [M ꢀ 2PF6
]
1560.18, found [M ꢀ 2PF6ꢀ]+ 1556.96.
Synthesis of 4,7-bis(7-(dibenzo-24-crown-8)-9,9-dioctylfluorene-2-yl)-
2,1,3-benzothiadiazole (3)
4,7-Bis(7-bromo-9,9-dioctylfluorene-2-yl)-2,1,3-benzothiadia-
zole (10) (1.3 g, 1.2 mmol), compound 5 (1.73 g, 3.0 mmol), and
Pd(PPh3)4 (40 mg) were added in a 100 mL two-necked round-
bottomed flask. Toluene (20 mL), THF (40 mL) and 40%
aqueous tetraethylammonium hydroxide (3 mL) were added to
the flask under argon. The mixture was refluxed for 36 h under
argon atmosphere. After the mixture had been cooled to room
temperature, the mixture was poured into brine, extracted with
dichloromethane three times and dried over MgSO4, evaporated,
and purified with column chromatography (silica gel, ethyl
acetate–dichloromethane (1/6) as eluent) to yield 0.58 g (27%) of
Synthesis of di-tert-butyl(((2,20:70,200-ter(9,9-dioctylflourene)-
7,700-diyl)bis(4,1-phenylene))bis(methylene))
bis(benzylcarbamate) (9)
Compound 8 (3.0 g, 2.2 mmol) and benzylamine (0.54 g, 5.0
mmol) were heated together in refluxing toluene (50 mL) for 20 h
under argon. The reaction mixture was cooled to room temper-
ature, and the solvent was evaporated under vacuum. The
residue was redissolved in THF (20 mL) and added in a 100 mL
two-necked round-bottomed flask. MeOH (60 mL) and NaBH4
(0.74 g, 15.0 mmol) were added, and the mixture was heated
under reflux for 8 h. The reaction mixture was quenched with
aqueous NaHCO3 and extracted with dichloromethane. The two
phases were separated, and the water phase was extracted twice
with dichloromethane. The combined organic extracts were
washed three times with water, dried over MgSO4 and filtered.
The filtrate was evaporated to dryness. The obtained oil was
stirred together with (Boc)2O (1.8 g, 8.0 mmol) and a catalytic
1
3 as a orange solid. H NMR (CDCl3, 300 MHz) d (ppm): 8.06
(dd, J ¼ 1.4, 7.9 Hz, 2H), 7.97 (s, 2H), 7.88 (d, J ¼ 8.6 Hz, 4H),
7.80 (d, J ¼ 8.0 Hz, 2H), 7.54 (d, J ¼ 7.8 Hz, 4H), 7.24–7.21 (m,
4H), 6.97 (d, J ¼ 8.5 Hz, 2H), 6.89 (s, 8H), 4.30–4.27 (m, 4H),
4.24–4.21 (m, 4H), 4.18–4.13 (m, 8H), 3.99–3.93 (m, 16H), 3.87
(s, 16H), 2.09–2.05 (m, 8H), 1.19–1.10 (m, 40H), 0.88–0.76 (m,
20H). 13C NMR (CDCl3, 100 MHz) d (ppm): 154.4, 152.0, 151.3,
149.2, 149.1, 148.7, 141.0, 140.0, 139.6, 136.1, 133.6, 128.2, 127.9,
125.8, 124.0, 121.5, 121.3, 120.3, 120.2, 119.7, 114.4, 114.3, 113.9,
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71.3, 70.0, 69.9, 69.6, 69.5, 55.3, 40.3, 31.8, 30.1, 29.2, 24.0, 22.6,
14.0. MADLI-TOF (m/z): calcd 1805.02, found [M]+ 1805.01, [M
+ Na]+ 1828.00, [M + K]+ 1843.98.
deposited on top of the TPBI to form the cathode. The current
density (J) and brightness (L) versus voltage (V) data were
collected using a Keithley 236 source meter and silicon photo-
diode. After typical encapsulation with UV epoxy and cover
glass, the devices were taken out from the dry box and the
luminance was calibrated by a PR-705 SpectraScan Spectro-
photometer (Photo Research) with simultaneous acquisitions of
the EL spectra.
Measurement and characterization
1H and 13C NMR were recorded by using a Bruker-300 and 400
spectrometer operating at 300 or 400 and 75 or 100 MHz at
295 K. Chemical shifts were reported as d values (ppm) relative to
an internal tetramethylsilane (TMS) standard. Time-of-flight
mass spectrometry (TOF-MS) was performed in the positive ion
mode with a matrix of dithranol using a Bruker-autoflex III
smartbeam. The differential scanning calorimetry (DSC)
measurements were carried out with a Netzsch DSC 204 under
N2 flow at heating and cooling rates of 10 ꢁC minꢀ1. The viscosity
was measured with a digital viscometer from Brookfield Engi-
neering Laboratories, Inc. (model LVDV-I+). UV-vis absorption
spectra were measured on a HP 8453 spectrophotometer. PL
spectra were recorded on an Instaspec IV CCD spectropho-
tometer (Oriel Co.) under 325 nm excitation of a HeCd laser. The
confocal laser scanning microscope micrograph was performed
on a Lecia TCS SP5 scanning microscope (Mannheim, Germany)
under 405 nm excitation of a 405 diode laser. The electrospinning
fibers were fabricated by an electrospinning forming machine
ESF-Y1.
Results and discussion
Synthesis and characterization
The synthetic routes of the monomers are shown in Scheme 2.
The host monomers 1 and 3 were synthesized from compounds
5 with 6 or 10 by Suzuki condensation reactions, respectively.
The guest monomer 2 was synthesized from compound 6 by
a four-step procedure with a high yield. Because of the presence
of a ring strain, the rigid aromatic linker units are favorable to
form the linear SLEPs compared with the flexible aliphatic
linker units.8g,h,13 Therefore, the critical polymerization
concentration (CPC) which exhibited a ring–chain transition
from the formation of the cyclic to linear polymers is low. To
get some insight into the complexation of the designed host and
guest monomers, the NMR studies were performed. The
concentration-dependent 1H NMR spectra of an equimolar
solution of host 1 and guest 2 in CDCl3–CD3CN (1/1, v/v) at
22 ꢁC are shown in Fig. 1. At low concentrations (below
10 mM), the benzyl protons H4 on the fast-exchanging units
(Scheme 1) have two sets of well-defined signals standing for the
cyclic and liner species. As the concentrations increased, the
signals for the cyclic dimer decreased, and further disappeared
at 20 mM. At high concentrations, all the signals are broad,
confirming the formation of high molecular weight aggregates
driven by host–guest interactions between host 1 and guest 2.8
Moreover, the large chemical shift change of benzyl protons H4
exhibited that a percentage of complexed species was concen-
tration-dependent.14
Electrospinning of SLEP 1 + 2 nanofiber
The concentration of SLEP 1 + 2 was 150 mM in CHCl3–
CH3CN (1/1, v/v). During electrospinning, the solution was
placed in a 1 mL syringe and was fed by a syringe pump at a rate
of 400 rpm. The metallic needle was connected to a grounded
counter electrode. A power supply was connected to the
aluminium collector. The distance between the tip of the capillary
and the collector was 13 cm. The collecting target was a quartz
substrate placed on the top of the aluminium electrode. The
spinning voltage was set at 13 kV, and all experiments were
carried out at room temperature.
The viscosity study can provide the direct physical evidence for
the formation of large supramolecular polymers by the mono-
mers. Therefore, viscosity measurements were performed in
CHCl3–CH3CN (1/1, v/v) solution. As shown in Fig. 2a, a double
logarithmic representation of specific viscosity versus the
concentration of equimolar solution of 1 and 2 was observed. In
the low concentration range, the curve had a slope of 0.176,
revealing a linear relationship between specific viscosity and
concentration, which is characteristic for noninteracting assem-
blies of constant size.15 As the concentrations increase, the curve
exhibited a sharp rise with a slope of 2.059, indicating the
formation of linear supramolecular polymers. The CPC for
SLEP 1 + 2 was about 10 mM as evidenced by the clear change of
slope, indicating a ring–chain transition from the formation of
cyclic dimer to ordered linear SLEP. It was found that the CPC
of the SLEPs based on a rigid aromatic linker was significantly
lower than those of the supramolecular polymers based on
a flexible aliphatic linker, owing to the presence of ring
strain.8g,h,13c,16 Clearly, the viscosity study results show that the
designed SLEPs will have good film formation abilities for
solution processed organic electronic devices.
Polymer light-emitting diodes fabrication and characterization
The ITO-coated glass substrates were ultrasonically cleaned
with deionized water, acetone, detergent, deionized water, and
isopropyl alcohol. Then a layer of 40 nm thick poly(3,4-ethyl-
enedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS)
(H.C.Stack, 4083) was spin-coated onto the pre-cleaned and O2-
plasma-treated ITO substrates. After that, the PEDOT:PSS
ꢁ
layer was baked at 150 C for 20 min to remove residual water,
and then the devices were moved into a glove box under the
argon-protected environment. SLEPs 1 + 2, 1 + 2 + 3 (10%),
and 1 + 2 + 3 (30%) (30 mg mLꢀ1 in o-DCB) were spin-coated
onto PEDOT:PSS at the speed of 2000 r per min to yield 90 nm
thickness light emitting layers. In SLEP 1 + 2 + 3 (10%), the
molar ratios of 1, 2, and 3 are 40%, 50% and 10%, respectively.
In SLEP 1 + 2 + 3 (30%), the molar ratios of 1, 2, and 3 are
20%, 50% and 30%, respectively. The samples were transferred
into a chamber and kept under vacuum (3.0 ꢂ 10ꢀ4 Pa) for 2 h.
30 nm TPBI thickness was sublimed as the hole blocking layer,
and then caesium fluoride (CsF) with a thickness of 1.5 nm and
aluminium with a thickness of 100 nm were subsequently
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Scheme 2 Synthesis of host 1, guest 2 and host 3.
Besides the film formation abilities, the film morphology
stabilities of the light-emitting polymers are also critical for
device application, which will greatly affect the resulting devices’
stabilities. Thus, the DSC measurement was carried out. Host 1 is
a crystalline solid with a melting point at around 90 ꢁC, while no
obvious endotherm–exotherm transition was found across the
entire scanning range for guest 2. In contrast, the solid obtained
from the evaporation of solutions 1 and 2 had a repeatable glass-
transition temperature (Tg) of 114.5 ꢁC, which was much higher
than those of the host–guest supramolecular polymers based on
the flexible aliphatic linker16 and comparable to polyfluorene
conjugated polymers.17 It is interesting that the designed SLEPs
based on 1 and 2 exhibited a relatively high Tg compared to
traditional conjugated polymers, which will ensure it has a good
film morphology stability for device application. Moreover,
although no fibers could be drawn from the individual highly
concentrated solutions of 1 and 2, rod like fibers with a regular
diameter of around 50 mm could easily be drawn from concen-
trated equimolar solutions of SLEP 1 + 2 (Fig. 3a and b).
Furthermore, long thin fibers can be processed by an electro-
spinning technique, which generally can only be applied for high
molecule weight polymers,18 providing another direct evidence of
the formation of supramolecular polymers with high molecular
weight (Fig. 3c). Most importantly, the luminescence properties
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Fig. 3 (a) A photo of a rod-like fiber drawn from a high concentration of
SLEP 1 + 2 in CHCl3–CH3CN (1/1, v/v) solution. (b) The confocal laser
scanning microscope micrograph of a rod-like fiber drawn from a high
concentration of SLEP 1 + 2 in CHCl3–CH3CN (1/1, v/v) solution. (c)
The confocal laser scanning microscope micrograph of the electrospun
SLEP 1 + 2 nanofibers.
Optical properties
1
Fig. 1 The stacked H NMR spectra (400 MHz, CDCl3–CD3CN, 1/1,
The absorption and PL spectra of the monomers and the SLEPs
in solid films are shown in Fig. 4. Both host 1 and guest 2
exhibited similar UV-visible absorption and PL spectra to those
of typical fluorene trimers.19 It was noted that guest 2 showed
more pronounced emission tails at around 500–600 nm
compared to host 1, probably due to the stronger intermolecular
interactions caused by the ionic groups among guest 2.20
Different from 1 and 2, 3 exhibited two absorption peaks and
a green emission with a peak at around 550 nm. The photo-
physical properties of the resulting SLEPs were also investigated.
The absorption spectrum of SLEP 1 + 2 is similar to those of the
monomers, while the SLEPs based on 1, 2 and 3 exhibited
a broader main absorption peak at around 360 nm and a small
v/v, 22 ꢁC) of solutions of 1 and 2 at different concentrations: (a) 1, (i) 2,
and equimolar solutions of 1 and 2 at concentrations of (b) 1, (c) 2, (d) 5,
(e) 10, (f) 20, (g) 30 and (h) 100 mM. Peaks of linear polymer, cyclic
dimer, and uncomplexed monomer are designated by lin, cyc, and uc,
respectively.
of the typical fluorene oligomers were well maintained in the
supramolecular systems and all these fibers exhibited strong blue
fluorescence under the excitation of the UV light, which can be
clearly observed by the confocal laser scanning microscope
micrograph (Fig. 3a–c).
Fig. 2 (a) Specific viscosity of equimolar mixtures of host 1 and guest 2
in CHCl3–CH3CN (1/1, v/v) solution; (b) DSC plots of host 1, guest 2,
and SLEP 1 + 2.
Fig. 4 (a) Absorption spectra and (b) PL spectra of host 1, guest 2, host
3, SLEP 1 + 2, SLEP 1 + 2 + 3 (10%) and SLEP 1 + 2 + 3 (30%).
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increased from 20.2% for SLEP 1 + 2 to 65% for SLEP 1 + 2 + 3
(10%) and 60% for SLEP 1 + 2 + 3 (30%), respectively. The
significantly enhanced PL efficiencies of SLEPs upon doping
with 3 were attributed to the efficient intra- and inter-molecular
energy transfer among the SLEPs.21
Electroluminescence properties
The electroluminescence (EL) properties of the SLEPs are
investigated. All the SLEPs were used as the emissive layers
(EMLs) in light-emitting devices. Fig. 5a compares the current
density (J) and brightness (L) versus voltage (V) characteristics
between different SLEPs. The SLEPs based on 1, 2 and 3
exhibited much higher device performances than the SLEPs
based on 1 and 2, which possess higher brightness and efficiencies
at the same voltage and current density. As shown in Table 1, the
devices of SLEPs based on 1, 2 and 3 showed maximum lumi-
nance efficiencies (LE) more than 3 cd Aꢀ1, which is much higher
than that of devices of SLEPs based on 1 and 2. Fig. 5b shows the
EL spectra of the devices. It can be found that compared to its PL
emission, the device of SLEPs based on 1 and 2 showed a largely
red-shifted EL emission, which is attributed to the excimer
formation as typical polyfluorene homopolymers.22 By using 3 as
an additional dopant host, the excimer emissions are completely
quenched and the resulting devices emit an exclusively green
emission of host 3. Besides, host 3 has a smaller band gap
compared to 1 and 2, and will act as a ‘‘charge trap’’ in the
resulting SLEPs. That will cause efficient energy transfer among
the SLEPs and enhance the hole–electron recombination ratios
in the devices, resulting in the greatly improved device perfor-
mances. It is interesting that all the SLEPs showed comparable
device efficiencies to those analogous traditional conjugated
polymers such as polyfluorene homopolymers17,23 or poly-
fluorene copolymers with 2,1,3-benzothiadiazole.24 It was noted
that the resulting devices’ brightness is not as high as those
analogous traditional conjugated polymers, which is probably
due to mobile ions among these SLEPs.25 Nevertheless, this can
be potentially addressed by tuning the counterions of the
SLEPs6i,25a and the related work is in progress.
Fig. 5 (a) Current density (J) and brightness (L) versus voltage (V)
characteristics (J–L–V) and (b) EL spectra of SLEPs 1 + 2, 1 + 2 + 3
(10%) and 1 + 2 + 3 (30%) in the devices with configuration of ITO/
PEDOT/EMLs/TPBI/CsF/Al.
absorption peak at around 440 nm, caused by the doped host 3.
The PL studies on the SLEPs interestingly show that SLEP 1 + 2
showed similar emission main peaks to those of monomers 1 and
2, while its emission tail at around 500–600 nm was largely
suppressed. That indicates that the intermolecular interactions
among monomers 1 and 2 were decreased in the SLEPs which
were desirable for PLED application. By doping 10% of 3 into
the supramolecule system, SLEP 1 + 2 + 3 (10%) exhibited
a green emission at around 550 nm with a small peak at around
420 nm, which is from the 1 + 2 hosts. With the increase in
content of 3, the emission of SLEP 1 + 2 + 3 (30%) is dominated
by a green emission that peaked at around 550 nm, indicating the
efficient energy transfer from 1 and 2 to 3, which is very similar to
those traditional conjugated copolymers.21 The PL quantum
yields of SLEPs were determined in an integrating sphere with
325 nm excitation of the HeCd laser. PL efficiencies were
Conclusions
In summary, the conjugated oligomers 1–3 were developed and
used as the starting monomers to build the SLEPs for PLED
application. The formation of the SLEPs was confirmed by the
NMR, viscosity and DSC studies. Photophysical studies indicate
that the luminescence properties of the typical conjugated olig-
omers can be well maintained in the supramolecular systems and
highly fluorescent fibers can be obtained by drawing or by
Table 1 EL Performance of the SLEPs in the devices with configuration of ITO/PEDOT:PSS/EML/TPBI/CsF/Al
Bb [cd mꢀ2
]
LEc [cd Aꢀ1
]
Bmax [cd mꢀ2
]
LEmax [cd Aꢀ1
]
a
Von [V]
d
e
EMLs
1 + 2
1 + 2 + 3 (10%)
1 + 2 + 3 (30%)
10
9.25
9.25
23.6
121.8
156.8
0.37
1.45
1.66
79.6
384.0
439.0
0.49
3.40
4.07
a
b
c
Turn-on voltage corresponding to 1 cd mꢀ2
.
Brightness at current density around 10 mA cmꢀ2
.
Luminous efficiency at current density around 10 mA
d
cmꢀ2
.
Maximum brightness. e Maximum luminance efficiency.
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ꢁ
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and A. P. H. J. Schenning, J. Am. Chem. Soc., 2009, 131, 833.
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electro-spinning from the SLEP solutions. The doping strategy
was successfully applied in SLEPs, resulting in largely enhanced
PL efficiencies. PLED device studies show that the designed
SLEPs had comparable device performances to those analogous
traditional conjugated polymers. Considering the perfectly
defined starting monomers and catalyst-free polymerization
process for the designed SLEPs, combining the good device
performances, the present study provides a promising alternative
route to develop solution processed semiconductors for opto-
electronic applications.
Acknowledgements
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The work was financially supported by the Ministry of Science
and Technology (no. 2009CB623601 and 2009CB930604) and
the Natural Science Foundation of China (no. 21125419,
50990065, 51010003, 51073058 and 20904011).
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