A series of blue supramolecular polymers with different counterions for polymer light-emitting diodes

Article abstract of DOI:10.1039/c4cc03080j

A series of blue supramolecular polymers with different counterions based on host-guest interactions were developed for polymer light-emitting diodes. It was found that the counterions play important roles in the resulting materials' supramolecular interactions as well as the device performance. This journal is the Partner Organisations 2014.

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A series of blue supramolecular polymers with different counterions
for polymer light-emitting diodes †
Jie Zhang,a Sheng Dong, a Kai Zhang, a Aihui Liang, a, b Xiye Yang, a Fei Huang* a and Yong Cao a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
fluorene-based oligomers FX with different counterions (FP6-,
50 Br-, BIm4-) were developed as the guests. The influences of
different counterions on the resulting linear supramolecular
polymer FSO-FX's supramolecular interactions, thermal
properties, optical physical properties as well as device
performance were studied.
A series of blue supramolecular polymers with different
counterions based on host-guest interactions was developed
for polymer light-emitting diodes. It was found that the
counterions play important roles on the resulting materials'
10 supramolecular interactions as well as the device
performance.
Supramolecular polymers have attracted considerable attention
due to their unique properties and potential applications in the
1
past decade.
Different from traditional polymers, the
15 supramolecular polymers are generally formed from the
monomeric units through noncovalent, mechanically interlocked
structures, but show good solution processibilty and comparable
mechanical properties in the bulk as those traditional polymers.2
Consequently, the supramolecular polymers can be potentially
20 promising candidates for solution processed organic devices, such
as polymer light-emitting diodes,3 polymer solar cells4 etc., which
can combine both the advantages of small molecular 5 and
polymer optoelectronic materials.6 For example, supramolecular
light-emitting polymers (SLEPs), based on the host-guest
25 interaction between dibenzo[24]crown-8 (DB24C8) and
dibenzylammonium salt (DBA) derivatives, have been developed
and applied as emission layer in PLEDs, where the SLEPs
exhibited good film forming ability and comparable device
performances to those traditional light-emitting polymers.7 Since
30 the DBA are usually used as the guests, there are a lot of
55
Scheme 1. Chemical structures of host momomer FSO and guest
momomers FX and Schematic illustration of the formation of
FSO-FX at high concentration by self-assembly.
The details of the synthesis of the host and guest oligomers
60 were demonstrated in the supporting information, where a
palladium-catalyzed Suzuki cross-coupling reaction produced the
host monomer FSO in a high yield of 72%. The guest monomer
FPF6 was prepared according to the reported procedure.7a The
guest monmomers FX with different counterions were obtained
65 from a ion-exchange procedure by adding an excess of a salt with
the counterion of interest into a FPF6 methanol/tetrahydrofuran
(THF) solution with vigorous stirring, followed by dialysis in
water, yielding FBr, and FBIm4 in 80%, and 74%, respectively.
1H NMR studies analysis confirmed quantitative replacement by
70 Br- and BIm4-(Fig. S1, ESI*). FPF6 and FBr have excellent
solubility in common organic solvents such as THF, CH3CN, and
dimethylsulfoxide (DMSO), etc., however, the solubility of
FBIm4 was poor. More than 10 mg of FPF6 and FBr can be
readily dissolved in 1 mL of DMSO at room temperature to give
75 a clear solution, while 1 mL of DMSO is only able to solve
around 1 mg of FBIm4. The concentration-dependent 1H NMR
studies of FSO and FX (Fig. 1, S2 and S3, ESI* ) provided
important insights into their self-assembly behavior by host-guest
interaction in solution. The interactions of FSO and FX were fast-
80 exchange complexations.12 As shown in Fig. 1, S2 and S3, at low
compensating
counterions
among
these
host-guest
supramolecular polymers. It has been reported that these
counterions can not only affect the supramolecular interactions of
these supramolecular materials,8 but also play important roles on
35 the device performance of the optoelectronic materials.9 Thereby,
to further develop high performance host-guest interaction based
supramolecular optoelectronic materials, it is critical to
understand the insight of the impact of counterions on the
resulting materials' supramolecular interactions and the device
40 performance.
Herein, we developed a series of blue-emitting SLEPs based on
the host-guest interaction of DB24C8 and DBA functionalized
conjugated oligomers.7, 10 The DB24C8 functionalized fluorene-
dibezothiophene-S,S-dioxide (SO) co-oligomer FSO was
45 developed as the host. The SO unit is a promising building block
for organic blue-emitting materials, because it can greatly
enhance the efficiency and electroluminescence (EL) spectra
stability of the resulting materials. 11 The DBA functionalized
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concentrations, the benzyl protons H had one set of well-defined 45 caused by the ionic groups among them.3b, 16 Compared to FPF6
signals for the cyclic and linear species. As the concentrations
increased, the signals for the cyclic dimer decreased, and the
signals for the linear increased. At high concentrations, all the
signals become broad, confirming the formation of high
and FBIm4, the FBr showed a much broader emission, indicating
a much stronger interchain interaction corresponding to the
smaller size of Br-, while the interchain interactions of FPF6 and
-
5
FBIm4 are largely suppressed by the "spacer" effect of larger FP6
-
molecular weight aggregates driven by host-guest interactions.7, 12, 50 and BIm4 counterions.9b The photophysical properties of the
13
resulting supramolecular polymer FSO-FX were also investigated.
Interestingly, despite of different counterions, all the FSO-FX
exhibited similar Uv-vis and PL spectra, and the emission tails at
around 500-600 nm of the FX guests completely disappeared.
55 That indicates the strong intermolecular interactions of FX were
greatly suppressed in FSO-FX, probably due to the host-guest
interactions, which were desirable for PLEDs application.
Consequently, the counterion-dependent PL quenching of those
traditional conjugated polyelectrolytes was not observed among
60 these supramolecular materials,16a and all the FSO-FX show
similar high quantum efficiencies, which are 53, 47, and 50% for
FSO-FPF6, FSO-FBr, and FSO-FBIm4, respectively.
(a)
(b)
FSO
FPF6
FBr
FBIm4
FSO-FPF6
FSO-FBr
FSO-FBIm4
FSO
FPF6
FBr
FBIm4
FSO-FPF6
FSO-FBr
FSO-FBIm4
1
0.8
0.6
0.4
0.2
0
1
0.8
0.6
0.4
0.2
0
1
Fig. 1 The stacked H NMR spectra (600 MHz, CDCl3/CD3CN 1/1, v/v,
10 25 ) of solutions of FSO and FBr at different concentrations: a) FSO, h)
FBr, and equimolar solutions b) 1, c) 2, d) 5, e) 10, f) 20, and g) 30mM. u:
uncomplexed monomer, c: cyclic polymer, and l: linear supramolecular
polymer.
300
350
400
Wavelength (nm)
450
500
550
350 400 450 500 550 600 650 700
Wavelength (nm)
The thermal properties of the resulting FSO-FX were studied
15 by the differential scanning calorimetry (DSC). FSO-FPF6
exhibited a high glass transition temperature (Tg) of 103.2 oC,
which was comparable to those of similar traditional conjugated
polymers.11a Interestingly, the Tg of FSO-FBr and FSO-FBIm4
was dramatically lower than those of FSO-PF6. The concentration
20 dependence of the solution viscosity provides important
information on the aggregation behavior of the supramolecular
polymers. The double logarithmic plots of specific viscosity vs.
the monomer concentration of all the resulting FSO-FX were
shown in Fig S5, ESI*. At low concentrations, the curves for
25 FSO-PF6, FSO-FBr, and FSO-FBIm4 had slopes of 0.18, 0.19,
and 0.32, respectively, which is characteristic for noninteracting
assemblies of constant size.14 As the concentrations increase, the
curves exhibited a sharp rise with slopes of 2.14, 2.02, and 2.35
for FSO-PF6, FSO-FBr, and FSO-FBIm4, respectively, indicating
30 the formation of linear supramolecular polymers. The critical
polymerization concentrations (CPC), corresponding to the
transition from the cyclic species to linear supramolecular
polymers, are approximately 13, 18, and 15 mmol L-1 for FSO-
PF6, FSO-FBr, and FSO-FBIm4, respectively. The viscosity study
35 results show that the resulting FSO-FX exhibit good film
formation capabilities for solution processed PLEDs applications.
The absorption and photoluminescent (PL) spectra of the
monomers FSO and FX and the supramolecular polymer FSO-FX
in solid films are shown in Fig. 2. The host FSO exhibited similar
40 UV-visible absorption and PL spectra to those of typical
fluorene-SO co-oligomers.11a The guest monomers FX showed
similar absorption, but more pronounced emission tails at around
500-600 nm compared to those of typical fluorene trimmers,15
which is probably due to the strong intermolecular interactions
Fig. 2 (a) UV-visible absorption and (b) PL spectra of host FSO, guest FX
and FSO-FX in thin films.
65
The EL properties of the FSO-FX were investigated in OLEDs
with device configuration of indium tin oxide (ITO)/ poly(3,4-
ethylenedioxythiophene):poly(styrene
sulfonic
acid)
(PEDOT:PSS)/FSO-FX/Al, where PSO-FX were used as the
70 emission layer and the high work-function metal Al was used as
the cathode. It should be noted that most of traditional light-
emitting materials exhibited very poor device performance in the
devices with stable high work-function metal cathodes (such as
Au, Ag, Al etc.) due to the large electron injection barrier.16, 17
75 Nevertheless, the highly polar charged groups and counterions
among FSO-FX may endow them good interface modification
capabilities and thus allow the using of high work-function
metals as the cathode in devices.9 Indeed, as shown in Table 1
and Fig. 3, all the FSO-FX exhibited good device performance
80 with maximum luminance efficiencies (LEs) more than 1 cd/A,
which is much higher than that of their polyfluorene analogue in
17
the same device configuration (~0.01 cd/A).
Moreover,
compared to previously reported all-fluorene based blue-emitting
SLEP,7a all the FSO-FX also exhibited much improved device
85 performance. For example, the device with FSO-FPF6 as
emission layer exhibited a turn voltage (Von) of 5.5 V, a
maximum brightness (Lmax) of 281 cd m-2, and a maximum LE of
2.60 cd A-1, which is much better than those of our previously
reported fluorene-based SLEP with a Von of 10.0 V, a Lmax of 79.6
90 cd m-2, and a maximum LE of 0.49 cd A-1.7a This is due to the
incorporation SO building blocks, which will cause efficient
charge transfer among the SLEPs and enhance the hole/electron
recombination ratios in the devices, resulting in the greatly
2
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DOI: 10.1039/C4CC03080J
improved device performances as well as colour stability.11c As
shown in Fig. 3, FSO-FPF6 exhibited a pure blue-emission peaked
at around 474 nm and the excimer emission of typical
of the FSO-FX, EL spectra dependences upon varied applied
current were carried out (Fig. S7, ESI*). It was found that with
accelerating applied current from 2 mA to 20 mA, the EL spectra
polyfluorene homopolymers in the long wavelength 10 of FSO-FPF6 were almost unchanged with emission peak located
disappeared,18 which is similar to those SO-based blue-emitting
conjugated polymers.11 In order to investigate the colour stability
5
at 474 nm, indicating its excellent colour stability.
Table 1 EL Performance of the FSO-FX in the devices with configuration of ITO/PEDOT:PSS/emission layer/Al.
LEmax[cd A−1]
Von[V]a)
L[cd m−2]b)
LE [cd A−1]c)
Lmax[cd m−2]
EQEmax(%)
1.78
Voc[V]
0.96
FSO-FPF6
FSO-FBr
5.5
86
85
0.92
281
2.60
6.0
0.84
435
1.14
0.74
0.82
FSO-FBIm4
4.7
151
1.51
777
1.53
1.24
1.02
a) The turn-on voltage at which luminescence reach 1 cd m-2. b) Brightness at current density around 10 mA cm-2. c) Luminous efficiency
at a current density around 10 mA cm-2.
The EL properties of the FSO-FX with different counterions 50 further develop high performance host-guest interaction based
15 were also investigated. It was found that the EL spectra of FSO-
supramolecular optoelectronic materials for optoelectronic
applications.
PF6 and FSO-FBIm4 were similar with their PL spectra, while
FSO-FBr showed a largely red-shifted EL emission compared to
its PL emission, which were attributed to the strong electrostatic
attractions in FSO-FBr. Since it has been found that counterions
20 can largely affect the electron injection properties of those
55
The work was financially supported by the Ministry of Science
and Technology (No. 2014CB643501), the Natural Science
Foundation of China (No. 21125419 and 51361165301) and
Guangdong Natural Science Foundation (Grant No.
S2012030006232). Dr J. Zhang thanks the support of China
traditional
conjugated
polyelectrolytes,9
photovoltaic
measurements were performed to determine the build-in potential
across the devices, which can reflect the barrier height of the
charge injection in devices and hence the charge injection 60 Postdoctoral Science Foundation (no. 2012M521598).
25 properties of the corresponding materials.16 It was found that the
open-circuit voltage (Voc) ranked in the order of FSO-FBr (0.82
Notes and references
a Institute of Polymer Optoelectronic Materials and Devices, State Key
Laboratory of Luminescent Materials and Devices, South China
V) < FSO-FPF6 (0.96 V) < FSO-FBIm4 (1.02V), which is
consistent with the turn-on voltage (Von) order of the OLED
devices (Table 1). Among all the FSO-FX, FSO-FBIm4 exhibited
30 the best comprehensive device performance with the lowest Von
of 4.7 V, a LE of 1.51 cd A-1 with a L of 281 cd m-2 at the current
density around 10 mA cm-2 and a more pure blue-emission
compared to FSO-PF6 and FSO-Br. Considering the relative high
PL quantum efficiencies of these materials, their device
35 performance may be further improved if used in the multilayer
PLEDs with suitable carrier transporting and blocking layers.
2500
University of Technology, 510640, Guangzhou, P. R. China. Fax: 86 20
65 87110606; Tel: 86 20 87114346; E-mail: msfhuang@scut.edu.cn
b College of Chemistry and Chemical Engineering, Jiangxi Normal
University, Nanchang 330022
† Electronic Supplementary Information (ESI) available: Experimental
details, 1H NMR, DSC. See DOI: 10.1039/b00000x/
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1
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