Influence of polyhedral oligomeric silsesquioxanes (POSS) on the luminescence properties of non-conjugated copolymers based on iridium complex and carbazole units

Article abstract of DOI:10.1039/c7ra07316j

Non-conjugated copolymers with iridium complex and carbazole groups and polyhedral oligomeric silsesquioxane (POSS) moieties as pendant groups were synthesized by free radical copolymerization. The luminescent properties of the polymeric hybrid materials

Full text of DOI:10.1039/c7ra07316j

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Influence of polyhedral oligomeric silsesquioxanes
(POSS) on the luminescence properties of non-
conjugated copolymers based on iridium complex
and carbazole units†
Cite this: RSC Adv., 2017, 7, 39512
*
*
Meijuan Lin,
Caiping Luo, Guang Xing, Longjie Chen and Qidan Ling
Non-conjugated copolymers with iridium complex and carbazole groups and polyhedral oligomeric
silsesquioxane (POSS) moieties as pendant groups were synthesized by free radical copolymerization.
The luminescent properties of the polymeric hybrid materials containing the carrier-transporting units
(carbazole groups), phosphorescence units (iridium complex) and nano-scale particles (POSS) were
studied to explore the effect of POSS on the luminescence of non-conjugated copolymer hybrid
materials. The results showed that the copolymers with POSS exhibited good solubility in common
organic solvents and better film-formation property. The photoluminescence decays of all copolymers in
solid state were bi-exponential indicating the presence of more than one emissive species, which was
attributed to intra-polymer chromophore interactions. The luminescence lifetimes of the copolymers
were 0.28–1.60 ms, indicating phosphorescence emission. The luminescence properties of the
copolymers were improved significantly by incorporating bulky POSS into the polymer side-chain, and
the maximum quantum efficiency was found to be 52.4% for ternary copolymer PCz-Ir1.0-POSS6, which
was more than six times than that of binary copolymer PCz-Ir1.0 (Fpl ¼ 7.7%). The grafted bulky POSS
can enhance the thermal stability, however, POSS had no influence on the electrochemical properties of
the copolymers.
Received 3rd July 2017
Accepted 4th August 2017
DOI: 10.1039/c7ra07316j
rsc.li/rsc-advances
heat evolution, and viscosity during processing.12 In the aspect
of its application in electroluminescent polymers, polymers
either side chain tethered13–17 or end-capped18 by POSS pendent
units, as well as uorescent emitter molecules covalently
attached to POSS core,19–22 improved the OLED's lifetime,
stability, external quantum efficiency (EQE), luminance and
color purity, because the thermal stability of the emitter could
be improved and the degree of aggregation and excimer
formation of the emitter in OLED could be reduced remarkably
by chemically incorporating bulky POSS into the conjugated
polymer chain.23
Phosphorescent iridium complexes give rise to more effi-
cient OLEDs as they can harvest both the singlet and the triplet
excitons formed during operation.24 However, the iridium
complexes used as electroluminescent (EL) materials suffer
from self-quenching in the solid state due to the interaction
aggregation with their neighboring complexes. To further
improve the performance of OLEDs, the iridium complexes
anchored POSS macromolecules were developed.25 Simulta-
neous attachment of the host materials and iridium or
platinum complex moieties to the POSS core should reduce
host-guest phase separation and decreased the interaction
among the iridium complexes. The devices based on dual-
functionalized POSS materials with hole-transporting
Introduction
Organic–inorganic hybrid materials have played an important
role in developing high-performance and functional materials.
The hybrid materials are expected to exhibit the advantages of
both organic and inorganic materials and may even possess
novel characteristics that are not found in the independent
organic and inorganic materials. Polyhedral oligomeric silses-
quioxane (POSS) is currently receiving much attention due to its
well-dened nano-scale organic–inorganic structure, which
makes it an ideal building block for constructing nano-
structured hybrid materials and nanocomposites.1,2 The appli-
cations of high-performance POSS nanocomposites can be used
as light emitting diodes,3 liquid crystals,4 photoresist mate-
rials,5 low-dielectric constant materials,6,7 luminescent mate-
rials,8,9 self-assembled block copolymers,10 and nanoparticles.11
The incorporation of POSS moieties into a polymeric mate-
rial can dramatically improve its mechanical properties (e.g.,
strength, modulus, rigidity) as well as reduce its ammability,
Fujian Key Laboratory of Polymer Materials, College of Chemistry and Materials,
Fujian Normal University, Fuzhou 350007, China. E-mail: mjlin@nu.edu.cn;
qdling@nu.edu.cn; Tel: +86 591 83464353
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra07316j
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(carbazole unit) and heavy-metal complex moieties were showed voltammetry (CV) in acetonitrile with Ag/AgCl as a reference
encouraging efficiency.26–29
electrode. The morphologies of the lms were observed by
Compared with the metal complexes, polymers are of great a eld-emission scanning electron microscope (FE-SEM JSM-
interest as they are more amenable to the solution processing 7500F) and scanning probe microscope (Bruker Dimension
techniques such as spin-coating and ink-jet printing that could Icon SPM) and thin lm samples were prepared by spin-coated
be used for low-cost and large-area device fabrication. Phos- of polymer solution (10 mg mLꢁ1) onto ITO-coated glass and
phorescent polymers with the metal complex covalently dried overnight. Thermal analyses were performed with a Met-
attached to the polymer backbone are mainly conjugated poly- tler 851e analysis system under air at a heating rate of 10 ꢂC
mers such as poly(uorene), poly(uorene-carbazole), and minꢁ1
poly(p-phenylene).24 However, the use of conjugated polymers
.
as hosts can be problematic because they can quench triplet
excitons from phosphors emitting. Therefore, polymers with
a non-conjugated backbone may be suitable for use with
phosphorescent emitters. In addition, non-conjugated poly-
mers generally have greater solubility than rigid-rod conjugated
polymers, thus facilitating their application to solution-
processed devices.30
In this study, non-conjugated polymers based on iridium
complex and carbazole groups and POSS moieties were
synthesized by free radical copolymerization. The luminescent
properties of polymeric hybrid materials containing simulta-
neous the carrier-transporting units (carbazole groups), phos-
phorescence units (iridium complex) and nano-scale particles
(POSS) were studied to explore the effect of POSS on the lumi-
nescence properties of non-conjugated polymer hybrid mate-
rials. The work has not been reported by others so far. The
results showed the signicantly enhanced luminescence, solu-
bility and stability of the copolymers were attributed to the
reduction of aggregation formation because the bulky POSS
group prohibited interchain and intrachain interactions.
Synthesis of the iridium complex monomer M2
The synthetic routes to the 3-[4-vinylbenzyl]pentane-2,4-dione
ligand 1 and the corresponding iridium complex monomer
M2 with vinyl diketone ligand, which is the actual unit in the
copolymers, are depicted in Scheme 1.
3-[4-Vinylbenzyl]pentane-2,4-dione ligand 1 was constructed
by reacting 4-chlorom-ethylstyrene with acetylacetone accord-
ing to literature procedures.30 Yield: 91.4%, 1H-NMR (CDCl3,
400 MHz), d: 7.28–7.36 (m, 2H, phenyl), 6.98–7.12 (m, 2H,
phenyl), 6.63–6.70 (m, 1H, CH]), 5.69–5.73 (d, 1H, ]CH2),
5.20–5.23 (d, 1H, ]CH2), 3.99 (t, 0.43H, (CO)2CH–CH2 keto-
form), 3.64 (s, 1H, phenyl-CH2C enol-form), 3.13–3.14 (d, 1H,
phenyl-CH2C keto-form), 2.07–2.13 (m, 6H, COCH3).
Cyclometalated Ir(III) m-chloro-bridged dimer 2 was synthe-
sized according to literature procedures,32,33 which involves
reuxing IrCl3$3H2O with 2–2.5 equiv. of cyclometalating ligand
2-phenylpyridine in a 3 : 1 mixture of 2-methoxyethanol and
water.
The iridium complex monomer M2 were synthesized
following literature procedures.24 The m-chloro-bridged dimer 2
(1.08 g, 1.0 mmol) was rst treated with silver tri-
uoromethanesulfonate (0.51 g, 2.0 mmol) in 30 mL acetone
heated at reux for 24 h under argon. The cloudy yellow solution
was gravity-ltered before 3-[4-vinylbenzyl]pentane-2,4-dione 1
(0.76 g, 3.5 mmol) and 0.53 g Na2CO3 were added sequentially.
The chelation proceeded very quickly at room temperature. The
iridium complex monomer was puried by column chromatog-
raphy over neutral alumina, followed by precipitation by pouring
a dichloromethane solution into hexane to afford iridium
complex M2 in a good yield of 71.6%. 1H-NMR (CDCl3, 400 MHz),
d: 8.60–8.61 (d, 2H, phenyl), 7.89–7.91 (d, 2H, phenyl), 7.79 (t, 2H,
phenyl), 7.58–7.60 (d, 2H, phenyl), 7.31–7.33 (m, 2H, phenyl),
7.20–7.21 (t, 2H, phenyl), 7.08–7.10 (d, 2H, phenyl), 6.84–6.85 (m,
2H, phenyl), 6.71–6.73 (m, 2H, phenyl), 6.29–6.31 (d, 2H, phenyl),
5.72–5.76 (m, 1H, CH]), 5.37–5.38 (d, 1H, ]CH2), 5.24 (d, 1H, ]
CH2), 3.31–3.73 (m, 2H, phenyl-CH2C), 1.82 (s, 6H, COCH3).
Experimental section
General information
All of the reagents and solvents used in the experiments, except
for acrylate carbazole monomer M1, were obtained from
commercial suppliers and were used without further purica-
tion unless otherwise stated. The acrylate carbazole monomer
(M1) was synthesized according to a previously reported
method.31
Thin layer chromatography was performed on G254 silica gel
plates from the Qingdao Haiyang Chemical. Column chroma-
tography was performed on Sorbent Technologies brand silica
gel (200–300 mesh).
The 1H NMR spectra were recorded on a Bruker AVIII-400
NMR spectrometer (in CDCl3). UV-vis absorption spectra were
obtained on a Shimadzu UV-2600 spectrophotometer (in
CH2Cl2, 1 ꢀ 10ꢁ5 M). The PL spectra were probed on a Shi-
madzu RF-5301 PC spectrophotometer. The luminescence life-
Synthesis of POSS monomer M3
time and absolute photoluminescence quantum efficiencies in Synthesis of POSS monomer M3 by corner capping of partially
solution and solid state were measured using an Edinburgh condensed silsesquioxanes [(C6H5)7Si7O9(OH)3] are depicted in
Instruments FLS920 Fluorescence Spectrometer. All photo- Scheme 2.
graphs were recorded on a Canon Powershot G7 digital camera
Modication of the procedures in the literature34 was used.
under 365 nm ultraviolet lamp irradiation. Cyclic voltammetry 1.86 g (2 mmol) (C6H5)7Si7O9(OH)3 and dry tetrahydrofuran
measurements were carried out on a CHI600D electro-chemical (40 mL) were added into a ask and the mixture was stirred for
analyzer using a three-electrode conguration under a nitrogen 15 min at ꢁ20 ꢂC under vacuum. Then g-(methacryloxy)propyl-
atmosphere. Oxidation potentials were measured by cyclic trimethoxysilane (0.62 g, 2.5 mmol) was added dropwise at
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Scheme 1 The synthetic routes of iridium complex monomer M2.
Scheme 2 The synthetic route of the POSS monomer M3.
ꢁ20 ꢂC. Aer the addition, the mixture was warmed to room without POSS. The ternary copolymers exhibit good solubility in
temperature and stirred for 7 d, and 20 mL of acetonitrile was common organic solvents such as petroleum ether, acetone,
added to precipitate the silsesquioxane-containing fraction. The ethyl acetate, chloroform, toluene, THF, DMF and DMSO and
off-white precipitate was collected and dried in vacuo at 50 ꢂC for can be spin-coated to give good quality neat thin lms. The
1
48 h with a yield of 48.4%. H-NMR (CDCl3, 400 MHz), d: 7.0– MWs of the copolymers were determined by size exclusion
7.92 (m, 35H, phenyl), 6.02–6.08 (d, 1H, ]CH2), 5.47–5.55 (d, chromatography (SEC) using polystyrene standards and the
1H, ]CH2), 4.09 (m, 2H, O–CH2), 1.88–1.93 (m, 3H, –CH3), GPC curves were showed in Fig. S1.† The polydispersity indexes
1.69–1.76 (m, 2H, –C–CH2–C), 0.73–0.88 (m, 2H, Si–CH2).
(PDIs) of polymers showed 1.4–2.2.
Synthesis of the copolymers
Results and discussion
IR spectra analysis
A series of binary copolymers and ternary copolymers were
synthesized from carbazole monomer M1, iridium complex
monomer M2 and POSS monomer M3 by free radical polymer- The IR spectra of the acrylate carbazole monomer M1, iridium
ization using 1 mol% 1,1-azobis (AIBN) as the initiator accord- complex monomer M2, POSS monomer M3 and binary copolymer
ing to literature procedures.31 The synthetic routes of the PCz-Ir2.5 and ternary copolymer PCz-Ir1.0-POSS6 are shown in
copolymers are depicted in Scheme 3.
Fig. 1. In the IR spectrum of POSS monomer M3, the absorption
The polymerizations were carried out at concentrations in band at 1130 cmꢁ1 is ascribed to the asymmetric stretching
the range of 0.35–0.40 M in tetrahydrofuran and the feed ratios vibration of Si–O–Si. The large peak area suggests that the ob-
of monomers (Table 1). Yield: 60–86%. The solubility of ternary tained POSS monomer M3 is mainly composed of Si–O–Si
copolymers with POSS is better than that of binary copolymers structure. At the same time, the characteristic absorption peaks
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Scheme 3 The synthetic route of the copolymers.
Table 1 Feed mole ratios of monomers and molecular weights and
their PDIs of the copolymers
incorporated into polymer. FT-IR curves of the copolymers (PCz-
Ir2.5 and PCz-Ir1.0-POSS6) and carbazole monomer M1 are
nearly the same due to the large amounts of carbazole added
and the overlap of the peaks. Compared with the spectrum of
the carbazole monomer M1, the stretching vibration peak of
carbazole C–N shis to 1151 cmꢁ1 and the strong characteristic
absorption peak at 1720 cmꢁ1 of C]O has a blue-shied to
1730 cmꢁ1 for the copolymers. Moreover, the characteristic
Feed mole ratio
[M1]/[M2]/[M3]
Copolymer
Yield/%
Mn (ꢀ104)
PDI
PCz-Ir0.5
PCz-Ir1.0
PCz-Ir2.5
PCz-Ir5.0
99.5 : 0.5
99 : 1
97.5 : 2.5
95 : 5
79.2
64.1
85.6
71.7
74.7
80.1
78.6
81.7
67.8
60.8
66.0
69.3
1.0324
0.8146
0.5110
1.1489
0.6112
0.7876
0.8169
0.5337
1.9396
0.7979
1.3763
0.9822
1.6
1.5
2.1
1.6
2.2
1.4
2.0
2.1
1.6
1.8
1.7
2.0
PCz-Ir10
90 : 10
PCz-Ir0.5-POSS2
PCz-Ir1.0-POSS2
PCz-Ir2.5-POSS2
PCz-Ir5.0-POSS2
PCz-Ir1.0-POSS4
PCz-Ir1.0-POSS6
PCz-Ir1.0-POSS8
99.5 : 0.5 : 2
99 : 1 : 2
97.5 : 2.5 : 2
95 : 5 : 2
99 : 1 : 4
99 : 1 : 6
99 : 1 : 8
of C]O at 1718 cmꢁ1 and the C]C stretching vibration at 1633
cmꢁ1 indicate that the C]C functional group has been intro-
duced. Moreover, the band at 850 cmꢁ1 may be assigned to
bending of Si–OH, was no longer seen in the IR spectrum of POSS
monomer, indicating that the cage was completely condensed.
In the IR spectrum of ternary copolymer PCz-Ir1.0-POSS6,
the characteristic bands of Si–O–Si stretching band of the
cubic cores of POSS at 1135 cmꢁ1, the deformation vibration of
POSS skeletal at 495 cmꢁ1 and the band at 699 cmꢁ1 of the Si–C
Fig. 1 IR spectra of monomers M1, M2, M3 and copolymers PCz-Ir2.5,
stretching vibration35 indicate that the POSS moiety is PCz-Ir1.0-POSS6.
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bands of the C]C out-of-plane rocking vibration at 947 cmꢁ1
and 815 cmꢁ1 and the peak at 1628 cmꢁ1 assigned to C]C
stretching vibration36 were not present, indicating that the
polymerization reaction was complete. The copolymers did not
appeared characteristic absorption bands of iridium complex
M2 due to its lower concentration.
UV-vis absorption spectra analysis
The UV-vis absorption spectra of the acrylate carbazole monomer
M1, iridium complex monomer M2, and copolymers PCz-Ir1.0,
PCz-Ir1.0-POSS2, PCz-Ir1.0-POSS6 in dichloromethane solution
(1 ꢀ 10ꢁ5 mol Lꢁ1) are shown in Fig. 2. The iridium complex M2
shows intense absorption from ligand p–p* and weak absorption
from MLCT transitions. The dominant absorption bands in the
range of 220–350 nm are assigned to spin-allowed ligand-centred
(LC) 1p–p* transitions of cyclometalated ligand. Both 1MLCT
(spin-allowed metal-to-ligand charge transfer) and 3MLCT (spin-
forbidden) bands are typically observed for the complex. The
high degree of spin–orbit coupling is evident in comparing the
oscillator strengths for the two MLCT bands. The singlet and
triplet MLCT bands fall at 405 and 460 nm, respectively.37
In the UV-vis spectrum of M1, the absorption peaks at
237 nm and 294 nm are attributed to the characterized
absorption bands of E2 and B of carbazole group and peak at
261 nm is ascribed to n–p* transition of carbonyl.
UV-vis spectra curves of the copolymers PCz-Ir1.0, PCz-Ir1.0-
POSS2, PCz-Ir1.0-POSS6 are almost the same as that of the
acrylate carbazole M1. The peaks at 237 nm, 261 nm, 294 nm,
328 nm and 342 nm, which come from the carbazole segment,
dominate UV-vis absorption of the copolymers. However, the
absorption strength of the copolymers is higher than that of the
acrylate carbazole M1. Meanwhile, the absorption peak of
iridium complex in copolymers is covered with carbazole
absorption peak due to iridium complex low concentration.
Fig. 3 PL spectra of the acrylate carbazole M1, iridium complex M2 and
the copolymers in CH2Cl2 (a), photographs of the copolymers without
POSS (b) and with 2 mol% POSS (c) in CH2Cl2 solution under 365 nm
ultraviolet lamp irradiation.
without POSS (b) and with 2 mol% POSS (c) in CH2Cl2 solution
under 365 nm ultraviolet lamp irradiation. As shown in Fig. 3a,
all of copolymers in CH2Cl2 solution exhibit dual emissions of
blue and green emission bands, in which the blue emission
bands at around 420 and 435 nm are attributed to the host
carbazole units M1 and the green emission band at around
530 nm is assigned to the guest iridium complex units M2. With
the increase of iridium complex content from 0.5 to 5.0 mol%,
the green emission increases, while the blue emission becomes
weak (Fig. 3). It is suggested that the iridium complex unit has
been incorporated into the polymer backbone and there is
energy transfer from the host (donor) carbazole units to guest
(acceptor) iridium complex units. The transfer's degree is con-
nected with content of the iridium complex and the POSS unit.
The green emission becomes stronger with the increase of
iridium complex content in the binary and ternary copolymers,
which indicates that more charges are trapped on the iridium
complex owing to its higher content.
Compared with binary copolymers, the energy transfer in the
ternary copolymers is more efficient even for iridium complex
content as low as 0.5 mol% and the intensity of the green
emission is higher at same mole ratio of carbazole units and
iridium complex units, which indicates that incorporated of the
POSS into the polymer backbone can promote the energy
transfer between donor and acceptor. However, the incorpora-
tion of the POSS does not affect the emission wavelength.
When the content of iridium complex is as high as 5.0 mol%,
the green emission of ternary copolymer PCz-Ir5.0-POSS2 is still
much strong than that of binary copolymer PCz-Ir5.0, which
indicates that the introduction of POSS units into the polymer
can efficiently separate or dilute phosphorescent emission
groups and effectively inhibit the aggregation of the iridium
complexes, thus concentration quenching phenomenon is
restrained further in solution.
Photoluminescence spectra analysis
Fig. 3 shows the photoluminescence spectra of the acrylate
carbazole M1, iridium complex M2, binary copolymers and
ternary copolymers with 2 mol% POSS in CH2Cl2 solution at
room temperature (a) and the photographs of the copolymers
The PL spectra and luminescent photographs of binary
copolymers and ternary copolymers with 2 mol% POSS in solid
powder are shown in Fig. 4. Compared with copolymers in
solution (Fig. 3a), the luminescence spectra of the copolymers
in solid state are some different from in solution. The results
show that the emissions are dominated by a green peak at
Fig. 2 UV-vis absorption spectra of monomers M1, M2 and the
copolymers PCz-Ir1.0, PCz-Ir1.0-POSS2, PCz-Ir1.0-POSS6 in CH2Cl2. around 557 nm responsible for iridium complex emission. It is
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Fig. 5 The PL spectra of copolymers with different content of POSS in
CH2Cl2 solution (a) and in solid state (b) (inset: luminescent photo-
graphs of the copolymers under 365 nm ultraviolet lamp irradiation).
Fig. 4 The PL spectra of binary copolymers (a) and ternary copoly-
mers (b) in solid state (inset: luminescent photographs of the copol-
ymers with different iridium complex content under 365 nm ultraviolet
lamp irradiation).
While the content of POSS is 8 mol%, the green intensity
decreases somewhat, which may result from the block of POSS
that may lead to incomplete energy transfer between host and
guest in intramolecular and intermolecular due to higher density
of POSS groups. The photophysical data of the monomers M1, M2
and the copolymers are summarized in Table 2.
suggested that the energy transfer from the host carbazole
segment to the iridium complex in solid states is more efficient,
compared with the polymers in CH2Cl2 solution. Energy trans-
fer is proportional to the molecular distance of the lumines-
cence materials involved. The distance between the host units
and the guest units is relatively xed by covalent bond, and
thus, intramolecular energy transfer effects can still be seen
even in dilute concentrations (Fig. 3a). However, no blue
emission or weak blue emission from host is seen due to
stronger intramolecular and intermolecular energy transfer in
the solid state as molecules are piled up and the distance
between guest–host systems is smaller.20
As shown in Fig. 4, with the increase of the iridium complex
content in the copolymers, the intensity of green emission
increases at rst and then decreases, and emission peaks are
slightly red-shied from 527 nm for PCZ-Ir0.5 to 550 nm for
PCz-Ir10 (Fig. 4a), and from 532 nm for PCZ-Ir0.5-POSS2.0 to
548 nm for PCZ-Ir5.0-POSS2.0 (Fig. 4b), respectively. Compared
to binary copolymers, the energy transfer in the ternary copol-
ymers in the solid state is more efficient even for iridium
complex content as low as 0.5 mol% and the intensity of the
green emission is much higher at same mole ratio of carbazole
units and iridium complex units, just as in solution.
In order to further explore the extents of energy transfer
between the host PCz (donor) and guest iridium complex
(acceptor), the time-resolved uorescence decay times of the
donor were measured from the decay curves of emission at
435 nm from the PCz donor in 10ꢁ5 M CH2Cl2 solution (Table 3,
Fig. S2†). As shown in Table 3, the lifetime of PCz is 4.01 ns and
there are a downtrend in the excited-state lifetime of the donor
with increased acceptor concentrations and the lifetimes of the
ternary copolymers PCz-Ir0.5-POSS2 (2.92 ns), PCz-Ir1.0-POSS2
(2.0 ns) are shorter than those of the binary copolymers PCz-
Ir0.5 (3.92 ns), PCz-Ir1.0 (3.41 ns). The energy transfer effi-
ciency (Et) can be calculated using the formula:38 Et ¼ 1 ꢁ s/s0,
where s0 and s are the lifetime of the donor (PCz) in the absence
and presence of the acceptor (iridium complex), respectively.
When the content of the guest iridium complex increases from
0.5% to 1.0%, the Et value is enhanced from 2.2% to 15.0% for
binary copolymers and from 27.2% to 50.1% for ternary copol-
ymers, respectively. This illustrates that there is a more efficient
energy transfer from the PCz host to the guest in ternary
copolymers than in binary copolymers.
To study the effect of the POSS content on the luminescent
property of the polymers, the PL spectra and luminescent
photographs of the copolymers with different content of POSS
both in solution and in solid state were measured. As shown in
Quantum efficiency
Fig. 5, along with an increase in POSS content from 0 to 6 mol%, Quantum efficiency (Fpl) measurements were carried out at 293
the green emission increases both in solution and in solid. When K in an integrating sphere and were taken as the average of
the content of POSS is 6 mol%, the green intensity of ternary three separate determinations and were reproducible within
copolymer PCz-Ir1.0-POSS6 is the highest among all copolymers. 10%. The phosphorescent quantum efficiencies for iridium
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Table 2 The photophysical data of the monomers M1, M2 and copolymers
Compound
labsa, (solution), (nm)
lemb, (solution), (nm)
lemc, (powder), (nm)
Fpld, (powder), (%)
M1
M2
PCz-Ir0.5
PCz-Ir1.0
PCz-Ir2.5
PCz-Ir5.0
PCz-Ir10
PCz-Ir0.5-POSS2
PCz-Ir1.0-POSS2
PCz-Ir2.5-POSS2
PCz-Ir5.0-POSS2
PCz-Ir1.0-POSS4
PCz-Ir1.0-POSS6
PCz-Ir1.0-POSS8
231, 237, 261, 294, 328, 343
232, 259, 343, 462
411, 434
530
416, 439
557
527
520
535
539
550
532
533
539
548
535
536
533
—
3.8
7.0
7.7
239, 261, 293, 328, 342
240, 261, 294, 328, 342
241, 261, 293, 328, 342
238, 261, 293, 328, 343
237, 261, 292, 329, 342
238, 261, 293, 328, 342
237, 261, 294, 328, 342
237, 261, 294, 328, 342
238, 261, 295, 328, 342
236, 261, 294, 327, 342
238, 261, 294, 328, 343
237, 261, 295, 328, 342
421(s), 443, 520
421(s), 443, 520
411, 436, 530(s)
421, 444, 530(s)
530(s)
419(s), 444, 529
411, 434, 530
410, 433, 530(s)
412, 436, 531
419, 446, 529
410, 433, 530(s)
415, 435, 530
15.1
24.2
19.2
26.0
39.6
26.5
24.5
43.8
52.4
46.1
a
b
Measured in CH2Cl2 at a concentration of 10ꢁ5 M at 298 K. Recorded in CH2Cl2 at a concentration of 10ꢁ5 M at 298 K at 375 nm excitation
wavelength. c Measured in powder at 375 nm excitation wavelength. d Fpl in powder were measured in an integrating sphere at 293 K.
Table 3 Time-resolved fluorescence decay times and energy transfer
copolymers improves signicantly by introduction of POSS, and
efficiency (Et) of the donor
the maximum Fpl is found to be 52.4% for PCz-Ir1.0-POSS6,
which is more than six times than that of binary copolymer
PCz-Ir0.5-
POSS2
PCz-Ir1.0-
POSS2
PCz-Ir1.0 (Fpl ¼ 7.7%).
Copolymer
PCz
PCz-Ir0.5
PCz-Ir1.0
It is noteworthy that more iridium complex is needed to
enhance luminescence for the binary copolymer, however, the
Fpl of the ternary copolymer is higher even for iridium complex
content as low as 0.5 mol%, thus less iridium complex can
reduce the cost and avoid “Roll down” phenomenon duo to high
content iridium complex.
sa (ns)
Etb (%)
4.01
—
3.92
2.2
3.41
15.0
2.92
27.2
2.00
50.1
Recorded in CH2Cl2 at a concentration of 10ꢁ5 M at 293 K at lex
375 nm and lem ¼ 435 nm. b Et ¼ 1 ꢁ s/s0.
¼
a
complex and copolymers in powder are listed in Table 2. The
results show that the quantum efficiencies of the copolymers
are higher than that of iridium complex (Fpl ¼ 3.8%), and
quantum efficiencies of the ternary copolymers (Fpl ¼ 24.5–
Phosphorescence decay
The time-resolved phosphorescence decay proles of the
copolymers were measured in powder at 293 K and phospho-
rescence decay followed the bi-exponential equation: F(t) ¼ Bl-
exp(ꢁt/s1) + B2 exp(ꢁt/s2), where s1 and s2 are the short and the
long decay components, respectively, and parameters B1 and B2
are the tting constants, indicating the presence of more than
one emissive species, which was attributed to intra-polymer
52.4%) are higher than those of the binary copolymers (Fpl
¼
7.0–24.2%). With the increase of the iridium complex content,
Fpl of copolymer increases at rst and then decreases, just as
luminescent intensity. For the binary polymer, the maximum
Fpl is found to be 24.2% for PCz-Ir5.0. The Fpl of the ternary
Table 4 Luminescence lifetimes and decay parameters of the copolymers
Copolymer
lem, (nm)
s1, (ns)
s2, (ns)
c2
B1, (%)
B2, (%)
s0a, (ms)
PCz-Ir0.5
PCz-Ir1.0
PCz-Ir2.5
PCz-Ir5.0
527
520
535
539
550
532
533
539
548
535
536
533
731.51
185.78
455.59
197.44
126.99
676.54
591.23
590.73
233.58
638.60
649.23
654.76
1612.08
1201.72
1160.72
717.15
1.179
1.095
1.204
1.079
0.993
1.022
1.058
1.057
1.133
1.077
1.095
1.230
18.17
7.72
81.83
92.28
79.77
75.83
55.03
83.79
84.47
84.50
78.91
84.93
80.35
79.02
1.45
1.12
1.02
0.59
0.28
1.54
1.25
1.25
0.65
1.60
1.34
1.31
20.23
24.17
44.97
16.21
15.53
15.50
21.09
15.07
19.65
20.98
PCz-Ir10
409.68
PCz-Ir0.5-POSS2
PCz-Ir1.0-POSS2
PCz-Ir2.5-POSS2
PCz-Ir5.0-POSS2
PCz-Ir1.0-POSS4
PCz-Ir1.0-POSS6
PCz-Ir1.0-POSS8
1707.71
1366.72
1366.37
766.06
1772.90
1502.94
1486.87
P
a
s0 ¼ si ꢀ Bi%.
39518 | RSC Adv., 2017, 7, 39512–39522
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annihilation in the phosphorescent OLEDs. Therefore, the
short phosphorescent lifetimes of these copolymers could
effectively restrain the phosphorescence self-quenching to show
high quantum yield. Meantime, excited-state lifetimes are in the
microsecond regime for all copolymers, which it clearly
suggests that the emitting state has triplet character, indicating
phosphorescence emission. With the increase of iridium
complex content, the phosphorescent lifetimes of these copol-
ymers show the tendency of decrease. The inuence of the POSS
content on phosphorescent lifetime is not obvious.
Electrochemical properties
Electrochemical properties of the copolymers including the
HOMO and LUMO energy levels were investigated to charac-
terize and compare the electronic properties of the copolymers
through cyclic voltammetry (CV) measurements. All copolymers
showed similar CV curves (Fig. S4†) and exhibited similar
reversible oxidation processes, which could be assigned to the
Fig. 6 TG curves of the copolymers PCz-Ir2.5 and PCz-Ir2.5-POSS2.
chromophore interactions.24 The luminescence lifetimes of the
copolymers have been determined from the decay curves
(Fig. S3†) and resulting lifetimes are given in Table 4.
As shown Table 4, it shows that the phosphorescence
lifetimes are 0.28–1.60 ms, which is shorter than that of popular
fac-[Ir(ppy)3] (ca. 4.75 ms). As we all know, the long life-time
would increase the possibility of intrinsic triplet–triplet
oxidation of electron-donating carbazole moiety, with the onset
ox
onset
potentials (E
) of 0.84–1.08 V. Compared with the ionization
potential of ferrocene at ꢁ4.8 eV, the HOMO level of the
copolymers could be calculated to be 5.64–5.88 eV. In addition,
the LUMO levels of the copolymers were estimated to be 2.33–
2.58 eV from their absorption edge. All results were listed in
Table S1.† The HOMO levels, the LUMO levels and the Eg of the
Fig. 7 SEM photographs of copolymers without POSS (a, b) and with POSS (c, d), (a) PCz-Ir1.0 (b) PCz-Ir2.5 (c) PCz-Ir2.5-POSS2 (d) PCz-Ir1.0-
POSS6.
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ternary copolymers PCz-Ir0.5-POSS2 ꢃ PCz-Ir1.0-POSS8 were bulky POSS can enhance the thermal stability of the luminescent
proximately close to those of binary copolymers PCz-Ir0.5 ꢃ materials due to the presence of inorganic Si–O cage shielding
PCz-Ir5.0. It was showed that the content of POSS had no effect, which limits the heat transfer and protects the organic
inuence on the electrochemical properties of the polymers part of polymer from the heat effect of attack.15,39
backbones, indicating that the presence of the POSS moieties in
the copolymers does not result in changes in the electronic
structure of the polymers, because POSS is inorganic nano-
particle and it has no conjugated structures.13,14,16
SEM and SPM photographs
Scanning electronic microscope (SEM) and scanning probe
microscope (SPM) are used to investigate the effect of POSS on
the surface morphology of the copolymer thin lms. The SEM
photographs and SPM 3D images of copolymer thin lms
Thermal properties
Thermal stability of the emissive materials is one of the most without POSS and with POSS are shown in Fig. 7 and 8,
important aspects to improve device lifetime and reliability. It is respectively. From Fig. 7, it is found that the surface
well-known incorporation of POSS into materials can improve morphologies of copolymer lms with POSS (PCz-Ir2.5-POSS2,
their thermal stability. Taking the binary copolymer PCz-Ir2.5 PCz-Ir1.0-POSS6) are uniform and smooth, whereas the
and ternary copolymer PCz-Ir2.5-POSS2 for example, from copolymers lms without POSS (PCz-Ir1.0, PCz-Ir2.5) show
TGA curves of PCz-Ir2.5 and PCz-Ir2.5-POSS2 (Fig. 6), it was rough surface with undissolved polymers particles. From the
observed that the POSS loading did not change the polymer 3D images (Fig. 8), it is found that the ternary copolymer lms
degradation path, and the thermal properties of ternary (PCz-Ir2.5-POSS2, PCz-Ir1.0-POSS6) show a smooth surface
copolymer PCz-Ir2.5-POSS2 was similar to those of binary with Ra values of 7.77 nm and 7.25 nm, respectively, and the Ra
copolymer PCz-Ir2.5. The thermal decomposition temperatures values of binary polymers (PCz-Ir1.0, PCz-Ir2.5) are 11.8 nm
(Td, 5% weight loss temperature) of ternary copolymer and 37.8 nm, respectively. These results imply the presence of
PCz-Ir2.5-POSS2 with POSS was 9 ꢂC higher than that of binary the POSS groups can effectively enhance the polymers solu-
copolymer PCz-Ir2.5 without POSS. It indicates that the graed bility and lm-forming property.
Fig. 8 SPM 3D-images of copolymers without POSS (a, b) and with POSS (c, d), (a) PCz-Ir1.0 (b) PCz-Ir2.5 (c) PCz-Ir2.5-POSS2 (d) PCz-Ir1.0-
POSS6.
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11 C. H. Lu, S. W. Kuo, C. F. Huang and F. C. Chang, J. Phys.
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Conclusion
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The non-conjugated copolymer hybrid materials containing the
carrier-transporting carbazole units, iridium complex phos-
phorescence units and nano-scale particles POSS were prepared
and studied. Compared to binary copolymers, the energy
transfer from host to guest in the ternary copolymers is more
efficient both in the solid state and in solution, and quantum
efficiency (Fpl ¼ 24.5–52.4%) of the ternary copolymers are
much higher than those of the binary copolymers (Fpl ¼ 7.0–
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the POSS as pendant group into the polymer backbone can
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lifetimes are 0.28–1.60 ms. The ternary copolymers exhibit good
solubility in common organic solvents, and better lm forma-
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there is a practical prospect for the free radical copolymeriza-
tion method which is convenient and easy to purify.
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Conflicts of interest
There are no conicts to declare.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21374017 and 21574021), Natural Science
Foundation of Fujian Province (2017J01683), Program for
Innovative Research Team in Science and Technology in Fujian
Province University (IRTSTFJ).
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