High-performance near-infrared (NIR) polymer light-emitting diodes (PLEDs) based on bipolar Ir(iii)-complex-grafted polymers

Article abstract of DOI:10.1039/d0tc04377j

Despite the cost-effective and large-area scalable advantages of NIR-PLEDs based on iridium(iii)-complex-doped polymers, the intrinsic phase-separation issue leading to inferior device performance is difficult to address. In this study, taking the vinyl-f

Full text of DOI:10.1039/d0tc04377j

Journal of
Materials Chemistry C
View Article Online
View Journal
PAPER
High-performance near-infrared (NIR) polymer
light-emitting diodes (PLEDs) based on bipolar
Ir(III)-complex-grafted polymers†
Cite this:DOI: 10.1039/d0tc04377j
Wentao Li,‡a Baowen Wang,‡a Tiezheng Miao,a Jiaxiang Liu,a Guorui Fu,*ab
b
Xingqiang Lu, *a Weixu Feng *c and Wai-Yeung Wong
*
¨
Despite the cost-effective and large-area scalable advantages of NIR-PLEDs based on iridium(III)-
complex-doped polymers, the intrinsic phase-separation issue leading to inferior device performance is
difficult to address. In this study, taking the vinyl-functionalized [Ir(iqbt)2(vb-ppy)] (Hqibt = 1-(benzo[b]-
thiophen-2-yl)-isoquinoline; vb-Hppy = 2-(40-vinylbiphenyl-4-yl)pyridine) as the polymerized complex
monomer, two series of Ir(III)-complex-grafted polymers Poly(NVK-co-[Ir(iqbt)2(vb-ppy)]) and
Poly((vinyl-PBD)-co-NVK-co-[Ir(iqbt)2(vb-ppy)]) (NVK = N-vinyl-carbazole; vinyl-PBD = 2-(4-(tert-butyl)-
phenyl)-5-(40-vinyl-[1,10-biphenyl]-4-yl)-2,5-dihydro-1,3,4-oxadiazole) are obtained, respectively. Moreover,
by using the bipolar Ir(III)-complex-grafted polymer further doped or grafted with an electron-transport unit
as the emitting layer (EML), reliable NIR-PLEDs are realized. In particular based on the concurrent covalent-
linkages of both the Ir(III)-complex and the vinyl-PBD towards the carrier-balanced NIR-PLED-III, the
Received 14th September 2020,
Accepted 11th November 2020
achievement of an almost negligible (o5%) efficiency roll-off does not sacrifice the attractive efficiency
DOI: 10.1039/d0tc04377j
max
EQE
(Z
= 3.6%). This finding makes bipolar Ir(III)-complex-grafted polymers a good platform to achieve
rsc.li/materials-c
high-performance NIR-PLEDs.
other triplet-utilized counterparts, together with the superiority
of a significantly alleviated efficiency roll-off, have particular
1. Introduction
Driven by the promising applications of near-infrared (NIR) organic/ appeal for application in NIR-OLEDs/PLEDs.
polymer light-emitting diodes (NIR-OLEDs/PLEDs) in night-vision
Until now, concrete C^N-cyclometalated Ir(III)-complexes
and information-security displays,1 telecommunications2 and photo- possessing neutral [Ir(C^N)3]-homoleptic6 or [Ir(C^N)2(L^X)]-
dynamic therapies,3 concerted efforts have been devoted to the heteroleptic (L^X = O^O7 or N^O8) and cationic [Ir(C^N)2(N^N)]+
development of new and efficient NIR-emitters. In this context, forms9 have been demonstrated for developing reliable NIR-
owing to the harvesting of both singlet and triplet excitons OLEDs/PLEDs, and the wavelength–ZEQE (external quantum effi-
towards a theoretical ZIQE (internal quantum efficiency) of ciency) relationship is summarized in Fig. 1 and Table S1 (ESI†).
100%, NIR-emissive transition-metal (Pt(II), Ir(III) or Os(II), etc.) Nonetheless, as constrained by the so-called ‘‘energy gap law’’,10
complex phosphors4 together with TADF (thermal activated it remains a real challenge to develop new Ir(III)-complex-based
delayed fluorescence) molecules5 capable of the facilitated NIR-emitters to achieve high efficiency. On the other hand, to
reverse intersystem crossing (RISC) are highly attractive. Impor- suppress the detrimental triplet–triplet annihilation (TTA)11 of
tantly, the octahedral Ir(III)-complexes with rather short triplet the Ir(III)-complex-based phosphors with narrow HOMO–LUMO
lifetimes and high efficiency which is competitive to those of band-gaps for the NIR emissions, it is necessary and also
challenging to dope one specific Ir(III)-complex into an appro-
priate small-molecule host for the vacuum-deposited/solution-
processed NIR-OLED (NIR-OLEDs-V/S) or polymeric matrix
a School of Chemical Engineering, Northwest University, Xi’an 710069, Shaanxi,
China. E-mail: lvxq@nwu.edu.cn
b Department of Applied Biology and Chemical Technology, The Hong Kong
for the NIR-PLEDs (Table S1, ESI† and Fig. 1), respectively.
In comparison, although cost-effective solution-processed NIR-
PLEDs with Ir(III)-complex-doped polymers as the EMLs are
more advantageous for the large-area scalability, the simple
doping suffers from an inevitable phase-separation issue,12
thereby leading to inferior device efficiency and serious efficiency
roll-off. Noticeably, despite the certain efficiency progress
Polytechnic University, Hung Hom, Hong Kong, China.
E-mail: guorui.fu@polyu.edu.hk, wai-yeung.wong@polyu.edu.hk
c School of Chemistry and Chemical Engineering, Northwestern Polytechnical
University, Xi’an 710029, Shaanxi, China. E-mail: fwxdk@nwpu.edu.cn
† Electronic supplementary information (ESI) available: Starting materials and
characterization; NMR, UV, and PL spectra. See DOI: 10.1039/d0tc04377j
‡ These authors contributed equally and should be considered co-first authors.
J. Mater. Chem. C
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
Journal of Materials Chemistry C
ligand Hiqbt was synthesized by the Suzuki coupling of
2-Cl-isoquinoline with benzo[b]thien-2-yl boronic acid as
reported in our recent literature.8e As to the m-chloro-bridged
dimeric intermediate [Ir(iqbt)2(l-Cl)]2, it was prepared according
to the typical Nonoyama procedure.15 For the vinyl-functionalized
HC^N2 ancillary ligand vb-Hppy, it was synthesized from the
improved Suzuki coupling reaction of 4-vinylphenyl-boronic acid
with 2-(4-bromophenyl)-pyridine as in the literature.16 The vinyl-
modified electron-transport monomer vinyl-PBD was obtained
through the dehydration cyclization17 and the subsequent
Suzuki coupling reaction.18
Synthesis of the vinyl-functionalized complex monomer
[Ir(iqbt)2(vb-ppy)]
Fig. 1 The lem-relative ZEQE comparison between the NIR-PLEDs-I–III in
this work with those from Ir3+-complexes doping in small-molecular or
polymer host with vacuum-deposition (NIR-OLEDs-V) or solution-
processing (NIR-OLEDs-S/PLEDs).
To a solution of the m-chloro-bridged dimeric intermediate
[Ir(iqbt)2(l-Cl)]2 (270 mg, 0.18 mmol) in mixed solvents of
CH2Cl2 (8 mL) and MeOH (4 mL), the synthesized vb-Hppy
(139 mg, 0.54 mmol) and AgCF3SO3 (138 mg, 0.54 mmol) were
added, and the mixture was heated at 45 1C under a dry N2
atmosphere for 24 h. After cooling to room temperature, the
white solid was removed and the residue was purified by
column chromatography on silica gel using CH2Cl2/acetonitrile
(v/v = 2 : 1) as the eluent. Yield: 35 mg (20%). Calc. for
appreciable from the supplementation of one electron-transport
layer (ETL) towards the facilitated carriers’ balance for some multi-
layer NIR-PLEDs,12 the issues of device instability and undesirable
efficiency-roll-off caused by the heterogeneity of the Ir(III)-complex-
doping polymer systems are still difficult to address.
To circumvent such problems, to some extent, we can rely on
a conceptual approach to use Ir(III)-complex-grafted polymers in
solution-processed multi-layer NIR-PLEDs. On one hand, benefit-
ing from the covalent-bonding linkage, the NIR-emitting Ir(III)-
complexes are molecularly dispersed into a hole-transporting
polymer host with a uniform phase. Meanwhile, further through
the doping or grafting of the electron-transport molecule, the
resultant Ir(III)-complex-grafted polymers could exhibit a bipolar
(electron/hole-transport ability) nature. In particular, through the
smooth feeding ratio tuning of both the Ir(III)-complex and
electron-transport molecule into the hole-transport polymer
matrix, much room can be achieved to facilely reform the carrier’s
balance within the bipolar polymer towards an optimized opto-
electronic feature. Noticeably, although bipolar Ir(III)-complex-
grafted polymers capable of showing monochromatic13 or
panchromatic14 emission in the visible-light range are achieved,
no examples of their fabrication for NIR-PLEDs, to our knowledge,
have been reported. Herein, taking the NIR-emitting [Ir(iqbt)2(vb-
ppy)] with one vinyl group as the polymerizable complex mono-
mer, as shown in Scheme 1, two series of Ir(III)-complex-grafted
polymers Poly(NVK-co-[Ir(iqbt)2(vb-ppy)]) (100 : 1, 150 : 1 or 200 : 1)
and Poly((vinyl-PBD)-co-NVK-co-[Ir(iqbt)2(vb-ppy)]) (15:150:1) are
obtained from its copolymerization with NVK and/or monomer
vinyl-PBD with the facilitated electron-transport, respectively.
Moreover, using the Ir(III)-complex-grafted polymer doped or
grafted with an electron-transport molecule as the EML, respec-
tively, the first-examples of bipolar Ir(III)-complex-grafted NIR-
PLEDs are also pursued.
C
53H34IrN3S2: C, 65.68; H, 3.54; N, 4.34%. Found: C, 65.63; H,
3.58; N, 4.30%. FT-IR (KBr, cmꢀ1): 3051 (w), 2953 (m), 2918 (m),
2851 (m), 2359 (w), 1618 (w), 1601 (w), 1582 (w), 1558 (w),
1541 (w), 1501 (w), 1468 (w), 1452 (w), 1435 (m), 1412 (s), 1375 (w),
1360 (w), 1335 (m), 1306 (w), 1288 (w), 1273 (w), 1231 (m),
1157 (w), 1148 (w), 1124 (w), 1067 (w), 1040 (w), 1020 (w), 988 (w),
962 (w), 910 (m), 862 (w), 845 (w), 806 (m), 779 (w), 760 (m),
727 (vs), 706 (w), 687 (s), 662 (m), 633 (w), 598 (w), 565 (w),
1
528 (w), 500 (w). H NMR (400 MHz, CDCl3): d (ppm) 9.27 (d,
1H, –Py), 8.80 (d, 1H, –Py), 8.51 (d, 1H, –Py), 8.06 (d, 1H, –Py), 8.02
(d, 1H, –Py), 7.95 (m, 7H, –Ph), 7.84 (t, 2H, –Ph), 7.69 (t, 2H, –Ph),
7.64 (d, 1H, –Py), 7.61 (d, 2H, –Ph), 7.58 (d, 1H, –Py), 7.53
(t, 2H, –Ph), 7.49 (d, 1H, –Ph), 7.39 (d, 1H, –Ph), 7.23 (d, 1H,
–Ph), 7.17 (m, 2H, –Ph), 7.11 (t, 1H, –Ph), 6.88 (t, 2H, –Ph),
6.82 (t, 1H, –Ph), 6.66 (t, 1H, –CHQ), 5.65 (d, 1H, QCH2), 5.15
(d,1H,QCH2). ESI-MS (in CH2Cl2) m/z: 970.21 (100%), [M + H]+.
Synthesis of the Ir3+-complex-grafted polymers Poly(NVK-co-
[Ir(iqbt)2(vb-ppy)]) (100 : 1, 150 : 1 or 200 : 1)
A mixture of NVK and the complex monomer [Ir(iqbt)2(vb-ppy)]
at a stipulated feed molar ratio (100 : 1, 150 : 1 or 200 : 1) in the
presence of AIBN (azobis(isobutyronitrile); 1.5 mol% of NVK)
was dissolved in toluene (30 mL), and the resultant homoge-
neous solution was purged with N2 for 10 min and sealed under
a reduced N2 atmosphere. The reaction mixture was heated to
80 1C with continuous stirring for 48 h. The viscous mixture was
diluted with toluene (15 mL) and precipitated with n-hexane
(50 mL) three times. The resulting solid products were collected
by filtration and dried at 45 1C under vacuum to constant
weight, respectively. For Poly(NVK-co-Ir(iqbt)2(vb-ppy)) (150 : 1):
yield: 92%. FT-IR (KBr, cmꢀ1): 3059 (w), 2968 (w), 2934 (w),
2. Experimental section
The information on starting materials and general character- 2359 (w), 1597 (w), 1483 (m), 1450 (s), 1325 (m), 1223 (m), 1157 (w),
ization methods is provided in the ESI.† The HC^N1 main 1124 (w), 1028 (w), 1003 (w), 926 (w), 829 (w), 745 (vs), 721 (s),
J. Mater. Chem. C
This journal is © The Royal Society of Chemistry 2020

View Article Online
Journal of Materials Chemistry C
Paper
Scheme 1 Synthetic scheme of the ligands Hiqbt, vb-Hppy, and vinyl-PBD, the complex monomer [Ir(iqbt)2(vb-ppy)] and their two series of Ir3+
-
complex-grafted polymers Poly(NVK-co-[Ir(iqbt)2(vb-ppy)]) and Poly((vinyl-PBD)-co-NVK-co-[Ir(iqbt)2(vb-ppy)]).
1
617 (w), 567 (w), 528 (w). H NMR (400 MHz, CDCl3): d (ppm) (150 : 1):PBD (30 wt%) (120 nm)/TmPyPB (15 nm)/LiF (1 nm)/
9.24 (m, 3H, –Ph), 8.10–6.04 (br, 1100H + 26H), 5.52–2.75 (br, Al (100 nm) and ITO/PEDOT:PSS (40 nm)/Poly((vinyl-PBD)-co-
138H), 2.38 (b, 1H), 1.65 (b, 2H), 1.30–0.88 (b, 276H). XPS result: NVK-co-[Ir(iqbt)2(vinyl-ppy)]) (15 : 150 : 1) (120 nm)/TmPyPB
0.80 mol% versus NVK. The characterization of the other (15 nm)/LiF (1 nm)/Al (100 nm), respectively. Their difference
Ir3+-complex-grafted polymers Poly(NVK-co-[Ir(iqbt)2(vb-ppy)]) lies in the usage of PVK:PBD:[Ir(iqbt)2(vb-ppy)] for the NIR-
(100 : 1 or 200 : 1) is provided in the ESI.†
PLED-I, Poly(NVK-co-[Ir(iqbt)2(vb-ppy)]) (150 : 1):PBD for the
NIR-PLED-II or Poly((vinyl-PBD)-co-NVK-co-[Ir(iqbt)2(vb-ppy)])
(15 : 150 : 1) for the NIR-PLED-III, respectively. TmPyPB (1,3,5-
tri[(3-pyridyl)-phen-3-yl]benzene) was used to further promote
the electron-transport ability in the NIR-PLEDs-I–III. Details of
the series of NIR-PLED fabrication and their testing are pre-
sented in the ESI.†
Synthesis of the bipolar Ir3+-polymer Poly((vinyl-PBD)-co-NVK-
co-[Ir(iqbt)2(vb-ppy)]) (15 : 150 : 1)
The bipolar Ir3+-polymer Poly((vinyl-PBD)-co-NVK-co-[Ir(iqbt)2(vb-
ppy)]) (15 : 150 : 1) was synthesized in the same way as the
Poly(NVK-co-[Ir(iqbt)2(vb-ppy)]) (150 : 1) except that the mixture
of the organic monomer vinyl-PBD, NVK and the complex mono-
mer [Ir(iqbt)2(vb-ppy)] at a stipulated feed molar ratio of 15 : 150 : 1
(1.5 mol% of AIBN relative to NVK) instead of the mixture of NVK
and the complex monomer [Ir(iqbt)2(vb-ppy)] at a feeding ratio of
150 : 1 (1.5 mol% of AIBN relative to NVK) was adopted. For
Poly((vinyl-PBD)-co-NVK-co-[Ir(iqbt)2(vb-ppy)]) (15 : 150 : 1): yield:
91%. FT-IR (KBr, cmꢀ1): 3051 (w), 2963 (w), 2932 (w), 2354 (w),
1598 (w), 1483 (m), 1452 (s), 1333 (m), 1225 (m), 1157 (w),
1124 (w), 1027 (w), 1003 (w), 924 (w), 829 (w), 742 (vs), 723 (s),
3. Results and discussion
Synthesis, characterization and photo-physical properties of
the Ir3+-complex monomer [Ir(iqbt)2(vb-ppy)]
Through the improved Suzuki coupling reaction8e of 2-Cl-iso-
quinoline (instead of 2-Br-isoquinoline7g) with benzo[b]thien-2-yl
boronic acid, the synthesized HC^N1 main ligand Hiqbt was
cyclometalated with IrCl3ꢁnH2O to give the m-chloro-bridged
dimeric intermediate [Ir(iqbt)2(l-Cl)]2 as specified in the
literature.15 Also as shown in Scheme 1, further based on the
cyclometalation of the intermediate [Ir(iqbt)2(l-Cl)]2 with another
vinyl-functionalized HC^N2 ancillary vb-Hppy by AgCF3SO3, the
chloride-free, vinyl-functionalized [Ir(C^N1)2(C^N2)] complex
monomer [Ir(iqbt)2(vb-ppy)] was obtained.
1
616 (w), 568 (w), 529 (w). H NMR (400 MHz, CDCl3): d (ppm)
8.13–5.93 (br, 1135H), 4.92–2.39 (b, 135H), 1.59 (s, 135H),
1.28–0.91 (b, 270H). XPS result: 0.78 mol% versus NVK.
Device designs of the doping-type NIR-PLED-I based on the
Ir3+-complex monomer and the grafting-type NIR-PLEDs-II–III
based on the Ir3+-polymers
Using a mixture of the complex monomer [Ir(iqbt)2(vb-ppy)] was well characterized via EA, FT-IR, 1H NMR and ESI-MS, despite
The vinyl-functionalized complex monomer [Ir(iqbt)2(vb-ppy)]
1
(5 wt%) and the co-host PVK : PBD (65 : 30, wt%; PVK = poly(N- the failure to produce its single-crystals. Evidently, in the H NMR
vinyl-carbazole), PBD = (2-(4-tert-butylphenyl)-5-(4-biphenylyl)- spectrum (Fig. S1, ESI†) of the complex monomer [Ir(iqbt)2(vb-ppy)],
1,3,4-oxadiazole)) as the EML, the doping-type NIR-PLED-I was the stipulated molar ratio of 2 : 1 between the C^N1 (iqbt)ꢀ and
fabricated with the configuration of ITO/PEDOT:PSS (40 nm)/ the C^N2 (vb-ppy)ꢀ proton resonances confirms its desirable
PVK:PBD:[Ir(iqbt)2(vb-ppy)] (120 nm)/TmPyPB (15 nm)/LiF [Ir(C^N1)2(C^N2)] component. Meanwhile, attributed to the incor-
(1 nm)/Al (100 nm) for comparison. As to the grafting-type poration of the asymmetric vinyl-functionalized HC^N2 ancillary vb-
NIR-PLEDs-II–III, they were fabricated with the configurations Hppy, the point group of its complex monomer [Ir(iqbt)2(vb-ppy)] is
of ITO/PEDOT:PSS (40 nm)/Poly(NVK-co-[Ir(iqbt)2(vb-ppy)]) C1, from which the two sets of doublet peaks at d = 8.80 and
J. Mater. Chem. C
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
Journal of Materials Chemistry C
8.51 ppm can be safely assigned to the two protons on the C Noticeably, the NIR-emissive lifetime (t = 0.25 ms) is remark-
atoms adjacent to N atoms in the pyridyl rings of the two ably shorter than those of the heteroleptic Ir3+-complexes
(iqbt)ꢀ-C^N1 ligands, respectively. Moreover, upon the Ir(III)- [Ir(iqbt)2(O^O)]7d,g or [Ir(iqbt)2(N^O)],8e which should be ori-
coordination, besides the double signal (d = 9.27 ppm) of the ginated from the stronger p-backbonding effect19 due to the
proton on the C atom adjacent to N atoms in the pyridyl ring of asymmetric C^N2-(vb-ppy) ancillary p-donor in the complex
the (vb-ppy)ꢀ-C^N2 ligand being significantly down-field shifted monomer [Ir(iqbt)2(vb-ppy)] with a restricted vibronic motion
compared to that (d = 8.70 ppm) for the free vb-Hppy, the slightly to the NIR-emitting excited-state. Accordingly, owing to the
high-field shifts (d = 6.66, 5.65 and 5.15 ppm) of the vinyl-terminal large radiative rate constant (kr = 7.6 ꢂ 105
s
ꢀ1), its NIR-
proton resonances for the complex monomer [Ir(iqbt)2(vb-ppy)] emissive efficiency of FPL = 0.19 is realized. Furthermore, as
relative to those (d = 6.78, 5.82 and 5.30 ppm) of the free vb-Hppy illustrated for the emission (85% of the lem Z 700 nm
further verify the successful vinyl-modification. Furthermore, the proportion; Fig. 2) with a well-resolved vibronic structure at
ESI-MS result of the complex monomer [Ir(iqbt)2(vb-ppy)] exhibits 77 K, the 0–0 transition at 704 nm and the 0–1 transition at
the strongest mass peak at m/z 970.21 assigned to the major 764 nm with small bathochromic shifts compared to the RT
species [M
+
H]+, indicating that its [Ir(C^N1)2(C^N2)]- one (Fig. 2) give a Huang–Rhys factor (SM) of 0.98, suggesting
that the complex monomer [Ir(iqbt)2(vb-ppy)] has a weak
characteristic unit can remain stable in solution.
The photo-physical properties of the complex monomer geometry distortion20 of the T1 state relative to the ground
[Ir(iqbt)2(vb-ppy)] were examined in degassed solution at RT or state. As a result, the thermal gravimetric (TG; Fig. S4, ESI†)
77 K, and the results are summarized in Table S2 (ESI†) and Fig. 2. analysis reveals that the complex monomer [Ir(iqbt)2(vb-ppy)]
As shown in Fig. 2, in contrast to the limited (lab o 400 nm; Fig. S2, exhibits a desirably good thermal stability with a comparable
ESI†) absorptions of the two kinds of C^N ligands, the decomposition temperature (Td, with 5 wt% weight loss) of
complex monomer [Ir(iqbt)2(vb-ppy)] exhibits significantly 384 1C to those of typical [Ir(C^N)3]-homoleptic6 complexes.
broadened UV-visible-NIR absorption: intense absorption
bands below 420 nm from the intraligand p–p* transitions,
Electronic structure calculations of the complex monomer
moderate absorption bands (lab = 456, 487 (sh), 518 and 557
[Ir(iqbt)2(vb-ppy)]
(sh) nm) assigned to the 1,3LLCT/1,3MLCT-admixed (LLCT =
ligand-to-ligand charge transfer; MLCT = metal-to-ligand
To explore the absorption nature of the complex monomer
[Ir(iqbt)2(vb-ppy)], DFT/TD-DFT (time-dependent density func-
tional theory) calculations based on its optimized S0 geometry
were performed, and the results are summarized in Table S3
(ESI†) and Fig. 3. As shown in Fig. 3, in contrast to the almost
entire contribution (92.76%) from one C^N1-(iqbt)ꢀ main ligand
to the LUMO, the HOMO is mainly (51.01% and 23.06%)
localized at the two C^N1-(iqbt)ꢀ main ligands and accompanied
by the substantial (23.73%) contribution from the Ir(III)-centre
and less (2.20%) contribution from the C^N2-(vb-ppy)ꢀ ancillary
ligand. However, different from the dominant (84.71%) contribu-
tion from one C^N1-(iqbt)ꢀ main ligand and the more substantial
(10.22%) contribution from the C^N2-(vb-ppy)ꢀ ancillary ligand
to the LUMO+1, the LUMO+2 is predominantly (86.93%) located
at the C^N2-(vb-ppy)ꢀ ancillary ligand. Meanwhile, besides the
prevalent (74.44%) contribution from the two C^N1-(iqbt)ꢀ main
charge transfer) transitions, and weak bands extending over
600 nm probably from the S0 - T1 excitation. Upon photo-
excitation at lex = 463 nm, the complex monomer [Ir(iqbt)2(vb-
ppy)] displays a strong NIR emission (58% of the lem
Z
700 nm proportion) peaking at 693 nm with a shoulder at
754 nm (Fig. 2). In contrast to the non-emissive character
(lem = 415 nm for the HC^N1 ligand Hiqbt and lem = 403 nm
for the HC^N2 ligand vb-Hppy; Fig. S2, ESI†) of the two C^N
ligands in the NIR range, the NIR emission of the complex
monomer [Ir(iqbt)2(vb-ppy)] should originate from the Ir3+
-
induced T1 state. Moreover, the time-decayed mono-
exponential lifetime of 0.25 ms (Fig. S3, ESI†) was obtained
at lem = 693 nm for the complex monomer [Ir(iqbt)2(vb-ppy)]
species, confirming the intrinsic NIR-phosphorescent nature.
Fig. 2 Normalized UV-visible-NIR absorption and emission spectra for
[Ir(iqbt)2(vb-ppy)] (lex = 463 nm) in degassed CH2Cl2 solution and PVK-
PBD (65 : 30, weight ratio; lex = 273 nm) in film at RT or 77 K.
Fig. 3 The HOMO and LUMO patterns for the complex monomer
[Ir(iqbt)2(vb-ppy)] based on its optimized S0 geometry.
J. Mater. Chem. C
This journal is © The Royal Society of Chemistry 2020

View Article Online
Journal of Materials Chemistry C
Paper
22
¨
ligands to the HOMOꢀ2 like the HOMOꢀ1 (97.79%), some transfer energy via the Forster mechanism to the low energy-
substantial contributions from the Ir(III)-centre (14.59%) and state Ir3+-complex-acceptor. Moreover, to further overcome the
the C^N2-(vb-ppy)ꢀ ancillary ligand (10.97%) are observed. electron-transport deficiency of the Ir3+-polymers Poly(NVK-co-
Further checking from Table S3 (ESI†), the calculated S0 - Sn [Ir(iqbt)2(vb-ppy)]), another grafting-type Ir3+-polymer Poly((vinyl-
(n = 1–4) transition absorption wavelengths of the complex PBD)-co-NVK-co-[Ir(iqbt)2(vb-ppy)]) (also Scheme 1) was designed,
monomer [Ir(iqbt)2(vb-ppy)] are predicted at 558, 512, 489 and where through the AIBN-assisted ternary copolymerization
445 nm, respectively. For the S0 - S1 transition absorption at of NVK, the complex monomer [Ir(iqbt)2(vb-ppy)] and the
558 nm, the population analysis of the HOMO - LUMO electron-transport monomer vinyl-PBD, the bipolar (electron/
1
(97.64%) transition verifies the partial (22.7%) MLCT, the sub- hole-transport)
Ir3+-polymer
Poly((vinyl-PBD)-co-NVK-co-
stantial (21.6%) 1ILCT (intraligand charge transfer) and the [Ir(iqbt)2(vb-ppy)]) (15 : 150 : 1) was obtained.
dominant (51.4%) 1LLCT feature from the p orbitals of one
To verify the AIBN-assisted radical copolymerization,23 both
C^N1-(iqbt)ꢀ main ligand to the p* orbitals of the other one. series of grafting-type Ir3+-polymers were characterized by FT-IR,
The calculated absorption peak at 512 nm, 489 nm or 445 nm 1H NMR and GPC (gel permeation chromatography) methods.
mainly results from the corresponding HOMO - LUMO+1 On one hand, in the 1H NMR spectrum (Fig. S1, ESI†) of the
(95.32%), HOMO - LUMO+2 (97.26%) or HOMOꢀ1 - LUMO representative Poly(NVK-co-[Ir(iqbt)2(vb-ppy)]) (150: 1) or
(76.73%) transition, respectively, also exhibiting the 1LLCT/1MLCT- Poly((vinyl-PBD)-co-NVK-co-[Ir(iqbt)2(vb-ppy)]) (15 : 150 : 1), the
admixed character. Hence, all the calculated absorptions featuring presence of the broadened proton resonances of the polymerized
1LLCT/1MLCT-admixed transitions are in good agreement with the [Ir(iqbt)2(vb-ppy)], NVK and/or vinyl-PBD, together with the dis-
experimental data (lab = 557, 518, 487 and 456 nm) of the complex appearance of their original vinyl-characteristic proton reso-
monomer [Ir(iqbt)2(vb-ppy)] in solution. Interestingly, due to the nances, indicate that the complex monomer [Ir(iqbt)2(vb-ppy)]
large contribution combined from the HOMO - LUMO (59.94%) and/or the vinyl-PBD are actually covalent-bonded into the
and the HOMOꢀ1 - LUMO (30.32%) transitions to the T1 state, corresponding PVK backbone. On the other hand, GPC results
the experimental S0 - T1 absorption (over 600 nm) transition can (Table S5, ESI†) show that all the PDIs (PDI = Mw/Mn) with
be reasonably assigned to the 3ILCT/3MLCT/3LLCT-admixed different feed molar ratios for the two kinds of the grafting-type
transitions.
Ir3+-polymers are in a relatively narrow range (o1.30) due to the
In order to definitely elucidate the NIR-emissive behaviour of AIBN-initiated radical copolymerization.23 Moreover, with regard
the complex monomer [Ir(iqbt)2(vb-ppy)], natural transition to the actual Ir3+-complex-grafting content, the XPS (X-ray photo-
orbital (NTO; Table S4 (ESI†) and Fig. 4) calculations were electron spectroscopy) quantitative analyses reveal that every
further performed on its optimized T1 geometry, where based Ir3+-complex-grafting content is found to be slightly higher than
on the entire (100%) hole - particle transition, the 3ILCT the corresponding initial feeding ratio, which probably arises from
dominated (73.8%) and the less prevalent (13.9%) 3MLCT transi- the loss of oligomeric PVK during the isolation of one specific Ir3+-
tions are responsible for its NIR-emitting phosphorescence.
polymer.24 Furthermore, the PXRD (powder X-ray diffraction) pattern
(Fig. S5, ESI†) of either Poly(NVK-co-[Ir(iqbt)2(vb-ppy)]) or Poly((vinyl-
PBD)-co-NVK-co-[Ir(iqbt)2(vb-ppy)]) just exhibits the PVK-based
amorphous peaks, suggesting the low-concentration homogeneous
dispersion of the monomers [Ir(iqbt)2(vb-ppy)] and/or vinyl-PBD into
the PVK backbone. TG and DSC (differential scanning calorimetry;
Fig. S4, ESI†) results of these grafting-type Ir3+-polymers show that
an improved (Td; 4 400 1C) thermal stability over that (384 1C)
of the complex monomer [Ir(iqbt)2(vb-ppy)] and a desirable Tg
(glass transition temperature) above 160 1C are observed.
The photo-physical properties of the two series of grafting-
type Ir3+-polymers Poly(NVK-co-[Ir(iqbt)2(vb-ppy)]) (100: 1, 150 : 1
or 200 : 1) and Poly((vinyl-PBD)-co-NVK-co-[Ir(iqbt)2(vb-ppy)])
(15 : 150 : 1) were investigated in solid-state or solution at RT,
and the data are summarized in Table S2 (ESI†) and Fig. 5 and
Fig. S6 (ESI†). As shown in Fig. S6 (ESI†), both the DR (diffuse
reflection) and the solution absorption spectra of all the grafting-
type Ir3+-polymers show significantly broader absorption bands
than that of the PVK, in which, besides the strong absorptions
below 400 nm attributed to the p–p* transitions from the organic
portions of PVK and the ligands, the absorptions across the
whole visible range should be assigned to the 1,3LC/1,3MLCT and
S0 - T1 admixed transitions of the grafted complex monomer
[Ir(iqbt)2(vb-ppy)]. Noticeably, owing to the significant spectral
overlap (also Fig. 2) between the absorption of the complex
Synthesis, characterization and photo-physical properties of
the two series of Ir3+-complex-grafted polymers
Considering the excellent physical properties (high thermal
stability, good mechanical intensity and excellent spin-coated
film-formability, etc.) of the semi-conducting PVK as a popular
polymer host,21 the grafting-type Ir3+-polymers Poly(NVK-co-
[Ir(iqbt)2(vb-ppy)]) with different feeding ratios (100 : 1, 150 : 1
or 200 : 1) were synthesized from the AIBN-initiated copolymer-
ization (Scheme 1) of NVK and the complex monomer
[Ir(iqbt)2(vb-ppy)]. As a matter of fact, not only does the PVK
host function as a hole-transport matrix, it with the signifi-
cantly higher T1 level also acts as an effective energy donor to
Fig. 4 The NTO pattern for the T1 - S0 emission of the complex
monomer [Ir(iqbt)2(vb-ppy)] based on its optimized T1 geometry.
J. Mater. Chem. C
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
Journal of Materials Chemistry C
Fig. 5 Normalized emission spectra of the Ir3+-polymers Poly(NVK-co-
[Ir(iqbt)2(vb-ppy)]) (100 : 1, 150 : 1 or 200 : 1) and Poly((vinyl-PBD)-co-
NVK-co-[Ir(iqbt)2(vb-ppy)]) (15 : 150 : 1) in solid-state at RT.
monomer [Ir(iqbt)2(vb-ppy)] and the emission of PVK, effective
22
¨
Forster energy transfer should occur. Convincingly, upon photo-
excitation, the resulting emissions (Fig. 4) of all the grafting-type Ir3+-
polymers do not show the simple addition spectra, but they are
highly related to the stipulated feeding ratio. For the Poly(NVK-co-
[Ir(iqbt)2(vb-ppy)]) (100 : 1 or 150 : 1), photo-excitation gives rise to the
almost entire NIR emission (lem = 696 nm), resembling that (Fig. 2)
of the complex monomer [Ir(iqbt)2(vb-ppy)] in solution. The absence
Fig. 6 Device structures and energy level diagrams for the doping-type
NIR-PLED-I (a) and the grafting-type NIR-PLEDs-II–III (b and c); normal-
ized EL spectra (d); R–J–V (e) and ZEQE–R curves (f).
¨
of the PVK-based blue-light is due to the effective Forster energy
transfer22 from the PVK to the Ir3+-complex-acceptor, giving rise to
the satisfactory FPL of 0.13 (100 : 1) or 0.16 (150 : 1). On further
increasing the feeding ratio up to 200:1, the dual-emitting (FPL
=
efficient NIR-emitting complex monomer [Ir(iqbt)2(vb-ppy)] as
the dopant (5 wt%) for the prototype NIR-PLED-I with the
configuration shown in Fig. 6(a). Attributed to the fact that
the experimental (Fig. S8, ESI†) HOMO (ꢀ5.17 eV) and LUMO
(ꢀ3.03 eV) levels of the complex monomer [Ir(iqbt)2(vb-ppy)]
aligned well within the band gap (ꢀ6.2 to ꢀ5.5 eV of HOMO
and ꢀ2.6 to ꢀ2.0 eV of LUMO) of PVK-PBD, the injected
electrons and holes through the PVK-PBD matrix are first
trapped, and then direct charge trapping29 should occur within
the NIR-emitting Ir(III)-complexes. As expected, as shown in
Fig. 6(d), the electroluminescence spectra of the NIR-PLED-I are
0.21) behaviour associated with the PVK-centered emission at
420 nm and the Ir3+-complex-based NIR emission (lem = 690 nm)
is observed, and the 28 ns of the PVK-centered lifetime together with
the Ir3+-complex-decayed lifetime of 1.29 ms further confirm the dual-
emitting character (Fig. S7, ESI†). Accordingly, based on the
equation25 FET = 1 ꢀ (tDA/tD) (tDA or tD is the donor’s amplitude-
weighted lifetime with and without acceptor, respectively; tD = 44 ns
(lem = 430 nm) for the pure PVK as in the literature26) for Poly(NVK-
¨
co-[Ir(iqbt)2(vb-ppy)]) (200 : 1), the Forster energy transfer FET of 36%
is qualitatively estimated. For comparison, accompanied by the
almost constant and mono-exponential Ir3+-complex-decayed life-
time (1.24 ms) for Poly(NVK-co-[Ir(iqbt)2(vb-ppy)]) (150 : 1) and the
significantly reduced lifetime of 0.97 ms for Poly(NVK-co-[Ir(iqbt)2(vb-
ppy)]) (100 : 1), the facilitated separation of the complex monomers
[Ir(iqbt)2(vb-ppy)] within the PVK backbone should occur at the
lower Ir3+-complex-grafting level (150 : 1 or 200 : 1), from which the
undesirable aggregation-caused quenching (ACQ)27 effect from
the high grafting content (100 : 1) is effectively suppressed. Interest-
ingly, with an appropriate amount of the electron-transport vinyl-PBD
further grafted for the bipolar Poly((vinyl-PBD)-co-NVK-co-[Ir(iqbt)2-
(vb-ppy)]) (15 : 150 : 1), besides the similar Ir3+-complex-based NIR
emission (lem = 693 nm) to that of Poly(NVK-co-[Ir(iqbt)2(vb-ppy)])
(150 : 1), its typical and comparable NIR-emitting phosphorescence
(t = 1.25 ms and FPL = 0.17) is also observed.
voltage-independent while just Ir(III)-complex-related NIR (lem
=
696 and 756 (sh) nm; ca. 70% of the lem Z 700 nm proportion)
emissions well resembled that (also Fig. 2) of the complex
monomer [Ir(iqbt)2(vb-ppy)] in solution. The absence of the
¨
PVK-PBD residual light indicates that the effective Forster
energy transfer22 also takes place within the doping EML upon
electrical driving. For the NIR-PLED-I, upon the turn-on
voltage (Von, defined as the voltage of the output irradiance
(R) = 5.0 W srꢀ1 cmꢀ2) of 9.0 V, as shown in Fig. 6(e), both the
R and the current density (J) monotonically increase with the
increase of the applied bias voltage (V), exhibiting an Rmax of
3772.1 W srꢀ1 cmꢀ2 with a Jmax of 452.8 mA cmꢀ2 at 21.0 V.
Meanwhile, the NIR-PLED-I exhibits the R-regulated waving
max
for the ZEQE (Fig. 6(f)), where the Z
of 4.1% with the
EQE
R = 65.5 W srꢀ1 cmꢀ2 at 12.0 V and about 30% efficiency-roll-
off in the higher radiance range of R = 65.5–3772.1 W srꢀ1 cmꢀ2
are observed. It is worthy of note that due to the contribution from
Device performance of NIR-PLEDs-I–III based on the complex
monomer [Ir(iqbt)2(vb-ppy)] and its Ir3+-polymers
Due to the suitability of PVK-PBD (65 : 30; wt%) with good hole/ more excitons confined within the broadened recombination zone
electron transport as the co-host,28 it is of interest to use the supplemented with the facilitated electron-transport TmPyPB,30
J. Mater. Chem. C
This journal is © The Royal Society of Chemistry 2020

View Article Online
Journal of Materials Chemistry C
Paper
the overall device performance of the NIR-PLED-I is at the top- Poly(NVK-co-[Ir(iqbt)2(vb-ppy)]) (150 : 1) doped with PBD and
level (also Fig. 1) and comparable to the best one7c among the the grafting system of the bipolar Poly((vinyl-PBD)-co-NVK-co-
previously reported NIR-PLEDs.
[Ir(iqbt)2(vb-ppy)]) (15 : 150 : 1) as the EML, reliable NIR-PLEDs-
Considering the almost identical Ir3+-complex-grafted content I–III are realized, respectively. Excitingly, for the NIR-PLED-III
between Poly(NVK-co-[Ir(iqbt)2(vb-ppy)]) (150 : 1) and Poly((vinyl- based on the bipolar Poly((vinyl-PBD)-co-NVK-co-[Ir(iqbt)2(vb-
PBD)-co-NVK-co-[Ir(iqbt)2(vb-ppy)]) (15: 150 : 1) comparable to ppy)]) (15 : 150 : 1), the superior device performance (the ZmEQaEx of
that of the doping system (PVK: PBD : [Ir(iqbt)2(vb-ppy)]; 3.6% and the negligible (o5%) efficiency roll-off) makes bipo-
65: 30 : 5, wt%) for the NIR-PLED-I, the bipolar Ir3+-polymers of lar Ir(III)-complex-grafted polymers a new platform to achieve
Poly(NVK-co-[Ir(iqbt)2(vb-ppy)]) (150: 1) and Poly((vinyl-PBD)-co- high-performance NIR-PLEDs.
NVK-co-[Ir(iqbt)2(vb-ppy)]) (15 : 150 : 1) further doped with PBD
and grafted with vinyl-PBD were used as the EML for the grafting
NIR-PLEDs-II–III (Fig. 6(b and c)), respectively. Through the
Conflicts of interest
further grafting of the vinyl-PBD for the Poly((vinyl-PBD)-co-
NVK-co-[Ir(iqbt)2(vb-ppy)]) (15 : 150 : 1), the electron-transport
There are no conflicts to declare.
promotion is reflected from its experimentally (Fig. S9, ESI†)
stabilized LUMO level (ꢀ3.19 eV) in comparison to that (ꢀ3.08 eV) Acknowledgements
of the Poly(NVK-co-[Ir(iqbt)2(vb-ppy)]) (150 : 1). Excitingly, for both
the NIR-PLED-II with the doping of PBD into Poly(NVK-co-
[Ir(iqbt)2(vb-ppy)]) (150 : 1) and the NIR-PLED-III based on
Poly((vinyl-PBD)-co-NVK-co-[Ir(iqbt)2(vb-ppy)]) (15:150:1), Ir(III)-
complex-exclusive NIR-emissive spectra similar to those of the
NIR-PLED-I or their photo-luminescence results (also Fig. 5) in
solid-states are observed. As compared with the NIR-PLED-I, due
to the deeper LUMO gap between PBD and Poly(NVK-co-
This work was funded by the National Natural Science Founda-
tion (21373160, 21173165), the State Key Laboratory of Struc-
ture Chemistry (20190026), the Guangdong Basic and Applied
Basic Research Foundation (2019A1515110527), the Wisteria
Scientific Research Cooperation Special Project of Northwest
University, the Hong Kong Research Grants Council
(PolyU153058/19P), the Hong Kong Polytechnic University (1-ZE1C,
YW4T) and the Endowed Professorship in Energy from Ms Clarea Au
[Ir(iqbt)2(vb-ppy)]) (150 : 1), the Von of the NIR-PLED-II is up to
max
EQE
(847S) in China.
11.0 V. Moreover, the decreased Z
of 2.5% and the Rmax of
1239.2 W srꢀ1 cmꢀ2 show a good trade off with the significantly
alleviated (ca. 3%) efficiency roll-off within the 12.0–21.0 range,
which should be attributed to the lower carrier-trapping probability
with the better carrier-balance within Poly(NVK-co-[Ir(iqbt)2(vb-ppy)])
(150 : 1). By contrast, using Poly((vinyl-PBD)-co-NVK-co-[Ir(iqbt)2(vb-
Notes and references
1 (a) L. T. Dou, Y. S. Liu, Z. R. Hong, G. Li and Y. Yang, Chem.
Rev., 2015, 115, 12633–12665; (b) A. Rogalski and
K. Chrzanowski, Opto-Electron. Rev., 2002, 10, 111–136.
2 J. Clark and G. Lanzani, Nat. Photonics, 2010, 4, 438–446.
3 F. Ding, Y. Fan, Y. Sun and F. Zhang, Adv. Healthcare Mater.,
2019, 8, 1900260.
4 (a) A. Zampetti, A. Minotto and F. Cacialli, Adv. Funct.
Mater., 2019, 29, 1807623; (b) M. Ibahim-Ouali and
F. Dumur, Molecules, 2019, 24, 1412; (c) C.-L. Ho, H. Li
and W.-Y. Wong, J. Organomet. Chem., 2014, 751, 261–285;
(d) H. F. Xiang, J. H. Cheng, X. F. Ma, X. G. Zhou and
J. J. Chruma, Chem. Soc. Rev., 2013, 42, 6128–6185.
5 J. H. Kim, J. H. Yun and J. Y. Lee, Adv. Opt. Mater., 2018,
6, 1800255.
6 (a) Y. X. Zhang, Q. Li, M. H. Cai, J. Xue and J. Qiao, J. Mater.
Chem. C, 2020, 8, 8484–8492; (b) J. Xue, L. J. Xin, J. Y. Hou,
L. Duan, R. J. Wang, Y. Wei and J. Qiao, Chem. Mater., 2017,
29, 4775–4782; (c) I. Shigeru, Y. Shigeyuki, M. Takeshi,
N. Hiroyuki, F. Hideki, K. Shiro and S. Yoshiaki, Inorg.
Chem. Commun., 2013, 38, 14–19.
¨
ppy)]) (15 : 150 : 1) as the bipolar EML for the NIR-PLED-III, Forster
energy transfer22 and carrier-trapping29 mechanisms also concur-
rently proceed within the TmPyPB-assisted recombination zone.30
Interestingly, for the NIR-PLED-III, besides the low Von at 7.5 V and
the ZmEQaEx up to 3.6% at 9.0 V, the high RMax of 6559.3 W srꢀ1 cmꢀ2
at 21.0 V is at the cost of the highest Jmax of 647.5 mA cmꢀ2
Nonetheless, the superior device performance of the NIR-PLED-
.
max
EQE
III is represented by the Z
of 3.6% (9.0 V) and the weak
(ca. 4%) efficiency roll-off with a preserved ZEQE of 3.4% at 21.0 V,
which means that the high-efficiency of the NIR-PLED-I and the
negligible efficiency roll-off of the NIR-PLED-II are well realized
for the NIR-PLED-III. Importantly, this result makes bipolar
Ir(III)-complex-grafted polymers a conceptual strategy to achieve
high-performance NIR-PLEDs.
4. Conclusions
In summary, through the copolymerization of NVK, the vinyl-
functionalized NIR-emitting monomer [Ir(iqbt)2(vb-ppy)] and/
or the electron-transporting monomer vinyl-PBD, two series of
Ir(III)-complex-grafted polymers Poly(NVK-co-[Ir(iqbt)2(vb-ppy)])
(100 : 1, 150 : 1 or 200 : 1) and Poly((vinyl-PBD)-co-NVK-co-
[Ir(iqbt)2(vb-ppy)]) (15 : 150 : 1) are obtained, respectively. Moreover,
using the doping system of PVK:PBD:[Ir(iqbt)2(vb-ppy)] or
7 (a) Z. Chen, H. Y. Zhang, D. W. Wen, W. H. Wu, Q. G. Zeng,
S. M. Chen and W.-Y. Wong, Chem. Sci., 2020, 11,
2342–2349; (b) C. F. You, D. H. Liu, M. B. Zhu, J. T. Yu,
B. Zhang, Y. Liu, Y. F. Wang and W. G. Zhu, J. Mater. Chem.
C, 2020, 8, 7079–7088; (c) C. F. You, D. H. Liu, F. Y. Meng,
Y. F. Wang, J. T. Yu, S. Wang, S. J. Su and W. G. Zhu, J. Mater.
Chem. C, 2019, 7, 10961–10971; (d) J. Zhou, G. R. Fu,
J. Mater. Chem. C
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
Journal of Materials Chemistry C
Y. N. He, L. N. Ma, W. T. Li, W. X. Feng and X. Q. Lu¨, 14 (a) C.-L. Ho and W.-Y. Wong, Top. Curr. Chem., 2016, 374,
J. Lumin., 2019, 209, 427–434; (e) H. U. Kim, S. Sohn,
W. Choi, M. Kim, S. U. Ryu, T. Park, S. Jung and
1–23; (b) L. Ying, C.-L. Hong, H. B. Wu, Y. Cao and W.-Y.
Wong, Adv. Mater., 2014, 26, 2459–2473.
K. S. Bejoymohandas, J. Mater. Chem. C, 2018, 6, 15 M. Nonoyama, Bull. Chem. Soc. Jpn., 1979, 52, 3749–3750.
10640–10658; ( f ) Y. Liu, Z. R. Han, F. Y. Meng, P. Wang, 16 Z. Zhang, Y. L. He, L. Liu, X. Q. Lu¨, X. J. Zhu, W.-K. Wong,
L. Yang, Y. F. Wang, Y. Pei and S. J. Su, Chem. Phys. Lett.,
M. Pan and C. Y. Su, Chem. Commun., 2016, 52, 3713–3716.
2018, 699, 99–106; (g) S. Kesarkar, W. Mroz, M. Penconi, 17 J. K. Augustine, V. Vairperrumal, S. Narasimhan, P. Alagarsamy
M. Pasini, S. Destri, M. Cazzaniga, D. Ceresoli, and A. Radhakrishnan, Tetrahedron, 2009, 65, 9989–9996.
P. R. Mussini, C. Baldoli, U. Giovanella and A. Bossi, Angew. 18 L. Liu, M. Y. Pang, H. T. Chen, G. R. Fu, B. N. Li, X. Q. Lu¨
Chem., Int. Ed., 2016, 55, 2714–2718; (h) X. S. Cao, J. S. Miao, and L. Wang, J. Mater. Chem. C, 2017, 5, 9021–9027.
M. R. Zhu, C. Zhong, C. L. Yang, H. B. Wu, J. G. Qin and 19 P.-N. Lai, C. H. Brysacz, M. K. Alam, N. A. Ayoub, T. G. Gray,
Y. Cao, Chem. Mater., 2015, 27, 96–104; (i) R. Tao, J. Qiao, J. Bai and T. S. Teets, J. Am. Chem. Soc., 2018, 140, 10198–10207.
´
G. L. Zhang, L. Duan, C. Chen, L. D. Wang and Y. Qiu, 20 P.-N. Lai and T. S. Teets, Chem. – Eur. J., 2019, 25, 6026–6037.
J. Mater. Chem. C, 2013, 1, 6446–6454; ( j) J. Qiao, L. Duan, 21 C. Tang, X. D. Liu, F. Liu, X. L. Wang, H. Xu and W. Huang,
L. T. Tang, L. He, L. D. Wang and Y. Qiu, J. Mater. Chem.,
2009, 19, 6573–6580; (k) E. L. Williams, J. Li and 22 D. R. Martir and E. Zysman-Colman, Coord. Chem. Rev.,
G. E. Jabbour, Appl. Phys. Lett., 2006, 89, 083506. 2018, 364, 86–117.
8 (a) H. U. Kim, H. J. Jang, W. Choi, S. Park, T. Park, J. Y. Lee 23 G. R. Fu, Y. N. He, W. T. Li, T. Z. Miao, X. Q. Lu¨, H. S. He,
and K. S. Bejoymohandas, J. Mater. Chem. C, 2020, 8, L. Liu and W.-Y. Wong, Chem. Sci., 2020, 11, 2640–2646.
4789–4800; (b) Y. N. He, G. R. Fu, W. T. Li, B. W. Wang, 24 G. R. Fu, L. Liu, W. T. Li, Y. N. He, T. Z. Miao, X. Q. Lu¨ and
T. Z. Miao, M. F. Tan, W. X. Feng and X. Q. Lu¨, J. Lumin., H. S. He, Adv. Opt. Mater., 2019, 7, 1900776.
2020, 218, 116847; (c) J. T. Yu, C. Xu, F. Y. Meng, H. Tan, 25 A. Kirch, M. Gmelch and S. Reineke, J. Phys. Chem. Lett.,
M. Q. Liu and W. G. Zhu, Dyes Pigm., 2019, 166, 307–313; 2019, 10, 310–315.
(d) Z. R. Hao, M. Li, Y. J. Liu, Y. F. Wang, G. H. Xie and 26 Z.-H. Wang, H. Lee and H.-N. Cui, J. Appl. Phys., 2012,
Y. Liu, Dyes Pigm., 2018, 149, 315–322; (e) G. R. Fu, H. Zheng, 111, 023512.
Y. N. He, W. T. Li, X. Q. Lu¨ and H. S. He, J. Mater. Chem. C, 27 M. Mauro and C. Cebrian, Isr. J. Chem., 2018, 58, 901–914.
Macromol. Chem. Phys., 2013, 241, 314–342.
2018, 6, 10589–10596.
28 (a) Z. R. Hao, H. G. Jiang, Y. Liu, Y. M. Zhang, J. T. Yu,
Y. F. Wang, H. Tan, S. J. Su and W. G. Zhu, Tetrahedron,
2016, 72, 8542–8549; (b) V. Jankus, K. Abdullah, G. C.
Griffiths, C. Gareth, H. Al-Attar, Y. H. Zheng, M. R. Bryce
and A. P. Monkman, Org. Electron., 2015, 20, 97–102;
(c) H. Al-Attar, G. C. Griffiths, T. N. Tom, M. Tavasli,
M. A. Fox, M. R. Bryce and A. P. Monkman, Adv. Funct.
Mater., 2011, 21, 2376–2382.
9 (a) L. J. Xin, J. Xue, G. T. Lei and J. Qiao, RSC Adv., 2015, 5,
42354–42361; (b) R. Tao, J. Qiao, G. L. Zhang, L. Duan,
L. D. Wang and Y. Qiu, J. Phys. Chem. C, 2012, 116,
11658–11664.
10 J. V. Caspar and T. J. Meyer, J. Phys. Chem., 1983, 87,
952–957.
11 Y. M. Zhang, Y. F. Wang, J. Song, J. L. Qu, B. H. Li, W. G. Zhu
and W.-Y. Wong, Adv. Opt. Mater., 2018, 6, 1800466.
12 F. Xu, H. U. Kim, J. Kim, B. J. Jung, A. C. Grimsdale and
D. Hwang, Prog. Polym. Sci., 2015, 47, 92–121.
29 (a) T. Y. Li, J. Wu, Z. G. Wu, Y. X. Zheng, J. L. Zuo and Y. Pan,
Coord. Chem. Rev., 2018, 374, 55–92; (b) X. L. Yang, G. J. Zhou
and W.-Y. Wong, Chem. Soc. Rev., 2015, 44, 8484–8575.
13 (a) Q. Zhao, S.-J. Liu and W. Huang, Macromol. Rapid Commun., 30 (a) T. Earmme and S. A. Henehke, J. Mater. Chem. C, 2012,
2010, 31, 794–807; (b) V. Marin, E. Holder, R. Hoogenboom and
U. S. Schubert, Chem. Soc. Rev., 2007, 36, 618–635.
22, 4660–4668; (b) T. L. Te, S. Y. Shao, J. S. Chem, L. X. Wang
and D. G. Ma, ACS Appl. Mater. Interfaces, 2011, 3, 410–416.
J. Mater. Chem. C
This journal is © The Royal Society of Chemistry 2020