A novel near-infrared-emitting cyclometalated platinum (II) complex with donor-acceptor-acceptor chromophores

Article abstract of DOI:10.1016/j.dyepig.2014.03.040

A novel near-infrared-emitting cyclometalated platinum (II) complex of (TPA-BT-Q)Ptpic with a donor-acceptor-acceptor (D-A-A) chromophores was synthesized and characterized, in which the TPA-BT-Q unit is a cyclometalated ligand of N,N-di(4-octyloxyphenyl)

Full text of DOI:10.1016/j.dyepig.2014.03.040

Dyes and Pigments 107 (2014) 146e152
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
A novel near-infrared-emitting cyclometalated platinum (II) complex
with donoreacceptoreacceptor chromophores
Junting Yu a, Keqi He a, Yanhu Li b, Hua Tan a, Meixiang Zhu a, Yafei Wang a, Yu Liu a,
,
b,
*
Weiguo Zhu a , Hongbin Wu
*
a College of Chemistry, Key Lab of Environment-Friendly Chemistry and Application in the Ministry of Education, Xiangtan University, Xiangtan 411105, PR
China
b Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel near-infrared-emitting cyclometalated platinum (II) complex of (TPA-BT-Q)Ptpic with a donor
eacceptoreacceptor (DeAeA) chromophores was synthesized and characterized, in which the TPA-BT-Q
unit is a cyclometalated ligand of N,N-di(4-octyloxyphenyl)-4-(7-(quinolin-2-yl)-benzo[c][1,2,5]thiadia-
zol-4-yl)-phenylamine and pic is picolinate anion. Its optophysical, electrochemical and electrolumi-
nescent characteristics were primarily studied. An intense UVevis absorption peak at 540 nm and a
strong near-infrared emission peak at 759 nm were observed for (TPA-BT-Q)Ptpic in dichloromethane.
Using (TPA-BT-Q)Ptpic as a single dopant and a blend of poly(vinylcarbazole) and 2-tert-butylphenyl-5-
biphenyl-1,3,4-oxadiazole as a host matrix, the single-emissive-layer polymer light-emitting devices
exhibited a near-infrared emission peaked at 760 nm with the maximum external quantum efficiency of
Received 28 January 2014
Received in revised form
22 March 2014
Accepted 29 March 2014
Available online 5 April 2014
Keywords:
Donoreacceptoreacceptor
Platinum (II) complexes
Near-infrared emission
Electrophosphorescence
Polymer light-emitting devices
Synthesis
0.12% at 16.6 mA cmꢀ2 and a radiance intensity of 112 W cmꢀ2 at 11.7 mA cmꢀ2 at the doping con-
m
centrations of 2.0 wt%. This work provides an efficient approach to realize near-infrared electro-
phosphorescent emission with high radiance intensity by employing platinum (II) complex with the DeA
eA structure.
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
organic light-emitting devices (OLEDs) with NIR emission in the
760e780 nm range. However, these EQE levels were typically
Recently, the research on near-infrared (NIR) organic lumi-
nescent materials has gained great attentions due to their diverse
potential applications in night-vision displays, sensors, optical
communication, and offering superior biocompatibility for med-
ical systems [1]. The developed NIR-emitting materials mostly
contain lanthanide complexes [2], fluorescent materials with a
donoreacceptor (DeA) structure [3], boron dipyrromethene dyes
[4] and transition-metal complexes [5]. Among these materials,
transition-metal complexes are available to exhibit higher emis-
sion efficiency due to their strong spin-orbit coupling in the
presence of heavy metals, which leads to an internal quantum
efficiency as high as 100% [6]. Metalloporphyrin is the most
typical class in the reported transition-metal complexes and has
recorded an external quantum efficiency (EQE) maximum of
2.49% for polymer light-emitting devices (PLEDs) and 9.2% for
obtained at very low current densities [7]. It is well known that
squareplanar platinum (II) complexes have rapidly developed in
OLEDs with high-efficiency red, green, blue and even white
emission by tuning their molecular structures [8]. But, few plat-
inum (II) complexes besides Pt-porphyrin complexes have dis-
played satisfactory EQE level in NIR emission. In order to develop
new NIR-emitting platinum (II) complexes, Gao et al. reported a
platinum complex of ppyPtq, which displayed an EL emission
peaked at 730 nm without EQE datum [5b]. Che et al. reported a
series of neutral platinum complexes containing substituted 8-
hydroxyquinoline, which gave a deep-red emission peak from
650 to 695 nm with another weak NIR emission peak from 705 to
755 nm in the device with an EQE of 1.7% [5e]. Recently, some
NIR-emitting organic and polymeric fluorescent materials with
donor (D) and acceptor (A) chromophores were developed
because the band gap levels and photoelectronic properties can
be readily tuned through a systematic variation between the D
and A units [1,9]. For example, Wang et al. reported a class of
* Corresponding authors. Tel.: þ86 731 58298280; fax: þ86 731 58292251.
E-mail addresses: zhuwg18@126.com (W. Zhu), hbwu@scut.edu.cn (H. Wu).
fluorescent materials with DepeAepeD type chromophore,
http://dx.doi.org/10.1016/j.dyepig.2014.03.040
0143-7208/Ó 2014 Elsevier Ltd. All rights reserved.

J. Yu et al. / Dyes and Pigments 107 (2014) 146e152
147
which displayed emission exclusively at 1080 nm with an EQE of
0.28% in the OLEDs [10]. Reynolds et al. reported a family of
conjugated oligomers with a multi-heterocycle DeAeD structure,
which displayed emission ranging from 651 to 1088 nm with an
EQE of 0.87% in the PLEDs [3a].
oxadiazole (PBD) as the host matrix, we fabricated the single-
emissive-layer (SEL) PLEDs by solution process and studied the
device performances. A NIR emission peaked at 760 nm with a
maximum EQE of 0.12% at 16.6 mA cmꢀ2 and an irradiance intensity
of 112
m
W cmꢀ2 (obtained at an applied current density of
As phosphorescent materials have exhibited higher external
quantum efficiency than fluorescent materials in OLEDs, it was al-
ways interesting in developing NIR-emitting platinum complexes.
In order to study effect of molecular structure of platinum com-
plexes on NIR-emitting property, a type of platinum complexes of
(Piq-G)Pt(acac) with DeA chromophores was obtained in our pre-
vious work, which exhibited an emission with a peak at 640 nm and
a shoulder at 700 nm [11]. There was about 40e60 nm red-shift
compared to that from the non-functionalized platinum (II) com-
plex of (Piq)Pt(acac). Based on this findings, we here designed
another novel platinum (II) complex of (TPA-BT-Q)Ptpic with DeAe
A type chromophores, in which a triphenylamine (TPA) was used as
an electron donor unit, a benzothiadiazole (BT) and a quinoline (Q)
were simultaneously employed as strong electron acceptor units. In
this (TPA-BT-Q)Ptpic, the non-planar TPA unit is available to
improve carrier-transporting properties and suppress aggregations,
the BT unit is a good class of luminescence units in OLEDs and
acceptor units in organic solar cells (OSCs) reported in recent years
[12], two alkoxy groups are benefit to improve solubility. Therefore,
the (TPA-BT-Q)Ptpic with DeAeA units should provide low-energy
near-infrared emission more easily than those counterparts with
DeA units. The synthetic route of (TPA-BT-Q)Ptpic is shown in
Scheme 1. For comparison, the counterpart of PQPtpic was made.
Using (TPA-BT-Q)Ptpic as a single dopant and a blend of poly-
(vinylcarbazole) (PVK) and 2-tert-butylphenyl-5-biphenyl-1,3,4-
11.7 mA cmꢀ2) were observed in the device at 2.0 wt% dopant
concentration. This work indicates that introducing DeAeA struc-
ture is an efficient approach to constructure NIR-emitting platinum
(II) complex and obtained high-efficiency NIR-emitting PLEDs with
high radiance intensity.
2. Experimental
2.1. General information
The solvents were carefully dried and distilled by standard
procedures before use. All chemicals, unless otherwise stated were
obtained from commercial sources and used as received. The
Suzuki coupling and cyclometalated reactions were carried out in
inert gas atmosphere and monitored by thin-layer chromatography
(TLC). 1H NMR spectra was recorded with a Bruker Dex-400 NMR
instrument using CDCl3 as a solvent. Elemental analysis was carried
out with a Harrios elemental analysis instrument. Mass spectrum
was recorded on a Voyager Depro MALDI-TOF spectrometer. UVe
vis absorption and photoluminescent spectra were recorded with a
Shimadzu UV-265 spectrophotometer and a PerkineElmer LS-50
luminescence spectrometer, respectively. Thermogravimetric
analysis (TGA) was conducted under a dry nitrogen gas flow at a
heating rate of 20 ꢁC minꢀ1 on a PerkineElmer TGA 7 instruments.
Surface morphologies were recorded by AFM on a Veeco, DI
Scheme 1. Synthetic route of (TPA-BT-Q)Ptpic and PQPtpic.

148
J. Yu et al. / Dyes and Pigments 107 (2014) 146e152
Multimode NS-3D apparatus in a trapping mode under normal air
condition at room temperature (RT). Cyclic voltammetry was per-
formed on a CHI600E electrochemical work station with a scan rate
of 100 mV sꢀ1 at room temperature under argon, in which a Pt disk,
Pt plate, and Ag/AgCl electrode were used as working electrode,
counter electrode, and reference electrode in n-Bu4NPF6 (0.1 M)
acetonitrile solution, respectively. For calibration, the redox po-
tential of ferrocene/ferrocenium (Fc/Fcþ) was measured under the
same conditions.
2.3.2. N,N-di(4-octyloxyphenyl)-4-(7-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)benzenamine (3)
A mixture of compound 2 (0.60 g, 0.84 mmol), bis(pinacolato)
diboron (1.71 g, 6.72 mmol), potassium acetate (0.82 g, 8.40 mmol)
and Pd(dppf)Cl$CH2Cl2 adduct (30 mg, 0.04 mmol) in anhydrous
tetrahydrofuran (50 mL) was heated at 60 ꢁC at nitrogen atmo-
sphere for 12 h. After cooled to RT, the mixture was poured into
water (150 mL) and extracted with DCM (3 ꢂ 30 mL). The combined
organic layer was dried over MgSO4 and then concentrated under
reduced pressure. The residue was purified by silica gel column
chromatography using DCM/petroleum ether (V/V ¼ 1/1) as eluent
to gain red viscous compound 3 (0.40 g, 62.9%). 1H NMR (400 MHz,
CDCl3, ppm): 8.26 (d, J ¼ 6.8 Hz, 1H), 7.84 (d, J ¼ 8.4 Hz, 2H), 7.72 (d,
J ¼ 6.8 Hz, 1H), 7.14 (d, J ¼ 8.6 Hz, 4H), 7.05 (d, J ¼ 8.6 Hz, 2H), 6.87
(d, J ¼ 8.6 Hz, 4H), 3.96e3.93 (t, J ¼ 6.0 Hz, 4H), 1.80e1.77 (m, 4H),
1.45e1.25 (br, 32H), 0.89e0.88 (t, J ¼ 3.2 Hz, 6H). MALDI-TOF MS
(m/z) for C46H60BN3O4S, Calcd: 761.440; Found, 761.396.
The cyclometalated platinum (II) complexes were also synthe-
sized according to the literature procedures [13]. A mixture of
^
K2PtCl4, CN-chelate ligand of TPA-BT-Q (or PQ) (1.2 equiv.), 2-
ethoxyethanol and distilled water (3:1, V/V) was stirred under ni-
trogen atmosphere at 80 ꢁC for 24 h. After cooled to RT, the pre-
cipitate was formed, then collected by filtration and washed
successively with water, ethanol and hexane, respectively. The
chloro-bridged dimer was obtained. This dimer was mixed with
picolinic acid (2.5 equiv.) and anhydrous Na2CO3 (6 equiv.) in 2-
ethoxyethanol. The resulting mixture was stirred under nitrogen
atmosphere at 100 ꢁC for 16 h. After cooled to RT, the resulting
precipitate was collected by filtration and washed successively with
water, ethanol and hexane, respectively. The residue was purified
by flash chromatography on silica gel using DCM/ethyl acetate (V/
V ¼ 5/1) as eluent to provide the cyclometalated platinum (II)
complexes.
2.3.3. N,N-di(4-octyloxyphenyl)-4-(7-(quinolin-2-yl)benzo[c][1,2,5]
thiadiazol-4-yl)benzenamine (TPA-BT-Q, 4)
A
mixture of compound 3 (0.30 g, 0.39 mmol), 2-
bromoquinoline (97 mg, 0.47 mmol), K2CO3 (6 mL, 2 M) and
Pd(PPh3)4 (20 mg, 0.02 mmol) in toluene (35 mL) and methanol
(6 mL) was heated at 80 ꢁC at nitrogen atmosphere for 12 h. After
cooled to RT, the mixture was poured into water (100 mL) and
extracted with DCM (3 ꢂ 30 mL). The combined organic layer was
dried over MgSO4 and then concentrated under reduced pressure.
The residue was purified by silica gel column chromatography us-
ing DCM/petroleum ether (V/V ¼ 1/2) as eluent to gain red viscous
compound 4 (0.24 g, 79.7%). 1H NMR (400 MHz, CDCl3, ppm): 8.75
(d, J ¼ 8.2 Hz, 1H), 8.65 (d, J ¼ 7.0 Hz, 1H), 8.36 (d, J ¼ 8.4 Hz, 1H),
8.23 (d, J ¼ 8.0 Hz, 1H), 7.91e7.86 (br, 4H), 7.79e7.75 (t, J ¼ 7.2 Hz,
1H), 7.60e7.57 (t, J ¼ 6.6 Hz, 1H), 7.15 (d, J ¼ 7.6 Hz, 4H), 7.08 (d,
J ¼ 7.8 Hz, 2H), 6.87 (d, J ¼ 7.8 Hz, 4H), 3.96e3.93 (t, J ¼ 6.4 Hz, 4H),
1.80e1.78 (m, 4H), 1.47e1.26 (br, 20H), 0.91e0.89 (t, J ¼ 3.8 Hz, 6H).
MALDI-TOF MS (m/z) for C49H54N4O2S, Calcd: 762.397; Found,
762.376.
2.2. PLEDs fabrication
The single-emissive-layer (SEL) PLEDs using (TPA-BT-Q)Ptpic as
dopant was fabricated by spin-coating and vacuum thermal evap-
oration. The device configuration is ITO/PEDOT:PSS, 40 nm/PVK-
30 wt% PBD:dopant, 80 nm/CsF, 1.5 nm/Al, 100 nm, where indium
tin oxide (ITO) acts as the anode, poly(3,4-ethylenedioxy
thiophene):poly(styrenesulfonate) (PEDOT:PSS) is used as an
anode buffer layer at the interface of ITO, LiF and Al are employed as
electron injection layer and cathode layer, respectively. The light-
emitting layer consists of PVK, PBD and dopant, where PVK acts
as the host material due to its excellent film-forming and hole-
transporting properties. To facilitate electron transport in the
light-emitting layer, PBD is simultaneously mixed with PVK. The
weight ratio of PVK and PBD is 70:30 (W/W). The dopant concen-
trations vary from 0.5 wt% to 8.0 wt%. The irradiance intensity of the
NIR PLEDs is measured by an integrating spheres coupled with UDT
A370 spectrometer.
2.3.4. (TPA-BT-Q)Ptpic (5)
(TPA-BT-Q)Ptpic was synthesized by the common method
described above for the general synthesis of the platinum complex
and obtained as violet black powder in a yield of 43.4%. 1H NMR
(400 MHz, CDCl3, ppm): 9.41 (d, J ¼ 8.8 Hz, 1H), 9.33 (d, J ¼ 8.8 Hz,
1H), 9.17e9.15 (t, J ¼ 4.6 Hz, 1H), 8.33 (d, J ¼ 8.8 Hz, 1H), 8.22e8.19
(t, J ¼ 6.8 Hz, 1H), 8.16e8.13 (t, J ¼ 7.4 Hz, 1H), 7.87e7.81 (br, 3H),
7.74e7.72 (t, J ¼ 4.0 Hz, 1H), 7.67 (d, J ¼ 6.4 Hz, 2H), 7.54e7.51 (t,
J ¼ 7.4 Hz,1H), 7.16 (d, J ¼ 8.8 Hz, 4H), 7.03 (d, J ¼ 7.8 Hz, 2H), 6.88 (d,
J ¼ 8.8 Hz, 4H), 3.97e3.94 (t, J ¼ 6.4 Hz, 4H), 1.81e1.78 (m, 4H),
1.48e1.28 (br, 20H), 0.91e0.88 (t, J ¼ 3.4 Hz, 6H). MALDI-TOF MS
(m/z) for C55H57N5O4PtS, Calcd: 1078.378; Found, 1078.375. Anal.
Calc. for C55H57N5O4PtS: C 61.21, H 5.32, N 6.49, S 2.97% Found: C
61.51, H 5.22, N 6.11, S 2.90%.
2.3. Syntheses
2.3.1. N,N-di(4-octyloxyphenyl)-4-(7-bromobenzo[c][1,2,5]
thiadiazol-4-yl)benzenamine (2)
A
mixture of compound 1 (1.00 g, 1.59 mmol), 4,7-
dibromobenzo[c][1,2,5]thiadiazole (0.56 g, 1.91 mmol), K2CO3
(8 mL, 2 M) and tetrakis(triphenylphosphine) palladium (60 mg,
0.05 mmol) in toluene (50 mL) and methanol (8 mL) was heated at
80 ꢁC under nitrogen atmosphere for 12 h. After cooled to RT, the
mixture was poured into water (150 mL), extracted with DCM
(3 ꢂ 30 mL). The combined organic layer was dried over MgSO4 and
then concentrated under reduced pressure. The residue was puri-
fied by silica gel column chromatography using DCM/petroleum
ether (V/V ¼ 1/5) as eluent to gain red viscous compound 2 (0.85 g,
60.7%). 1H NMR (400 MHz, CDCl3, ppm): 7.88 (d, J ¼ 7.6 Hz, 1H), 7.74
(d, J ¼ 8.4 Hz, 2H), 7.51 (d, J ¼ 7.6 Hz,1H), 7.13 (d, J ¼ 8.6 Hz, 4H), 7.03
(d, J ¼ 8.4 Hz, 2H), 6.86 (d, J ¼ 8.6 Hz, 4H), 3.96e3.93 (t, J ¼ 6.4 Hz,
4H), 1.85e1.75 (m, 4H), 1.45e1.26 (br, 20H), 0.89e0.88 (t, J ¼ 3.2 Hz,
6H).
2.3.5. 2-phenylquinoline (PQ, 6)
2-phenylquinoline was synthesized according to the above
procedure of compound 4 and obtained as white solid in a yield of
82.4%. 1H NMR (400 MHz, CDCl3, ppm): 8.24 (d, J ¼ 8.6 Hz, 1H),
8.19e8.16 (br, 3H), 7.90 (d, J ¼ 8.4 Hz, 1H), 7.85 (d, J ¼ 8.0 Hz, 1H),
7.75e7.71 (t, J ¼ 7.4 Hz, 1H), 7.55e7.52 (br, 3H), 7.47e7.45 (t,
J ¼ 7.2 Hz, 1H).
2.3.6. PQPtpic (7)
PQPtpic was synthesized according to the above general pro-
cedure of platinum complex and obtained as an orange powder in a
yield of 45.9%. 1H NMR (400 MHz, CDCl3, ppm): 9.47 (d, J ¼ 8.8 Hz,

J. Yu et al. / Dyes and Pigments 107 (2014) 146e152
149
1H), 9.23 (d, J ¼ 5.4 Hz, 1H), 8.33 (d, J ¼ 8.4 Hz, 1H), 8.25 (d,
J ¼ 7.4 Hz, 1H), 8.16e8.14 (t, J ¼ 4.2 Hz, 1H), 8.00e7.89 (t, J ¼ 3.4 Hz,
1H), 7.84e7.82 (t, J ¼ 4.4 Hz, 1H), 7.80e7.78 (t, J ¼ 4.0 Hz, 1H), 7.63e
7.60 (br, 3H), 7.34e7.26 (t, J ¼ 5.4 Hz, 1H), 7.24 (d, J ¼ 3.4 Hz, 2H).
MALDI-TOF MS (m/z) for C21H14N2O2Pt, Calcd: 521.070; Found,
521.110.
TPA-BT-Q free ligand due to incorporation of the substituent
PQPtpic acceptor unit.
The PL spectra of (TPA-BT-Q)Ptpic, TPA-BT-Q and PQtpic in dilute
DCM are displayed at RT in Fig. 2. The corresponding data are
summarized in Table 1. Under photo-excitation at 390 nm, the
parent complex of PQPtpic displayed two intrinsic structured
emission peaks at 555 nm and 590 nm, as well as the slightly
structureless excimer emission at 705 nm. This inherent emission is
attributed to a mixed emission from MLCT and LC state [13a].
However, TPA-BT-Q and (TPA-BT-Q)Ptpic exhibited a wide red and
NIR emission profiles with a peak at 580 nm and 760 nm under
photo-excitation at 400 nm and 540 nm, respectively. Compared to
PQPtpic and TPA-BT-Q, (TPA-BT-Q)Ptpic presented a bathochromic
PL spectrum by 170 nm due to the additional DeA and stronger De
AeA interactions, respectively. This indicates that the intra-
molecular DeA effect is available to make its platinum complexes
exhibit red-shifted PL spectra.
3. Results and discussion
3.1. Synthesis and characterization
Compound 1 was prepared according to the literature pro-
cedures [14]. Compound 2, TPA-BT-Q and PQ were synthesized
through Suzuki couplings. The (TPA-BT-Q)Ptpic and PQPtpic were
synthesized using the previous method with two-step procedures,
which contain a cyclometalation of TPA-BT-Q, PQ and a cleavage of
the chloride groups in the corresponding dimers with picolinic
acid. The made (TPA-BT-Q)Ptpic and PQPtpic were characterized by
1H NMR, MALDI-TOF mass spectra and element analysis to confirm
their well-defined chemical structures.
3.3. Thermal properties and dispersibility
Thermal properties of (TPA-BT-Q)Ptpic and PQPtpic were char-
acterized by thermal gravimetric analysis (TGA) under a nitrogen
atmosphere with a scanning rate of 20 ꢁC minꢀ1. The recorded TGA
curves are shown in Fig. S1 (see Supporting Information, SI). The
onset decomposition temperatures for 5% weight loss (Td) were
287 ꢁC and 202 ꢁC for (TPA-BT-Q)Ptpic and PQPtpic, respectively. It
indicates that (TPA-BT-Q)Ptpic has higher thermal stability.
To clarify the dispersibility of this platinum complex in the
polymer matrix, the (TPA-BT-Q)Ptpic-doped PVK-PBD films at
different doping concentrations from 0.5 wt% to 8.0 wt% were
made. The surface morphology was recorded by atomic force mi-
croscopy (AFM) and is shown in Fig. S2 (see the SI). Roughness
value of Ra ¼ 0.287 nm, 0.274 nm, 0.262 nm, 0.264 nm and
0.321 nm were observed at doping concentrations of 0.5 wt%,1.0 wt
%, 2.0 wt%, 4.0 wt% and 8.0 wt%, respectively. This result means that
the DeAeA type platinum (II) complexes exhibited a good dis-
persibility in the PVK-PBD matrix at these given doping
concentrations.
3.2. Photophysical properties
Fig. 1 shows the normalized UVevis spectrum of (TPA-BT-Q)
Ptpic in dichloromethane (DCM). For comparison, the normalized
UVevis spectra of the TPA-BT-Q free ligand and the parent plat-
inum (II) complex of PQPtpic in DCM were insetted in Fig. 1. Three
typical absorption peaks at 314 nm, 371 nm and 540 nm were
observed for (TPA-BT-Q)Ptpic. The intense high-lying absorption
peak is ascribed to the spin-allowed ligand-central (LC)
pep*
transitions of TPA-BT-Q, the moderate-lying one from 371 nm to
450 nm is mainly attributed to the mixed spin-allowed and spin-
forbidden singlet metal-to-ligand charge transfer (1MLCT and
3MLCT) transitions. The intense and broad low-lying one around
540 nm with an extinction coefficient (
3
) of 1.5 ꢂ 105 L molꢀ1 cmꢀ1
is assigned to the intramolecular charge transfer (ICT) transition
from TPA unit to the BT and PQPtpic chromophores. Compared to
the TPA-BT-Q free ligand, (TPA-BT-Q)Ptpic exhibited a significant
red-shift low-lying absorption peak (ca. 65 nm), which is due to the
additional MLCT transition and enhanced electron acceptor effect
of the PQPtpic chromophore. The red-shift low-lying absorption
peak implies that (TPA-BT-Q)Ptpic with DeAeA architecture has an
extensional conjugated system and more intense ICT effect than the
3.4. Electrochemical properties
The redox properties of (TPA-BT-Q)Ptpic was characterized by
cyclic voltammetry (CV) method using ferrocene as an internal
Fig. 1. Normalized UVevis absorption spectra of TPA-BT-Q, (TPA-BT-Q)Ptpic and
PQPtpic in DCM at RT.
Fig. 2. Normalized PL spectra of TPA-BT-Q, (TPA-BT-Q)Ptpic and PQPtpic in DCM at RT.

150
J. Yu et al. / Dyes and Pigments 107 (2014) 146e152
Table 1
the PVK-PBD. With further increasing dopant concentrations, the
PVK-PBD emission was gradually decreased, but the (TPA-BT-Q)-
based LC and (TPA-BT-Q)Ptpic emissions were gradually enhanced.
When the dopant concentration reached to 2.0 wt%e8.0 wt%, the
PVK-PBD emission was quickly quenched and strong NIR emission
from (TPA-BT-Q)Ptpic at 764 nm was observed with a shoulder at
630 nm.
Photophysical, thermal and electrochemical properties of (TPA-BT-Q)Ptpic.
UVevis
l
/nma
PL
l
/nma
Td (ꢁC)c
EHOMO/eVd
ELUMO/eVd
b
(
3
max/L molꢀ1 cmꢀ1
)
314 (310,526),
371 (144,737),
540 (152,632)
759
287
ꢀ4.88
ꢀ2.91
a
Measured in DCM at 298 K at a concentration of 10ꢀ6 mol Lꢀ1
.
The EQE versus current density characteristic of the (TPA-BT-Q)
Ptpic-doped devices is shown in Fig. 4 at different dopant con-
centrations from 0.5 wt% to 8.0 wt%. The corresponding device
performance data are summarized in Table 2. The EQEs values were
decreased with increasing dopant concentrations. The maximum
EQEs of 0.06% at current density of 24.4 mA cmꢀ2 and 0.12% at
16.6 mA cmꢀ2 were observed in the device at the dopant concen-
trations of 4.0 wt% and 2.0 wt%, respectively. In order to understand
why the roll-off of EQEs was occurred with increasing dopant
concentrations, we measured the PL efficiency (FPL) of the (TPA-BT-
Q)Ptpic-doped PVK-PBD blend films at the different dopant con-
centrations from 0.5 wt% to 8.0 wt%. The FPL values of 8.1%, 4.6% and
1.4% were obtained at the dopant concentrations of 0.5 wt%, 1.0 wt%
and 2.0 wt%, respectively. However, the FPL values at 4.0 wt% and
8.0 wt% dopant concentrations were not detected by the instru-
ment. It is sure that the FPL values are also decreased in the blend
films with increasing dopant concentrations. Therefore, the
decreasing EQEs with increasing dopant concentrations here is
related to the concentration quenching of (TPA-BT-Q)Ptpic. How-
ever, the EQE level displayed small roll-off at high current densities,
which is favorable for practical applications of OLEDs [17]. We
noted that the Pt-metalloporphyrin complexes-based devices
usually gave the same phenomenon because this class of planar
platinum (II) complexes has relatively long lifetimes which easily
result in the dominant exciton decay channel by tripletetriplet
annihilation [7,18]. In this case, the (TPA-BT-Q)Ptpic with the DeAe
A structure containing the bulky non-planar TPA unit effectively
exhibited a suppressed tripletetriplet annihilation at high current
densities. The TPA group with alkoxy chain may exert a positive
b
c
Molar extinction coefficient.
Temperature at 5% weight loss measured by TGA at a heating rate of 20 ꢁC minꢀ1
under nitrogen.
d
EHOMO ¼ ꢀ(4.40 þ Eox) eV, ELUMO ¼ ꢀ(4.40 þ Ered) eV.
standard. An irreversible oxidation wave (Eox) at 0.48 eV and a
reversible reduction wave (Ered) at ꢀ1.49 eV were observed versus
Fc/Fcþ (see supporting information, Fig. S3). According to the re-
ported literature, the oxidation originates from the metal center
[13a]. On the basis of Eox and Ered values, we can calculate the
highest occupied molecular orbital (HOMO) and the lowest unoc-
cupied molecular orbital (LUMO) energy levels (EHOMO and ELUMO
)
of (TPA-BT-Q)Ptpic based on the empirical formula [15]. The
resulting CV data are summarized in Table 1. As the LUMO and
HOMO energy levels were ꢀ2.20 eV/ꢀ5.80 eV for PVK
and ꢀ2.46 eV/ꢀ6.20 eV for PBD [8b], (TPA-BT-Q)Ptpic exhibited a
matched energy level with the PVK-PBD blend, which is available
for (TPA-BT-Q)Ptpic to play a carrier trap role in the PVK-PBD-
hosted PLEDs. In order to conveniently analyze the energy trans-
fer of the guest and host in the PLEDs, the HOMOeLUMO energy
levels of all materials used and the device configuration are shown
in Fig. S4 (see the SI).
3.5. Electroluminescence properties
Fig. 3 shows the electroluminescent (EL) spectra of the (TPA-BT-
Q)Ptpic-doped devices at different dopant concentrations from
0.5 wt% to 8.0 wt%. Three distinct EL peaks at about 429 nm, 630 nm
and 764 nm were observed, which are assigned to the PVK-PBD, the
(TPA-BT-Q) ligand center (LC) and (TPA-BT-Q)Ptpic emissions
compared to the EL spectra of the (TPA-BT-Q)-doped devices in
Fig. S5, as well as the corresponding PL spectra of the (TPA-BT-Q)-
based DCM solution at an excitation wavelength of 400 nm in Fig. 2
and at different excitation wavelengths in Fig. S6, respectively [16].
At low doping concentration of 0.5 wt%, the EL was dominated by
effect for the sluggish roll-off of the EQEs due to reducing
p-
stacking of dopants and improving dispersibility at the same times.
The current densities (J)evoltage (V) characteristics of the (TPA-
BT-Q)Ptpic-doped PVK-PBD devices are shown in Fig. 5 and the EL
data are summarized in Table 2. The turn-on voltage of the devices
increased from 7.3 V to 14.2 V with increasing doping levels from
0.5 wt% to 8.0 wt%. It indicates that the devices are mainly operated
Fig. 3. EL spectra of the (TPA-BT-Q)Ptpic-doped PLEDs at dopant concentrations from
Fig. 4. The external quantum efficiency versus current density characteristics of the
0.5 to 8.0 wt%.
(TPA-BT-Q)Ptpic-doped PLEDs.

J. Yu et al. / Dyes and Pigments 107 (2014) 146e152
151
Table 2
The EL parameters of the (TPA-BT-Q)Ptpic-doped PLEDs.
Dopant (wt%)
0.5
1.0
2.0
4.0
8.0
Von (V)a
7.3
8.8
10.7
427,637,760
0.12
16.61
112
14.2
422,637,762
0.06
24.42
53
14.2
655,766
0.03
47.62
29
lEL (nm)b
432,625,759
0.16
11.36
153
429,633,760
0.14
12.41
141
EQEmax (%)c
J (mA cmꢀ2
)
d
R (m )
W cmꢀ2
e
a
Von: turn-on voltage at 1 cd cmꢀ2
lEL: the maximum EL emission peak.
.
b
c
EQEmax: the maximum external quantum efficiency.
Current density at maximum EQE.
d
e
Radiant intensity obtained at an applied current density of 11.7 mA cmꢀ2
.
Postgraduate Science Foundation for Innovation in Hunan Province
(CX2012B249).
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2014.03.040.
References
[1] (a) Qian G, Wang ZY. Near-infrared organic compounds and emerging appli-
cations. Chem Asian J 2010;5:1006e29;
(b) Xiang HF, Cheng JH, Ma XF, Zhou XG, Chruma JC. Near-infrared phos-
phorescence: materials and applications. Chem Soc Rev 2013;46:6128e85.
[2] (a) Eliseeva SV, Bünzli JCG. Lanthanide luminescence for functional materials
and bio-sciences. Chem Soc Rev 2010;39:189e227;
(b) Artizzu F, Mercuri ML, Serpe A, Deplano P. NIR-emissive erbium-
quinolinolate complexes. Coord Chem Rev 2011;255:2514e29.
[3] (a) Ellinger S, Graham KR, Shi P, Farley RT, Steckler TT, Brookins RN, et al.
Donoreacceptoredonor-based
p-conjugated oligomers for nonlinear optics
and near-IR emission. Chem Mater 2011;23:3805e17;
(b) Qian G, Dai B, Luo M, Yu DB, Zhan J, Zhang ZQ, et al. Donoreacceptore
donor charge-transfer heteroquinoid-based chromophores: near infrared
photoluminescence and electroluminescence. Chem Mater 2008;20:6208e16.
[4] (a) Jiao C, Huang KW, Wu J. Perylene-fused BODIPY dye with near-IR ab-
sorption/emission and high photostability. Org Lett 2011;13:632e5;
(b) Ma X, Mao XR, Zhang SW, Huang XB, Cheng YX, Zhu CJ. Nucleation ability
of multiwall carbon nanotubes in polypropylene composites. Polym Chem
2013;4:520e7.
[5] (a) Wu XG, Liu Y, Wang YF, Wang L, Tan H, Zhu MX, et al. Highly efficient near-
infrared emission from binuclear cyclo-metalated platinum complexes
bridged with 5-(4-octyloxyphenyl)-1,3,4-oxadiazole-2-thiol in PLEDs. Org
Electron 2012;13:932e7;
Fig. 5. The current densityevoltage (JeV) curves of the (TPA-BT-Q)Ptpic-doped devices
at different concentrations from 0.5 to 8.0 wt%.
by the carrier-trapping mechanism based on the identical phe-
nomenon observed previously by Chang et al. [19]. On the other
hand, the light outputs of 53
m m
W cmꢀ2 and 112 W cmꢀ2 at
11.7 mA cmꢀ2 were obtained in the device at 4.0 wt% and 2.0 wt%
doping concentrations, respectively. These radiant emittances are
comparable to those levels of the reported metallo-porphyrins.
(b) Yang CJ, Yi C, Xu M, Wang JH, Liu YZ, Gao XC, et al. Red to near-infrared
electrophosphorescence from
a platinum complex coordinated with 8-
hydroxyquinoline. Appl Phys Lett 2006;89:233506;
(c) Lee TC, Hung JY, Chi Y, Cheng YM, Lee GH, Chou PT, et al. Rational design of
charge-neutral, near-infrared-emitting osmium(II) complexes and OLED
fabrication. Adv Funct Mater 2009;19:2639e47;
4. Conclusions
(d) Mori H, Tanaka T, Osuka A. Fused porphyrinoids as promising near-
infrared absorbing dyes. J Mater Chem C 2013;1:2500e19;
(e) Xiang HF, Xu ZX, Roy VAL, Yan BP, Chan SC, Che CM, et al. Deep-red to
near-infrared electrophosphorescence based on bis(8-hydroxyquinolato)
platinum(II) complexes. Appl Phys Lett 2008;92:163305;
(f) Tao R, Qiao J, Zhang GL, Duan L, Wang LD, Qiu Y. Efficient near-infrared-
emitting cationic iridium complexes as dopants for OLEDs with small effi-
ciency roll-off. J Phys Chem C 2012;116:11658e64;
In summary, a novel DeAeA platinum (II) complex of (TPA-BT-
Q)Ptpic with NIR emission at 760 nm was obtained. Employing it as
a dopant, the NIR-emitting PLEDs showed a suppressed efficiency
roll-off with increasing operating current density. The best device
performances were presented in the device at 2.0 wt% dopant
concentration. The maximum EQE of 0.12% at 16.6 mA cmꢀ2 and an
(g) Tao R, Qiao J, Zhang GL, Duan L, Chen CC, Wang LD, et al. High-efficiency
near-infrared organic light-emitting devices based on an iridium complex
with negligible efficiency roll-off. J Mater Chem C 2013;1:6446e54.
[6] Baldo MA, O’Brien DF, You Y, Shoustikov A, Sibley S, Thompson ME, et al.
Highly efficient phosphorescent emission from organic electroluminescent
devices. Nature 1998;395:151e4.
[7] Borek C, Hanson K, Djurovich PI, Thompson ME, Aznavour K, Bau R, et al.
Highly efficient, near-infrared electrophosphorescence from a Pt-metallo-
porphyrin complex. Angew Chem Int Ed 2007;46:1109e12.
irradiance intensity of 112
observed.
m
W cmꢀ2 at 11.7 mA cmꢀ2 were
Acknowledgments
This work was supported by financial support from the National
Natural Science Foundation of China (51273168, 21202139,
50973093), the Innovation Group in Hunan Natural Science Foun-
dation (12JJ7002), the Scientific Research Fund of Hunan Provincial
Education Department (10A119, 11CY023, 10B112), Natural Science
Foundation of Hunan (12JJ4019, 11JJ3061), the Scientific Research
Fund of Xiangtan University (11QDZ18, 2011XZX08), and the
[8] (a) Shi DY, Wang YF, Liu Y, Zhang ZY, Luo J, He J, et al. Star-shaped trinuclear
cyclometalated platinum (II) complexes as single-component emitters in
white-emitting PLEDs. Chem Asian J 2012;7:2096e101;
(b) Shigehiro T, Yagi S, Maeda T, Nakazumi H, Fujiwara H, Sakurai Y. Photo-
and electroluminescence from 2-(dibenzo[b,d]furan-4-yl)pyridine-based
heteroleptic cyclometalated platinum (II) complexes: excimer formation
drastically facilitated by an aromatic diketonate ancillary ligand. J Phys Chem
C 2013;117:532e42;

152
J. Yu et al. / Dyes and Pigments 107 (2014) 146e152
(c) Fukagawa H, Shimizu T, Hanashima H, Osada Y, Suzuki M, Fujikake H.
Highly efficient and stable red phosphorescent organic light-emitting diodes
using platinum complexes. Adv Mater 2012;24:5099e103;
platinum (II) complex containing alkyl-trifluorene picolinic acid as emitter for
single-layer white PLEDs. Org Electron 2010;11:1954e9.
[14] (a) Li JM, Deng XP, Zhang ZY, Wang YF, Liu Y, He KQ, et al. Tuning photovoltaic
(d) Kui SCF, Hung FF, Lai SL, Yuen MY, Kwok CC, Low KH, et al. Luminescent
organoplatinum(II) complexes with functionalized cyclometalated C^N^C li-
gands: structures, photophysical properties and material applications. Chem
Eur J 2012;18:96e109.
performance
of
9,9-dioctylfluorene-alt-5,7-bis(thiophen-2-yl)-2,3-
biphenylthieno[3,4-b]pyrazine copolymeric derivatives by attaching addi-
tional donor units in pendant phenyl ring. J Polym Sci Pol Chem 2012;50:
4686e94;
[9] Shimizu M, Kaki R, Takeda Y, Hiyama T, Nagai N, Yamagishi H, et al. 1,4-
Bis(diarylamino)-2,5-bis(4-cyanophenylethenyl)benzenes: fluorophores
(b) Zeng WD, Cao Y, Bai Y, Wang YH, Shi YS, Zhang M, et al. Efficient dye-
sensitized solar cells with an organic photosensitizer featuring orderly con-
jugated ethylenedioxythiophene and dithienosilole blocks. Chem Mater
2010;22:1915e25;
exhibiting efficient red and near-infrared emissions in solid State. Angew
Chem Int Ed 2012;51:4095e9.
[10] Qian G, Zhong Z, Luo M, Yu DB, Zhang ZQ, Wang ZY, et al. Simple and efficient
near-infrared organic chromophores for light-emitting diodes with single
electroluminescent emission above 1000 nm. Adv Mater 2009;21:111e6.
[11] Hu ZY, Wang YF, Shi DY, Tan H, Li XS, Wang L, et al. Highly-efficiency red-
emitting platinum(II) complexes containing 40-diarylamino-1-phenyl-iso-
quinoline ligands in polymer light-emitting diodes: synthesis, structure,
photoelectron and electroluminescence. Dyes Pigments 2010;86:166e73.
[12] (a) Zhang BH, Qin CJ, Ding JQ, Chen L, Xie ZY, Cheng YX, et al. High-perfor-
mance all-polymer white-light-emitting diodes using polyfluorene containing
phosphonate groups as an efficient electron-injection layer. Adv Funct Mater
2010;20:2951e7;
(c) Medina A, Claessens CG, Rahman GMA, Lamsabhi AM, Mó O, Yáñez M, et al.
Accelerating charge transfer in a triphenylamineesubphthalocyanine donore
acceptor system. Chem Commun 2008;15:1759e61.
[15] Dou L, Gao J, Richard E, You J, Chen CC, Cha KC, et al. Systematic investigation
of benzodithiophene-and diketopyrrolopyrrole-based low-bandgap polymers
designed for single junction and tandem polymer solar cells. J Am Chem Soc
2012;134:10071e9.
[16] He Z, Wong W-Y, Yu X, Kwok H-S, Lin Z. Phosphorescent platinum (II)
complexes derived from multifunctional chromophores: synthesis,
structures, photophysics, and electroluminescence. Inorg Chem 2006;45:
10922e37.
(b) Luo J, Li XZ, Hou Q, Peng JB, Yang W, Cao Y. High-efficiency white-light
emission from a single copolymer: fluorescent blue, green, and red chromo-
phores on a conjugated polymer backbone. Adv Mater 2007;19:1113e7;
(c) Zhu WH, Wu YZ, Wang ST, Li WQ, Li X, Chen J, et al. Organic DeAepeA
solar cell sensitizers with improved stability and spectral response. Adv Funct
[17] (a) Gao ZQ, Mi BX, Tam HL, Cheah KW, Chen CH, Wong MS, et al. High effi-
ciency and small roll-off electrophosphorescence from a new iridium complex
with well-matched energy levels. Adv Mater 2008;20:774e8;
(b) Jou JH, Wang YS, Lin CH, Shen SM, Chen PC, Tang MC, et al. Nearly non-roll-
off high efficiency fluorescent yellow organic light-emitting diodes. J Mater
Chem 2011;21:12613e8.
Mater 2011;21:756e63;
(d) Wu YZ, Zhu WH. Organic sensitizers from De
p
eA to DeAe
peA: effect of
[18] Sommer JR, Shelton AH, Parthasarathy A, Ghiviriga I, Reynolds JR, Schanze KS.
the internal electron-withdrawing units on molecular absorption, energy
levels and photovoltaic performances. Chem Soc Rev 2013;42:2039e58.
[13] (a) Brooks J, Babayan Y, Lamansky S, Djurovich PI, Tsyba I, Bau R, et al. Syn-
thesis and characterization of phosphorescent cyclometalated platinum
complexes. Inorg Chem 2002;41:3055e66;
Photophysical properties of near-infrared phosphorescent p-extended plat-
inum porphyrins. Chem Mater 2011;23:5296e304.
[19] Chang SY, Kavitha J, Li SW, Hsu CS, Chi Y, Yeh YS, et al. Platinum(II) com-
plexes with pyridyl azolate-based chelates: synthesis, structural character-
ization, and tuning of photo- and electrophosphorescence. Inorg Chem
2006;45:137e46.
(b) Wang YF, Liu Y, Li XS, Qi HR, Zhu MX, Wang L, et al. Novel cyclometalated