A feasible approach to obtain near-infrared (NIR) emission from binuclear platinum(II) complexes containing centrosymmetric isoquinoline ligand in PLEDs

Article abstract of DOI:10.1016/j.orgel.2020.105902

Organic light-emitting diodes (OLEDs) of deep-red (DR)/near-infrared (NIR) emission have become an emerging hot topic in applications for medical and night-vision devices. In this article, one novel symmetrical binuclear platinum(II) complexes as well as

Full text of DOI:10.1016/j.orgel.2020.105902

Organic Electronics 87 (2020) 105902
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Organic Electronics
journal homepage: http://www.elsevier.com/locate/orgel
A feasible approach to obtain near-infrared (NIR) emission from binuclear
platinum(II) complexes containing centrosymmetric isoquinoline ligand
in PLEDs
a
b
b
b
a
a
a,b,*
Kai Zhang , Yajun Liu , Zhaoran Hao , Gangtie Lei , Suqian Cui , Weiguo Zhu , Yu Liu
a
School of Materials Science and Engineering, Jiangsu Collaboration Innovation Center of Photovoltaic Science and Engineering, Jiangsu Engineering Laboratory of Light-
Electricity-Heat Energy-Converting Materials and Applications, National Experimental Demonstration Center for Materials Science and Engineering, Changzhou
University, Changzhou, 213164, PR China
b
College of Chemistry, Key Lab of Environment-Friendly Chemistry and Application in Ministry of Education, Xiangtan University, Xiangtan, 411105, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Organic light-emitting diodes (OLEDs) of deep-red (DR)/near-infrared (NIR) emission have become an emerging
hot topic in applications for medical and night-vision devices. In this article, one novel symmetrical binuclear
Binuclear platinum(II) complexes
Isoquinoline ligands
Deep-red/near-infrared emission
OLEDs
2
platinum(II) complexes as well as its mononuclear analogues, namely (DIQB)[Pt(DPM)] and (DIQB)Pt(DPM),
involving big rigid planar ligand 1,4-di(isoquinolin-1-yl)benzene (DIQB) and auxiliary ligand dipivaloylmetha-
nato (DPM), were successfully synthesized and characterized. Intrinsic DR emission peaked at 618 nm with a
photoluminescence quantum yield (Φ) of 2.42% and lifetime of 0.37
solution. Wondrously, an outstandingly 112 nm red-shifted emission peaked at 730 nm with a Φ of 0.77% and
lifetime of 0.26 s was observed in (DIQB)[Pt(DPM)] solution. Density functional theory (DFT/TD-DFT) cal-
μ
s was obtained in the (DIQB)Pt(DPM)
μ
2
3
3
culations were carried out to reveal the emission process and a predominant ILCT/ MLCT characteristics. As a
result, the emission of platinum(II) complexes is tuned from DR to NIR via appending an additional platinum(II)
ion. OLEDs based on (DIQB)Pt(DPM) exhibited an efficient DR emission at 666 nm with a maximum external
quantum efficiency (EQE) of 2.86% and a brightness of 1632 cd/cm2 at dopant concentration of 3 wt %, In
contrast, an outstandingly 80 nm red-shifted NIR emission at 746 nm with a EQE of 0.58% and a radiance of up
to 10036 mW/Sr/m2 was obtained for the (DBIQ)[Pt(DPM)]
over, the efficiency roll-off was efficiently inhibited in the (DIQB)[Pt(DPM)]
device at the same dopant concentration. More-
-doped devices. This work dem-
2
2
∧
∧
onstrates that binuclear platinum(II) complexes dominated by centrosymmetric type C N-C N tetradentate big
rigid planar ligand is an effective strategy for obtaining NIR luminescent materials.
1
. Introduction
Inspired by the successful advancement of visible organic light
been employed as DR/NIR emitters. For the iridium(III) complexes, a
DR-emitting OLED based on an iridium(III) complex presented an EQE
of 11.2% [11]. The highest efficient NIR-emitting OLEDs based on
iridium(III) complexes were obtained with EQEs of 3.4% at 702 nm and
3.1% at 714 nm [16]. Among these of the reported platinum(II) com-
plexes [17–28], not only have both singlet and triplet excitons, but also
help to harvest unity internal quantum efficiency. Therefore, enormous
researches based on platinum(II) complexes have been founded with
tunable emission color from DR to NIR in their high-efficiency devices
[17–28]. Nevertheless, in terms of the low energy NIR phosphorescent
emitting materials, vibrational coupling between the ground and excited
emitting devices (OLEDs), the researches on deep-red (DR) to near-
infrared (NIR) OLEDs involved in electromagnetic radiation wave-
length range from 650 nm to 2500 nm have been drawn tremendous
efforts in the recent years, mainly due to their special applications in
photodynamic therapy, night-vision readable displays, optical commu-
nication and chemical sensors [1–5]. So far, several kinds of DR-NIR
OLEDs focused on organic molecules [6–8], conjugated polymer [9],
lanthanide complex [10], and transition metal complexes [11–31], have
*
Corresponding author. School of Materials Science and Engineering, Jiangsu Collaboration Innovation Center of Photovoltaic Science and Engineering, Jiangsu
Engineering Laboratory of Light-Electricity-Heat Energy-Converting Materials and Applications, National Experimental Demonstration Center for Materials Science
and Engineering, Changzhou University, Changzhou, 213164, PR China.
E-mail addresses: lgt@xtu.edu.cn (G. Lei), zhuwg18@126.com (W. Zhu), liuyu03b@126.com (Y. Liu).
https://doi.org/10.1016/j.orgel.2020.105902
Received 7 May 2020; Received in revised form 7 July 2020; Accepted 23 July 2020
Available online 12 August 2020
1
566-1199/© 2020 Elsevier B.V. All rights reserved.

K. Zhang et al.
Organic Electronics 87 (2020) 105902
Scheme 1. Synthetic route of platinum(II) complexes.
electronic states are strong and accompanied with fast nonradiative
decay [32]. Therefore, the efficiencies of large amounts NIR emission
materials still lag far behind that of visible light emission materials to
date.
ligand, mainly due to its highly extended
π
conjugation and more rigid
chelating architectures. Meanwhile, DPM with dendritic structure was
chosen as an auxiliary ligand, which is mainly beneficial for its effec-
tively inhibit intermolecular interactions and enhance solubility [26,
28], their photophysical, electrochemical, thermal as well as electrolu-
minescent (EL) properties were systematically investigated. Further
density functional theory (DFT) and time-dependent DFT (TD-DFT)
calculations are engaged to clarify their luminous behavior. As expected,
(DIQB)Pt(DPM) presented DR emission at 618 nm with a photo-
As transition metal complexes, platinum(II) complexes have
demonstrated efficient emission in NIR emitting OLEDs due to its unique
8
d electron structure [17–31]. This structure ensures that the platinum
(
II) complex molecules exhibit a high degree of planarity, variety of
excited states, and strong intermolecular interactions. And have been
developed quite high EQEs of 2.0–24% with peak emissions between
luminescence quantum yield (ФPL) of 2.42% and a lifetime of 0.37
dichloromethane (DCM) solution. Encouragingly, a NIR emission at 730
nm with a ФPL of 0.77% and a short lifetime of 0.26 s was obtained in
(DIQB)[Pt(DPM)] solution. Shorter phosphorescent lifetime means
μ
s in
6
50 and 800 nm. For instance, two more efficient NIR OLEDs were
fabricated based on cyclometallated platinum(II) complexes with the
EQEs of 13.9% and 16.7%, respectively [26]. So far, the most efficient
NIR-emitting non-doped OLEDs based on platinum(II) complex exhibi-
ted an unusual EQE of up to 24% with peak at 740 nm [27]. Never-
theless, these results suffering from easy aggregation and rather long
phosphorescence lifetimes, and all these platinum(II) complexes present
serious efficiency roll-off in devices at high current density. Conse-
quently, it is still a challenge to exploit DR and NIR emitting platinum(II)
complexes with high efficiency and low roll-off in OLEDs.
μ
2
fewer triplet-triplet annihilation (TTA) processes and thus lower roll-off
efficiency [33,34]. As a result, a DR emission peak at 666 nm with a
2
maximum EQE of 2.86% and a brightness of 1632 cd/m was obtained
for the device containing 3 wt % (DIQB)Pt(DPM). Also, the (DIQB)[Pt
2
(DPM)] -doped devices afford a red-shifted 80 nm NIR isolated dual
emissions at about 624 nm and 746 nm with a maximum EQE of 0.58%
2
and a radiance of 10036 mW/Sr/m . Moreover, its efficiency roll-off was
As is well known, binuclear platinum(II) complexes can occasionally
extending the conjugation length as well as increasing the spin-orbit
coupling (SOC) effect induced by its incorporated d orbital, which
seems to be an inspiring approach to obtain efficient NIR platinum(II)
emitters [18,23–25]. In our previous work, a binuclear platinum(II)
complex was incorporated using pyrenyl-dipyridine (BuPyrDPy) as
cyclometalating ligand, which is bridged with an ancillary ligand of
dipivaloylmethanato (DPM) [28]. The results demonstrated that the DR
to NIR emission was realized by appending additional metal ion.
Inspired by above mentioned merit. It is expected this types of binuclear
platinum(II) complexes could obtain NIR emission by grafting addition
metal ion, we dedicated our efforts to devise novel NIR emission ma-
terials based on binuclear platinum (II) complexes bearing centrosym-
metric big rigid planar ligand 1,4-di(isoquinolin-1-yl)benzene (DIQB)
and auxiliary ligand DPM. In this work, one novel symmetrical binuclear
platinum(II) complexes as well as its mononuclear homologous, namely
efficiently inhibited in the (DIQB)[Pt(DPM)]
2
-doped devices. These
findings will open novel avenue to exploit highly efficient
a
NIR-emitting devices via binuclear platinum(II) complexes with
centrosymmetric large rigid planar ligand.
2. Experimental section
2.1. Materials and synthesis
Unless otherwise stated, all chemicals and reagents were purchased
from commercial sources without further purification. All reactions
were implemented under nitrogen protection and monitored by thin-
layer chromatography. As shown in Scheme 1, the ligand DIQB was
synthesized by a common Suzuki coupling reaction [28,33]. The target
2
platinum(II) complexes of (DIQB)Pt(DPM) and (DIQB)[Pt(DPM)] were
synthesized by an one-pot method [24,25]. The chemical structure of
ligand DIQB was fully confirmed with 1H NMR and MALDI-TOF mass
spectrometry. Furthermore, the complex (DIQB)Pt(DPM) was confirmed
with 1H NMR and high resolution MS spectrometry. Particularly, the
(
2
DIQB)[Pt(DPM)] and (DIQB)Pt(DPM), are primarily synthesized and
characterized. Synthetic routes of the platinum(II) complexes are shown
in Scheme 1. We selected DIQB unit as rigid expanded cyclometalated
2

K. Zhang et al.
Organic Electronics 87 (2020) 105902
(
DIQB)[Pt(DPM)]
2
was determined by 1H NMR, MALDI-TOF mass
diffraction studies were performed on a Bruker SMART Apex CCD
spectrometry and single-crystal X-ray diffraction.
diffractometer equipped with graphite monochromatized (Mo K ) ra-
α
diation, and the structures were solved by direct methods, and refined
by a full-matrix least-squares technique using the SHELXL-97 crystal-
lographic software package. The UV–Vis absorption spectra and pho-
toluminescence (PL) spectra were obtained from the DCM solution and
recorded with Shimadzu UV-2600 and Perkin-Elmer LS50B fluorescence
spectrometers, respectively. Steady-state fluorescence measurements
were recorded on a PerkinElmer LS45 instrument, while performing
time-resolved fluorescence measurements using a time-dependent single
photon counting (TCSPC) spectrometer (DeltaFlex-01-DD/HORIBA)
with a Delta diode laser (510 nm) as excitation sources. Thermal analysis
(TGA) was conducted on a NETZSCHSTA449 instrument at a heating
2
.1.1. Synthesis of 1,4-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)
benzene (M1)
Under nitrogen atmosphere protection, a mixture of 1,4-dibromo-
benzene (5.0 g, 21.37 mmol), bipinate borate (13.56 g, 53.42 mmol),
KOAc (12.58 g, 128.22 mmol) and Pd(dppf)Cl
2
(781 mg, 1.07 mmol) in
◦
1
,4-dioxane(40 mL) were stirred at 80 C for 24 h. After being cooled to
room temperature (RT), then the mixture was washed with saturated
brine, extracted with DCM, and the combined organic layer was dried
4
over anhydrous MgSO . The organic solvent was removed by rotary
evaporation and the residue was passed through a flash silica gel column
using petroleum ether (PE)/dichloromethane (DCM) (V/V, 3/1) as the
eluent to afford white solid, then recrystallized from n-hexane to obtain
white needle-like crystals (4.80 g, 68.51%). 1 H NMR (400 MHz, CDCl
◦
rate of 20 C/min. The Cyclic voltammetry (CV) was performed using
3
CHI630E at a scan rate of 50 mV/s in CH CN solutions. All experiments
3
,
were carried out in a three-electrode compartment cell with a Pt-wire
counter electrode, a Pt-disk working electrode and Ag/AgCl reference
electrode.
TMS), δ(ppm): 7.80 (s, 1H), 1.35 (s, 6H). (MALDI-TOF) MS (m/z) for
C
+
18
H
28
2
B O
4
, Calcd: 330.220; Found, 330.012 [M+H] .
2
.1.2. Synthesis of 1,4-di(isoquinolin-1-yl) benzene (DIQB)
A mixture of 1-bromoisoquinoline (1.0 g, 4.81 mmol), M1 (722.0 mg,
2.3. PLED fabrication and measurement
2
.19 mmol) and K
2
CO
3
(1.81 g, 13.10 mmol) in 6.5 mL H
2
O, 6.5 mL
◦
The structure of the doped devices is ITO/PEDOT:PSS(40 nm)/poly-
TPD(30 nm)/[PVK:OXD-7(7:3)]:Complexes (x wt %, 50 nm)/TmPyPb
(50 nm)/CsF(1.2 nm)/Al (120 nm). The PEDOT:PSS is used as a hole
injection layer and poly-TPD is used as a hole transport layer (HTL).
PVK:OXD-7 has good hole and electron transport ability as a mixed host.
All devices with the emitting layers (EMLs) based on PVK:OXD-7(7:3)
doped with platinum(II) complexes, with different doping concentra-
tions of 3 wt%, 6 wt% and 9 wt%, respectively. TmTyPB is used as an
electron transport layer (ETL) and a hole blocking layer (HBL), and CsF
is used as an electron injection layer. PEDOT:PSS(40 nm) films covered
by poly-TPD (30 nm) were spin-coated on precleaned ITO glass sub-
EtOH and 10 mL toluene were stirred at 80 C for 24 h under nitrogen
protection. After cooled to RT and washed with methanol (3 × 30 mL).
The organic solvent was removed by rotary evaporation and then
recrystallized from methanol to obtain white crystals (680 mg, 93.8%).
1
H NMR (400 MHz, CDCl
3
, TMS), δ(ppm): 8.67 (d, J = 5.7 Hz, 2H), 8.22
(
dd, J = 8.5, 0.6 Hz, 2H), 7.95–7.87 (m, 6H), 7.77–7.67 (m, 4H),
7
.63–7.54 (m, 2H). MALDI-TOF MS (m/z) for C24
16 2
H N , Calcd: 332.410,
+
Found, 333.000 [M+H] .
2
.1.3. Synthesis of complex (DIQB)Pt(DPM) and (DIQB)[Pt(DPM)]
Under nitrogen protection, a mixture of DIQB (200 mg, 0.60 mmol)
and K PtCl (550 mg, 1.32 mmol) were dissolved in 2-ethoxyethanol
12 mL) and stirred at reflux for 48 h. After cooling to RT, the mixture
2
◦
strates and annealed at 120 C for 20 min then the light emitting layer
2
4
was spin-coated onto the PEDOT substrate from a mixture of PVK:OXD-7
and phosphors. After that, TmPyPb was evaporated onto the active
layer. Finally, the CsF/Al layer was deposited on the top of the emitting
layer. In order to prevent degradation and emission quenching caused
by oxygen and water, all the above operations are performed in a ni-
(
was poured into water and filtered, then the precipitate was washed
with water and a small amount of methanol, and dried under high
vacuum to get a crude dimer (320 mg). Without further purification, a
mixture of dimer (320 mg, 0.17 mmol) and DPM (443 mg, 2.41 mmol)
ꢀ
4
with Na
2
CO
3
(638 mg, 6.02 mmol) were suspended in 15 mL THF so-
trogen atmosphere or a vacuum state (1 × 10 Pa), and the PLED is
encapsulated before characterization. The EL spectra and current den-
sity (J)–voltage (V)–radiance (R) curves were obtained using a PHO-
TORESEARCH Spectra Scan PR 735 photometer and a KEITHLEY 2400
Source Meter constant current source at room temperature. The EQE
values were calculated by assuming a Lambertian distribution.
lution and stirred at reflux for 12 h. The reaction was quenched with
water and the mixture was extracted with DCM (3 × 30 mL), and the
4
combined organic layer was dried over anhydrous MgSO . The residue
was purified by chromatography on a silica gel column using PE/DCM
V/V, 3/1) as the eluent. Finally, an orange solid (DIQB)Pt(DPM) (100
mg, 23.0%) and a dark red solid (DIQB)[Pt(DPM)] (150 mg, 22.9%)
were obtained, respectively. (DIQB)Pt(DPM): 1H NMR (400 MHz,
CDCl
(
2
3. Result and discussion
3
, TMS), δ(ppm): 9.07 (d, J = 6.5 Hz, 1H), 9.01 (d, J = 8.6 Hz, 1H),
8
1
.69 (d, J = 5.6 Hz, 1H), 8.53 (d, J = 8.3 Hz, 1H), 8.33 (d, J = 8.3 Hz,
H), 8.22 (d, J = 1.8 Hz, 1H), 7.91 (d, J = 8.3 Hz, 2H), 7.82 (t, J = 7.0
3.1. Synthesis, characterization, thermal and single-crystal properties
Synthetic routes for the platinum(II) complexes of (DIQB)Pt(DPM)
Hz, 1H), 7.71 (d, J = 14.4, 10.8, 4.5 Hz, 4H), 7.55 (t, J = 7.1 Hz, 2H),
5
.84 (s, 1H), 1.43–0.93 (m, 18H). High resolution MS (m/z) for
and (DIQB)[Pt(DPM)] are outlined in Scheme 1. DPM was employed as
2
+
C
35
H
34
N
2
O
2
Pt, Calcd: 709.230; Found, 710.233 [M+H] . (DIQB)[Pt
the ancillary ligand to restrain intermolecular interactions and improve
1
(
DPM)]
2
: H NMR (400 MHz, CDCl
3
, TMS), δ(ppm): 9.26 (d, J = 8.7 Hz,
solubility of the complexes. 1,4-benzenediboronic acid bis(pinacol) ester
′
′
′
′
2
H), 9.04 (d, J = 6.5 Hz, 2H), 8.49 (s, 2H), 7.85 (d, J = 8.0 Hz, 2H), 7.76
(M1) was prepared via 1,4-dibromobenzene and 4,4,4 ,4 ,5,5,5 ,5 -
′
(
(
t, J = 7.5 Hz, 2H), 7.64 (t, J = 7.8 Hz, 2H), 7.48 (d, J = 6.5 Hz, 2H), 5.85
octamethyl-2,2 -bi(1,3,2-dioxaborolane). 1,4-di- (isoquinolin-1-yl)ben-
s, 2H), 1.34 (d, J = 18.3 Hz, 36H). MALDI-TOF MS (m/z) for
zene (DIQB) was synthesized by Suzuki coupling reaction with M1 and
1-bromoisoquinoline. Subsequently, based on the DIQB two coordina-
tion sites, one or two platinum (II) complexes can be prepared by one pot
+
C
46
H
52
N
2
O
4
Pt
2
, Calcd: 1086.320; Found, 1086.633 [M+H] .
[
24,25
2
.2. Instrumentation
method reported in the literature,
corresponding complexes were fully characterized by 1H NMR, the
structures of DIQB and (DIQB)[Pt(DPM)] were determined by MALDI-
and the structures of DIQB and
1H NMR spectra was recorded on a Bruker DRX-400 spectrometer at
2
4
00 Hz using CDCl
3
as solvent and tetramethylsilane (TMS) as the in-
TOF mass spectra and the (DIQB)Pt(DPM) was carried out on high res-
olution MS, which confirmed their well-defined chemical structures
(Fig. S1-S6, see ESI†). Particularly, the structure of the (DIQB)[Pt
ternal standard. Mass spectrometry (MS) results were obtained on a
Bruker Autoflex MALDI-TOF instrument. High resolution MS was ach-
ieved on waters Xevo G2-Xs QTof instrument. Single-crystal X-ray
2
(DPM)] was determined by single-crystal X-ray diffraction.
3

K. Zhang et al.
Organic Electronics 87 (2020) 105902
Fig. 1. a) ORTEP diagram of (DIQB)[Pt(DPM)]
2
with the atom numbering scheme (ellipsoids are drawn at the 50% probability level), b) Crystal structures and C)
2
Intermolecular stacking of (DIQB)[Pt(DPM)] , respectively.
Table 1
Photophysical and thermal parameters of platinum(II) complexes.
a
b
b
b
c
d
d
Complex
λ
abs nm, [
ε
×
λ
PL
Φ
p
τ
k
r
k
nr
T
d
ꢀ
4
ꢀ 1
ꢀ 1
4
6
o
1
0
M
cm
]
[nm]
[%]
[μ
s]
[10
[10
ꢀ 1
[ C]
ꢀ
1
s
]
s
]
Pt-1
413(0.97),369
618
2.42
0.37
6.54
2.64
306
(
(
2.15),352
1.81),
3
25(2.22),270
(
(
3.06),280
2.87)
Pt-2
570(6.6),535
730
0.77
0.26
2.96
3.82
380
(
(
6.1),450
1.3),424(1.2),
3
92(2.8),375
(
(
2.0),310
3.4),286(4.0)
a
(
2
DIQB)Pt(DPM) and (DIQB)[Pt(DPM)] replaced with Pt-1 and Pt-2,
respectively.
b
Measured in CH
2
Cl
2
solution.
Cl
c
Fig. 2. Absorption and photoluminescence spectra of ligand and platinum(II)
complexes in CH Cl solution.
Measured in degassed CH
2
2
relative to Ir(piq)
2
(acac) (Φ
p
= 0.2, λex = 475
2
2
c
nm) and calculated according to the equation Φ
s
= Φ
r
(
η
s
I
s
A
r
/
η
r
r
I A
s
). Measured
2
2
in degassed toluene solution.
d
Radiative decay rate k
r
= Φ/
τ
and nonradiative decay rate knr = (1-Φ)/
τ.
3
.2. Thermal stability
Thermal properties of both complexes were employed by thermog-
2
packing plot of (DIQB)[Pt(DPM)] .
ravimetric analysis (TGA) (Fig. S7, see ESI†). High thermal decomposi-
◦
tion temperatures (T
d
) of 306 for (DIQB)Pt(DPM) and 380 C for (DIQB)
3
.4. Photophysical properties
The absorption spectra of ligand DIQB and both platinum(II) com-
[
2
Pt(DPM)] were obtained at 5% weight loss, respectively, demon-
strating that the introduction of the additional metal platinum(II) ion is
beneficial to enhance the thermal stability of its platinum(II) complex.
plexes in dilute DCM solutions are displayed in Fig. 2, and the elaborated
absorption data are listed in Table 1. The absorption spectrum of (DIQB)
Pt(DPM) showed a typical profiles of platinum(II) complex, in which an
3
.3. Single-crystal structures
intense high-lying band under 350 nm is corresponding to
π
- * electron
π
A purplish red flaky single crystal of (DIQB)[Pt(DPM)]
2
was formed
transition absorption ascribed with the DIQB ligand. While the lower-
energy absorption band range from 350 to 630 nm was ascribed to the
mixing spin-allowed and spin-forbidden metal-to-ligand charge-transfer
by slow diffusion of their methanol/dichloromethane solutions at room
temperature. The ORTEP diagram, molecular structures and crystal-
packing diagram are shown in Fig. 1. Crystal data and refinement pa-
rameters and selected bond lengths and angles are summarized in
1
3
states ( MLCT and MLCT), somewhat with the ligand-centered
transition or ligand-to-ligand charge transfer (LLCT) transition.
Furthermore, the (DIQB)[Pt(DPM)] presented a more intense absorp-
tion coefficients from the spin-allowed * and MLCT transitions,
π
–π*
Table S1 and S2. The corresponding platinum-ligating atom bond
2
∧
lengths in (DIQB)[Pt(DPM)]
2
are similar to many C N bidentate plat-
π–π
inum complexes. And the Pt–C bonds are slightly shorter than the Pt-N
bonds, also exhibit obvious trans effect to the corresponding trans Pt-
O bonds. As shown in Fig. 1, the whole ligand frameworks execute
which was resulted from the extended conjugate molecular framework
introduced by additional metal ion [24–26,28]. Furthermore, the optical
opt
energy gaps (E
g
) are 2.44 eV for (DIQB)Pt(DPM) and 1.99 eV for
much planarity that welded by the chelated metal ions in (DIQB)[Pt
(DIQB)[Pt(DPM)]
2
according to their absorption edges, a narrower band
, owing to the enhanced
◦
(
DPM)]
2
and a torsion angle of 12.60 was observed between benzene
gap is observed for (DIQB)[Pt(DPM)]
2
π
and isoquinoline unit and intra-ligand charge transfer character (ILCT)
from benzene to isoquinoline can be expected in such a donor-acceptor-
donor ligand system. A large Pt– Pt separation of 6.43 Å between the
adjacent platinum centers also means the absence of metal–metal-to-
ligand charge transfer (MMLCT) in the crystal states in the crystal-
conjugation system based on extra metal ion [24–26,28]. Their transi-
tions were also evidenced by the time-dependent density functional
theory (TD-DFT) calculations at the S
S9, see ESI†). The overlapping absorption bands of 350–500 nm in
(DIQB)[Pt(DPM)] shows an around 20 nm red-shifted emission in
0
geometry structure (Fig. S8 and
2
4

K. Zhang et al.
Organic Electronics 87 (2020) 105902
Table 2
Electrochemical data of the platinum(II) complexes.
a
b
c
d
opt
g
e
cal
f
cal
g
f
Complexes
λ
oneset (nm)
Eox (V)
EHOMO (eV)
E
LUMO (eV)
E
(eV)
HOMO/LUMO (eV)
E
(eV)
Pt-1
Pt-2
509
623
0.78
0.70
ꢀ 5.41
ꢀ 5.33
ꢀ 2.97
ꢀ 3.34
2.44
1.99
ꢀ 5.41/-2.14
ꢀ 4.86/-2.28
3.27
2.58
a
Absorption edges of the films.
b
c
+
Data are versus Fc/Fc (Fc is ferrocene) estimated from the onset oxidation curve.
Deduced from the equation EHOMO = -(Eox - 0.47)-5.1, where 0.47 V is the potential for ferrocene vs. Ag/AgCl and 5.1 eV is the energy level of ferrocene to the
vacuum energy level.
d
opt
E
LUMO = EHOMO + ΔE
g
.
e
f
opt
ΔE
g
= 1240/λoneset
.
Calculated by DFT.
comparison with those of the (DIQB)Pt(DPM), which is arising from the
increased in conjugating length and the enhanced donor-acceptor
characteristics of the ligand induced by the introduction to a second
metal platinum ion [26,35,36].
As also shown in Fig. 2 and summarized in Table 1, the PL spectra of
both complexes in DCM solution presented typical characteristic of
platinum(II) species [17–28]. The (DIQB)Pt(DPM) exhibits an intense
DR emission peak at 618 nm with a shoulder around 655 nm under
photo-excitation at 413 nm in DCM solution. As compared, a remarkably
NIR emission at 730 nm with a shoulder around 800 nm was obtained
from binuclear (DIQB)[Pt(DPM)]
Obviously, the (DIQB)[Pt(DPM)]
2
under photo-excitation at 570 nm.
displays an outstandingly 112 nm
2
red-shifted emission compared to its mononuclear complex. And illus-
trating that grafting an additional metal ion would effectively tune
molecular conjugation of its corresponding complex. Meanwhile, the
Ф
PL of 2.42% and 0.77% were detected in DCM solution for (DIQB)Pt
DPM) and (DIQB)[Pt(DPM)] at RT, and together with excited-state
s (Fig. S10, see ESI†), respectively. The
(
2
lifetimes of 0.38 and 0.26
μ
r
quite decreased k value towards to longer emission wavelength in the
complexes is chiefly assigned to a fast decay passage in terms of a
vibrionic coupling between ground and excited states, which is sub-
jected by “energy gap law” [32]. The shorter phosphorescent lifetimes
1 1
Fig. 3. Calculate S , T , HOMO and LUMO energies, as well as HOMO and
LUMO topologies of platinum(II) complexes.
2
for the (DIQB)[Pt(DPM)] would decrease its TTA and triplet-polaron
annihilation (TPA) [33,34].
continuum model (PCM) for DCM as the solvent. Representative frontier
orbitals of the both platinum(II) complexes as shown in Fig. 3, and more
iso-surface contour plots are shown in Fig. S12 and S13, further related
data are summarized in Table S3. The calculated HOMO and LUMO
3
.5. Electrochemical properties
Using ferrocene as the internal standard, the electrochemical char-
acteristics of both platinum(II) complexes were investigated by cyclic
voltammetry (CV) in CH
CN solution (Fig. S11, see ESI†), and their
detailed parameters are listed in Table 2. The irreversible onset oxida-
energy levels of (DIQB)Pt(DPM) and (DIQB)[Pt(DPM)]
2
are ꢀ 5.41
eV/ꢀ 2.14 eV and ꢀ 4.86 eV/ꢀ 2.28 eV, respectively (Table 2). The trend
3
of energy gaps are perfectly reasonable, and the lowered LUMO level of
(
DIQB)[Pt(DPM)]
2
matches well with most bimetallic species [27,
stacking shows more
tion potentials (Eox) of (DIQB)Pt(DPM) and (DIQB)[Pt(DPM)]
2
are
3
7–39]. As shown in Fig. 3, the (DIQB)[Pt(DPM)]
2
+
observed at 0.78 V and 0.70 V vs Fc/Fc , respectively. According to the
following equation: EHOMO = -(Eox+ 4.63) eV [25–27], their highest
occupied molecular orbit (HOMO) energy levels (EHOMO) are ꢀ 5.41 eV
◦
littler dropped dihedral angles (10.86 ) between the centre ring and
◦
second isoquinoline unit than that of the (DIQB)Pt(DPM) (43.63 ). This
implies that appending an additional platinum(II) ion could enhance the
intramolecular planarity, which is beneficial to enhancing conjugated
degree. Moreover, to further understand the absorption spectra of the
both complexes, their computational absorption spectra are shown in
Fig. S8 and S9. The transition energy and oscillator strength calculated
and ꢀ 5.33 eV for (DIQB)Pt(DPM) and (DIQB)[Pt(DPM)]
2
, respectively.
opt
Based on their corresponding optical energy gaps (E
levels, the lowest unoccupied molecular orbital (LUMO) energy levels
LUMO) are estimated to be ꢀ 2.97 eV for (DIQB)Pt(DPM) and ꢀ 3.34 eV
for (DIQB)[Pt(DPM)] , respectively. The orderly lowered energy gaps
from (DIQB)Pt(DPM) to (DIQB)[Pt(DPM)] are associated with the
g
) and EHOMO
(
E
2
by TD-DFT of the two complexes in the S
the profile of the experimental absorption spectra. The S
both complexes have almost exclusively HOMO→LUMO characteristics,
but (DIQB)[Pt(DPM)] shows a larger delocalized region than (DIQB)Pt
DPM), which demonstrates that the low energy of the lowest-energy
absorption bands happened in (DIQB)[Pt(DPM)] . According to the
0
geometry are agree well with
2
1
states of the
related increased HOMO energy levels, which is mainly triggered by the
increased molecular planarity and the additional metallic d orbital of the
binuclear complex, agreed much well with the mentioned above
red-shifted UV spectra [25].
2
(
2
energy level diagram (Fig. 4), the electron cloud of LUMO are distrib-
uted in the DIQB ligand and central platinum(II) ion. Therefore, both the
platinum(II) complexes have a similar LUMO energy levels. In contrast,
the electron cloud of the HOMO is mainly located on the central plat-
inum(II) ion, ligand and donor benzene unit.
3
.6. Theoretical calculations
Density functional theory (DFT) calculations were carried out to
understand the geometry and optical properties on complexes using the
Gaussian 09W program at the B3LYP/6-31G(d)/LanL2DZ level [28], and
further analyze its optical properties with a conductor-like polarizable
As shown in Fig. S12 and Fig. S13, the lowest triplet state T of the
1
5

K. Zhang et al.
Organic Electronics 87 (2020) 105902
Fig. 4. Energy level diagrams and chemical structures of related materials in PLED device.
Fig. 5. EL spectra of platinum(II) complexes doped devices at various concentrations from 3 to 9 wt %.
1 1
which may further facilitate SOC between S and T states [40].
Table 3
EL parameters of platinum(II) complexes doped devices with different dopant
concentrations.
3
.7. Electroluminescence (EL) properties
The device structures and energy level diagrams of all materials
a
b
c
d
e
f
g
Complexes
wt %)
V
on
J
λ
EL
EQEmax
EQE
L
max /Rmax
2
(
(V)
(mA/
(nm)
(%)
(%)
(cd/m )/
2
2
cm )
(mW/Sr/m )
employed in devices are depicted in Fig. 4. The HOMO/LUMO levels of
both platinum(II) complexes are well matched between the HOMO level
of PVK (ꢀ 5.8 eV) and the LUMO level of OXD-7 (ꢀ 3.0 eV), an efficient
energy transfer from the host to the guest should happen in the emitting
layer. Spin-coated OLEDs were fabricated with a configuration of ITO/
PEDOT:PSS (40 nm)/poly-TPD (30 nm)/[PVK:OXD-7(7:3)]: complexes
f
3
6
9
3
6
9
wt% Pt-1
wt% Pt-1
wt% Pt-1
wt% Pt-2
wt% Pt-2
wt% Pt-2
11.6
11.2
10.1
12.8
12.0
11.2
4.97
666
622
622
746
745
745
2.86
2.49
2.11
0.58
0.51
0.45
1.84
1.70
1.52
0.53
0.47
0.42
1632
f
6.73
1752
f
8.70
1658
g
23.69
91.36
33.49
10036
g
9667
g
9046
(
x wt %, 50 nm)/TmPyPb(50 nm)/CsF (1.2 nm)/Al (120 nm). The EL
a
b
c
d
e
f
2
Turn-on voltage at 1 cd/cm .
spectra of OLEDs based on platinum(II) complexes with different con-
centrations from 3 wt % to 9 wt %, and all devices as shown by the EL
spectra in Fig. 5, further detailed devices data are summarized in
Table 3. The maximum EL peaks located at 623 and 666 nm with wide
band for the (DIQB)Pt(DPM) doped device, and 746 nm accompanied by
Current density at maximum EQE.
The maximum EL emission peak.
The maximum external quantum efficiency.
ꢀ 2
EQE value at 100 mA cm
Brightness.
.
g
2
a shoulder at around 810 nm for (DIQB)[Pt(DPM)] doped device,
Radiant intensity.
respectively. With increasing dopant concentration of (DIQB)Pt(DPM),
the DR emission at 666 nm is preliminarily decreased, which is caused
by the steadily larger LUMO separation of the EML to the ETL and the
gradually smaller HOMO separation of that to HTL, and ultimately
achieving a more balanced electron/hole transportation in device [41].
both complexes was investigated. TD-DFT calculations on T
DIQB)Pt(DPM) displays that it has mainly HOMO→LUMO character-
istics (62.2%) with a certain influence from HOMO-1→LUMO (23.2%)
, the T state
1
state of
(
and HOMO-4→LUMO (5.8%). For (DIQB)[Pt(DPM)]
2
1
2
While the (DIQB)[Pt(DPM)] -doped devices presents a NIR emission
mainly originates from the contribution of HOMO→LUMO (90.2%) and
minor from HOMO-1→LUMO (5.6%). The triplet states mixing with an
peak at 746 nm with a shoulder at around 810 nm within the all doping
concentrations, which is largely associated with its much suitable
HOMO/LUMO levels in devices (Fig. 5b). Upon increasing the dopant
concentration from 3 wt % to 9 wt %, a minor high-lying emission peak
at around 429 nm from the host blend were decreased in the both de-
vices, which provided an efficient forward energy transfer from host to
1
MLCT states that included different d orbitals can effectively promote
the spin-orbital coupling (SOC) characteristic of metal complexes [23,
4
0]. It is worth noting that the HOMO, HOMO-1, and HOMO-4 orbitals
of the two complexes all involved different d orbitals of platinum(II) ion,
6

K. Zhang et al.
Organic Electronics 87 (2020) 105902
Fig. 6. J–V–R profiles of platinum(II) complexes doped devices at various concentrations from 3 to 9 wt %.
Fig. 7. EQE-J characteristics of platinum(II) complexes doped devices at various concentrations from 3 to 9 wt %.
2
2
dopant [25]. Meanwhile, the EL spectra exhibit somewhat red-shift in all
devices, illustrating the strong polarization influence and intermolecular
interactions of these complexes [25,42].
density from 23.69 mA/cm to 100 mA/cm , which should be triggered
by the increased rigidity skeleton and reduced phosphorescence lifetime
originated from the introduction of an additional platinum(II) ion [24,
25,42].
To demonstrate balanced electron/hole transportation, their elec-
tron- and hole-only devices had been also fabricated. Their electron-only
and hole-only devices were fabricated with the structures of ITO/ZnO
4. Conclusion
(
30 nm)/PVK:OXD-7(7:3)]:neat film of Pt complexes (40 nm)/Ca (30
nm)/Al and ITO/PEDOT:PSS (30 nm)/PVK:OXD-7(7:3)]:neat film of Pt
complexes (40 nm)/MoO (10 nm)/Al, respectively. As shown in
In summary, based on simply centrosymmetric isoquinoline ligand, a
binuclear platinum(II) complex of (DIQB)[Pt(DPM)]2 and its mono-
nuclear homologous of (DIQB)Pt(DPM) were successfully obtained. The
3
Fig. S14, the current densities of electron-only and hole-only devices
based on (DIQB)Pt(DPM) were much higher than those devices based on
binuclear (DIQB)[Pt(DPM)] presents a red-shifted PL spectrum by 112
2
(
DIQB)[Pt(DPM)]
2
, and demonstrating that the higher electron trans-
nm and higher ФPL of 2.42% than the mononuclear (DIQB)Pt(DPM). At
port ability of (DIQB)Pt(DPM) neat film than that of (DIQB)[Pt(DPM)]
2
the 3 wt % dopant concentration, the (DIQB)Pt(DPM)-doped device
presented a DR emission with a peak at 666 nm and a maximum EQE of
2.86%. In contrast, an outstandingly 80 nm red-shifted NIR emission at
746 nm with a EQE of 0.58% was obtained in the (DIQB)[Pt(DPM)]2-
doped device. Moreover, an efficiency roll-offs are efficiently suppressed
neat film. Consequently, the (DIQB)Pt(DPM) device could display more
balanced hole and electron transport behavior, and thereby obtain
higher EQE [26,43].
The J–V–R and EQE–J characteristics of the both platinum(II) com-
plexes for all devices are shown in Figs. 6 and 7, respectively, and
summarized the relevant data of all EL in Table 3. The turn-on voltages
in the (DIQB)[Pt(DPM)] -based devices. This work provides that high-
2
efficiency NIR-emitting binuclear platinum(II) complexes can be ach-
(
V
on) of the all devices preliminary dropped in turn with increasing
ieved by grafting centrosymmetric big rigid planar ligand.
dopant concentrations, which implies that the energy transfer processes
of the both platinum(II) complexes-doped devices are mainly dominated
by the Dexter transfer mechanism rather than the carrier-trapping
mechanism [44]. At dopant concentration of 3 wt%, the maximum
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
EQE and brightness (radiant emittance) were obtained to be
2
2
.86%/1632 cd/m2 and 0.58%/10036 mW/Sr/m for the (DIQB)Pt
(
DPM) and (DIQB)[Pt(DPM)]
compared with the (DIQB)Pt(DPM)-doped devices, an efficiency roll-offs
are efficiently suppressed in the (DIQB)[Pt(DPM)] -based devices. For
instance, at 3 wt % dopant concentration of (DIQB)[Pt(DPM)] , a slight
dropped in its EQEs level from 0.58 to 0.53% with increasing current
2
doped device, respectively. Furthermore,
Acknowledgments
2
2
We thank the financial supports from the National Natural Science
Foundation of China (U1663229, 51473140, 51573154); the Major
7

K. Zhang et al.
Organic Electronics 87 (2020) 105902
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