Triazine-phosphine oxide electron transporter for ultralow-voltage-driven sky blue PHOLEDs

Article abstract of DOI:10.1039/c4tc02993c

1,3,5-Triazine derivatives xTPOTZ with aryl phosphine oxide (APO) groups at the ortho-, meta- and para-positions were designed and prepared to demonstrate an APO modification strategy for developing n-type semiconductors. Optical analysis showed that the

Full text of DOI:10.1039/c4tc02993c

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ARTICLE TYPE
Triazole-Phosphine Oxide Electron Transporter for Ultralow-Voltage-
Driven Sky Blue PHOLEDs
Jilin Jia,a Liping Zhu,b Ying Wei,a,c Zhongbin Wu,b Hui Xu,*a,c Dongxue Ding,a Runfeng Chen,*c Dongge
Ma,*b Wei Huangc
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
1,3,5-Triazole derivatives xTPOTZ with aryl phosphine oxide (APO) groups at ortho-, meta- and para-
positions were designed and prepared to demonstrate an APO modification strategy for developing n-type
semiconductors. Optical analysis showed that the involvement of APO groups can hardly alter excited
10 state characteristics of xTPOTZ, rendering their triplet energy beyond 2.95 eV for effective exciton
blocking in sky blue phosphorescent organic light-emitting diodes. The strong electron-withdrawing
effect of P=O can effectively enhance electroactivity of the materials, on the basis of suitable substitution
configuration. mTPOTZ and pTPOTZ with the low-lying lowest unoccupied molecular orbitals at ~-3.4
eV show the improved electron affinity, as well as the reduced highest occupied molecular orbital at -6.6
15 eV for effective hole blocking. The estimated electron mobility of mTPOTZ and pTPOTZ reaches to the
order of 10-4 cm2 V-1 s-1. Furthermore, owing to the strongest intramolecular interactions and the exposed
triazole core under para-substitution configuration, pTPOTZ achieves the lowest LUMO at -3.46 eV and
the highest e of 3.4×10-4 cm2 V-1 s-1, which is among the highest values reported so far for high-energy-
gap electron-transporting materials (ETM). As the result, pTPOTZ endowed its typical mCP and FIrpic-
20 based sky blue PHOLEDs with the ultralow onset voltage of 2.5 V and favourable external quantum
efficiency up to ~20%, revealing the effectiveness of APO modification for high-performance ETM
development.
45 electron transporting layers (ETL)15. In this case, ETM should
have triplet energy higher than those of phosphors to block triplet
exciton diffusion and the highest occupied molecular orbital
deeper than those of EML materials to restrain hole leakage.17
Therefore, in common sense, ETM development is focused on
50 improving e, triplet energy and electron injecting/hole blocking
ability.18-19
1. Introduction
Electrophosphorescence is believed as one of the most promising
25 technologies competent for the environmentally friendly flat-
panel displays and lightings with the advantage of simultaneously
utilizing singlet and triplet excitons.1-8 Since exciton is formed
through hole and electron recombination, balanced carrier
injection and transportation is one of the core concerns for
30 achieving high-efficiency phosphorescent light-emitting diodes
(PHOLEDs).9-10 However, as conjugated systems with
delocalized  electrons, most of organic semiconductors prefer to
transport hole rather than electron, causing unbalanced charge
flux in their devices.11 Actually, hole mobility (h) of common
35 hole transporting materials, such as di-[4- 1,1-bis[(di-4-
tolylamino)phenyl]cyclohexane (TAPC, 10-2 cm2 V-1 s-1)12 and
4,4'-bis[N-(p-tolyl)-N-phenyl-amino]biphenyl (TPD, 10-3 cm2 V-1
s-1)13, is one to four orders of magnitude larger than electron
mobility (e) of conventional electron transporting materials
40 (ETM), e.g. 1,3,5-tris(N-phenylbenzimidizol-2-yl)benzene (TPBI,
10-6 cm2 V-1 s-1)14, 4,7-diphenyl-1,10-phenanthroline (Bphen, 10-4
cm2 V-1 s-1)15 and 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene
(TmPyPB, 10-4 cm2 V-1 s-1)16, making exciton recombination
zone shift to interfaces between emissive layers (EML) and
For blue PHOLEDs, the first triplet energy level (T1) of ETM
should be over 2.8 eV to suppress triplet exciton diffusion,
rendering the employment of meso14, 20 or twisted21 linkages. In
55 contrast, high e requires planar configurations with extended
conjugations and strong intermolecular interactions for facile
charge hopping. This reflects the intrinsic contradiction between
optical and electrical properties of high-energy-gap ETMs.
Accordingly, nitrogen-bearing heterocycles seem superior as
60 building blocks for ETMs with large electron affinity, planar
configuration and high triplet energy. Represented by TmPyPB16,
pyridine22-24, pyrimidine25 and 1,3,5-triazole26 have been adopted
to construct high-energy-gap ETMs for blue PHOLEDs. Low
driving voltages, such as 3.0 V for onset, and high efficiencies,
65 e.g. external quantum efficiency (EQE) more than 10% were
already realized through conventional device configurations.
Nevertheless, it is noteworthy that although meso linkage is
utilized during system extension, triplet energy of these ETMs is
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Journal of Materials Chemistry C
Page 2 of 13
still around 2.8 eV, which should be elevated for efficient
suppression of exciton diffusion.
View Article Online
DOI: 10.1039/C4TC02993C
5
Fig. 1 Chemical structures of xTPOTZ (x = o, m or p) (a) and single-crystal structures of oTPOTZ (b) and mTPOTZ (c).
In recent years, aryl phosphine oxide (APO) group is of great
corresponding to the substitution positions on TPPO groups),
concern as a unique electroactive insulating linkage.27-29 Its 30 were designed and prepared (Fig. 1a). Compared to parent
strong electron-withdrawing inductive effect can polarize
molecules and enhance electron affinity, while C-P bond can
molecule 2,4,6-triphenyl-1,3,5-triazine (TPTZ), the involvement
of APO groups in xTPOTZ remarkably improves their thermal
stability during operation, including the temperatures of
10 effectively block intramolecular electronic communications to
restrain influences of functionalization on triplet energy.30-36 In
decomposition (Td) over 400 C and melting points (Tm) more
o
this sense, APO group is ideal for constructing high-energy-gap 35 than 250 oC. Meanwhile, by virtue of insulating characteristics of
ETMs. After primary attempts to improve electron injection and
transportation with ETMs of triphenylphosphine oxide (TPPO)37
15 and its pyrenyl38 and spiro-fluorenyl derivatives39, m-terphenyl40-
PO linkage, xTPOTZ completely inherit optical properties of
TPTZ. Their triplet energy beyond 2.95 eV makes them
competent as high-energy-gap ETM for sky blue PHOLEDs with
41
and tetraphenylsilicanyl42 are modified by diphenylphosphine
bis(4,6-(difluorophenyl)pyridinato-N,C2)picolinate
iridium(III)
oxide (DPPO) on account of high triplet energy for applications 40 (FIrpic, T1 = 2.65 eV) as phosphor. According to density function
in sky blue PHOLEDs. Although high efficiencies were realized
due to the effective exciton confinement in EMLs, the weak
20 electron affinity of m-terphenyl and tetraphenylsilicanyl gave rise
to the big driving voltages and low luminance of the devices.
theory (DFT) calculation, although DPPO groups are only
involved in their highest occupied molecular oribtals (HOMO),
both of the HOMOs and the lowest unoccupied molecular orbitals
(LUMO) of mTPOTZ and pTPOTZ remarkably descend,
Regarding to this case, it is rational and imperative to develop 45 indicating the enhanced electron injecting/hole blocking ability
feasible strategies to combine nitrogen-bearing heterocyclic
skeletons and APO groups for the integration of their respective
25 superiorities in electrical performance and triplet energy
preservation.
by the strong electron-withdrawing effect of P=O. This is
consistent with cyclic voltammetric (CV) analysis results that due
to its twisted configuration, oTPOTZ shows the HOMO and
LUMO energy levels identical with those of TPTZ, while the
In this contribution, a series of 1,3,5-triazole and TPPO 50 frontier molecular orbital (FMO) energy levels of mTPOTZ and
hybrids, collectively named xTPOTZ (x = o, m and p,
pTPOTZ are reduced for more than 0.12 eV, respectively.
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Estimated by I-V characteristics of the nominal electron-only
potential of APO-substituted heterocycles as n-type
devices, pTPOTZ achieves the e of 3.4×10-4 cm2 V-1 s-1, which
is among the highest values reported so far for high-energy-gap
ETMs.43 To manifest the advantage of xTPOTZ as ETMs, their
FIrpic-based sky blue PHOLEDs with a common host 1,3-
bis(9H-carbazol-9-yl)benzene (mCP) were fabricated without any
structural optimization. Compared to the four-layer device of
commercially available TmPyPB44, pTPOTZ endowed its
trilayer device with the dramatically reduced driving voltages of
semiconductors for various optoelectronic applications.
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DOI: 10.1039/C4TC02993C
2. Experimental Section
5
15 2.1 Materials and Instruments
All the reagents and solvents used for the synthesis of the
compounds were purchased from Aldrich and Acros companies
and used without further purification.
10 2.6 V for onset and 4.0 V at 100 cd m-2 and the comparable EQE
with maximum of ~13%, convincingly reflecting the great
20
Scheme 1 Synthetic procedure of xTPOTZ (x = o, m or p).
1H, 13C and DEPT135o NMR spectra were recorded using a
Phosphorescence spectra were measured in dichloromethane
Varian Mercury plus 400NB spectrometer relative to 55 using an Edinburgh FPLS 920 fluorescence spectrophotometer at
tetramethylsilane (TMS) as internal standard. Molecular masses
were determined by a Bruker microflex MALDI-TOF Mass
25 Spectrometry. Elemental analyses were performed on a Vario EL
III elemental analyzer. The crystal suitable for single-crystal
77 K cooling by liquid nitrogen with a delay of 300 μs using
Time-Correlated Single Photon Counting (TCSPC) method with
a microsecond pulsed Xenon light source for 0.1 μs-10 s lifetime
measurement, the synchronization photomultiplier for signal
XRD analysis was obtained by diffusion method at room 60 collection and the Multi-Channel Scaling Mode of the PCS900
temperature. All diffraction data were collected at 295 K on a
Rigaku Xcalibur diffractometer with graphite
fast counter PC plug-in card for data processing.
E
2.2 Synthesis
30 monochromatized Mo Kα (λ = 0.71073 Å) radiation in ω scan
mode. All structures were solved by direct method and difference
Fourier syntheses. Non-hydrogen atoms were refined by full-
matrix least-squares techniques on F2 with anisotropic thermal
parameters. The hydrogen atoms attached to carbons were placed
35 in calculated positions with C−H = 0.93 Å and U(H) = 1.2Ueq(C)
in the riding model approximation. All calculations were carried
out with the SHELXL97 program. Absorption and
photoluminescence (PL) emission spectra of the target compound
were measured using a SHIMADZU UV-3150 spectrophotometer
40 and a SHIMADZU RF-5301PC spectrophotometer, respectively.
The molar extinction coefficient () was estimated according to
2.2.1 General synthetic procedure of 2,4,6-tri(bromophenyl)-
1,3,5-triazine
65 In Ar2, bromobenzonitrile (4.55 g, 25 mmol) in anhydrous
chloroform
(150
mL)
was
dropwise
added
to
trifluoromethanesulfonic acid (7.50 g, 50 mmol) at 0 oC. The
resulted mixture was warmed to room temperature and stirred for
24 h. Then, water (250 mL) was added. After a further stir for 2 h,
70 the precipitate was filtrated and washed with water (3 × 10 ml)
and cold chloroform (3 × 10 ml), which was then dried in air to
afford the bromides.
2,4,6-Tri(2-bromophenyl)-1,3,5-triazine (oTBTZ): 2.74 g of
1
white powder with a yield of 60%. H NMR (TMS, CDCl3, 400
the Lambert-Beer's Law as
, where A is absorbence,
  A C  L
C is molar concentration in mol L-1 and L is optical distance in
cm. Thermogravimetric analysis (TGA) and differential scanning
45 calorimetry (DSC) were performed on Shimadzu DSC-60A and
DTG-60A thermal analyzers under nitrogen atmosphere at a
heating rate of 10 °C min-1. Cyclic voltammetric (CV) studies
were conducted using an Eco Chemie B. V. AUTOLAB
potentiostat in a typical three-electrode cell with a platinum sheet
50 working electrode, a platinum wire counter electrode, and a
silver/silver nitrate (Ag/Ag+) reference electrode. All
electrochemical experiments were carried out under a nitrogen
atmosphere at room temperature in dichloromethane.
75 MHz): δ = 8.118-8.096 (dd, J1 = 7.6 Hz, J2 = 1.2 Hz, 3H), 7.762
(d, J = 8.0 Hz,3 H), 7.506-7.469 (t, J = 7.6 Hz,3 H), 7.392-7.350
ppm (td, J1 = 8.0 Hz, J2 = 1.2 Hz, 3 H); LDI-TOF: m/z (%): 545
(100) [M+], 547 (100) [M+2+]; elemental analysis (%) for
C21H12Br3N3: C 46.19, H 2.22, N 7.70; found: C 46.21, H 2.23, N
80 7.75.
2,4,6-Tri(3-bromophenyl)-1,3,5-triazine (mTBTZ): 2.97 g of
white powder with a yield of 65%. 1H NMR (TMS, CDCl3, 400
MHz): δ = 8.838 (s, 3 H), 8.672 (d, J = 8.0 Hz,3 H), 7.757 (d, J =
8.0 Hz,3 H), 7.485-7.445 ppm (t, J = 8.0 Hz,3 H); LDI-TOF: m/z
85 (%): 545 (100) [M+], 547 (100) [M+2+]; elemental analysis (%)
for C21H12Br3N3: C 46.19, H 2.22, N 7.70; found: C 46.23, H 2.25,
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Journal of Materials Chemistry C
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N 7.77.
25 135.365(CH, observed from DEPT135o), 134.817(C), 133.752(C),
2,4,6-Tri(4-bromophenyl)-1,3,5-triazine (pTBTZ): 3.20 g of
white powder with a yield of 70%. H NMR (TMS, CDCl3, 400
133.573(CH), 131.900(CH), 131.574(CH), 131.116(CH),
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1
130.109(CH), 128.106(CH) ppm; LDI-TOFD: OmI:/z10(.%10)3:9/9C049TC(100209)93C
MHz): δ = 8.606 (d, J = 8.8 Hz, 6H), 7.713 ppm (d, J = 8.8Hz,
[M+]; FTIR (KBr pellet): 3053, 1589 (C=N stretching), 1567
(C=C stretching), 1517, 1483, 1436, 1359, 1196 (P=O stretching),
5
6H); LDI-TOF: m/z (%): 545 (100) [M+], 547 (100) [M+2+];
elemental analysis (%) for C21H12Br3N3: C 46.19, H 2.22, N 7.70; 30 1117, 1070, 844, 748, 717, 698, 545, 516 cm-1; elemental analysis
found: C 46.20, H 2.22, N 7.72.
2.2.2 General procedure for the palladium-catalyzed
phosphorylation
(%) for C57H42N3O3P3: C 75.24, H 4.65, N 4.62, O 5.28; found: C
75.26, H 4.68, N 4.77, O 5.35.
2,4,6-Tris(3-(diphenylphosphoryl)phenyl)-1,3,5-triazine
(mTPOTZ): 0.54 g of white crystal with a yield of 60%. 1H NMR
35 (TMS, CDCl3, 400 MHz): δ = 8.981 (d, J = 8.4 Hz,,3H), 8.744-
8.721 (dd, J1 = 8.0 Hz, J2 = 1.2 Hz, 3H), 7.917-7.869 (m, 3H),
7.750-7.699 (m, 12H), 7.670-7.625 (td, J1 = 7.6 Hz, J2 = 2.8 Hz,
3H), 7.581-7.482 ppm (m, 18H); 13C NMR (TMS, DMSO, 100
MHz): δ = 170.924(C), 136.183(CH), 135.987(C), 133.877(C),
40 132.847(CH), 132.554(CH), 132.207(CH), 132.110(CH),
131.683(C), 129.113(CH), 128.766(CH) ppm; LDI-TOF: m/z (%):
909 (100) [M+]; FTIR (KBr pellet): 3052, 1594 (C=N stretching),
1580 (C=C stretching), 1520, 1483, 1436, 1357, 1200 (P=O
stretching), 1116, 721, 694, 641, 544 cm-1; elemental analysis (%)
45 for C57H42N3O3P3: C 75.24, H 4.65, N 4.62, O 5.28; found: C
75.29, H 4.66, N 4.71, O 5.37.
10 In Ar2, 2,4,6-tri(bromophenyl)-1,3,5-triazine (0.546 g, 1 mmol),
NaOAc (0.27 g, 3.3 mmol), Pd(OAc)2 (6.7 mg, 0.03 mmol),
diphenylphosphine (0.6 mL, 3.3 mmol) was dissolved in N,N-
o
dimethylformamide (5 mL). The mixture was heated to 130 C
and then stirred for 24 h. After cooled to room temperature, the
15 mixture was poured into water (50 mL) and extracted with
CH2Cl2 (3 × 10 ml). The combined organic phase were dried with
anhydrous Na2SO4 and concentrated in vacuo. The resident was
further purified by flash column chromatography (5:1
EtOAc/EtOH).
20
2,4,6-Tris(2-(diphenylphosphoryl)phenyl)-1,3,5-triazine
(oTPOTZ): 0.64 g of white powder with a yield of 70%. 1H NMR
(TMS, DMSO, 400 MHz): δ = 7.770-7.743 (m, 3H), 7.601-7.523
(m, 6H), 7.492-7.458 (m, 6H), 7.371-7.256 ppm (m, 27H); 13C
NMR (TMS, DMSO, 100 MHz): δ = 170.932(C), 140.193(C),
Fig. 2 Single-crystal packing diagram of oTPOTZ and mTPOTZ.
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2,4,6-Tris(4-(diphenylphosphoryl)phenyl)-1,3,5-triazine
(pTPOTZ): 0.50 g of white powder with a yield of 55%. H 50 electron affinity. Even directly substituted with electron-donating
1,3,5-Triazole is an extremely electron-deficient ring with high
1
NMR (TMS, CDCl3, 400 MHz): δ = 8.824-8.797 (dd, J1 = 8.4 Hz,
J2 = 2.4 Hz, 6H), 7.921-7.872 (d, J = 11.6 Hz,, 6H), 7.740-7.689
(m, 12H), 7.599-7.559 (t,d, J1 = 7.6 Hz, J2 = 1.2 Hz, 6H),7.515-
7.471 ppm (t,d, J1 = 7.6 Hz, J2 = 2.4 Hz,12H); 13C NMR (TMS,
groups, such as carbazole and diphenylamine, its derivatives still
exhibit favorable electron injecting/transporting ability.26, 48 Su et
al. showed that electron affinity of 2,4,6-tri(biphenyl-3-yl)-1,3,5-
triazine is as high as 3.0 eV, which can be further increased to 3.3
5
DMSO, 100 MHz): δ = 171.291(C), 138.767(C), 137.900(C), 55 eV by incorporating peripheral pyridine groups.49 However,
136.893(C),
132.579(CH),
132.260(CH),
132.122(CH),
triplet energy of these meso-linked materials remains too low as
2.7-2.8 eV to afford the positive energy transfer to FIrpic,
indicating T1 energy of TPTZ core highly sensitive to
conjugation extension and substitution with electron-withdrawing
128.919(CH), 128.752(CH) ppm; LDI-TOF: m/z (%): 909 (100)
10 [M+]; FTIR (KBr pellet): 3053, 1572, 1516, 1436, 1356, 1198,
1114, 808, 728, 714, 694, 558, 546 cm-1; elemental analysis (%)
for C57H42N3O3P3: C 75.24, H 4.65, N 4.62, O 5.28; found: C 60 groups. Meanwhile, we and other groups have demonstrated that
75.27, H 4.64, N 4.68, O 5.33.
the introduction of APO group in conjugated systems can
polarize molecules, accompanied with negligible T1 variation less
than 0.1 eV. Therefore, it is rational to utilize DPPO as the
secondary electron-withdrawing group with superiority in carrier
65 injection enhancement and triplet energy preservation.
Furthermore, through making use of its big steric hindrance,
various substitution positions of DPPO group in xTPOTZ can
modulate the planarity and conjugation of TPTZ core and adjust
the intermolecular interactions, thereby establishing a flexible
70 platform to investigate the correlations between molecular
configuration of ETMs and their optoelectronic properties. From
another point of view, xTPOTZ are actually combinations of two
conventional high-energy-gap electron-deficient units, namely
TPPO and 1,3,5-triazole, through ortho-, meta- and para-linkages,
75 respectively, with different inter-group interplays (Fig. 1a),
which would testify the optimal linkage style to integrate
superiorities of TPPO and 1,3,5-triazole to the greatest extent.
xTPOTZ were conveniently prepared from bromobenzonitrile
through cyclization and phosphorylation with moderate overall
80 yields of ~40%, respectively (Scheme 1). Their chemical
structures were characterized and certified on the basis of mass
spectrometry, NMR spectroscopy and elemental analysis. The
molecular configurations and intermolecular interactions of
oTPOTZ and mTPOTZ were further confirmed by X-ray
85 diffraction of their single crystals (Fig. 1b, 1c and 2). The mesa-
substitution configuration of mTPOTZ renders its almost planar
TPTZ core with small dihedral angles between three phenyls and
triazole of 6.77(1)o, 12.73(6)o and 13.68(4)o, respectively; while
dihedral angles of TPTZ core in oTPOTZ are remarkable larger
90 as 28.05(0)o, 43.11(5)o and 54.20(8)o, respectively, verifying the
twisted configuration and reduced conjugation of oTPOTZ due
to its ortho-linked DPPOs. Close inspection on packing diagrams
reveals one-dimensional chains of oTPOTZ and mTPOTZ.
P=O … H and edge-to-centroid distances between DPPO and
95 TPTZ groups are 2.892(9) and 3.757(9) Å for oTPOTZ and
2.3 Density function theory (DFT) Calculations
15 DFT computations were carried out with different parameters for
structure optimizations and vibration analyses. The ground states
and triplet states of molecules in vacuum were optimized by the
restricted and unrestricted formalism of Beck's three-parameter
hybrid exchange functional45 and Lee, and Yang and Parr
20 correlation functional46 (B3LYP)/ 6-31G(d) respectively. The
ground-state configurations of o-TPOTZ and m-TPOTZ were
generated according to single crystal data. The fully optimized
stationary points were further characterized by harmonic
vibrational frequency analysis to ensure that real local minima
25 had been found without imaginary vibrational frequency. The
total energies were also corrected by zero-point energy both for
the ground state and triplet state. The spin density distributions
were visualized with Gaussview 3.0. All computations were
performed using the Gaussian 03 package.47
30 2.4 Device Fabrication and Testing
Before loading into a deposition chamber, the ITO substrate was
cleaned with detergents and deionized water, dried in an oven at
120 °C for 4 h, and treated with UV-ozone for 20 min. Devices
were fabricated by evaporating organic layers at a rate of 0.1-0.3
35 nm s-1 onto the ITO substrate sequentially at a pressure below
1×10-6 mbar. Onto the TPBI layer, a layer of LiF with 1 nm
thickness was deposited at a rate of 0.1 nm s-1 to improve electron
injection. Finally, a 100-nm-thick layer of Al was deposited at a
rate of 0.6 nm s-1 as the cathode. The emission area of the devices
40 was 0.14 cm2 as determined by the overlap area of the anode and
the cathode. The EL spectra and CIE coordinates were measured
using a PR650 spectra colorimeter. The current-density-voltage
and brightness–voltage curves of the devices were measured
using a Keithley 2400/2000 source meter and a calibrated silicon
45 photodiode. All the experiments and measurements were carried
out at room temperature under ambient conditions.
3.324(8) and 3.288(6)
Å for mTPOTZ, indicating weak
hydrogen bond and - stacking interactions between adjacent
molecules. Obviously, the interactions between peripheral DPPOs
and TPTZ cores establish the effective channels for
100 intermolecular electron transport. Although single crystal of
3. Results and Discussions
3.1 Design and Synthesis
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pTPOTZ was not obtained, it is convincible that its para-
triazole and phenyls of TPTZ cores and →* transition of
substituted and protrudent DPPO groups would result in the
planar TPTZ core and considerable intermolecualr DPPO-TPTZ
phenyls in DPPO groups (Fig. 4 and Table 1). Obviously, ,
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DOI: 10.1039/C4TC02993C
interactions. In this sense, mTPOTZ and pTPOTZ with 45 TPTZ core-originated absorptions for xTPOTZ are determined
5
extended conjugations and planar configurations would be
superior to oTPOTZ in electron transportation.
by conjugation degrees of TPTZ cores. In this case, with
absorption of TPTZ at 270 nm as reference, the weaker
absorption of oTPOTZ at 264 nm is perfectly consistent with its
twisted TPTZ core, while the absorption band of mTPOTZ at
50 272 nm has the comparable coefficient, revealing the unchanged
configuration of its TPTZ core due to mesa-substitution of DPPO
groups. For pTPOTZ, this band bathochromicly shifts to 280 nm
with enhanced coefficient, which should be ascribed to slight
conjugation extension by para-substituted P=O groups.
55 Simultaneously, since the strong electron-withdrawing inductive
effect of triazole on para-DPPO groups, the DPPO-attributed
absorption of pTPOTZ is distinctly weaker than those of
oTPOTZ and mTPOTZ, which reflects the more effective
intramolecular electronic communications in pTPOTZ.
60 Nevertheless, the energy levels of the first singlet states (S1) for
xTPOTZ are identical as ~4.0 eV, estimated by their absorption
edges, which are 0.16 eV lower than that of TPTZ due to
electron-withdrawing effect of DPPO groups. In spin-coated thin
films, the fine structures of absorption spectra for xTPOTZ
65 remain with slight bathochromic shifts due to the limited
aggregation, validating their excited energy preservation in solid
states (Fig. S1).
3.2 Thermal Stability
Thermal performance is significant for ETMs, which would
influence stability of their thin films during device operation.
10 Thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) of xTPOTZ and TPTZ were measured to Fig.
out the contribution of APO groups to thermal property
improvement (Fig. 3 and Table 1). It is convincing that no matter
of substitution position, the involvement of steric DPPO groups
15 in xTPOTZ should decrease the intermolecular interactions
between their TPTZ cores. However, even for nonplanar
oTPOTZ, its melting point (Tm) of 273 C is 40 C bigger than
that of TPTZ, revealing DPPO-induced stronger intermolecular
interactions, namely P=O … H hydrogen bonds and -
20 interactions between DPPO and TPTZ core as shown by single
crystal result. It is noteworthy that the melting points of oTPOTZ
and mTPOTZ are comparable, but Tm of pTPOTZ is improved
o
o
o
to 320 oC with a further increase of ~40 C, which rationally
manifests its strongest intermolecular interactions in xTPOTZ.
25
TGA curves show that the decomposition temperatures (Td) of
o
xTPOTZ at weight loss of 5% are more than 420 C, which are
o
dramatically improved for more than 170 C in comparison to
that of TPTZ. The residues in crucibles after heating evidence
the decomposition of these compounds under heating rather than
30 sublimation or volatilization. Furthermore, on the basis of the
same building blocks, Td of pTPOTZ is about 70 and 30 oC
higher than those of oTPOTZ and mTPOTZ, respectively,
which is consistent with intramolecular tensions in these
molecules. Consequently, device fabrication of xTPOTZ is
35 feasible through vacuum evaporation.
70 Fig. 4 UV/vis absorption, fluorescence (FL) and phosphorescence (PH,
inset) spectra of xTPOTZ in CH2Cl2 (10-6 M). PH spectra were measured
at 77 K after a delay of 300 μs.
The fluorescence (FL) spectrum of TPTZ in dilute solution
consists of two peaks at 366 and 475 nm, corresponding to
75 monomer and excimer emissions50-51, respectively (Fig. 4, S2 and
Table 1). Contrarily, only single-molecule emissions from dilute
solutions of xTPOTZ are observed around 367 nm, owing to
suppressed intermolecualr aggregation by steric hindrance of
peripheral DPPO groups. The wavelengths and profiles of
80 monomer emission of xTPOTZ and TPTZ are identical, in
accord with their similar electronic states. However, in films, FL
spectra of xTPOTZ and TPTZ are exactly the same in whole
range, including a main peak at 379 nm and a broad excimer
emission from 400 to 700 nm. Compared to spectra in solution,
85 the bathochromic shift of 20 nm and enhanced excimer emissions
Fig. 3 TGA and DSC (inset) curves of xTPOTZ.
3.3 Optical Performance
Electronic absorption spectra of xTPOTZ in dilute solutions (10-6
40 mol L-1 in CH2Cl2) comprise two bands around 270 and 230 nm,
ascribed to combined n→* and →* transitions between
6
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Journal of Materials Chemistry C
reflect the stronger intermolecular interactions of xTPOTZ in
reduction of 0.05 eV should be ascribed to either structural
solid states, which is beneficial to charge transport in their thin
films.
relaxation of triplet excited state for oTPOTZ or slight
View Article Online
conjugation extension by para-substitution of P=O groups in
DOI: 10.1039/C4TC02993C
Significantly, phosphorescence (PH) spectra of xTPOTZ were 15 pTPOTZ, which is in accord with the negligible influence of
5
recorded at 77 K with a time-resolved approach to evaluate their
triplet-state characteristics (inset in Fig. 4). Compared to TPTZ,
DPPO-substitution in xTPOTZ only slightly changes the profiles
of their PH spectra, implying the similar locations of their triplet
P=O substitution on triplet energy of other unipolar systems.52
Consequently, T1 values of xTPOTZ are comparable with that of
the conventional high-energy-gap host mCP (2.95 eV) and ~0.3
eV higher than that of sky blue phosphor FIrpic (2.65 eV),
states. Estimated from the 0-0 transitions, T1 value of mTPOTZ 20 affording effective suppression of exciton diffusion from EML in
10 is as high as 3.0 eV, identical to that of TPTZ; while, T1 levels of
oTPOTZ and pTPOTZ reach to 2.95 eV. The negligible triplet
PHOLEDs.
Fig. 5 Contours and energy levels of frontier molecular orbitals of xTPOTZ and spin density distributions of their triplet states according to DFT
calculation.
25
Therefore, with the feature of insulating linkage, DPPO
properties, especially T1 energy, ignorable for their device-
substitution successfully restrains conjugation-extension induced 30 performance discrepancy. Meanwhile, although not as dramatic,
energy loss of excited states for xTPOTZ, rendering their similar
pTPOTZ exhibits the enchanced intramolecualr electronic
optical properties, which sequentially makes their optical
communication at the same time of excited energy preservation,
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Journal of Materials Chemistry C
Page 8 of 13
which would improve its electron injecting/transporting ability.
injecting and hole blocking ability of mTPOTZ and pTPOTZ.
Among xTPOTZ, the lowest energy of unoccupied MOs for
3.4 DFT simulation
View Article Online
pTPOTZ further manifest its strongest intramolecualr electronic
DOI: 10.1039/C4TC02993C
To understand the nature of substitution effect of DPPO groups
on optoelectronic properties of xTPOTZ, the characteristics of
their ground states and T1 states were simulated with DFT
method at the level of 6-31G(d) (Fig. 5 and Table 1).
For TPTZ, electron cloud densities of its occupied molecular
orbitals (MOs) and LUMO+2 are uniformly dispersed in whole
molecule, while its LUMO and LUMO+1 are contributed by
communications between DPPO and TPTZ segments on the
35 basis of para-linkage.
5
It is noteworthy that with the predominant contributions to the
unoccupied MOs, TPTZ cores of xTPOTZ establish the main
electron transport channels. Therefore, the increased distributions
of unoccupied MOs on outer phenyls of their TPTZ cores would
40 benefit the intermolecular charge exchange. In this sense,
pTPOTZ should be superior in electron transportation with more
exposed configuration and more uniform distributions of
unoccupied MOs on its TPTZ core, owing to its para-
substitution DPPO groups.
10 triazole and respective two phenyls. It is showed that energy
difference between the LUMO and LUMO+1 is negligible as
0.001 eV, making whole TPTZ involved in electron injection.
Furthermore, energy levels of its HOMO and LUMO are -6.648
and -1.802 eV, revealing the strong electron injecting and hole
15 blocking ability. Contrarily, the asymmetrically twisted TPTZ
core of oTPOTZ induces the different distribution of its
unoccupied MOs. The LUMO is localized on triazole and two
phenyls with smaller dihedral angles, while the LUMO+1 is
dispersed on whole TPTZ core. Accordingly, the energy gap
20 between the LUMO and LUMO+1 remarkably increases to 0.3
eV. With planar TPTZ cores, the situation for unoccupied MO
distributions of mTPOTZ and pTPOTZ is similar to that for
TPTZ. Differently, the contributions of phenyls in their TPTZ
cores to these MOs are increased due to the electron-withdrawing
25 inductive effect of P=O, giving rise to their dramatically reduced
LUMO and LUMO+1 energy levels of -2.203 and -2.196 eV for
mTPOTZ and -2.227 and -2.226 eV for pTPOTZ, respectively.
Their HOMO and HOMO-1 are simultaneously decreased for
0.25 eV in comparison to those of TPTZ. Therefore, the
45
T1 energy levels of xTPOTZ were estimated as the energy
gaps between their ground states and T1 excited states, which are
2.975, 3.093 and 3.031 eV for oTPOTZ, mTPOTZ and
pTPOTZ, respectively. These values are in accord with optical
analysis results and high enough for suppressing triplet exciton
50 diffusion in blue PHOLEDs. According to spin density
distribution (SDD), T1 states of TPTZ and oTPOTZ are more
extended with the locations on triazole and two phenyl groups,
which are overlapped with their LUMOs. Contrarily, the stronger
inductive effect of DPPO groups in mTPOTZ and pTPOTZ
55 makes their T1 states concentrated to triazole and one phenyl
moieties and thereby thoroughly embedded in molecular
frameworks. Considering the charge-exchange Dexter mechanism
of triplet energy transfer, the shielded triplet states of mTPOTZ
and pTPOTZ would be superior in blocking triplet exciton
60 diffusion from EMLs in their devices.
30 employment of DPPO groups indeed enhances the electron
Table 1 Physical properties of xTPOTZ and TPTZ.
f
eg
Compound
oTPOTZ
mTPOTZ
pTPOTZ
TPTZ
Absorption
(nm)
Emission
(nm)
S1
T1
Tg/Tm/Td
(oC)
-/273/429
HOMO
(eV)
LUMO
(eV)
ER
(cm2 V-1 s-1)
4.1×10-6
(eV)
4.01c
4.54d
4.01c
4.68d
3.98c
4.64d
4.16c
4.99d
(eV)
2.95e
2.96d
3.00e
3.09d
2.95e
3.03d
3.00e
3.10d
(eV)
0.565
228, 263a
228, 270b
228, 272a
229, 278b
227, 281a
228, 289b
222, 270a
224, 281b
367a
-6.40g
-6.25d
-6.64g
-6.89d
-6.64g
-6.87d
-6.24g
-6.26d
-3.26g
-1.71d
-3.34g
-2.20d
-3.46g
-2.23d
-3.22g
-1.18d
295, 379b
367a
-/284/466
-/320/500
-/232/259
0.373
0.379
0.277
1.1×10-4
3.4×10-4
-
295, 379b
367a
295, 379b
366, 482a
295, 379b
a in CH2Cl2 (10-6 mol L-1); b in film; c estimated according to the absorption edges; d DFT calculated results; e calculated according to the 0-0 transitions of
the phosphorescence spectra; g calculated according to the equation HOMO/LUMO = 4.78 + onset voltage; f anion reorganization energy calculated by
DFT method; g estimated according to field-dependent SCLC model on the basis of electron-transporting-only devices.
65
Consequently, FMO and triplet-state locations of xTPOTZ are
for the single reversible peaks of oTPOTZ and mTPOTZ at -
successfully adjusted in virtue of steric hindrance and inductive 80 1.67 and -1.60 V correspond to the LUMO energy levels of -3.26
effects of DPPO groups and substitution position variation, which
rationally corresponds to the improved electron affinity and
exciton blocking ability of mTPOTZ and pTPOTZ.
and -3.34 eV, respectively. Compared to TPTZ, the 0.12 eV
reduced LUMO of mTPOTZ can be rationally attributed to
enhanced electron affinity of its TPTZ core by the electron-
withdrawing inductive effect of P=O groups, which also
85 neutralizes remedy the defect of oTPOTZ with twisted TPTZ
core. As expected, the first reversible reduction peak of pTPOTZ
dramatically shifts to -1.49 V with an onset voltage of -1.32 V,
corresponding to the LUMO of -3.46 eV. Therefore, in
comparison with mesa-substituted mTPOTZ, a further LUMO
90 reduction of 0.12 eV for pTPOTZ is achieve by enhanced
inductive effect of P=O groups at para positions of TPTZ, which
is in accord with DFT simulated results. The amplified
intramolecular interactions between TPTZ core and DPPO
70 3.5 Electrical Performance
Electrochemical analysis of xTPOTZ was performed to
experimentally validate improved carrier injecting ability of these
materials (Fig. 6 and Table 1). Reduction voltammograms of
TPTZ consisted of one reversible peak at -2.00 V with an onset
75 voltage of -1.56 V, corresponding to the LUMO energy of -3.22
eV. The reduction behaviours of oTPOTZ and mTPOTZ are
ascribed to their TPTZ cores, verified by their voltammograms
similar to that of TPTZ. The onset voltages of -1.52 and -1.44 V
8
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Journal of Materials Chemistry C
groups of pTPOTZ sequentially result in its unique second
where V is electrical voltage, d is film thickness, 0 is the average
mobility, 0 is the permittivity of free space (8.854×10-12 F m-1),
and  is dielectric constant of the filmD. OTI:h1e0.1e0s3tVi9mie/Cwa4tAeTrdtCic0le2O9e n9lo3infCe
oTPOTZ is 4.1×10-6 cm2 V-1 s-1, which is comparable to that of
reversible DPPO-attributed reduction peak at -2.06 V, revealing
the auxiliary function of peripheral DPPO groups for electron
injection and transportation in pTPOTZ.
5
As ETMs, the cycle-to-cycle stability under reduction 45 TPBI. Inspiringly, the e of pTPOTZ is dramatically improved to
processes is one of the key indicators. The reduction
voltammograms of xTPOTZ was measured in a range of -2.5-0
V with 10 times of repetition (Fig. S3). The peak position and
profile of the curves are unchanged during whole cyclic processes,
3.4×10-4 cm2 V-1 s-1 as 3 and 100 folds of mTPOTZ and
oTPOTZ, owing to its exposed TPTZ core and enhanced
intramolecular electronic coupling under para-substitution
(Table 1). Consequently, accompanied with its much higher
10 indicating the excellent charge-discharge reversibility of 50 triplet energy, pTPOTZ simultaneously achieves the e
xTPOTZ, which benefits to not only electron injection but also
comparable to that of TmPyPB.
intermolecular electron transfer.
It is showed that APO groups as insulating linkage with strong
polarizability can be promising building blocks for high-energy-
gap ETMs. Although their steric configurations would impede
55 intermolecular interactions, at appropriate substitution position,
APO groups can increase molecular electron affinity and assist
electron transport on the basis of enhanced intramolecualr
electronic coupling. Therefore, PO modification can indeed
effectively integrate and harmonize high excited-state energy and
60 electron transporting ability for high-energy-gap ETMs, as
evidenced by mTPOTZ and pTPOTZ with the T1 value of ~3.0
eV and e on the order of 10-4 cm2 V-1 s-1.
15 Fig. 6 Cyclic voltammograms of xTPOTZ and TPTZ measured in THF
with tetra-n-butylammonium hexafluorophosphate as supporting
electrolyte (0.1 mol L-1) at the scanning rate of 100 mV S-1.
On the other hand, according onset voltages of their irreversible
oxidation peaks, the HOMO energy levels of TPTZ and
20 xTPOTZ are estimated as -6.24, -6.40, -6.64 and -6.64 eV
(Table 1), respectively, exhibiting the effect of DPPO
substitution on hole blocking enhancement. Especially for
mTPOTZ and pTPOTZ, their remarkably low-lying HOMOs
make them favourable among the most efficient hole blocking
25 materials, such as 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline
(BCP, -6.50 eV) and TmPyPB (-6.68 eV).
Fig. 7 I-V characteristics of xTPOTZ-based electron-only devices in the
65
range of operating voltages for their PHOLEDs. Symbols show the
experimental data, while lines are the fitting curves according to field-
dependent SCLC model.
The single-layer electron-only transporting devices of
xTPOTZ were fabricated to estimate their intrinsic electron
transporting characteristics with a configuration of ITO|LiF (1
30 nm)|xTPOTZ (100 nm)|LiF (1 nm)|Al, in which LiF served as
electron-injecting layer. Ampere-volt (I-V) characteristics of these
electron-only transporting devices reveal the same dependence on
molecular structures of xTPOTZ, as deduced from single crystal
data and DFT simulation results (Fig. 7). Current density (J) of
35 pTPOTZ-based devices was about one and two orders of
magnitude larger than those of mTPOTZ and oTPOTZ-based
devices, respectively. The current density in these devices follows
the field-dependent space charge limited current (SCLC) model
of Mott-Gurney law, which is given as53:
3.6 Device Performance
Encouraged by the excellent optoelectronic properties of
70 xTPOTZ, especially their high T1 value of ~3.0 eV, xTPOTZ
are competent as ETMs in sky blue PHOLEDs. Therefore,
PHOLEDs A, B and C by employing mCP as host and FIrpic as
sky blue phosphor were fabricated with the most conventional
trilayer configuration of ITO|MoO3 (10 nm)|TAPC (70
75 nm)|mCP:FIrpic (10%, 20 nm)|xTPOTZ (35 nm)|LiF (1 nm)|Al,
where TAPC served as hole transporting/exciton blocking layer
and xTPOTZ was utilized as electron transporting/exciton
blocking material (Scheme 2). The control device D with
TmPyPB as ETL was also fabricated for comparation.
80 Electroluminescence (EL) spectra were ascribed to the pure sky-
blue emissions from FIrpic with the main peak at 475 nm and
favorable Commission Internationale Ed I'eclairage (CIE)
coordinates of (0.15, 0.30) (inset in Fig. 8a), reflecting the
2







9
8
V
d
V
d
40
eq.1

JSCLC

00 exp 0.89
3
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Journal of Materials Chemistry C
Page 10 of 13
effective exciton confinement in EML.
that compared to B, the increase tendency of luminance for C was
slowed down at high J, showing the worsened quenching in C at
View Article Online
DOI: 10.1039/C4TC02993C
40 high exciton concentrations.
Scheme 2 Configuration and energy level diagram of PHOLEDs
and the chemical structures of employed materials.
5
According to energy levels diagram of these sky blue
PHOLEDs (Scheme 2), the electron-injecting barrier between
FIrpic and cathode was 0.8 eV, which was 0.5 eV lower than that
between mCP and cathode. In this case, the dominant electron
10 injection channel in EML would be direct charge capture by
FIrpic. It is noteworthy that the LUMO energy levels of
xTPOTZ are between the LUMO and Femi energy levels of
FIrpic and cathode, dividing the energy gap into two parts of ~0.4
eV. In this case, xTPOTZ can facilitate the direct electron
15 capture on FIrpic as intermediates to provide two sequential
electron-transfer steps with reduced injection barriers. As
expected, xTPOTZ endowed their devices with the extremely
low turn-on voltages of ~2.6 V, which were equivalent with the
photon energy as 2.65 eV of FIrpic-originated emission according
20 to its first emission peak at 475 nm (Fig. 8a and Table 2).
Compared to D, the threshold voltages of A, B and C were
dramatically reduced for 1.0 V, reflecting the barrier-free electron
injection in these devices through xTPOTZ with more low-lying
LUMOs. Furthermore, at practical luminance of 100 and 1000 cd
25 m-2, operating voltages of C with pTPOTZ as ETL were less
than 3.8 and 5.2 V, respectively, which were 0.7 V lower than
those of B. On account of the major carrier in these devices as
hole, the highest e of pTPOTZ facilitated charge flux balance in
its devices and enhance exciton recombination for high
30 luminance under the same voltages. For the same reason,
oTPOTZ with the lowest e induced the smallest luminance of
its devices. This correlation was further indicated by the
proportionality between brightness of A, B and C and their J. The
tendency of J for these devices was approximately consistent
35 with that of I-V characteristics for xTPOTZ-based electron-only
transporting devices, further verifying the effectiveness of
xTPOTZ to electron transporting and hole blocking. It is notable
Fig. 8 a) J-Voltage-Luminance characteristics and EL spectra (inset) of
mCP and FIrpic based devices A, B, C and D with xTPOTZ and
TmPyPB as ETL; b) Efficiencies curves of A, B, C and D versus
45
luminance.
The uniform triplet energy and triplet-state locations of
xTPOTZ exclude the correlation of triplet characteristics with
their various device performance, establishing the direct
correspondence between charge flux balance and EL efficiencies
50 and efficiency stability. Consequently, beneficial to the most
balanced carrier flux by pTPOTZ with the strongest electron
injecting and transporting ability, the efficiencies of C were the
highest among these devices with maxima of 39.3 cd A-1 for
current efficiency (CE), 34.0 lm W-1 for power efficiency (PE)
55 and 19.9% for EQE, which were improved for more than 50%
compared to those of TmPyPB-based D (Fig. 8b and Table 2).
The maximum CE and EQE of C were achieved at a practical
luminance more than 100 cd m-2, accompanied with alomost
negligible roll-offs less than 4% at 1000 cd m-2. Because of 3
60 times low e for mTPOTZ, the maximum efficiencies of its
device B were remarkably reduced to 23.8 cd A-1, 20.1 lm W-1
and 11.8%, respectively, which were only about a half of those of
C, but 1.6 folds of those of A. The worst EL performance of A
among these devices should be ascribed to its most unbalanced
65 charge flux, due to the lowest e of oTPOTZ among xTPOTZ.
At high voltages, J of these devices followed SCLC model, in
10
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Journal of Materials Chemistry C
direct proportion to charge mobility of whole organic
reduced roll-offs. This work demonstrated an effective strategy of
semiconductor layer (eq. 1). Considering h of TAPC as hole 50 APO modification for enhancing electrical performance of n-type
transporting layer (HTL) on order of 10-3 cm2 V-1 s-1, which is
three orders of magnitude larger than e of oTPOTZ as ETL,
hole would be extremely excessive in A at high voltages,
worsening charge unbalance and triplet exciton-polaron
quenching (TPQ) in its emissive layer. Therefore, serious roll-
offs of A at 1000 cd m-2 were 5 and 20 folds of those of B and C,
respectively. The worse efficiency stability of B than that of C
semiconductors without influencing their optical proVpieewrtiAertsic.le Online
DOI: 10.1039/C4TC02993C
5
Acknowledgement
This project was financially supported by NSFC (51373050 and
61176020), New Century Excellent Talents Supporting Program
55 of MOE (NCET-12-0706), Key Project of MOE (212039),
Education Bureau of Heilongjiang Province (1252-NCET-005
and 2014CJHB005), Science and Technology Bureau of
Heilongjiang Province (ZD201402) and the Fok Ying-Tong
Education Foundation for Young Teachers in the Higher
60 Education Institutions of China (141012).
10 further verified this correlation between charge flux balance and
efficiency roll-offs.
Consequently, compared to the classical high-energy-gap ETM
TmPyPB, pTPOTZ endowed its conventional mCP and FIrpic
based sky blue PHOLEDs with the much reduced driving
15 voltages and the 1.5-fold larger efficiencies, which should be the
cumulative result of its triplet energy of ~3.0 eV, the low-lying
LUMO at -3.5 eV and high e on order of 10-4 cm2 V-1 s-1.
Notes and references
a Key Laboratory of Functional Inorganic Material Chemistry, Ministry
of Education, School of Chemistry and Materials, Heilongjiang
University, 74 Xuefu Road, Harbin 150080, P.R. China. E-mail:
65 hxu@hlju.edu.cn
Table 2 EL performance of mCP and FIrpic-based devices A-D.
ETM
Operating
Maximum
Efficiency Roll-Offs (%)c
CE PE EQE
b State Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences, 5625
People Street, Changchun 130022, P.R. China. E-mail:
mdg1014@ciac.jl.cn
Voltage (V)a Efficienciesb
<2.6, <4.9, 16.9,14.3,8.3 <9, <75 <31, <90 <10, <75
<9.3
oTPOT
Z
mTPOT
Z
pTPOT
Z
<2.7, <4.5,
<6.3
23.8, 20.1,
11.8
-, <15
<17, <50
<9, <40
-, <4
-, <15
70 c Institute of Advanced Materials, Nanjing University of Posts and
Telecommunications, 66 Xinmofan Road, Nanjing 211816, P. R. China.
E-mail: iamrfchen@njupt.edu.cn
<2.5, <3.8,
<5.6
39.3, 34.0,
19.9
-, <4
-, <4
a Electronic Supplementary Information (ESI) available: Cif files of
single crystals of oTPOTZ and mTPOTZ, solid-state optical properties
75 and electrochemical stability of xTPOTZ. See DOI: 10.1039/b000000x/
1. H. Xu, R. Chen, Q. Sun, W. Lai, Q. Su, W. Huang and X. Liu, Chem.
Soc. Rev., 2014, 43, 3259-3302.
<3.4, <4.9,
<6.8
24.7, 12.4,
13.1
-, <0.5
-, <0.5
TmPyP
B
a in the order of onset, 100 and 1000 cd m-2; b in the order of CE (cd A-1),
20 PE (lm W-1) and EQE (%); c in the order of 100 and 1000 cd m-2.
2. M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M. E.
Thompson and S. R. Forrest, Nature, 1998, 395, 151-154.
80 3. S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B.
Lussem and K. Leo, Nature, 2009, 459, 234-238.
4. M. C. Gather, A. Köhnen and K. Meerholz, Adv. Mater., 2011, 23,
233-248.
4. Conclusions
In summary,
a series of TPPO substituted 1,3,5-triazole
derivatives xTPOTZ were designed and prepared as n-type
semiconductors. APO modification remarkably enhances the
25 morphological and thermal stability of xTPOTZ with Tm beyond
5. K. T. Kamtekar, A. P. Monkman and M. R. Bryce, Adv. Mater., 2010,
85
90
95
22, 572-584.
o
o
250 C and Td more than 400 C. As an insulating polar group,
DPPO substituents endow xTPOTZ with the preserved optical
properties of their TPTZ cores, such as T1 of ~2.95 eV for
effective exciton blocking, and improved electrical performance,
30 including higher electron affinity and mobility. Both DFT
simulation and CV analysis showed the low-lying LUMO and
HOMO energy levels of xTPOTZ, compared to those of TPTZ,
revealing the enhanced electron injecting and hole blocking
ability after APO modification. It is verified that the substitution
35 position of DPPO groups in xTPOTZ correlates to their electrical
performance. The twisted configuration of oTPOTZ by steric
hindrance of ortho-substituted DPPO groups rendered its weakest
electron affinity and smallest electron mobility, while planar
mTPOTZ and pTPOTZ achieve the LUMO and HOMO energy
40 levels of about -3.4 and -6.6 eV, respectively, accompanied with
the extremely high e on the order of 10-4 cm2 V-1 s-1. Among
xTPOTZ, pTPOTZ shows the strongest electroactivity, owing to
its exposed TPTZ core and enhanced intramolecular electronic
communication through its para-substitution configuration. On
45 the basis of trilayer conventional mCP and FIrpic-based sky blue
PHOLEDs, pTPOTZ revealed its superiority as ETL with the
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Page 13 of 13
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C4TC02993C
Triazole and triphenylphosphine oxide (TPPO) hybrids xTPOTZ were constructed as
electron-transporting materials in blue phosphorescent light-emitting diodes to achieve high triplet
energy beyond 2.95 eV, low-lying lowest unoccupied molecular orbitals at ~-3.4 eV and high
electron mobility up to 3.4×10-4 cm2 V-1 s-1, which are impressive among high-energy-gap
electron-transporting materials reported so far. As the result, pTPOTZ endowed its typical mCP
and FIrpic-based blue PHOLEDs with the ultralow onset voltage of 2.6 V and favourable external
quantum efficiency up to ~13%, revealing the effectiveness of APO modification for
high-performance ETM development.