Extremely condensing triplet states of DPEPO-type hosts through constitutional isomerization for high-efficiency deep-blue thermally activated delayed fluorescence diodes

Article abstract of DOI:10.1039/c5sc04848f

The similarity of thermally activated delayed fluorescence (TADF) dyes and their hosts as pure organic molecules makes hosts predominant in intermolecular interactions and crucial to exciton harvesting and utilization in TADF diodes. DPEPO is the most pop

Full text of DOI:10.1039/c5sc04848f

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ARTICLE
Extremely Condensing Triplet States of DPEPO‐Type Hosts through
Constitutional Isomerization for High‐Efficiency Deep‐Blue
Thermally Activated Delayed Fluorescence Diodes
Received 00th January 20xx,
Accepted 00th January 20xx
Jing Zhang, Dongxue Ding, Ying Wei* and Hui Xu*
The similarity of thermally activated delayed fluorescence (TADF) dyes and their hosts as pure organic molecules makes
hosts predominant in intermolecular interactions and crucial to exciton harvesting and utilization in the TADF diodes.
DPEPO is the most popular high‐energy‐gap blue TADF host with steric ortho‐substituted diphenylphosphine oxide (DPPO)
groups for intermolecular interaction suppression, but suffering from serious efficiency roll‐offs due to its weak
electroactivity. On contrary, para‐substituted DPPO with small steric hindrance is superior in intramolecular electronic
coupling. In this work, four constitutional isomers of DPEPO are constructed as diphenylether (DPE) with two
diphenylphosphine oxide (DPPO) groups substituted at either 2 or 4 positions, namely 22'DPEPO (viz. DPEPO), 24DPEPO,
24'DPEPO and 44'DPEPO, respectively. On the basis of separation configuration, the steric effect and electroactivity of
ortho‐ and para‐substituted DPPOs are successfully integrated in 24'DPEPO, accompanied with the remarkably reduced
intermolecular interactions by its unsymmetrical configuration. Compared to its congeners, 24'DPEPO shows the rigid
structure and locally excited states similar to 22'DPEPO for interaction suppression and improved charge mobility
comparable to 44'DPEPO for charge flux balance. Significantly, in virtue of the predominant orientation effect of ortho‐
DPPO on T1 location, its T1 state is extremely condensed to a single phenyl, protected by its rest five phenyls from
intermolecular interactions at maximum extent. Consequently, 24'DPEPO endowed its DMAC‐DPS‐based deep‐blue
devices with the state‐of‐the‐art performance, including high color purity with chromaticity coordinates of (0.16, 0.17),
external quantum efficiency (EQE) beyond 20% and EQE roll‐off as low as 32% at 1000 cd m‐2. It is showed that device
performance of 24'DPEPO was far beyond simple integration of those of 22'DPEPO and 44'DPEPO, verifying the
significance of host optimization.
DOI: 10.1039/x0xx00000x
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adopted to dilute emitters and restrain exciton quenching in
TADF diodes.6 However, in contrast to phosphorescent
counterparts, as the same pure organic materials, TADF dyes
1. Introduction
Exciton harvesting and utilization is crucial to realize efficient
organic light‐emitting diodes (OLEDs).2 After the great success
of phosphorescent heavy‐metal complexes in harvesting triplet
excitons through spin‐orbital coupling,3 thermally activated
delayed fluorescence (TADF) dyes emerge in recent years,
whose near‐zero singlet‐triplet splitting can facilitate triplet‐to‐
singlet conversion, in virtue of reverse intersystem crossing
(RISC).4 TADF dyes are mostly pure organic compounds
featured donor‐acceptor (D‐A) structures with high molecular
polarity and populated excited states characteristic of charge
transfer (CT).5 On account of the involvement of triplet exciton
in electroluminescence process and the strong intermolecular
interactions of TADF dyes, host materials are commonly
show the molecular components and excited characteristics
similar to their hosts, especially their comparable triplet
lifetimes. It is rational that as the majority of emissive layers
(EML), host is dominant in intermolecular interplays, including
host‐host and host‐dopant interactions.8 In this case, besides
of the basic functions for host materials, viz. carrier flux
balance and energy transfer, their structures and
optoelectronic properties would exert great influences on
collision‐induced exciton quenching effects, such as triplet‐
triplet annihilation (TTA, Fig. 1a) and triplet‐polaron quenching
(TPQ, Fig. 1b).9 Therefore, compared to phosphorescent OLEDs,
host materials in TADF diodes play a more vital role in exciton
harvesting and utilization.8, 10
The collision between two triplet excitons can render
TTA.11 That is, one exciton is annihilated through transferring
its energy to the other, making the latter excited to a higher
level and then recovered to the first triplet (T1) state through
internal conversion (IC) (Fig. 1a). Meanwhile, TPQ occurs
during collision between exciton and polaron, which induces
the nonradiative deactivation of the former through charge
a. Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education
& School of Chemistry and Material Science, Heilongjiang University, 74 Xuefu
Road, Harbin 150080, P. R. China. E‐mail: hxu@hlju.edu.cn and ywei@hlju.edu.cn.
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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optoelectronic properties of blue TADF host materials is really
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challenging, since many key issues are sDtiOllI:u1n0c.1l0e3a9r/,Ce5SsCp0e4c8ia4l8lyF
the influences and correlations of their electrical properties
and excited‐state characteristics and their merits.19 In our
previous works, it is showed that these two factors strongly
depend on the functional linkage site and molecular
sequence.20 In this sense, the isomerization of DPEPO may
provide a feasible way to clarify the key determinants of EL
performance for deep blue TADF host materials.
In this contribution, four constitutional isomers of DPEPO
with a collective name of
m
DPEPO are constructed as
bis(diphenylphosphoryl)
diphenylether with two
diphenylphosphine oxide (DPPO) groups at either 2 or 4
positions of their diphenylether (DPE) cores, named 22'DPEPO
(viz. DPEPO), 24DPEPO
, 24'DPEPO and 44'DPEPO, respectively
(
Fig. 1d). DPPO at 2‐position indicates the strong steric effect
and dominant localization effect on T1 state, while DPPO at 4‐
position is superior in enhancing intramolecular electronic
coupling. In virtue of the asymmetrical and separated
structure for 24'DPEPO, the functions of its two kinds of DPPO
groups are successfully integrated. Significantly, its T1 state is
extremely condensed to a single phenyl, embedded and
protected by its rest five phenyls at maximum extent.
Meanwhile, compared to 22'DPEPO, electron mobility of
24'DPEPO is dramatically improved for about 50 times.
Consequently, its DMAC‐DPS‐based deep‐blue devices realized
the state‐of‐the‐art performance, including high color purity
with CIE coordinates of (0.16, 0.17), EQE beyond 20% and EQE
roll‐off as low as 32% at 1000 cd m‐2. The efficiencies of
24'DPEPO‐based devices were improved for 20% in
comparison to those employing 22'DPEPO as host; meanwhile,
the roll‐offs of the former were halved. It is clear that host
engineering is a gateway to resolve the key issues of TADF
devices in exciton harvesting and utilization.
Fig. 1. Mechanisms of triplet‐triplet annihilation (TTA,
a) and triplet‐
polaron quenching (TPQ, b), two T1 state protection strategies through
surface modification and location condensation (
DPEPO‐type constitutional isomers DPEPO (d).
c) and molecular design of
m
exchange (Fig. 1b).12 In consequence, three approaches would
be effective for quenching suppression: (i) embedding and/or
segregating T1‐localized moieties of host and dopant molecules
to protect triplet excitons from collision quenching with
surrounding modification13 and T1 state condensation14, in
spite of only few reports about T1 condensation (Fig. 1c); (ii)
reducing collision probability through shortening exciton
lifetime; (iii) enhancing charge flux balance and unifying
exciton recombination to decrease polaron concentration. In
this case, the excited‐state characteristics and carrier‐
transporting ability of host materials should be determinant to
electroluminescence (EL) efficiencies.
It is known that CT‐type exciton is labile and highly
sensitive to environment.15 Consequently, although 100%
internal quantum efficiencies were already achieved for TADF
devices, most of them suffered from serious efficiency roll‐offs,
especially for blue TADF diodes utilizing high‐energy excitons.
DPEPO is the most popular host for blue TADF diodes.4a, 5e, 5i, 5j,
16 Its multi‐insulating structure and steric effect of P=O groups
give rise to its high T1 value of ~3.0 eV for efficient energy
transfer to blue TADF dyes, e.g. bis[4‐(9,9‐dimethyl‐9,10‐
dihydroacridine)phenyl]sulfone (DMAC‐DPS) (T1 = 2.90 eV)16a
and quenching suppression.17 The external quantum efficiency
(EQE) of its deep‐blue TADF devices with DMAC‐DPS as emitter
reached to ~20%, however, accompanied by remarkable roll‐
off more than 90% at 1000 cd m‐2, indicating the serious
exciton quenching.18 Nevertheless, how to optimize
2. Results and Discussions
2.1. Design and Synthesis
DPPO is an electron‐withdrawing group with big steric
hindrance. Its dominant function bears on its substitution
positions. For instance, DPPOs at ortho‐positions of 22'DPEPO
are superior in steric effect; while, DPPOs in 44'DPEPO can
enhance intramolecular coupling with electron‐donating ether
bridge at para‐position (Fig. 1c). In this case, intermolecular
interaction in 22'DPEPO at the DPPO‐substituted side can be
thoroughly blocked to effectively suppress TTA. However,
electronic coupling in 22'DPEPO is remarkably weaker due to
its ortho‐substitution configuration.17 In contrast, sufficient
electronic coupling in 44'DPEPO can support balanced hole
and electron flux in its EMLs, facilitating exciton recombination
and reducing TPQ.21 Nonetheless, its DPE chromophore is
completely exposed to intermolecular interactions, worsening
TTA. Therefore, it is well‐reasoned to combine these two kinds
of DPPO substitutions in one single molecule for integrating
their advantages and thereby simultaneously suppressing TTA
and TPQ.
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DOI: 10.1039/C5SC04848F
Fig. 2. DFT and TDDFT simulations of ground, S1 and T1 excited states for
and the spin density distributions of T1 states.
m
DPEPO: configurations, the LUMO and HOMO orbital distributions of ground states
More significantly, in our recent works, it was showed that the molecules can be condensed into a designated location by
DPPO substitution can regulate T1 location, which should be DPPO with dominant orientation effect. The narrow T1 location
attributed to the influence of P=O group on electronic cloud can amplify the protective action of surrounding groups and
distribution.14, 20a, 22 In this sense, the orientation effect of further reduce its involvement probability in intermolecular
DPPO on T1 state would be related to its substitution position. interactions. Regarding to DPEPO‐type analogues, since all of
Therefore, with asymmetrical DPPO substitutions, T1 states of their phenyls are separated by insulating linkages, it can be
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DOI: 10.1039/C5SC04848F
Fig. 3. Natural transition orbitals (NTO) of the S1 and T1 states for
mDPEPO.  and f refer to the associated weight and oscillator strength, respectively.
expected to confine their T1 states into a single phenyl, which 24'DPEPO shows Td equivalent with that of 44'DEPPO and Tm
is almost the minimum for conjugated compounds with comparable to that of 22'DPEPO. Therefore, one ortho‐
respect to spatial scale.
substituted DPPO in 24'DPEPO already revealed strong enough
With these considerations, two other asymmetrical molecular rigidity.
isomers, namely 24DPEPO and 24'DPEPO, incorporating both
ortho‐ and para‐linked DPPO groups were designed. Two
2.2. DFT and TDDFT Simulation
DPPOs in 24DPEPO are bonded with the same phenyl, while For insight into the nature for the effects of DPPO substitution
those in 24'DPEPO are respectively bonded with two different on electrical and excited‐state characteristics of
mDPEPO, DFT
phenyls of DPE. The superposition of two DPPOs in 24DPEPO and TDDFT calculations were performed at the level of
may render the mutual acceleration or counteraction of their B3LYP/6‐31+g(d, p), taking into account computational
effects. On contrary, in virtue of insulating ether bridge, two accuracy and cost.
separated DPPOs of 24'DPEPO can perform their functions
independently. In consequence, it is the most likely for ground (S0), the first singlet (S1) and T1 states are showed in Fig.
24'DPEPO to integrate merits of 22'DPEPO and 44'DPEPO . Ascribed to the biggest steric hindrance of its two ortho‐
Simultaneously, multi‐insulating linkage establishes the substituted DPPOs, the dihedral angle of DPE in 22'DPEPO at
similarity of DPEPO to the maximum extent, excluding the S0 state is the largest among DPEPO, which is almost
The optimized molecular configurations of mDPEPO at
.
2
m
m
interferences from undesirable intramolecular interplays and preserved at its S1 and T1 states. The dihedral angles of DPE in
conjugation variation, which simplifies the influencing factors 44'DPEPO at these states are the smallest, in accord with the
of their device performance into electrical and excited‐state smaller steric hindrance of its DPPOs at para‐position.
characteristics.
Compared to ground state, its excited‐state configurations are
m
DPEPO can be conveniently prepared through Pd‐ adjusted. Nevertheless, 24DPEPO shows the largest
catalyzed phosphorylation with good yields more than 50%. configuration variation at excited states, which is ascribed to
Their chemical structures were fully characterized on the basis the rotational phenyl of its DPE. In this sense, both ortho‐ and
of NMR spectra, mass spectra and elementary analysis. Among para‐ DPPO substitution can reduce excited‐state structural
m
DPEPO, the biggest steric hindrance in 22'DPEPO increases relaxation. Consequently, the molecular configurations of
its intramolecular tension, rendering its lowest temperature of 24'DPEPO at S0, S1 and T1 states are almost unchanged,
decomposition (Td) as 322 oC; while its rigid and accompanied by the dihedral angles of DPE close to those of
monosymmetrical structure results in its highest melting point 22'DPEPO. In comparison to 24DPEPO, the fixed molecular
o
(Tm) of 280 C (Fig. S1 and Table 1). On contrary, owing to configurations of 24'DPEPO and 22'DPEPO make them
negligible steric hindrance in 44'DPEPO, its Td is remarkably superior in reducing relaxation‐induced excited‐energy loss,
improved to 417 oC, however, accompanied with a reduced Tm which is beneficial to improve luminescent efficiencies.
o
of 190 C. The rotatable phenyl of DPE in 24DPEPO make its
The contours of the highest occupied and the lowest
thermal properties similar to those of 44'DPEPO. Importantly, unoccupied molecular orbitals (HOMO and LUMO) for
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DPEPO reveal the influence of DPPO substitution position on
hole and electron injecting ability, respectively. Although the
frontier molecular orbitals (FMO) of 22'DPEPO are respectively
localized on its DPE and DPPO groups, its LUMO energy level is
m
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DOI: 10.1039/C5SC04848F
the highest among
withdrawing ability of its DPPOs at 2‐position (Fig. 2
m
DPEPO, indicating the weakest electron‐
Fig. S2
,
and Table 1). The situations of 44'DPEPO and 24DPEPO are
similar with the partially separated HOMO and LUMO.
However, the superposition of DPPOs in 24DPEPO renders the
simultaneous reduced HOMO and LUMO energy levels. In
contrast, 44'DPEPO possesses the HOMO and LUMO almost
equivalent to that of 22'DPEPO and 24DPEPO, respectively,
manifesting the enhanced intramolecular electronic coupling
in 44'DPEPO and its ambipolar characteristics. As expected,
through incorporating the para‐substituted DPPO, in
comparison to 22'DPEPO, the HOMO energy level of 24'DPEPO
is preserved, while its LUMO energy level is reduced for 0.12
eV, which is similar to that of 44'DPEPO. Therefore, it is
showed that despite of its overlapped HOMO and LUMO
distributions, the charge injecting ability of 24'DPEPO is mainly
determined by its para‐linked DPE‐DPPO segment.
The calculated S1 and T1 energy of
mDPEPO is almost same
as ~4.7 and ~3.5 eV, supporting positive energy transfer to
DMAC‐DPS with the S1 and T1 value of ~2.9 eV (Fig. 2 and
Table 1). The similar excited energy of
mDPEPO is attributed to
their multi‐insulating structures. It is rational that on the basis
of monosymmetric configurations and two equivalent DPPOs,
the T1 states of 22'DPEPO and 44'DPEPO are uniformly
dispersed on their DPE groups. In accord with our previous
reports, DPPO substitution has the effect on T1 location
regulation.20a T1 state of 24DPEPO is mainly contributed by the
phenyl substituted with two P=O groups. Inspiringly, 24'DPEPO
shows T1 state thoroughly localized on the 2‐DPPO substituted
phenyl, reflecting the dominant orientation effect of ortho‐
substituted DPPO on T1 state. The T1 state of 24'DPEPO is
extremely condensed on a single phenyl as one of the smallest
conjugated units, which is embedded by its rest five phenyls.
Compared to its isomers, the exposure degree of T1 state of
24'DPEPO is doubtlessly the smallest, which can support the
most effective suppression of collision‐induced quenching
effects.
Fig. 4. (a) Electronic absorption spectra, room‐temperature emission
spectra of
m
DPEPO in CH2Cl2 (10‐6 M). Inset shows the time‐resolved
phosphorescence spectra of
100 s; (b) absorption and fluorescence (FL) spectra of polycrystalline
powder for DPEPO; (c) Time decay curves and emission spectra (inset) of
DMAC‐DPS‐doped DPEPO films (10%, 100 nm) through vacuum‐
evaporation.
mDPEPO in CH2Cl2 glass at 77 K after a delay of
The nature of electronic transitions for excited states of

m
m
DPEPO were further evaluated with natural transition
m
orbitals (NTO) of the first singlet and triplet excitations (Fig. 3
and Fig. S4).23 In the cases of NTOs about S0→S1 states for
"particle" locations. In accord with DFT results, triplet states of
22'DPEPO and 44'DPEPO are dispersed on their DPE, while
only a single phenyl of 24DPEPO and 24'DPEPO is involved in
their triplet transitions. Regarding to CT character of excitons
on TADF dyes, LE‐dominant excited states of hosts can provide
uniform and apparently neutral matrixes to restrain Coulomb
interaction induced CT exciton dissociation.
It is showed that 24'DPEPO successfully integrates the
merits of ortho‐ and para‐DPPO substitutions, including
excited structural stability and singlet characteristics
comparable to 22'DPEPO and FMO energy levels close to
44'DPEPO. In addition of significantly reinforced T1 state
m
DPEPO, "holes" are thoroughly localized on their DPE.
However, the distributions of "particles" are various that for
22'DPEPO and 24'DPEPO, "particles" are also dispersed on
their DPE, featuring locally excited (LE) state transition; while,
for 24DPEPO, "hole" is concentrated to one phenyl of DPE, and
for 44'DPEPO, "hole" is partially dispersed on DPPO groups,
characteristic of hybridized local and charge transfer excited
states. Nevertheless, the biggest oscillator strength (f) suggests
LE transition of DPE as the major part for 44'DPEPO, while CT
transition between two phenyls of DPE is dominant for
24DPEPO with the smallest f. NTOs of S0→T1 states for
m
DPEPO reveal LE character with the overlapped "hole" and
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protection through extremely condensing T1 location on the further investigated to exclude the solvent effeVcitesw A(rFtiicgle.O4nblin)e.
DOI: 10.1039/C5SC04848F
minimum unit phenyl, 24'DPEPO shows superiority in The main absorption bands are preserved in the solid‐state
quenching suppression and charge balance. In contrast, spectra, reflecting the limited intermolecular interactions.
24DPEPO exists some shortages of serious excited structural However, in contrast to the unchanged fluorescence emission
relaxation and CT‐dominant S1 characteristic.
of 22'DPEPO in solid state, 44'DPEPO shows an additional
broad aggregation‐induced emission band in long‐wavelength
range from 325 to 450 nm, in accord with the weak steric
effect of its para‐DPPOs. The flexible DPE in 24DPEPO also
2.3. Optical Properties
The absorption spectra of
m M
DPEPO in dilute solutions (10‐6
in CH2Cl2) consist of three bands around 230, 270 and 300 nm, facilitates aggregation, rendering a similar broad band around
corresponding to * transitions of DPPO and DPE and 340 nm. As expected, in virtue of the strong steric effect of
n→ * transition of DPE, respectively (Fig. 4a). The spectra of ortho‐substituted DPPO in 24'DPEPO with separation
22'DPEPO 24DPEPO and 24'DPEPO are almost identical, but configuration, the aggregation in solid state is successfully
different to that of 44'DPEPO. As indicated by DFT result, the suppressed, making one of its solid‐state emission bands at
→

,
smallest dihedral angle of DPE in 44'DPEPO facilitates

‐
315 nm almost overlapped with its emission in solution.
interactions between two phenyls of DPE and decreases p‐

Significantly, solid‐state emissions of 24'DPEPO and 24DPEPO
conjugation between O atom and phenyls, accordingly contain additional distinct multi‐peak bands coincident with
enhancing * transition and weaken n→ * transition. blue parts of their phosphorescence, whose lifetimes ( ) are
Nevertheless, the optical energy gaps of DPEPO estimated also comparable as ~7 s (Fig. S3); while, for 22'DPEPO and
by absorption edges are almost identical as 3.9 eV, consistent 44'DPEPO, their solid‐state phosphorescence is fully quenched

→



m

with TDDFT results (Table 1).
through nonradiative transitions due to their exposed T1 states.
In dilute solutions, the small volumes of solvent molecules
In the same way, the fluorescence (FL) spectra of 22'DPEPO
,
24DPEPO and 24'DPEPO in dilute solutions (10‐6 M in CH2Cl2) make collision with every parts of solute molecules facile,
are exactly the same in range and profile. While, FL spectrum resulting in serious solvent quenching effects on T1 states,
of 44'DPEPO reveals blue shift of ~10 nm. Nonetheless, there which can be excluded in the solid state. Recently, room‐
are large‐range spectral overlaps between FL emissions of temperature phosphorescence from crystalline pure organic
m
DPEPO and absorption of DMAC‐DPS from 275 to 350 nm, molecules are realized on the basis of D‐A systems with
facilitating Fӧrster resonance energy transfer (FRET). The low‐ overlapped S1 and T1 locations.24 It is rational to attribute the
temperature time‐resolved phosphorescence (PH) spectra of visible room‐temperature solid‐state phosphorescence of
m
DPEPO are identical with the same peaks and profiles, which 24'DPEPO and 24DPEPO to their extremely condensed T1 state,
is in accord with their similar T1 transition characteristics as effectively protected from interaction‐induced triplet
shown by TDDFT simulation (inset of Fig. 4a). Estimated with quenching, which validates T1 condensation as a feasible and
0→0 transitions, T1 value of
supporting the efficient triplet energy transfer to DMAC‐DPS
The similar excited energy of these DPEPO‐type isomers fully
mDPEPO is equivalent as 2.98 eV, effective strategy to achieving and enhancing room‐
.
temperature phosphorescence from pure organic systems.
The host‐dopant energy transfer was further investigated
verifies the effectiveness of multi‐insulating linkage in excited through steady‐state and transient photoluminescence (PL)
energy preservation.
spectra of vacuum‐evaporated DMAC‐DPS‐doped
mDPEPO
The optical properties of
mDPEPO in solid state were films (Fig. 4c). The emission spectra are almost identical with
Table 1. Physical properties of mDPEPO.
22'DPEPO
24DPEPO
24'DPEPO
44'DPEPO
Absa (nm)
227, 274, 284, 300b
228, 274, 284, 300b
228, 253, 274, 284, 294b
228, 250, 273, 283b
225, 283c
225, 283c
225, 283c
225, 278c
Emd (nm)
S1/T1 (eV)
312b/309, 389c
3.92e/2.98f
4.82/3.64g
‐/280/322
311b/316, 349, 418, 447c
3.94e/2.98f
310b/315, 412, 433, 461c
3.92e/2.98f
301b/302, 343c
3.94e/2.97f
4.75/3.53g
‐/190/417
4.73/3.53g
‐/203/405
4.76/3.62g
‐/250/417
Tg/Tm/Td (oC)
HOMO/LUMO (eV)
‐6.53/‐2.53h
‐6.27/‐0.76g
0.2312j/0.4353k
7.03×10‐8j/1.40×10‐9k
470
‐6.51/‐2.79h
‐6.40/‐0.97g
0.7237j/0.5550k
7.72×10‐9j/1.25×10‐9k
470
‐6.65/‐2.63h
‐6.34/‐0.89g
0.2884j/1.0176k
4.02×10‐6j/0.99×10‐8k
470
‐6.65/‐2.63h
‐6.27/‐0.93g
0.1823j/0.3129k
5.15×10‐6j/1.20×10‐7k
470
ERi (eV)
l (cm2/V/s)
Emm (nm)
ms)
7.2
85
7.4
59
7.5
89
8.6
77
PLQYm (%)
b
a Absorption peaks; in CH2Cl2 (10‐6 mol L‐1); c in polycrystalline powder; d fluorescence peaks at room temperature; e estimated according to the absorption
g
edges; f calculated according to the 0‐0 transitions of the phosphorescence spectra; TDDFT calculated results; h calculated according to the equation
i
j
l
HOMO/LUMO = ‐(4.78 + onset voltage) eV1; reorganization energy of electron; for electron; k for hole; electron mobility estimated by I‐V characteristics of
electron‐only devices according filed‐dependent SCLC model7; m data of vacuum‐evaporated mDPEPO:DMAC‐DPS (10%wt.) films with thickness of 100 nm.
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the maxima at 470 nm, corresponding to the pure DMAC‐DPS
originated emission, which indicates the same positive energy
transfer from DPEPO consistent with their equivalent S1 and
T1 energy. As mentioned above, of T1 states for DPEPO and
emissions of their DMAC‐DPS‐doped films is by the same order
of magnitude, making DPEPO‐involved interactions
dominant in triplet quenching processes. of 44'DPEPO‐based
film is 8.6 s, about 1 s longer than those of the other
‐
View Article Online
DOI: 10.1039/C5SC04848F
m

m
m



isomers based films, revealing the effect of 44'DPEPO with
partial CT‐featured S1 state on stabilizing DMAC‐DPS excitons.8
In spite of CT‐dominant S1 state,
 of 24DPEPO‐based film is
much shorter due to its structural relaxation‐induced exciton
quenching, which is further manifested by its lowest PL
quantum yield (PLQY) of only 59% (Table 1). Furthermore, it is
known that longer lifetime would worsen collision‐induced
exciton quenching. Therefore, in comparison to PLQY of 77%
for 44'DPEPO‐based film, 22'DPEPO and 24'DPEPO endow
their films with PLQYs as high as 85 and 89%, respectively.
In consequence, the optical properties of 24'DPEPO and
22'DPEPO are identical, originated from their similar excited‐
state characteristics as TDDFT simulated.
2.4. Electrical Properties
The influence of DPPO substitution position on the HOMO and
LUMO energy levels is experimentally investigated with cyclic
voltammetry (CV) analysis
(Fig. 5a). The oxidation
voltammograms of DPEPO consist of two irreversible peaks
m
characteristic of DPE and DPPO, respectively. Estimated with
onset voltages of anodic peaks, the HOMO energy level of
24'DPEPO is equivalent with that of 44'DPEPO as ‐6.65 eV,
which is 0.1 eV lower than those of 22'DPEPO and 24DPEPO
Fig. 5. (a) Cyclic voltammograms of
m
DPEPO measured in tetrahydrofuran
for reduction and dichloromethane for oxidation, respectively, at room
temperature with tetrabutylammonium hexafluorophosphate as
electrolyte under the scanning rate of 100 mV s−1; (b) volt‐ampere
characteristics of single‐carrier only transporting devices based on
(
Table 1). On the other hand, the reduction voltammograms of
22'DPEPO 24'DPEPO and 44'DPEPO only contain single
,
m
DPEPO and the corresponding fitting curves according to field‐dependent
irreversible cathodic peaks, while 24'DPEPO shows two
irreversible reduction peaks attributed to DPE and DPPO,
respectively. It is showed that the reduction curves of
24'DPEPO and 44'DPEPO are almost overlapped with the same
onset voltages of ‐2.15 V, corresponding to the LUMO of ‐2.63
eV. The superposition of two DPPOs in 24DPEPO further
reduces its LUMO to ‐2.79 eV, which is in accord with DFT
result. It is clear that in comparison to 22'DPEPO, the stronger
electron‐withdrawing effect of para‐DPPO substitution
endows 24'DPEPO and 44'DPEPO with the deeper LUMOs.
Significantly, the identical FMO energy levels of 24'DPEPO and
44'DPEPO verify the dominant effect of para‐DPPO in FMO
energy levels regulation.
SCLC model.
as 1.20×10‐7 cm2 V‐1 s‐1, which is 10 folds of that of 24'DPEPO
and 100 folds of those of 22'DPEPO and 24DPEPO
On account of the DPE‐localized HOMOs for DPEPO and the
electron‐transporting character of DPPO, the hole transporting
abilities of DPEPO should be in direct proportion to the
(Table 1).
m
m
exposure degree of their DPE as the main hole transporting
channel. Although the superposition of two DPPOs renders the
deepest LUMO to 24DPEPO, its electron mobility (

e) is the
lowest as 7.72×10‐9 cm2 V‐1 s‐1, which is only one tenth of that
of 22'DPEPO. In contrast,

e of 24'DPEPO as 4.02×10‐6 cm2 V‐1
s‐1 is dramatically improved by 3 orders of magnitude, which is
almost equivalent to that of 44'DPEPO. According to DFT
results, both DPE and DPPO are involved in unoccupied
molecular orbitals and incorporated into electron
The nominal single‐carrier transporting devices with the
single‐layer configurations of ITO|MoO3 (6 nm)|
nm)|MoO3 (6 nm)|Al for hole only and ITO|LiF (1
nm)| DPEPO (100 nm)|LiF (1 nm)|Al for electron only, in
mDPEPO (100
m
transportation (Fig. S2). In this case, the various e of mDPEPO
which where MoO3 and LiF respectively served as hole‐ and
electron‐injecting layers, were fabricated to evaluate the
actually corresponds to their different intermolecular
interplays. Compared to the over‐concentration of DPPO with
big steric hindrance in 24DPEPO and 22'DPEPO, in virtue of
relatively weaker steric effect of para‐substituted DPPO and
separation configurations, the intermolecular interplays of
24'DPEPO and 44'DPEPO are more effective to facilitate
intrinsic carrier transporting ability of
mDPEPO (Fig. 5b). All of
these materials show the electron predominant characteristics
with electron‐only current density (J) remarkably larger than
hole‐only J. According to field‐dependent space charge limited
current model, the hole mobility (h) of 44'DPEPO is estimated
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Journal Name
View Article Online
DOI: 10.1039/C5SC04848F
Fig. 6. (a) Device configuration and energy level diagram of
mDPEPO‐based deep blue TADF diodes and the chemical structures of employed materials; (b)
Luminance‐J‐Voltage curves and EL spectra (inset) of DMAC‐DPS‐based devices using
m
DPEPO as hosts; (c) Efficiency vs. Luminance curves of the deep blue
TADF devices.
charge hopping between adjacent molecules. Furthermore, it blocking layers, respectively
(Fig. 6a). The doping
is showed that J of 24'DPEPO‐based devices is intermediate concentration was optimized as 10%wt. (Fig. S5). In contrast to
between those of 22'DPEPO and 44'DPEPO‐based devices, the large energy barriers of ~0.5 eV between FMOs of
revealing carrier transporting ability of 24'DPEPO as a
mDPEPO and mCP and Bphen, the HOMO and LUMO energy
combined result of steric and electronic coupling effects of its levels of DMAC‐DPS perfectly match with the corresponding
ortho‐ and para‐substituted DPPOs. Nevertheless, on the FMO energy levels of mCP and Bphen, making the charge
contrary to 24DPEPO with superposition configuration, the capture and exciton recombination on DMAC‐DPS dominant in
separation configuration of 24'DPEPO takes the superiority of exciton harvesting. At high operation voltages, high‐energy
its para‐DPPO in electrical performance enhancement.
carriers can surmount the barriers to recombine on mDPEPO,
In general, through asymmetrical separation configuration making host‐to‐dopant energy transfer considerable. In this
incorporating both ortho‐ and para‐DPPO substitutions, case, exciton quenching by host‐host and host‐dopant
24'DPEPO successfully integrates the complementary interactions can directly influence the device performance.
advantages of 22'DPEPO and 44'DPEPO in favorable optical
All of the devices showed the deep blue emissions peaked
characteristics and improved electrical performance, at 460 nm at 1000 cd m‐2 (inset in Fig. 6b). In comparison to PL
respectively, revealing its great potential in exciton harvesting spectra, the hypochromatic shift of EL emissions should be
and utilization.
attributed to optical microcavity effect of nanometer‐scaled
devices. 22'DPEPO and 24'DPEPO‐based devices achieved the
highest color purity with Commission Internationale Ed
2.5. Device Performance of Deep‐Blue TADF Diodes
The well‐controlled and differentiated optoelectronic I'eclairage (CIE) coordinates of (0.16, 0.17), owing to their rigid
properties of constitutional isomers DPEPO establish the structures and LE‐featured excited states (Table 2). In contrast,
m
basis to selectively investigate the determinants on EL the relatively flexible structures and partially CT‐type excited
performance of blue TADF host materials. The devices were states of 24DPEPO and 44'DPEPO rendered the broadened EL
fabricated with a configuration of ITO|MoO3 (6 nm)|NPB (70 spectra with CIE coordinates of (0.16, 0.20) and (0.16, 0.18),
nm)|mCP
10%wt.)|
which MoO3 and LiF served as hole and electron‐injecting layer, was actually due to the decreased intensity of short‐
NPB and Bphen are 4,4'‐bis[N‐(1‐naphthyl)‐N‐ wavelength part, which is readily quenched through
phenylamino]biphenyl and 4,7‐diphenyl‐1,10‐phenanthroline nonradiative transitions during host‐dopant interactions.
as hole and electron transporting layers and mCP (N,N'‐ I‐V characteristics of these light‐emitting devices showed
(5
nm)|
m
DPEPO
:
DMAC‐DPS
(20
nm, respectively. The remarkably increased intensity of long‐
m
DPEPO (5 nm)|Bphen (30 nm)|LiF (1 nm)|Al, in wavelength part in EL emission from 24DPEPO‐based devices
dicarbazole‐3,5‐benzene) and
m
DPEPO were used as exciton‐ the trend consistent with the charge mobility of their host
8 | J. Name., 2012, 00, 1‐3
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Page 9 of 13
Journal Name ARTICLE
materials (Fig. 6b). 44'DPEPO endowed its devices with the quenching through structural relaxation‐inducedVnieownArrtaicdleiaOtnilvinee
DOI: 10.1039/C5SC04848F
highest J, while J of 24'DPEPO‐based devices secondary. Along transitions.
with operation voltage increased, the effect of 44'DPEPO and
Despite larger maximum efficiencies, 22'DPEPO‐based
24'DPEPO on enhancing J became more prominent. However, devices showed the serious efficiency reduction with the roll‐
the difference between J of these devices were remarkably offs as large as 18 and 50% at 100 and 1000 cd m‐2,
smaller than that between J of the single‐carrier transporting respectively. With the best electrical performance among
devices, especially at low driving voltages, which indicated the
mDPEPO, 44'DPEPO facilitated the charge flux balance in EML
significant contribution of DMAC‐DPS to carrier injection and of its devices, enhanced exciton recombination and reduced
transportation. Nevertheless, 44'DPEPO and 24'DPEPO redundant charge concentration, thereby decreasing exciton‐
endowed their devices with the maximum luminance of ~6500 polaron collision probability for TPQ suppression.
cd m‐2, which was more than 2000 cd m‐2 higher than that of Consequently, on the contrary to 22'DPEPO
, 44'DPEPO
22'DPEPO‐based devices; while, when using 24DPEPO as host, supported its devices with the remarkably reduced roll‐offs as
the maximum luminance was less than 1000 cd m‐2. The higher low as 7 and 30% at 100 and 1000 cd m‐2, respectively.
Luminance of 44'DPEPO and 24'DPEPO‐based devices Significantly, 24'DPEPO‐based devices also realized the low
corresponded to higher exciton concentrations owing to larger roll‐offs of 8 and 32% at 100 and 1000 cd m‐2, respectively,
and balanced charge flux in their EMLs. Furthermore, the comparable to those of 44'DPEPO‐based analogues. The
driving voltages of 44'DPEPO‐based devices were 3.6, 5.1 and efficiency roll offs of 24'DPEPO and 44'DPEPO‐based devices
7.5 V for onset, 100 and 1000 cd m‐2, respectively, which were are among the lowest values reported so far for deep‐blue
lower than those of 22'DPEPO‐based analogues (Table 2). TADF diodes.4a, 5d, 5e, 5i, 5j, 16a If proportional to their charge
24'DPEPO further reduced the driving voltages of its devices to mobility, TPQ suppression in these devices can be roughly
3.5, 5.1 and 7.0 V, which were about 1 V lower than those of evaluated
24DPEPO‐based devices. In comparison to 44'DPEPO with 44'DPEPO
in
24'DPEPO
the
order
of
>
>
22'DPEPO 24DPEPO. On the other hand,
>
stronger electroactivity, the lower driving voltages achieved by compared to the exposed T1 states of 22'DPEPO and 44'DPEPO
24'DPEPO actually reflected the higher exciton harvesting the extremely condensed T1 state of 24'DPEPO can more
,
efficiency of its devices.25
effectively restrain collision‐induced quenching, which offsets
the slight inferiority of 24'DPEPO to 44'DPEPO in charge
On the basis of the four‐layer device structure, 22'DPEPO
‐
based devices realized the maximum efficiencies of 25.3 cd A‐1 mobility. It would be rational that to some extent the
for current efficiency (CE), 21.5 lm W‐1 for power efficiency (PE) enhanced efficiency stability at high luminance would reflect
and 16.7% for EQE, which were about 20% higher than those the improved device durability, since the device aging can be
of 44'DPEPO‐based devices (Fig. 6c and Table 2). The accelerated at high luminance to generate charge traps and
superiority of 22'DPEPO in the maximum efficiencies should be exciton quenching sites, making both luminance and efficiency
26
attributed to the effectively suppressed collision‐induced decrease.5e,
In this case, 24'DPEPO with the reduced
quenching through reducing host‐dopant interactions in virtue efficiency roll‐offs would be dominant to 22'DPEPO in device
of the strong steric effect of its ortho‐DPPOs. Significantly, in stability.
addition to the advantages of ortho‐substituted DPPO,
The state‐of‐the‐art performance of 24'DPEPO‐based
unsymmetrical separation structure of 24'DPEPO further devices clearly shows that for high‐energy‐gap blue TADF host
weakens intermolecular interactions, improving the maximum materials, strong steric effect is beneficial to suppress TTA for
efficiencies to 30.6 cd A‐1, 27.5 lm W‐1 and 20.1%, which are high efficiencies; while, under high driving voltages, the
among the best results of deep‐blue TADF devices reported so improved charge mobility can facilitate the flux balance in EML
far.4a, 5d, 5e, 5i, 5j, 16a Therefore, compared to 22'DPEPO, the to suppress TPQ. Significantly, with effectively protected T1
asymmetric configuration of 24'DPEPO gives rise to the state with extremely condensed location, EL performance of
remarkable efficiency increase for 20%. In contrast, the asymmetric 24'DPEPO was far beyond simple integration of
maximum efficiencies of 24DPEPO‐based devices were the those of 22'DPEPO and 44'DPEPO, verifying the great
lowest among these devices, suffering from the serious exciton importance of suppressing host‐involved intermolecular
Table 2. EL performance of
mDPEPO‐based devices employing DMAC‐DPS as TADF emitter.
Host
Operation Voltagea (V)
Maximum Efficiencyb
Efficiency Roll‐Offc (%)
Emission Peak (nm)
CIE (x, y)
CE
PE
EQE
22'DPEPO
24DPEPO
24'DPEPO
44'DPEPO
3.7, 5.4, 7.6
4.3, 6.5, ‐
25.3, 21.5, 16.7
5.7, 4.2, 3.7
18, 50
44, ‐
44, 75
63, ‐
19, 50
44, ‐
460
460
460
460
0.16, 0.17
0.16, 0.20
0.16, 0.17
0.16, 0.18
3.5, 5.1, 7.0
3.6, 5.1, 7.5
30.6, 27.5, 20.1
20.8, 18.1, 13.1
8, 33
7, 30
37, 66
35, 53
8, 32
7, 30
a Operating voltages for onset, 100 and 1000 cd m‐2; b the maximum efficiencies of CE (cd A‐1), PE (lm W‐1) and EQE (%); c at 100 and 1000 cd m‐2.
This journal is © The Royal Society of Chemistry 20xx
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Journal Name
interactions. The sky‐blue devices of another TADF dye 1,2‐ Ph2PH (1.1 mmol) were dissolved in DMF (10 mLV)ieawnAdrtichleeOantleinde
DOI: 10.1039/C5SC04848F
employing to reflux for 24h. After cooled to room temperature, water (10
bis(carbazol‐9‐yl)‐4,5‐dicyanobenzene
(
2CzPN
)
24'DPEPO also showed the higher efficiencies and reduced mL) was added to the system, which was then extracted with
roll‐offs in contrast to 22'DPEPO, manifesting the universality CH2Cl2 (3×3 mL). The organic layer was combined and dried
of host design strategy for 24'DPEPO
(
Fig. S6).
with anhydrous sodium sulfate. The solvent was removed in
vacuo to afford the crude phosphine intermediate product.
Then, the phosphine was dissolved in CH2Cl2 (10 mL). H2O2
(30%, 4 mL) was added to the solution in dropwise and stirred
for 2h at 0 oC. Then, the mixture was extracted CH2Cl2 (3×3 mL).
The organic layer was combined and dried with anhydrous
sodium sulfate. The solvent was removed in vacuo, and then
the residue was purified by flash column chromatography to
afford the phosphine oxide product.
3. Experimental Section
Materials and Instruments: All the reagents and solvents used
for the synthesis were purchased from Aldrich and Acros
companies and used without further purification. DPEPO and
DPESPO bromides were prepared according to our previous
report20d
.
1H NMR spectra were recorded using a Varian Mercury plus
400NB spectrometer relative to tetramethylsilane (TMS) as
internal standard. Molecular masses were determined by a
FINNIGAN LCQ Electro‐Spraying Ionization‐Mass Spectrometry
(ESI‐MS), or a MALDI‐TOF‐MS. Elemental analyses were
performed on a Vario EL III elemental analyzer. Absorption and
photoluminescence (PL) emission spectra of the target
compounds were measured using a SHIMADZU UV‐3150
2,4‐bis(diphenylphosphoryl)diphenylether (24DPEPO): 393
mg of white powder with a yield of 69%. 1H NMR (TMS, CDCl3,
400 MHz):
 = 8.009‐7.957 (t, J = 10.4 Hz, 1H), 7.840‐7.775 (t, J
= 12.8 Hz, 1H), 7.717‐7.606 (m, 8H), 7.566‐7.284 (m, 14H),
6.862‐6.821(q, J1 = 4.4 Hz, J2= 8.4Hz, J3 = 1.2 Hz, 1H), 6.572‐
6.550 ppm (d, J = 10.0 Hz, 2H); LDI‐TOF: m/z (%): 570 (100)
[M+]; elemental analysis (%) for C36H28O3P2: C 75.78, H 4.95;
found: C 75.82, H 4.97.
2,4'‐bis(diphenylphosphoryl)diphenylether (24'DPEPO): 439
spectrophotometer
and
a
SHIMADZU
RF‐5301PC
mg of white powder with a yield of 77%. 1H NMR (TMS, CDCl3,
spectrophotometer, respectively. Thermogravimetric analysis
(TGA) and differential scanning 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 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. Phosphorescence spectra
were measured in dichloromethane using an Edinburgh FPLS
920 fluorescence spectrophotometer at 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 10 μs‐10 s lifetime measurement, the
synchronization photomultiplier for signal collection and the
Multi‐Channel Scaling Mode of the PCS900 fast counter PC
plug‐in card for data processing. The absolute PLQY of the
films was measured with a integrating sphere.
400 MHz):
 = 8.062‐8.007 (q, J1 = 7.6 Hz, J2 = 12.8 Hz, J3= 1.6
Hz, 1H), 7.767‐7.719 (q, J1 = 6.8 Hz, J2 = 12.4 Hz, J3= 1.6 Hz, 4H),
7.667‐7.616 (q, J1 = 7.2 Hz, J2 = 12.0 Hz, J3 = 1.2 Hz, 4H), 7.589‐
7.534 q, J1 = 7.2 Hz, J2 = 14.8 Hz, J3= 1.2 Hz, 3H), 7.506‐7.315 (m,
3H), 6.930‐6.897 (q, J1 = 5.2 Hz, J2 = 8.0 Hz, 1H), 6.665‐6.638
ppm (dd, J1 = 2.0 Hz, J2= 8.8Hz, 2H); LDI‐TOF: m/z (%): 570 (100)
[M+]; elemental analysis (%) for C36H28O3P2: C 75.78, H 4.95;
found: C 75.80, H 4.95.
4,4‐bis(diphenylphosphoryl)diphenylether (44'DPEPO): 428
mg of white powder with a yield of 75%. 1H NMR (TMS, CDCl3,
400 MHz):
 = 7.691‐7.621 (m, 12H), 7.563‐7.523 (td, J1 =1.2 Hz,
J2= 7.2 Hz, 4H), 7.482‐7.438 (td, J1 = 2.8 Hz, J2=7.6 Hz, 8H),
7.105‐7.078 (dd, J1 = 2.4 Hz, J2 = 8.8 Hz, 4H); LDI‐TOF: m/z (%):
570 (100) [M+]; elemental analysis (%)for C36H28O3P2: C 75.78,
H 4.95; found: C 75.77, H 4.94.
Density functional theory (DFT) and time‐dependent DFT
(TDDFT) Calculations: 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
General phosphorylation procedure of bromides: In Ar2,
bromide (1 mmol), NaAc (1.1 mmol), Pd(Ac)2 (0.05 mmol) and
Scheme 1. Synthetic procedure of 24DPEPO, 24'DPEPO and 44'DPEPO. (i) Ph2PH, Pd(Ac)2, NaAc, DMF, 130 oC, 24 h; (ii) CH2Cl2, 30%H2O2, 0 oC.
10 | J. Name., 2012, 00, 1‐3
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Journal Name ARTICLE
functional27 and Lee, and Yang and Parr correlation delicate and purposeful modulation on optoelectronic
View Article Online
functional28 (B3LYP)/6‐31+G(d, p), respectively. The fully properties of host materials is crucial DfOorI: 1d0e.1v0e39lo/Cp5inSCg04h8ig48hF‐
optimized stationary points were further characterized by performance TADF diodes.
harmonic vibrational frequency analysis to ensure that real
local minima had been found without imaginary vibrational
frequency. The total energies were also corrected by zero‐
Acknowledgements
point energy both for the ground state and triplet state.
Natural transition orbital (NTO) analysis was performed on the
basis of optimized ground‐state geometries at the level of
(B3LYP)/6‐31+G(d, p).23 The contours were visualized with
Gaussview 5.0. All computations were performed using the
Gaussian 09 package.29
JZ and DD contributed equally to this work. HX thanks Prof.
Runfeng Chen (Nanjing University of Posts and
Telecommunication) for his assistance in TDDFT simulation.
This project was financially supported by NSFC (61176020 and
51373050), New Century Excellent Talents Supporting Program
of Ministry of Education (China) (NCET‐12‐0706), Program for
Innovative Research Team in University (MOE) (IRT‐1237),
Science and Technology Bureau of Heilongjiang Province
(ZD201402 and JC2015002), Education Bureau of Heilongjiang
Province (2014CJHB005), the Fok Ying‐Tong Education
Foundation for Young Teachers in the Higher Education
Institutions of China (141012) and Harbin Science and
Technology Bureau (2015RAYXJ008).
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 oxygen plasma for 3 min. Devices were
fabricated by evaporating organic layers at a rate of 0.1‐0.2 nm
s‐1 onto the ITO substrate sequentially at a pressure below
4×10‐4 Pa. Onto the electron‐transporting 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 was 0.09 cm2 as determined by
the overlap area of the anode and the cathode. After
fabrication, the devices were immediately transferred to a
glove box for encapsulation with glass cover slips using epoxy
glue. 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 4200 source meter and a calibrated silicon
photodiode. All the measurements were carried out at room
temperature under ambient conditions. For each structure,
five devices were fabricated to confirm the performance
repeatability. To make conclusions reliable, the data reported
herein were most close to the average results.
Notes and references
1
D. M. de Leeuw, M. M. J. Simenon, A. R. Brown and R. E. F.
Einerhand, Synthet. Met., 1997, 87, 53.
2
(a) C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987,
51, 913; (b) J. Kido, M. Kimura and K. Nagai, Science, 1995,
267, 1332; (c)
Forrest, Nature, 2000, 403, 750; (d)
M. A. Baldo, M. E. Thompson and S. R.
C. D. Muller, A.
Falcou, N. Reckefuss, M. Rojahn, V. Wiederhirn, P. Rudati, H.
Frohne, O. Nuyken, H. Becker and K. Meerholz, Nature,
2003, 421, 829; (e) Y. R. Sun, N. C. Giebink, H. Kanno, B. W.
Ma, M. E. Thompson and S. R. Forrest, Nature, 2006, 440
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908; (f) S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K.
Walzer, B. Lussem and K. Leo, Nature, 2009, 459, 234; (g) F.
B. Dias, K. N. Bourdakos, V. Jankus, K. C. Moss, K. T.
Kamtekar, V. Bhalla, J. Santos, M. R. Bryce and A. P.
Monkman, Adv. Mater., 2013, 25, 3707; (h)
G. Schwartz,
4. Conclusions
A series of DPEPO‐type host materials named
constitutional isomers with two DPPO groups substituted on
DPE core at either ortho‐ or para‐positions were designed and
prepared. The steric effect of ortho‐substituted DPPO and
electron coupling effect of para‐substituted DPPO are
successfully integrated through separation configuration of
S. Reineke, T. C. Rosenow, K. Walzer and K. Leo, Adv. Funct.
Mater., 2009, 19, 1319; (i) Q. Wang and D. Ma, Chem. Soc.
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(a) M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S.
Sibley, M. E. Thompson and S. R. Forrest, Nature, 1998, 395
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Kong, Q. Gong and J. Kido, Adv. Mater., 2011, 23, 926; (d) K.
S. Yook and J. Y. Lee, Adv. Mater., 2012, 24, 3169; (e) Y.
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H. Xu, R. Chen, Q. Sun, W. Lai, Q. Su, W. Huang and X. Liu,
mDPEPO as
3
,
24'DPEPO
, giving rise to its harmonious excited‐state
characteristics and electrical performance inherited from
22'DPEPO and 44'DPEPO, respectively. The asymmetrical
configuration and the dominant orientation effect of ortho‐
substituted DPPO on T1 location extremely condense the T1
state of 24'DPEPO on a single phenyl, completely protected
from intermolecular interactions. Consequently, 24'DPEPO
endowed its DMAC‐DPS‐based deep blue TADF diodes with
the state‐of‐the‐art performance featured high EQE beyond
20% and low roll‐offs as 32% at 1000 cd m‐2, which was
dramatically improved in comparison to those of 22'DPEPO
and 44'DPEPO and successfully demonstrated. It is showed
that besides of common energy level optimization, more
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