Synergistic tuning of the optical and electrical performance of AIEgens with a hybridized local and charge-transfer excited state

Article abstract of DOI:10.1039/c9tc01453e

The reported organic light-emitting diodes (OLEDs) based on hybridized local and charge-transfer (HLCT) state emitters mostly exhibit low photoluminescence quantum yields (PLQYs) in aggregates, failing to achieve optimal device efficiency although they ex

Full text of DOI:10.1039/c9tc01453e

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DOI: 10.1039/C9TC01453E.
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PAPER
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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
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DOI: 10.1039/C9TC01453E
ARTICLE
Synergistic Tunings on Optical and Electrical Performance of
AIEgens with Hybridized Local and Charge-Transfer Excited State
Han Zhang,a Jiajie Zeng,a Wenwen Luo,a Haozhong Wu,a Cheng Zeng,a Kexin Zhang,a Weiqiang Feng,a
Received 00th January 20xx,
Accepted 00th January 20xx
Zhiming Wang,*,a,c Zujin Zhao*,a and Ben Zhong Tang*,a,b
DOI: 10.1039/x0xx00000x
The reported organic light-emitting diodes (OLEDs) based on hybridized local and charge-transfer (HLCT) state emitters
mostly exhibt low photoluminescence quantum yields (PLQYs) in aggregates, failing to achieve the best device efficiency
although they exhibit good exciton utilization efficiencies (EUEs). In this work, by introducing aggregation-induced emission
(AIE) moiety (tetraphenylethene, TPE) and cyano group (CN) to HLCT-typed core, phenanthroimidazole (PI), six luminescent
compounds with different conjugation patterns at C2 and N1 substituent positions are obtained, and their excited states are
regulated effectively. Based on systematic photophysical analysis, the impacts of molecular conjugation patterns on
regulation of the locally excited (LE) and charge-transfer (CT) component are disclosed, and their AIE characters that ensure
high PLQYs of these compounds in aggregates are observed. Exciton conversion channels from triplets to singlets via excited
states tunings are proposed based on theoretical calculations. The non-doped OLEDs based on these compounds exhibit
excellent performances with maximum luminance, current efficiency, and external quantum efficiency of up to 31070 cd
m‒2, 18.46 cd A‒1, and 7.16 %, respectively, and very small efficiency roll-off of 4.0 % at 1000 cd m‒2 luminance. The successful
design in these HLCT-based AIEgens not only gives more optimization choices for OLED emitters, but also proves that a
strategy of reasonably superposing AIE unit to HLCT emitters is feasible in materials design.
Therefore, harvesting triplet excitons for light emission has been
recoganized an effective strategy to increase EL efficiencies in OLEDs.
Currently, thermally activated delayed fluorescence (TADF),3
Introduction
Organic light-emitting diodes (OLEDs) have been extensively
developed for the practical applications in flat-panel display and
solid-state lighting technologies owing to various superior features
such as low energy cost, high brightness, high-quality color, light
weight, and flexibility, etc.1 To achieve high electroluminescence (EL)
efficiencies, two main factors of the luminescent materials,
photoluminescence quantum yields (PLQY) in solid state and exciton
utilization efficiency (EUE), play crucial roles in the working process,
which deserve particular attentions in materials design. In fact, the
EUEs of most traditional fluorescent emitters are theoreticallly
limited to 25% (a maximum) from electro-generated singlet excitons
according to the spin statistics,2 but a greater proportion of triplet
excitons (75%) are failed to generate photons and lost by means of
non-radiative decay because of spin forbidden from triplet to singlet.
triplet-triplet annihilation (TTA)4 and hybridized local and charge-
transfer (HLCT) state5 mechanisms are developed to solve the
problem of triplet exciton utilization in pure organic emitters.
Actually, considerable successes have been achieved based on these
mechanisms, leading to greatly boosted EL efficiencies of noble-
metal free ognaic emitters. Among them, HLCT mechanism has
drawn increasing attention in recent years because it is conducive to
both PLQY and EUE.6 In HLCT emitters (Fig. 1A), the dominant locally
excited (LE) in the hybridized state at low-lying S1 is liable to fluoresce
intensely due to a large overlap between hole and electron wave
functions, and generates a large singlet–triplet energy splitting
between S1 and T1 (ΔES1T1); while at high-lying levels, the charge
transfer (CT) component in hybridized state leads to activating the
“hot exciton” channels with a negligible ΔESmTn (m≥1, n≥2), enabling
reverse intersystem crossing (RISC) process and thus increasing
EUE.6d The EUEs of many reported HLCT emitters are close to 100 %,
but their PLQYs are not high enough in neat films in practice due to
aggregation-caused quenching (ACQ) phenomenon, which
undermines their EL performance.5d,6c In addition, seeking for
molecular design strategies and excited state modulation principles
for HLCT emitters is still a challenging task.
a.SCUT-HKUST Joint Research Institute, Guangzhou International Campus, Center for
Aggregation-Induced Emission, State Key Laboratory of Luminescent Materials and
Devices, South China University of Technology, Guangzhou 510640, China.
*E-mail: wangzhiming@scut.edu.cn; mszjzhao@scut.edu.cn
b.Department of Chemistry, Hong Kong Branch of Chinese National Engineering
Research Center for Tissue Restoration and Reconstruction, The Hong Kong
University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong,
China. *E-mail: tangbenz@ust.hk
c. School of Petrochemical Engineering, Shenyang University of Technology, Liaoyang
111003, China.
Recently, aggregation-induced emission (AIE) offers high
possibility to solve the ACQ problem of most traditional
chromophores,7 and the luminogens with AIE characteristics
(AIEgens)8 can fluoresce intensively when they are fabricated into
neat films, which are considered as ideal light-emitting materials for
†
Electronic Supplementary Information (ESI) available: Experimental details, X-ray
crystallography, device fabrication, TGA curves and DSC spectra, PL spectra in THF,
additional AIE curves, time-resolved fluorescence decay curves in THF solutions and
neat films, detailed theoretical calculations data, mechanochromism properties
measurement and 1H and 13C NMR spectra. See DOI: 10.1039/x0xx00000x
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DOI: 10.1039/C9TC01453E
Fig. 1 (A) Typical energy level character of HLCT emitter with “hot exciton” channel; (B) The long-short axis design based on
phenanthroimidazole in this work. (C) Construction of LE state and CT state in the PI derivative (taking ppCTPI as an example).
non-doped OLEDs.9 Hence, it is a promising strategy to achieve a win- 16.32 lm W–1, respectively. In addition, ultra-low roll-off of 4.0% at
win situation via combining HLCT process with AIE nature.10 Herein, 1000 cd m–2 luminance was observed. These results demonstrated a
we chose a typical HLCT unit, phenanthroimidazole (PI), to function feasible strategy of tuning excited state distributions by reasonably
as an emissive core in molecular design (Fig. 1B and 1C).11 And a star combining HLCT materials with AIE nature for the development of
AIE molecule, TPE, is introduced into the C2-position of imidazole high-efficiency emitters for OLEDs.
ring in PI (defined as the long axis in molecular structure) with
different patterns (para- and meta- linkages). Thereby, we can tune
the LE component in hybridized excited states and luminescence
Results and discussion
behaviors in the aggregated states via different conjugated degrees. Synthesis and crystal structure
At the N1-position (defined as the short axis in molecular structure),
a benzonitrile with cyano group (CN) at meta- or para- pattern was
inserted to tune the CT component in HLCT state. Then, six AIEgens,
pTPI,12a mpCTPI, ppCTPI, mTPI, mmCTPI and pmCTPI, were
Phenanthrenequinone, the corresponding aromatic amines,
aromatic aldehydes and ammonium acetate were chosen to obtain
the target compounds via a one-pot reaction in good yields. The
detailed synthetic routes, preparation methods and characterization
synthesized and characterized (Fig. 2). The substituent effects on
data of these final products are described in the Supporting
their photophysical and electrochemical properties of the six
Information (Fig. S1†). Single crystals of mTPI were grown from the
compounds were systematically investigated. They showed
ethanol and dichloromethane mixture by slow solvent diffusion and
prominent AIE characteristics and high PLQYs in aggregates. Their
evaporation. As depicted in Fig. 3, a twisted conformation with a big
EUEs can be enhanced by tuning excited state to promote exciton
dihedral angel of 78.51o and a planar conformation with a small
conversion channels based on theoretical calculations, which was
dihedral angel of 33.59o are observed at N1-position and C2-position
confirmed in subsequent EL performances. The non-doped OLED
using ppCTPI as an emitter exhibited the best EL performance with
excellent external quantum efficiency (7.16%, EQE) and EUEs (>48%),
and their maximum luminance, current efficiency, and power
efficiency (L, ηc, and ηp) are of up to 31070 cd m‒2, 18.46 cd A–1 and
Fig. 3 (a) ORTEP drawing of the crystal structure (CCDC 1903655) of
Fig. 2 Chemical structures of the six PI derivatives.
2 | J. Name., 2012, 00, 1-3
mTPI and (b, c) packing pattern of mTPI in crystal.
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Table 1 Photophysical and thermal properties of the compounds. a
ARTICLE
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DOI: 10.1039/C9TC01453E
λem (nm)
aggr film
ΦF (%)
τ (ns) [kr (ns‒1), knr (ns‒1)]
λabs
(nm)
Tg/Td
(oC)
αAIE
soln
soln aggr film
soln
film
pTPI
mpCTPI
ppCTPI
mTPI
mmCTPI
pmCTPI
365 432,465 483
361 433,461 485
361 433,460 475
362 389,410 465
359 389,412 464
493
503
501
491
493
492
0.3
0.5
0.5
0.5
0.6
1.2
69.4 88.5 231.3 0.73 (0.004, 1.366)
64.2 67.2 128.4 0.84 (0.006, 1.185)
61.2 59.5 122.4 1.06 (0.005, 0.939)
2.63 (0.337, 0.044)
2.37 (0.284, 0.138)
2.85 (0.209, 0.142)
4.19 (0.114, 0.125)
3.97 (0.060, 0.192)
4.26 (0.048, 0.187)
nd/389
nd/443
nd/415
114/402
126/408
128/414
39.4 47.6
33.5 23.8
31.3 20.5
78.8
55.8
26.1
1.03 (0.005, 0,966)
1.94 (0.003, 0.512)
1.14 (0.011, 0.867)
359
423
467
a soln = THF solution (10−5 M); aggr = nanoaggregate formed in H2O/THF mixture with a fw of 90%; film = vacuum-deposited neat film; ΦF =
absolute PL quantum yield determined by a calibrated integrating sphere; αAIE = value of AIE effect, calculated by ΦF (aggr)/ΦF (soln); τ = PL
lifetimes measured at room temperature in air; kr = radiative decay rate (kr = ΦF/τ); knr = nonradiative decay rate (knr = (1 − ΦF/τ)); nd = not
detectable.
of mTPI, respectively. The molecular packing of mTPI shows an meta-linked ones. Interestingly, there was negligible change in
antiparallel arrangement, in which TPE and PI units form a head- absorption spectra before and after cyano group introduction, which
to-tail stacking. As a result, the twisted TPE unit can interrupt the π- indicated that ICT effect along N1-substitution direction had little
π stacking interactions among PI units with interplane distance of influence on absorption process and optical band gaps of the
3.636 Å. The distances of 2.775 Å and 2.917 Å between the hydrogen compounds.
atoms of the TPE phenyl rings and the π-electrons of PI indicate C–
H···π hydrogen-bond interactions existed in crystals, which can
restrict and lock the molecular rotations of TPE and lead to reduction
of the non-radiative energy loss via rotational relaxation.
pTPI
mpCTPI
ppCTPI
A
mTPI
mmCTPI
pmCTPI
Thermal properties
The thermal properties of the six compounds were examined using
thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) methods under N2 atmosphere. As shown in Fig.
S2† and Table 1, all the compounds exhibited high thermal
decomposition temperatures (Td, corresponding to 5% weight loss)
in the range of 389–443 oC. While, the four cyano-containing
compounds had better thermal stabilities, like our previous
findings.11a DSC measurements were carried out from 30 C to 360
o
250
300
350
400
450
oC, and the three TPE meta-linked compounds, mTPI, mmCTPI and
Wavelength (nm)
pmCTPI, revealed distinct glass transition temperatures (Tg) of 114
o
o
oC, 126 C and 128 C, respectively. No obvious Tg peaks could be
pTPI
B
mpCTPI
ppCTPI
mTPI
mmCTPI
pmCTPI
detected in their counterparts, but crystallization temperatures (Tc)
o
o
of 202 C for pTPI and 210 C for mpCTPI were observed. These
results indicated the good thermal and morphological stabilities of
the six compounds, which would have positive effects on the
performance of their OLED devices.
Photophysical properties
Fig. 4A shows the ultraviolet-visible (UV-vis) absorption spectra of
the compounds in tetrahydrofuran (THF, [c] = 1×10–5 M). The sharp
absorptions at 260 nm in all compounds were considered as typical
π-electron response of the phenanthrene derivative.11b The long-
wavelength absorption bands at 300–370 nm might be attributed to
the π–π* transitions at the long-axis direction.11b,13 Red-shifted and
400 450 500 550 600 650
Wavelength (nm)
enhanced absorption bands were found in TPE para-linked Fig. 4 (A) Absorption spectra in THF solutions (10−5 M) and (B) PL
compounds because of their more effective conjugation than TPE spectra in neat films of the compounds.
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The photoluminescence (PL) spectra of these compounds in dilute TPE para-linked compounds in neat films were higherVtiehwaAnrttichleoOsenlionef
THF solutions (10−5 M) and neat films were displayed in Fig. S3† and their counterparts because of enlarged degree of effective π-
DOI: 10.1039/C9TC01453E
Fig. 4B. In THF solutions, three TPE para-linked compounds exhibited conjugation at long-axis, and pTPI had a highest value of 88.5%. As
blue emissions ranging from 409–487 nm with hyperfine vibrational the addition of cyano group, the ΦFs of the emitters were decreased,
structures in their PL spectra, indicating LE components were indicating the increment of CT-component had negative effect on
dominant in their HLCT states according to the previous reports their luminescence efficiencies. It was worth noting that the six
about PI-based deactivates.5a,12a When the linkage of TPE unit was compounds exhibited mechanochromism properties.12a The PL
changed into meta-linked form, their conjugation degree at long-axis emissions of neat films were closer to the ones from ground states
direction was decreased, resulting in bluer emissions and less LE (see ESI), indicating the fromation of amorphous states in vacuum
components. However, their hyperfine vibrational structure deposition films. Their absolute ΦFs in different states were listed in
vanished gradually after cyano group insertion, and the emission was Table 1. Solvatochromic experiments of PL were employed to exam
red-shifted at the same time, implying increased CT proportion at complex HLCT changes in composition.5a,14 However, the emissions
short-axis direction. In the vacuum-deposited neat films, pTPI, in some high-polar solvents were too weak to detect accurately
mpCTPI and ppCTPI showed broad PL peaks at 493, 503 and 501 nm, because of their faint fluorescence caused by their AIE nature.
respectively, and the other three ones emitted sky-blue PL ranging Therefore, the tuning processes of LE and CT distribution and
from 491–493 nm. In comparison with those in solutions, the proportion were confirmed by theoretical calculation instead.
emission peaks in films are much red-shifted. This is because the
For investigating their AIE characteristics, water, a poor solvent of
molecule packing in film can decrease the degree of twisting of TPE the compounds, was added into their THF solutions in different
unit, and thus prolong the conjugation length and give rise to a large proportions (Fig. 5 and Fig. S4†). The emissions of the six compounds
redshift. All compounds were weakly fluorescent with low absolute were very weak in a low water fraction (fw), but became stronger as
PL quantum yields (ΦF) of 0.3–1.2 % in THF solutions, but emitted the increment of fw. Consequently, their ΦFs got dozens and even
intensely in neat films, demonstrating notable AIE nature. The ΦFs of hundreds of times enhancement in aqueous condition. The greatly
fw (vol%)
fw (vol%)
A
B
90
80
60
40
20
0
90
80
60
40
20
0
400 450 500 550 600 650
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
80
60
40
20
0
pTPI
mpCTPI
ppCTPI
mTPI
mmCTPI
pmCTPI
C
D
40
20
0
0
20
40
60
80
100
0
20
40
60
80
100
Water Fraction (vol %)
Water Fraction (vol %)
Fig. 5 PL spectra of (A) ppCTPI, (B) pmCTPI in THF/water mixtures (10−5 M) with different water fractions (fw); (C) and (D) PLQY versus fw
curves; Inset: photos of the compounds in THF/water mixtures (fw = 0 and 90%), taken under 365 nm excitation.
4 | J. Name., 2012, 00, 1-3
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enhanced PL efficiencies should be attributed to the restriction of
rotational motions by the spatial constraint in the aggregated state
and thus the suppression of nonradiative decay of the excited state.15
To further understand the relevant photophysical process, the
transient fluorescence spectra were measured (Fig. S5†). As listed in
Table 1, all the compounds showed longer fluorescence lifetimes in
neat films than in THF solutions. Their non-radiative decay rates (knr)
were decreased greatly from solutions to films, which enabled them
to emit more efficiently in aggregates with obvious PLQYs
improvement. The compounds showed single index decay of lifetime
without delayed component, which confirmed that there were no
TADF or TTA mechanisms in luminescence process of these
compounds.16
Table 2 Electrochemical data of the compounds. a
View Article Online
DOI: 10.1039/C9TC01453E
ox
onset
red
onset
e
o
E
E
HOMO
(eV)
LUMO
(eV)
Eg
Eg
(V)
(V)
(eV)
3.05
3.01
3.01
3.22
3.12
3.13
(eV)
3.13
3.14
3.14
3.34
3.34
3.34
pTPI
mpCTPI 0.97
ppCTPI
mTPI
mmCTPI 1.06
pmCTPI 1.07
0.92
–2.29
–2.20
–2.19
–2.38
–2.22
–2.24
–5.49
–5.54
–5.55
–5.56
–5.63
–5.64
–2.44
–2.53
–2.54
–2.34
–2.51
–2.50
0.97
0.99
ox
red
a Eonset: onset oxidation potential; Eonset: onset reduction potential; Eg
= LUMO – HOMO; Ego = 1240/λ, λ gained from absorption spectra.
e
energy levels of pTPI and mTPI were estimated to be –2.44 eV and –
2.34 eV, and probably populated on the TPE (as an acceptor
compared to PI). The other four compounds with cyano group had
lower LUMO energy levels of –2.53, –2.54, –2.51, and –2.50 eV,
respectively, indicating that the distribution of LUMOs had changed.
Electrochemical properties and theoretical calculations
The CV measurements were conducted in CH2Cl2 for oxidation
potentials and DMF for reduction potentials using 0.1 M tetra-n-
butylammonium-hexafluoro-phosphate
(n-Bu4NPF6)
as
the
electrolyte. The HOMO and LUMO energy levels were determined
using ferrocene (Fc) as the reference (4.8 eV) and were calibrated
with respect to E1/2 (Fc/Fc+) in every measurement.17 Their cyclic
voltammograms were given in Fig. S6† and the corresponding data
were summarized in Table 2. The HOMO energy levels of pTPI,
mpCTPI, ppCTPI, mTPI, mmCTPI and pmCTPI were calculated to be –
5.49, –5.54, –5.55, –5.56, –5.63, and –5.64 eV, respectively, which
were similar to those of reported PI-based derivatives.11 The LUMO
e
For further comparison, the electrical bandgaps (Eg ) of the
compounds became narrower after cyano group insertion, quite
o
different from their optical bandgaps (Eg ) discussed in absorption
spectra. Such measurement results indicated that there might be
separations of electrical and optical energy gaps character in cyano-
containing compounds, which could be beneficial to constructing
lower injection/transport barrier in wide-band gap luminescent
materials.18
Fig. 6 Relevant molecular orbital amplitude plots, energy levels and possible transition sketch maps of TPE para-linked compounds.
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Fig. 7 Natural transition orbitals and properties of S0→S1 and S0→S2 for pTPI, mpCTPI and ppCTPI, calculated by M06-2X hybrid functional at
the basis set level of 6-31G (d, p).
To further understand their different energy gaps obtained from 75° for ppCTPI, 80° for mTPI, 77° for mmCTPI and 76° for pmCTPI)
absorption spectra and CV measuements, the density functional gave considerable contributions to the separation of the MOs.19
theory (DFT) calculations were carried out at level of M0-62X/6-
31G(d,p) employing the Gaussian 09 package. The geometries, Excited States Properties
molecular orbitals (MOs) and calculated energy levels were shown in
Fig. 6 and Fig. S7†. The HOMOs of the compounds were mainly
located on the PI unit and TPE moiety at long-axis direction, and little
decrements were observed in their calculated HOMO values after
cyano group was inserted. The LUMOs of both mTPI and pTPI were
mostly populated on TPE acceptor, which well agreed with the
analysis above. After inserting cyano group, the distribution of
LUMOs was transferred into N1-imidazole position owing to their
enhanced electron-withdrawing abilities,11c and these lower LUMO
values of cyano-containing compounds were agreed to the results in
electrochemical measurements. Interestingly, the LUMO+1 levels of
compounds with cyano group located mostly in TPE unit, similar to
their respective matrix situations (LUMOs in mTPI and pTPI). Their
caculated energy gaps from HOMO to LUMO+1 were 5.63 eV for
mpCTPI, 5.71 eV for ppCTPI, 5.94 eV for mmCTPI, and 5.97 eV for
pmCTPI, respectively, and these values were close to energy gaps
from HOMO to LUMO in pTPI and mTPI (5.60 eV and 5.91 eV),
respectively. Exactly, these resluts were consitent with optical
bandgaps gained from absorption spectra. Now, the hypothesis of
separation of electrical and optical energy gaps was confirmed. In
fact, the transitions from HOMO to LUMO in non-cyano-containing
compounds and from HOMO to LUMO+1 in cyano-containing
compounds were allowed because of their effective overlaps in the
MOs. However, the transitions from HOMO to LUMO in the latter
were nearly forbidden in photo-absorption process, owing to the
complete separation characters. Wherein, the large twisting dihedral
angles between N1-phenyl and PI plane (78° for pTPI, 77° for mpCTPI,
In order to examine the excited states properties of the
compunds, natural transition orbitals (NTOs) for the S0 Sn and
Tn (n = 1–8) transition were performed using time-dependent
density functional theory (TD-DFT) method (Table S2-7 and Fig.
S8† →S1 transition in pTPI, the
).20 As shown in Fig. 7, for the S0
“hole” and “particle” were mostly located on imidazole ring and
TPE moiety. This distribution was assigned to LE transition, and
the large overlap of the NTO in TPE implied the compouds
would have high fluorescence efficiency and AIE
characteristic.12a A homologous LE character on PI moiety was
also observed in S0→S2 transition. Along with the introduction
of cynao group into N1-phenyl, the ICT process was activitaed
and the distribution of NTOs became more complicated. In
→
mpCTPI, the S0→S1 transition exhibited nearly the same LE
configuration with pTPI, but the distinct CT transition with the
“hole” in PI and “particle” in N1-CN-phenyl could be found in
S0→S2 transition, implying the high-energy CT state was
constructed in its excited states as our expected. When the
cyano group was linked at the para-position, the increased
electron-withdrawing ability at the short axis direction
generated a larger CT component in ppCTPI, which could
contribute to S0→S1. The“hole” and “particle” NTOs for S1 and
S2, taking on partial separation and partial overlap, were
considered as class HLCT states.5d,6b Similar situation arose in
TPE meta-linked compounds (Fig. S9†), wherein, mmCTPI and
pmCTPI contained better mixed LE/CT component than mTPI as
well. However, the higher CT components in mmCTPI and
pmCTPI were observed because of the lower conjugation
degree caused by TPE meta-linked pattern. Importantly, the
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oscillator strength of S0
→
S1 (fS0-S1) that was considered to be
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DOI: 10.1039/C9TC01453E
related to the PLQYs of emitters was reduced as the addition of
cyano group, with values of 0.9570 for pTPI, 0.9490 for mpCTPI
and 0.8102 for ppCTPI, respectively (In counterparts, 0.3324 for
mTPI, 0.2711 for mmCTPI and 0.0642 for pmCTPI). These
obvious decreases in oscillator strength were in good
agreement with the PLQYs in experimental measurements
(Table 1).
Compared with the common materials based on D‒A or D–
π–A system, this two-dimensional vertical conformation based
long-short axis had a larger regulation space for CT proportion.
The lowest luminous state remianed LE-like characters when
the cyano group was inserted to N1-position at the short-axis,
and there was only increment of the CT-component proportion
in every excited state energy level. The feature had been
mentioned in our previous reports: at the long axis of the
emitter, the LE-dominated emissive state based on large π–
conjugated structure needed to be constructed; and at its
vertical direction, a short axis with obvious CT characteristic was
built with large steric hindrance (the conjugation degree was
not too large, or this short axis would become the emission
axis).6c,11 Hence, tunable CT-components could be incorporated
into LE emissive state to form HLCT state. Combining AIE
characteristics in further, the PLQYs in solid-state would be
enhanced synchronously.
Why was the CT state required in the process of
luminescence? In fact, the CT component does have negative
effect on PLQYs, but it is liable to increase EUEs via RISC
channels.3a,5a,6c For the “hot exciton” based on HLCT
mechanism,
the complete conversion of excitons from triplet to singlet must
meet three requirements in EL process: small ΔESmTn (m≥1, n≥2),
CT component with weak exciton binding energy and obstacle
for exciton relaxation from Tn to T1.6d,21 Wherein, the CT-typed
exciton ususally have lower binding energy, and its generation
is easier than LE-typed exciton after carrier injection and
recombination, differnent from PL process. In other words, the
formation of T4 (CT-dominant) for ppCTPI was esaier than T1 (LE-
dominant) in OLEDs, and excitons might be converted into
singlet from T4 if there was a fast conversion channle. So,
intrinsically, the appropriate proportion of CT component was
very important to enhance the EUEs. By comparing all the
detailed results of the excited states (Table S8-S13†), some
valuable tendencies were observed. In pTPI and mTPI, every
excitation in eight NTOs for triplet states mainly exhibited LE
characters, as a result of noneffective RISC path to S1 or S2
(TLE
→SLE). For the high-lying S2 and T8 in mpCTPI, we found their
ΔEST was very close with value of 0.05 eV and they both showed
obvious CT character from NTOs, so the triplet excitons might
be converted to singlet excitons via the RISC channel
Fig. 8 Probable “hot exciton” mechaniam caused by HLCT states for
(THLCT
→SCT). As the CT component became larger in other three
TPE para-linked compounds. Here, S: singlet state; T: triplet state; cyano-containing compounds, the NTOs of both S1 and S2 took
on HLCT character and more CT channels, such as S1 and T4
(THLCT SHLCT), S2 and T8 (THLCT SHLCT), could be found and
generated more singlet excitons by more effective exciton up-
conversion channels as shown in Fig. 8 and Fig. S10 . It was
LE: local excited-state; CT: charge-transfer state; ΔEST: singlet–triplet
energy splitting; KIC: internal conversion rate; KRISC: reverse
intersystem crossing rate.
→
→
†
worth noting that the existence of several lower triplets (like T2
This journal is © The Royal Society of Chemistry 20xx
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Journal of Materials Chemistry C
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ARTICLE
and T3 in ppCTPI) were unfavorable to RISC process and thus phenyl)cyclohexan)
Journal Name
and
TmPyPB(1,3,5-TriV(miew-pArytirciledOinn-li3ne-
DOI: 10.1039/C9TC01453E
might make loss of harvesting triplet excitons.
ylphenyl)benzene) worked as the hole- and electron-
transporting layers, respectively; 4,4′,4′′-tris(carbazol-9-yl)-
triphenylamine (TCTA) was used as the exciton blocking layer,
and ITO (indium tin oxide) and Al were used as the anode and
electrode, respectively. The schematic energy level diagrams of
the devices, the EL spectra at 10 mA cm–2, the current density–
voltage–luminance (J–V–L) characteristics, the EQE versus
luminance curves and the current efficiency versus luminance
curves of the non-doped OLEDs were displayed in Fig. 9. The key
data of device performances were summarized in Table 3, in
which EUE was calculated by Eqn-1:
EL performances
If the discussions above were correct, the gradual increment of
EUEs could be expected in OLED performance. To evaluate their
EL properties, we fabricated non-doped OLEDs with a device
configuration of ITO/HATCN (5 nm)/TAPC (40 nm)/TCTA (5 nm)/
emitters (20 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (120 nm)
(emiters = pTPI, device
mTPI, device IV; mmCTPI, device
HATCN (dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-
hexacarbo-nitrile) and LiF served as hole- and electro-injection
layers, respectively; TAPC (di-(4-(N,N-ditolyl-amino)-
I; mpCTPI, device II; ppCTPI, device III
;
V; pmCTPI, device VI), where
ƞEL =ƞrec × ƞs × ƞPL × ƞout (Eqn-1)
Table 3 EL performances of the OLEDs based on the compounds.
λEL
(nm)
494
506
503
492
493
489
Von
(V)a
2.9
2.9
3.1
3.1
3.5
2.9
L
ηc
(cd A‒1
16.52
18.27
18.46
8.87
ηp
(lm W‒1)b
15.55
19.12
16.32
8.70
EUE
(%)
EQEmax
(%)b
EQE100
0 (%)
6.07
6.26
6.83
3.23
2.14
2.04
RO
(%)
4.0
6.7
4.6
18.8
5.7
17.7
Device
CIE (x, y) c
Compounds
(cd m–2)b
40290
36030
31070
12310
5386
)
b
I
II
III
IV
V
(0.226, 0.423)
(0.243, 0.447)
(0.211, 0.402)
(0.201, 0.339)
(0.213, 0.345)
(0.195, 0.342)
28.6
39.9
48.1
33.4
38.2
48.4
6.32
6.71
7.16
3.98
2.27
2.48
pTPI
mpCTPI
ppCTPI
mTPI
mmTPI
pmTPI
5.07
5.44
3.93
5.34
VI
6748
a Von is the turn-on voltage at 1 cd m–2. b The luminescence (L), current efficiency (ηc), power efficiency (ηp) are the maximum values of the
devices. c CIE coordinates at 10 mA cm–2.
Fig. 9 (A) and (B) Energy level diagrams; (C) EL spectra at 10 mA cm–2; (D) current density–voltage–luminance (J-V-L) characteristics; (E)
current efficiency versus luminance curves; and (F) EQE versus luminance curves of the non-doped OLEDs based on the six AIEgens.
8 | J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry C
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Journal Name
ARTICLE
where, ƞEL is the EQE, ƞrec is the electron-hole recombination
proportion (assumed to be 100%), ƞs is the EUE, ƞPL is the PLQY of
neat film, and ƞout is the light out coupling efficiency (usually
results demonstrated that, a strategy of reasonably superposing
AIE unit to HLCT emitters via construcing a conformation of
vertical cross axes is practically feasible in materials design.
View Article Online
DOI: 10.1039/C9TC01453E
estimated from 20% to 30%, and calculated using 25% here ).22
As shown in the EL spectra, the devices based on TPE para- and
meta-linked compounds exhibited bluish green and sky-blue
emissions, respectively, and their EL peaks are close to their PL peaks
of neat films. All the devices exhibited low turn-on voltages (Von)
under 3.5 eV, thanks to the bipolar properties from PI unit.23 And
among them, the TPE para-linked compounds displayed lower Von
than meta-linked ones because of more balanced carrier transport
stem from their larger molecular conjugation.24 Enjoying the higher
PLQYs, the former exhabited better EL performances than the latter.
The EQEmax of pTPI, mpCTPI and ppCTPI were 6.32%, 6.71%, and
7.16%, respectively, much higher than those of mTPI (3.98%),
mmCTPI (2.27%) and pmCTPI (2.48%). In addition, the cynao group
had a very positive effect on increasing EUE. For pTPI and mTPI, the
EUE values were calculated to be 28% and 33.4%, respectively, which
had slightly exceeded the theoretical limit of spin statistics of
fluorescent emitters. For the cyano-containing compounds, the EUE
was increased obviously because of larger CT component in the HLCT
states. When the cynao group was linked at the para position, the
EUE reached excellent values of 48.1 % for ppCTPI and 48.4% for
pmCTPI. The improvment in EUEs was consitent with the above
analysis of more excitons conversion channels. Furthermore, taking
the advantage of AIE property, low efficiency roll-off was also
achieved in these non-doped OLEDs. The non-doped OLED based on
ppCTPI exhibited maximum L, ηc, ηp and EQE of up to 31070 cd m‒2,
18.46 cd A–1, 16.32 lm W–1 and 7.16 %, respectively. Notably, the
deivce showed excellent efficiency stability as evidenced by a very
small efficiency roll-off of 4.0 % at 1000 cd m‒2.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work is financially supported by National Natural Science
Foundation of China (21788102, 51673118 and 51603127), the
Guangdong Natural Science Funds for Distinguished Young
Scholar (2014A030306035), Science & Technology Program of
Guangzhou (201804010218, 201804020027, 201704030069),
the Innovation and Technology Commission of Hong Kong (ITC-
CNERC14S01), and the Fundamental Research Funds for the
Central Universities (2017JQ013).
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8
In order to enhance the PLQYs of HLCT-based compounds in
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DOI: 10.1039/C9TC01453E
TOC
Combining HLCT and AIE characters, six phenanthroimidazole derivatives are