Purely Organic Crystals Exhibit Bright Thermally Activated Delayed Fluorescence

Article abstract of DOI:10.1002/anie.201906371

Thermally activated delayed fluorescent (TADF) materials generally suffer from severe concentration quenching. Efficient non-doped TADF emitters are generally highly twisted aromatic amine-based compounds with isolated chemical moieties. Herein we demonst

Full text of DOI:10.1002/anie.201906371

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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International Edition
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Accepted Article
Title: Purely Organic Crystal Exhibiting Bright Thermally Activated
Delayed Fluorescence
Authors: Xinyi Cai, Zhenyang Qiao, Mengke Li, Xiao Wu, Yanmei He,
Xiaofang Jiang, Yong Cao, and Shi-Jian Su
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201906371
Angew. Chem. 10.1002/ange.201906371
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10.1002/anie.201906371
Angewandte Chemie International Edition
RESEARCH ARTICLE
Purely Organic Crystal Exhibiting Bright Thermally Activated
Delayed Fluorescence
Xinyi Cai+, Zhenyang Qiao+, Mengke Li, Xiao Wu, Yanmei He, Xiaofang Jiang, Yong Cao and Shi-Jian
Su*
Abstract: Thermally activated delayed fluorescent (TADF) materials
generally suffered from severe concentration quenching. At present,
efficient non-doped TADF emitters none the less were largely
twisted aromatic amine-based compounds with isolating chemical
moieties. Herein, we demonstrated even with co-facial packing and
strong π-π intermolecular interactions, bright TADF emissions in
non-doped film and crystalline states could possibly be realized in a
special compound, 2,4-diphenyl-6-(thianthren-1-yl)-1,3,5-triazine
(oTE-DRZ). According to quantum chemistry simulation, a disperse
outer-orbital of sulfur atom, a folded thianthrene plane for a reduced
donor-acceptor distance and an n-π* character of triazine acceptor
generated a spatial conjugated transition exhibiting a small singlet-
triplet splitting energy. Considering a highly emissive non-doped film,
adopting host-guest emitting layer (EML) strategy, aiming at
replacing the noble-metal containing phosphorescent
counterparts.[2] Specifically, exploring TADF emitters showing
high photoluminescent quantum yields (PLQYs) in non-doped
state none the less attracts global attention since they are highly
promising for low-cost and stable display and illumination
applications from the broad exciton recombination zone within
the EML and an extremely simple device fabrication
procedure.[3] In light of this, more efforts are demanded as
performances of these non-doped devices yet lagged far behind
their doped counterparts.
Challenges remained as TADF materials generally suffered from
severe aggregation concentration quenching (ACQ).[3a, 3b] In view
of a complicated molecular packing in solid state, the key is to
block multiple ACQ channels. Fortunately, optoelectronic
structure-property relationship in crystalline state had been
extensively investigated on organic dyes such as polythiophene,
tetracene, perylene imide derivatives, et al.[4] Among them,
corresponding organic light-emitting diode achieved
a 20.6%
external quantum efficiency, verifying its potential for high-
performance optoelectronic applications. In crystalline state, intra-
and intermolecular dual TADF were verified assisted by a hidden
room temperature phosphorescent state. This state could both well
reserve the long-lived excitons from highly suppressed non-radiation
and serve as a “spring-board” for the cascade up-conversion
processes, which compensate the energy loss from Dexter energy
transfer-induced bimolecular annihilation. Moreover, in spite of a “H-
aggregate” packing, Förster energy transfer for radiative exciton
delocalization was effectively blocked from a large Stokes shift for
minimized transfer integral and a special “M-aggregate” for negligible
dimer dipole resonance. Consequently, oTE-DRZ crystal showed
greenish-blue emission with a very high photoluminescent quantum
yield of ~87%, the highest amongst all reported TADF crystalline
counterparts.
traditional H-, J- and special M-type aggregates were
4b, 4e]
disclosed.[4a,
One of the strategies was blocking Förster
energy transfer (FRET) channel along the packed molecules.
Fast exciton migration would compete violently with the radiative
process, leading to fluorescence quenching from reabsorption,
trapping at chemical or structural defect sites and other
nonradiative processes.[5] For typical H- and J-aggregates,
considering a large dipole relative orientation factor (κ2), strong
dipole resonance is expected assuming a large individual
transition dipole, a small interacting dipole distance and a
prominent overlap of vibronic states obeying the conservation
rules of angular momentum and energy.[6] Generally, designing
TADF materials with strong intramolecular charge transfer (CT)
state could suppress such a FRET process as the separation of
frontier molecular orbitals (FMOs) results in a large Stokes shift
and a small transition dipole ocsillator strength (f).[3a, 5b] Energy
losses of TADF materials in non-doped state should also be
Introduction
Nowadays, purely-organic thermally activated delayed
fluorescent (TADF) electroluminescence (EL) with 100% internal
quantum efficiency (IQE) has been realized.[1] Aided by ambient
thermal energy, the radiative-forbidden triplet excitons could be
effectively up-converted to the radiative-allowed singlet state
owing to a tiny singlet-triplet splitting energy (ΔEST). Over the last
decade, the external quantum efficiencies (EQEs) of TADF
organic light-emitting diodes (OLEDs) had surpassed 30% and
are heading to the 40% milestone with regards to devices
illustrated by electron-exchange interaction governed by Dexter
7]
energy transfer (DET) mechanism.[3b,
As close electronic
wavefunction contact is necessary to collision-induced ACQ, the
realization of bright TADF solids acquires moderate
intermolecular isolations. At present, the successful cases
nonetheless possessed amine-based largely twisted molecular
backbones with “inert” chemical isolating fragments, significantly
restricting our thinking of developing new bright luminophores in
non-doped state.
[*]
Dr. X. Y. Cai, Z. Y. Qiao, M. K. Li, X. Wu, Y. M. He, Dr. X. F. Jiang,
Prof. Dr. Y. Cao, Prof. Dr. S.-J. Su
State Key Laboratory of Luminescent Materials and Devices and
Institute of Polymer Optoelectronic Materials and Devices, South
China University of Technology
Wushan Road 381, Tianhe District, Guangzhou 510640,
Guangdong Province, P. R. China
E-mail: mssjsu@scut.edu.cn
Not until recently, our group unveiled that even eliminating any
aromatic amine units, TADF exciplex emission could be realized
[+]
These authors contributed equally to this work.
using
a
special
compound,
3,3’-(2-methyl-1,3-
phenylene)diphenoxathiine (mPXTMP) as the electron donor.[8]
Nevertheless, the long-range intermolecular CT state with weak
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201906371
Angewandte Chemie International Edition
RESEARCH ARTICLE
exciton-binding energy caused low PLQYs, hampering the
realization of high-performance devices.[1b, 9] Referencing to the
development of efficient TADF materials, introducing strong
intramolecular CT state is promising to address this issue
because of the radiative recombination-inclined character of
Frenkel exciton with strong Columbic interaction between hole
and electron.[10] In this work, an electron donor thianthrene (TE)
was selected to directly bond with 1,3,5-triazine (TRZ), yielding a
compound 2,4-diphenyl-6-(thianthren-1-yl)-1,3,5-triazine (oTE-
DRZ). (Scheme 1) TE was chosen owing to its strong electron-
donating ability, high triplet state (T1) and high-level outer-
orbitals (principle quantum number (n=3)) of sulfur “heavy”
atoms involved in transitions for a strong spin-orbital coupling
(SOC). To highlight the importance of the ortho-substituted TE,
reference compounds 2,4-diphenyl-6-(thianthren-2-yl)-1,3,5-
triazine (mTE-DRZ) and 2-(dibenzo[b,d]thiophen-4-yl)-4,6-
A folded TE unit and a large size sulfur atom twisted the D-A
backbone by steric repulsion interaction. As a result, a direct
spatial conjugated transfer channel opened up since the electron
wavefunctions could overlap spatially from a short positive and
negative centers distance and a disperse spatial outmost-orbital
(n=3) of sulfur atom. In contrast, only π-conjugated channel left
for mTE-DRZ with a linear and planar molecular backbone for a
large D-A distance. (Figure S1 and S2) Concerning oDBT-DRZ,
despite the five-member ring dibenzothiophene (DBT) with weak
electron donating ability was 4-yl-substituted to the acceptor, a
rather planar geometry formed owing to a small steric hindrance
effect. (Figure S3a and S3b) This brought about a separation of
local positive and negative extremum centers either in ground
state (S0) or excited state, yielding solely π-conjugated transition
diphenyl-1,3,5-triazine
(oDBT-DRZ)
were
synthesized.
Surprisingly, oTE-DRZ exhibited a high PLQY even with a close
intermolecular π-π packing in solid state. Whereafter, electronic
structure, exciton dynamic process, device performance and
condense state characters of these compounds were
investigated to gain insight into the inner mechanism.
(a)
-25
+25
(kcal mol-1)
-22.32 (S+N)
(back)
-15.95 (S)
-12.35 (S)
R=donor
n orbital
spatial conjugated
channel
2 8 6
S
-20.23 (N)
-17.80 (N)
n = 3
[Ne] 3s2 3p4
DBT
TE
TRZ
+7.63 (TRZ)
+9.32 (TRZ)
Donors
Acceptor
(back)
π transition
channels
π & spatial conjugated
weak e- donor & π
transition channels
transition channels
(b)
π*
Particle
mTE-DRZ
F
oTE-DRZ
TADF
oDBT-DRZ
F
f=0.0755
93.4%
oTE-DRZ
f=0.2466
94.7%
mTE-DRZ
Scheme 1. Electronic structures of sulfur atom (n denotes the principle
quantum number of outer-most orbital), dibenzothiophene (DBT), thianthrene
(TE) and triazine (TRZ), molecular structures of the investigated compounds
and design strategies for the realization of TADF.
Hole
n
Results and Discussion
Initially, electronic structures of mTE-DRZ, oDBT-DRZ and oTE-
DRZ were computed by density functional theory (DFT) and
Figure 1. (a) ESP mapped molecular van der Waals surface of oTE-DRZ
(unit: kcal mol-1). The cyan and orange spheres labeled by contributive atoms
represent the surface local minima and maxima of ESP. The spatial
conjugated channel was indicated by green arrow; (b) The natural transition
occupied (hole) and unoccupied (particle) orbital distributions, oscillator
strengths (f) and eigenvalues of NTO pairs of mTE-DRZ and oTE-DRZ. The
green arrow and circles indicate n-π* character (S0→S1 transition of oTE-
DRZ) localizing on TRZ unit.
time-dependent
density
functional
theory
(TD-DFT)
simulation.[11] The inner-built spatial conjugated channel of oTE-
DRZ could be verified by observing its molecular geometry and
electrostatic potential (ESP) mapped van der Waals surface.[12]
(Figure 1a) The local minima and maxima located around sulfur
atoms of TE moiety and the centroid of TRZ plane, respectively.
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10.1002/anie.201906371
Angewandte Chemie International Edition
RESEARCH ARTICLE
channel. Visualization of hole and particle distributions and
transition probability analysis further verified such a deduction.
phosphorescent profile was a mixture of 3LE of TE fragment and
3CT emissions. This could be explained by
a
fast
Linear π-conjugation and planar D-A backbone generally results
phosphorescent rate (kphos) of TE moiety owing to a “heavy
atom” effect of sulfur atoms for an enhanced SOC between S0
and T1 states.[15] Since long-lived triplet excitons gradually
diffuse into low-energy sites, T1 of mTE-DRZ was dominated by
3CT emission after several millisecond (ms), yielding a
of 0.38 eV.[8, 16] (Figure S12) For oTE-DRZ in toluene, however,
in large ocsillator strength (f) and FMOs overlap.[6,
As
13]
expected, mTE-DRZ and oDBT-DRZ exhibited larger f values
comparing to that of oTE-DRZ. (Figure 1b and S3c) Though
moderate FMOs overlap was noted, a small S0→S1 transition f
value of oTE-DRZ could be illustrated by its n-π* transition
character locating on TRZ unit, which was beneficial for
achieving a small ΔEST. (Figure 1b, green square)
on account of a hardly recognized fluorescent profile at 77 K, it
was challenging to reliably estimate its ΔEST
.
The theoretical prediction was affirmed by measuring the photo-
physical properties in solution state. In diluted toluene solution,
the maximum molar extinction coefficient (ε) of the mTE-DRZ CT
absorption band was over threefold larger than that of oTE-DRZ,
implying large FMOs overlaps and transition probability with
planar molecular geometry and linear π-conjugated transition
channel. (Figure 2a) Meanwhile, the photoluminescence (PL)
profile of oTE-DRZ was more bathochromic than that of mTE-
DRZ, suggesting the spatial-conjugated channel led to a
stronger intramolecular CT state. Surprisingly, a vast Stokes
shift of 172 nm in toluene and 219 nm in methanol solutions
were noted for oTE-DRZ. (Figure S9a) A significant internal
conversion (IC) should happened, accompanied with a severe
geometry deformation after optical excitation. (Figure S9b)
Formation of this low-energy CT state was advantageous to
achieve a small ΔEST given that the locally triplet excited state
(3LE) of either the donor or acceptor was over 0.15 eV higher
α~12.9°
β~168.2°
β~142.7°
α~10.7°
D-A complanation for
large Stokes shift
S
T
spin flip for geo-
metry deformation
ISC/RISC
Sn
IC
S1 (1CT*)
T1 (3CT*)
Strong spin-orbital
coupling (SOC)
α~34.0°
VA
VE
S0
than its CT triplet state (3CT).[1d, 14] (Figure S11) In contrast, CT
3
β~126.7°
twisted D-A
backbone & TE plane
of mTE-DRZ was 0.32 eV lower than that of oTE-DRZ, whose
Scheme 2. Schematic diagram of oTE-DRZ potential surface in S0, S1 and T1
states. Optimized geometries, exciton dynamic processes of singlet (green
circles) and triplet (blue circle) excitons, reaction paths (marked by arrows)
were shown, in which VA, VE, IC, ISC, RISC, 1CT*, 3CT*, α and β denotes
vertical absorption, vertical emission, internal conversion, intersystem crossing,
reverse intersystem crossing, the singlet and triplet charge transfer excited
states, dihedral angles within the folded TE plane and between the donor and
acceptor.
(a)
UV-vis
mTE-DRZ
3LETE
Fluo@298 K
Fluo@77 K
Phos
3CT
10
1
TE_Phos
0
0
1
3CT
UV-vis
oTE-DRZ
Fluo@298 K
Fluo@77 K
Phos
10
To comprehend the vast Stokes shifts and reverse of S1 and T1
energies in toluene, oTE-DRZ excited state geometries were
simulated. (Scheme 2) In S1, the “butterfly” shape TE fragment
flattened, yielding a bending angle (β) of 142.7° and a D-A
dihedral angle (α) of 12.9°. In T1, α was further reduced to 10.7°
while β increased to 168.2°. The significant deformation in
excited state should be responsible for the severe energy loss.
Rationality of such a deduction could be confirmed by the good
agreement between the experimental results and theoretical S1
transition orbitals and energies.[17] (Table S1 and S3) In spite of
a similar α, the almost coplanar TE unit resulted in a highly
twisted molecular backbone in T1 than that in S1. These T1
conformers could be even more tortile if relatively small steric
hindrance interaction and complicate solid circumstance were
taken into account. Specifically, unlike those in solution, a
0
0
400
500
600
700
(b)
Wavelength (nm)
UV-vis
TE_RTP@5 ms
Fluo@298 K
Fluo@77 K
Phos
1.0
0.5
0.0
1
oTE-DRZ
Neat film
0
300
400
500
600
700
Wavelength (nm)
Figure 2. (a) UV-vis absorption, PL at 298 and 77 K, phosphorescence
spectra at 77 K of mTE-DRZ and oTE-DRZ dissolved in toluene solutions
(0.01 mM) and (b) oTE-DRZ neat film. Phosphorescence of TE fragment in
toluene at 77 K and neat film at room temperature were also shown.
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10.1002/anie.201906371
Angewandte Chemie International Edition
RESEARCH ARTICLE
reduced Stokes shift, a hypsochromic PL and a high PLQY of
oTE-DRZ neat film were found since rigid environment could
well suppress geometric relaxation in excited state.[14a] (Table 1)
Similar phenomenon was observed for mTE-DRZ and oDBT-
DRZ and their ΔEST values in neat films were reckoned to be
0.501 and 0.569 eV, respectively
microsecond (μs) to 100 ms, TRPL unveiled almost constant PL
3
profiles, which could be ascribed to CT emission as the room
temperature phosphorescent state of TE film (RTPTE) was
energetically higher than this 3CT state.
In view of a remarkable difference between the delayed
components of the oTE-DRZ neat film at 298 and 5 K, the long-
lived ones should originate from different radiative species,
which was further verified by temperature-dependent transient
and steady-state PL measurements. (Figure 4a, 4c and S13b)
Accompanied with a rising temperature from 220 to 250 K, the
delayed component increased monotonically, followed by slight
decreases when increasing to 300 K because of an enhanced
nonradiation.[18] This long-lived component could also be
effectively quenched by O2.[19] (Figure S14b and S14e) The
RTPTE state was deduced less likely to contribute largely to this
delayed component as kphos of purely organic compounds was
generally in the order of 10-2~101 s-1 only.[20] (Figure 3) With a
delayed lifetime of 230 μs and a very large ISC ratio (Φisc), the
delayed radiative rate would at least be in the order of 103 s-1,
Consequently, this delayed profile could be rationally ascribed to
TADF. Since RISC rate (krisc) is proportional to exp(-ΔEa/kBT)
according to Arrhenius relationship, in which ΔEa, kB and T
represents thermal activation energy, Boltzmann constant and
temperature, respectively, ΔEa was fitted to be 181 meV.[1b, 1c]
(Figure 4d) Besides, the aggregation of oTE-DRZ molecules
was found not necessary condition to induce TADF. Both
thermally activated temperature-dependent and oxygen
sensitive properties were observed when oTE-DRZ was doped
into a wide bandgap host bis[2-(diphenylphosphino)phenyl] ether
oxide (DPEPO) at a very low concentration (1 wt%), for which
bimolecular interaction was minimized.[21] (Figure S13a, S14a
and S14d) Moreover, transient PL of oTE-DRZ in different
solutions exhibited a second-order exponential decay with τTADF
of hundreds of ns to several μs. (Figure S10a and Table S4)
Therefore, the oTE-DRZ monomer was verified TADF active, in
good agreement with a small ΔEST of 0.304 eV simulated by K-
OHF methodology.[22] (Figure S4 and Table S3) In contrast, only
one-order exponential decay with several ns excited state
0-1 ns
2.2 ns
5 ns
298 K
TADF
400 ns
100 μs
1 ms
1
S1
RTPTE
0
1
0-1 ns
2.2 ns
500 μs
100 ms
5 K
Phos
S1
0
450
500
550
600
650
700
Wavelength (nm)
Figure 3. Time-resolved PL spectra of the oTE-DRZ neat film measured at
298 and 5 K, respectively. For contrast, room-temperature phosphorescence
profile with 5 ms time delay of the TE neat film was shown.
Analogous to the case in frozen toluene, phosphorescence of
the oTE-DRZ neat film dominated the PL profile at 77 K owing to
a very fast intersystem crossing rate (kisc) of >108 s-1. To
dissociate prompt fluorescence (PF) from phosphorescence to
estimate ΔEST, time-resolved PL (TRPL) spectra were measured
at 298 and 5 K, respectively. (Figure 3) At 298 K, S1
fluorescence initially showed up with an emitting onset at 442
nm. Several nanoseconds (ns) later, the PL onset pinned at 468
nm, followed by a slight redshift when prolonging the detection
time. In contrast, owing to a highly rigid circumstance with
negligible thermal energy at 5 K, the PF profile slightly blue
shifted owing to a further suppressed IC process from geometry
relaxation. When elongating the detection time from 500
Table 1. Summary of the photo-physical properties of the investigated compounds in neat film and crystal states, respectively.
[a]
ΦPL
ΦPF
ΦTADF
ΦISC+ΦIC
τPF
τTADF
(μs)
krS
kic+kisc
krisc
kTADF
kCQ
ΔEST
(%)
(%)
(%)
(%)
(ns)
(107 s-1)
(107 s-1)
(104 s-1)
(103 s-1)
(103 s-1)
(eV)
neat film
oDBT-DRZ
mTE-DRZ
oTE-DRZ
3.6
3.6
n.a.
n.a.
72.3
96.4
56.1
91.8
1.04
11.1
1.31
n.a.
3.46
3.95
4.95
92.7
5.05
71.5
n.a.
n.a.
5.20
n.a.
n.a.
3.4
n.a.
n.a.
0.99
0.568[b]
43.9
78.8
43.9
6.5
n.a.
0.501[b]
229.8
0.181[c]/0.16[d]
crystal
oTE-DRZ
87.0
13.1
73.9
84.9
1.02
131.3
12.9
85.1
4.93
6.6
1.14
0.182[c]/0.16[d]
[a] ΦPL was estimated in vacuum condition. [b] ΔEST were evaluated by experimental fluorescent and phosphorescent spectra at 77 K, [c] fitting Arrhenius
equation and [d] time-resolved PL slices of TADF and phosphorescence, respectively.
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10.1002/anie.201906371
Angewandte Chemie International Edition
RESEARCH ARTICLE
lifetimes were detected concerning mTE-DRZ and oDBT-DRZ
either in solution or film state owing to the large ΔEST verified
experimentally and theoretically. (Figure S10b, S11, S15-S16)
TRPL also unveiled that at 298 K, S1 of the oTE-DRZ neat film
was 0.27 eV higher than T1, much larger than the fitted ΔEa of
0.181 eV. As mentioned, the PL profile of the oTE-DRZ neat film
red-shifted within several ns and stabilized, suggesting the
achieved.
Notably, oTE-DRZ in solid state exhibited
a
special
concentration independent property. When increasing the
dopant concentration, τTADF shortened while krS and krisc
gradually increased, accompanied with
a
rather small
concentration quenching rate (kCQ). (Figure 4b and Table S5)
PLQY of the oTE-DRZ neat film was over 78%, indicating
possible efficient non-doped OLEDs application. To verify this
concept, devices were fabricated with an architecture shown
(Figure 4(e)), in which 4,4’-cyclohexylidenebis[N,N-bis(4-
possibility of forming highly tortile D-A conformers and low-
23]
energy radiative states.[14a,
(Figure 3 and S5) A thermal
equilibrium intermediate radiative state was assumed gradually
formed as T1 geometry significantly differed from the S0 and S1
ones. Besides, RTPTE state was found hidden between this
methylphenyl)benzenamine]
carbazolyl)benzene (mCP),
(TAPC),
DPEPO,
1,3-bis(N-
tri[3-(3-
3
intermediate state and CT of the oTE-DRZ neat film. Such an
intermediate triplet state from aggregation of TE (3LETE) could
serve as a “springboard” to facilitate the cascade up-conversion
process for fast reverse internal conversion (RIC) and strong
SOC between 1CT and 3LETE states.[16a, 24] This explained why in
spite of a relatively large FMOs overlap, a small D-A dihedral
angle and a short D-A distance of oTE-DRZ in contrast to its
aromatic amine counterparts, an efficient RISC process was still
pyridyl)mesityl]borane (3TPYMB), lithium fluoride (LiF) and
aluminum (Al) served as hole transporting, hole transporting and
electron blocking, electron transporting and hole blocking,
electron transporting, electron injection materials and cathode,
respectively. (Figure S20 to S23 and Table S6) The oTE-DRZ
based non-doped device turned on at 3.3 V with a maximum
EQE of 20.6%.[3a-c, 3e, 25] Assuming a light outcoupling efficiency
(a)
(b)
(c)
102
1.5
oTE-DRZ
neat film
77 K
x wt% oTE-DRZ: DPEPO
515 nm
104
103
102
101
100
0.8
0.6
0.4
0.2
0.0
1.0
0.5
0.0
1.0
0.5
0.0
1 wt%
10 wt%
25 wt%
50 wt%
75 wt%
100 wt%
101
100
10-1
10-2
TADF
542 nm
298 K
77 K
single crystal
Phos
crystal
502 nm
528 nm
ΦDF τTADF
~ 480 nm
film
crystal
krS(107)
krisc(104)
kTADF(104)
kCQ(103)
TADF
298 K
Phos
0
25
50
75 100
450
500
550
600
650
0
2
4
6
8
10
Wavelength (nm)
Time (ms)
Doping concentration (%)
(d)
(e)
(f)
-2.1
-2.2
-2
-3
-4
-5
-6
11
10
9
EQE~20.6%
101
100
-2.6
-2.84
-2.92
1
8
-3.1
1
m
e
V
1
8
2
-4.2
m
e
V
-5.1
10-1
10-2
oTE-DRZ
non-doped EML
mTE-DRZ
8
-5.5
450 600 750
Wavelength (nm)
neat film
crystal
-5.8 -5.8
oTE-DRZ
-6.1
7
-6.5
-6.7
E (eV)
10-3
10-2 10-1
100
101
102
3.5 4.0 4.5 5.0
103/T (K-1)
Current Density (mA cm-2)
Figure 4. (a) Transient PL decay curves of x wt% oTE-DRZ: DPEPO doped films (x=1, 10, 25, 50, 75, 100) and crystal. The arrows denote the evolving tendency
of TADF lifetime (τTADF) and TADF ratio (ΦDF) when increasing the doping concentration; (b) ΦPL and rate constants of exciton dynamic processes of the doped
films and crystal; (c) Temperature-dependent PL spectra of the oTE-DRZ neat film and crystal. The blue triangles labeled the emission peaks ascribing to
different emitting species while the red and black arrows for the rise and fall of TADF and phosphorescent components; (d) Fitting natural logarithm relationship of
reverse intersystem crossing rate (ln(krisc)) plotted versus different temperatures to estimate thermally activated energy (ΔEa) according to Arrhenius relationship;
(e) Device architectures of the OLEDs with non-doped EMLs; (f) External quantum efficiency-current density (EQE-J) curves of OLEDs. Insets were the
electroluminescent spectra detected at the current density of 10 mA cm-2 (left) and photos of the lighting devices (right).
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10.1002/anie.201906371
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RESEARCH ARTICLE
of 30% and a charge carrier balance factor of 1.0, corresponding
exciton utilization ratio (ηr) reached 87%.[26] In sharp contrast,
the maximum EQE of the mTE-DRZ based OLED was only
3.8% with a ηr of 28.7%, which slightly exceeded the theoretical
25% singlet exciton production rate in EL device according to
spin-statistic rule.[27] (Table S7) This might be interpreted by the
transition dipole orientation of mTE-DRZ in solid state owing to
its stick-like, rigid and planar molecular backbone.[28]
in which the electronic orbitals delocalized and dispersed on
both of the adjacent monomers. (Figure 5b and S6) Simulation
of the trimer unveiled that the intermolecular transition could
hardly step over
a monomer, suggesting the intra- and
intermolecular CT transitions dominated the radiative processes.
(Figure S7)
Temperature-dependent PL measurement of oTE-DRZ crystal
solidly affirmed such a deduction. As the temperature dropped
from 300 to 220 K, the ~480 nm emission decreased rapidly
while the one at 502 nm remained unchanged. (Figure 4c) On
account of a high PLQY of 87% at ambient temperature, the
nonradiative process should be highly suppressed when further
lowered the temperature. Thus, the decreasing emitting intensity
should be ascribed to inadequate thermal energy for efficient
RISC. (Figure S13c) Such energetically high species were highly
sensitive to temperature, which could be ascribed to transition
profile of the monomer. (Table S2) The other was tentatively
Considering the complicated molecular stacking in films (Figure
S24), oTE-DRZ single crystal was analyzed alternatively to
illustrate the possible mechanism for bright emission in non-
doped state. (Figure S25 and Table S8) X-ray crystallographic
analysis unveiled the molecules were packing face-to-face with
a small slide adjacent to each other.[29] The average distances
between the adjacent TE and TRZ fragments were only 3.496
and 3.199 Å, respectively. (Figure 5a and 5c) With an angle (θ1)
between the individual transition dipole and dipole alignment
direction of ~88.4°, a typical “H-aggregate” could be termed.[4b, 4d, assumed to be the dimer transition profile as intermolecular
6, 30]
To gain insight into the excited state electronic structures,
oTE-DRZ monomer, dimer and trimer chemical structures were
extracted for simulation. (Table S2) As expected, the S0→S1
transition of the monomer consist of intramolecular CT transition
from TE to TRZ unit. For the dimer, an additional transition from
TE of one monomer to TRZ of the adjacent one was disclosed,
long-range CT state was beneficial to realize a small ΔEST,
which was energetically lower than that of the monomer
according to TD-DFT results. In this case, efficient RISC could
be expected even with a reduced thermal energy. Exciton
population from down-conversion of high-energy monomer
excitons would result in its less sensitivity to temperature as well.
58.6%
Crystal
3.199 Å
Cell
HONTO
LUNTO
3.496 Å
36.9%
HONTO-1
LUNTO+1
(a)
(b)
(c)
(d)
H-Aggregation
κ=3(cosθ)2-1
2
1
2
1
2
Intermolecular
transition dipole
1
θ=54.7°
J
θ~0°
H
θ~90°
1
2
θ1= 88.4°
θ2
0
0
Intramolecular
transition dipole
Dipole
alignment
Magic angle
θ2 = 58.6°±1.7
-1
-1
90
κ2 = 0.039 ±0.029
0
30
60
θ (°)
Figure 5. (a) Crystal cell and H-aggregated molecular packing of oTE-DRZ single crystal. The white and cyan balls denote the centroids of two adjacent
monomers for the evaluation of intermolecular distances; (b) The natural transition occupied (hole) and unoccupied (particle) orbital distributions and eigenvalues
of NTO pairs of the monomer and dimer; (c) Angle (θ) between intra- or intermolecular transition dipole moment and the corresponding dipole alignment direction;
(d) Schematic diagram of dipole orientation factor (κ) versus angles between the resonance dipoles (θ) according to Kasha’s model.[30b] Regarding typical H- and
J-type aggregation, θ approached 0° and 90°, respectively, leading to significant dipole resonance effect. Whereas, for M-aggregation with a θ close to 54.7°, κ2
was minimized with negligible FRET transfer integral in spite of a very small intermolecular distance.
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10.1002/anie.201906371
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When further lowered the temperature to below 220 K, a
significantly reduced thermal energy hampered the up-
conversion processes, leading to reduction of the dimer
emission intensity. When temperature was below 150 K, the
phosphorescence enhanced remarkably owing to a highly
suppressed nonradiative process and enhanced triplet exciton
population from blocked up-conversion channels. The deduction
of forming multiple radiative channels of oTE-DRZ in neat film
and crystal states could be further validated via analyzing their
krS, which were much larger than those of the 1 wt% oTE-DRZ:
DPEPO doped film and in diluted solutions, respectively. (Figure
4b, Table 1, S4 and S5) In amorphous film state, owing to a
disordered molecular alignment and diverse conformers, a
mixed intra- and intermolecular transitions could be supposed
although only one broad PL profile was observed at 298 K.
1) Notably, the fitted ΔEa was close to the one estimated from
TRPL profiles, implying the radiative state from thermal
equilibrium S1 conformers dominated the TADF emission.
(Figure 3 and Scheme 2) Different from the oTE-DRZ neat film,
the close π-π stacking of the molecules in crystalline state could
significantly suppress the excited state geometry relaxation
considering a small red-shift of TRPL when prolonging the
detection time. According to the design strategy for bright
materials in non-doped state, it was critical to isolate adjacent
molecules via introducing “inert” chemical units.[2h, 3b] Normally, a
close intermolecular van der Waals contact was deemed
unfavorable to fluorescence owing to the formation of low-
energy dark states or excimer.[31] Nevertheless, with regards to
molecules exhibiting a special “M-aggregate”, dipole resonance
could be minimized even with a very small intermolecular
30]
distance.[6,
In particular, the angle (θ2) between the dimer
It is worth mentioning that achieving a high PLQY in molecular
systems with closely intermolecular stacking was now scarce
amongst reported TADF materials. Apart from the inner built
multiple radiative channels, the fitted ΔEa was as small as 182
mV, leading to a large krisc of 4.93×104 s-1. (Figure 4d and Table
transition dipole and the corresponding dipole alignment
direction approached 58.6°, very close to a “Magic-angle” of
~54.7°. (Figure 5c and 5d) Thus, the orientated factor (κ2) was
minimized (~0.039) for a negligible energy transfer integral of the
dimer transitions. For transitions of the oTE-DRZ monomer with
1CTmonomer
(a) (b)
Sn
3CTmonomer
IC
ISC/RISC
Min
IC/RIC
IC/RIC
1CTdimer
ISC/RISC
3CTdimer
Stable T1,TE
Dual TADF
VA
F+TADF
F+TADF
very slow NR
RTP
RTP trailing
298 K
oTE-DRZ crystal
Dual TADF + RTP
Max
S0
(c)
(d)
Excitation
TE_RTP
Fluo@298 K
Fluo@77 K
Phos
0-1 ns
1.9 ns
100 ns
400 μs
1 ms
Crystal
1.5
1.5
1.0
0.5
0.0
Phos@5 ms
Phos@50 ms
1.0
0.5
0.0
1
0
Crystal
300
400
500
600
700
450 500 550 600 650 700
Wavelength (nm)
Wavelength (nm)
Figure 6. (a) Electronic processes of oTE-DRZ single crystal under optical excitation, in which VA, F, TADF, IC, ISC, RISC, NR, RTP, CTmonomer, CTdimer and T1, TE
denote vertical absorption, fluorescence, thermally activated delayed fluorescence, internal conversion, intersystem crossing, reverse intersystem crossing,
nonradiative, room-temperature phosphorescence, charge transfer state of the monomer and dimer, the lowest stable triplet state of TE crystal, respectively; (b)
2D contour map of time-resolved PL spectra of oTE-DRZ crystal at 298 K; (c) UV-vis absorption, steady-state PL at 298 and 77 K, phosphorescence spectra oTE-
DRZ crystal. For contrast, RTP spectra of TE crystal was shown; (d) Time-resolved PL slices of oTE-DRZ crystal at different time point and corresponding steady-
state phosphorescent spectra at 5 and 50 ms for comparison.
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10.1002/anie.201906371
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RESEARCH ARTICLE
vast Stokes shifts, FRET process was also highly suppressed,
generating highly localized Frenkel excitons favorable for
fluorescence.
reabsorption, which was detrimental to the performance of
device at high brightness.[7b]
Conclusion
In addition to the suppressed energy transfer channels, the
hidden RTP state was found underneath the PL profile of the
oTE-DRZ crystal as its low temperature phosphorescence
agreed well with the long-lasting components at 298 K. (Figure 5
and S17) Since the packing of TE units within oTE-DRZ crystal
was different from that of pure TE crystal, the corresponding
RTP profile of oTE-DRZ crystal was similar but with a slight red-
shift in contrast to that of the RTP-active TE crystal.[32] (Figure
S26 and Table S8) The evaluated kphos of oTE-DRZ crystal was
approaching 5.2×101 s-1, which was relatively fast among the
reported purely organic RTP materials owing to a strong SOC
induced by the sulfur-rich environment.[20a, 20b, 33] (Figure S18) As
a result, triplet excitons from fast ISC and IC processes could be
well preserved in the “H-aggregate” induced stable T1 state.[20d]
A rather rigid crystalline circumstance resulted in negligible
nonradiative processes, which was importance to the protection
of long-lived excitons. Considering small energy gaps (<0.30 eV)
between this stabilized T1 and the upper singlet or triplet states
of the dimers and monomers, the phosphorescence was
reasonably less competitive to the RISC process. (Figure 5a)
Therefore, multiple RIC/RISC cycles could be expected,
eventuating in efficient TADF emissions. In spite of an obvious
difference of molecular packing between amorphous and
crystalline states, both were exhibiting similar ΔEa, much larger
krS as well as higher PLQYs comparing to those in doped films
and solutions, indicating a strong π-π interaction in neat film
could be rationally assumed.
In summary, based on
a
nonaromatic-amine-containing
compound oTE-DRZ, non-doped film and crystal exhibiting
bright TADF emissions were realized in spite of a strong
intermolecular π-π interaction. Briefly, inner-built multiple
radiative channels, formation of intermediate radiative state from
thermal equilibrium conformers, highly localized radiative
excitons, a rigid molecular circumstance, strong SOC effect from
“heavy-atom” effect of sulfur element and a small ΔEST for
efficient RISC should be all taken into account. Specifically,
thermally activated radiative channels were proved assisted by
an additionally under-lying RTP state. Owing to small energy
gaps between this state and those up-lying ones, the RTP state
played an important role in both reserving long-lived triplet
excitons from highly suppressed nonradiative process and
serving as a key “springboard” for the cascade up-conversion.
Employing such a strategy, corresponding oTE-DRZ crystal
achieved a very high PLQY of 87%. Although amorphous
morphology was noted regarding the oTE-DRZ neat film, an
intermolecular close packing similar to that in crystalline state
could be assumed on account of their similar ΔEa, large krS as
well as high PLQYs. With a high PLQY of 78% in non-doped
state, corresponding TADF-OLED achieved an EQE of 20.6%
and a ηr of 87%, which were comparable to the reported
aromatic-amine-based counterparts.
It is noteworthy that issues remained unaddressed for the
efficiency roll-off of this non-doped device at high brightness
were severe. Apart for realizing a high PLQY in non-doped state,
further reduction of ΔEST for rapid RISC would be critical.
Moreover, as the DET-induced bimolecular annihilation was
verified to be the dominant pathway for energy loss in organic
material systems with close intermolecular interaction, the inner
mechanism on well preserving long-lived excitons in the
stabilized T1 state should be further illustrated in addition to a
highly suppressed nonradiative process. Beyond that,
introducing a hidden RTP state could be a promising strategy for
the realization of highly emissive organic materials with strong
intermolecular interaction according to our results. In view of the
coexistence of a high PLQY and a close intermolecular π-π
interaction, future optoelectronic applications such as efficient
organic light-emitting field effect transistors (OLEFETs) could be
anticipated and related works are under investigation now!
However, issues remained as the bimolecular annihilation
should be severe owing to fast DET from efficient electron
6]
exchange processes.[3b,
(Figure S8) As mentioned, for the
oTE-DRZ neat film and crystal, the hidden RTP states should
play vital roles in both preserving triplet excitons and serving as
an intermediate “springboard”. Because of a much lower triplet
exciton density under optical excitation in contrast to the case
under electrical stress, the bimolecular annihilation process was
less competitive to the rapid up-conversion and multiple
radiative processes considering a rather small kCQ of ≤103 s-1.[34]
(Table 1) Nevertheless, luminance loss was still noted even with
negligible FRET and nonradiative processes. In some extend,
bimolecular annihilation dissipated long-lived excitons within
only several ms. In EL condition, a severe efficiency roll-off was
noted at high current densities. As up-conversion rate plays a
“rate-limited” role in shortening τTADF, severe efficiency roll-off
should be ascribed to a relatively large ΔEST and a τTADF of
several hundreds of μs.[18b] High exciton densities would lead to
severe singlet-triplet annihilation (STA) and triplet-triplet
annihilation (TTA) in view of a significant overlap between the
PL and excited state absorption profile from transient absorption
(TA) spectra.[34a, 35] (Figure S19) Thus, single step bimolecular
annihilated process would lead to severe excited state
Acknowledgements
The authors greatly appreciate the financial support from the
National Key R&D Program of China (2016YFB0401004), the
National Natural Science Foundation of China (91833304,
51625301,
U1601651,
and
51573059),
973
Project
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Entry for the Table of Contents
RESEARCH ARTICLE
Purely organic crystal exhibiting
both a bright thermally activated
delayed fluorescence (TADF)
(photoluminescent quantum yield up
to 87%) and a strong intermolecular
π-π stacking is firstly demonstrated.
Owing to inner-built spatial
conjugated intra- and intermolecular
transitions, dual TADF emissions
assisted by a hidden room
Xinyi Cai+, Zhenyang Qiao+, Mengke
Li, Xiao Wu, Yanmei He, Xiaofang
Jiang, Yong Cao and Shi-Jian Su*
spatial
conjugated
RISC
RIC
Dual TADF + RTP
NR
Page No. – Page No.
Intra- &
intermolecular
transition
Purely Organic Crystal Exhibiting
Bright Thermally Activated Delayed
Fluorescence
monomer
dimer
RTP
TADF crystal with
strong π-π packing
ΦPL = 87%
temperature phosphorescent state is
unveiled.
ΔEST = 0.182 eV
kRISC > 104 s-1
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