Mechanochromic Delayed Fluorescence Switching in Propeller-Shaped Carbazole–Isophthalonitrile Luminogens with Stimuli-Responsive Intramolecular Charge-Transfer Excited States

Article abstract of DOI:10.1002/anie.202005584

Herein, the universal design of high-efficiency stimuli-responsive luminous materials endowed with mechanochromic luminescence (MCL) and thermally activated delayed fluorescence (TADF) functions is reported. The origin of the unique stimuli-triggered TADF

Full text of DOI:10.1002/anie.202005584

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: Mechanochromic Delayed Fluorescence Switching in Propeller-
Shaped Carbazole–Isophthalonitrile Luminogens with Stimuli-
Responsive Intramolecular Charge-Transfer Excited States
Authors: Minlang Yang, In Seob Park, Yasuhiro Miyashita, Katsunori
Tanaka, and Takuma Yasuda
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202005584
Link to VoR: https://doi.org/10.1002/anie.202005584

10.1002/anie.202005584
Angewandte Chemie International Edition
RESEARCH ARTICLE
Mechanochromic Delayed Fluorescence Switching in Propeller-
Shaped Carbazole–Isophthalonitrile Luminogens with Stimuli-
Responsive Intramolecular Charge-Transfer Excited States
Minlang Yang,[a,b] In Seob Park,[a] Yasuhiro Miyashita,[c] Katsunori Tanaka,[c] and Takuma Yasuda*[a,b]
[a]
M. Yang, Dr. I. S. Park, Prof. Dr. T. Yasuda
INAMORI Frontier Research Center (IFRC), Kyushu University
744 Motooka, Nishi-ku, Fukuoka 819-0395 (Japan)
E-mail: yasuda@ifrc.kyushu-u.ac.jp
[b]
[c]
M. Yang, Prof. Dr. T. Yasuda
Department of Applied Chemistry, Graduate School of Engineering, Kyushu University
744 Motooka, Nishi-ku, Fukuoka 819-0395 (Japan)
Y. Miyashita, K. Tanaka
Odawara Research Center, Nippon Soda Co., Ltd.
345 Takada, Odawara, Kanagawa 250-0280 (Japan)
Supporting information for this article is given via a link at the end of the document.
Abstract: Herein, the universal design of high-efficiency stimuli-
responsive luminous materials endowed with mechanochromic
luminescence (MCL) and thermally activated delayed fluorescence
(TADF) functions is reported. The origin of the unique stimuli-
triggered TADF switching for a series of carbazole–isophthalonitrile-
based donor–acceptor (D–A) luminogens is demonstrated based on
systematic photophysical and X-ray analyses, coupled with
theoretical calculations. It was revealed that a tiny alteration of the
intramolecular D–A twisting in the excited-state structures governed
by the solid morphologies is responsible for this dynamic TADF
switching behavior. This concept is applicable to the fabrication of
bicolor emissive organic light-emitting diodes using a single TADF
emitter.
recently reported a highly emissive MCL material based on a
donor–acceptor (D–A) electronic system with intramolecular
charge transfer (ICT) characteristics and revealed that its MCL
behavior originated from changes in the planar and twisted
excited-state D–A structures formed in the different solid
phases.[6] In addition, we demonstrated two-color-emissive
organic light-emitting diodes (OLEDs) using this D–A-type MCL
material as a single fluorescent emitter. Ma and coworkers
reported a D–A-type fluorophore with ICT, which exhibited
unique multi-color MCL behavior caused by the change in the
twisting angles between the D and A units.[7] Compared with
conventional MCL materials that inevitably involve strong
intermolecular interactions or stacking, such D–A-type MCL
systems induced by variable intramolecular twisting, therefore,
have huge potential to demonstrate greater ΦPL along with
definite color changes even in the aggregated states.[6,7]
Introduction
Meanwhile, thermally activated delayed fluorescence
(TADF) materials have emerged as a promising alternative to
traditional fluorescent and phosphorescent emitters in the field of
OLEDs[8,9] and related optoelectronic devices[10] owing to their
ability to produce high internal quantum efficiencies (ηint)
approaching 100% without using noble metals. To achieve an
effective up-conversion of the non-emissive triplet (T1) into
emissive singlet (S1) excitons, TADF materials are required to
possess a tiny energy splitting (ΔEST) between the lowest
excited S1 and T1 states.[8,9] The most common strategy for
designing efficient TADF molecules is to construct highly twisted
D–A structures within a single molecule, rendering them to have
spatially separated frontier molecular orbitals without
compromising the radiative ICT nature. With remarkable exciton
utilization capability and quantum efficiencies, the merging of
TADF and MCL functions into the D–A-type molecular systems
will be a fascinating approach for the development of high-
performance luminous smart materials and devices.
Furthermore, the solid-state emission colors and emission
lifetimes of such MCL-TADF materials can be manipulated by
minutely controlling their ICT excited states while minimizing the
ACQ effect through rational material design.
Solid-state luminescence is
a
conspicuous area of
fundamental and applied research. In particular, smart organic
luminescent materials that are capable of switching their
emission colors in response to external stimuli, such as
mechanical force, heat, and solvent fuming, have attracted great
attention toward their applications in sensors, probes, and
security and storage devices.[1] Over the past decade, a wide
variety of mechanochromic luminescent (MCL) materials
including π-conjugated small molecular dyes,[2] liquid crystals,[3]
polymers,[4] and organometallic complexes,[5] which can switch
their emission colors (wavelength), intensity, and/or lifetime
upon mechanical grinding, shearing, and pressing, have been
extensively explored. The stimuli-responsive characteristics of
most reported MCL materials primarily originate from the
alteration of intermolecular interactions, such as π–π stacking,
J- or H-aggregation, and excimer formation within the
condensed states. Accordingly, the photoluminescence (PL)
quantum yields (ΦPL) of such MCL materials are typically rather
low due to severe aggregation-caused quenching (ACQ), which
restricts their functionality as actual emitters. The development
of novel MCL systems that display clear MCL color changes
together with high ΦPL in their aggregated solid states is of vital
importance for their future practical applications. Our group
Herein, we present a versatile design strategy for high-
efficiency MCL-TADF materials featuring stimuli-responsive
twisted ICT excited states.
A
novel mechanism for
1
This article is protected by copyright. All rights reserved.

10.1002/anie.202005584
Angewandte Chemie International Edition
RESEARCH ARTICLE
mechanochromic TADF switching in carbazole–isophthalonitrile-
based luminogens 1–3 (Figure 1a) is reported. Thus far, some
TADF materials have been reported to exhibit distinct MCL color
changes in response to external forces.[11−17] The key design
strategy adopted in most of these previous reports was the use
of structurally deformable D units such as phenothiazine, which
played a vital role in their MCL responsivity via the quasi-axial
and quasi-equatorial conformational changes.[11−16] In contrast,
our present MCL-TADF systems rely on the stimuli-triggered
alteration of intramolecular D–A twisting (or torsion angles) in
their ICT excited states and do not entail significant changes in
noncovalent intermolecular interactions, resulting in notably high
ΦPL values (75%–98%) even after the MCL color changes in the
condensed states.
state (Figure 1b,c). The planar Cz units in both molecules
adopted a highly twisted arrangement but not orthogonal with
respect to the central isophthalonitrile core, as evidenced by the
large dihedral angles of 56–79°. These propeller-shaped D–A
arrangements can ensure effective spatial separation of the
frontier molecular orbitals, leading to distinct ICT excited states
and small ΔEST.
Intriguingly, all 1–3 solid samples showed distinct MCL
behavior in response to external stimuli, such as grinding. As
presented in Figure 2, the pristine powders of 1 and 2 obtained
by recrystallization from chloroform/methanol exhibited intense
blue PL emissions peaking at λPL = 470 and 466 nm, with ΦPL as
high as 98% and 82%, respectively. When these samples were
subjected to mechanical grinding with mortar and pestle, the λPL
positions were found to redshift to 515 and 512 nm, respectively,
displaying similar intense green PL emissions. It is worth noting
that ground 1 and 2 still retained considerably high ΦPL values of
80% and 75%, respectively, indicating that ACQ could be well
suppressed as opposed to conventional fluorescent MCL
materials. For 3 with four Cz units, the emission color changed
Results and Discussion
Compounds 1–3 were prepared via nucleophilic aromatic
substitution reactions using fluorinated isophthalonitrile
precursors and 3,6-di-tert-butylcarbazole.[8a,18] The peripheral
tert-butyl (tBu) groups were introduced to suppress the
intermolecular interactions and exciton deactivation arising from
collisional Dexter energy transfer,[19] thereby boosting ΦPL in the
solid states. Detailed synthetic procedures and characterization
data are described in the Supporting Information. While the
synthesis and basic TADF characteristics for 3 were previously
reported by Lee et al.,[18] its MCL-TADF properties have not
been revealed thus far. More recently, a polymorphic behavior of
3 arising from dimer formation was also reported by Etherington
et al.[20]
clearly from green (λPL = 525 nm; ΦPL = 92%) to yellow (λPL
=
557 nm; ΦPL = 80%) upon mechanical grinding. The original
emission colors of 1–3 could be recovered by recrystallization or
exposure to tetrahydrofuran (THF) or chloroform vapor,
confirming the reversibility of their mechano/vapochromic
processes.
Figure 1. a) Molecular structures of propeller-shaped MCL-TADF luminogens
1–3. Single-crystal X-ray structures of b) 1 and c) 3 (CCDC 1990823 and
1990824, respectively) with thermal ellipsoids at 50% probability. Hydrogen
atoms and disordered tBu groups are omitted for clarity.
Figure 2. a–c) Steady-state PL spectra and d–f) transient PL decay profiles for
the pristine and ground powders of 1–3 measured at room temperature.
Insets: photographs showing MCL color changes observed for 1–3 in
response to mechanical grinding under UV irradiation at 365 nm.
X-ray crystallographic analyses for 1 and 3 provided in-
depth insight into their stable molecular structures and the
relative arrangement of the carbazolyl (Cz) units in the ground
2
This article is protected by copyright. All rights reserved.

10.1002/anie.202005584
Angewandte Chemie International Edition
RESEARCH ARTICLE
Table 1. Photophysical data of MCL-TADF luminogens 1–3 in different states
[a]
[b]
[c]
[c]
[d]
[d]
[f]
[g]
[h]
[h]
[i]
Compound
State
Sol[j]
PL
PL
p
d
p
d
kr[e]
kISC
[s−1
kRISC
[s−1
ES
ET
EST
[nm]
500
470
515
514
485
466
512
507
528
525
557
557
[%]
49
98
80
98
42
82
75
89
93
92
80
74
[%]
[%]
47
96
57
66
40
80
42
54
76
86
51
44
[ns]
14
7.3
16
17
14
6.0
18
17
15
7.8
16
16
[s]
[s−1
]
]
]
[eV]
[eV]
[eV]
1
2
11
6.5 × 106
1.5 × 106
1.5 × 107
2.7 × 107
8.9 × 106
3.0 × 106
1.8 × 107
1.5 × 107
1.1 × 107
7.6 × 106
1.8 × 107
2.2 × 107
7.6 × 107
1.4 × 108
4.9 × 107
5.6 × 107
7.0 × 107
1.6 × 108
3.7 × 107
5.0 × 107
5.5 × 107
1.1 × 108
4.3 × 107
4.0 × 107
6.8 × 105
4.5 × 106
4.7 × 105
5.5 × 105
4.7 × 105
1.6 × 106
2.5 × 105
3.6 × 105
1.8 × 106
5.2 × 106
1.2 × 106
1.2 × 106
–
–
–
Pristine
Ground
Film[k]
2
20
2.85
2.72
2.73
–
2.82
2.68
2.69
–
0.03
0.04
0.04
–
23
32
2
6.8
5.4
37
2
3
Sollj]
Pristine
Ground
Film[k]
2
22
2.90
2.74
2.75
–
2.83
2.69
2.70
–
0.07
0.05
0.05
–
33
35
17
6
10
9.8
3.0
2.6
2.1
1.8
Sollj]
Pristine
Ground
Film[k]
2.57
2.48
2.50
2.49
2.44
2.47
0.08
0.04
0.03
29
30
[a] PL emission maximum. [b] Absolute PL quantum yield evaluated using an integrating sphere under N2. [c] Fractional quantum yields for prompt
fluorescence (Φp) and delayed fluorescence (d); p + d = PL. [d] Emission lifetimes for prompt fluorescence (p) and delayed fluorescence (d). [e] Rate
constant of fluorescence radiative decay (S1 → S0): kr = p/p. [f] Rate constant of ISC (S1 → T1): kISC = (1 − p)/p. [g] Rate constant of RISC (T1 → S1): kRISC
= d/(kISC∙p∙d∙p). [h] Lowest excited singlet (ES) and triplet (ET) energies estimated from the onsets of the fluorescence and phosphorescence spectra. [i]
Singlet–triplet energy splitting: EST = ES – ET. [j] Measured in deoxygenated toluene solution (10−5 M). [k] Measured as a vacuum-deposited neat film.
Time-resolved PL measurements revealed that both the
pristine and ground powders of 1–3 exhibited noticeable two-
component transient PL decay composed of a nanosecond
prompt fluorescence and a microsecond delayed fluorescence
(Figure 2d–f). These are typical characteristics of TADF
involving spin-converting intersystem crossing (ISC; S1 → T1)
and reverse intersystem crossing (RISC; T1 → S1) processes.[8,9]
The prompt fluorescence lifetimes (τp) for the ground samples of
1–3 were measured to be 15–20 ns, which were longer than
those for the pristine ones (τp = 6.0–7.8 ns). Conversely, the
corresponding delayed fluorescence lifetimes (τd) appeared to
have shortened upon grinding. The detailed MCL-TADF and
relevant photophysical data are summarized in Table 1. These
peculiar stimuli-triggered photophysical switching in terms of
TADF colors and lifetimes can be mainly attributed to the
variations in the ICT-based S1 and T1 excited energies (ES and
ET) in the solid states before and after applying the mechanical
forces, as will be discussed later.
Powder X-ray diffraction (XRD) analysis provided useful
insight into the stimuli-triggered morphological changes in 1–3.
As shown in Figure 3a (see also Supporting Information), the
XRD patterns of the pristine samples of 1–3 showed sharp
diffraction peaks and aligned with the simulated patterns based
on the crystallographic data, implying that these initial materials
are essentially microcrystalline solids. After mechanical grinding,
the pronounced diffraction peaks disappeared, suggesting that
each sample transformed into an amorphous state. Hence, the
MCL-TADF behaviors in 1–3 can be envisaged to be
accompanied by the crystalline-to-amorphous transformation
within the solid states. The morphological changes could also be
confirmed by differential scanning calorimetry (DSC). As can be
seen from Figure 3b, the ground sample of 1 exhibited an
exothermic peak at 158 °C with a small transition enthalpy (ΔH
~3 J g−1) upon heating, which originated from the cold-
recrystallization of the mechano-amorphized powder, whereas
its pristine unground sample showed neither an exothermic nor
an endothermic peak until 250 °C before thermal decomposition.
This observation suggests that the ground amorphized state is
metastable and tends to return to a thermodynamically stable
crystalline state through molecular reorganization upon heating.
Now the question arises as to how the foregoing solid-state
morphological changes in 1–3 can affect their MCL color
changes. To clarify this and understand the underlying
mechanism of the MCL-TADF phenomena, we computationally
simulated the geometric structures in the ground (S0) and
excited S1 and T1 states, as well as the electronic transition
characteristics for 1–3, using density functional theory (DFT) and
time-dependent DFT (TDDFT) methods. As depicted in Figure 4,
the dihedral angles between the 2-Cz unit and the central
isophthalonitrile unit (θ2) were calculated to be ~66° for 1 and 2
in the optimized S0 geometries.[21] Importantly, in their S1 and T1
states, θ2 substantially increased to 82–90° to form full-twisted
Figure 3. a) Powder XRD patterns of 1 in the form of pristine powder (blue
line) and ground powder (green line). The simulated result based on the
crystallographic data (black line) is also included. b) DSC thermograms of a
pristine sample of 1 upon first heating and a ground sample of 1 in a heating
and cooling cycle at a scanning rate of 10 °C min−1 under N2.
3
This article is protected by copyright. All rights reserved.

10.1002/anie.202005584
Angewandte Chemie International Edition
RESEARCH ARTICLE
Figure 4. Optimized molecular geometries of 1–3 at S0, S1, and T1 states (left) and hole and electron distributions of the natural transition orbitals (NTOs)
and the adiabatic excitation energies for the S1 and T1 states of 1–3 (center, right), calculated at the PBE0/6-31G(d) level in gas phase. HLCT = hybrid local
and charge-transfer excited state.
ICT excited states with orthogonal D–A arrangements. Note that
for 2, there is no significant contribution from the opposite 5-Cz
unit to the hole density distribution in its S1 and T1 states,
resulting in electronic transition characteristics analogous to 1.
Consequently, the hole and electron densities are localized on
the 2-Cz and diphenyl-isophthalonitrile moieties, respectively, in
the optimized excited S1 and T1 states, which can cause
complete charge separation and lower (or stabilize) the resulting
ICT energy levels. However, 3 with additional Cz units had
different geometric and electronic features in comparison with 1
and 2. While θ2 in 3 underwent almost no change upon
electronic excitation, the calculated dihedral angles between the
4,5,6-Cz units and the isophthalonitrile unit (θ4, θ5, and θ6) were
found to increase significantly. In this case, the 4,5,6-Cz units
serve as the actual D units, and hence, the hole density in the S1
state is solely distributed on the 4,5,6-Cz units instead of the 2-
Cz unit.[22] Based on these electronic configurations, all 1–3
were calculated to have sufficiently small adiabatic ΔEST values
in the range of 18–52 meV (Figure 4) to induce TADF. These
TDDFT results unambiguously indicate that the ICT
excitation/emission processes of 1–3 should be accompanied by
large variations of intramolecular D–A twisting due to the Cz
rotation. In the crystalline states, such structural relaxation would
be strongly suppressed by constraints from the dense and rigid
molecular packings that make the D and A units remain the
initial relatively less twisted arrangements even in the excited
state, as can be observed in the X-ray structures (Figure 1b,c).
Therefore, the pristine microcrystalline powders of 1–3 led to
higher-energy (or blue-shifted) TADF emissions. In contrast,
because the disordered amorphous states are generally less
dense and possess a relatively large free volume, the specific
Cz units in 1–3 can more freely rotate to afford the orthogonal
excited D–A structures with a more stabilized ICT excited state,
as predicted by the TDDFT calculations (Figure 4).
Consequently, the PL spectra underwent conspicuous red-shifts
along with TADF color changes via the crystalline-to-amorphous
transformation.
As expected, for the vacuum-deposited neat films, the
TADF properties of 1–3 closely resembled those of the ground
amorphous powders (Table 1). Their ΦPL values were as high as
98%, 89%, and 74%, individually, which renders them practically
useful
for
device
fabrication.
To
investigate
the
electroluminescence (EL) performances of 1–3, nondoped
OLEDs were fabricated using the vacuum deposition method,
with the configuration of indium tin oxide (ITO)/2,3,6,7,10,11-
hexacyano-1,4,5,8,9,12-hexaazatriphenylene
(HAT-CN,
10
nm)/1,1-bis[4-[N,N-di(p-tolyl)amino]phenyl]cyclohexane (TAPC,
50 nm)/9-phenyl-3,9'-bicarbazole (CCP, 10 nm)/1,3-bis(N-
carbazolyl)benzene (mCP, 10 nm)/1–3 (20 nm)/ 2,8-
bis(diphenylphosphoryl)dibenzo[b,d]furan (PPF, 10 nm)/1,3-
bis[3,5-di(pyridin-3-yl)phenyl]benzene (B3PyPB, 40 nm)/8-
hydroxyquinoline lithium (Liq, 1 nm)/Al (100 nm).[23] The EL
spectra,
current
density–voltage–luminance
(J–V–L)
characteristics, and external EL quantum efficiency (ηext) versus
J plots for this set of devices are depicted in Figure 5, and the
key EL parameters are listed in Table 2.
Similar to the PL spectra of the thin films and ground
powders, devices A–C employing 1–3 as a neat emission layer
(EML) displayed green to yellow structure-less EL with peaks
(λEL) at 509, 512, and 546 nm, respectively. Although 1 had the
4
This article is protected by copyright. All rights reserved.

10.1002/anie.202005584
Angewandte Chemie International Edition
RESEARCH ARTICLE
highest ΦPL in the neat film, the EL performance of device A was
limited due to a rather high turn-on voltage (Von ~7 V), probably
due to its relatively low hole injection and hole/electron transport
properties. As revealed by photoelectron yield spectroscopy
(Supporting Information), the ionization potential of 1 (IP = 6.0
eV) was indeed higher than those of 2 and 3 with multiple Cz
units (IP = 5.7 and 5.5 eV, respectively). As a result, device A
showed a relatively low external EL quantum efficiency (ηext) of
less than 10% due to poor charge balance. However, with a very
low Von of 3.2 V, device B based on analogous 2 demonstrated
the best EL performance with a maximum ηint of 18.2%, current
efficiency (CE) of 54.8 cd A−1, and power efficiency (PE) of 47.4
lm W−1. The maximum ηext values were in the order of device B
(18.2%) > device C (13.5%) > device A (9.6%). Moreover, small
efficiency roll-offs were attained in devices B and C. In particular,
the ηext of device B was maintained as high as 16.2% even at a
high luminance of 1000 cd m−1, corresponding to ~11% roll-off
relative to the maximum ηext value. Note also that in comparison
with these nondoped devices, the doped OLEDs using 1–3 with
a high-T1 PPF host showed excellent EL performances with a
maximum ηext of 17.4%–21.1% (Supporting Information).
[a] Turn-on voltage at a luminance above 1 cd m−2. [b] EL emission maximum
at 10 mA cm−1. [c] Commission Internationale de l'Éclairage (CIE) color
coordinates recorded at 10 mA cm−2. [d] Maximum luminance. [e] Maximum
external EL quantum efficiency. [f] External EL quantum efficiency at
luminance of 100 and 1000 cd m−2. [g] Maximum current efficiency. [h]
Maximum power efficiency.
Motivated by the efficient color-switchable TADF properties,
we attempted to produce bicolor emissive OLEDs using both the
amorphous and crystalline states of 1 as an EML. To this end,
nondoped solution-processed OLEDs were fabricated with a
configuration
of
ITO/poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate):tetrafluoroethyle
neperfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid copolymer
(PEDOT:PSS:PFI, 30 nm)/poly(N-vinylcarbazole) (PVK, 20
nm)/amorphous or crystalline 1 (20 nm)/PPF (10 nm)/1,3,5-
tris(N-phenylbenzimidazol-2-yl)benzene (TPBi, 40 nm)/Liq (1
nm)/Al (100 nm).[6] In the device fabrication (Figure 6a), the neat
film of 1 was initially spin-coated onto the ITO/PEDOT:PSS:PFI-
coated substrates to obtain an amorphous green-emissive EML.
To convert it to a blue-emissive EML, the obtained 1-coated
substrate was then exposed to THF (or chloroform) vapor.
Subsequently, the electron-transporting PPF and TPBi layers, as
well as the Liq/Al cathode, were vacuum deposited on each as-
spun amorphous layer and vapor-fumed microcrystalline layer of
1, affording devices D and E, respectively. Significantly, green
and blue bicolor EL emissions with λEL of 509 and 492 nm were
attained using a single emitter (Figure 6b,c). However, devices
D and E showed lower maximum ηext values (3.0% and 1.5%,
respectively) than that of device A. The EL spectrum of device E
was slightly red-shifted compared to the corresponding PL
spectrum of microcrystalline 1 (Figure 2a), which can be
ascribed to incomplete amorphous-to-crystalline transformation
in the thin EML with a large surface area, unlike bulk powdery
sample. Although some MCL and MCL-TADF materials have
recently been applied as emitters in OLEDs,[6,12a,14,16,17,24] most of
these devices showed mere monochromatic EL. Furthermore, to
date, there have been few reports on bicolor emissive OLEDs
based on a single emitter.[6,24] We therefore believe that high-
efficiency multi-color OLEDs based on a single emitter can be
realized in the future by further sophisticating the design of
mechano/vapochromic TADF luminogens, as well as the device
fabrication processes.
Figure 5. EL characteristics of nondoped OLEDs (devices A–C) based on 1–3
as MCL-TADF emitters: a) EL spectra at 10 mA cm−1, b) photographs of EL
emissions, c) J–V–L characteristics, and d) ηext versus J plots.
Table 2. Performances of nondoped OLEDs based on 1–3 as emitters
Device A (1)
7.0
Device B (2)
3.2
Device C (3)
3.2
Von[a] [V]
EL[b] [nm]
509
512
546
CIE[c] (x, y)
Lmax[d] [cd m−2
ηext,max[e] [%]
(0.25, 0.51)
4400
(0.25, 0.53)
13000
18.2
(0.40, 0.57)
17300
13.5
]
9.6
ηext,100/1000[f] [%]
CE[g] [cd A−1
PE[h] [lm W−1
9.6/8.0
28.2
18.2/16.2
54.8
13.5/12.7
45.5
]
]
11.7
47.4
41.5
Figure 6. a) Schematic drawing representing the conversion from an
amorphous EML of 1 (for device D) into a microcrystalline EML (for device E)
5
This article is protected by copyright. All rights reserved.

10.1002/anie.202005584
Angewandte Chemie International Edition
RESEARCH ARTICLE
via THF vapor annealing. b) EL spectra and c) ηext versus J plots for devices D
and E (inset: photographs of bicolor EL emissions from 1).
Chem. Asian J. 2011, 6, 808−811; h) X. Luo, J. Li, C. Li, L. Heng, Y. Q.
Dong, Z. Liu, Z. Bo, B. Z. Tang, Adv. Mater. 2011, 23, 3261−3265; i) S.-
J. Yoon, S. Y. Park, J. Mater. Chem. 2011, 21, 8338−8346; j) Y.
Ooyama, Y. Harima, J. Mater. Chem. 2011, 21, 8372−8380; k) Y. Dong,
B. Xu, J. Zhang, X. Tan, L. Wang, J. Chen, H. Lv, S. Wen, B. Li, L. Ye,
B. Zou, W. Tian, Angew. Chem. Int. Ed. 2012, 51, 10782−10785;
Angew. Chem. 2012, 124, 10940−10943; l) X. Luo, W. Zhao, J. Shi, C.
Li, Z. Liu, Z. Bo, Y. Q. Dong, B. Z. Tang, J. Phys Chem. C 2012, 116,
21967−21972; m) K. Nagura, S. Saito, H. Yusa, H. Yamawaki, H.
Fujihisa, H. Sato, Y. Shimoikeda, S. Yamaguchi, J. Am. Chem. Soc.
2013, 135, 10322−10325; n) S. Yagai, S. Okamura, Y. Nakano, M.
Yamaguchi, K. Kishikawa, T. Karatsu, A. Kitamura, A. Ueno, D.
Kuzuhara, H. Yamada, T. Seki, H. Ito, Nat. Commun., 2014, 4, 4013; o)
R. Yoshii, K. Suenaga, K. Tanaka, Y. Chujo, Chem. Eur. J. 2015, 21,
7231-7237; p) Z. Wang, H. Nie, Z. Yu, A. Qin, Z. Zhao, B. Z. Tang, J.
Mater. Chem. C 2015, 3, 9103−9111; q) S. K. Park, I. Cho, J.
Gierschner, J. H. Kim, J. H. Kim, J. E. Kwon, O. K. Kwon, D. R. Whang,
J. H. Park, B. K. An, S. Y. Park, Angew. Chem. Int. Ed. 2016, 55,
203−207; Angew. Chem. 2016, 128, 211−215; r) J. Wu, Y. Cheng, J.
Lan, D. Wu, S. Qian, L. Yan, Z. He, X. Li, K. Wang, B. Zou, J. You, J.
Am. Chem. Soc. 2016, 138, 12803−12812; s) S. Nagai, M. Yamashita,
T. Tachikawa, T. Ubukata, M. Asami, S. Ito, J. Mater. Chem. 2019, 7,
4988–4998.
Conclusion
We have presented a universal design for high-efficiency
MCL-TADF luminogens based on renowned carbazole–
isophthalonitrile D–A systems and revealed the mechanism for
their MCL behaviors from the molecular level to the condensed
states. The resulting functional materials cannot only switch their
TADF colors and emission lifetimes in response to external
stimuli but can also retain high ΦPL values (75%–98%) by
minimizing the ACQ effect in the solids. It was found that a tiny
alteration of the intramolecular D–A twisting in the ICT excited
states causes drastic changes in the TADF and photophysical
properties. Additionally, we have successfully demonstrated
high-performance nondoped OLEDs with a high ηint and small
efficiency roll-off, as well as two-color emissive OLEDs, using a
single TADF emitter. Thus, this work provides a new guideline
for designing advanced MCL-TADF materials with tunable
photophysical properties.
[3]
a) Y. Sagara, T. Kato, Angew. Chem. Int. Ed. 2008, 47, 5175−5178;
Angew. Chem. 2008, 120, 5253−5256; b) Y. Sagara, S. Yamane, T.
Mutai, K. Araki, T. Kato, Adv. Funct. Mater. 2009, 19, 1869−1875; c) M.
Mitani, S. Ogata, S. Yamane, M. Yoshio, M. Hasegawa, T. Kato, J.
Mater. Chem.
C 2016, 4, 2752−2760; d) A. Seki, M. Yoshio,
Acknowledgements
ChemPhysChem 2020, 21, 328−334.
[4]
[5]
a) Y. Sagara, M. Karman, E. Verde-Sesto, K. Matsuo, Y. Kim, N.
Tamaoki, C. Weder, J. Am. Chem. Soc. 2018, 140, 1584−1587; b) Y.
Sagara, M. Karman, A. Seki, M. Pannipara, N. Tamaoki, C. Weder,
ACS Cent. Sci. 2019, 5, 874−881.
This work was supported in part by Grant-in-Aid for JSPS
KAKENHI Grant No. JP18H02048 (T.Y.) and JP19K15651
(I.S.P.), the Research Foundation for the Electrotechnology of
Chubu (T.Y.), Hoso Bunka Foundation (T.Y.), and Hirose
International Scholarship Foundation (I.S.P.). The authors are
grateful for the support provided by the Cooperative Research
Program “Network Joint Research Center for Materials and
Devices” and the computer facilities at the Research Institute for
Information Technology, Kyushu University.
a) S. Mizukami, H. Houjou, K. Sugaya, E. Koyama, H. Tokuhisa, T.
Sasaki, M. Kanesato, Chem. Mater. 2005, 17, 50−56; b) H. Ito, T. Saito,
N. Oshima, N. Kitamura, S. Ishizaka, Y. Hinatsu, M. Wakeshima, M.
Kato, K. Tsuge, M. Sawamura, J. Am. Chem. Soc. 2008, 130,
10044−10045; c) H. Ito, M. Muromoto, S. Kurenuma, S. Ishizaka, N.
Kitamura, H. Sato, T. Seki, Nat. Commun. 2013, 4, 2009; d) M.
Krikorian, S. Liu, T. M. Swager, J. Am. Chem. Soc. 2014, 136,
2952−2955; e) J. Liang, Z. Chen, L. Xu, J. Wang, J. Yin, G.-A. Yu, Z.-N.
Chen, S. H. Liu, J. Mater. Chem. C 2014, 2, 2243−2250; f) S. Yagai, T.
Seki, H. Aonuma, K. Kawaguchi, T. Karatsu, T. Okura, A. Sakon, H.
Uekusa, H. Ito, Chem. Mater. 2015, 28, 234−241; g) T. Seki, T. Ozaki,
T. Okura, K. Asakura, A. Sakon, H. Uekusa, H. Ito, Chem. Sci. 2015, 6,
2187−2195.
Conflict of interest
The authors declare no conflict of interest
[6]
[7]
[8]
K. Isayama, N. Aizawa, J. Y. Kim, T. Yasuda, Angew. Chem. Int. Ed.
2018, 57, 11982–11986; Angew. Chem. 2018, 130, 12158–12162.
Y. Zhang, K. Wang, G. Zhuang, Z. Xie, C. Zhang, F. Cao, G. Pan, H.
Chen, B. Zou, Y. Ma, Chem. Eur. J. 2015, 21, 2474–2479.
a) H. Uoyama, K. Goushi, K. Shizu, H. Nomua, C. Adachi, Nature 2012,
492, 234–238; b) S. Y. Lee, T. Yasuda, H. Nomura, C. Adachi, Appl.
Phys. Lett. 2012, 101, 093306; c) A. Endo, K. Sato, K. Yoshimura, T.
Kai, A. Kawada, H. Miyazaki, C. Adachi, Appl. Phys. Lett. 2011, 98,
083302.
Keywords: mechanochromism • thermally activated delayed
fluorescence • charge transfer • organic light-emitting diodes •
solid-state luminescence
[1]
For reviews, see: a) Y. Sagara, T. Kato, Nat. Chem. 2009, 1, 605−610;
b) Z. Chi, X. Zhang, B. Xu, X. Zhou, C. Ma, Y. Zhang, S. Liu, J. Xu,
Chem. Soc. Rev. 2012, 41, 3878−3896; c) X. Zhang, Z. Chi, S. Liu, J.
Xu, J. Mater. Chem. C 2013, 1, 3376–3390; d) J. Gierschner, S. Y.
Park, J. Mater. Chem. C 2013, 1, 5818–5832; e) J. Mei, Y. Hong, J. W.
Y. Lam, A. Qin, Y. Tang, B. Z. Tang, Adv. Mater. 2014, 26, 5429–5479;
f) Y. Sagara, S. Yamane, M. Mitani, C. Weder, T. Kato, Adv. Mater.
2016, 28, 1073−1095; g) P. Xue, J. Ding, P. Wang, R. Lu, J. Mater.
Chem. C 2016, 4, 6688–6706.
[9]
For reviews, see: a) Y. Tao, K. Yuan, T. Chen, P. Xu, H. Li, R. Chen, C.
Zheng, L. Zhang, W. Huang, Adv. Mater. 2014, 26, 7931–7958; b) M. Y.
Wong, E. Zysman-Colman, Adv. Mater. 2017, 29, 1605444; c) Y. Im, M.
Kim, Y. J. Cho, J.-A. Seo, K. S. Yook, J. Y. Lee, Chem. Mater. 2017, 29,
1946–1963; d) Y. Liu, C. Li, Z. Ren, S. Yan, M. R. Bryce, Nat. Rev.
Mater. 2018, 3, 18020; e) X. Cai, S.-J. Su, Adv. Funct. Mater. 2018, 28,
1802558.
[2]
a) C. Löwe, C. Weder, Adv. Mater. 2002, 14, 1625-1629; b) T. Mutai, H.
Satou, K. Araki, Nat. Mater. 2005, 4, 685−687; c) Y. Sagara, T. Mutai, I.
Yoshikawa, K. Araki, J. Am. Chem. Soc. 2007, 129, 1520−1521; d) J.
Kunzelman, M. Kinami, B. R. Crenshaw, J. D. Protasiewicz, C. Weder,
Adv. Mater. 2008, 20, 119−122; e) G. Zhang, J. Lu, M. Sabat, C. L.
Fraser, J. Am. Chem. Soc. 2010, 132, 2160−2162; f) C. Dou, L. Han, S.
Zhao, H. Zhang, Y. Wang, J. Phys. Chem. Lett. 2011, 2, 666−670; g) X.
Zhang, Z. Chi, H. Li, B. Xu, X. Li, W. Zhou, S. Liu, Y. Zhang, J. Xu,
[10] H. Tachibana, N. Aizawa, Y. Hidaka, T. Yasuda, ACS Photonics 2017,
4, 223−227.
[11] a) S. Xu, T. Liu, Y. Mu, Y.-F. Wang, Z. Chi, C.-C. Lo, S. Liu, Y. Zhang,
A. Lien, J. Xu, Angew. Chem. Int. Ed. 2015, 54, 874−878; Angew.
Chem. 2015, 127, 888−892; b) Z. Xie, C. Chen, S. Xu, J. Li, Y. Zhang,
S. Liu, J. Xu, Z. Chi, Angew. Chem. Int. Ed. 2015, 54, 7181−7184;
Angew. Chem. 2015, 127, 7287−7290; c) J. Yang, J. Qin, P. Geng, J.
6
This article is protected by copyright. All rights reserved.

10.1002/anie.202005584
Angewandte Chemie International Edition
RESEARCH ARTICLE
Wang, M. Fang, Z. Li, Angew. Chem. Int. Ed. 2018, 57, 14174−14178;
Angew. Chem. 2018, 130, 14370−14374.
[12] a) M. Okazaki, Y. Takeda, P. Data, P. Pander, H. Higginbotham, A. P.
Monkman, S. Minakata, Chem. Sci. 2017, 8, 2677−2688; b) Y. Takeda,
T. Kaihara, M. Okazaki, H. Higginbotham, P. Data, N. Tohnai, S.
Minakata, Chem. Commun. 2018, 54, 6847−6850.
[13] B. Xu, Y. Mu, Z. Mao, Z. Xie, H. Wu, Y. Zhang, C. Jin, Z. Chi, S. Liu, J.
Xu, Y.-C. Wu, P.-Y. Lu, A. Lien, M. R. Bryce, Chem. Sci. 2016, 7,
2201−2206.
[14] H. Tsujimoto, D.-G. Ha, G. Markopoulos, H. S. Chae, M. A. Balso, T. M.
Swager, J. Am. Chem. Soc. 2017, 139, 4894−4900.
[15] B. Huang, W.-C. Chen, Z. Li, J. Zhang, W. Zhao, Y. Feng, B. Z. Tang,
C.-S. Lee, Angew. Chem. Int. Ed. 2018, 57, 12473−12477; Angew.
Chem. 2018, 130, 12653−12657.
[16] a) Z. Cheng, J. Liang, Z. Li, T. Yang, C. Lin, X. Mu, Y. Wang, J. Mater.
Chem. C 2019, 7, 14255−14263; b) P. Ganesan, R. Ranganathan, Y.
Chi, X.-K. Liu, C.-S. Lee, S.-H. Liu, G.-H. Lee, T.-C. Lin, Y.-T. Chen, P.-
T. Chou, Chem. Eur. J. 2017, 23, 2858−2866; c) X. Zeng, T. Zhou, J.
Liu, K. Wu, S. Li, X. Xiao, Y. Zhang, S. Gong, G. Xie, C. Yang, Adv. Opt.
Mater. 2018, 6, 1801071; d) K. Zheng, H. Yang, F. Ni, Z. Chen, S.
Gong, Z. Lu, C. Yang, Adv. Opt. Mater. 2019, 7, 1900727.
[17] a) P. Rajamalli, N. Senthilkumar, P. Gandeepan, C.-Z. Ren-Wu, H.-W.
Lin, C.-H. Cheng, J. Mater. Chem.
C 2016, 4, 900−904; b) R.
Pashazadeh, P. Pander, A. Lazauskas, F. B. Dias, J. V. Grazulevicius,
J. Phys. Chem. Lett. 2018, 9, 1172−1177; c) Y. Chen, S. Wang, X. Wu,
Y. Xu, H. Li, Y. Liu, H. Tong, L. Wang, J. Mater. Chem. C 2018, 6,
12503−12508.
[18] Y. J. Cho, K. S. Yook, J. Y. Lee, Adv. Mater. 2014, 26, 6642−6646.
[19] J. Lee, N. Aizawa, M. Numata, C. Adachi, T. Yasuda, Adv. Mater. 2017,
29, 1604856.
[20] M. K. Etherington, N. A. Kukhta, H. F. Higginbotham, A. Danos, A. N.
Bismillah, D. R. Graves, P. R. McGonigal, N. Haase, A. Morherr, A. S.
Batsanov, C. Pflumm, V. Bhalla, M. R. Bryce, A. P. Monkman, J. Phys.
Chem. C 2019, 123, 11109−11117.
[21] For 1, TDDFT calculations were also performed using its single-crystal
structure with θ2 of 79° as the initial S0 geometry (see Supporting
Information). The excited structures and electronic configurations for
both S1 and T1 states converged in almost the same as those
calculated with the DFT-optimized S0 structure with θ2 of 66° (see
Figure 4), indicating the validity of the simulation results.
[22] H. Noda, X.-K. Chen, H. Nakanotani, T. Hosokai, M. Miyajima, N.
Notsuka, Y. Kashima, J.-L. Brédas, C. Adachi, Nat. Mater. 2019, 18,
1084−1090.
[23] a) J. Lee, N. Aizawa, T. Yasuda, Chem. Mater. 2017, 29, 8012−8020;
b) I. S. Park, K. Matsuo, N. Aizawa, T. Yasuda, Adv. Funct. Mater. 2018,
28, 1802031; c) K. Matsuo, T. Yasuda, Chem. Sci. 2019, 10,
10687−10697.
[24] a) B. Huang, Z. Li, H. Yang, D. Hu, W. Wu, Y. Feng, Y. Sun, B. Lin, W.
Jiang, J. Mater. Chem. C 2017, 5, 12031−12034; b) B. Huang, D. Jiang,
Y. Feng, W.-C. Chen, Y. Zhang, C. Cao, D. Shen, Y. Ji, C. Wang, C.-S.
Lee, J. Mater. Chem. C 2019, 7, 9808−9812.
7
This article is protected by copyright. All rights reserved.

10.1002/anie.202005584
Angewandte Chemie International Edition
RESEARCH ARTICLE
Entry for the Table of Contents
A mechanism of delayed fluorescence switching for propeller-shaped twisted donor–acceptor systems driven by mechanical force is
reported. By applying this concept, blue and green bicolor emissive light-emitting devices can be produced using a single emitter
endowed with stimuli-responsive luminescence and delayed fluorescence properties.
8
This article is protected by copyright. All rights reserved.