Versatile Direct Cyclization Constructs Spiro-acridan Derivatives for Highly Efficient TADF emitters

Article abstract of DOI:10.1002/anie.202103187

Spiro-acridan (SpA) derivatives possess a great potential in preparing efficient thermally activated delayed fluorescent (TADF) emitters. However, the conventional synthetic routes are cost-expensive and time-consuming. The development of a simple procedure to synthesize SpAs is still in urgent pursuing appreciated by academic and industrial communities. In this contribution, we present a feasible acid-catalyzed solvent-free metal-free cyclization between diarylamines and ketones to construct SpAs. The as-constructed moieties provide a wide possibility to assemble efficient TADF emitters. As an example, D2T-TR with high photoluminescent quantum yield and proper TADF character is applied in organic light emitting diodes (OLEDs) which achieves a maximum external quantum efficiency (EQE) of 27.1 %. This work shows us a bright inspiration on developing excellent organic optoelectronic materials and an effective tool to realize it.

Full text of DOI:10.1002/anie.202103187

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How to cite:
International Edition: doi.org/10.1002/anie.202103187
German Edition: doi.org/10.1002/ange.202103187
Fluorescence
Hot Paper
Versatile Direct Cyclization Constructs Spiro-acridan Derivatives for
Highly Efficient TADF emitters
He Liu, Zhiwen Liu, Ganggang Li, Huaina Huang, Changjiang Zhou, Zhiming Wang,* and
Chuluo Yang*
Abstract: Spiro-acridan (SpA) derivatives possess a great
potential in preparing efficient thermally activated delayed
fluorescent (TADF) emitters. However, the conventional
synthetic routes are cost-expensive and time-consuming. The
development of a simple procedure to synthesize SpAs is still in
urgent pursuing appreciated by academic and industrial
communities. In this contribution, we present a feasible acid-
catalyzed solvent-free metal-free cyclization between diaryl-
amines and ketones to construct SpAs. The as-constructed
moieties provide a wide possibility to assemble efficient TADF
emitters. As an example, D2T-TR with high photoluminescent
quantum yield and proper TADF character is applied in
organic light emitting diodes (OLEDs) which achieves a max-
imum external quantum efficiency (EQE) of 27.1%. This
work shows us a bright inspiration on developing excellent
organic optoelectronic materials and an effective tool to realize
it.
leading to a sufficient separation of the frontier molecular
orbitals (FMOs). Together with a moderate electron donating
ability, SpAs earn their unique superiority in assembling
TADF emitters with varied emission and high efficiencies.[10]
In addition, the extended structure also endows the emitters
with an oriented alignment which further boosts external
quantum efficiency (EQE) beyond 30%.[11] Recent works
benefit from the advanced electronic and geometrical char-
acteristics of SpA to construct excellent emitters via spatial
confined charge transfer.[12] Such units show increasing
importance in OLEDs application. However, the synthesis
of SpAs still involves complex procedures. First, the active N-
H of diarylamine generally needs protection before treating
with lithium agents (Scheme 1). Then, the following lithium–
T
hermally activated delayed fluorescent (TADF) emitters
have aroused recent interest owing to their great potential in
display and lighting,[1] lasering,[2] time-resolved imaging,[3] and
scintillators.[4] Especially in organic light-emitting diodes
(OLEDs), TADF emitters have achieved comparable electro-
luminescent performance to the phosphorescent dyes.[5–7]
Generally, efficient TADF emitters contain donor (D) and
acceptor (A) connected together via various kinds of bridges
to achieve a small energy difference (DEST) between the first
singlet (S1) and triplet (T1) excited states in order to activated
a fast reversed intersystem crossing (RISC).[8]
Scheme 1. Comparison of synthetic route by conventional method and
this work.
Spiro-acridan (SpA) is one of the most attractive D
building blocks in developing TADF emitters.[9] On one hand,
the two perpendicular planes connected via a sp3 hybridized
carbon increase the rigidity which will suppress the non-
radiative decay and also help avoiding severe aggregation. On
the other hand, the central six-membered ring of acridan
provides large steric effect when connected with A units
halogen exchanging reaction demands delicate operation
under low temperature (e.g. À708C) to avoid unfavorable
side-reactions and extra caution due to the combustible
agents. At last, a further ring closing is necessary to afford the
final compounds.[11b,12a] A [2+2] cycloaddition/ring-opening
procedure can also generates SpAs, but it usually involves in
complicated precursors.[13] Such cost-expensive and time-
consuming routes make it even more difficult to assemble
SpAs with advanced structures. A breakthrough in develop-
ing a simple synthetic procedure would benefit revealing full
potential of SpAs in organic electronics and the further
commercialization.
Herein, we present a feasible preparation of SpA
derivatives via an acid-catalyzed metal-free solvent-free
ring-cyclizing reaction at an elevated temperature between
diarylamines and ketones (such as fluorenones, anthraqui-
nones). Encouragingly, such procedure completes in short
[*] Dr. H. Liu, Z. Liu, H. Huang, Dr. C. Zhou, Prof. Dr. C. Yang
College of Materials Science and Engineering, Shenzhen University
Shenzhen 518055 (P. R. China)
E-mail: clyang@szu.edu.cn
G. Li, Dr. Z. Wang
State Key Laboratory of Luminescent Materials and Devices, Center
for Aggregation-Induced Emission, Guangzhou International
Campus, South China University of Technology (SCUT)
Guangzhou 510640 (P. R. China)
E-mail: wangzhiming@scut.edu.cn
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202103187.
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time for 30 min heating. Mass production at 15–20 mmol is
also verified for model compounds with satisfied yield of 64–
70%. The good tolerance of functional groups ensures myriad
possibilities in extending the structure of SpAs. More
important, SpAs with multiple amines or ketones also succeed
which could greatly extent their variety. To estimate their
applications in OLEDs, 10-(4-(tert-butyl)phenyl)-10’’-(4-(4,6-
diphenyl-1,3,5-triazin-2-yl)-phenyl)-10H,10’’H-dispiro [acri-
dine-9,10’-indeno[2,1-b]fluorene-12’,9’’-acridine] (D2T-TR)
was synthesized and evaluated as an example. EQE as high
as 27.1% have been achieved in multi-layered devices
illustrating the excellence of D2T-TR as a TADF emitter.
This procedure opens a practical perspective for assembling
SpAs for organic optoelectronic materials.
The general procedure contains two steps: pre-heating of
diarylamines with acid at 1408C under inner atmosphere for
about 15 min, then adding ketones and increasing the
temperature for 30 min reaction. Further prolonging the
reaction time contributes less to yield but more to the
byproducts. One-pot heating of all the agents would also give
the demanding compounds but with lower yields. p-Toluene-
sulfonic acid (TsOH) was used owing to its strong acidity and
low oxidability. The reaction is first investigated by screening
a series of reacting temperatures. No obvious products obtain
when the temperature is below 1608C. Meanwhile, a sharp
increase of unidentifiable byproducts and severe carboniza-
tion occurs when the reaction heated higher than 2208C. The
optimized temperature is set at 2008C.
With the optimized condition, the substrate scope with
regards to diarylamines was examined (Supporting Informa-
tion, Figure S1). As shown in Scheme 2, a series of diaryl-
amines successfully underwent the cyclization with fluore-
none (2a, Supporting Information, Figure S2) to afford SpAs
in good yields (4a–4g). This protocol shows a tolerance of
functional group including symmetrical/unsymmetrical
methyl, phenyl, t-butyl and active fluoro/chloro groups
which is important for further extension of these molecules.
Bromo substituted diarylamine (bis(4-bromophenyl)amine
(1j, Supporting Information, Figure S1)) fails the reaction
may due to high reactivity of bromo or the active N-para
position. This failure also inspires us to lock the para position
of diarylamines to avoid unfavorable reactions. Product 4d
and 4g were obtained with low yield, which may attribute to
the steric hinderance of the bulky t-butyl and m-methyl
groups on the substrates. Moreover, model compound 10H-
spiro[acridine-9,9’-fluorene] (4a) is performed under scale of
15 mmol with a satisfied yield of 64% which demonstrates the
capability of this procedure in commercial application. As
shown in the Supporting Information, Figure S3, compared
with 4a, 4b possess a little red-shifted absorption which could
ascribe to the electron-donating properties of methyl. Both
donors show identical phosphorescence, indicating their same
triplet energy level. Such properties suggest 4b could be
a useful candidate for TADF emitters.
Scheme 2. Scope of substrates with isolated yields, *: yield of mass
production (4a at 15 mmol, see supporting information, section of
synthesis and characterization), **: total yield of the inseparable
isomers.
in 2,7- and 3,6-position enabling a huge potential in post-
functionalization. Benzophenone (2j, Supporting Informa-
tion, Figure S2) with free rotating phenyl also succeeds,
indicating the possibility of assembling more flexible ketone
agents. Furthermore, anthraquinones (3a–d, Supporting
Information, Figure S4) were tested to generalize the variety
of ketones. Even better, excellent yield of 80% was reached
by 2,7-dimethyl-10H,10’H-spiro[acridine-9,9’-anthracen]-10’-
one (5a). While, bromo decorated anthraquinones could
generate SpAs with moderate yield of 44–48%. Reaction
between 1b and 2-bromo-6-phenylanthracene-9,10-dione
(3d, Supporting Information, Figure S4) generates insepara-
ble isomers 5d and 5e with a molar ratio of 0.85:1 which can
1
be identified in their H NMR spectrum (Supporting Infor-
mation). Furthermore, the combination of unsymmetrical
arylamines and ketones could afford chiral SpAs which can be
applied in developing chiral emitters.
The successful assembly of simple SpAs enlightens us to
develop more complicated structures. Multi amines N1, N3-
diphenylbenzene-1,3-diamine (1h), N1, N3-di-p-tolylbenzene-
1,3-diamine (1i, Supporting Information, Figure S1) and
ketones indeno[2,1-b]fluorene-10,12-dione (2k), indeno[1,2-
b]fluorene-6,12-dione (2l, Supporting Information, Fig-
ure S2) are examined as example. Multi-amines react well
with fluorene with good yields. More importantly, the reaction
of 1h and fluorenone could provide (Scheme 3) N-phenyl-
10H-spiro[acridine-9,9’-fluoren]-3-amine (4q) or 5’,7’-dihy-
Next, the scope of fluorenone was explored using di-p-
tolylamine (1b, Supporting Information, Figure S1) with
methyl to eliminate the extra active para position of aryl-
amines as inspired. Surprisingly, this reaction shows good
tolerance with single or double bromo units on fluorenes both
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triazine. The structure is fully characterized by H, 13C NMR
and high-resolution mass spectrum. Theoretical simulation
shows an almost perpendicular geometry with a dihedral
angle of 888 between D and A which ensures a large
separation of the FMOs. As expected, the lowest unoccupied
molecular orbital (LUMO) is located on electro-deficient
triazine (TR) unit and the highest occupied molecular orbital
(HOMO) mainly spreads on the other acridan unit connected
with t-butylbenzene (Ac2), while HOMO-1 focuses on the
acridan directly attached with TR unit (Ac1) (Figure 1b). TR
unit may weaken the electron donating property of Ac1
compared with that of Ac2 resulting in such distribution of the
FMOs. Natural transition orbital analysis confirms that S1 and
T1 originate from the transition between Ac1 and TR
(Supporting Information, Figure S6). The energy of S1 and
T1 are calculated as 2.595 eVand 2.586 eV giving a small DEST
value of 9 meV which will ensure a fast triplet up-conversion.
Furthermore, a local excited triplet state (3LE) centered at
indenofluorene is also observed with energy of 2.658 eV
(Supporting Information, Figure S6). Such energetically
1
Scheme 3. Scope of SpAs from multi amines and ketones.
drodispiro[fluorene-9,12’-quinolino[3,2-b]acridine-14’,9’’-flu-
orene] (4r) under the ratio of 1:1 or 1:2 suggesting the
controllability of this procedure. Massive synthesis of 2’,10’-
dimethyl-5’,7’-dihydrodispiro-[fluorene-9,12’-quinolino[3,2-
b]acridine-14’,9’’-fluorene] (4s) can be obtained at 20 mmol
scale with yield of 70% further revealing its potential in
commercialization. Diarylamine equipping with multi-
ketones also shows favorable results. Both 4t and 4u can be
successfully assembled. Noteworthy, 4r–4u are not reported
yet which could ascribe to the complicated synthesis via
conventional method. This new procedure provides an easy
and effective way to further extend SpAs with more advanced
structures.
According to the verification of different substrates,
a plausible mechanism of a two-step Friedel–Crafts reaction
is speculated. As shown in the Supporting Information,
Figure S5, first, an electrophilic attack of protonized fluore-
none to 2-positon of diarylamine and the loss of equivalent
water gives a carbon cation, which then attacks the other
phenyl for cyclization to afford the final compound. In brief
summary, this approach could generate SpAs with advanced
structures and ensure mass production which demonstrates its
potential towards academic and commercial research.
To demonstrate the application of SpAs in TADF
emitters, 10H,10’’H-dispiro[acridine-9,10’-indeno[2,1-b]fluo-
rene-12’,9’’-acridine], prepared from diphenylamine and
indeno[2,1-b]fluorene-10,12-dione (Supporting Information,
Scheme S2), was used as an example to assemble D2T-TR
(Figure 1a) via Buchwald–Hartwig coupling with 4-bromo-
3
closed LE may also facilitate the RISC process.[14]
Photophysical property was further investigated using
ultraviolet-visible (UV/Vis) absorption and photolumines-
cence (PL) spectra in solution and in bis[2-(diphenylphos-
phino)phenyl] ether oxide (DPEPO) doped film with
a doping concentration of 20 wt%. The intense absorption
in diluted toluene located at 344, 328 and 300 nm is attributed
to the local p–p* transition from indenofluorene, TR and Ac
units. Weak and broad peak around 400 nm ascribes to charge
transfer absorption from Ac1 to TR. The PL spectrum in
doped film shows a bluish-green emission at 490 nm and high
photoluminescent quantum yield (PLQY) of 99%. The
singlet (ES1) and triplet (ET1) energy levels from the onset
of fluorescent and phosphorescent spectra are 2.86 eV and
2.76 eV, respectively. Thus, a small DEST of 0.1 eV is achieved
endowing an efficient RISC process in D2T-TR. A transient
PL decay spectrum (Figure 2) confirms the TADF character-
istic with clear two components of 28 ns and 2.1 ms which
originates from prompt and delayed fluorescence. Together
with the data of PLQY, the rate constant of radiative decay
(kr), intersystem crossing (kISC) and RISC (kRISC) could be
calculated as 1.11 ꢀ 107 sÀ1, 2.48 ꢀ 107 sÀ1 and 1.09 ꢀ 105 sÀ1,
Figure 1. a) Chemical structure and the twist angle of D2T-TR. b) Dis-
tribution of the FMOs calculated with density function theory (DFT)
under B3LYP/def2svp level (cyan: LUMO, pink: HOMO, red: HOMO-
1), c) Theoretical simulated energy levels of S1, T1 and DEST.
Figure 2. a) UV/Vis absorption in dilute toluene (10À5 M), fluorescence
(Fl) and phosphorescence (Ph) spectra in doped film (20 wt% in
DPEPO) under room temperature and 77 K, respectively. b) Transient
PL decay profile in DPEPO (20 wt%) under inert atmosphere.
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respectively. Fast kr and kRISC could promote an efficient
usage of excitons.[8]
efficient TADF emitter, and more importantly, presents the
infinite potential of SpAs.
Considering the FMO levels of D2T-TR estimated from
cyclic voltammetry profile (Supporting Information, Table S1
and Figure S8), multi-layer device was fabricated using D2T-
TR as emitter under following structure of ITO/MoO3 (1 nm)/
TAPC (40 nm)/TcTa (10 nm)/mCP (10 nm)/DPEPO: 20 wt%
D2T-TR (25 nm)/DPEPO (5 nm)/Bphen (30 nm)/LiF (1 nm)/
Al (1100 nm) by vacuum-deposition to evaluate its EL
performance. The energy diagram and chemical structures
of (1,1-bis[4-[N, N’-di(p-tolyl)-amino]phenyl]cyclohexane)
(TAPC), (4,4’,4’’-tri(9-carbazoyl)-triphenylamine) (TcTa),
1,3-bis(N-carbazolyl)benzene (mCP), DPEPO, 4,7-diphenyl-
1,10-phenanthroline (Bphen) are illustrated in Figure 3.
TAPC and TcTa served as hole-transporting layers (HTL)
while DPEPO acted as host matrix and Bphen were used as
electron transporting layer (ETL). A thin layer of mCP and
DPEPO were inserted between respective HTL/emission
layer (EML) and EML/ETL to confine excitons. As shown in
Figure 3c, EL spectrum exhibits a single blueish-green
emission peaked at 490 nm suggesting well confinement of
the excitons. The maximum EQE, current efficiency (CE) and
power efficiency (PE) reach 27.1%, 69.2 cdAÀ1 and
63.9 lmWÀ1, respectively, thanks to the outstanding PLQY,
efficient up-conversion of triplet excitons via TADF mecha-
nism and carefully designed device structure. Relieved
efficiency roll-off of 11% at 100 cdmÀ2 and 33% at
1000 cdmÀ2 are also observed which could ascribe to the
fast RISC process. Such result indicates D2T-TR as an
In conclusion, we have provided a simple metal-free
solvent-free strategy to synthesize SpA derivatives with
advanced structures featuring fast construction, good con-
trollability and mass production which may help in revealing
the potentiality of SpAs in organic electronics. A proof-of-
concept TADF emitter D2T-TR is further prepared using the
assembled SpA as D unit which exhibits high PLQY, obvious
TADF characteristic and consequently acquires excellent EL
performance. This work not only demonstrates the wide
possibility of SpAs in constructing TADF emitters but also
points out a practical way to achieve it.
Acknowledgements
This work was supported by National Natural Science
Foundation of China (No. 51803125, 91833304, 51673118,
and 201975077), the Shenzhen Science and Technology
Program (KQTD20170330110107046) and Science and Tech-
nology
Innovation
Commission
of
Shenzhen
(JCYJ20180507182244027). We thank the Instrumental Anal-
ysis Center of Shenzhen University for analytical support.
Conflict of interest
The authors declare no conflict of interest.
Figure 3. a) Device structure and energy-level diagram of the OLED. b) Chemical structures of the compounds used in devices. c) EL spectra of
the device recorded at 10 mAcmÀ2, inner: corresponding CIE coordinates. d) Current-density–voltage–brightness (J-V-L) profile. e) Power efficiency
and EQE vs. brightness curves and efficiency roll-off at 100 cdmÀ2 and 1000 cdmÀ2
.
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Accepted manuscript online: March 26, 2021
Version of record online: && &&, &&&&
Angew. Chem. Int. Ed. 2021, 60, 1 – 6
ꢀ 2021 Wiley-VCH GmbH
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Fluorescence
A versatile direct cyclization is demon-
strated to proceed by substituted diaryl-
amines with various ketones under
simple solvent-free and metal-free con-
ditions. It shows huge potential in pre-
paring spiro-acridan derivatives and
TADF emitters.
H. Liu, Z. Liu, G. Li, H. Huang, C. Zhou,
Z. Wang,* C. Yang*
&&&& — &&&&
Versatile Direct Cyclization Constructs
Spiro-acridan Derivatives for Highly
Efficient TADF emitters
6
www.angewandte.org
ꢀ 2021 Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 1 – 6
These are not the final page numbers!