Fused twin-acridine scaffolds as electron donors for thermally activated delayed fluorescence emitters: Controllable TADF behavior by methyl substitution

Article abstract of DOI:10.1039/c9cc08679j

Fused twin-acridine scaffolds of TMQAC and MeTMQAC were designed as novel donors to construct new organic emitters. TMQAC-based emitters were TADF active, while the TADF character was turned off in MeTMQAC-based emitters by the extra methyl group. The TMQ

Full text of DOI:10.1039/c9cc08679j

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Fused twin-acridine scaffolds as electron donors
for thermally activated delayed fluorescence
emitters: controllable TADF behavior by methyl
substitution†
Cite this: Chem. Commun., 2019,
55, 15125
Received 7th November 2019,
Accepted 19th November 2019
Danyang Chai,a Yang Zou, *a Yepeng Xiang,b Xuan Zeng,b Zhanxiang Chen,b
DOI: 10.1039/c9cc08679j
b
ab
Shaolong Gong
and Chuluo Yang
*
rsc.li/chemcomm
Fused twin-acridine scaffolds of TMQAC and MeTMQAC were
So far, a vast number of acceptors with an electron-
designed as novel donors to construct new organic emitters. TMQAC- withdrawing nature have been intensively investigated for
based emitters were TADF active, while the TADF character was turned constructing TADF emitters.4 In contrast, the number of donor
off in MeTMQAC-based emitters by the extra methyl group. The moieties used for TADF emitters is far less than that of
TMQAC-based emitters exhibited high OLED performances, with acceptors.5 In this context, new design strategies for donors
external quantum efficiencies (EQEs) of up to 20.7%.
used for TADF emitters are highly demanded.
Among all the arylamine type electron donor moieties,
Exploring emitting materials with excellent device performance 9,9-dimethyl-9,10-dihydroacridine (DMAC) has been the most
and low cost is currently of concern for the development of widely and intensively investigated. TADF emitters based on
organic light emitting diodes (OLEDs). Among all the emitters, DMAC usually exhibit small DESTs, high PLQYs, and good
cost-effective purely organic thermally activated delayed fluores- device performances.6 TADF emitters based on DMAC deriva-
cence (TADF) emitters are recognized to have great potential tives bearing a rigid spiral skeleton have been demonstrated to
owing to their triplet harvesting capability in devices which can be excellent emitters for OLEDs.7 Aiming to construct efficient
realize a theoretical 100% internal quantum yield like noble TADF emitters, in this study we designed a new electron donor
metal-based phosphorescent emitters through a reverse inter- by combining two DMAC moieties together, namely 12,12,14,14-
system crossing (RISC) process.1 The key to enabling the TADF tetramethyl-5,7,12,14-tetrahydro-quinolino[3,2-b]acridine (TMQAC),
character for an emitter is to minimize the singlet–triplet splitting in which two nitrogen atoms located at the meta position to each
energy (DEST) between the molecular lowest singlet (S1) and the other to regulate its electron donating ability, and the two DMAC
triplet state (T1).2 According to the quantum theory, the ideal units shared one central benzene ring to form a rigid ladder-type
molecular design for TADF emitters is to realize a spatial separa- coplanar framework. When the TMQAC donor is equipped with
tion between the highest occupied molecular orbital (HOMO) and two acceptors via C–N connection, the resulting emitter bearing a
lowest unoccupied molecular orbital (LUMO) distributions. In a double D–A structure meets the demand for TADF molecules. First,
typical donor–acceptor (D–A) type molecule, the HOMO and the twisted D–A structure can endow the molecule with small DEST
LUMO generally locate at the electron donor and acceptor, that enabled TADF. Second, it is known that connecting multiple
respectively. Based on this constitution, numerous efficient D–A emitter moieties together in a single molecule may enhance
TADF emitters bearing a D–A architecture with sterically the absorption and photoluminescence quantum yield (PLQY) of
hindered bridges have been developed.3
the emitters.8 Third, high molecular rigidity is crucial for boosting
the PLQY as it can minimize the non-radiative transition process
in the molecular excited state. For TMQAC-based emitters, two
chromophore counterparts are locked up in a fused fashion; thus
high molecular rigidity can be guaranteed. As expected, two
TMQAC-based TADF emitters featuring a twin D–A structure with
diphenyl sulphone (DPS) and triazine (TRZ) as acceptors exhibited
an obvious TADF character and a high PLQY, and the resulting
OLED devices achieved maximum quantum efficiencies (EQEs) of
up to 20.7%. For comparison, another similar donor to TMQAC,
namely MeTMQAC, in which an extra methyl group was attached
on the central benzene ring, was designed and used to construct
a Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials
Science and Engineering, Shenzhen University, Shenzhen 518060, People’s
Republic of China. E-mail: yangzou@szu.edu.cn, clyang@szu.edu.cn
b Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Department of
Chemistry, Wuhan University, Wuhan, 430072, People’s Republic of China.
E-mail: clyang@whu.edu.cn
† Electronic supplementary information (ESI) available: Experimental section,
NMR spectra, TGA and electrochemical CV curves and temperature-dependent
transient fluorescence spectra and OLED device data for TADF materials. CCDC
1961049–1961051. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c9cc08679j
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geometry from TMQAC-based emitters (Fig. 1c). The donor
moiety in DPS-MeTMQAC was greatly distorted into a twist-
boat conformation with large dihedral angles of 1391 and 1381,
while the acceptor moieties of DPS were less twisted to the donor
with dihedral angles of 531 and 481 compared with TMQAC-based
emitters.
Scheme 1 Molecular structures of the emitters.
The distinct single crystal X-ray structures of these emitters
were in good agreement with their optimal molecular structures.
As shown in Fig. 2, according to the optimized geometry obtained
by density functional theory (DFT) and time dependent density
functional theory (TD-DFT) with the B3LYP hybrid functional, the
donor of TMQAC showed a nearly coplanar conformation, and
large dihedral angles of 861 and 861 between the TRZ acceptors
and the TMQAC donor were observed. In contrast, the MeTMQAC
donor part was greatly distorted into a twist-boat conformation
due to the steric hindrance of the extra methyl group, and thus
resulted in much smaller twisting angles of 461/501 between the
TRZ acceptors and the TMQAC donor. Similar results were
obtained when DPS was selected as an acceptor (Fig. S14, ESI†).
All the calculated data are summarized in Table S1 (ESI†).
Noticeably, the difference in molecular geometry led to distinct
molecular energy levels, orbital distributions and DESTs. A very
good HOMO–LUMO separation was predicted for TRZ-TMQAC,
resulting in an ultra-small DEST of 0.01 eV that could facilitate
RISC. However, for TRZ-MeTMQAC, the remarkably overlapped
HOMO–LUMO levels on the bridging benzene ring between the
donor and acceptor led to a large DEST of 0.33 eV. Similar
calculated results were acquired for the two emitters with the
DPS acceptor as well (Fig. S14 and Table S1, ESI†).
The distinct molecular structures of TMQAC- and MeTMQAC-
based emitters revealed by simulation and X-ray crystallography
implied their remarkably different properties. As investigated by
cyclic voltammetry (CV) experiments (Fig. S16, ESI†), TMQAC-
based emitters exhibited much shallower HOMO levels (À5.11 eV
for TRZ-TMQAC and À5.23 eV for DPS-TMQAC) than their
analogues with the MeTMQAC donor (À5.53 eV for TRZ-
MeTMQAC and À5.64 eV for DPS-MeTMQAC). According to
the UV-vis spectroscopy (Fig. 3a), TRZ-TMQAC showed a strong
absorption band at 300–350 nm corresponding to localized
p–p* and n–p* transitions, along with a broad characteristic band
at 350–450 nm assigned to intramolecular charge transfer (ICT).
two compounds, namely DPS-MeTMQAC and TRZ-MeTMQAC.
Unexpectedly, the methyl substitution remarkably affects the mole-
cular geometry, and thus disables the TADF.
The molecular structures of TMQAC- and MeTMQAC-based
emitters are depicted in Scheme 1, and their synthetic routes
are shown in Schemes S1–S4 (see the ESI†). After treating
methyl dicarboxylate with methyllithium, the obtained crude
diol was directly reacted with a catalytic amount of boron
trifluoride to obtain TMQAC via the intramolecular Friedel–
Crafts cyclization process. MeTMQAC was synthesized in a
similar way, except for using the corresponding ethylene as
the ring closing substrate.9 After that, four emitters with a
twin D–A structure were successively prepared through double
Buchwald–Hartwig C–N coupling reactions between the corres-
ponding donors and arylbromides. All the intermediates and
the final products were obtained in good yields, and they are
fully identified by 1H and 13C nuclear magnetic resonance
(NMR) spectroscopy as well as high-resolution electrospray
ionization mass spectrometry (HR-ESI-MS), which are in good
agreement with the theoretical results (Fig. S1–S12, ESI†).
All four emitters exhibited excellent thermal stability with high
decomposition temperatures (Td, corresponding to 5% weight
loss) over 400 1C as investigated by thermogravimetric analysis
(Fig. S13, ESI†), which allowed them to be further refined by
vacuum sublimation for device characterization.
Crystals of TRZ-TMQAC, DPS-TMQAC and DPS-MeTMQAC
suitable for crystallography were grown by slowly evaporating
their dichloromethane solutions. As shown in Fig. 1a, the donor
moiety in TRZ-TMQAC adopted a nearly planar conformation,
with one DMAC ring being quasi-equatorial, but the other
one being quasi-axial.10 The two triazine acceptor moieties in
TRZ-TMQAC were almost parallel to each other with a close
distance of 4.1 Å, and they are highly twisted to the donor with
dihedral angles of 771 and 871. A similar molecular conforma-
tion was found in DPS-TMQAC as well (Fig. 1b). Unexpectedly,
MeTMQAC-based emitters presented an entirely different
Fig. 2 The optimal geometric structures and molecular frontier orbital
distributions of TRZ-TMQAC and TRZ-MeTMQAC obtained by theoretical
calculations.
Fig. 1 X-ray crystal structures of (a) TRZ-TMQAC (CCDC: 1961049), (b) DPS-
TMQAC (CCDC: 1961050) and (c) DPS-MeTMQAC (CCDC: 1961051).†
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their DF components were almost quenched by oxygen under
aerated conditions, which obviously proved their TADF character.
Similar decay curves with typical double-exponential character
were found for their films in DPEPO (Fig. S19, ESI†), with high
PLQYs of 73% and 74% for TRZ-TMQAC and DPS-TMQAC,
respectively. In sharp contrast, no long lifetime on a micro-
second scale could be observed for either TRZ-MeTMQAC or
DPS-MeTMQAC even under degassed conditions (Fig. 3d and
Fig. S18c, ESI†), suggesting that the RISC process was disabled,
which also led to their low PLQYs in the DPEPO films (Table 1).
The distinct photoluminescent properties of these emitters
could reflect on their device performances. To validate this,
OLEDs with an ITO/HAT-CN (5 nm)/TPAC (30 nm)/mCP (10 nm)/
EML (20 nm)/DPEPO (10 nm)/TmPyPB (30 nm)/Liq (1.5 nm)/Al
(100 nm) device configuration were fabricated, where ITO and
Al acted as an anode and a cathode, respectively; 1,4,5,8,9,11-
hexaazatriphenylene-hexacarbonitrile (HAT-CN) and lithium
8-quinolinolate (Liq) were used as hole and electron-injection
layers, respectively; 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane
(TAPC) and 1,3,5-tri(m-pyrid-3-ylphenyl)benzene (TmPyPb) were
Fig. 3 Optical properties of TRZ-TMQAC and TRZ-MeTMQAC: (a) UV-vis
and fluorescence spectra in toluene solution, (b) fluorescence at 300 K and
phosphorescence at 77 K of 10 wt% emitters doped into a DPEPO host,
and (c and d) the transient photoluminescence decay curves of the
emitters in toluene (10À5 M) under degassed and aerated conditions.
On the other hand, for TRZ-MeTMQAC, no clear ICT absorption used as hole- and electron transport materials, respectively; and
band was observed,11 and only a localized p–p* and n–p* 1,3-bis(N-carbazoyl)benzene (mCP) was used as an electron/
transition peak at 364 nm was found, which was typical for a exciton blocking layer. DPEPO was employed as the host owing
D–A emitter with a sterically hindered donor.12 In the fluores- to its high energy level of 4.1 eV, and all four emitters were
cence spectrum (Fig. 3a), TRZ-TMQAC showed a green emission doped into the DPEPO host with a doping concentration of
peak at 510 nm in toluene with clear ICT character proved by 10 wt% to act as the emitting layer. The devices results are
solvatochromism experiments (Fig. S16, ESI†). In contrast, summarized in Table S2 (ESI†).
because of the highly distorted donor arising from methyl
As shown in Fig. 4c, both TMQAC-based emitters showed
substitution, TRZ-MeTMQAC exhibited a blue-shifted spectrum identical electroluminescence (EL) spectra to their photolumines-
with an emission peak at 488 nm in toluene, along with a weak cence (PL) spectra. TRZ-TMQAC displayed green emission with
emission at 400 nm that originated from the local emission of Commission International de l’Eclairage (CIE) coordinates of (0.30,
the donor.12 According to the phosphorescence spectra of their 0.56), while DPS-TMQAC showed sky-blue emission with CIE
films in bis[2-(diphenylphosphino)phenyl]ether oxide (DPEPO) coordinates of (0.19, 0.33). More importantly, both TMQAC-based
recorded at 77 K (Fig. 3b), the triplet energy can be obtained devices presented significantly superior device performance to the
(Table 1). Accordingly, TMQAC-based emitters exhibited ultra- MeTMQAC-based ones. For example, TRZ-TMQAC-based device
small DESTs less than 0.10 eV, while MeTMQAC-based emitters displayed a maximum external quantum efficiency (EQE) of
exhibited large DESTs more than 0.40 eV.
20.7%, a maximum current efficiency (CEmax) of 66.1 cd AÀ1, and
To further illustrate the impact of methyl substitution on a maximum power efficiency (PEmax) of 51.9 lm WÀ1. However,
photophysical properties, the transient photoluminescence TRZ-MeTMQAC under the same device configuration exhibited a
decay curves of the emitters in toluene solution were measured. much lower EQEmax of 5.5%, CEmax of 8.3% and PEmax of 6.5%. The
As shown in Fig. 3c and Fig. S18b (ESI†), both TMQAC-based same trend was found for DPS-TMQAC- and DPS-MeTMQAC-based
emitters exhibited clear double-exponential decay curves under devices (EQEmax of 14.3% vs. 1.2%, Fig. S20, ESI†). The superior
an argon atmosphere, accompanied by fitting prompt and device performance with an EQEmax of over 20% further proved
delayed fluorescence (DF) lifetimes of 37.68 ns/1.62 ms for the TADF character for TMQAC-based emitters. In contrast,
TRZ-TMQAC and 24.89 ns/1.55 ms for DPS-TMQAC. In addition, MeTMQAC-based emitters are only traditional fluorescent emitters.
Table 1 The physical properties and kinetic parameters of the compounds
HOMO/LUMOc [eV]
S1/T1/DEST
tPF/tDF [ns]/[ms]
DF ratioe (%)
FPL
a
b
d
e
f
Compound
labs [nm]
lem [nm]
TRZ-TMQAC
TRZ-MeTMQAC
DPS-TMQAC
296, 406
364
295, 320, 372
310
508/522
487/454
473/471
453/418
À5.11/À2.48
À5.53/À2.44
À5.23/À2.33
À5.64/À2.20
2.74/2.72/0.02
2.97/2.93/0.44
3.07/2.63/0.04
3.49/3.06/0.43
39/1.01
—–
30/3.09
—
78
—
90
—
73
43
74
33
DPS-MeTMQAC
a
b
c
Measured in toluene solutions (10À5 M) at 298 K. Measured in toluene solutions/in the DPEPO films. The HOMOs were calculated from CV;
LUMO = HOMO + Eogpt
.
Calculated from the onset wavelengths of the fluorescence (298 K) and phosphorescence (77 K) spectra of both emitters in
d
e
f
the DPEPO host. The fitting lifetimes of the prompt and DF components and the DF ratios of both emitters in toluene solution. PLQYs of
10 wt% both emitters doped into the DPEPO host.
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Conflicts of interest
There are no conflicts to declare.
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This journal is ©The Royal Society of Chemistry 2019