Methoxy substituents activated carbazole-based boron dimesityl TADF emitters

Article abstract of DOI:10.1039/c9tc07020f

N-Borylated emitters bearing both boron dimesityl acceptor (Mes)₂B and phenoxazine or acridine donors are a class of efficient TADF emitters; however, switching to a carbazole donor nullifies the TADF characteristics. This work is targeted at i

Full text of DOI:10.1039/c9tc07020f

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Methoxy Substituents Activated Carbazole-Based Boron Dimesityl TADF Emitters
Paramaguru Ganesan,a,† Deng-Gao Chen,b,† Wen-Cheng Chen, Premkumar Gnanasekaran,a
c
b
b
b
,c
,b
Jia-An Lin, Chun-Ying Huang, Meng-Chi Chen, Chun-Sing Lee,* Pi-Tai Chou,* and Yun
Chi*,a,d
a
Dr. Paramaguru Ganesan, Premkumar Gnanasekaran, Prof. Yun Chi
Department of Chemistry and Frontier Research Center on Fundamental and Applied Sciences
of Matters, National Tsing Hua University, Hsinchu 30013, Taiwan
b
Deng-Gao Chen, Jia-An Lin, Chun-Ying Huang, Meng-Chi Chen, Prof. Pi-Tai Chou
Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan; E-mail:
chop@ntu.edu.tw
c
Dr. Wen-Cheng Chen, Prof. Chun-Sing Lee
Center of Super-Diamond and Advanced Films (COSDAF) and Department of Chemistry, City
University of Hong Kong, Hong Kong SAR; E-mail: apcslee@cityu.edu.hk
d
Prof. Yun Chi
Department of Materials Science and Engineering and Department of Chemistry, City
University of Hong Kong, Kowloon 999077, Hong Kong SAR; E-mail: yunchi@cityu.edu.hk
†
These authors contributed equally.
ORCID
Paramaguru Ganesan: 0000-0001-8413-6893
Deng-Gao Chen: 0000-0001-6406-2209
Wen-Cheng Chen: 0000-0003-3788-3516
Chun-Sing Lee: 0000-0001-6557-453X
Pi-Tai Chou: 0000-0002-8925-7747
Yun Chi: 0000-0002-8441-3974
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Keywords: boron, carbazole, charge transfer, TADF, OLEDs
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ABSTRACT
N-borylated emitters bearing both boron dimesityl acceptor (Mes) B and phenoxazine
2
or acridine donors are a class of efficient TADF emitters; however, switching to carbazole
donor nullifies the TADF characteristics. This work is targeted at improving the TADF
characteristics of the carbazole based (Mes) B emitters by introduction of up to two methoxy
2
substituents at the carbazoles. We hereby report the design and synthesis of three methoxy
substituted carbazoles (Cz-1, Cz-2 and Cz-3) and the corresponding directly N-borylated
emitters, CzBM-1, CzBM-2 and CzBM-3. Moreover, a p-phenylene spacer was also introduced
between the (Mes) B unit and carbazole, giving CzPBM-2 and CzPBM-3. As confirmed by the
2
bi-exponential transient decay analyses, all titled compounds exhibit prominent TADF
character, while the parent molecule CzBM-0 and CzPBM-0 without methoxy groups show no
TADF characteristics. The results manifest the importance of donor strength of substituents
and degrees of spatial orthogonality in harnessing the charge transfer property, minimalizing
S -T energy gap and facilitating reversed intersystem crossing process. OLED devices with
1
1
doped CzBM-2 and CzPBM-3 emitters exhibited a maximum external quantum efficiency of
2.5 % and 13.3 %, respectively, confirming the potential of methoxy substituents in
improving their TADF characteristics.
1
1
. Introduction
Organic light emitting diodes (OLEDs) has been rapidly developed in the past three
decades and found promising applications in flat panel displays and solid state lighting
luminaires.1-5 Two major luminescent materials can achieve unitary internal quantum
efficiency (IQE) via converting both singlet and triplet excitons into light, achieving very high
external quantum efficiencies (EQE) that are extensively tested for the next generation
OLEDs.6-8 On the one hand, noble-metal-based organometallic phosphors constitute one class
of such materials due to the highly efficient phosphorescence.9 In this scenario,
phosphorescent OLEDs exhibited EQE higher than 30% with varied emission colors were
documented.10-14 On the other hand, Adachi and coworkers reported distinctive organic
compounds possessing unitary quantum efficiency via thermally activated delayed
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fluorescence (TADF).15 Furthermore, with the assistance of the horizontal aligned transition
dipole moment, external quantum efficiency (EQE) of over 37% with simple planar
1
6
architecture has been successfully obtained for TADF based OLEDs. For both the full-color
display and lighting applications, it is essential to achieve emission with all three elementary
1
7
colors, i.e. blue, green and red. Although the library of green and red emitting materials are
widely established, there is a strong demand for stable, high efficient blue emitters.18 Pure
blue emission requires enlarged HOMO/LUMO energy gap and sets a tough challenge in
emitters. However, TADF materials showing both true blue and blue emissions are emerging
1
9
in the past few years, despite that the wide energy gap of blue emitters is expected to lead
exciton instability, limited charge injection and poor device lifetime.20
In view of molecular design, traditional TADF molecules consist of electron donating
and accepting moieties, which are linked in different arrangements such as donoracceptor
(DA), DAD, ADA and etc., enabling the triplet excitons to undergo fast reversed
intersystem crossing (RISC) that leads to unitary IQE.21 Typical donors such as phenoxazine
and phenothiazine give red emission, while acridine and analogues provided the required
blue shifted emission.22-24 However, in view of long term stability it will be better to switch to
alternative donor such as carbazole and functional analogues, which are also widely utilized
in blue emitting TADF.25 Among acceptors, electron deficient triazine, pyrimidine, pyridine,
sulfone, phosphine oxide, polycyclic borazine, boron dimesityl, etc. were successfully utilized
in achieving the needed TADF characters.26-31 In recent years, functional boron based
acceptors have received much attention in the development of TADF emitters.32, 33 These
boron compounds can be categorized into the boron-fused heteroaromatics,34 three
coordinate35-37 and four coordinate boron compounds. The electron accepting property of
the three coordinate boron compounds is related to its lower lying vacant p-orbital that
facilitates the charge transfer upon excitation.35 Earlier reports on three coordinate boron
TADF compounds generally possess a phenylene spacer between the boron dimesityl
acceptor and donor segment.39 The relative position between donor and boron dimesityl
fragments was an important factor in realizing TADF. For instance, the utilization of p-
substituted linkage affords only fluorescence, while switching the o-appended phenylene
38
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yields the desired TADF.40 Hence, it is believed that the sterically congested o-arrangement
gives the vertical arranged donor and acceptor and, hence, smaller gap between S and T
1
1
4
1,
states (i.e. ΔE ), faster intersystem crossing (ISC) rates and notable delayed fluorescence.
ST
4
2
Recently, Thilagar, his co-workers and our groups have independently reported the
directly N-borylated stimuli and shape responsive materials and efficient TADF emitters based
on phenothiazine, acridine and phenoxazine donors.43-45 However, upon switching to
carbazole, although charge transfer character was clearly observed for the corresponding N-
4
6
borylated compound, no TADF properties has been detected. Based on the aforementioned
discussion and hence a goal to realize the possible TADF behavior, we then shifted gear to
study the carbazole substituted N-borylated compounds. In this study, the methoxy
substituent(s) were strategically added to carbazole at various position(s) in an attempt to
achieve the desired electronic and steric properties (c.f. Scheme 1).47-49 Furthermore, the
tradeoff between steric interaction and donor strength is critical in maintaining the required
5
0
blue emission and TADF character. Detail of results and discussion is elaborated as follows.
MeO
NH
NH
NH
OMe
OMe
OMe
MeO
Cz-1
Cz-2
Cz-3
Scheme 1. Structural drawings of carbazole with varied methoxy substituents.
Scheme 1 displays our designs of carbazole donor molecules. The methoxy groups are
inserted at the selected location in maximizing the resonance effect, while the 1-position will
also afford large steric encumbrance to the adjacent fragment, thereby satisfying the
condition of higher donor strength and better orthogonality for realizing TADF character. As
depicted in Scheme 1, Cz-1 possesses 1-methoxy substituent and, to further enhance the
donor strength, we also introduced a second 3-methoxy substituent in giving Cz-2, while Cz-
3
comprises of the 1,6-dimethoxy substituents. Hence, comparison of Cz-2 and Cz-3 will offer
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the distinctive structural effect. Next, we synthesized both the boron dimesityl (BM) and
phenylene bridged boron dimesityl (PBM) type of emitters, giving CzBM-1, 2 and 3 series and
CzPBM-2 and 3 series, respectively, which are depicted in Scheme 2. In sharp contrast to the
4
6
39
reference CzBM-0 and CzPBM-0 compounds (see Scheme 2) that were reported to have
no TADF character, remarkable TADF was observed for all titled methoxy-substituted
compounds. Among them, some exhibit the highly demanding blue emission, which adds
another niche to this investigation. Detail of results and discussion is elaborated as follows.
R6
R6
6
2
N
B
N
B
1
R1
R1
R3
R3
R1
CzBM-0 : H
CzBM-1 : OMe
R3
H
H
R6
H
R1
CzPBM-0 : H
CzPBM-2 : OMe OMe
CzPBM-3 : OMe
R3
R6
H
H
H
H
CzBM-2 : OMe OMe
CzBM-3 : OMe
H
H
OMe
H
OMe
Scheme 2. Structural drawings of various carbazole based TADF emitters.
2
2
. RESULTS AND DISCUSSION
.1. Syntheses and Characterization.
All carbazoles are synthesized in accordance to the protocols shown in Scheme S1 of
Supporting information. As indicated, Cz-1 and Cz-2 were synthesized using 2-haloanilines
and methoxy substituted halobenzenes. Subsequent Buchwald-Hartwig amination yields
intermediates which are subjected to C-H activation and ring closure to form the required
Cz-1 and Cz-2.51 For Cz-3, we employed Pd(II)-catalyzed oxidative carbon-carbon bond
formation for the needed ring closure reaction.52 The directly N-borylated BM compounds,
CzBM-1 to 3, were synthesized via in situ deprotonation of respective carbazole with n-BuLi
in THF at −78°C, followed by treatment with B(Mes) F under reflux. The N-phenyl carbazoles
2
were next synthesized by Ullmann coupling of dimethoxy substituted Cz-2 and Cz-3 and 1-
bromo-4-iodobenzene, and the PBM compounds (CzPBM-2 and -3) were prepared from
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aforementioned N-phenyl carbazole and B(Mes) F at RT. All the BM and PBM compounds are
2
air-stable and, hence, enable facile purification by sequential column chromatography and
vacuum sublimation. Finally, all boron compounds were characterized by H and 13C NMR
spectroscopies, FAB mass spectrometry and elemental analysis.
1
1
Notably, H NMR spectral patterns of both the o- and p-methyl groups in the mesityl
unit are strongly influenced by their individual structures (Figure S1). The o-methyl signals of
CzBM-3 appeared as four broadened signals, while those of CzPBM-3 appeared as one sharp
signal at RT. This difference is mainly due to a large barrier of the restricted rotation of mesityl
groups in solution at RT. Introducing the phenylene spacer to CzBM-3 in forming CzPBM-3
reduces the rotational barrier and gives rise to only one sharp signal. This observation was
akin to the NMR spectral behaviors observed for other three coordinate N-borylated
compounds reported in literature.53
CzBM-1, CzBM-3, and CzPBM-3 were characterized by single-crystal X-ray diffraction
study, and their molecular structures are displayed in Figures 1 and 2. Generally speaking,
both CzBM-1 and CzBM-3 display trigonal planar core arrangement as evidenced from the
sum of all angles around B(1) atom being 360. The observed B-N distance of 1.458 Å and
1
.454 Å for CzBM-1 and CzBM-3, respectively, falls in the range of 1.4421.472 Å as observed
for other N-borylated pyrrole derivatives, which is shorter than the tetravalent borylated
3
1
2
compounds. This is mainly due to the larger s-character of the sp -hybridized trivalent boron
3
atom that enhances the B-N bond strength compared to the sp -hybridized tetravalent boron
compounds. The torsional angles between the N(1)-C(1)-C(12) and B(1)-C(15)-C(25) planes of
CzBM-1 and CzBM-3 are measured to be 38.75 and 40.37, respectively, confirming the
reduced p-orbital overlap between the N-donor and B-acceptor.
Concurrently, introducing phenylene in forming CzPBM-3 does not alter the trigonal
geometry of (Mes) B as displayed in Figure 2. The dihedral angle between the carbazole and
2
phenylene is measured to be 58.73 which is akin to the relevant structural parameters
reported in literature.32, 39 Although this dihedral angle is smaller than that of the phenoxazine
or acridine donors, our attached methoxy substituents enhance the electron donating ability
of carbazole that, in turn, assists the localization of the HOMO on the carbazole moiety,
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reduces the spatial overlap between the HOMO and LUMO as well as favors smaller ΔE and
ST
better TADF character (vide infra).54, 55
Their thermal and morphological properties were analyzed using thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC) at a heating rate of 10 C/min. The
corresponding TGA and DSC graphs are shown in Figure S2 and the numeric data are
summarized in Table S1. The decomposition temperature (T ) for CzBM-2 and CzBM-3
d
occurred at 276 and 286 C respectively, which was slightly higher than that of 1-methoxy
substituted CzBM-1 (264 C). In addition, T of CzPBM-3 occurred at 308 C, revealing an
d
increase of 22 C in comparison to that of CzBM-3, which is attributed to the addition of a
phenylene spacer. The glass transition temperature (T ) of CzBM-1 to 3 was observed
g
between 192 to 196 C, while the introduction of phenylene spacer decreases T to 136 and
g
1
41 C for CzPBM-2 and 3, respectively.
2
.2. Photophysical properties
The UV–vis absorption and photoluminescence (PL) spectra were measured in
solvents with different polarity, as well as in neat film and doped film to validate the influence
on their photophysical properties. The corresponding spectra and data are displayed in
Figures 3 and 4 and Table 1. It appears that alternation of methoxy substituents and solvent
polarity renders no significant influence in their absorption behavior. All the N-borylated
compounds possess two absorption bands, among which the higher energy above 300 nm is
attributed to π  π* transitions, while the lower energy band below 320 nm is assigned to a
charge transfer (CT) transition. Red shift of absorption from 330 to 370 nm was observed
in respective BM and PBM compounds that correspond to the extended π-conjugation
imposed by the inserted p-phenylene fragment.
As can be seen, addition of methoxy substituents exerts significant influence on their
photoluminescence. For example, the emission maximum of 1-methoxy substituted CzBM-1
in toluene is observed at 462 nm. Upon introducing 1,3-dimethoxy substituents in affording
CzBM-2, the emission maximum is shifted to 501 nm, revealing an increased electron
donation on Cz-2 compared to Cz-1 as shown in Figure 3. Furthermore, the emission of CzBM-
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3
is blue shifted by 9 nm from CzBM-2 in toluene, indicating a slightly reduction of charge
transfer. Moreover, their emissions are highly sensitive to the solvent polarity. For example,
CzBM-1 displays a prominent red shifting of 40 nm by switching from cyclohexane (440 nm)
to CH Cl (480 nm), while CzBM-2 and CzBM-3 exhibit a red shifting of 49 nm and 43 nm,
2
2
respectively. CzPBM-2 and CzPBM-3 with phenylene bridge show an emission max. at 417 nm
and 406 nm in cyclohexane and at 519 nm and 499 nm in CH Cl . This change of luminescence
2
2
confirms the extensive charge transfer character at the excited state, which is one important
criterion for the TADF behavior.
Although the decrease in the emission energy gap upon increasing solvent polarity in
both BM and PBM series of compounds can be attributed to the solvent relaxation, i.e.
solvatochromism, their emissive hues have greatly different from each other. For instance,
both CzPBM-2 and CzPBM-3 exhibit emission max. at 464 nm and 452 nm in toluene, both
are blue-shifted by approx. 40 nm in comparison to their N-borylated counterpart CzBM-2
(501 nm) and CzBM-3 (492 nm). Hence, in addition to the solvatochromism, another
relaxation process is considered to take place in BM based compounds to account for the
large Stokes-shifted emission in nonpolar solvent (i.e. toluene). Accordingly, from a molecular
structure point of view, the lack of phenylene in BM compounds may induce the large steric
hindrance between carbazole and (Mes) B unit, resulting in a more rigid molecular structure.
2
Furthermore, upon optical excitation in all solution states, the rigid structures can undergo
structural relaxation in its S state potential energy surface to relief the steric constrains,
1
obtaining the large Stokes-shifted emission (see Figure 3). On the contrary, owing to the
addition of phenylene bridge in PBM compounds, they can retain their original, more relaxed
geometries upon excitation and exhibit less Stokes-shifted emission (see Figure 4).31
Verifications of the structural relaxations and its corresponding geometries are discussed in
the theoretical calculations sections (vide infra).
TADF characteristics were preliminarily confirmed by measurement of their
photoluminescence properties in aerated versus degassed solutions. As displayed in Table 1,
the quantum yields were found to increase by two to three folds in degassed solutions,
endorsing the suppression of oxygen quenching and efficient reversed intersystem crossing
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(
RISC) from T  S excited states. To validate these properties, the time-resolved delayed PL
1
1
spectra were conducted for all the molecules in CH Cl as shown in Figure S3. The prompt
2
2
emission components can be acquired at zero delay time and a gate width of 50 ns by an
intensified charge-coupled detector, whereas the delayed emission components were
acquired at a delayed time of 10 µs and a gate width of 10 µs. The proximity of the spectral
onsets between fluorescence and the delayed components apparently affirm the appearance
of delayed fluorescence and the possibility of TADF character. Next, transient PL decay
dynamics were then measured in different solvent, and as neat film and doped film. The
pertinent lifetime results are tabulated in Table 1, while the spectra taken in degassed
solutions are displayed in Figures S4 to S8. All the compounds exhibited bi-exponential decays
comprising prompt and delayed components. For example, CzBM-2 exhibited two decay
kinetics in degassed CH Cl , with a short time constant of 11 ns that corresponds to S  S
2
2
1
0
fluorescence and the other long decay of 3.5 s that arises from RISC, followed by the delayed
S  S emission. These observations confirm the TADF property in the BM compounds and
1
0
validates the usefulness of methoxy groups. Moreover, the long decay lifetime of CzPBM-2
1.6 µs) and CzPBM-3 (1.2 µs) in neat film confirm their TADF characters, which is in contrast
(
to the parent CzPBM-0,55, 56 for which the lack of TADF reveals the positive influence imposed
by methoxy substituents.
2
.4 Theoretical calculation
Computations were performed to gain further insights into the basic photophysical
and electronic properties of titled molecules. Their ground-state geometries were optimized
using density function theory (DFT) calculation, while the excited-state properties were
computed by the time-dependent-DFT method with a m062x hybrid functional. In this
section, the S  S absorption and the vertical S  S transition were calculated from the
0
1
1
0
structurally optimized S and S states to obtain the transitional energy gap as tabulated in
0
1
Table S4 to S14, respectively. For a fair comparison, the reported CzBM-046 and CzPBM-039
were also computed based on the same functional and basis sets. Detailed procedures are
provided in the SI.
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As shown in Figure 5, for all CzBM and CzPBM series, the results of computational data
indicate that the electron distributions in HOMO and LUMO are mainly localized at carbazole
(donor) and boron dimesityl (acceptor) moieties, respectively. Therefore, the emission
possessing charge transfer (CT) character for all titled boron complexes is unambiguous.
However, as for CzBM-0 and CzPBM-0 (see Figure 5), careful analysis of frontier orbital
indicates their subtle difference from the rest of BM and PBM compounds anchored with
methoxy groups, in which CzBM-0 and CzPBM-0 has non-negligible overlap between HOMO
and LUMO at CzBM-0’s N(1), C(1) and C(2) as well as CzPBM-0’s C(6) and C(8) atoms (see
Figure 5). The result indicates an appreciable electronic coupling between locally excited (LE)
and CT states, diminishing the TADF character by its enlarged ΔE . Conversely, the overlap
ST
between HOMO and LUMO at C(6) and C(8) atoms is nearly non-observable for CzPBM-2 and
CzPBM-3, indicating mainly a CT character in their emissive states. The difference, in a
qualitative manner, can be rationalized by the anchoring of methoxy group in CzPBM-2 and
CzPBM-3 to raise the carbazole HOMO level. The relatively large CT/LE mixing in CzPBM-0
increases the electron exchange energy and hence leads to a large energy splitting between
5
3-54
S and T state, i.e. ΔE ,
making TADF vanished. To the contrary, CzPBM-2 and CzPBM-3
1
1
ST
with predominant CT character reveal prominent TADF. Similar dominant CT character was
resolved for CzBM-1, 2 and 3, consistent with their observable TADF behavior.
Despite the sameness of the CT characters of all molecules in this study, the distinct
differences lie in the optimized geometries of both BM and PBM compounds. It is notable
that CzBM-1, 2 and 3 exhibits smaller torsional angle (θ ) of 41.13, 38.36 and 38.27 (cf.
t
8
8.46 and 85.63 of CzPBM-2 and 3) as shown in Figure 6, respectively. These results indicate
that the phenylene bridge provides better orthogonality between carbazole donor and
Mes) B acceptor. Furthermore, at the optimized geometries of at the lowest lying excited
(
2
state, the θ angle increased in all N-borylated compounds, together with the expansion of
t
the bending angle in (Mes) B fragments (θ ). The orthogonal orientation may result in a poor
2
b
donor-acceptor π-orbital overlap, to which the resulting smaller exchange energy is expected
to lower ΔE and facilitate RISC. In order to increase θ angle for better orthogonality, the B-
ST
t
N distance is elongated in BM molecules, combining with the increased θ angle in the boron
b
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dimesityl fragments. For example, θ was changed from 38.36 into 59.90 in ground/excited-
t
state optimized structure of CzBM-2 as shown in Figure 6. Moreover, the B-N distance and θb
angle were varied from 1.452 Å to 1.545 Å and from 121.13 to 128.93 in CzBM-2,
respectively. Similar structurally relaxed processes were also observed in CzBM-1 and CzBM-
3
. The computational results clearly support the occurrence of excited-state structural
relaxations in N-borylated molecules (i.e. CzBM-1 to 3), including rotation of the donor and
acceptor moieties, elongation of B-N distance and expansion of the boron dimesityl
fragments.
On the contrary, owing to the phenylene spacer in PBM molecules, CzPBM-2 and 3
already exhibit nearly orthogonal orientations in their optimized ground-state geometries, in
which θ and θ are recorded as 88.46 and 121.46 for CzPBM-2 and 85.63 and 121.65 for
t
b
CzPBM-3 as shown in Figure 6. Moreover, CzPBM-2 and 3 show very small structural
relaxation upon excitation, indicating the less steric restriction in PBM molecules compared
to the BM series. These observations indicate that they had reached their stable geometries
in an absence of steric confinements, which are consistent with the small Stokes-shifted
emission and the previously reported PBM type of TADF molecules.31
2
.4. Electrochemical properties
Redox parameters of the title compounds are necessary with an aim to access
optimized device architecture for OLEDs. Accordingly, cyclic voltammetry measurements
were conducted to analyze their electrochemical behavior, for which the voltammograms and
numerical data are displayed in Figure S9 and Table S3. As can be seen, the anodic sweep
resulted in two reversible oxidation peaks for CzBM-2 and CzPBM-2, both possess 1,3-
dimethoxy substituted carbazole, while only one reversible oxidation peak was observed for
their isomeric 1,6-derivatives CzBM-3 and CzPBM-3. Specifically, the first oxidation peak of
CzBM-2 and CzPBM-2 appeared at 0.37 V and 0.44 V, indicating the shallow HOMO energy
level of 1,3-dimethoxy compounds in comparison to that of 1,6-dimethoxy substituted CzBM-
3
and CzPBM-3 (i.e. 0.56 V and 0.61 V). Meanwhile, both phenylene bridged CzPBM-2 and
CzPBM-3 exhibit more stabilized HOMO than the corresponding N-borylated derivatives
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DOI: 10.1039/C9TC07020F
(
CzBM-2 and CzBM-3, respectively). In good agreement with these observation, we also
measured the oxidation potentials of the parent carbazoles Cz-2 and C-3, which occurred at
.36 V and 0.57 V, respectively. These results confirmed the higher oxidation potentials of all
0
Cz-3 based N-borylated emitters as well as the blue-shifting of emission in comparison to their
Cz-2 analogues.
Moreover, CzBM-1 exhibited the highest oxidation peak potential at 0.66 V, indicating
the lowest donor strength for its carbazole among all studied derivatives. As for the reduction
process, the peak potentials occurred between 2.57 to 2.65 V and 2.40 to 2.48 V for the
BM and PBM derivatives respectively, affirming the greater stabilization of LUMO by π-
conjugated phenylene.
2
.5 OLED device fabrication
Electroluminescent (EL) properties of the five TADF blue emitters were then studied
with a device structure of indium tin oxide (ITO)/N,N'-bis(naphthalen-1-yl)-N,N'-
bis(phenyl)benzidine (NPB, 40 nm)/4,4',4"-tris(carbazol-9-yl)triphenylamine (TCTA, 20
nm)/1,3-bis(carbazol-9-yl)benzene (mCP, 10 nm)/bis[2-(diphenylphosphino)phenyl]ether
oxide (DPEPO) : 6 wt% emitter (20 nm)/DPEPO (10 nm)/1,3,5-tri[(3-pyridyl)-phen-3-
yl]benzene (TmPyPB, 35 nm)/LiF (1 nm)/Al (100 nm). The device performances are shown in
Figure 7 and the key performance parameters are listed in Table 2. Based on the current
density-voltage-luminance (J-V-L) curves shown in Figure 7(a), the fabricated devices
presented an approximately ten-time difference in current efficiency as recorded in Table 2.
-2
Figure 7(b) shows the EL spectra measured at 100 cd m . As indicated, all the EL characters
were akin to their PL measured in thin films that possess an identical composition to the
doped emitting layer in OLED. Among them, CzBM-1 emitted
a
deep-blue
electroluminescence with a CIE(y) value as low as 0.11 with emission peak wavelength located
at 444 nm. However, owing to low PLQY and wide energy gap of CzBM-1, the associated
devices showed the lowest current efficiency (1.4 cd/A), power efficiency (1.1 lm/W) and
external quantum efficiency (1.8%) among all fabricated TADF OLED devices. Alternatively,
CzBM-2, CzBM-3, CzPBM-2 and CzPBM-3 exhibit much better EL performances with EQEs
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DOI: 10.1039/C9TC07020F
ranging from 9.5 to 13.3% (Fig. 7c), owing to better TADF characteristics and higher PLQYs in
doped thin films. The highly twisted molecular conformation induced by the phenylene spacer
between N-donor and B-acceptor in PBM molecules may influence their electrical properties
and carrier balances in the emissive layer, leading to higher turn-on voltages in the devices
based on CZBM-2 and CZBM-3. Although their EL peak wavelength spanned the true-blue
region from 444 nm to 468 nm, their max. EQE values are higher than those of the EQE
expected for conventional fluorescent OLED (i.e. 5%). These results affirm their characteristics
in harvesting both the singlet and triplet excitons via RISC and, thus, giving direct evidence for
the TADF characters.
3
. Conclusion
In summary, we have designed and synthesized three methoxy substituted carbazoles
Cz-1, Cz-2 and Cz-3) in an attempt to enhance the electron donating strength and to create
(
a relatively enlarged steric encumbrance. These donors were then linked to boron dimesityl
acceptor, forming the BM emitters (CzBM-1, CzBM-2 and CzBM-3) and phenylene bridged
PBM emitters (CzPBM-2 and CzPBM-3). Absorption and photoluminescence measurements
in different solvents reveal strong solvatochromism induced by the CT processes. Drastically
different from lack of TADF for reference compounds CzBM-0 and CzPBM-0 being reported
previously, all synthesized emitters showed notable delayed fluorescence, validating the
methoxy substitution as one useful strategy to harness the electron donating strength and
special orthogonality in achieving the TADF characteristics. Furthermore, these compounds
were then employed as dopant in fabrication of OLEDs, in which their EL peak wavelengths
span the region from 444 nm to 468 nm. OLED based on CzBM-2 and CzPBM-3 achieved max.
EQEs of 12.5% and 13.3% respectively, demonstrating their potentials in the development of
highly efficient blue TADF emitters.
4
. Supporting Information
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Detailed synthetic conditions for both donor and acceptor moieties as well as the
DOI: 10.1039/C9TC07020F
electrochemical, computational and spectroscopic data of the studied TADF molecules are
recorded.
Crystallographic data. The X-ray crystallographic coordinates for the compounds reported in
this article has been deposited at the Cambridge Crystallographic Data Centre (CCDC) under
deposition number CCDC 1900690 (CzBM-1), 1900691 (CzBM-3) and 1900692 (CzPBM-3).
These data can be obtained free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac
5
. Notes
The authors declare no competing financial interests.
ORCID
Chun-Sing Lee: 0000-0001-6557-453X
Deng-Gao Chen: 0000-0001-6406-2209
Pi-Tai Chou: 0000-0002-8925-7747
Yun Chi: 0000-0002-8441-3974
6
. Corresponding Author
Chun-Sing Lee, E-mail: apcslee@cityu.edu.hk
Pi-Tai Chou, E-mail: chop@ntu.edu.tw
Yun Chi, E-mail: yunchi@cityu.edu.hk
7
. Author Contributions
†These authors contributed equally
8
. Acknowledgment
Prof. Chi and Chou thanks for Ministry of Science Technology (MOST) of Taiwan and
the Higher Education Sprout Project administrated by Ministry of Education (MOE) of Taiwan
for finical supports. This work was also supported by the funding from City University of Hong
Kong, Hong Kong SAR. The authors are grateful to the National Center for the High-
performance Computing (NCHC) of Taiwan for the computer facilities.
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Journal of Materials Chemistry C
Table 1. Selected photophysical properties of the studied boron compounds in both solution and thin film states at RT.
b
state
λem [nm]
440
PLQY [%] a
11 / 13
τ [ns] and pre-exp. factor
4.19 (1.000)
ΔE , (eV)
ST
CzBM-1
CzBM-2
cyclohexane
toluene
CH Cl
462
18 / 26
9.8 (0.996), 212(0.004)
14.57 (0.998), 764 (0.002)
0.113
0.131
480
20 / 33
2
2
Neat film
cyclohexane
toluene
442
475
501
524
481
478
467
492
510
479
476
417
464
519
475
474
406
452
499
463
458
30
24 / 42
21 / 57
16 / 26
32
5.97 (0.999), 721.4 (0.001)
10.98 (0.997), 629(0.003)
15.29 (0.992), 2840 (0.008)
11.53 (0.998), 3520 (0.002)
1.13 (0.997), 4259 (0.003)
4.94 (0.999), 26633 (0.001)
9.98 (0.997), 868 (0.003)
15.48 (0.997), 3087 (0.003)
20.77 (0.997), 6955 (0.003)
3.58 (0.992), 2657 (0.008)
8.47 (0.999) 5578 (0.001)
6.42 (1.000)
16.40 (0.998), 10000 (0.002)
18.42 (0.998), 10008 (0.002)
12.31 (0.999), 1609 (0.001)
18.11 (0.999), 2218 (0.001)
5.65 (0.999), 10000 (0.001)
12.59 (0.999), 5582 (0.001)
21.81 (0.998), 19277 (0.002)
12.49 (0.997), 1251 (0.003)
31.00 (0.997), 9880 (0.003)
0.149
0.121
0.096
0.131
0.121
0.149
0.121
0.121
0.121
0.096
0.149
CH Cl
2
2
Neat film
DPEPO doped film
cyclohexane
toluene
55
CzBM-3
20 / 25
20 / 51
15 / 42
57
CH Cl
2
2
Neat film
DPEPO doped film
cyclohexane
toluene
70.3
CzPBM-2
CzPBM-3
53 / 88
55 / 100
50 / 71
68
0.131
0.131
0.149
0.149
0.149
0.149
0.131
0.121
0.121
CH Cl
2
2
Neat film
DPEPO doped film
cyclohexane
toluene
94
70 / 95
75 / 100
42 / 100
72
CH Cl
2
2
Neat film
DPEPO doped film
94
a
b
The solution quantum yields are recorded in both the aerated and degassed conditions and separated by slash.
The energy differences were analyzed by the equations as descripted in Table S2.
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Journal of Materials Chemistry C
Page 16 of 24
Table 2. Key OLED device performance parameters.
c
CEmaxd (cd/A)
1.4, 0.1
PEmaxd (lm/W)
1.1, 0.2
EQEmaxd (%)
1.8, 0.4
Emitter
CzBM-1
CzBM-2
CzBM-3
Vona (V)
3.9
λELb (nm)
444
CIE (x, y)
0.16, 0.11
0.18, 0.24
0.17, 0.22
3.7
468
19.8, 7.3
19.3, 9.2
17.3, 4.4
12.5, 4.2
11.7, 5.6
3.5
468
16.8, 5.8
CzPBM-2
CzPBM-3
4.5
5.0
-2
467
467
0.19, 0.27
0.18, 0.19
-2
13.8, 4.6
9.4, 2.2
9.6, 2.1
6.5, 0.8
9.5, 2.8
13.3, 1.8
a
b
c
-2 c
Voltage measured at 1 cd m ; color index measured at 100 cd m ; electroluminescence peak at 100 cd m ; device performances measured at maximum
-2
and 100 cd m , respectively.
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DOI: 10.1039/C9TC07020F
(
a) CzBM-1
(b) CzBM-3
Figure 1. Structural drawing of (a) CzBM-1, B(1)-N(1) = 1.458(3) Å; torsional angle ( ) between
t
the N(1)-C(1)-C(12) and B(1)-C(13)-C(22) planes: 38.75 and (b) CzBM-3, B(1)-N(1) = 1.454(16)
Å; torsional angle ( ) between the N(1)-C(1)-C(12) and B(1)-C(15)-C(25) planes: 40.37. The
t
ellipsoids were shown at the 30% probability.
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DOI: 10.1039/C9TC07020F
Figure 2. Structural drawing of CzPBM-3 with ellipsoids shown at the 30% probability level.
B(1)-C(4) = 1.570(17) and C(1)-N(1) = 1.427(15) Å; dihedral angle between the C(6)-C(1)-C(2)
and C(7)-N(1)-C(18) planes: 58.73.
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0.20
0.15
0.10
0.05
0.00
1.2
a) CzBM-1
0.9
DOI: 10.1039/C9TC07020F
(
0.6
0.3
0.0
3
00
00
00
400
400
400
500
500
500
600
700
0
0
0
0
.20
.15
.10
.05
.00
1.2
0.9
0.6
0.3
(
b) CzBM-2
0
0
0.0
1.2
3
600
700
.20
.15
.10
.05
.00
(
c) CzBM-3 1.0
0
0
0
0
0
.8
0.6
0
.4
.2
0
0.0
700
3
600
Wavelength (nm)
Figure 3. Absorption and emission spectra of (a) CzBM-1, (b) CzBM-2 and (c) CzBM-3, in
cyclohexane (blue), toluene (red), CH Cl (brown), neat film (black) and doped film (magenta)
2
2
at 298 K.
0.20
0.15
0.10
0.05
0.00
1.2
0.9
0.6
0.3
0.0
(a) CzPBM-2
300
400
500
600
700
0.20
0.15
0.10
0.05
0.00
1.2
0.9
0.6
0.3
0.0
(b) CzPBM-3
300
400
500
600
700
Wavelength (nm)
Figure 4. Absorption and emission spectra of (a) CzPBM-2, and (b) CzPBM-3 in cyclohexane
blue), toluene (red), CH Cl (brown), neat film (black) and doped film (magenta) at 298 K.
(
2
2
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Figure 5. Optimized molecular structures in their ground-states and the associated HOMO
and LUMO for CzBM-0 to 3 and CzPBM-0, 2 and 3.
Figure 6. Optimized molecular structures in the ground-states and excited-states for CzBM-1
to 3 and CzPBM-2 and 3.  indicates the torsional angle between the mean planes C(1)-N(1)-
t
C(2) and C(3)-B(1)-C(4) of the carbazole donor and the (Mes) B unit, respectively. 
2
b
represents the bending angle between the two mesityl groups in the acceptor, which is
recorded by the angle of C(3)-B(1)-C(4).
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Figure 7. (a) current density-voltage-luminance characteristics; (b) electroluminescent
-2
spectra at 100 cd m ; (c) EQE-current density curves.
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TOC Graphic
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