Isomeric spiro-[acridine-9,9′-fluorene]-2,6-dipyridylpyrimidine based TADF emitters: Insights into photophysical behaviors and OLED performances

Article abstract of DOI:10.1039/c8tc03645d

TADF molecules with a higher horizontal-dipole ratio have recently been realized to show a large conversion efficiency in organic light-emitting diodes (OLEDs) and hence great promise for their application in display and solid state lighting sources. The

Full text of DOI:10.1039/c8tc03645d

View Article Online
View Journal
Journal of
Materials Chemistry C
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: P. Ganesan, D.
Chen, J. Liao, W. Li, Y. Lai, D. Lo, C. Chang, C. Ko, W. Hung, S. Liu, G. Lee, P. Chou and Y. Chi, J. Mater.
Chem. C, 2018, DOI: 10.1039/C8TC03645D.
This is an Accepted Manuscript, which has been through the
Volume
4 Number 1 7 January 2016 Pages 1–224
Royal Society of Chemistry peer review process and has been
accepted for publication.
Journal of
Materials Chemistry C
Accepted Manuscripts are published online shortly after
Materials for optical, magnetic and electronic devices
www.rsc.org/MaterialsC
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 2050-7526
PAPER
~
Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
rsc.li/materials-c

Page 1 of 32
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C8TC03645D
Isomeric spiro-[acridine-9,9'-fluorene]-2,4-dipyridylpyrimidine based TADF emitters: Insights
into photophysical behaviors and OLED performances
Paramaguru Ganesan,‡,a Deng-Gao Chen,‡,b Jia-Ling Liao,a Wei-Cheng Li,c Yi-Ning Lai,c Dian Luo,c
Chih-Hao Chang,*,c Chang-Lun Ko,d Wen-Yi Hung,*,d Shun-Wei Liu,e Gene-Hsiang Lee,b Pi-Tai
Chou,*,b and Yun Chi*,a,f
a Dr. Paramaguru Ganesan, Dr. Jia-Ling Liao, 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, Prof. Pi-Tai Chou
Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan; E-mail:
chop@ntu.edu.tw; ORCID ID: orcid.org/0000-0002-8925-7747
c Wei-Cheng Li, Yi-Ning Lai, Dian Luo, Prof. Chih-Hao Chang
Department of Photonics Engineering, Yuan Ze University, Chung-Li 32003, Taiwan; E-mail:
chc@saturn.yzu.edu.tw; ORCID id: orcid.org/0000-0002-5586-9526
d Chang-Lun Ko, Prof. Wen-Yi Hung
Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 202, Taiwan; E-
mail: wenhung@mail.ntou.edu.tw
e Prof. Shun-Wei Liu
Department of Electronic Engineering, Ming Chi University of Technology, New Taipei 24301,
Taiwan
f Prof. Yun Chi
Department of Materials Science and Engineering and Department of Chemistry, City University
of Hong Kong, Hong Kong SAR; E-mail: yunchi@cityu.edu.hk; ORCID id: orcid.org/0000-0002-
8441-3974
− 1 −

Journal of Materials Chemistry C
Page 2 of 32
View Article Online
DOI: 10.1039/C8TC03645D
Keywords: thermally activated delay fluorescence, charge transfer, solvatochromism
-------------------------------------------------------------------------------------------------------------------------------
Abstract
TADF molecules with higher horizontal-dipole ratio have recently been realized to show
large conversion efficiency in organic light-emitting diodes (OLEDs) and hence great promise for
their application in display and solid state lighting sources. The current work focuses on fine-
tuning the structure of parent 2,4-diphenylpyrimidinyl compound PhPMAF, and its derivatives
with a series of 2,4-dipyridylpyrimidine acceptor; namely: 2NPMAF, 3NPMAF and 4NPMAF,
among which the intramolecular CH···N bonding interaction was confirmed for 2NPMAF,
showing good agreements with the NMR results. The solvatochromism and TADF behaviors,
together with TD-DFT computations, of the titled compounds were explored to gain insights
into the structure-photophysics relationship. Eventually, these functional 2,4-
dipyridylpyrimidine acceptor based TADF compounds exhibit electroluminescence ranging from
deep-blue to sky-blue, among which an external quantum efficiency (EQE) of 24.9 % was
achieved for derivative 3NPMAF. The high horizontal - dipole ratios (∼ 0.86 - 0.91) obtained for
these class of compounds elucidates superior OLED performance. Moreover, a non-doped OLED
architecture achieves an EQE of 14.1 % for 3NPMAF, which is superior to that of PhPMAF with
EQE of 5.1 %, demonstrating the significance of dipyridylpyrimidine as the acceptor for future
structural design of TADF emitters.
1. Introduction
Efficient luminescent materials are much demanded for constructing organic light-
emitting diodes (OLEDs) that are widely used in high-resolution flat panel displays and lighting
luminaries.1-6 One key parameter for the materials displaying red, green and blue emission is
the achievement of unitary internal quantum efficiency (IQE). Early OLED materials mainly
focused on fluorescent emitters; however, the spin statistics produce only 25 % singlet excitons
− 2 −

Page 3 of 32
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C8TC03645D
and the remaining 75 % are triplet excitons that decay through nonradiative process, limiting
the overall internal quantum efficiency within 25 %. Alternatively, noble-metal-based
organometallic phosphors, especially Ir(III) and Pt(II) metal complexes, are known to exhibit
rapid intersystem crossing between singlet and triplet states and, thus, can afford the near-unity
internal electroluminescence.7-10 In recent years, a new class of compounds possessing small
energy gap between the singlet and triplet excited states (ΔEST) also exhibit unitary quantum
efficiency via thermally activated delayed fluorescence (TADF).11-15 It aids in converting the
electrically generated triplet excitons to the singlet excitons via reverse intersystem crossing.
The typical molecular design can be accomplished by linking the donor and acceptor groups at
the spatially orthogonal position.14 Typical donor units are functional acridine,16, 17 carbazole,18,
19
phenoxazine,20 etc., while suitable acceptors may include: electron deficient aromatics,21
sulfone,22 phosphine oxide,23 dimesitylboryl,24 polycyclic borazine,25 etc.
Moreover, among various acceptors, pyrimidine and functional analogues are
considered to be superior. This is confirmed by earlier reports that pyrimidine-containing
compounds with extended π-conjugation show remarkable solvatofluorochromism,26 while
those with strong electron donating groups are expected to display intramolecular charge
transfer (ICT) character upon excitation.27, 28 Moreover, the electron deficiency of pyrimidine fell
in between that of pyridine and triazine fragment, making it an ideal candidate for further
tuning of emission color.29, 30 In general, these TADF emitters include either a simple pyrimidine
terminus or a 4,6-diphenylpyrimidine core, of which the phenyl appendages are expected to
increase thermal stability, but exert only minor influence to the photophysical properties.31
Other successful designs with either pyrimidine acceptor or equivalent are composed of simple
D-A alignment,32-35 D-A-D arrangement,36-39 A-D-D-A skeleton40 or D-A-A-D architectures,41, 42
among which the donor groups are aligned in a linear or butterfly arrangement to the
pyrimidine core.
Nowadays, TADF OLEDs have occasionally achieved an external quantum efficiency (EQE)
of over 30 % that is comparable to the best results of phosphorescent OLEDs. In addition, high
− 3 −

Journal of Materials Chemistry C
Page 4 of 32
View Article Online
DOI: 10.1039/C8TC03645D
EQE can be further boosted by enhancing the out-coupling coefficient (ηout) according to the
equation:
ηext = ηint × ηout
It has been reported that ηout can be tuned by intrinsic elements such as refraction index, layer
thickness and horizontal transition dipole moment ratio (Θ); the latter is capable to enhance the
out-coupling efficiency, achieving a remarkably high EQEs for both phosphorescent and TADF
materials.43 Such a phenomenon has been observed in Pt(II) based complex [Pt(fppz)2], for
which the vacuum deposited thin film exhibits highly ordered morphology and a remarkable Θ
of 93%, rendering a very high EQE of 38.8%.44 Alternatively, sky-blue emitting TADF material
bearing a spiro[acridine-9,9'-fluorene] donor also exhibits Θ of 83 % and a maximum EQE of 37
%.45
Furthermore, Lee and coworkers reported a correlation between the donor-acceptor
orientation and transition dipole moment, for which the para-linked donor exhibits higher Θ of
85 % than that of the ortho (71 %) and meta (75 %) analogues.46 Adachi et al. utilizes the CH···N
intramolecular H-bonding of dipyridyl terminals in planar Bpy-OXD to regulate the charge
transport characteristics.47 Kido et al. investigated the electron mobilities (µe) of three isomeric
pyridine-based electron transport materials, and demonstrated that the stacking of pyridyl units
had crucial influence to harness the carrier mobility.48, 49 Bearing these observations in mind,
we design a series of TADF molecules with spiro[acridine-9,9'-fluorene] as the electron donor
and dipyridylpyrimidine based electron acceptor with varied pyridyl orientation shown in
Scheme 1. The orientations of pyridyl groups of the target TADF materials, in combination with
the associated intra- and inter-molecular H-bonding interaction, are expected to influence the
horizontal dipole orientations, photophysical properties and OLED performances,50 the results
of which are elaborated in the following sections.
− 4 −

Page 5 of 32
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C8TC03645D
Scheme 1. Structural drawings of various spiro[acridine-9,9-fluorene]-pyrimidine based TADF
emitters.
2. Results and discussion
Synthesis and characterization. Synthetic strategy to all target molecules are shown in
Scheme 2. Parent phenyl chalcone intermediate 1 was routinely obtained by aldol condensation
of phenyl aldehyde and acetophenone in the presence of NaOH. However, application of NaOH
lowered the synthetic yield, particularly, in the case of pyridine substituted chalcones (2 - 4);
hence we switched from NaOH to Na2CO3 which gives higher yields of corresponding pyridyl
chalcone derivatives 2 - 4. Next, phenyl amidine (5) and functional pyridyl amidine derivatives (6
- 8) were allowed to react with pyridyl chalcones 1 - 4 in affording the desired acceptor units,
i.e. 6-phenyl-2,4-dipyridylpyrimidine (9 - 12).51 Addition of spiro[acridine-9,9'-fluorene] (AF) via
Buchwald-Hartwig amination afforded targeted TADF emitters, namely: PhPMAF, 2NPMAF,
3NPMAF and 4NPMAF. These materials were purified by column chromatography, followed by
recrystallization and temperature gradient sublimation. Their chemical structures were
1
confirmed using H and 13C NMR, mass spectroscopy and elemental analysis. It is notable that,
in the present study, we focused only on the asymmetrical 2,6-diphenyl and 2,6-
dipyridylpyrimidine acceptor rather than its symmetrical counterpart, i.e. those with
corresponding 4,6-substituents for the sake of inducing stronger ICT characteristics that
eventually afford an improved performances for the as-fabricated OLEDs.52, 53
− 5 −

Journal of Materials Chemistry C
Page 6 of 32
View Article Online
DOI: 10.1039/C8TC03645D
Scheme 2. Synthesis of pyrimidine-acceptors and final TADF emitters. Reaction conditions: i)
Na2CO3, H2O/MeOH(2:1), RT, 12 h; ii) KOH, EtOH/water, reflux, 12 h; iii) 10H-spiro[acridine-9,9-
fluorene], tBuONa, P(tBu)3, [Pd2(dba)3] (dba = dibenzylideneacetone), dry toluene, reflux, 48 h.
1H NMR chemical shifts of the central pyrimidinyl unit are strongly influenced by their
individual structures (Figure S1). First, the parent PhPMAF possesses two phenyl pendants and,
hence, the central pyrimidinyl proton appeared at δ 8.15 ppm. When the dual phenyl pendants
were replaced by the 2-pyridyl substituents in 2NPMAF, this unique proton signal shifted to δ
8.97 by 0.82 ppm due to the formation of intramolecular CH···N bonding between pyrimidine
and nearby pyridyl fragment as represented in Scheme 3.54, 55 Alternatively, no obvious CH···N
1
bonding was observed in 3NPMAF and 4NPMAF, for which the H NMR signal occurred at δ
8.23 and 8.30 ppm, which is akin to that of PhPMAF.
− 6 −

Page 7 of 32
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C8TC03645D
Scheme 3. Possible intramolecular CH···N bonding existed in 2NPMAF.
The needle-like single crystal of 3NPMAF was successfully grown from a mixed CHCl3 and
hexane solution at RT and the structure was characterized by single-crystal X-ray diffraction
study. As depicted in Figure 1, the spiro[acridine-9,9'-fluorene] donor is nearly orthogonal
(81.69°) to the phenylene that is also connected to the pyrimidine acceptor. Hence, these two
units are arranged in mutual orthogonal disposition, which is essential for minimizing orbital
overlap and giving small ΔEST.19 The dihedral angles between pyrimidine and phenylene and
between pyrimidine and pyridine were found to be 5.88° and 1.42°, respectively, as shown in
Figure 1. These values reveal the planar arrangement that will aid extended π-conjugation over
the whole 6-phenylene-2,4-dipyridylpyrimidine acceptor unit.
Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) at a
heating rate of 10 °C were employed to analyze their thermal stability. The corresponding TGA
and DSC graphs are displayed in Figure S2 and the numeric data are summarized in Table S1.
These compounds possess higher glass transition temperature (Tg) around 199 to 203 °C, and
decomposition temperature (Td), corresponding to 5 % of weight loss, was observed between
426 to 443 °C. The introduction of dual pyridine units gave Td at around 426 °C, which is slightly
increased by 9 to 17 °C compared to the parent compound PhPMAF. Introduction of
spiro[acridine-9,9'-fluorene] donor in PhPMAF enhanced Td by 44 °C vs. the analogues bearing
dimethyl acridine donor.52 This behavior could present one more advantage of utilizing the
spiro[acridine-9,9'-fluorene] donor.
Photophysical properties
The influence of pyridyl orientation in determining the photophysical properties was first
analyzed by UV/Vis absorption and photoluminescence (PL) studies in different solvents and as
thin film at RT. Figure 2 displays the recorded spectra and their photophysical data are
summarized in Table 1. All compounds exhibit a high molar extinction coefficient band (ε ≈ 4 - 7
− 7 −

Journal of Materials Chemistry C
Page 8 of 32
View Article Online
DOI: 10.1039/C8TC03645D
× 104 M−1 cm−1) around 300 nm that originate from the π → π* transition and a less intense
absorption band around 350 − 400 nm that can be attributed to the intramolecular charge
transfer (CT) transition.56
Despite the similarity in absorption spectra, their photoluminescence spectra are
significantly different. The phenyl substituted derivative PhPMAF showed a broadened emission
peak with max. at 441 nm in toluene. Upon switching to various pyridine substituents, the
emission max. at 482 nm, 480 nm and 495 nm for 2NPMAF, 3NPMAF and 4NPMAF are red
shifted by 41 nm, 39 nm and 54 nm, respectively, relative to PhPMAF. Similar shifting trend in
emission was observed in more polar CH2Cl2, and the emission peak wavelength of PhPMAF,
2NPMAF, 3NPMAF and 4NPMAF occurred at 500 nm, 566 nm, 568 nm and 596 nm. Hence, all
compounds exhibit the solvent polarity dependent luminescence, which served as the first
indication for having excited-state charge transfer characteristics. Moreover, the solid state
photoluminescence (sublimed powder) of all four compounds occurred at 447 nm (PhPMAF),
477 nm (2NPMAF), 480 nm (3NPMAF) and 509 nm (4NPMAF), for which the max. are relatively
closer to those recorded in toluene solution.
Figures 3 and 4 display the transient PL decay profile of all these compounds in
deaerated CH2Cl2 solution and as doped thin film, while the spectra taken in deaerated toluene
and sublimed powder are displayed in Figures S3 and S4 and their pertinent lifetime data are
listed in Table 1. As displayed in these figures, all the compounds exhibit bi-exponential decay
comprising the prompt and delayed components. For example, the transient PL decay of
3NPMAF in degassed CH2Cl2 solution exhibits a short lifetime component of 28 ns that
corresponds to the conventional fluorescence transition from S1 → S0 and another long decay
component of 2.08 ꢀs; the latter is arising from the T1 → S1 reverse intersystem crossing
followed by the delayed S1 → S0 emission, as its lifetime is drastically reduced upon aerating the
solvent. For other derivatives the long decay component has time constant of 3.1 ꢀs, 1.7 ꢀs and
0.8 ꢀs for PhPMAF, 2NPMAF and 4NPMAF, respectively, in degassed CH2Cl2 solution. In general,
assuming a pre-equilibrium between S1 and T1, TADF originates from thermal equilibrium
− 8 −

Page 9 of 32
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C8TC03645D
between the S1 and T1 states, and the difference in ΔET-S can be estimated according to the
relationship of ΔET-S and its equilibrium constant Keq, which expressed as Keq = Kisc/Krisc, ΔET-S
=
−RTln(Keq/3), where a factor of 3 stands for the triplet degenerate states and the equilibrium
constant Keq can be deduced from the pre-exponential factor for prompt fluorescence versus
delayed components. Based on this, ΔET-S is then deduced to be −2.45, −1.88, −1.73 and −1.91
kcal mol−1 for PhPMAF, 2NPMAF, 3NPMAF and 4NPMAF in CH2Cl2, respectively.
The TADF characters are also identified in solid state, which is highly important to OLED
fabrications. Hence, all powdery samples display the prompt (∼6 - 22 ns) and delayed
fluorescence (∼0.3 - 1.3 ꢀs) in their transient PL spectra as depicted in Figures S3 and S4. The PL
quantum yields of the sublimed powder of PhPMAF, 2NPMAF, 3NPMAF and 4NPMAF were
found to be 35 %, 40 %, 85 % and 44 % respectively. Of particular interest is the derivative
3NPMAF, which exhibits high PL quantum yield compared to those obtained in degassed
solution. This may arise from the suppressed non-radiative transitions in rigid medium,
envisaged for many TADF materials in literature.31, 45
Similar measurement was next extended to co-doped DPEPO thin film for PhPMAF,
2NPMAF, 3NPMAF and DCDPA thin film for 4NPMAF with 22 wt.% of dopant, for which the
spectral data were displayed in Figures 3 and 4 and Table 1. Again, introduction of pyridyl
substituents resulted a red shift of emission max. by ∼25 - 31 nm compared to that of PhPMAF,
which occurred at ∼457 nm. The absolute quantum yields were found to be 47, 73, 86 and 75 %
for PhPMAF, 2NPMAF, 3NPMAF and 4NPMAF, respectively. Moreover, all the transient PL
profiles in doped DPEPO consist of a prompt fluorescence with a lifetime of ∼10 - 20 ns, and a
delayed component with a lifetime of ∼1.6 - 1.7 μs. The detailed intersystem crossing and the
reversed intersystem crossing kinetic factors are calculated and showed in Table S2. The
calculated ΔET-S in the doped DPEPO matrix was found to be −3.95, −2.59, −2.95 and −2.43 kcal
mol−1 , which is in the same trend as that in non-doped solid and solution. It seems that
2NPMAF, 3NPMAF and 4NPMAF where pyridyl substituents are added to the acceptor
fragment significantly increase their emission quantum yield and reduced ΔET-S in comparison to
− 9 −

Journal of Materials Chemistry C
Page 10 of 32
View Article Online
DOI: 10.1039/C8TC03645D
that of the parent compound bearing only phenyl substituents, i.e. PhPMAF. Further support of
the delayed fluorescence property is provided by the time-resolved photoluminescence (PL)
spectra of these co-doped films as shown in Figure S5. The prompt emission component
acquired by intensified charge couple detector at zero delay time and a gate width of 10 ns is
contributed to fluorescence, whereas the component acquired at a delay time of 10 μs and gate
width 10 μs is ascribed to the delayed fluorescence.
Electrochemical behavior of these compounds was analyzed using cyclic voltammetry
and their corresponding voltammograms and data are represented in Figure S10. A reversible
oxidation peak was observed for all compounds in region between 0.62 V to 0.83 V that could
be ascribed to the formation of oxidized radical cation of acridine-fluorene donor while the
cathodic sweep resulted in a reversible reduction peak ranging from −2.15 V to −2.32 V. The
existence of reversible anodic oxidation and cathodic reduction potentials fulfills the needed
bipolar characteristics for better emissive dopants in electroluminescence applications.57, 58
Theoretical calculation
In this computational approach, time-dependent density functional theory (TD-DFT)
calculations were performed with an aim of gaining further insight into the basic electronic
properties. In this approach, the ground state geometries for the titled molecules were
optimized by DFT method. The excited-state photophysical properties were conducted by the
TD-DFT method with a CAM-B3LYP hybrid function in combination with a polarizable continuum
model (PCM) in cyclohexane. To access the excited-state properties, the So → S1 absorption was
calculated directly from the Franck-Condon excitation of the geometry optimized ground state.
Then the vertical S1 → S0 transition was calculated from the structurally optimized S1 state to
obtain the emission energy gap. Moreover, the 6-31+G(d,p) basis set was employed for all the
atoms. All the theoretical calculations were performed using the Gaussian 09 program. The
calculated absorption and emission data are listed in Table S3.
On the basis of the DFT and TD-DFT calculation, the electron density distributions in
− 10 −

Page 11 of 32
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C8TC03645D
HOMO and LUMO are mainly localized at spiro[acridine-9,9-fluorene] (donor) and 2,6-
disubstituted (both phenyl and functional pyridyl) pyrimidine (acceptor) fragment as shown in
Figure 5, respectively, indicating the charge transfer property in the lowest lying excited state
for all studied molecules.
Despite the sameness of the charge transfer properties of the titled molecules (vide
supra), the distinct difference lies in the fully geometry-optimized structures in the ground state
as shown in Figure 6. The dihedral angles between the spiro[acridine-9,9’-fluorene] donor and
the phenylene units are 89.87°, 89.79°, 89.74° and 89.84° for PhPMAF and 2-4NPMAF. Their
values, defined as θt, are not the main focus here. Instead, we attempt to discuss the dihedral
angles between the pyrimidine and central phenylene units (recorded as θa) as well as between
the pyrimidine and pyridyl appendages (recorded as θb). Once the dual phenyl pendants are
replaced by the 2-pyridyl substituents, the dihedral angle θb changes dramatically from 25.37°
(in PhPMAF) to 1.82° for 2NPMAF, but maintains a larger value of 23.18° and 25.08° for
3NPMAF and 4NPMAF, respectively. The H···N non-bonding distance was calculated to be 2.474
Å in 2NPMAF, but enlarged to 2.746 Å, 2.760 Å and 2.748 Å in corresponding PhPMAF,
1
3NPMAF and 4NPMAF, respectively. These results further support the H NMR data (vide
supra), confirming the strong intramolecular CH···N bonding interaction between the nearby 2-
pyridyl and the pyrimidinyl fragment in 2NPMAF.
OLED Device Fabrication
Since these spiro[acridine-9,9-fluorene]-pyrimidine based TADF compounds reveal
emission colors ranging from deep-blue to sky-blue, the device architectures should be
optimized in accordance to the individual emitter. Several wide triplet-energy-bandgap host
materials were used to examine their ability for fulfilling the exothermic energy transfer to the
studied emitters; they include 3-bis(9-carbazolyl)benzene (mCP),59 3,5-di(9H-carbazol-9-yl)-N,N-
diphenylaniline (DCDPA),60 9-(4-tertbutylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi),61
and bis(2-(diphenylphosphino)phenyl)ether oxide (DPEPO).62 In addition, the exciton
− 11 −

Journal of Materials Chemistry C
Page 12 of 32
View Article Online
DOI: 10.1039/C8TC03645D
confinement also greatly influences device performance. On the basis of our previous
experience in deep-blue OLED fabrication, double exciton confining layers (ECLs) should be
adopted for preventing the unwanted loss of energy, which consists of two wide energy
bandgap materials with opposite carrier transport behaviors.63 Hence, the emitting layer (EML)
was sandwiched between two ECLs to construct the basic framework, giving devices A1 and B1
with emitters PhPMAF and 2NPMAF. The hole-transport and electron-transport materials
adopted for ECLs were CzSi and DPEPO, respectively. In addition, device C1 with light-blue
emitting 3NPMAF possesses the same architecture except for the electron transporting ECL (i.e.
DPEPO). Furthermore, a simpler device architecture without dual ECLs is adopted for sky-blue
emitting 4NPMAF, for which the smaller energy bandgap facilitates direct exciton confinement;
hence, 1,1-bis[(di-4-tolylamino)phenyl] cyclohexane (TAPC) was selected as the hole transport
layer (HTL) because of its high carrier transport mobility of ∼10-2 cm2/Vs. Moreover, a p-type
hole injection layer which consisted of TAPC doped with 20 wt.% MoO3 was introduced for
facilitating hole injection and decreasing operation voltages.64 Furthermore, 4,4’,4’’-
tris(carbazol-9-yl)-triphenylamine (TCTA)65 was used to build a stepwise HTL to achieve better
carrier balance, while 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB)66 was selected as
electron transport layer (ETL) because of its wide triplet-energy-bandgap as well as high carrier
transport capability. Finally, DPEPO was used as the host for blue-emitting PhPMAF, 2NPMAF,
and 3NPMAF; while DCDPA with a relative smaller energy bandgap was selected for sky-blue
emitting 4NPMAF.
All layer thicknesses and doping concentration of EML were carefully fine-tuned for
better radiation output. Architectures for devices A1 and B1 are: ITO (120 nm)/TAPC doped with
20 wt.% of MoO3 (20 nm)/TAPC (45 nm)/TCTA (5 nm)/CzSi (2 nm)/DPEPO doped with 22 wt.% of
PhPMAF or 2NPMAF (20 nm)/DPEPO (3 nm)/TmPyPB (50 nm)/LiF (0.8 nm)/Al (150 nm), where
LiF and aluminum respectively serve as the electron injection layer and reflective cathode.
Furthermore, the light-blue emitting device C1 was developed with the architecture of ITO (120
nm)/TAPC doped with 20 wt.% MoO3 (20 nm)/TAPC (45 nm)/TCTA (5 nm)/CzSi (2 nm)/DPEPO
− 12 −

Page 13 of 32
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C8TC03645D
doped with 22 wt.% of 3NPMAF (20 nm)/ TmPyPB (50 nm)/LiF (0.8 nm)/Al (150 nm), while
device D1 consists of ITO (120 nm)/TAPC doped with 20 wt.% of MoO3 (20 nm)/TAPC (45
nm)/DCDPA (5 nm)/DCDPA doped with 22 wt.% of 4NPMAF (20 nm)/TmPyPB (50 nm)/LiF (0.8
nm)/Al (150 nm). Figure 7 shows the schematic illustration of all fabricated devices.
Figure 8 and Table 2 summarize the electroluminescent (EL) characteristics and
numerical data of the as-fabricated devices. Normalized EL spectra of all devices at 1000 cd/m2
were depicted in Figure 8(a). As indicated, all EL spectra exhibited solely the emission from the
studied TADF materials and are slightly red-shifted compared to their PL spectra measured in
CH2Cl2 solution. In addition, there is no exciton diffusion to the adjacent layers, a result of
effective hole transport and exciton confinement.67 As shown in Figure 8(b), the current
density-voltage-luminance (J-V-L) curves revealed that the devices using sky-blue emitting
4NPMAF possessed the highest current density. The higher luminance may be derived from the
simpler device architecture of D1 and the higher responses of human eyes to this emission
color. The maximum luminance of D1 reached 46095 cd/m2 at 10.8 V, exceeding that of A1
under similar condition (i.e. 12258 cd/m2 at 10.6 V). On the other hand, all devices A1, B1 and
C1 showed very low turn-on voltages of 2.8 - 2.9 V, which were lower than those of many
reported blue-emitting TADF OLEDs.23 Basically, the donor-acceptor based TADF emitters tend
to exhibit better carrier transport capability. Therefore, the higher performances were mainly
deriving from the higher doping concentration of emitters in EML.
Figure 8(c) and (d) depicted device efficiencies, for which their maxima reached 19.7%
(36.3 cd/A and 40.7 lm/W), 23.6% (50.6 cd/A and 56.7 lm/W), 24.9% (60.7 cd/A and 68.2 lm/W),
and 20.4% (52.5 cd/A and 58.7 lm/W) for PhPMAF, 2NPMAF, 3NPMAF and 4NPMAF,
respectively. Considering their PLQY in doped DPEPO matrix, the high EQE of devices illustrated
the achievement of nearly 100% internal quantum efficiency for all studied architectures.
Furthermore, at a practical luminance of 102 cd/m2, EQE of 14.4 %, 18.4%, 21.2%, and 19.1%
were obtained for devices A1, B1, C1, and D1, respectively. As the result, the efficiency decay at
102 cd/m2 were estimated to be 27%, 22%, 15%, and 6%, respectively. The lowered efficiency
− 13 −

Journal of Materials Chemistry C
Page 14 of 32
View Article Online
DOI: 10.1039/C8TC03645D
roll-off in devices C1 and D1 was probably due to the simpler device architecture, which
preventing carrier/exciton accumulation at the ECL/EML interfaces and thus mitigate triplet-
triplet annihilation (TTA) as well as triplet-polaron quenching.68 Furthermore, the high doping
conc. of these devices (i.e. 22 wt.%) maintain very high efficiencies at high luminance, implying
insignificant aggregation induced quenching. One possibility lies in the higher electron
mobilities provided by the pyridyl substituents and balanced carrier transport in EML. The other
may be attributed to the very short excited lifetimes of < 20 ns, as shown in Table 1. Thus, we
attempted to fabricate a similar set of devices using non-doped (neat) EML.69 Their EL
characteristics were depicted in Figure 9 and Table 2.
Figure 9(a) exhibit normalized EL spectra of these non-doped devices and with a slight
red-shift of EL by 0 - 13 nm (cf. Table 2), indicating an insignificant degree of solvation. As shown
in Figure 9(b), the J-V-L curves revealed the higher current densities of devices B2, C2 and D2
than that of A1, mainly due to the higher energy bandgap of PhPMAF. Nevertheless, the turn-
on voltages of all tested devices remain low at ∼2.9 - 3.0 V, due to their excellent carrier
transport capability. As shown in Figures 9(c) and (d), the peak efficiencies of devices A2, B2, C2,
and D2 still maintained at 5.1% (10.0 cd/A and 10.2 lm/W), 12.4% (30.3 cd/A and 30.4 lm/W),
14.1% (38.5 cd/A and 37.8 lm/W), and 11.2% (35.4 cd/A and 34.4 lm/W), respectively. Similar
phenomena of the mitigated efficiency roll-off were observed in the second series of devices
with non-doped EML, which could maintain high efficiency up to 100 cd/m2. Hence, the high
efficiencies obtained in these non-doped devices suggested the presence of certain
enhancement in light extraction. Therefore, these molecules may form a well aligned packing
arrangement during deposition,70 resulting from the strong coupling of dipyridylpyrimidine
fragments between molecules.
Thus, the angle-dependent PL measurement was executed to investigate the transition
dipole orientations.43 The measurement setup for the orientation factor of transition dipole
moment was described in Figure S11 of the Supporting Information. Figure 10 shows the angle-
dependent PL intensities of the p-polarized light at the emission peak max. of these TADF
− 14 −

Page 15 of 32
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C8TC03645D
emitters, along with the simulation results based on the classical dipole model. These data were
well fitted by the horizontal-dipole ratios (Θ) of 0.90 for PhPMAF, 0.87 for 2NPMAF, 0.86 for
3NPMAF, and 0.91 for 4NPMAF, where Θ was defined as the ratio of the horizontal dipoles to
the total transition dipoles of each emitter. These results also explained the superior
performance obtained in all OLED devices discussed. The resulted dipole ratios Θ are also
comparable to the best data reported in the spiro[acridine-9,9'-fluorene] and triazine based
TADF emitters,40, 45, 71-74 showing the high potential for this series of molecular design in
improving the light harvesting efficiencies without installment of any external out-coupling
structure.
3. Conclusion
In summary, we have designed and synthesized a new class of 2,4-dipyridylpyrimidine
based acceptor systems. Theses acceptors are then linked with the bulky and rigid spirophenyl
acridine donor by varying the position of the nitrogen atom in the pyridine ring, forming
2NPMAF, 3NPMAF and 4NPMAF to make a comparative study with the parent 2,4-
1
diphenylpyrimidine derivative, PhPMAF. Affirmed by H NMR and computational approaches
2NPMAF possesses an intramolecular CH···N bonding interaction. The observed delayed
fluorescence with microseconds decay time constants validates the presence of TADF
phenomenon for all studied compounds. These pyrimidine functionalized TADF compounds
render emission colors ranging from deep-blue to sky-blue and are suitable to serve as dopant
in fabrication of OLED devices. High horizontal transition dipole ratio of 0.86 − 0.91 assist in
achieving higher EQEs of 19.7 %, 23.6 %, 24.9 % and 20.4 % for PhPMAF, 2NPMAF, 3NPMAF
and 4NPMAF, respectively. Interestingly, in the non-doped (neat) device architecture, the
maximum EQE of 12.4%, 14.1% and 11.2%, obtained for 2NPAMF, 3NPMAF and 4NPMAF, are
significantly higher than parent diphenylpyrimidine derivative with EQE of only 5.1 %. These
results demonstrate the promising advantage of this new class of dipyridylpyrimidine based
acceptors in the development of highly efficient TADF molecules for display and lighting
− 15 −

Journal of Materials Chemistry C
Page 16 of 32
View Article Online
DOI: 10.1039/C8TC03645D
applications.
4. Notes
The authors declare no competing financial interests.
5. Corresponding Author
Yun Chi, E-mail: yunchi@cityu.edu.hk
Pi-Tai Chou, E-mail: chop@ntu.edu.tw
Chih-Hao Chang, E-mail: chc@saturn.yzu.edu.tw
Wen-Yi Hung, E-mail: wenhung@mail.ntou.edu.tw
6. Author Contributions
‡ P.G. and D.-G. C contributed equally.
7. Acknowledgment
This work was supported by the funding from Ministry of Science and Technology (MOST),
featured areas research program within the framework of the Higher Education Sprout Project
administrated by Ministry of Education (MOE) of Taiwan, and City University of Hong Kong,
Hong Kong SAR. We are grateful to the National Center for the High-performance Computing
(NCHC) of Taiwan for the valuable computer time and facilities.
− 16 −

Page 17 of 32
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C8TC03645D
Figure 1. Structural drawing of 3NPMAF with ellipsoids shown at the 30% probability level. The
curved arrows depict the dihedral angle between each of the adjacent rings.
− 17 −

Journal of Materials Chemistry C
Page 18 of 32
View Article Online
DOI: 10.1039/C8TC03645D
6
4
2
0
1.2
0.8
0.4
0.0
(a) PhPMAF
300
300
300
300
400
(b) 2NPMAF
500
500
500
600
600
600
600
700
700
700
700
1.2
0.8
0.4
0.0
9
6
3
0
400
(c) 3NPMAF
9
6
3
0
1.2
0.8
0.4
0.0
400
1.2
0.8
0.4
0.0
9
6
3
0
(d) 4NPMAF
400
500
Wavelength (nm)
Figure 2. Absorption and emission spectra of (a) PhPMAF, (b) 2NPMAF, (c) 3NPMAF and (d)
4NPMAF, in toluene (blue), CH2Cl2 (red), sublimed powder (black) and doped DPEPO for
PhPMAF, 2NPMAF, 3NPMAF and DCDPA for 4NPMAF (green) at 298 K.
− 18 −

Page 19 of 32
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C8TC03645D
10000
1000
100
10
22 wt.% of PhPMAF in doped thin film
IRF
Fitting
PhPMAF in degassed DCM
IRF
Fitting
10000
1000
100
10
1
0
5000
10000
Time (ns)
15000
20000
2000
4000
6000
8000
10000
Time (ns)
10000
1000
100
10
2NPMAF in degassed DCM
IRF
Fitting
22 wt.% of 2NPMAF in doped thin film
IRF
Fitting
1000
100
10
1
0
2500
5000
Time (ns)
7500
10000
2000
4000
6000
Time (ns)
8000
10000
Figure 3. Transient photoluminescence characteristics of PhPMAF (top) and 2NPMAF (bottom)
in degassed CH2Cl2 solution and 22 wt.% doped in DPEPO (λex = 374 nm). IRF = instrument
response function (red).
− 19 −

Journal of Materials Chemistry C
Page 20 of 32
View Article Online
DOI: 10.1039/C8TC03645D
10000
1000
100
10
3NPMAF in degassed DCM
IRF
Fitting
22 wt.% of 3NPMAF in doped thin film
IRF
Fitting
10000
1000
100
1
0
2000
4000
6000
8000
10000
0
2000
4000
6000
8000
10000
Time (ns)
Time (ns)
10000
1000
100
10
4NPMAF in degassed DCM
IRF
Fitting
22 wt.% of 4NPMAF in doped thin film
IRF
Fitting
10000
1000
100
1
0
2000
4000
Time (ns)
6000
8000
0
2000
4000
6000
8000
10000
Time (ns)
Figure 4. Transient photoluminescence characteristics of 3NPMAF (top) and 4NPMAF (bottom)
in degassed CH2Cl2 solution and 22 wt.% doped in DPEPO for 3NPMAF and in DCDPA for
4NPMAF (λex = 374 nm). IRF = instrument response function (red).
− 20 −

Page 21 of 32
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C8TC03645D
Figure 5. Optimized molecular structures and the associated HOMO (cyan) and LUMO (red) in
the ground states for (a) PhPMAF, (b) 2NPMAF, (c) 3NPMAF and (d) 4NPMAF.
− 21 −

Journal of Materials Chemistry C
Page 22 of 32
View Article Online
DOI: 10.1039/C8TC03645D
Figure 6. Optimized molecular structures in the ground states for (a) PhPMAF, (b) 2NPMAF, (c)
3NPMAF and (d) 4NPMAF. Angle θt indicated the torsional angle between the spiro[acridine-
9,9’-fluorene] donor and the phenylene unit. The dihedral angles between the pyrimidine and
the phenylene were recorded as θa, and the angles between the pyrimidine and the pyridines
was recorded as θb. (i.e. in PhPMAF (a), θa was recorded by dihedral angle in C(30)-C(29)-C(32)-
C(35), θb was recorded by dihedral angle in C(43)-C(37)-C(34)-N(3))
− 22 −

Page 23 of 32
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C8TC03645D
Figure 7. Schematic architectures of OLEDs for different TADF emitters.
− 23 −

Journal of Materials Chemistry C
Page 24 of 32
View Article Online
DOI: 10.1039/C8TC03645D
(a)
(b)
105
104
103
102
101
100
10-1
800
@ 1000 cd/m2
1.0
700
600
500
400
300
200
100
0
Device A1
Device B1
Device C1
Device D1
A1
B1
C1
D1
0.8
0.6
0.4
0.2
0.0
10-2
10
0
2
4
6
8
400
500
600
700
Wavelength (nm)
Voltage (V)
(c)
(d)
70
60
50
40
30
20
10
0
70
60
50
40
30
20
10
0
28
24
20
16
12
8
Device A1
Device A1
Device B1
Device C1
Device D1
Device B1
Device C1
Device D1
4
0
10-1
100
101
102
103
104
105
10-1
100
101
102
103
104
105
Luminance (cd/m2)
Luminance (cd/m2)
Figure 8. EL characteristics of the blue TADF OLEDs: (a) normalized EL spectra; (b) current
density-voltage-luminance (J-V-L) curves; (c) external quantum efficiency vs luminance; (d)
luminance efficiency and power efficiency of devices A1, B1, C1 and D1.
− 24 −

Page 25 of 32
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C8TC03645D
(b)
(a)
105
104
103
102
101
100
10-1
1000
@ 1000 cd/m2
1.0
A2
B2
C2
D2
Device A2
Device B2
Device C2
Device D2
800
600
400
200
0
0.8
0.6
0.4
0.2
0.0
10-2
10
0
2
4
6
8
400
500
600
700
Wavelength (nm)
Voltage (V)
(c)
(d)
40
35
30
25
20
15
10
5
40
16
14
12
10
8
Device A2
Device B2
Device C2
Device D2
Device A2
Device B2
Device C2
Device D2
35
30
25
20
15
10
5
6
4
2
0
0
0
10-1
100
101
102
103
104
105
10-1
100
101
102
103
104
105
Luminance (cd/m2)
Luminance (cd/m2)
Figure 9. EL characteristics of the TADF OLEDs with neat EML: (a) normalized EL spectra; (b)
current density-voltage-luminance (J-V-L) curves; (c) external quantum efficiency vs luminance;
(d) luminance efficiency and power efficiency of devices A2, B2, C2 and D2.
− 25 −

Journal of Materials Chemistry C
Page 26 of 32
View Article Online
DOI: 10.1039/C8TC03645D
(a)
(b)
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.4
1.2
1.0
0.8
0.6
PhPMAF
Measurement
Fit ( =90%)
Horizontal(
=90%)
Isotropic(
2NPMAF
Measurement
Fit ( =87%)
Horizontal(
=90%)
Isotropic(
0.4
0.2
0.0
Θ
Θ
Θ
=100%)
Θ
=100%)
(
Θ
(
Θ
Θ
=67%)
Θ
=67%)
0
10 20 30 40 50 60 70 80 90
0
10 20 30 40 50 60 70 80 90
Angle (degree)
Angle (degree)
(c)
(d)
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
3NPMAF
4NPMAF
Measurement
Fit ( =91%)
Horizontal(
=90%)
Isotropic(
Measurement
Fit (Θ=86%)
Horizontal(Θ=100%)
(Θ=90%)
Θ
Θ=100%)
(
Θ
Isotropic(Θ=67%)
Θ=67%)
0
10 20 30 40 50 60 70 80 90
0
10 20 30 40 50 60 70 80 90
Angle (degree)
Angle (degree)
Figure 10. Measured horizontal transition dipole moment ratios of (a) DPEPO : PhPMAF, (b)
DPEPO : 2NPMAF, (c) DPEPO : 3NPMAF, (d) DCDPA : 4NPMAF at their emission peak max. from
a 100 nm of co-doped emissive layer with DPEPO or DCDPA host and 22 wt.% of dopant. Black
dot and red dash line represent the experimental data and theoretical fitting, respectively.
− 26 −

Page 27 of 32
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C8TC03645D
Table 1. Selected photophysical properties of PhPMAF, 2NPMAF, 3NPMAF and 4NPMAF under
various states at RT.
λem
[nm]
PLQY
[%]
ΔET S,
‑
state
τ [ns] and pre-exp. factor
(kcal/mol)
PhPMAF
toluene
441
500
447
31
51
35
12 (0.9999), 10000 (0.0001) -4.86
CH2Cl2
23 (0.9947), 3133 (0.0053)
6 (0.9991), 364 (0.0009)
-2.45
-3.52
sublimed powder
Co-doped DPEPO film
457
482
566
477
482
480
568
508
486
495
596
509
488
47
26
36
40
73
10
15
85
86
69
27
44
75
10 (0.9996), 1682 (0.0004)
17 (0.9997), 5712 (0.0003)
29 (0.9863), 1776 (0.0137)
8 (0.9363), 361 (0.0637)
19 (0.9958), 1754 (0.0042)
17 (0.9996), 5354 (0.0004)
28 (0.9824), 2083 (0.0176)
22 (0.9754), 1338 (0.0246)
15 (0.9977), 1707 (0.0023)
-3.95
-4.08
-1.88
-0.94
-2.59
-4.04
-1.73
-1.53
-2.95
2NPMAF toluene
CH2Cl2
sublimed powder
Co-doped DPEPO film
3NPMAF toluene
CH2Cl2
sublimed powder
Co-doped DPEPO film
4NPMAF toluene
19 (0.9981), 13073 (0.0019) -3.06
CH2Cl2
23 (0.9870), 868 (0.0130)
12 (0.9820), 570 (0.0180)
20 (0.9946), 1668 (0.0054)
-1.91
-1.72
-2.43
sublimed powder
Co-doped DCDPA film
− 27 −

Journal of Materials Chemistry C
Page 28 of 32
Table 2. EL characteristics of the fabricated blue TADF OLEDs.
EQE (%)
[a], [b]
LE (cd/A)
[a], [b]
PE (lm/W)
[a], [b]
Von
max. Lumin.
λmax.
CIE1931
Device
Emitter
PhPMAF
2NPMAF
3NPMAF
4NPMAF
(V)[c] in cd/m2 (V)
(nm)[d]
coordinates [d]
(0.168, 0.250)
(0.172, 0.231)
(0.187, 0.343)
A1 (22 wt%)
A2 (neat)
1
9.7, 14.4
36.3, 26.5
10.0, 9.3
50.6, 39.4
30.3, 29.3
60.7, 51.6
38.5, 35.9
52.5, 49.3
35.4, 34.3
40.7, 21.4
10.2, 7.0
2.9
3.
12258 (10.6) 467
13977 (10.6) 46
11845 (10.4) 481
18165 (9.8) 494
19234 (10.6) 49
5.1, 4.7
2
7
B
B
C
C
1 (22 wt%)
23.6, 18.4
12.4, 12.0
24.9, 21.2
14.1, 13.1
20.4, 19.1
11.2, 10.9
56.7, 33.9
30.4, 25.5
68.2, 46.0
37.8, 31.3
58.7, 43.0
34.4 , 30.0
2.8
3.0
2.8
2.9
2.8
2.9
2 (neat)
(
0.225, 0.423)
1 (22 wt%)
2 (neat)
2
(0.204, 0.399)
(0.266, 0.496)
(0.217, 0.452)
(0.297, 0.565)
36844 (10.8) 505
46095 (10.8) 494
34523 (10.6) 514
D
D
1 (22 wt%)
2 (neat)
[a] Maximum efficiency; [b] recorded at 102 cd/m2; [c] turn-on voltage measured at 1 cd/m2; [d] recorded at 103 cd/m2.
− 28 −

Page 29 of 32
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C8TC03645D
References
1.
2.
3.
4.
C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913-915.
J. Kido, M. Kimura and K. Nagai, Science, 1995, 267, 1332-1334.
B. W. D'Andrade and S. R. Forrest, Adv. Mater., 2004, 16, 1585-1595.
Y. Sun, N. C. Giebink, H. Kanno, B. Ma, M. E. Thompson and S. R. Forrest, Nature, 2006,
440, 908-912.
5.
S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Luessem and K. Leo, Nature,
2009, 459, 234-238.
6.
7.
H. Sasabe and J. Kido, Chem. Mater., 2011, 23, 621-630.
H. Yersin, A. F. Rausch, R. Czerwieniec, T. Hofbeck and T. Fischer, Coord. Chem. Rev., 2011,
255, 2622-2652.
8.
9.
B. Minaev, G. Baryshnikov and H. Agren, Phys. Chem. Chem. Phys., 2014, 16, 1719-1758.
Y. Im, S. Y. Byun, J. H. Kim, D. R. Lee, C. S. Oh, K. S. Yook and J. Y. Lee, Adv. Funct. Mater.,
2017, 27, 1603007.
10.
11.
12.
13.
G. Baryshnikov, B. Minaev and H. Ågren, Chem. Rev., 2017, 117, 6500-6537.
H. Uoyama, K. Goushi, K. Shizu, H. Nomura and C. Adachi, Nature, 2012, 492, 234-238.
M. Y. Wong and E. Zysman-Colman, Adv. Mater., 2017, 29, 1605444.
Z. Yang, Z. Mao, Z. Xie, Y. Zhang, S. Liu, J. Zhao, J. Xu, Z. Chi and M. P. Aldred, Chem. Soc.
Rev., 2017, 46, 915-1016.
14.
Y. Im, M. Kim, Y. J. Cho, J.-A. Seo, K. S. Yook and J. Y. Lee, Chem. Mater., 2017, 29, 1946-
1963.
15.
16.
T. J. Penfold, F. Dias and A. P. Monkman, Chem. Commun., 2018, 54, 3926-3935.
J. Li, R. Zhang, Z. Wang, B. Zhao, J. Xie, F. Zhang, H. Wang and K. Guo, Adv. Opt. Mater.,
2018, 6, 1701256.
17.
W. Zeng, H.-Y. Lai, W.-K. Lee, M. Jiao, Y.-J. Shiu, C. Zhong, S. Gong, T. Zhou, G. Xie, M.
Sarma, K.-T. Wong, C.-C. Wu and C. Yang, Adv. Mater., 2018, 30, 1704961.
B. Wex and B. R. Kaafarani, J. Mater. Chem. C, 2017, 5, 8622-8653
L.-S. Cui, H. Nomura, Y. Geng, J. U. Kim, H. Nakanotani and C. Adachi, Angew. Chem. Int.
Ed., 2017, 56, 1571-1575.
18.
19.
20.
H. F. Higginbotham, C.-L. Yi, A. P. Monkman and K.-T. Wong, J. Phys. Chem. C, 2018, 122,
7627-7634.
21.
22.
X. Cao, D. Zhang, S. Zhang, Y. Tao and W. Huang, J. Mater. Chem. C, 2017, 5, 7699-7714.
N. Jürgensen, A. Kretzschmar, S. Höfle, J. Freudenberg, U. H. F. Bunz and G. Hernandez-
Sosa, Chem. Mater., 2017, 29, 9154-9161.
23.
24.
C. Duan, J. Li, C. Han, D. Ding, H. Yang, Y. Wei and H. Xu, Chem. Mater., 2016, 28, 5667-
5679.
D.-G. Chen, T.-C. Lin, C.-L. Chen, Y.-T. Chen, Y.-A. Chen, G.-H. Lee, P.-T. Chou, C.-W. Liao, P.-
C. Chiu, C.-H. Chang, Y.-J. Lien and Y. Chi, ACS Appl. Mater. Interfaces, 2018, 10, 12886-
12896.
25.
S. Nakatsuka, H. Gotoh, K. Kinoshita, N. Yasuda and T. Hatakeyama, Angew. Chem. Int.
Ed., 2017, 56, 5087-5090.
26.
27.
K. Itami, D. Yamazaki and J.-i. Yoshida, J. Am. Chem. Soc., 2004, 126, 15396-15397.
S.-i. Kato, Y. Yamada, H. Hiyoshi, K. Umezu and Y. Nakamura, J. Org. Chem., 2015, 80,
9076-9090.
28.
L. Skardziute, J. Dodonova, A. Voitechovicius, J. Jovaisaite, R. Komskis, A. Voitechoviciute,
J. Bucevicius, K. Kazlauskas, S. Jursenas and S. Tumkevicius, Dyes Pigm., 2015, 118, 118-
128.
29.
30.
S.-J. Su, C. Cai and J. Kido, Chem. Mater., 2011, 23, 274-284.
K.-C. Pan, S.-W. Li, Y.-Y. Ho, Y.-J. Shiu, W.-L. Tsai, M. Jiao, W.-K. Lee, C.-C. Wu, C.-L. Chung,
T. Chatterjee, Y.-S. Li, K.-T. Wong, H.-C. Hu, C.-C. Chen and M.-T. Lee, Adv. Funct. Mater.,
2016, 26, 7560-7571.
− 29 −