Functional Pyrimidine-Based Thermally Activated Delay Fluorescence Emitters: Photophysics, Mechanochromism, and Fabrication of Organic Light-Emitting Diodes

Article abstract of DOI:10.1002/chem.201604883

A new series of molecules, T1–T4, possessing thermally activated delayed fluorescence (TADF) have been strategically designed and synthesized. Molecules T1–T4 contain the dimethyl acridine as the electron donor, which is linked to either symmetrical or un

Full text of DOI:10.1002/chem.201604883

A Journal of
Accepted Article
Title: Functional Pyrimidine-based Thermally Activated Delay
Fluorescence Emitters: Photophysics, Mechanochromism and
Fabrication of Organic Light-Emitting Diodes
Authors: Paramaguru Ganesan, Revathi Ranganathan, Yun Chi, Xiao-
Ke Liu, Chun-Sing Lee, Shih-Hung Liu, Gene-Hsiang Lee,
Tzu-Chieh Lin, Yi-Ting Chen, and Pi-Tai Chou
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To be cited as: Chem. Eur. J. 10.1002/chem.201604883
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10.1002/chem.201604883
Chemistry - A European Journal
Functional Pyrimidine-based Thermally Activated Delay Fluorescence Emitters: Photophysics,
Mechanochromism and Fabrication of Organic Light-Emitting Diodes
Paramaguru Ganesan,a,‡ Revathi Ranganathan,a,‡ Yun Chi,a,* Xiao-Ke Liu,b Chun-Sing Lee,b,*
Shih-Hung Liu,c Gene-Hsiang Lee,c Tzu-Chieh Lin,c Yi-Ting Chen,c and Pi-Tai Chou,c,*
a
Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan; E-mail:
ychi@mx.nthu.edu.tw
b
Center of Super-Diamond and Advanced Films (COSDAF) & Department of Physics and
Materials Science, City University of Hong Kong, Hong Kong Special Administrative Region, P. R.
China; E-mail: apcslee@cityu.edu.hk
c
Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan; E-mail:
chop@ntu.edu.tw
‡ Authors with equal contribution.
TOC Entry
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Abstract
A new series of molecules T1-T4 possessing thermally activated delay fluorescence (TADF) have
been strategically designed and synthesized. T1-T4 bear the dimethyl acridine as the electron
donor, which is linked to either symmetrical or unsymmetrical diphenyl pyrimidine as an
acceptor. In comparison to the ubiquitous triazine acceptor, the selection of pyrimidine as an
acceptor takes its advantage of facile functionalization and less stabilized unoccupied π-orbitals
so that the energy gap toward blue can be accessed. Together with the acridine donors, the
resulting D-A functional materials reveal remarkable TADF properties. In solid, all T1-T4 exhibit
intriguing mechanochromism. The crystal structures, together with the spectroscopy and
dynamics acquired upon application of stressing, lead us to propose two types of structure
arrangement that give distinct emission properties, one with and the other without TADF. Upon
fabricating OLEDs the T1-T4 films prepared from sublimation all exhibit dominant TADF
behavior, accounting for their high performance where the 490 nm EL emission with an EQE of
14.2% can be attained when T2 is used as an emitter.
1. Introduction
Organic light-emitting diodes (OLEDs) are deemed to be the auspicious technology for
lighting applications and new generation flexible displays.[1] Typically OLEDs are composed of
electroluminescent materials and carrier transport materials embedded between metal
cathode and transparent conducting oxide anode. Internal quantum efficiency (IQE) of the
electroluminescent materials plays key role in determining the practical performance of the
OLEDs. Theoretically, due to the fast rate of singlet-triplet intersystem crossing and allowed T1
→ S0 transition, organometallic phosphorescent emitters are able to harvest both singlet and
triplet excitons and can achieve 100% IQE, superior to the fluorescent materials with limited
IQE of 25%.[2] However, rare availability and high cost of phosphorescent materials hinder their
practical applications, motivating researchers to develop new types of emitters with lower costs
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and better performance. In light of this scenario, Adachi et al. pioneered a new generation low-
cost and high-efficiency emitters based on the mechanism of thermally activated delayed
fluorescence (TADF).[3] TADF emitters have extremely small singlet-triplet energy gap, allowing
up-conversion of triplet excitons to the radiative singlet excited state via reverse intersystem
crossing by harvesting environmental thermal energy. Without employing heavy metal atoms,
TADF emitters are able to harvest both singlet and triplet excitons for light emission, providing
an efficient pathway to achieve 100% IQE.[4]
TADF materials with minimized spatial overlap between the donor (D) and acceptor (A)
fragments can aid in reducing the energy gap between the singlet and triplet states (ET-S,
defined as T1 minus S1 in the lowest lying excited state). Tremendous efforts have been made to
couple donor and acceptor units for realizing highly efficient TADF materials.[5] Electron
deficient aromatic systems such as triazole,[6] oxadiazole,[7] anthraquinone,[8] cyano,[9]
sulfones,[10] phosphine oxide,[11] and etc., are generally regarded as adequate accepting
fragments. Of particular interest is the 1,3,5-triazine that has been widely utilized as acceptor
unit of D-A system that exhibits remarkable TADF characteristic.[12] More recently, pyrimidine
has emerged as an alternative acceptor fragment.[13] This selection is mainly due to its electron
deficient character and easy functionalization of the donor moieties, which aid in constructing
the required D-A arrangement. It is also expected that the pyrimidine acceptor possesses the
less stabilized empty π-orbitals compared with the triazine based acceptor. Hence, by
combining with suitable donors, the pyrimidine based D-A functional materials would provide a
suitable motif for efficient sky-blue and blue emission.
Furthermore, the donor should attach to the acceptor in a partially orthogonal
orientation to reduce the electron correlation and hence reduced ET-S will be achieved.[14]
From the view point of molecular design, carbazole has long been considered as an excellent
donor fragment. Unfortunately, the central pyrrole ring is not capable of inducing sufficiently
large steric effect against the pyrimidine to maintain the perpendicular arrangement upon
excitation.[4, 15] We thus decided to switch to the dimethyl acridine, for which the six membered
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central ring is expected to increase spatial encumbrance with the acceptor fragment as
envisaged in the literature.[6, 16] Scheme 1 displays the structural sketching of these pyrimidine
based TADF molecules, T1-T4, among which T1 and T2 are endowed with the dimethyl acridine
donor linked to either symmetrical or unsymmetrical diphenyl pyrimidine acceptor, and T3 and
T4 are derivatives of T1 and T2 but with reduced π-conjugation at one phenyl group by
introducing an ortho-methyl substituent. These structural changes within the emitters are
expected to alter the associated optoelectronic properties systematically.
Scheme 1. Structural drawing of diphenyl pyrimidine based TADF molecules.
2. Results and discussion
2.1. Synthesis and characterization
The synthetic strategy for achieving the target molecules are shown in Scheme 2.
Chalcone type intermediates (1 - 4) were prepared by simple Aldol condensation reactions with
the corresponding aldehyde and acetophenone in good yields.[17] Accordingly, the prepared
chalcones further reacted with respective amidine hydrochloride salts to yield the pyrimidine
based acceptor unit (5 - 8). The donor fragment was then coupled with the acceptor unit via
Buchwald Hartwig amination to yield the desired products (T1, T2, T3 and T4). They were
subjected to recrystallization, followed by temperature gradient vacuum sublimation. Chemical
1
structures of the final products were confirmed using H and 13C NMR, mass spectroscopy and
elemental analysis.
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Scheme 2. Synthesis of pyrimidine accepting unit and final TADF molecules. Experimental
t
conditions: (i) KOH, EtOH, RT; (ii) KOH, EtOH/water, reflux. (iii) dimethylacridine, BuONa,
P(tBu)3, Pd2(dba)3, dry toluene, reflux, 24 h.
T1 and T2 were also examined by single-crystal X-ray diffraction and, to the best of our
knowledge, they represent the first set of structural study based on pyrimidine TADF system,
which should provide in-depth insight into the overall molecular structure and the relative
orientation between the donor and acceptor in the ground state. As depicted in Figures 1 and 2,
the planar dimethyl acridine for both molecules is nearly orthogonal (88.76 and 85.49 for T1
and T2 respectively) to the middle phenylene substituent that links to the central pyrimidine,
fulfilling the requirement of minimizing orbital overlap between HOMO and LUMO (vide infra).
The measured dihedral angles between each phenyl group and the central pyrimidine unit were
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found to be 21.93, 5.15, 15.28 and 19.41, 6.05, 17.87 for T1 and T2, respectively, which
confirm the formation of pseudo-planar arrangement for the central pyrimidine acceptor unit
that is favorable for the extended π-conjugation.
Thermal properties of T1, T2, T3 and T4 were investigated by using thermal gravimetric
analysis (TGA) and differential scanning calorimetry (DSC) at a heating rate of 10 C min1. The
TGA and DSC graphs were displayed in Figure S1 and their relevant data are listed in Table S1.
The temperature of decomposition (Td, which corresponds to 5% weight loss) was observed in
the range of 357 to 382 C, evidencing their robustness and excellent thermal stability under
investigation. The insertion of methyl group in the pyrimidine acceptors (T3 and T4) slightly
lower its Td compared to their parent analogues T1 and T2. The glass transition temperature
(Tg) of all samples was recorded to be around 95 C.
2.3 TADF properties via photophysical measurements
Photophysical properties of these compounds were firstly analyzed by UV/Vis
absorption and photoluminescence studies in different solvents at room temperature. The
corresponding spectra are displayed in Figure 3 and their relevant photophysical data are
summarized in Table 1. All T1-T4 compounds exhibit a higher energy band around 200 – 300 nm
with high molar extinction coefficient ( 4 - 6  104 M-1 cm-1) arising from the π → π*
transition and a lower lying band around 350 - 400 nm with relatively much weaker molar
extinction coefficient ( 103 M-1 cm-1) that can be attributed to the intramolecular charge
transfer transition from the donor unit to the acceptor fragment as envisaged in literature.[13d,
13e]
We then performed the time-dependent density functional theory (TD−DFT) calculations in
aim to gain insight into the basic photophysical properties. All pertinent assignments of the
lowest lying singlet S0 → S1 optical absorption transitions for T1 - T4 are listed in Table S2 - S5,
Figure 4 and Figure S2 - S5 (Supporting Information). As a result, the S0 → S1 transitions are
assigned to HOMO → LUMO+1 for T1 and T3 and HOMO → LUMO for T2 and T4. The electron
density distribution of HOMO is mainly localized at the planar dimethyl acridine fragments for
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all compounds, while the electron density distribution of LUMO+1 (for T1 and T3) and LUMO
(for T2 and T4) is localized at the central phenylene and the diphenyl pyrimidine fragments.
Therefore, the S0 → S1 transition for T1 - T4 is unambiguously assigned to the charge transfer
transition, in which HOMO and LUMO+1 (for T1 and T3) or LUMO (for T2 and T4) are spatially
separated by the dimethyl acridine and the phenylene substituent. The dihedral angles
between the dimethyl acridine and the central phenylene substituent are calculated to be 90°,
firmly supporting the orthogonal configuration (Figure 4). Because the calculated oscillator
strengths are rather small (10-4), in a result, the S0 → S1 transition is expected to reveal a
charge-transfer character with small absorptivity, consistent with the rather weak low-lying
absorption band observed experimentally (vide supra).[18]
The photoluminescence spectra of all titled compounds reveal significant solvent
dependent luminescence, i.e., solvatochromism, leading to a trend of bathochromic shift upon
increasing the solvent polarity from toluene to CH2Cl2 and CH3CN. For example, the emission
maximum of T2 appears at 473 nm in toluene, which is red shifted by 93 nm and 142 nm in
CH2Cl2 and CH3CN, respectively. Similar solvent dependent excited-state behavior was observed
for T1, T3 and T4. It has been well established that solvatochromism is related to the large
change of dipole moment in the electronically excited state versus that of the ground state,
supporting the observed luminescence originating from the intramolecular charge transfer (ICT)
transition,[19] which is the prerequisite characteristic to the TADF molecules.
The TADF behavior can be evidenced indirectly by the changes of steady state emission in
aerated solution versus that in degassed solution. The delay fluorescence resulting from the T1
 S1 reverse intersystem crossing should be drastically quenched by T1 to dioxygen energy
transfer process. Accordingly, the photoluminescence quantum yields (PLQY) of these
compounds in both aerated and degassed conditions were measured and the data are
summarized in Table 1. Apparently, except for T1 and T3 in toluene (vide supra), the results
shown in Table 1 reveal a significant increase in quantum yields under degassed conditions. For
instance, PLQY of the charge transfer emission for T1, T2, T3 and T4 in degassed CH2Cl2 were
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recorded to be 63%, 43%, 80% and 88 % versus 14%, 16%, 25% and 25%, respectively,
measured in aerated condition, supporting their potential TADF property. In a direct manner,
the transient photoluminescence decay dynamics was then measured in different solutions to
analyze their TADF behaviors. Figures 5 and 6 display the PL decay profiles in the both degassed
toluene and CH2Cl2, and their corresponding lifetime data are listed in Table 1. For example, the
emission of T2 in degassed CH2Cl2 exhibited two single-exponential decay kinetics with time
constants fitted to be 25 ns and 2.5 µs. Upon aerating the solution, in sharp contrast, the
emission revealed only single exponential decay kinetics with a time constant of 18 ns (Figure
S6). Therefore, the 2.5 s long decay component is drastically quenched by O2, confirming its
origin from the triplet state. Also, the required low excitation intensity of TSPC eliminates the
possibility of the long-lived emission from triplet-triplet (T-T) annihilation. Therefore, the
assignment of the 2.5 s decay component to TADF is unambiguous. Accordingly, the 25 ns
decay component can be assigned to the prompt fluorescence.
Except for T1 and T3 that did not show double exponential decay in degassed toluene,
similar TADF character, i.e. the prompt and long decay component, was observed for all other
compounds in the studied solvents. The lack of TADF for T1 and T3 in toluene from both steady
state and kinetic measurements may be rationalized by their lower charge transfer ability than
that of T2 and T4. As elaborated in the cyclic voltammetry measurement (Table S7 in SI), the
HOMO energy, within experimental error, was measured to be about the same (-5.34 eV), while
the LUMO energy is T1 (-2.45 eV) > T2 (-2.48 eV) and T3 (-2.37 eV) > T4 (-2.40 eV). Therefore,
the charge transfer T2 and T4 seems to be more exergonic than that for T1 and T3, respectively.
Recent experimental evidence in the literature supports the involvement of local triplet excited
3
3
states from the donor or acceptor units, D or A, other than the triplet CT in the reverse
intersystem crossing (rISC). Therefore, the reverse intersystem crossing (rISC) may not occur
between 1CT and 3CT but between 1CT and 3A/3D.[20] Accordingly, we reason that for T1 and T3,
3
due to their lower charge transfer ability, A/3D may be below 3CT in low polarity solvent such
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3
1
as toluene. In this case, the A/3D → CT rISC is inefficient. Conversely, in high polarity solvent
such as CH2Cl2, 3CT is lower than 3A/3D and 3CT → 1CT rISC is rather efficient.
In theory, under the assumption of pre-equilibrium between S1 and T1 states, the difference
in energy ET-S (defined as T1 minus S1) can be derived according to the relationship between
ET-S and S1-T1 equilibrium constant Keq expressed as ET-S = -RTln(Keq/3) where the equilibrium
constant Keq can be deduced from the pre-exponential factors for prompt fluorescence versus
delay fluorescence decay components. A factor of 3 stands for the triplet degenerate states,[21]
ET-S was then deduced to be ‒2.56, ‒1.79, ‒3.09 and ‒1.94 kcal/mol for T1, T2, T3 and T4 in
CH2Cl2, respectively. For organic molecules, the TADF efficiency is mainly limited by the
nonadiabatic electronic state coupling matrix associated with intersystem crossing. A designed
strong charge-transfer character with small overlap of the frontier orbitals not only minimizes
ET-S, but also induces a mixing of the singlet and triplet states and thus enhances intersystem
crossing for a high TADF efficiency. From our molecule design, comparing T1 and T2, the central
pyrimidine acceptor of T1 is expected to countervail the electron withdrawing ability.
Comparing the pair of compounds T1/T3 and T2/T4, the additional o-methyl group on T3 and T4
showed the slightly increased electron donation and reduced π-conjugation to the attached
phenyl substituent, i.e. the reduced charge transfer ability. Overall, it can be speculated from
the steady state and lifetime measurements that the derivative T2 has significant
solvatochromism property and the smallest energy gap. As can be seen, the charge transfer
ability follows the order of T2  T4  T1  T3, consistent with early conclusion.
2.4 The solid state property
Prior to present the TADF behavior for T1 - T4 in solid state, an intriguing phenomenon,
namely the mechanochromism, is worth to report.[22] Herein, we take T2 as an example for
detailed elaboration. The single crystal of T2 exhibits an emission band with peak maximum at
433 nm (Figure 7). Upon applying the mechanical force such as grinding, as shown in Figure 7,
the maximum is red shifted to 483 nm indicating the alternation of solid packing arrangement
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and hence the electronic transition upon changing the morphology of solids. Such a change of
emission can be visualized by the naked eyes. Figure 8 shows the photographic images of the
luminescence color of T2 in response to mechanical grinding with a mortar and pestle.
Apparently, upon UV lamp (366 nm) excitation the emission of the crystal sample (before
grinding) appears to be blue, whereas the emission of the ground powder turns out to be prone
to cyan. The phenomenon is much more dramatic when T2 is sublimed, for which the emission
peak wavelength is completely shifted to 490 nm such that the luminescence appears to be
greenish. Upon re-dissolving the ground powder or sublimed sample in solvent such as toluene,
followed by slow recrystallization, the green-emitting sample is then recovered back to the blue
emitting sample.
To gain further insight into the associated mechanochromism we then performed T2
transient PL decays of these three solid samples (see Figure S7 and Table S6). As for the
crystalline sample, upon monitoring at the deep-blue region of 430 nm, the fluorescence
exhibits solely single exponential decay kinetics with a lifetime fitted to be 5.1 ns. In stark
contrast, the emission monitored at the long wavelength region of e.g. 550 nm reveals multiple
decay components, consisting of a fast decay component with a lifetime of 5.1 ns, a relatively
slow decay of 219 ns and a much longer decay component of 8.9 µs. The 5.1 ns decay
component is similar to that monitored at 430 nm and can be attributed to the same 430 nm
band that tails down to 550 nm. Accordingly, we ascribe the 219 ns and 8.9 µs components to
the same fluorescence band that possesses TADF property. Due to the small pre-exponential
value, the long wavelength band should not be obvious in the steady state measurement, as
seen in Figure 7 where almost only a blue 435 nm band appears for T2 in the crystalline form. In
the ground powder this long-wavelength band appears around 490 nm (Figure 7), which is also
supported by the increase of 219 ns and 8.9 µs components (see Figure S7). In the sublimed
sample, shown in Figure S7, the emission kinetics is dominated by the 219 ns and 8.9 µs
components, clearly revealing the dominant TADF property. In other words, the sublimed
sample exhibits a blue-greenish 490 nm fluorescence that has thorough TADF property. Note
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that sublimation was also the method used in the fabrication of vacuum deposited OLEDs (vide
infra). Mechanochromism is also observed for T1, T3 and T4 and similar excitation wavelength
dependent relaxation dynamics with that of T2 are resolved, which are depicted in Figure S8
with pertinent data summarized in Table S6.
According to the results and discussion elaborated above, it is reasonable to propose
the existence of two kinds of structure alignment in solid state, for which the intermolecular
interaction plays a key role to account for the observed mechanochromism and hence different
photophysics. To gain more insight into the associated intermolecular interaction, we have then
carefully examined the crystal packing of T1 and T2. As depicted in Figure S9, the crystal
structure of T1 and T2, which contains four molecules in the unit cell, it belongs to the
monoclinic crystal system and with space group P21/C. There exist a pair of molecules that
adopt a head-to-head stacking and with a close π-π intermolecular interaction showing a
distance in the range of 3.5 - 3.8 Å. This H-type like interaction (the h aggregation), in theory,
should lead to a blue shift of the lowest lying transition in both singlet and triplet states.[23]
Moreover, the π-π stacking interaction may enhance theπ-electron correlation and hence
increase the electron exchange energy. The net result is to increase the singlet-triplet (S1-T1)
energy gap such that the T1  S1 is thermally inaccessible and TADF is no longer observed. The
other packing adopts a loosely head-to-tail alignment via perhaps the weak C-H∙∙∙π interactions.
This loosely packing mode has negligible effect to the electronic property of T1 - T4 per se,
which then exhibits TADF behavior similar to that observed in the solution.[24]
Upon exerting the mechanic force to the crystal such as grinding, the well aligned h-like
dimers collapse, in part, and become loosely packed, or both packing modes even turn to the
amorphous structure under a more radical scenario. Accordingly, grinding T1 and T2 increases
the single molecule entity free from strong intermolecular interaction and hence increases the
contribution of long-wavelength emission band with strong TADF property. Especially, the
sublimation of T1 - T4 causes the fast condensation and hence the formation of amorphous
structure, resulting in the dominant TADF property, which is what we observed in the OLED
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fabrication, particularly when T1 - T4 films are prepared by sublimation. Detail is elaborated
below.
2.5 Electrochemical properties and OLED fabrication
For the application of OLED, the information of electrochemical properties is necessary,
which was scrutinized by using cyclic voltammetry measurements and the voltammograms. The
results are depicted in Figure S10 and the corresponding numerical data are listed in Table S7.
All the compounds exhibit reversible oxidation process in the range of 0.53  0.54 V and
reduction process lies between 2.32 and  2.43 V versus FcH+/FcH couple, for which the small
variation of oxidation potential is due to the presence of same donor fragment in all
compounds. The insertion of methyl group to T1 and T2 slightly increases their reduction
potential by 80 mV for T3 and T4, respectively.
Electroluminescent properties of the four materials were investigated with a device
configuration of ITO/TAPC (50 nm)/mCP:10 wt% dopant (15 nm)/DPEPO:10 wt% dopant (15
nm)/TmPyPB (60 nm)/LiF (1 nm)/Al (100 nm). As shown in Figure 9a and Table 2, these devices
show similar turn-on voltages of 3.0 - 3.2 V. Figure 9b shows their EQE vs. luminance. T1 shows
a maximum EQE of 7.2% in device, whereas T2, T3 and T4 based OLEDs deliver higher EQEs of
11.8 - 12.6%. The EQE of T2 based device could be improved to 14.2% by using an emitting layer
of 12 nm. Considering that OLEDs based on conventional fluorescent emitters will deliver
maximum EQEs of 5 - 7.5% assuming an optical out-coupling efficiency of 20 - 30%,[25] the high
EQEs (> 10%) of the T2, T3 and T4 based OLEDs suggest the contribution of the triplet excitons
that are thermally repopulated back to the singlet manifold to account for the enhanced light
emission. The finding confirms the existence of TADF in these materials. Figure 9c reveals EL
spectra of the OLEDs, which show that EL peaks for T1, T2, T3 and T4 are 472, 492, 464 and 476
nm, respectively. The results indicate that all four materials are efficient blue emitters. By
inserting an orange-red phosphorescent emitting layer of DPEPO:4 wt% Ir(2-phq)2acac (5 nm)
into the blue OLEDs, where Ir(2-phq)2acac is bis(2-phenylquinoline)(acetylacetonate)
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iridium(III),[26] four white OLEDs were fabricated. As shown in Figure 10 and listed in Table 2,
the white OLEDs turned on at ca. 3.0 V. High EQEs over 10% have been realized in these devices.
3. Conclusion
In summary, we report the design and synthesis of a new series of T1 - T4 molecules
bearing dimethyl acridine and pyrimidine as the electron donor and acceptor, respectively. In
comparison to the ubiquitous triazine acceptor, the selection of pyrimidine as an acceptor is
due to its less stabilized unoccupied π-orbitals and facile functionalization. The resulting D-A
functional materials reveal remarkable TADF properties in solution, among which T2 exhibited
the smallest ET-S, which could be due to the asymmetric arrangement of pyrimidine nitrogen
atoms. The crystal structures, together with the intriguing mechanochromism observed for T1 -
T4, lead us to propose two type of structure arrangement that give distinct emission properties,
showing emission lifetimes in both the ns and μs regime. OLEDs based on T1 - T4 exhibit
dominant TADF behavior; in particular, the device incorporating T2 as the emitter delivers blue
emission with a high EQE of 14.2%. By inserting an orange-red phosphorescent emitting layer
into the blue devices, the white OLEDs with turn-on voltage ca. 3.0 V and EQEs over 10% were
realized. These results suggest pyrimidine (A)/acridine(D) hybrids as promising materials for
mechanochromism and OLEDs.
Supporting information. Details of synthetic procedures, theoretical calculation,
photophysical and electrochemical measurements, single crystal X-ray diffraction studies and
procedures for OLED device fabrication.
Acknowledgments. The authors gratefully acknowledge the financial support from
Ministry of Science and Technology of Taiwan.
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5.15
21.93
15.28
88.76
Figure 1. Structural drawing of T1 with ellipsoids shown at the 30% probability level. The curved
arrows depict the dihedral angle between the adjacent rings.
19.41
85.49
6.05
17.87
Figure 2. Structural drawing of T2 with ellipsoids shown at the 30% probability level. The curved
arrows depict the dihedral angle between the adjacent rings.
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8
6
4
2
1.2
0.8
0.4
T1
T2
0
0.0
1.2
300
400
400
400
500
500
500
600
600
600
700
700
700
800
6
0.8
0.4
0.0
4
2
0
300
800
1.2
0.8
0.4
0.0
6
4
2
T3
T4
0
300
800
800
6
4
2
1.0
0.5
0.0
0
300
400
500
600
700
Wavelength (nm)
Figure 3. Absorption and emission spectra of a) T1, b) T2, c) T3 and c) T4, in toluene (blue),
CH2Cl2 (red), acetonitrile (brown) and solid state (black) at 298 K.
‒ 15 ‒
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T1: HOMO → LUMO+1, 88.91°
T2: HOMO → LUMO, 90.38°
T3: HOMO → LUMO+1, 89.97°
T4: HOMO → LUMO, 89.99°
Figure 4. Frontier molecular orbitals pertinent to the lowest optical transitions for T1-T4 in
CH2Cl2. Pink and green meshes represent HOMO and LUMO (or LUMO+1), respectively. The
calculated optimized dihedral angles between the planar dimethyl acridine to the central
phenylene substituent are also displayed.
‒ 16 ‒
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10.1002/chem.201604883
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T1 in degassed toluene
IRF
fitting
T1 in degassed DCM
IRF
fitting
10000
1000
100
10000
1000
100
10
10
0
10000
20000
Time (ns)
30000
40000
0
10000
20000
Time (ns)
30000
40000
10000
1000
100
10000
1000
100
T2 in degassed DCM
IRF
Fitting
T2 in degassed toluene
IRF
fitting
10
10
0
10000
20000
30000
40000
0
2000 4000 6000 8000 10000
Time (ns)
Time (ns)
Figure 5. Transient photoluminescence characteristics of T1 (upper) and T2 (lower) in degassed
toluene and CH2Cl2 (ex = 355 nm).
10000
1000
100
10
10000
1000
100
10
T3 in degassed DCM
IRF
fitting
T3 in degassed toluene
IRF
fitting
1
1
0
10000
20000
30000
40000
0
10000
20000
30000
40000
Time (ns)
Time (ns)
10000
1000
100
10
10000
1000
100
10
T4 in degassed toluene
IRF
fitting
T4 in degassed DCM
IRF
fitting
1
1
0
10000
20000
30000
40000
0
5000
10000
15000
20000
Time (ns)
Time (ns)
Figure 6. Transient photoluminescence characteristics of T3 (upper) and T4 (lower) in degassed
toluene and CH2Cl2 (λex = 355 nm).
‒ 17 ‒
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T2 Crystal
T2 Powder
T2 Subimed
T2 Ground
1.0
0.8
0.6
0.4
0.2
0.0
400
450
500
550
600
650
700
Wavelength (nm)
Figure 7. Photoluminescence spectra of T2 in single crystal (black), powder (red), sublimation
state (blue) and ground state (green) respectively.
Figure 8. Photographic images of the color and luminescence changes of T2 in response to
mechanical grinding: (a) and (c) unground sample, (b) and (d) ground sample. Photographs
were taken under ambient light and UV irradiation (365 nm).
‒ 18 ‒
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0
2
4
6
8
10
12
104
(a)
(b)
100
10
10
103
102
101
100
1
0.1
0.01
1
T1
T2
T3
T4
1E-3
1E-4
1E-5
1E-6
T1
T2
T3
T4
0.1
100
101
102
103
104
0
2
4
6
8
10
12
Luminance (cd m-2)
Voltage (V)
(c)
1.0
0.8
0.6
0.4
0.2
0.0
T1
T2
T3
T4
400
500
600
700
800
Wavelength (nm)
Figure 9. (a) Current density-voltage-luminance, (b) external quantum efficiency-luminance
plots and (c) EL spectra of the blue OLEDs.
‒ 19 ‒
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10.1002/chem.201604883
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0
2
4
6
8
10
(a)
(b)
104
103
102
101
100
100
10
10
1
1
0.1
T1
T2
T3
T4
0.01
1E-3
1E-4
1E-5
1E-6
T1
T2
T3
T4
0.1
100
101
102
103
104
0
2
4
6
8
10
Luminance (cd m-2)
Voltage (V)
Figure 10. (a) Current density-voltage-luminance and (b) external quantum efficiency-
luminance plots of the white OLEDs. The device structure of the white OLEDs consists of a
multilayer structure of ITO/TAPC (50 nm)/mCP:10 wt% dopant (15 nm)/DPEPO:10 wt% dopant
(15 nm)/DPEPO:4 wt% Ir(2-phq)2acac (5 nm)/TmPyPB (60 nm)/LiF (1 nm)/Al (100 nm), where
dopant is T1 - T4, respectively.
‒ 20 ‒
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Table 1. Selected photophysical properties of T1, T2, T3 and T4 in toluene, CH2Cl2 and CH3CN at
RT.
compound
(solvent)
ET-S
kisc;
CT / nm Q.Y. / %
 (pre-exp factor)
[kcal/mol]
krisc (s-1)
T1 (toluene)
471
24.3,[a] 33.9[b]
6.48ns (-)
18.2ns (0.9956),
5.49107;
2.43105
T1 (CH2Cl2)
T1 (CH3CN)
T2 (toluene)
525
575
473
14.0,[a] 63.2[b]
1.0,[a] 1.8[b]
-2.56
16.5s (0.0044)
4.23ns (0.9997),
78.5s (0.0003)
25.0ns (0.9839),
2.52s (0.0161)
2.36108;
6.22104
3.93107;
6.42105
16.0,[a] 33.6[b]
-4.23
-1.79
T2 (CH2Cl2)
566
15.74,[a] 42.83[b]
T2 (CH3CN)
615
469
4.4,[a] 7.4[b]
T3 (toluene)
24.8,[a] 38.5[b]
8.76ns (-)
24.3ns (0.9982),
24.5s (0.0018)
4.10107;
7.42104
T3 (CH2Cl2)
T3 (CH3CN)
T4 (toluene)
517
563
479
25.4,[a] 79.6[b]
4.9,[a] 12.5[b]
-3.09
2.76ns (0.9980),
61.5s (0.0020)
22.7s (0.9875),
3.79s (0.0125)
3.62108;
7.39105
4.35107;
5.51105
21.31,[a] 77.75[b]
-3.02
-1.94
T4 (CH2Cl2)
T4 (CH3CN)
558
605
24.65,[a] 88.39[b]
4.9,[a] 10.1[b]
[a] those measured in aerated solution, and [b] measured in degassed solution.
‒ 21 ‒
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Table 2. The EL data of the OLEDs using T1 - T4 as the emitters.
Von
(V)
3.0
3.2
3.0
3.0
3.0
3.0
3.0
3.1
3.1
Lmax
(cd/m2)
2868
7606
7385
1269
1867
3458
5401
1727
2969
EQE (%)[c]
ηc
ηp
CIE(x,y)[d]
(cd∙A-1)[c]
13.4/10.4
30.4/30.3
34.2/28.1
18.8/6.5
24.0/21.0
18.5/12.2
23.8/18.9
20.3/9.5
29.3/21.4
(lm∙W-1)[c]
12.0/8.2
23.2/22.4
29.8/21.0
19.6/4.7
21.5/15.5
19.4.8.5
T1[a]
T2[a]
7.2/5.6
12.6/12.6
14.2/11.5
11.8/4.2
11.8/10.3
10.7/7.1
11.3/9.0
11.9/5.7
14.3/10.6
(0.17,0.27)
(0.21,0.39)
(0.20,0.39)
(0.17,0.21)
(0.18,0.30)
(0.34,0.30)
(0.34,0.37)
(0.33,0.28)
(0.30,0.34)
T2[b]
T3[a]
T4[a]
T1[a] (white)
T2[b] (white)
T3[a] (white)
T4[a] (white)
24.9/13.8
21.2/6.1
26.3/15.2
[a] EMLs: mCP:10 wt% dopant (15 nm)/DPEPO:10 wt% dopant (15 nm).
[b] EMLs: mCP:10 wt% dopant (12 nm)/DPEPO:10 wt% dopant (12 nm).
[c] Efficiencies at maximum and 100 cd/m2.
[d] At 100 cd/m2.
‒ 22 ‒
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