Comparative study of multi-functional luminogens with 1,3,5-triazine as the core and phenothiazine or phenoxy donors as the peripheral moieties for non-doped/doped fluorescent and red phosphorescent OLEDs

Article abstract of DOI:10.1016/j.dyepig.2019.107793

A series of three-armed fluorescent emitters based on a 1,3,5-triazine acceptor as the central core and phenothiazine or phenoxy moieties as the different peripheral units, are designed and synthesized. All of them exhibit aggregation-induced emission enhancement and thermally activated delayed fluorescence. Photoluminescence quantum yields exceeded 37 and 67% for non-doped and doped solid-state samples respectively. The compounds were found to be capable of transporting both holes and electrons. The highest mobility values of more than 1 × 10^?4 cm²V^?1s^?1 were obtained for the compound having three phenothiazine moieties at electric field of more than 1 × 10⁶ Vcm^?1. Additionally, three types of organic light emitting diodes based on these materials were fabricated. The device containing non-doped emission layer with the compound containing single phenothiazine moiety as the emitter gave maximum external quantum efficiency of 5.4%. The device with doped emission layer containing 5% solid solution of the compound having three phenothiazine moieties in an appropriate host gave maximum external quantum efficiency of 9.9%. The phosphorescent device hosted by the compound with single phenothiazine unit exhibited maximum external quantum efficiency of 10.3% and a low-efficiency roll-offs of 1% at 1000 cd/m².

Full text of DOI:10.1016/j.dyepig.2019.107793

DyesandPigments173(2020)107793
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Comparative study of multi-functional luminogens with 1,3,5-triazine as the
core and phenothiazine or phenoxy donors as the peripheral moieties for
non-doped/doped fluorescent and red phosphorescent OLEDs
Xiao-Feng Tana,b,1, Pei-Pei Wanga,1, Ling Lua, Oleksandr Bezvikonnyib,c, Dmytro Volyniukb,
Juozas Vidas Grazuleviciusb,∗, Qing-Hua Zhaoa,∗∗
a College of Materials Science and Engineering, Huaqiao University, Xiamen, 361020, China
b Department of Polymer Chemistry and Technology, Kaunas University of Technology, K. Baršausko g. 59, Kaunas, Lithuania
c Experimental Physics Department, Faculty of Physics, Taras Shevchenko National University of Kyiv, pros. Akademika Glushkova 4b, Kyiv, Ukraine
A B S T R A C T
A series of three-armed fluorescent emitters based on a 1,3,5-triazine acceptor as the central core and phenothiazine or phenoxy moieties as the different peripheral
units, are designed and synthesized. All of them exhibit aggregation-induced emission enhancement and thermally activated delayed fluorescence.
Photoluminescence quantum yields exceeded 37 and 67% for non-doped and doped solid-state samples respectively. The compounds were found to be capable of
transporting both holes and electrons. The highest mobility values of more than 1 × 10−4 cm2V−1s−1 were obtained for the compound having three phenothiazine
moieties at electric field of more than 1 × 106 Vcm−1. Additionally, three types of organic light emitting diodes based on these materials were fabricated. The device
containing non-doped emission layer with the compound containing single phenothiazine moiety as the emitter gave maximum external quantum efficiency of 5.4%.
The device with doped emission layer containing 5% solid solution of the compound having three phenothiazine moieties in an appropriate host gave maximum
external quantum efficiency of 9.9%. The phosphorescent device hosted by the compound with single phenothiazine unit exhibited maximum external quantum
efficiency of 10.3% and a low-efficiency roll-offs of 1% at 1000 cd/m2.
1. Introduction
unoccupied molecular orbital (LUMO) of molecules [5]. In most cases,
donor-acceptor systems or conjugation-breaking molecular structures
are used to obtain small ΔEST for TADF emitters [6]. As for kr, it is
related to the overlapping density (ρ10) between the electronic wave
functions of the ground state (S0) and the lowest excited singlet state
(S1) [7]. Usage of phenyl linkage or multiple donor/acceptor systems
can increase the overlapping density and allow to achieve high EL ef-
ficiency [8]. Specifically, extending the molecular orbitals while lim-
iting the overlap between HOMO and LUMO can suppress a decrease in
kr while lowering ΔEST [6].
Most of the pure organic TADF molecules are of donor-acceptor
(D–A) type with the twisted structures or big steric hindrance between
the donor and acceptor units [9], such as derivatives of triazine
[10,11], thioxanthone [12], oxadiazole [13] and sulfones [14]. In
general, most of the discovered TADF molecules require to be doped
into some rigid host materials with high triplet energy levels [15].
However, majority of TADF materials commonly suffer from aggrega-
tion-caused quenching in the condensed state, which makes EL device
structures more complicated due to the requirement of doping of
Thermally activated delayed fluorescence (TADF) has attracted
significant attention because of easily activated reverse intersystem
crossing (RISC) by the thermal motion of atoms [1]. The real break-
through of harvesting both singlet and triplet excitons, as well as an
internal quantum efficiency (IQE) of 100% for organic light emitting
diodes (OLEDs), was achieved by Adachi et al., in 2011 [2]. An ideal
TADF material needs to have at least two important features: (i) a large
RISC rate constant (krisc) for better utilization of non-emissive triplet
excitons; (ii) a relatively large radiative rate constant (kr) for achieving
high electroluminescence (EL) efficiency [3]. However, the large krisc
and kr sometimes are conflicting, which make the design of TADF
molecules knotty and difficult. Numerous theoretical researches dis-
closed that krisc is inversely proportional to the energy gap (ΔEST) be-
tween singlet excited state (S1) and triplet excited state (T1) [4]. The
interaction of electron-donating and electron-accepting moieties re-
sulting in intramolecular charge transfer leads to small ΔEST by spatially
separating highest occupied molecular orbital (HOMO) and lowest
∗ Corresponding author.
∗∗ Corresponding author.
E-mail address: juozas.grazulevicius@ktu.lt (J.V. Grazulevicius).
1 Xiaofeng Tan and Peipei Wang share the same contribution for this article.
https://doi.org/10.1016/j.dyepig.2019.107793
Received 29 March 2019; Received in revised form 18 June 2019; Accepted 12 August 2019
Availableonline05September2019
0143-7208/©2019PublishedbyElsevierLtd.

X.-F. Tan, et al.
DyesandPigments173(2020)107793
emitter molecules in some properly selected host materials [16]. In
2001, Tang et al. [17,18][] demonstrated the mechanism of aggrega-
tion-induced emission (AIE) in which light emission can be dramati-
cally enhanced in the condensed phase. After that, numerous AIE lu-
minophores have attracted extensive attention for their potential
application in OLEDs [19,20] and fluorescent molecular sensors
[21,22]. Therefore, it is of great interest to obtain enhanced light
emission in the condensed phase and generate efficient OLEDs with
simple non-doped device structures, if TADF materials also posses AIEE
ability. Based on the design concept, several groups have reported the
successful cases of the combination of prominent TADF and AIEE
properties [23]. Recently, Tang et al. [24] synthesized a new tailor-
made material DCPDAPM based on carbazole as the skeleton, with 9,9-
dimethyl-9,10-dihydroacridine as the donor group and benzophenone
as the acceptor group. The non-doped device with the material gave a
maximum external quantum efficiency (EQE) of 8.15%, and doped
device with DCPDAPM doped into CBP exhibited a maximum EQE of
19.67%. Among the reported AIEE-TADF materials, only a few linear or
two-dimensional molecule structures have been investigated [23].
Thus, the development of new AIEE-TADF materials with different
types of structures is of great interest.
In this work, a series of three-armed AIEE-TADF materials based on
a 1,3,5-triazine acceptor as the core and a phenothiazine donor or a
phenoxy donor as the different peripheral units, were developed for use
in OLEDs. Triazine (TRZ) was widely used as the electron-withdrawing
moiety of D–A materials because of its high electronic deficiency and
three scattered modifiable positions [25]. Phenothiazine moiety linked
the triazine core via phenyl bridge or phenoxy group linked the triazine
core via oxygen atom were selected as the different peripheral units to
constitute AIEE-TADF materials. The effect of the different peripheral
units on the properties of obtained materials were explored in detail.
Compared with the common carbazole donor, phenothiazine moiety
brings more steric repulsion because of its morpholine-like six-mem-
bered ring and can better localize the electron density distribution of
the HOMO onto the donor unit [26]. Additionally, phenoxy unit is also
beneficial for realizing the TADF character for the materials, as it was
previously reported [27].
electron distribution [28]. It is obvious that the S1 states of all three
compounds are pure CT states and the transition can be well re-
presented by a pair of natural transition orbitals. However, since holes
and electrons are almost entirely localized on phenothiazine and tria-
zine units, the transition from hole to electron is hard to be noticed,
because the oscillator strength is close to zero. In order to analyze the
quantitative characterization of hole and electron distribution differ-
ences, two parameters were introduced. Sr index was used to char-
acterize the overlapping extent of hole and electron, while t index was
designed to measure separation degree of hole and electron in CT di-
rection. The two parameters are written as:
S index = S (r)dr ≡
ρhole (r)ρele (r)dr
∫
∫
r
r
t index = D index − HCT
where ρhole and ρele refer to the density of hole and electron, D index
refers to the total magnitude of CT length, HCT refers to the average
degree of spatial extension of hole and electron distribution in CT di-
rection [28,29]. PTZ-TROZ showed the largest Sr index, which means
that PTZ-TROZ has the best overlapping of hole and electron. Bis-PTZ-
TROZ showed the largest t index which implies that bis-PTZ-TROZ has
the best separation of holes and electrons which was allowed to predict
smaller ΔEST [30,31].
2.4. Electrochemical properties
HOMO and LUMO of the molecules can be estimated by measuring
ionization potentials (IP) and electron affinities (EA) by cyclic vol-
tammetry (CV), since they have same absolute values. CV curves of
PTZ-TROZ, bis-PTZ-TROZ, and tri-PTZ-TROZ (Fig. 3) exhibited an
oxidation peak with the onset oxidation potential (EOX) at 0.75 eV,
0.73 eV, 0.68 eV (Table 1). The values of IP were calculated to be
5.55 eV, 5.53 eV, 5.48 eV, respectively from the empirical formula
E
IP = Eox+ 4.8 eV. The energies of EA were obtained to be 2.71 eV,
2.68 eV, 2.61 eV, respectively from the empirical formula EEA = EIP
opt, where Eopt is an estimate of the energy of the first singlet excited
-
E
state measured from the onset position of their absorption spectra from
Fig. 4a.
2. Results and discussions
2.5. Photophysical properties
2.1. Synthesis
UV–vis absorption and photoluminescence (PL) spectra of toluene
solutions and vacuum deposited films of the studied compounds are
plotted in Fig. 4a and S1. Summary of the main photophysical prop-
erties of PTZ-TROZ, bis-PTZ-TROZ, and tri-PTZ-TROZ are presented in
Tables 2 and 3. All three compounds show π-π* transition absorption
band with the maximum at 256 nm and ICT absorption bands at 354,
368 and 370 nm for PTZ-TROZ, bis-PTZ-TROZ, and tri-PTZ-TROZ re-
spectively. The red-shift of ICT absorption bands with the increasing
number of PTZ donor units can prove that the ICT was stabilized and
delocalized over the PTZ donor units and TRZ core unit. With the in-
crease of solvent polarity, significant red shifts of the emission peaks
occurred (Fig. S2), which indicate that the emission originated from the
intramolecular charge transfer state. In PL spectra of the films, the
fluorescence intensity maxima wavelengths are red-shift with the in-
crease of the number of PTZ donor units from 551 nm for PTZ-TROZ to
557 nm for bis-PTZ-TROZ and 563 nm for tri-PTZ-TROZ. The en-
hancement of the donor property by increasing number of PTZ donor
units induced the delocalization of the ICT state over the D-A mole-
cules. Such stabilization of the ICT states leads to a red-shift of ICT
emission. The half-widths of emission bands of the films and solutions
showed no big difference. This observation is apparently the result of
two factors competing with each other: (i) strengthening of ICT caused
by the increasing number of PTZ donor units and (ii) weakening of
vibrational motion caused by the decreasing number of phenoxy units.
Dilute toluene solutions of compounds PTZ-TROZ, bis-PTZ-TROZ,
Synthesis of PTZ-TROZ, bis-PTZ-TROZ, and tri-PTZ-TROZ is shown
in Scheme 1. 1,3,5-triazine-based compounds can be easily synthesized
by Suzuki coupling reaction. The detailed synthesis procedures are
given in the experimental section. The target compounds were char-
acterized by 1H and 13C NMR, and high-resolution mass spectrometry.
2.2. Thermal characterization
Thermogravimetric analysis (TGA) and differential scanning ca-
lorimetry (DSC) measurements in a wide range of temperature were
performed for the samples of PTZ-TROZ, bis-PTZ-TROZ, and tri-PTZ-
TROZ. The results are collected in Fig. 1 and Table 1. With the in-
creasing number of PTZ units, 5% weight loss temperatures of the
compounds increased in the range of 328 °C, 365 °C, 421 °C (Fig. 1a).
Similarly, DSC curves showed the increase of glass transition tem-
peratures in the range of 75 °C, 95 °C, 114 °C (Fig. 1b). The thermal
stability of PTZ-TROZ, bis-PTZ-TROZ, and tri-PTZ-TROZ gradually im-
proved due to the increase of rigid phenothiazine units.
2.3. Theoretical studies
Natural transition orbitals (Fig. 2a) and hole/electron distribution
(Fig. 2b) for the S1 state of the compounds were depicted by Multiwfn
(a multifunctional wavefunction analyzer) to study the hole and
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X.-F. Tan, et al.
DyesandPigments173(2020)107793
Scheme 1. Synthesis of PTZ-TROZ, bis-PTZ-TROZ, and tri-PTZ-TROZ.
and tri-PTZ-TROZ exhibited yellow emission under UV excitation (Fig.
S1). Slight red-shifts of the PL spectra of the solutions were observed
with the increasing number of PTZ substituents (Table 2). PL spectra of
the vacuum-deposited films of PTZ-TROZ, bis-PTZ-TROZ, and tri-PTZ-
TROZ exhibited significant red shifts in comparison to PL spectra of the
toluene solutions apparently due to the higher polarity of the studied
compounds in solid state comparing to that of toluene (ε = 2.38)
(Fig. 4a, Table 2). Such red-shifts are usual for donor-acceptor com-
pounds exhibiting intramolecular charge transfer (ICT) character of
emissions [32].
The deoxygenated solution of PTZ-TROZ, bis-PTZ-TROZ, and tri-
PTZ-TROZ in toluene demonstrated low PLQY values of 4.3, 2.4, and
6.4%, respectively. Small differences of PLQY values of PTZ-TROZ, bis-
PTZ-TROZ, and tri-PTZ-TROZ can be explained by the differences in
their dipole moments induced by the different number of PTZ sub-
stituents. Much higher PLQY values of 37, 24, and 15% were recorded
for vacuum-deposited films of PTZ-TROZ, bis-PTZ-TROZ, and tri-PTZ-
TROZ, respectively (Table 2). The considerable increase in the emission
intensity of the solid state compared to the solution, is usually related to
AIEE phenomenon [33]. The additional evidence indicating that the
emission of PTZ-TROZ, bis-PTZ-TROZ and tri-PTZ-TROZ compounds in
solid state is related to AIEE will be provided in the following sections.
It is worth noting that for the doped layers of PTZ-TROZ, bis-PTZ-TROZ,
and tri-PTZ-TROZ using mCP as the host, approximately twice higher
PLQY values were obtained. This observation can be explained by the
enhancement of TADF efficiency of compounds in the host compared
with that in non-doped layers, due to the possible changes in dihedral
angles between donor and acceptor moieties of PTZ-TROZ, bis-PTZ-
TROZ, and tri-PTZ-TROZ. Similar hosting effects on TADF emitters were
studied elsewhere [34]. In addition, mCP host, which molecular
structure is not appropriate for the hydrogen bonding, may restricts
intermolecular interactions (such as the π-π stacking or hydrogen bonds
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X.-F. Tan, et al.
DyesandPigments173(2020)107793
Fig. 1. TGA (a) and DSC (b) curves of PTZ-TROZ, bis-PTZ-TROZ, and tri-PTZ-TROZ.
[35]) between neighboring molecules in solid state of PTZ-TROZ, bis-
PTZ-TROZ, and tri-PTZ-TROZ, which molecular structures are well
appropriate for the hydrogen bonding since they includes oxygen and
sulfur atoms. As a result, higher PLQY values of 64, 67 and 63% for the
doped films mCP:PTZ-TROZ, mCP:bis-PTZ-TROZ, and mCP:tri-PTZ-
TROZ were obtained in comparison to PLQY values for the non-doped
films PTZ-TROZ, bis-PTZ-TROZ, and tri-PTZ-TROZ, respectively
(Table 2). PLQYs of 28, 33 and 37% for the films of molecular mixtures
DPEPO:PTZ-TROZ, DPEPO:bis-PTZ-TROZ, and DPEPO:tri-PTZ-TROZ,
respectively, were additionally measured using bis[2-(diphenylpho-
sphino)phenyl] ether oxide (DPEPO) as a host molecular structure of
which is favourable for hydrogen bonding. This result partly supports
above described assumption that the restriction of intermolecular in-
teractions can cause PLQY enhancement even for AIEE emitters.
PL spectroscopy measurements were performed for degassed and
non-degassed solutions of the compounds (Fig. 4b and c). PL decay
curves with typical TADF material characteristics were obtained for the
studied compounds PTZ-TROZ, bis-PTZ-TROZ and tri-PTZ-TROZ
(Fig. 4c). In addition, for the deoxygenated toluene solution and films,
an increase in the intensity of the long-lived emission component was
detected, indicating the contribution of the triplet excitons (Fig. 4c and
S8). Fig. 4b and c shows that, with the increasing number of PTZ sub-
stituents, the contribution of delayed fluorescence in the fluorescence
spectra increased. This observation may be explained by the large de-
localization of molecular orbitals in the structures with well-separated
HOMO and LUMO and by increased dipole moment of transition be-
tween S0 and S1. To explain the increasing intensity of the long-lived
emission components, we consider that the increase of PTZ units sta-
bilized the ICT state.
respectively (Table 2). Taking into account the relatively large ΔEST
values, inefficient TADF is expected for the solutions of PTZ-TROZ, bis-
PTZ-TROZ, and tri-PTZ-TROZ confirmed by small PLQY values
(Table 2). With the increase of the number of PTZ units and with the
decrease of the number of phenoxy units in the studied molecules, their
triplet state energy values decreased gradually.
Due to the similarity of fluorescence and phosphorescence spectra,
practically identical PL spectra were recorded for the films of the
compounds at different temperatures (Fig. 5a and S3). In contrast to the
relatively high ΔEST values observed for the THF solutions, small ΔEST
values of 0.07, 0.02, and 0.07 eV were recorded for the solid films of
PTZ-TROZ, bis-PTZ-TROZ, and tri-PTZ-TROZ, which promoted the
generation of TADF (Fig. 5b, Table 2). Therefore, PLQYs of the undoped
films were found to be considerably higher than those of the solutions.
PL decay curves recorded at different temperatures indicate that in-
tensity of the long-lived emission component increases with the de-
creasing temperature. This observation confirms the TADF character of
delayed fluorescence (Fig. 5c and S3).
To analyze the relative contribution of prompt and delayed com-
ponents to the whole relaxation processes, we fitted PL decays of
deoxygenated toluene solutions and of the films of molecular mixtures
PTZ-TROZ:mCP, bis-PTZ-TROZ:mCP, tri-PTZ-TROZ:mCP (Fig. 4b and
S7b). By fitting the formula shown below, the radiation transition rates
of the doped films of the compounds were calculated [36].
η
η
DF
PF
kPF
kDF
=
kISC
=
kPF
τ
η
+ η
PF
PF
DF
k
η
τ
η
⋅ k
DF
DF
DF PF DF
=
kRISC
=
⋅
η
k
ISC
PF
where kPF
,
kDF
,
k
ISC, and kRISC are rate constants of prompt and delayed
To evaluate the singlet-triplet splitting for the tested compounds, PL
and phosphorescence spectra of the solutions in THF were recorded at
77 K (Fig. 4d). From the difference between the onsets of fluorescence
and phosphorescence spectra, ΔEST was determined to be 0.42 eV,
0.38 eV, and 0.43 eV for PTZ-TROZ, bis-PTZ-TROZ, and tri-PTZ-TROZ,
components, intersystem crossing (ISC) and RISC processes, respec-
tively; ηPF and ηDF are prompt and delayed PLQYs which can be dis-
tinguished from the total PLQY by comparing the integrated intensity of
the prompt and delayed components in the transient photo-
luminescence spectra (Table 3).
Table 1
Energies of frontier molecular orbitals and ground/excited states of PTZ-TROZ, bis-PTZ-TROZ, and tri-PTZ-TROZ.
HOMOa (eV)
LUMOa (eV)
b
Sample
Td (°C)
Tg (°C)
Eopt (eV)
Eox (eV)
PTZ-TROZ
bis-PTZ-TROZ
tri-PTZ-TROZ
328
365
421
75
95
114
2.84
2.85
2.87
0.75
0.73
0.71
−5.55
−5.53
−5.48
−2.71
−2.68
−2.61
a
Calculated from the empirical formula EHOMO = −(EOX+4.8), ELUMO = EHOMO-Eopt
Estimated from the onset of the absorption edge of the thin films.
.
b
4

X.-F. Tan, et al.
DyesandPigments173(2020)107793
Fig. 2. Natural transition orbital (a) and hole/electron distribution (b) of PTZ-TROZ, bis-PTZ-TROZ, and tri-PTZ-TROZ.
films exhibited higher TADF efficiency than the solutions (Table 3). The
studied compounds in solid state exhibited relatively high kRISC values
which are comparable with those of highly efficient TADF emitters
[37].
2.6. Aggregation induced emission enhancement
PL spectra of the dispersions of the compounds in the THF: water
systems were recorded for estimation of their AIEE properties [38]. The
most representative experiment to prove AIE characteristics. PTZ-
TROZ, bis-PTZ-TROZ, and tri-PTZ-TROZ showed good solubility in
THF, but they were insoluble in water (Fig. 6 and S4). In contrast to
very weak emission of THF solutions of the compounds, dispersions of
PTZ-TROZ, bis-PTZ-TROZ, and tri-PTZ-TROZ in THF and water mix-
tures exhibited strong emission after formation of aggregates in the
mixtures with high water fraction (> 80%). The AIEE effect was ob-
served for the compounds due to the restriction of rotations and vi-
brations of PTZ and phenoxy units relative to triazine moieties in solid
aggregates. When the molecules are in solution, the peripheral phe-
nothiazine and phenoxy can be freely rotated by a single bond around
the center of the 1,3,5-triazine [39]. This process consumes the excited
state energy in a non-radiative transition, resulting in weak fluores-
cence. However, in the aggregate state, the “propeller” configuration of
the molecule can prevent the π-π stacking and inhibit the fluorescence
quenching. At the same time, due to the space constraints, the in-
tramolecular rotation limitation inhibits the non-radiative transition
Fig. 3. CV curves of PTZ-TROZ, bis-PTZ-TROZ, and tri-PTZ-TROZ.
Because of smaller ΔEST values of the solid samples of the com-
pounds than those of the solutions (Table 2), higher kDF and kRISC by one
order of magnitude were obtained for the films of PTZ-TROZ:mCP, bis-
PTZTROZ:mCP, and tri-PTZ-TROZ:mCP. For this reason, the doped
5

X.-F. Tan, et al.
DyesandPigments173(2020)107793
Fig. 4. UV and PL spectra of vacuum deposited films of PTZ-TROZ, bis-PTZ-TROZ, and tri-PTZ-TROZ (a); PL spectra (b) and emission decay curves (c) of the degassed
and non-degassed solutions in toluene at room temperature; and PL and phosphorescence spectra (d) of the solutions in THF recorded at 77 K. Excitation wavelengths
have to be given for PL spectra.
channel of the excited state, opens the radiation transition channel,
thereby enhancing fluorescence [40,41].
mobilities (μ) were determined by photoelectron emission spectroscopy
and time-of-flight (ToF) measurements, respectively. By extrapolation
of liner part of photoelectron emission spectra of vacuum-deposited
films of PTZ-TROZ, bis-PTZ-TROZ, and tri-PTZ-TROZ to abscissa axis
(Fig. 7), practically identical IPEP values of ca. 5.4 eV were obtained for
the compounds. This observation shows that they are appropriate for
efficient hole-injection from the common ITO anode of the optoelec-
2.7. Charge-injecting and charge-transporting properties
With the purpose to unclose the potential of PTZ-TROZ, bis-PTZ-
TROZ, and tri-PTZ-TROZ for optoelectronic application, charge-in-
jecting and charge-transporting properties of their solid samples were
investigated. The key parameters of organic electroactive materials, i.e.,
ionization potentials (IPEP), electron affinities (EAPE) and charge-drift
PE
tronic devices. Using optical band gaps Eg, close EA values of ca.
2.57 eV were obtained for the samples of PTZ-TROZ, bis-PTZ-TROZ,
and tri-PTZ-TROZ by the equation EA = IPPE-Eg. These values enable to
Table 2
Summary of photophysical properties of PTZ-TROZ, bis-PTZ-TROZ, and tri-PTZ-TROZ.
Sample
λmax,abs (nm)
λmax,PL (nm)
S1a (eV)
T1a (eV)
ΔEST (eV)
PLQY (%)
Toluene/neat film
THF/neat film
Toluene/neat film/mCP doped film
PTZ-TROZ
bis-PTZ-TROZ
tri-PTZ-TROZ
256, 355/261, 363
255, 364/260, 374
256, 365/262, 374
549/585
558/596
562/591
3.21/2.59
3.12/2.49
3.11/2.57
2.79/2.53
2.74/2.47
2.68/2.5
0.42/0.07
0.38/0.02
0.43/0.07
4.9/37/64
2.4/24/67
6.4/15/63
a
To calculate the singlet (S1) and triplet (T1) energy levels of the studied compounds, empiric formula ES = 1239/λS, and ET = 1239/λT was used, where λS and
λT are wavelengths of onsets of fluorescence and phosphorescence spectra (Fig. 4a).
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X.-F. Tan, et al.
DyesandPigments173(2020)107793
Table 3
Rate constants of prompt and delayed components of the deoxygenated toluene solutions and doped films of PTZ-TROZ, bis-PTZTROZ, and tri-PTZ-TROZ.
PF , s−1
k
ISC, s−1
k
DF , s−1
k
RISC, s−1
ηPF
ηDF
Compound
τ
PF , ns
τDF
,
μs
k
In toluene
PTZ-TROZ,
bis-PTZ-TROZ
tri-PTZ-TROZ
In mCP
PTZ-TROZ,
bis-PTZ-TROZ
tri-PTZ-TROZ
8.75 (78%)
5.13 (83%)
12 (72%)
0.3 (22%)
0.26 (17%)
0.36 (28%)
0.038
0.02
0.046
0.011
0.004
0.018
7.8 × 106
8.3 × 106
7.2 × 106
1.75 × 106
1.38 × 106
2 × 106
0.4 × 105
0.15 × 105
0.5 × 105
0.52 × 105
0.18 × 105
0.7 × 105
11 (34%)
7.3 (69%)
8 (33%)
1.1 (66%)
0.8 (31%)
1 (67%)
0.22
0.46
0.21
0.44
0.21
0.42
2 × 107
1.4 × 107
2 × 107
4 × 105
1.14 × 106
0.29 × 106
1.26 × 106
6.3 × 107
2.6 × 107
2.6 × 105
4.2 × 105
1.73 × 107
predict efficient electron injection from the common LiF/Al cathode.
These energy levels are in excellent agreement with the data of CV
measurements described in the previous section. The increase in the
number of PTZ units practically leads to tiny shifts of HOMO and LUMO
values.
To study the impact of the number of donor substituents on charge
carrier transport in the vacuum-deposited layers of the compounds, ToF
measurements we performed [42]. Depending on the polarity of applied
voltages, holes or electrons were generated in the vacuum-deposited
layers of PTZ-TROZ, bis-PTZ-TROZ, and tri-PTZ-TROZ by light excita-
tion through the ITO electrode using a pulsed laser (λ = 355 nm).
Under different external electric fields, the photogenerated either holes
or electrons were transported through the layer from the ITO electrode
to the opposite Al electrode. The transit times (ttr) were well-observed
for charges in the corresponding low-dispersive photocurrent transients
of the tested samples (Fig. 7a and S5). Taking ttr from photocurrent
plots in log-log scale as it was described earlier [43], hole (μh) and
electron (μe) mobilities were calculated and presented in Fig. 7b
Fig. 5. PL and phosphorescence spectra (a) of the solid films of PTZ-TROZ, bis-PTZ-TROZ, and tri-PTZ-TROZ recorded at 77 K; PL spectra (b) and PL decay curves (c)
of the solid film of PTZ-TROZ recorded at different temperatures.
7

X.-F. Tan, et al.
DyesandPigments173(2020)107793
the studied compounds, the highest hole mobility values were observed
for the layers of tri-PTZ-TROZ. They exceeded 1 × 10−4 cm2V−1s−1 at
electric fields higher than 1 × 106 Vcm−1 (Fig. 7b).
2.8. Electroluminescent properties
2.8.1. Non-doped fluorescent OLEDs
Because of the appropriate charge-injection properties, HOMO/
LUMO levels, bipolar charge transport, AIEE and TADF characteristics,
the solid films of the compounds were used as non-doped light-emitting
layers in OLEDs with the structure ITO/MoO3(1 nm)/NPB(30 nm)/
TCTA(15 nm)/mCP(5 nm)/light-emitting layer (25 nm)/TSPO1(5 nm)/
TPBi(30 nm)/LiF(0.5 nm)/Al. Emitting layers of PTZ-TROZ, bis-PTZ-
TROZ or tri-PTZ-TROZ were deposited in devices A, B or C, respec-
tively. Energy levels of the functional layers of the fabricated devices
are displayed in Fig. 8a. To provide appropriate charge-injection and
charge-transport from electrodes to the light-emitting layers, the layers
of MoO3 and LiF were used as the hole and electron injection layers.
NPB/TCTA and TPBi were employed as the hole and electron trans-
porting materials, respectively. As a result, a turn-on voltage lower than
4 V was observed for the devices (Fig. 8b). To block excitons within the
light-emitting layer [46], the mCP and TSPO1 layers were deposited.
The fabricated non-doped devices were characterized by extremely
high brightness exceeding 30000 cd/m2 (Fig. 8b). At the brightness of
1000 cd/m2, EQEs of devices A, B, and C were found to be 4.92, 4.77,
and 4.57%, respectively, i.e., they were only slightly lower than their
maximum values (5.4, 4.85, and 5.10%) (Table 4). At the brightness of
10000 cd/m2, the values of EQE were found to be 3.40, 4.38, and
4.00%, respectively. Low roll-off efficiencies of 9, 2, and 10% at the
brightness of 1000 cd/m2, and 37, 10, and 22% at the brightness of
10000 cd/m2 were observed for devices A, B, and C, respectively
(Fig. 8c, Table 4). Because of superior charge transporting ptoperties of
bis-PTZ-TROZ and tri-PTZ-TROZ (Fig. 7b), the low-efficiency roll-off
was obtained for devices B and C in contrast to that observed for device
A.
Fig. 6. PL intensity versus water fraction in THF: water mixtures. Insets: PL
spectra and photos under UV excitation of bis-PTZ-TROZ dispersed in THF:
water mixtures with the different water content.
according to the Poole–Frenkel mobility-electric field (E) relationship
μ = α0eαE1/2, where α is the field dependence parameter [44]. PTZ-
TROZ containing single PTZ unit was characterized by μh of 3.2 × 10−6
cm2V−1s−1 and μe of 3.3 × 10−5 cm2V−1s−1 at an electric field of
9.4 × 105 Vcm−1. Thus PTZ-TROZ exhibited by ca. one order of mag-
nitude lower hole mobility in comparison to its electron mobility. In
contrast, bis-PTZ-TROZ and tri-PTZ-TROZ showed close values of hole
and electron mobilities indicating well-balanced charge transport over
a large range of electric fields (Fig. 7b). The similar electron mobility
values of 1.1 × 10−5, 2.2 × 10−5 and 3.8 × 10−5 cm2V−1s−1 were
observed for the layers of PTZ-TROZ, bis-PTZ-TROZ, and tri-PTZ-TROZ,
respectively at an electric field of 6.4 × 105 Vcm−1. The slight increase
of electron mobilities with the increase of the number of PTZ moieties
may be apparently explained by the different molecular packing [45].
Considerable effect of the number of PTZ substituents on hole mobilities
was observed. At the electric field of 6.4 × 105 Vcm−1, hole mobilities
Yellow to orange emission with the wavelengths of electro-
luminescence intensity maxima of 555, 570, and 580 nm, corre-
sponding to the Commission Internationale de l’Eclairage (CIE 1931)
chromaticity coordinates (x, y) of (0.43, 0.55), (0.47, 0.52), and (0.49,
0.5), were observed for devices A-C (Fig. 8d, Table 4). The slight red-
shifts of EL spectra of devices B and C in comparison to that of device A
were caused by the increasing number of PTZ substituents of the
increased in the range 1 × 10−6
˂ ˂
1.5 × 10−5 2.2 × 10−5
cm2V−1s−1 with an increasing number of phenothiazine units. Among
Fig. 7. Photoelectron emission spectra (a) and charge mobility electric field dependencies (b) for vacuum-deposited films of PTZ-TROZ, bis-PTZ-TROZ, and tri-PTZ-
TROZ. Insets: Hole and electron. time-of-flight current transients for the vacuum-deposited film of PTZ-TROZ.
8

X.-F. Tan, et al.
DyesandPigments173(2020)107793
Fig. 8. Equilibrium energy diagram (a), current density–voltage-brightness (b) and EQE–brightness (c) characteristics and EL spectra recorded at 7 V (d) for devices
A–G. Insets: photos of PTZ-TROZ-based devices A, D, and G.
Table 4
Output characteristics of non-doped/doped fluorescent and phosphorescent OLEDs.
a
Device
Light-emitting layer
Von
V
,
Max. brightness, cd/m2
EQEmax
%
Roll-off,b
%
λmaxEL, nm
CIE,c (x, y)
Non-doped fluorescent OLEDs using PTZ-TROZ, bis-PTZ-TROZ, or tri-PTZ-TROZ as the AIE/TADF emitters
A
B
C
PTZ-TROZ
bis-PTZ-TROZ
tri-PTZ-TROZ
3.7
3.8
4.0
49000
33000
30000
5.4
4.85
5.1
9/37
2/10
10/22
555
570
580
0.43, 0.55
0.47, 0.52
0.49, 0.5
Doped fluorescent OLEDs using PTZ-TROZ, bis-PTZ-TROZ, or tri-PTZ-TROZ as the AIE/TADF emitters
D
E
F
PTZ-TROZ:mCP
bis-PTZ-TROZ:mCP
tri-PTZ-TROZ:mCP
4.1
4.0
3.9
9800
16000
21000
5.4
7.0
9.9
4/74
1/37
2/34
540
540
540
0.33, 0.53
0.32, 0.5
0.33, 0.53
Red phosphorescent OLED using PTZ-TROZ as the AIE/TADF host
G
PTZ-TROZ:Ir(piq)2(acac)
3.6
17000
10.3
1/11
624
0.62, 0.37
a
b
c
Turn-on voltages at 10 cd/m2.
Efficiency roll-off efficiency at 1000/10000 cd/m2 relatively to EQEmax
.
CIE for EL spectrum recorded at 7 V.
9

X.-F. Tan, et al.
DyesandPigments173(2020)107793
materials used as the emitters. The stable EL spectra at different applied
voltages were in perfect agreement with the PL spectra of vacuum-de-
posited films of PTZ-TROZ, bis-PTZ-TROZ, or tri-PTZ-TROZ (Fig. S6).
Due to the excellent charge-blocking and exciton-blocking properties of
the devices, no additional emission was observed in EL spectra of the
devices A-C at the different applied voltages (Fig. S5).
Because of the AIE and TADF effects observed for the emitters which
were characterized by relatively high PLQYs of 15–37% in solid-state,
EQEs of devices A-C exceeded the theoretical limit (5%) of fluorescent
OLEDs without outcoupling (Table 4) [47]. The EQE values of devices
A-C did not exceed the best EQE values of the published yellow-orange
non-doped OLEDs. However the combinations of maximum brightness,
EQE and efficiency roll-off values are among the best reported up to
now for non-doped OLEDs [48,49].
(Fig. 8b). EL spectra of Device G were mostly related to the emission of
Ir(piq)2(acac) (Fig. 8d) [54,55]. However, the high-energy band which
was related to PTZ-TROZ emission could be recognized. As the host
material of PhOLEDs, PTZ-TROZ exhibited great potential. Maximum
EQE of 10.3%, low-efficiency roll-offs of 1% at 1000 cd/m2, and low-
efficiency roll-offs of 11% at 10000 cd/m2 were observed for device G
(Table 4).
3. Conclusions
A series of newly designed AIE-TADF materials based on a 1,3,5-
triazine acceptor as the core and a phenothiazine donor or a phenoxy
donor as the different peripheral units were synthesized and analyzed.
These materials were characterized by thermally activated delayed
fluorescence and aggregation induced emission enhancement.
Photoluminescence quantum yields of up to 37 and 67% were recorded
for non-doped and doped solid-state samples. All the synthesized
compounds exhibited bipolar charge-transporting properties. The
strong effect of phenothiazine substituents on charge-transporting
properties of the compounds was demonstrated. The highest charge
mobility values were observed for the layers of the compound con-
taining three phenothiazine moieties. They exceeded 1 × 10−4
cm2V−1s−1 at electric fields higher than 1 × 106 Vcm−1. Three types of
OLEDs based on the synthesized materials were fabricated. The non-
doped yellow device with the compound containing single phenothia-
zine unit as an emitter gave a maximum external quantum efficiency of
5.4%. OLED with the emissive layer containing 5% solid solution of the
compound containing three phenothiazine moieties in 1,3-bis(N-car-
bazolyl)benzene exhibited maximum external quantum efficiency of
9.9%. Red phosphorescent OLED containing the compound with single
phenothiazine moiety as a host showed maximum external quantum
efficiency of 10.3% and low efficiency roll-offs of 1% at 1000 cd/m2.
2.8.2. Doped fluorescent OLEDs
In guest-host systems, TADF properties of emitters can be improved
by using appropriate host due to the host polarity effect giving high-
efficiency RISC [50]. Recently, the enhancement of PLQY for AIEE
emitter 4,6-di(9,9-dimethylacridan-10-yl)isophthalonitrile was ob-
served due to its smaller ΔEST in hosted films, which lead to more ef-
ficient TADF emission in comparison to that observed for non-doped
films [34]. Light-emitting layers containing 5% of each luminophore in
mCP were deposited in devices D, E, and F. Higher maximum EQE
values of 5.4, 7.0, and 9.9% were observed for devices D, E, and F in
comparison to those observed for non-doped devices A, B, and C, cor-
respondingly (Fig. 8c, Table 4). This result can be mainly explained by
the higher PLQY values of 64, 67, and 63% for the solid-state molecular
mixtures of PTZ-TROZ: mCP, bis-PTZ-TROZ: mCP, and tri-PTZ-TROZ:
mCP, respectively. Apparently, TADF efficiency of the studied AIE/
TADF luminophores was improved due to the decrease of their ΔEST
values in mCP hosted films [51]. At the brightness of 1000 cd/m2, de-
vices D, E, and F exhibited the low-efficiency roll-off values of 4, 1, and
2%, respectively, which were similar to their maximum EQE values.
However, at the brightness of 10000 cd/m2, considerably higher effi-
ciency roll-off values of 74, 37, and 34% were observed (Table 4). In
addition, compared with the non-doped devices, the doped devices
exhibited the lower maximum brightness. These observations can be
explained by dis-balance of hole-electron pairs in the light-emitting
layers of the doped devices at high electric fields since the hole-trans-
porting material (mCP) was used as the host [52]. The higher efficiency
roll-off values of PTZ-TROZ-based devices A and D can be explained by
the different hole and electron mobilities of PTZ-TROZ (Fig. 7b).
Due to the polarity effect of TADF emitters in the guest-host mix-
tures [53], green electroluminescence was observed for the devices D-F
(Fig. 8d). Corresponding to CIE color coordinates (0.33, 0.53), (0.32,
0.5), and (0.33, 0.53), the EL spectra of devices D, E, and F were in good
agreement with the PL spectra of the films of the molecular mixtures
PTZ-TROZ:mCP, bis-PTZ-TROZ:mCP, and tri-PTZ-TROZ:mCP (Fig. S7).
Acknowledgements
This research was funded by the European Regional Development
Fund according to the supported activity ‘Research Projects
Implemented by World-class Researcher Groups’ under Measure No.
01.2.2-LMT-K-718. This work was also supported by the Natural
Science Foundation of Fujian Province (Grant No. 2016J01233), and
Graphene Powder & Composite Research Center of Fujian Province,
2017H2001).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.dyepig.2019.107793.
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