Through-space charge transfer in luminophore based on phenyl-linked carbazole- and phthalimide moieties utilized in cyan-emitting OLEDs

Article abstract of DOI:10.1016/j.dyepig.2019.107833

Two new carbazole-containing aromatic imides were synthesized by three-step synthetic pathway with the moderate yields up to 76%. The compounds were investigated theoretically. TD-DFT computational studies revealed low singlet-triplet energy differences. The compounds were found to have relatively high thermal stability with 5% mass loss temperatures in the range of 280–310 °C. Phthalimide-based compound exhibited aggregation-induced emission enhancement and thermally activated delayed fluorescence with photoluminescence quantum efficiency of 20% in the solid state. Structure-properties relationship of this compound was investigated and it was found that charge-transfer through space mechanism is responsible for the emission. Series of green-emitting doped and non-doped electroluminescent devices were fabricated based on the carbazole-phthalimide derivative were fabricated to reveal best-performing mCP-doped device demonstrating maximum external quantum efficiency of 2.4% with current efficiency of 6.6 cd/A and power efficiency of 4.0 lm/W with maximum brightness of 8300 cd/m².

Full text of DOI:10.1016/j.dyepig.2019.107833

DyesandPigments172(2020)107833
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Through-space charge transfer in luminophore based on phenyl-linked
carbazole- and phthalimide moieties utilized in cyan-emitting OLEDs
Yan Danyliva, Dmytro Volyniuka, Oleksandr Bezvikonnyia, Iryna Hladkaa, Khrystyna Ivaniukb,
Igor Helzhynskyyb, Pavlo Stakhirab, Ausra Tomkevicienea, Levan Skhirtladzec,
Juozas V. Grazuleviciusa,∗
a Department of Polymer Chemistry and Technology, Kaunas University of Technology, Radvilenu Pl. 19, LT-50254, Kaunas, Lithuania
b Lviv Polytechnic National University, Stepan Bandera 12, 79013, Lviv, Ukraine
c Ivane Javakhishvili University, Institute Macromolecular Chemistry & Polymer Materials, Ilia Chavchavadze Ave 13, Tbilisi, 0179, Georgia
A R T I C L E I N F O
A B S T R A C T
Keywords:
Two new carbazole-containing aromatic imides were synthesized by three-step synthetic pathway with the
moderate yields up to 76%. The compounds were investigated theoretically. TD-DFT computational studies
revealed low singlet-triplet energy differences. The compounds were found to have relatively high thermal
stability with 5% mass loss temperatures in the range of 280–310 °C. Phthalimide-based compound exhibited
aggregation-induced emission enhancement and thermally activated delayed fluorescence with photo-
luminescence quantum efficiency of 20% in the solid state. Structure-properties relationship of this compound
was investigated and it was found that charge-transfer through space mechanism is responsible for the emission.
Series of green-emitting doped and non-doped electroluminescent devices were fabricated based on the carba-
zole-phthalimide derivative were fabricated to reveal best-performing mCP-doped device demonstrating max-
imum external quantum efficiency of 2.4% with current efficiency of 6.6 cd/A and power efficiency of 4.0 lm/W
with maximum brightness of 8300 cd/m2.
Thermally activated delayed fluorescence
Aggregation-induced emission enhancement
Through-space charge transfer
Aromatic imide
Cyan-emitting OLED
1. Introduction
design of compounds with the desired wavelength of emission is se-
lection of appropriate donor for the selected acceptor.
Employment of new design strategies and synthetic approaches for
the development of electroactive materials is of high importance for the
achievement of high efficiency of organic light-emitting diodes
(OLEDs). Suitable donor-acceptor materials for highly effective OLEDs
based on delayed fluorescence must meet a set of requirements in-
cluding thermal and electrochemical stability, morphological stability
of the layers, low ΔES-T values, good charge-transporting abilities, high
solid-state photoluminescence quantum yields (PLQY). Effect of ag-
gregation induced emission enhancement (AIEE) can be additional
advantage of such materials.
In this work new donor-acceptor materials were designed and syn-
thesized with the aim of obtaining TADF and AIEE properties im-
plemented via charge transfer occurring through space. Carbazole, as
electron-rich heterocycle and well-known semiconductor moiety was
selected as electron-donating unit, while aromatic imides were used as
acceptor moieties. Aromatic imides were reported as materials for
fluorescent probes [3] and memory devices [4]. They also appeared to
be attractive in the synthesis of materials for OLED [5] and OPVC [6]
application. Donor-acceptor materials based on aromatic imides can be
obtained by simple and inexpensive synthesis.
New approach in the design can also be realized through alternative
ways of intramolecular charge transfer (ICT) in donor-acceptor mate-
rials, e. g. through the formation of intermolecular exciplexes or
through charge transfer through space [1].
2. Results and discussion
2.1. Theoretical calculations
Usually charge transfer through space in any type of exciplex is
provided by bonds between electron pairs of electron-rich moiety and
electron-deficient acceptor moiety [2]. The biggest challenge in the
Computational studies applying time dependent density functional
theory (TD-DFT) were carried out to predict electrochemical and
∗ Corresponding author.
E-mail address: juozas.grazulevicius@ktu.lt (J.V. Grazulevicius).
https://doi.org/10.1016/j.dyepig.2019.107833
Received 22 April 2019; Received in revised form 14 July 2019; Accepted 22 August 2019
Availableonline23August2019
0143-7208/©2019ElsevierLtd.Allrightsreserved.

Y. Danyliv, et al.
DyesandPigments172(2020)107833
Table 1
carbonyl group of respective imide were also measured. For CzPhPI this
distance was estimated to be 3.227 Å, and for CzPhNI it was 3.423 Å.
The value of N–C interatomic distance for CzPhPI was slightly less than
the sum of the van der Waals radii of carbon and nitrogen atoms
(3.25 Å) which makes possible n→π* interaction between free electron
pair of nitrogen atom of carbazole with unoccupied orbital of carbon
atom of carbonyl group [9,10].
TD-DFT calculation data for CzPhPI and CzPhNI.
Compound
HOMO, eV
LUMO, eV
E (S1), eV
E (T1), eV
ΔE (S1-T1), eV
CzPhPI
CzPhNI
−5.7
−5.6
−2.6
−2.8
2.5143
2.2819
2.5017
2.2781
0.0126
0.0038
Additionally, molecular packings of CzPhPI and CzPhNI were
analyzed (Fig. S1). The molecular packing diagrams shows that there
are no additional intermolecular interactions in the crystals of CzPhPI
which can have influence on the charge transfer process due to per-
pendicular packing and relatively high (more than 4 Å) distances be-
tween carbazole and imide moieties of adjacent molecules. In contrast,
CzPhNI has sandwich-like packing, although, stacking of naphthyla-
mide planes apparently can not cause any intermolecular charge
transfer.
photophysical properties of CzPhPI and CzPhNI. Spartan 14′ software
package [7] was used for the implementation of the calculations. Basis
set B3LYP/6-311 + G** was used for the calculation of triplet level
energy values (ET) and equilibrium geometries. The computed values of
parameters (ES, ET, ΔES-T, HOMO, LUMO) are presented in Table 1,
while delocalization of HOMOs and LUMOs are depicted on Fig. 1.
LUMOs are strictly located on the electron-deficient aromatic imide
fragments while HOMOs are mostly located on the electron-rich car-
bazole fragments.
Theoretically obtained delocalization of electron orbitals indicates
potential ability of transporting both electrons and holes by the com-
pounds. Electron-donating and electron-withdrawing fragments are
twisted with respect to phenyl linker with torsion angle of ca. 75° in the
ground state, which significantly reduce conjugation and leads to the
expectation of a low singlet-triplet energy splitting (ΔES-T). The position
of nitrogen atom with respect of the carbon atom of carbonyl group of
imide moiety allows to predict the possible interactions between free
electron pair of nitrogen atom with double bond of carbonyl group.
Such n→π* interactions were previously reported [8–10], however,
charge transfer between carbazole and phthalimide units through this
interaction was not yet observed.
2.3. Thermal characterization
Thermal gravimetric analysis (TGA) and differential scanning ca-
lorimetry (DSC) were utilized to investigate thermal stability and de-
termine temperatures of the morphological transitions. After the
synthesis, both compounds were isolated as crystalline substances. DSC
scans of CzPhPI and CzPhNI are depicted on Fig. 3. Endothermic peaks
at 232 °C and 249 °C for CzPhPI and CzPhNI, respectively, can be
characterized as melting temperatures. Exothermic peak which is ob-
served at 154 °C for CzPhPI on cooling scan can be characterized as
crystallization temperature, while cooling scan of CzPhNI revealed two
close peaks of crystallization at 158 °C and 162 °C. In addition, 2nd
heating scan for CzPhNI revealed exothermic peak at 180 °C. This be-
havior, apparently, can be explained as existence of two crystalline
forms of CzPhNI. On the cooling scan two crystallization peaks are
responsible for the formation of two types of different crystals, and on
the following heating scan solid-solid phase transition between crys-
talline forms can be observed. Melting points (Tm), crystallization
temperatures (Tc), solid-solid phase transition temperature (Ts-s) and
5% weight loss temperatures (Td) are summarized in Table 2.
TGA curves of the studied materials are shown on Fig. S2. Results of
the measurements revealed 5% weight loss temperatures of 280 °C and
310 °C for CzPhPI and CzPhNI, respectively, which can be character-
ized as relatively high. Practically, full loss of weight of the sample in
the temperature range between 360 °C and 380 °C shows that the pro-
cess of sublimation was recorded by TGA but not thermal decomposi-
tion.
2.2. Synthesis and structural characterization
The synthetic pathways for CzPhPI and CzPhNI contained three
steps (Fig. 2 a). The first step was aromatic nucleophilic substitution,
the second step was a reduction of nitro group to amino group using
nickel bromide hydrate as a catalyst and the last one was formation of
cyclic imides.
After recrystallization, target compounds were obtained as crystal-
line substances. Mass spectrometry, 1H and 13C NMR spectroscopy and
single crystal X-ray analysis were utilized for confirmation of the che-
mical structures of the materials. CzPhPI demonstrated moderate so-
lubility in polar organic solvents, such as chloroform, dichloromethane,
dimethylformamide (DMF), acetonitrile and acetone while solubility in
toluene was very low. CzPhNI was found to be considerably less soluble
than CzPhPI. It could be dissolved only in DMF and DMSO.
Single crystal X-Ray analysis of compounds CzPhPI and CzPhNI
(Fig. 2 b) was performed. It revealed that torsion angles for CzPhPI
were 69° between carbazole and benzene ring planes and 64° between
phthalimide plane and benzene ring plane. For CzPhNI these angles
were 86° between carbazole plane and benzene ring plane and 78°
between napththalic imide plane and benzene ring plane. Interatomic
distances between nitrogen atom of carbazole and carbon atom of
2.4. Electrochemical and photoelectrical properties
Cyclic voltammetry (CV) was used to study electrochemical prop-
erties of DMF solutions of the compounds for experimental estimation
of the ionization potentials (IPCV) and electron affinities (EACV). IPCV
and EACV of the solutions of the compounds were calculated by equa-
tions IPCV
= (Eox + 4.8) and EACV = (Ered + 4.8) using
Fig. 1. HOMOs and LUMOs delocalization for CzPhPI and CzPhNI.
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Y. Danyliv, et al.
DyesandPigments172(2020)107833
Fig. 2. Synthetic pathway for synthesis (a) and ORTEP structures (b) of compounds CzPhPI and CzPhNI.
electrochemical oxidation and reduction onset potentials of the solu-
tions and ferrocene/ferrocenium as a standard. CV curves of CzPhPI
and CzPhNI are depicted on Fig. 4 a. Irreversible oxidation processes
were observed at 0.99 eV for CzPhPI and at 0.94 eV for CzPhNI. Irre-
versibility of oxidation processes can be explained by absence of sub-
stituent in active 3rd and 6th positions of carbazole units [11]. CV
curves of CzPhPI and CzPhNI revealed reversible reduction at
−1.77 eV and −1.57 eV respectively (Table 3.).
The compounds demonstrated comparable IPCV values of
5.74–5.79 eV. EACV values were estimated to be 3.03 eV and 3.23 eV for
CzPhPI and CzPhNI, respectively. Electrochemical band gaps were
estimated to be 2.76 eV for CzPhPI and 2.51 eV for CzPhNI (Table 3.).
Electron photoemission spectra measurements were utilized for the
measurement of the ionization potentials (IPPE) of the solid layers of
CzPhPI and CzPhNI [12]. Photoelectron emission spectra of the layers
of the materials are plotted in Fig. 4 b. The IPPE values for compounds
CzPhPI and CzPhNI were taken from the photoelectron spectra extra-
polating their liner parts to Y = 0. The solid-state IPPE values were
estimated to be 6.03 eV and 6.09 eV for CzPhPI and CzPhNI, respec-
tively (Table 3). Relatively high difference between the IP values of the
solid samples and of the solutions can be explained by the existence of
Table 2
Thermal characteristics of CzPhPI and CzPhNI.
Compound
Tm, °C
Tc, °C
Ts-s, °C
Td, °C
CzPhPI
CzPhNI
232
249
154
158, 162
–
180
280
310
gap values (Eg) of 2.95 and 2.67 eV taken from onsets of absorption
spectra of the layers of CzPhPI and CzPhNI and the constant of 1.37
was used due to the reasons described earlier [12] (Fig. 5, Table 3).
The trends of IPPE and EAPE values for solid-state samples of CzPhPI
and CzPhNI are in good agreement with the trends of IPCV and EACV
values estimated for solutions of CzPhPI and CzPhNI and with the
trends of theoretically calculated HOMO and LUMO energy values
(Tables 1 and 3).
2.5. Photophysical properties
Photophysical properties of the solutions of CzPhPI and CzPhNI in
tetrahydrofuran (THF) and toluene (concentration 1 × 10−5 mol/L)
and of vacuum deposited neat films were studied. Photoluminescence
(PL) and absorption spectra of the solutions and of the neat films are
plotted in Fig. 5. The corresponding values are collected in Table 4.
intermolecular interactions in the solid layers. Electron affinities (EAPE
)
of 2.03 and 2.44 eV were estimated for the solid samples of CzPhPI and
CzPhNI using the formula EAPE = IPPE – 1.37 × Eg. The optical band-
Fig. 3. DSC thermograms of CzPhPI and CzPhNI.
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Y. Danyliv, et al.
DyesandPigments172(2020)107833
Fig. 4. Cyclic voltammograms of the solutions (a) and photoelectron emission spectra (b) of the solid films of CzPhPI and CzPhNI.
Table 3
Electrochemical characteristics of CzPhPI and CzPhNI.
Compound
Eox, eV
Ered, eV
IPCV, eV
EACV, eV
EgCV, eV
IPPE, eV
EAPE, eV
CzPhPI
CzPhNI
0.99
0.94
−1.77
−1.57
5.79
5.74
3.03
3.23
2.76
2.51
6.03
6.09
2.03
2.44
these tails may be attributed to intramolecular through-space CT tran-
sitions of CzPhPI and CzPhNI because of their donor-acceptor mole-
cular structures. However, these through-space intramolecular CT
transitions in ground states should be very weak similarly to the very
weak CT transitions of exciplex-forming mixtures in ground states
which are usually not detectible by normal reflection–transmission
measurements [16,17]. Thus, intramolecular CT transitions in ground
states should be not easily detectable in absorption spectra of CzPhPI
and CzPhNI. Therefore, the tails in the absorption spectra of the films
of CzPhPI and CzPhNI have to be mainly attributed to aggregation
effects.
Both solid samples and toluene solutions of CzPhPI and CzPhNI
demonstrated PL with the intensity maxima in the range of
512–520 nm. While, photoluminescence spectra of the CzPhPI and
CzPhNI solutions in more polar tetrahydrofuran were shifted to the
low-energy region showing positive solvatochromism (Fig. 5). This in-
dicates a larger change in dipole moment and stronger intramolecular
CT character in the excited state of both compounds CzPhPI and
CzPhNI. Moreover, that intramolecular CT was occurred through space
(but not trough the phenyl bridge) between carbazole and either
phthalimide or napththalic imide units. That intramolecular through-
space CT of compounds CzPhPI and CzPhNI is similar to the inter-
molecular CT (exciplex) formed between separated electron donating
and accepting molecules [18,19]. The following evidences confirm in-
tramolecular through-space CT for CzPhPI and CzPhNI: 1) emissions of
CzPhPI and CzPhNI were characterized by broad PL spectra (full
widths at half maxima (FWHM) of 128 and 110 nm, respectively) and
PL decays which are similar to those of exciplexes (Fig. 5) [20]; 2)
emissive exciplex formation between carbazole and phthalimide units is
Fig. 5. UV and PL spectra of toluene/THF solutions and neat films of CzPhPI
and CzPhNI.
Absorption spectra of the solutions of CzPhPI and CzPhNI were
characterized by the maxima which were similar to those of carbazole
(ca. 255, 290, and 233 nm) (Fig. 5, Table 4) [13]. The shapes of ab-
sorption spectra were also similar to those of other carbazole deriva-
tives showing small dependence on acceptor substitutions [14,15].
Low-energy band of the spectrum of CzPhNI is related to absorption of
the acceptor. In contrast to absorption spectra of the toluene solutions
of CzPhPI and CzPhNI, low-energy tails were well observed in ab-
sorption spectra of the solid samples (Fig. 5). One may suppose that
Table 4
Photophysical characteristics of CzPhPI and CzPhNI.
Ф
Tol/Фfilm
film
film
Compound
λabsTol/λabs
(nm)
λemTol/λemTHF/λem
(nm)
CzPhPI
CzPhNI
245, 291, 336/236, 295, 335
245, 292, 333/232, 294, 337
517/538/514
520/561/512
< 0.01/0.20
< 0.01/0.01
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Y. Danyliv, et al.
DyesandPigments172(2020)107833
possible and was recently reported [21]; 3) our statement is in agree-
ment with recently published one [22]; 4) propyl-linked carbazole and
phthalimide derivative (which definitely has not conjugation between
donor and acceptor) was characterized by very similar photophysical
properties of the developed here compound CzPhPI which emission,
therefore, is related to intramolecular through-space CT [23].
insoluble aggregates with higher PL intensity. These observations
clearly indicate the effect of AIEE.
TD-DFT calculations revealed very low theoretical value of ΔES-T for
the both the structures (Table 1). To estimate experimental ΔES-T values
and to estimate possibility of upconversion of triplet excitons, PL and
phosphorescence (Ph) spectra were recorded for ethanol solutions of
CzPhPI and CzPhNI which form a good glass at 77 K (Fig. 7 a). To
understand phosphorescence behavior of CzPhNI and CzPhPI at 77 K,
individual phosphorescence spectra of carbazole (Cz), phthalimide (PI)
and naphthalic anhydride (NI) were additionally measured. Phosphor-
escence spectra of the solutions of CzPhPI and CzPhNI are well re-
solved and indicates a localized triplet (3LE) state (Fig. 7 a). The 3LE
state of CzPhPI is related to Cz since the lowest triplet level of Cz is
lower than that of PI. The 3LE state of CzPhNI is related to NI since the
lowest triplet level of Cz is higher than that of NI. Low PLQY of CzPhNI
can be partly attributed to its low triplet LE levels. High triplet levels of
donating and accepting molecules are required for high PLQYs of ex-
ciplexes [26,27]. In contrast, high triplet LE level of CzPhPI is appro-
priate for its high PLQYs. Also, excited-state lifetimes for the solutions
of both materials for aerated and non-aerated solutions were measured
(Fig. S3), results are collected in Table S1.
Additionally, PL and Ph spectra were taken of the films of the solid
solution of CzPhPI in mCP. From these spectra it was possible to esti-
mate ΔES-T of 0.03 eV for CzPhPI (Fig. 7 b). Such small ΔES-T value can
induce reverse intersystem crossing (RISC) resulting in TADF. Indeed,
intensity of delayed fluorescence of the film of molecular mixture
CzPhPI and mCP boosted with the raising temperature (Fig. 8). The
shape of PL spectrum of the film of molecular mixture of CzPhPI and
mCP film was the same at various temperatures indicating the similar
shapes of fluorescence and Ph spectra of CzPhPI. The shape of PL decay
curve of the film of the solid solution of CzPhPI and mCP at 300 K was
similar to those of many published TADF emitters with intramolecular
CT [28,29].
To prove that TADF of CzPhPI is related to intramolecular CT but
not to intermolecular CT, photophysical behavior of the molecular
dispersion of CzPhPI in ZEONEX was studied. PL spectrum of the neat
film of CzPhPI was compared with the spectrum of 1% wt solid solution
of CzPhPI in ZEONEX (Fig. 9 a). The amount of the emissive material
used for both film preparations is the same. The comparison revealed
considerably higher PL intensity and hypsochromic shift of emission
peak of the spectrum of molecular mixture in comparison to those of the
neat film. These results clearly indicate intramolecular nature of the
charge through-space transfer. The low concentration of CzPhPI in
ZEONEX apparently exclude any possible intermolecular interactions.
Photoluminescence quantum yield of the solid solution (0.26) was
found to higher than that of the neat film (0.2) due to the blocking of
intramolecular rotations/vibrations and elimination of intermolecular
interactions (Table 5). The enhancement was estimated by division of
integral square of emission spectrum of CzPhPI/ZEONEX film by in-
tegral square of emission spectrum of the film of CzPhPI. Lower
emission intensity of neat film of CzPhPI can be explained by ag-
gregation-caused quenching. Hypsochromic shift of emission peak of
CzPhPI/ZEONEX film relative to that of the neat film of CzPhPI was
apparently caused by the decrease of dielectric constant.
Toluene solutions of CzPhPI and CzPhNI showed very low PL
quantum yields (QY). They were lower than 1%. Weak overlapping of
HOMO and LUMO orbitals observed for CzPhPI and CzPhNI apparently
was the reason of low PLQYs (Fig. 1). Similarly, the solid sample of
CzPhNI demonstrated weak photoluminescence with negligible value
of PLQY. Considerably higher PLQY value of 20% was observed for the
solid sample of CzPhPI. Higher PLQY value observed for the solid
sample, was apparently caused by existence of n→π* interaction be-
tween unoccupied orbital of carbon atom of carbonyl group and free
electron pair of nitrogen atom of carbazole moiety in CzPhPI which
makes efficient charge-transfer through space. Since the corresponding
interatomic distances are considerably higher in case of compound
CzPhNI, its charge-transfer through space is less efficient than that of
compound CzPhPI. These differences result in the different PLQY va-
lues in solid-state (Fig. 2, Table 4). Summarizing, the different intera-
tomic distances of respective atoms in CzPhPI and CzPhNI leads to
different overlapping of HOMO and LUMO orbitals through space, and
as a result, their PLQY values are significantly different. Because of
higher PLQY of CzPhPI in solid-state in contrast to that of CzPhNI,
emission behavior of CzPhPI was investigated in more details.
The emission enhancement of CzPhPI in solid-state comparing to
that of solution also can be explained by the restriction of in-
tramolecular rotations without the planarization of the molecular
structure as it demonstrated for emitters exhibiting aggregation in-
duced emission enhancement (AIEE) [24]. To study AIEE phenomenon
for CzPhPI, PL spectra of the dispersions in THF/H2O systems with
different amount of H2O with the same concentration of CzPhPI were
recorded (Fig. 6). CzPhPI formed solid aggregates with increase of H2O
amount in THF/H2O system due to much lower solubility in water
comparable to that in water. Also, with the increase of H2O fraction, PL
intensity considerably decreased as well as emission maximum bath-
ochromically shifted from 542 nm to 618 nm. Apparently, increased
polarity of the THF/H2O system was responsible for the change of
photophysical properties. One more reason of decreasing of PL intensity
could also be enhancement of H-bonding which lead to the dynamic
quenching of the excited states, as it was reported earlier [25]. How-
ever, the further increase of H2O fraction to 90% lead to considerable
increase of PL intensity accompanied with hypsochromic shift of PL
maximum to 523 nm, which can be explained by rapid formation of
PL decay curves of the neat film of CzPhPI and of the film of mo-
lecular mixture CzPhPI/ZEONEX are shown in Fig. 9 b. Short- and long-
lived components, corresponding to prompt and delayed fluorescence,
respectively, can be identified in PL decay curves. Excited-state life-
times of both short-lived and long-lived parts are considerably longer in
case of the film of CzPhPI doped in ZEONEX compared to those of the
neat film of CzPhPI (Table 5). This difference apparently is caused by
the absence of intermolecular interactions in the film of molecular
dispersion in ZEONEX which leads to the suppression of non-radiative
decay in the polymer film [30].
Fig. 6. Emission spectra of dispersion of CzPhPI in THF/H2O system with the
various H2O fractions. Inset: effect of water concentration on emission effi-
ciency of CzPhPI.
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Y. Danyliv, et al.
DyesandPigments172(2020)107833
Fig. 7. PL and Ph spectra of ethanol solutions of CzPhNI, CzPhPI, Cz, PI and NI (a) and of solid solutions of CzPhPI in mCP (b).
Table 5
PL decay data of the of the films of CzPhPI and CzPhPI:ZEONEX.
Film
τ1, ns/%
τ2, ns/%
χ2
Ф
CzPhPI neat
CzPhPI:ZEONEX
17/73
469/18
119/27
2349/82
1.005
1.259
0.20
0.26
CzPhNI. The results revealed bipolar charge transporting ability of the
layers of the materials. Transit times (ttr) for both electrons and holes
were easily recognized in the log-log graphs of current transients (Fig.
S4 a, b). The current transients were found to be very dispersive. Drift
mobilities of holes and electrons versus electric field for the layers of
CzPhPI and CzPhNI are plotted in Fig. 10. CzPhPI was characterized
by hole mobility of 4.9 × 10−4 cm2V−1s−1 and electron mobility of
6.7 × 10−4 cm2V−1s−1 at an electric field of ca. 3 × 105 Vcm−1
.
CzPhNI was characterized by much lower hole mobility of 8.9 × 10−5
cm2V−1s−1 and electron mobility of 2.2 × 10−5 cm2V−1s−1 at the
same electric field (Fig. 10).
Fig. 8. PL decay curves and PL spectra (insets) of the film of the solid solution
of CzPhPI in mCP recorded at different temperatures.
2.7. Electroluminescent properties
2.6. Charge-transporting properties
Since CzPhPI showed relatively high PLQY in solid state, its elec-
troluminescent properties were investigated in non-doped device A and
in doped devices B, C and D using mCP, TCTA and oCzmOXD as the
hosts, respectively. For comparison of electroluminescent properties of
Time-of-flight (ToF) technique were used for estimation of charge-
transporting properties of vacuum-deposited layers of CzPhPI and
Fig. 9. PL spectra (a) and PL decay curves (b) of CzPhPI neat film and of the film of 1% wt solid solution of CzPhPI in ZEONEX.
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Y. Danyliv, et al.
DyesandPigments172(2020)107833
LiF (0.5 nm)/Al was used except light-emitting layers which were
CzPhPI,
CzPhPI(10%):mCP,
CzPhPI(10%):TCTA
or
CzPhPI
(10%):oCzmOXD in devices A, B, C, or D, respectively (Fig. 11 a).
Taking into account high ionization potential of 6.03 eV of CzPhPI,
hole-injection layer MoO3 and hole-transporting layers of NPB and
TCTA were used in the structures of OLEDs lowering energy barriers
between anode and light-emitting layers (Fig. 11 a). Electron-injection
layer of LiF and electron-transporting layer of TPBi were used for the
decrease of energy barrier between cathode and light-emitting layers
taking into account EAPE of 2.03 eV observed for CzPhPI. Additionally,
exciton-blocking layers of mCP and TSPO1 were used, thus forbidding
leakage of triplet excitons from the emitter CzPhPI to the charge-
transporting layers.
Relatively strong effect of different hosts on electroluminescence
(EL) spectra of devices A-D was observed (Fig. 11 b). Non-doped device
A was characterized by green emission with EL maximum at 529 nm
and commission Internationale de l’Eclairage (CIE 1931) chromaticity
coordinates (x, y) of (0.3, 0.51) (Fig. S6 b, Table 6).
Due to the low-polarity of the hosts mCP and oCzmOXD (ε = 2.84
for mCP [31]), devices B and D were characterized by greenish blue
electroluminescence. Device C containing relatively high-polarity host
TCTA (ε = 5.61 for TCTA [31]) emitted yellow light. Stable EL spectra
were recorded at different voltages (Fig. S5). The doped devices B-D
were characterized by lower turn-on voltages (at 10 cd/m2) than device
Fig. 10. Plots of charge-carrier drift mobilities versus electric field for the layers
CzPhPI and CzPhNI.
CzPhPI as AIEE/TADF emitter in different media, the same device
structure ITO/MoO3 (1 nm)/NPB (20 nm)/TCTA (20 nm)/mCP
(10 nm)/light-emitting layer (30 nm)/TSPO1 (10 nm)/TPBi (40 nm)/
Fig. 11. Equilibrium energy diagrams (a), EL and PL spectra of the devices and light-emitting layers, respectively (b), current density and brightness versus voltage
plots (c), EQEs and power efficiencies versus brightness (d).
7

Y. Danyliv, et al.
DyesandPigments172(2020)107833
Table 6
OLEDs parameters.
b
Device
Host
λ
EL, nm
CIE (x, y)
Lv,a cd/m2
Von
,
V
CEmax, cd/A
PE max, lm/W
EQEmax, %
A
B
C
D
Non-doped
mCP
TCTA
529
500
504
544
(0.3, 0.51)
(0.23, 0.41)
(0.35, 0.53)
(0.25, 0.46)
3400
8300
2400
2300
6.1
4.9
4.7
5.0
4.8
6.6
4.5
3.8
2.2
4.0
2.6
2.4
1.7
2.4
1.5
1.5
oCzmOXD
a
At 10 V CIE, CIE of electroluminescence spectra of devices A-D recorded at 8 V.
At 10 cd/m2.
b
A due to the lower energy barriers between light-emitting layers and
mCP and TSPO1 layers (Fig. 11 a). The best OLED performance was
observed for Device B which showed brightness (at 10 V) of 8300 cd/
m2, and current, power, external quantum efficiencies of 6.6 cd/A, 4.0
lm/W, 2.4%, respectively (Fig. 11 c, d). This result is mainly related to
the small ΔES-T of 0.03 eV of the molecular dispersion of CzPhPI in mCP
host resulting in efficient TADF (Fig. 7). As it was previously shown for
TADF emitters including TADF emitters exhibiting AIEE, TADF prop-
erties of such emitters can be improved using hosts of appropriate po-
larity [32,33]. Consequently, mCP is the most appropriate host for
CzPhPI among tested ones (Fig. S6a).
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Acknowledgement
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This research was funded by a grant No. S-MIP-17-96 from Research
Council of Lithuania.
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Appendix A. Supplementary data
[20] Guzauskas M, Volyniuk DY, Tomkeviciene A, Pidluzhna A, Lazauskas A,
Grazulevicius JV. Dual nature of exciplexes: exciplex-forming properties of carba-
zole and fluorene hybrid trimers. J Mater Chem C 2019;7(1):25–32https://doi.org/
10.1039/c8tc04708a.
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.dyepig.2019.107833.
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