White hyperelectrofluorescence from solution-processable OLEDs based on phenothiazine substituted tetraphenylethylene derivatives

Article abstract of DOI:10.1039/d0tc03136d

Mechanochromic emitters with an appropriate combination of properties for white hyperfluorescent solution-processable organic light emitting diodes (OLEDs) were developed involving phenothiazine, tetraphenylethylene, and electron withdrawing (cyano (-CN))

Full text of DOI:10.1039/d0tc03136d

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White hyperelectrofluorescence from solution-processable OLEDs based on
phenothiazine substituted tetraphenylethylene derivatives
Faizal Khana, Ervinas Urbonas,b Dmytro Volyniuk,b* Juozas V. Grazuleviciusb Shaikh M. Mobina, Rajneesh Misraa*
,
aDepartment of Chemistry, Indian Institute of Technology Indore, Indore- 453552, India; *e-mails:
rajneeshmisra@iiti.ac.in
bDepartment of Polymer Chemistry and Technology, Kaunas University of Technology, Barsausko 39, LT- 51423,
Kaunas, Lithuania; *e-mails: dvolynyuk@gmail.com
Abstract
Mechanochromic emitters with appropriate combination of properties for white hyperfluorescent
solution-processable organic light emitting diodes (OLEDs) were developed involving
phenothiazine, tetraphenylethylene, and electron withdrawing (cyano (-CN)) or phenyl
acrylonitrile (-CHC(CN)Ph) groups. Among four studied compounds, the best performance in
white hyperfluorescence OLEDs was shown by one compound (named PTZTPE-3) due to its
orange emission with high photoluminescence quantum yield of 66% observed for toluene solution
and of 39% for doped films, low-dispersity hole-transporting properties, appropriate ionization
potential (5.5 eV) and electron affinity (3.24 eV). The perfect combination of these characteristics
is required for efficient white hyperfluorescence. Mechanoluminescent properties with
hypsochromically shifted emission under external stimuli were additionally detected and studied
in detail for the developed PTZTPE compounds discovering their multifunctionality for either
sensing, imaging, security, memory or other proposes. Using PTZTPE compounds
hyperfluorescence systems were developed and utilized in simply processed white organic light-
emitting diodes involving conventional host, host with thermally activated delayed fluorescence
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(TADF) properties and singlet emitters. Due to the Förster resonance energy transfer from co-hosts
to blue and orange emitters, the hyperfluorescence based white OLEDs were developed. They
were characterized by high quality of white electroluminescence with color rendering index of 67
and CIE1931 color coordinates of (0.28, 0.38), in the best case. The maximum external quantum
efficiency (8.2%) of the solution-processed OLED was higher than the theoretical limit (5–7.5%)
of the devices based on the conventional fluorescent emitters.
Introduction
Organic small molecules with intense solid state emission have attracted considerable research
interest on account of their promising applications in the field of data storage devices, organic light
emitting diodes (OLEDs), security inks, solid state lasers and non-linear optics (NLO) [1-9]. The
change of emission wavelength in response to mechanical stimuli such as smearing, mechanical
pressure, stretching, shearing etc. is called mechanofluorochromism (MFC) [10-14]. To date, only a
few molecules have been reported which exhibit hypsochromically shifted emission in response
to mechanical grinding [15-17]. It is widely accepted that the solid state optical properties of MFC
materials is strongly depend on the molecular packing arrangements hence by regulating the
intermolecular interactions, molecular conformation and packing modes between the molecules it
is possible to tune the optical properties of MFC materials.[18-20] The PTZTPE-1 and PTZTPE-4
showed bathochromic shift in their emission on grinding with mortar and pestle which was
reported earlier.[21] In order to tune the molecular packing arrangements, the molecular structure
of PTZTPE-1 was extended by cyano and phenyl acrylonitrile moieties to synthesize PTZTPE-
2 and PTZTPE-3 (Figure 1a). The single crystal X-ray analysis reveals that the presence of
electron withdrawing substituents affect the intermolecular interactions and hence resulted in more
versatile mechanochromic properties of PTZTPE-2 and PTZTPE-3. The cyano group bearing
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derivative (PTZTPE-2) shifts its emission hypsochromically on mechanical grinding while the
phenyl acrylonitrile group bearing derivative (PTZTPE-3) shows a small hypsochromic shift in
its emission and then emission shifts to longer wavelength on strong grinding. On one hand, this
result on hypsochromic shift of emission of PTZTPE-3 predictively has potential to expand
applicability of phenothiazine based tetraphenylethylene derivatives as optical functional materials
for either sensing, imaging, security, memory or other proposes.
a)
b)
N
S
H
PTZTPE-1
S
S
N
N
N
NC
S
PTZTPE-2
N
NC
N
N
PTZTPE-4
S
S
S
PTZTPE-3
Figure 1 (a) Chemical structures of PTZTPE 1–4 and (b) Schematical visualization of the proposed
approach for white hyperelectrofluorescence based on compound PTZTPE-3.
On the other hand, applicability of compounds PTZTPE 1–4 as electronic functional materials for
OLEDs was additionally studied. According to the spin statistics, positively charged hole and
negatively charged electron annihilate forming one singlet exciton for every three triplet excitons.
It is schematically shown as formation 25% singlets and 75% triplets in Figure 1b during the
process of hole and electron recombination in organic semiconductor materials [22]. Because of
that, maximum internal quantum efficiency for OLEDs based on conventional fluorescent emitters
is limited to 25%, while, external quantum efficiency (EQE) is limited to 5–7.5% even if emitters
are characterized by photoluminescence quantum yield (PLQY) of 100% [23]. In order to overcome
this limitation, several approaches for efficient harvesting of triplet excitons were proposed. The
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most well-established route is a usage of phosphorescent emitters [24]. These emitters contain heavy
metal atoms (e.g., Ir and Pt) which form strong spin-orbit interaction determinant very efficient
intersystem crossing (ISC). Because of that, these compounds can radiatively harvest triplet
excitons ensuring 100% internal quantum efficiency of OLEDs [25,26]. Moreover, such
phosphorescent emitters can be used as triplet sensitizers for fluorescent emitters yielding internal
quantum efficiency of 45% under electrical excitation [27,28]. Unfortunately, organometallic
systems have some intrinsic problems, such as toxicity and instability in aqueous environments
[29]. An alternative route to generate additional singlet excitons is triplet–triplet annihilation (TTA)
[30,31]. However, only 50% of triplet excitons can be converted into singlets which results in
theoretical limit of internal quantum efficiency of 62.5% [32]. Another way to harvest triplet
excitons is to employ thermally activated delayed fluorescence (TADF) [33]. TADF is based on
reverse intersystem crossing (RISC) which is allowed by small difference between the first excited
singlet and triplet energy levels. During RISC triplet excitons are converted into singlets
recombining in radiative way. Therefore, purely organic TADF molecules enable to achieve up to
100% internal quantum efficiency under electrical excitation and at the same time allow to avoid
disadvantages of phosphorescent emitters [34]. It should be noted that TADF emitters usually face
the same challenges as phosphorescent emitters such as poor lifetime (mainly for blue) and broad
electroluminescence spectra of TADF based OLEDs [35]
.
Recent approach for efficient harvesting of triplet excitons in OLEDs is hyperfluorescence which
theoretically allows to achieve internal quantum efficiency of 100% [36]. In all organic
hyperfluorescence systems, TADF molecules are employed as assistant dopants (co-hosts) in
fluorescent dopants and wide bandgap host systems solving several problems at the same time
[37,38]. In comparison to conventional fluorescene, hyperfluorescence suggests higher internal
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quantum efficiencies and higher EQE of OLEDs. In comparison to TADF systems,
hyperfluorescence based systems allow to achieve higher color purity and, more importantly,
higher operational stability of OLEDs.
[29-31]
Mechanism of hyperfluorescence consist of several main steps
(Figure 1b). Firstly, the
injected charge carriers are transported in the wide band gap conventional host. Then, injected
holes and electrons are eventually trapped on co-host (TADF molecule) if its HOMO is shallower
and LUMO is deeper compared with those of the conventional host material. In the third step,
triplet excitons created on TADF co-host can be converted into singlet excitons via RISC. In the
next step, singlet excitons of TADF host are transferred to the singlet state of conventional
fluorescence emitter via Förster resonance energy transfer (FRET). Finally, efficient radiative
decay from singlet level of the fluorescent emitter can be achieved. In order to construct an
efficient hyperfluorescence organic systems, at least the following requirements have to be
fulfilled for the selection of its components (host, TADF co-host and fluorescent emitter) [29-31]
:
 Lower HOMO and higher LUMO of host than those of the TADF co-host; TADF co-host
has to have wider energy gap then those of fluorescent emitter (Figure 1b). Selection of
host materials with high HOMO level is critical.
 To ensure efficient FRET, the ground state absorption of fluorescent emitter and emission
of TADF co-host have to be characterized by big spectral overlap.
 To avoid direct charge carriers trapping on the fluorescent emitter, low its doping
concentrations (0.5–2%) and much higher concentrations of TADF co-host (10–50%)
have to be selected.
 To ensure short delayed lifetime of the TADF co-host, the selected TADF molecules
should be characterized by high RISC rate.
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Taking into account, to the above requirements, PTZTPE 1–4 based hyperfluorescence systems
were developed. In design strategy of such emitters, well-known aggregation induced emission
(AIE)-active tetraphenylethylene (TPE) moiety and strong electron donor phenothiazine (PTZ)
unit were involved. To tune their energy levels, additional substitution by different electron
withdrawing group was exploited. Thus, we have synthesized two derivatives of PTZ and TPE
(PTZTPE-2 and PTZTPE-3) in which the PTZ moiety contains TPE unit on one side and an
[39,40]
electron withdrawing group on the other side. 1,3-Bis(N-carbazolyl)benzene (mCP)
was
selected as the conventional host and 4,4’-(9H,9’H-[3,3’-bicarbazole]-9,9’-diyl)bis(3-
[41-43]
(trifluoromethyl)benzonitrile) (pCNBCzoCF3)
as TADF co-host. Despite the fact that
hyperfluorescence is mainly used to improve EQE of single-color fluorescent OLEDs, it can be
also employed to construct white OLEDs [44,45]. Most of the reported hyperfluorescence-based
WOLEDs contain single emission layer with two fluorescent emitters [46]. In this study we propose
new approach for the design of solution-processable white hyperfluorescent OLEDs using two
separate blue and orange emitting layers (Figure 1b). In the developed device structure, the hole-
electron pairs recombine within the orange light-emitting layer on mCP and pCNBCzoCF3 co-
hosts forming 25% of singlet excitons and 75% of triplet excitons. Then, triplets can be converted
to singlets due to RISC of TADF co-host. In the same time, singlets can be transferred from co-
hosts mCP and pCNBCzoCF3 to conventional blue and orange emitters via FRET since there is
overlapping between emission spectra of co-hosts and absorption spectrum of emitters as well as
there is small enough distance between sensitizing co-hosts and emitters for the long-range FRET
(up to ca. 8.5 nm) [47]. These processes in the proposed structure result in white
hyperelectroluminescence with theoretical internal quantum efficiency of 100% because of
harvesting of triplets via TADF. For the optimized devices based on blue poly(9,9-dioctylfluorene-
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alt-N-(4-sec-butylphenyl)-diphenylamine) (TFB) and orange PTZTPE-3 emitters, white
hyperelectrofluorescence with external quantum efficiency of 8.2%, CIE color coordinate of (0.28,
0.38) and color rendering index of 67 were achieved.
Results and discussion
The synthetic route for the target compounds (PTZTPE-2 and PTZTPE-3) is outlined in Scheme
1. The reaction of 4-(1,2,2-triphenylvinyl)-phenylboronic acid pinacol ester (TPE-Bpin) with 7-
bromo-10-propyl-10H-phenothiazine-3-carbonitrile (5) was carried out by Pd-catalyzed Suzuki
cross coupling reaction to furnish PTZTPE-2 in 78% yield. The TPE-Bpin when reacted with 2-
phenyl-3-(10-propyl-10H-phenothiazin-3-yl)acrylonitrile (6), it resulted in the formation of a
mixture of the E and Z isomers of PTZTPE-3 in 69% yield. Due to almost negligible polarity
difference between the E and Z isomeric forms of PTZTPE-3, we tried several attempts but could
not separate them into individual isomers. The detailed synthetic procedure for the intermediates
involved is illustrated in scheme S1. The crystalline samples of PTZTPE-2 were obtained by
vapor diffusion method using n-pentane/dichloromethane binary solvent system and for PTZTPE-
3, the crystalline samples was prepared by recrystallization from dichloromethane and n-hexane.
The chemical structures of newly synthesized compounds PTZTPE-2 and PTZTPE-3 were well
characterized by 1H-NMR, 13C-NMR, high resolution mass spectrometry and single crystal XRD
techniques.
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NC
Br
N
S
K2CO3, Pd(PPh3)4
H2O/EtOH/Toluene
Reflux, 14h
CN
O
S
B
N
O
5
PTZTPE-2
(78%)
TPE-Bpin
N
K2CO3, Pd(PPh3)4
Br
CN
S
H2O/EtOH/Toluene
O
NC
S
N
B
Reflux, 14h
O
6
PTZTPE-3
(69%)
TPE-Bpin
Scheme 1. Synthetic scheme for PTZTPE-2 and PTZTPE-3.
Photophysical properties
The electronic absorption and emission spectra of PTZTPE-2 and PTZTPE-3 are depicted in
Figure 2 and the corresponding data are illustrated in Table 1. The absorption spectra of PTZTPE-
2 and PTZTPE-3 in THF solution (1x10-5 M) display a sharp absorption band in shorter wavelength
region at 280 nm and 304 nm, respectively, which could be attributed to the π-π* transitions and a
charge transfer (CT) band in longer wavelength region was observed at 322 nm and 423 nm for
PTZTPE-2 and PTZTPE-3, respectively. The CT nature of absorption peak was then examined by
solvatochromism. The solvatochromic nature of fluorescence of PTZTPE-2 and PTZTPE-3
reveals stronger donor acceptor character of PTZTPE-3 compared to PTZTPE-2. The PTZTPE-3
exhibits 101 nm red shifted absorption compared to PTZTPE-2. Normally, TPE derivatives are
non-emissive in solution due to the non-radiative energy loss of excited states by the free rotation
of TPE phenyl rings but the electron withdrawing moieties bearing phenothiazine substituted
derivatives of TPE (PTZTPE-2 and PTZTPE-3) are emissive even in solutions.
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a).
b).
c).
d).
Figure 2 (a) Absorption and (b) normalized emission spectra of the solutions of compounds
PTZTPE-2 and PTZTPE-3 in THF (1 x 10-5 M) (λexc = 350 nm and 440 nm for PTZTPE-2 and
PTZTPE-3 respectively) (c) Emission spectra of compounds PTZTPE-2 and PTZTPE-3 in
different THF- water mixtures. (d) Plot of fluorescence intensity Vs. % of water fraction for the
compounds PTZTPE-2 and PTZTPE-3.
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Table 1. Photophysical properties of PTZTPE-2 and PTZTPE-3.
b
Compounds
λ
(nm), ε (Lmol-1cm-1)a
λ
(nm)a Stokes shift
(cm-1)
Δλ
Φfc
abs
em
em
(nm)
PTZTPE-2
PTZTPE-3
280 (50600), 322 (23350)
304 (30200), 423 (10250)
482
10309
35
0.290
0.273
585
6547
08
a
b
Recorded in dry THF (1 x 10-5 M concentration). difference of the emission maxima in the
crystalline state and in solution, c fluorescence quantum yields were measured using quinine sulphate
as a standard in 0.5 M H2SO4 solution for PTZTPE-2 and rhodamine-6G as standard in ethanol for
PTZTPE-3.
In case of compound PTZTPE-2, PL spectra at similar wavelengths were recorded for its THF
solution and for the dispersions in THF:water mixtures with the different fractions of water (fw)
(Figure 2c). At water fractions ranging from 30 to 85%, PL intensity constantly increased with the
increase of water fraction due to the formation of PTZTPE-2 aggregates (Figure 2d). This result is
related to AIE effect caused by TPE group. At fw above 85%, PL intensity of the dispersions in
THF:water mixtures of PTZTPE-2 decreased. This observation can be explained by precipitation of
heavy PTZTPE-2 aggregates but not by any kind of aggregation-induced quenching. In case of
compound PTZTPE-3, PL spectra of its dispersions in THF:water mixtures PL intensity decreased
the spectra red-shifted when water fractions were changed from 0% to 50%. When aggregates started
to grow due to poor solubility of PTZTPE-3 in water (fw >50%), PL intensity of the corresponding
mixtures increased demonstrating AIE effect. Different PL spectra and PL intensity trends of the
dispersions of PTZTPE-2 and PTZTPE-3 in THF:water mixtures result from their different
solvatochromism which is discussed in the next section.
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Solvatochromism
The effect of polarity of different solvents on the absorption and emission spectra of PTZTPE-2 and
PTZTPE-3 was investigated and the results are shown in Figure S1, Figure 3 and Table S1. On
increasing polarity of the solvents, the PTZTPE-2 and PTZTPE-3 do not show much variation in
their absorption spectra but their emission spectra show remarkable bathochromic shifts. This is
because polar solvents stabilize the excited states more than the ground states which results in
decrease in the energy of excited states. In non-polar solvents such as toluene, the emission peak of
PTZTPE-2 and PTZTPE-3 was centered at 480 nm (blue) and 563 nm (yellow), respectively. With
increasing polarity, the emission spectra show bathochromic shift and in polar solvent such as DMF
the emission peak was observed at 492 nm (cyan) and 613 nm (red) for PTZTPE-2 and PTZTPE-3,
respectively. The emission spectra of PTZTPE-3 shows large red shift and significant color change
(Figure 3 and Figure S2) compared to PTZTPE-2 as the solvent was changed from toluene to DMF
indicating better D-A character in former.
a).
b).
Figure 3 Emission spectra of compounds (a) PTZTPE-2 and (b) PTZTPE-3 in solvents of
different polarity. (excitation wavelength for PTZTPE-2 and PTZTPE-3 is 350 nm and 440 nm
respectively).
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Computational analysis
In a view to get insight of the molecular geometries and to evaluate the electron density
distribution, the ground state optimization of the PTZTPE-2 and PTZTPE-3 was carried out using
the basis set B3LYP/6-31G(d,p) [48-50]. The frontier orbitals of PTZTPE-2 and PTZTPE-3 are
shown in Figure S5 and the corresponding data are listed in Table S4. The optimized molecular
structures of PTZTPE-2 and PTZTPE-3 show bent shape for phenothiazine. The TPE phenyl
ring and the PTZ moiety has a dihedral angle of 37.58° and 35.69° in PTZTPE-2 and PTZTPE-
3, respectively. The molecular orbital of PTZTPE-2 shows that the electronic density of both the
HOMO state and the LUMO state is localized over the entire molecular framework, while for
PTZTPE-3, the electronic density of the HOMO state is concentrated on the phenothiazine moiety
and that of the LUMO state is stretched towards the phenyl acrylonitrile moiety. The separation of
electron density on HOMO-LUMO energy levels suggested that the PTZTPE-3 has better D-A
character compared to PTZTPE-2. The HOMO-LUMO gap values were calculated for PTZTPE-
2 and PTZTPE-3 which is found to be 3.75 eV and 2.99 eV, respectively. Furthermore, the time
dependent density functional theory (TD-DFT) calculations were done using B3LYP/6-31G(d,p)
level to study the excited state energies and transitions. The transitions with composition, oscillator
strength and assignments are shown in Table S3.
Mechanochromism
The mechanochromic properties of PTZTPE-2 and PTZTPE-3 were explored by grinding the
crystalline samples in mortar with a pestle. The solid-state emission spectra of PTZTPE-2 and
PTZTPE-3 under various external stimuli are shown in Figure 4 and the related data are listed in
Table S5. The study shows that the PTZTPE-2 and PTZTPE-3 exhibit substituent dependent
mechanochromic properties. Earlier, we have reported the mechanochromic properties of
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PTZTPE-1 and PTZTPE-4 in which both the synthesized derivatives upon mechanical grinding
shift their emission to longer wavelength region with a spectral shift of +44 nm and +47 nm,
respectively. [21] In this work, the incorporation of cyano and phenyl acrylonitrile moieties affects
the intermolecular interactions and hence resulted in more versatile mechanochromic properties of
PTZTPE-2 and PTZTPE-3. The pristine form of PTZTPE-2 was ground mechanically using
mortar and pestle and the solid-state emission spectra was recorded using fluorescence
spectroscopy. On grinding, the PTZTPE-2 hypsochromically shifts its emission from 517 nm to
489 nm and shows grinding induced spectral shift (Δλ) of -28 nm. On fuming the ground sample
of PTZTPE-2 with dichloromethane for about 5–6 minutes, the emission spectra shift to 512 nm.
The powder XRD analysis were carried out for the pristine, ground and fumed samples of
PTZTPE-2 which shows that the spectral shift is associated with the phase transformation from
the crystalline to amorphous state. On the other hand, the PTZTPE-3 shows different
mechanochromic phenomena upon gentle grinding and strong grinding. The crystalline form of
PTZTPE-3 was considered as pristine-o form which is orange in color and emits at 593 nm. Gentle
grinding of pristine-o form of PTZTPE-3 hypsochromically shifted its emission to 590 nm (Δλ =
-3 nm) and shows yellow emission. The yellow form of PTZTPE-3 was considered as pristine-y
form which on strong grinding, bathochromically shifts its emission to 611 nm and shows grinding
induced spectral shift (Δλ) of +21 nm. The pristine-y form was not stable, and it reversibly converts
to the pristine-o form within a very short period of time hence we were not able to characterize the
pristine-y form but the small blue shift observed on gentle grinding could be explained on the basis
of results obtained from single crystal X-ray analysis which reveals that the pristine-o form
contains a dimeric unit of PTZTPE-3 molecules in which the S atom of phenothiazine of one
PTZTPE-3 molecule interacts with the CN group of a neighbouring PTZTPE-3 molecule through
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C(9)–N(2)...S(1) interaction. On gentle grinding the pristine-o form of PTZTPE-3, the
intermolecular CN…S interaction is likely to be destroyed which causes disruption of the dimeric
structure resulting in a small hypsochromic shifted band [16]. The self-reformation of these dimeric
interactions converts the pristine-y form again to pristine-o form. The ground sample of PTZTPE-
3 when fumed with dichloromethane for about 5–6 minutes, revert to the yellow colour and shifts
its emission to 586 nm (Figure 4). Since the pristine-y form was not stable hence we were not able
to get a distinct PXRD pattern for this form and hence the PXRD analysis were carried out for the
pristine-o, ground and fumed samples of PTZTPE-3. The sharp diffractions for the pristine-o
samples of PTZTPE-3 (Figure S3) suggesting its crystalline nature. The ground sample exhibits
extremely weak diffractions but the diffraction peaks are not completely disappeared which reveals
loss of crystallinity of pristine-o form on mechanical grinding. The fumed samples however exhibit
recovered sharp diffractions thus reveals the restoration of regular crystalline nature. We found
that the PTZTPE-3 can maintain its crystallinity to some extent after being ground. Hence, the
PXRD study suggests that the switchable conversion from the one crystalline state to another
crystalline powder with low crystallinity should be responsible for the reversible
mechanochromism of PTZTPE-3.
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a)
b)
Figure 4 Solid state emission spectra of compounds (a) PTZTPE-2 and (b) PTZTPE-3 in pristine,
grinded and fumed forms. Photographs (inset) taken under UV illumination. (c) UV and (d)
daylight images of pristine-o and pristine-y forms of PTZTPE-3 in a single frame.
Single crystal XRD analysis
To investigate the underlying mechanism for the observed mechanochromic behaviour of
PTZTPE-3, single crystal XRD analysis were performed (Figure 5 and Figure S4) and the data
obtained from X-ray analysis were summarized in Table S2. Our attempt to separate E and Z
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isomers of PTZTPE-3 through recrystallization was not successful. However, during
crystallization, we obtained good quality crystal of Z isomer of PTZTPE-3 which was suitable for
X-ray diffraction analysis. The crystal structure of PTZTPE-3 shows bent shape of phenothiazine
and propeller shape of tetraphenylethylene unit (Figure 5(A)). The phenothiazine moiety bends
through S and N atoms with an angle of 139.59° (Figure 5(B)). The phenothiazine moiety shows
a dihedral angle of 52.61° with phenyl acrylonitrile unit and 16.04° with tetraphenylethylene unit
(Figure 5(D) and Figure S4(E)), indicating the twisted confirmation of the molecule. The
PTZTPE-3 crystallizes in the triclinic, P-1 space group. The crystal structure of PTZTPE-3
shows four intermolecular CH…π interactions and one π-π interaction with the neighboring
molecules.
The benzene ring of phenyl acrylonitrile moiety of one PTZTPE-3 molecule interacts with the
two neighboring PTZTPE-3 molecules through two CH…π intermolecular interactions. The one
CH…π intermolecular interaction was observed with the CH group of tetraphenylethylene moiety
of a neighboring PTZTPE-3 molecule by head to tail arrangement making a linear chain of
PTZTPE-3 molecules as shown by blue dashed lines in Figure S4(A). The second CH…π
intermolecular interaction was observed with the CH group of phenothiazine moiety of another
neighboring PTZTPE-3 molecule as shown by green dashed lines in Figure S4(B). A slipped π-π
(4.633 Å) intermolecular interaction was observed between the phenothiazine moiety of one
PTZTPE-3 molecule with the benzene ring of phenyl acrylonitrile moiety of another PTZTPE-3
molecule (Figure S4(C)). The same phenothiazine moiety also interacts with CH groups of
tetraphenylethylene moiety of other neighboring PTZTPE-3 molecule through two CH…π
interactions as shown by green dashed lines in Figure S4(D). The crystal packing diagram of
PTZTPE-3 shows a dimeric unit in which the S atom of phenothiazine of one PTZTPE-3
16

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molecule interacts with the CN group of a neighboring PTZTPE-3 molecule through C(9)–
N(2)...S(1) interaction (3.327 Å) (Figure 5(C)). The loose molecular packing (CH…π
intermolecular interactions with lengths of 3.125-3.468 Å) and the slipped π-π intermolecular
interactions could be the inherent reason for making them readily disorganized by the external
stimuli. Hence, are responsible for their mechanochromic behaviour.
Figure 5 (A) Single crystal structure of PTZTPE-3 (B) Single crystal structure of PTZTPE-3
showing bent shape of phenothiazine (C) A dimeric unit of PTZTPE-3 showing C(9)–N(2)...S(1)
interaction (D) Single crystal structure of PTZTPE-3 showing dihedral angle between
phenothiazine and phenyl acrylonitrile moiety.
Charge-transporting properties, ionization potentials and electron affinities of
the solid samples.
To estimate potential of compounds PTZTPE 1–4 as semiconductors for various organic/hybrid
electronic applications, charge-transporting properties of thick vacuum-deposited films of
PTZTPE 1–4 with the thicknesses (d) higher than 2 µm were studied by the time-of-flight (TOF)
experiments recording TOF current transients under positive (for holes) or negative (for electrons)
applied external voltages (V) using the previously described setups (Figure 6 a,b, S7) [51]. Transit
17

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times (ttr) for holes can be well observed from the corresponding TOF transients indicating low
dispersity of hole transport. However, transit times for electrons were not observed meaning
electron transport was not proved for PTZTPEs by TOF measurements. Thus PTZTPE 1–4 can
be considered as hole-transporting compounds. Taking transit times for holes at the different
applied voltages, hole mobilities were calculated using the formula µ=d2/ V·ttr. Their dependences
on electric field are plotted in Figure 6c. Practically the same hole mobilities of 4.7–7.6×10-6
cm2/Vs at electric field of 4.6×105 V/cm were observed for compounds PTZTPE-2-PTZTPE-4
indicating small effect of electron withdrawing substituents (Table 2). Relatively low hole
mobilities may by predictively explained by non-planar molecular structures of PTZTPE-2-
PTZTPE-4 which do not allow their close molecular packing in solid-state as it was discussed in
the previous section. Because of slightly different values of field dependence parameter (α), their
zero-field mobilities were found in relatively large range of 1.5-43× 10-9 cm2/Vs. In contrast, by
ca. two orders of magnitude higher hole mobility of 1.67×10-4 cm2/Vs at the same electric field of
4.6×105 V/cm was observed for compound PTZTPE-1. Such difference can be related to the
different molecular packing in solid films of compound PTZTPE-1, and, as a result, more
appropriate HOMO-HOMO overlapping for hole hopping between neighboring molecules, in
comparison to those of compounds PTZTPE-2-PTZTPE-4 (Figure S4) [21]. Hole-transporting
properties of compounds PTZTPE 1–4 mainly suggest their potential applications as emitters but
not as hosts or charge-transporting materials which charge mobility commonly exceeded 10-3
cm2/Vs [52]
.
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a)
b)
120V
110V
100V
90V
80V
70V
60V
50V
40V
160V
140V
120V
10-1
100V
10-1
10-2
10-3
80V
60V
40V
10-2
PTZTPE-2: holes
d=2.9 m
PTZTPE-3: holes
d=2.6 m
10-3
10-5
10-4
10-3
10-2
10-5
10-4
10-3
Time, s
10-2
Time, s
c)
d)
1.2
IEPP=5.48 eV
PTZTPE-1
PTZTPE-2
PTZTPE-3
PTZTPE-4
PTZTPE-1
PTZTPE-2
PTZTPE-3
PTZTPE-4
IEPP=5.63 eV
IEPP=5.56 eV
IEPP=5.59 eV
10-3
1.0
0.8
0.6
0.4
0.2
0.0
10-4
10-5
10-6
10-7
10-8
Poole–Frenkel type fitting
0
200
400
600
800
1000
5.0
5.2
5.4
5.6
5.8
6.0
6.2
6.4
E1/2 (V1/2/cm1/2
)
Energy, eV
Figure 6 (a, b) TOF signals for holes for the layers of PTZTPE-2 and PTZTPE-3. (c) Hole
mobilities versus electric fields and (d) photoelectron emission spectra of vacuum-deposited films
of compounds PTZTPE 1–4.
The results of photoelectron emission spectroscopy measurements of solid samples PTZTPE 1–4
(Figure 6 d, Table 2) show that electron withdrawing substituents expectedly have effect on energy
levels of the compounds. The values of ionization potential (IpPESA) of the PTZTPE-2-PTZTPE-4
having were higher in comparison to that of compound PTZTPE-1. It should be noted that values
of both IpPESA and electron affinity (Eafilm) for all studied materials are in perfect ranges of 5.48–
5.59 and 2.6–3.24 eV for hole and electron injection from electrodes of charge-transporting layers,
respectively.
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Table 2 Energy levels and hole mobility data for the vacuum-deposited layers of compounds
PTZTPE 1–4.
Compound
µh (cm2 V-1 s-1)*
µ0 (cm2 V-1 s-1)
α (cm V-1)1/2
IpPESA (eV)
PTZTPE-1 PTZTPE-2 PTZTPE-3 PTZTPE-4
1.67 × 10-4
4.2 × 10-6
0.0055
5.48
6.5 × 10-6
1.5 × 10-9
0.009
5.63
7.6 × 10-6
3.3 × 10-9
0.0081
5.56
4.7 × 10-6
4.3 × 10-8
0.0069
5.59
Egfilm (eV)
2.88
2.73
2.32
2.64
Eafilm (eV)
2.6
2.9
3.24
2.95
*values recorded at electric field of 4.6×105 V/cm. µh is hole mobility. µ0 is zero-field
mobility. Field dependence parameter (α) of a Poole–Frenkel type mobility predicted by the
훼퐸1/2
PESA
relationship
. Ip
is ionization potential taken for vacuum-deposited films by
휇 = 휇0푒
photoelectron spectroscopy in air (PESA), Eg is optical band gap taken from low-energy
onsets of absorption spectra in solid state, Eafilm is electron affinity calculated by formula
film
Eafilm= Ip- Eg
.
Electroluminescent properties.
Aiming to study electroluminescent properties of compounds PTZTPE 1–4, vacuum-deposited
and solution-processed non-doped OLEDs were fabricated exploiting device structures ITO/HAT-
CN(8 nm)/TCTA(32 nm)/EML (24 nm)/TSPO1(8 nm)/TPBi(40 nm)/LiF(0.5 nm):Al and
ITO/HAT-CN (8 nm)/EML (40 nm)/TSPO1 (8 nm)/TPBi (48 nm)/LiF(0.5 nm):Al, respectively.
The EML were vacuum-deposited or spin-coated light-emitting layers of compounds PTZTPE-1,
PTZTPE-2, PTZTPE-3 or PTZTPE-4. Hole-injector hexaazatriphenylenehexacabonitrile
(HAT-CN), hole-transporter tris(4-carbazoyl-9-ylphenyl)amine (TCTA), hole-blocker
diphenyl[4-(triphenylsilyl)phenyl]phosphine oxide (TSPO1); electron-transporter 2,2’,2’’-(1,3,5-
benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), electron-injector fluorolithium (LiF) were
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selected for the vacuum-deposited devices. The hole-transporting layer of TCTA was not used in
solution-processed OLEDs to prevent its washing-out during spin-coating of EML of emitters of
PTZTPE family. The relationship between energy levels of the selected functional materials are
demonstrated in Figure 7 a. According this relationship, good hole/electron injection to the light-
emitting layers and hole/electron blocking within these layers is expected. This expectation is in
very good agreement with the electroluminescence (EL) spectra of non-doped devices which were
completely related to emission of PTZTPE emitters (Figure 7). The similar EL spectra were
recorded at different voltages meaning that hole-electron recombination zone was not shifted to
the hole or electron transporting layers (Figure S8). The CIE colour coordinates of the vacuum-
processed non-doped OLEDs were related to greenish-blue for device based on PTZTPE-2, green
for device based on PTZTPE-1, yellowish-green for device based on PTZTPE-4, and orange for
device based on PTZTPE-3 (Figure S9, Table 3). Interestingly, PTZTPE-1-based vacuum-
processed OLEDs and PTZTPE-4-based solution-processed OLEDs showed EL spectra with two
bands. This observation is apparently related to MCL properties of the compounds when two kind
of aggregates are formed under the vacuum deposition/solution processing [21]. There are evidences
in the literature that different electroluminescence spectra can be obtained using the same emitter
with mechanofluorochromic properties [53-55]. Such electroluminescence colour changes were
obtained due to treatments of light-emitting layers of MCL emitter-based OLEDs. Different
electroluminescence spectra can be obtained when the light-emitting layer will be spin coated
using different solvents for spin coating of MCL emitter [56]. Such result was explained by
formation of different conformers of the same MCL molecule depending on the fabrication
technology of light-emitting layers. Apparently, different deposition methods (solution or vacuum-
processed ones) caused formation of different ratio of either MCL forms or molecular conformers
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in the light-emitting layers resulting in different EL spectra of PTZTPE-1 and PTZTPE-4 non-
doped OLEDs under electrical excitation. Such effect is more evident for compounds PTZTPE-1
and PTZTPE-4 since their MCL forms were characterized by more different emission spectra that
those of compounds PTZTPE-2 and PTZTPE-3.
a)
b)
1.0
0.8
0.6
0.4
0.2
0.0
Vacuum-processed
EL
400
450
450
450
500
500
500
550
550
550
600
600
600
650
700
750
1.0
0.8
0.6
0.4
0.2
0.0
Solution-processed
EL
400
650
700
750
1.0
0.8
0.6
0.4
0.2
0.0
PL
PTZTPE-1
PTZTPE-2
PTZTPE-3
PTZTPE-4
400
650
700
750
Wavelength, nm
Figure 7 (a) Energy levels of the functional layers of non-doped (fluorescent) and doped
(hyperelectrofluorescence based) devices. The energies are given in eV. The figure includes
chemical structure of the conventional blue fluorescent polymeric emitter TFB and TADF co-host
pCNBCzoCF3. (b) EL spectra at 6V of vacuum-processed and solution-processed non-doped
OLEDs based on the corresponding PTZTPE emitters and PL spectra of their neat layers.
Maximum EQE values of vacuum-processed OLEDs were found in the range 1.03–1.73% (Table
3). This result is in good agreement with PLQY values of the corresponding light-emitting layers
(24% for PTZTPE-1, 28% for PTZTPE-2 and for PTZTPE-3, 19% for PTZTPE-4) taking into
account a formula ηext=γ×PLQY×χ×ηout (Formula 1) where the charge balance factor γ is of 1 for
the perfectly optimized device structure, the efficiency of exciton production χ is of 0.25 for
conventional fluorescent emitters such as PTZTPEs, the outcoupling efficiency ηout is of 0.2-0.3
for glass subtract based devices [23]. Maximum EQE values of solution-processed OLEDs were
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found to be considerably lower apparently because of the absence of hole-transporting layers in
their structures leading to decrease of the charge balance factor γ. Very low maximum EQE of
0.027% was obtained for solution-processed PTZTPE-4-based OLEDs in contrast to that of
vacuum-deposited ones. This result is related to the quality of spin-coated films based on
compound PTZTPE-4. Similarly, efficiency of PTZTPE-4:pCNBCzoCF3:mCP-based device
was much lower than efficiencies of PTZTPE-1:pCNBCzoCF3:mCP-based and PTZTPE-
3:pCNBCzoCF3:mCP-based analogs (Table 3).
PLQY values of 3, 30, 66, and 6% were obtained for low-polarity toluene solutions of PTZTPE-
1, PTZTPE-2, PTZTPE-3 and PTZTPE-4, respectively. Taking into account PLQYs of the
corresponding solutions, the usage of appropriate hosts can predictively allow to enhance PLQYs
values in solid state at least for compounds PTZTPE-2 and PTZTPE-3. Indeed, PLQY values of
30, 24, 39, and 17% were obtained for mCP-doped films of PTZTPE-1, PTZTPE-2, PTZTPE-3
and PTZTPE-4, respectively. However, these values may allow only slightly to increase the EQEs
of the PTZTPEs-based devices. To improve the efficiency of exciton production from χ=0.25 to
χ=1 in the PTZTPEs-based OLEDs, TADF exhibiting compound pCNBCzoCF3 was selected as
co-host (Figure 7a) [41,42]. pCNBCzoCF3 was selected taking into account to the requirements for
hyperfluorescence discussed in the introduction. For example, HOMOPTZTPE-3
>
HOMOpCNBCzoCF3>HOMOmCP and LUMOPTZTPE-3<LUMOpCNBCzoCF3<LUMOmCP (Figure 7a). The
different absorption-emission spectral overlaps of co-hosts:emitter systems was identified for
compounds PTZTPE-3 and PTZTPE-2 (Figure S10a). PL spectra of non-doped films of
PTZTPE-1, PTZTPE-3 and PTZTPE-4 were red-shifted in comparison to PL spectrum of the
non-doped film of pCNBCzoCF3 supposing efficient co-host→emitter FRET (Figure S10b). In
contrast, PL spectrum of film PTZTPE-2 is blue-shifted comparing to that of the film of
23

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pCNBCzoCF3 supposing inefficient FRET from pCNBCzoCF3 to PTZTPE-2. In addition, it is
not possible to get white electroluminescence mixing the blue emissions of TFB and PTZTPE-2.
Therefore, compound PTZTPE-2 was not used for the development of pCNBCzoCF3-based
hyperfluorescence systems. The doped films of the systems PTZTPE-1:pCNBCzoCF3:mCP,
PTZTPE-3:pCNBCzoCF3:mCP and PTZTPE-4:pCNBCzoCF3:mCP with hyperfluorescence
properties were prepared by spin coating with the optimized concentrations of 2, 15 and 83 wt%
of PTZTPEs, pCNBCzoCF3 and mCP respectively. To develop white hyperelectrofluorescent
OLEDs, the solution-processing was selected mainly because of the possibility to better control
the doping concentration in comparison to vacuum based one. It is important especially in our case
when the three component- based light-emitting layer PTZTPE-3(2 wt%):pCNBCzoCF3(15
wt%):mCP(83 wt%) with precisely optimized concentration was used in the device structure.
Moreover, there is practical interest for solution-processing technology since it is appropriate for
large area devices and it has its own advantages (e.g., high processing efficiency, low-cost, low-
waste of materials, easy scalability) in comparison to the vacuum technology [57]. PL spectra of the
films of co-host containing systems PTZTPE-1:pCNBCzoCF3:mCP, PTZTPE-
3:pCNBCzoCF3:mCP and PTZTPE-4:pCNBCzoCF3:mCP were completely related to emission
of compounds PTZTPE-1, PTZTPE-3 or PTZTPE-4 proving perfect energy transfer from hosts
to the emitter of low concentration (2%), (Figure 8a). Meanwhile, PL decays of co-host-based
films were characterized by longer lifetimes in comparison to the prompt fluorescence lifetimes of
PTZTPEs (Figure 8b, Table S6). This observation may be explained by triplet harvesting via
TADF host pCNBCzoCF3 which typically is attributed to hyperfluorescence [30-32]. PL decay of
the system PTZTPE-3:pCNBCzoCF3:mCP was fitted by single exponential function with the
lifetime of 6.7 ns due to the efficient FRET. The systems PTZTPE-1:pCNBCzoCF3:mCP and
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PTZTPE-4:pCNBCzoCF3:mCP systems were characterized by double exponentials PL decays
(Figure 8b, Table S6). The efficient FRET of the system PTZTPE-3:pCNBCzoCF3:mCP can be
explained big absorption-emission spectral overlap with the emission-absorption of pCNBCzoCF3
(Figure S10). Hyperfluorescence allows to obtain the efficiency of exciton production with
theoretical limit of 100% under electrical excitation[29-31]. Predictively, EQEs of PTZTPEs-based
OLEDs can be significantly improved according to their hyperfluorescence with χ of 1 (Figure 8,
Formula 1).
a)
b)
10000
1000
100
10
pCNBCzoCF3:mCP
1.0
0.8
0.6
0.4
0.2
0.0
pCNBCzoCF3:mCP
PTZTPE-1:pCNBCzoCF3:mCP
PTZTPE-1:mCP
PTZTPE-1:mCP
PTZTPE-1:pCNBCzoCF3:mCP
fitting
fitting
1
10
10
100
1000
pCNBCzoCF3:mCP
10000
1000
100
10
400
400
400
450
450
450
500
500
500
550
550
550
600
600
600
650
700
750
1.0
0.8
0.6
0.4
0.2
0.0
pCNBCzoCF3:mCP
PTZTPE-3:pCNBCzoCF3:mCP
PTZTPE-3:mCP
PTZTPE-3:mCP
PTZTPE-3:pCNBCzoCF3:mCP
1
100
1000
650
700
750
10000
1000
100
10
pCNBCzoCF3:mCP
1.0
0.8
0.6
0.4
0.2
0.0
pCNBCzoCF3:mCP
PTZTPE-4:pCNBCzoCF3:mCP
PTZTPE-4:mCP
PTZTPE-4:mCP
PTZTPE-4:pCNBCzoCF3:mCP
fitting
10
1
650
700
750
100
1000
Wavelength, nm
Time, ns
Figure 8. PL spectra (a) and PL decay curves (b) of the doped spin-coated films co-host:host
(pCNBCzoCF3:mCP),
emitter:host
(PTZTPEs:mCP)
and
emitter:co-host:host
(PTZTPEs:pCNBCzoCF3).
Mixing orange hyperfluorescence of the system PTZTPE-3:pCNBCzoCF3:mCP with blue
emission of the layer of polymer TFB [58] (Figure 9a, S10), it is predictively possible to obtain
white hyperelectroluminescence similar to that of many other types of white OLEDs (Figure 9a)
[59-61]. The corresponding coordinates can be found in Figure 9a. If to draw a line through those
CIE colour coordinates of PTZTPE-3:pCNBCzoCF3:mCP and TFB, this line will go near the
coordinate of nature white (0.33, 0.33). This analyse means that white electroluminescence can be
25

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obtained by mixing emission of TFB and PTZTPE-3 if the device structure is well optimized. To
prove this prediction, OLEDs with the structure
ITO/MoO3(2nm)/TFB(40nm)/EML(40nm)/TSPO1(8nm)/TPBi(40)/LiF:Al were fabricated using
the systems PTZTPE-1:pCNBCzoCF3:mCP, PTZTPE-3:pCNBCzoCF3:mCP or PTZTPE-
4:pCNBCzoCF3:mCP as EML. Step-by-step spin-coating of the TFB and EML layers was possible
since the layer TFB was insoluble after its thermal treatment at 150 °C during 30 min in nitrogen
atmosphere. In solution-processed OLEDs, the layer of TFB acted not only as blue emitter but also
as hole-transporting and electron-blocking layer due to its high hole mobility (ca. 2×10-3 cm2/Vs
at the electric field of 2.5×105 V/cm), low ionization potential (5.3 eV) and high electron affinity
(2.3 eV) [62]. Because of both good balance of hole and electron mobilities within EML and
hyperelectroluminescence phenomenon, maximum EQEs of solution processed OLEDs based on
the systems PTZTPE-1:pCNBCzoCF3:mCP, PTZTPE-3:pCNBCzoCF3:mCP or PTZTPE-
4:pCNBCzoCF3:mCP were much improved in comparison to EQEs of non-doped PTZTPEs-
based solution processed devices (Figure 9b, Table 3). EQE of 8.2% was obtained for OLED based
on the system PTZTPE-3:pCNBCzoCF3:mCP. The low turn-on voltage of 4.5 V and the high
maximum brightness of 3400 cd/m2 were recorded for the device (Figure 9c, Table 3).
Electroluminescence of TADF and/or phosphorescent (rare-earth-atom-containing) emitters is
mainly exploited in state-of-art white OLEDs [63, 64]. Meanwhile, examples of efficient white
electroluminescence from simple fluorescent emitters only (from hyperfluorescence only, thus
without TADF or phosphorescence) are very limited due to the best of our knowledge. Those
examples are related to vacuum-deposited white OLEDs with maximum EQE of 12.1 and 12.9%
in the best cases [46, 47]. EQE (8.2 %) of the developed white solution-processed PTZTPE-3-based
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hyperelectrofluorescence-based OLEDs is not far below of the efficiencies of the mentioned
examples.
a)
b)
CIE 1931
10
1
0.8
PTZPTE-3:pCNBzoCF :mCP
3
0.6
0.4
0.2
0.0
(0.467, 0.512)
5V
(0.289, 0.403)
9V
(0.265, 0.349)
75
70
65
60
55
nature white
(0.33, 0.33)
0.1
0.01
PTZTPE-1: pCNBCzoCF3:mCP
PTZTPE-3: pCNBCzoCF3:mCP
PTZTPE-4: pCNBCzoCF3:mCP
TFB
(0.15, 0.11)
5
6
7
8
Voltage, V9
0.0
0.1
0.2
0.3
0.4
x
0.5
0.6
0.7
0.8
0.1
1
10
100
Current density, mA/cm2
c)
d)
500
450
400
350
300
250
200
150
100
50
1.0
0.8
0.6
0.4
0.2
0.0
PTZTPE-1: pCNBCzoCF3:mCP
5V
6V
7V
8V
9V
PTZTPE-1:pCNBCzoCF3:mCP
PTZTPE-3:pCNBCzoCF3:mCP
PTZTPE-4:pCNBCzoCF3:mCP
10000
(2:15:83wt%)
400
450
450
450
500
500
500
550
550
550
600
650
700
1.0
0.8
0.6
0.4
0.2
0.0
1000
100
10
PTZTPE-3:
pCNBCzoCF3:mCP
(2:15:83wt%)
400
600
650
700
1.0
0.8
0.6
0.4
0.2
0.0
PTZTPE-4: pCNBCzoCF3:mCP
(2:15:83wt%)
0
0
1
2
3
4
5
6
7
8
9
10
11
12
400
600
650
700
Voltage, V
Wavelength, nm
Figure 9 CIE1931 colour coordinates (a) for blue and orange emissions of spin-coated light-
emitting layers of TFB and PTZTPE-3: pCNBCzoCF3:mCP as well as for EL spectra recorded at
different voltages (the step is 1V) of OLEDs based on those layers. The inset shows CRI-applied
voltage dependence of OLEDs. EQE-current density (b) voltage-current density and voltage-
brightness (c) characteristics as well as EL spectra at different applied voltages of the solution
processed hyperelectroluminescence based devices.
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OLEDs based on the system PTZTPE-3:pCNBCzoCF3:mCP were characterized by EL spectra
with CIE1931 colour coordinates ((0.28, 0.38) at 6V) close to the CIE colour coordinates (0.33,
0.33) of nature white (Figure 9a,d, Table 3). White hyperelectroluminescence with CRI value of
67 at 6 V was obtained for such OLEDs (Figure 9a, inset, Table 3). In the well-optimized device
structure, such result can be explained by overlapping of blue emission of TFB and orange
emission of PTZTPE-3 of the similar intensities. It should be noted that PL spectrum of
pCNBCzoCF3:mCP film shows a peak of ~440 nm (Figure 8a). The EL spectra of PTZTPE-3-
based OLEDs are characterized by the high-energy band peaked at wavelength of ~440 nm (Figure
9d). Despite similar maximum wavelengths (~440 nm) of PL spectrum of pCNBCzoCF3: mCP
film and EL spectra of PTZTPE-based OLEDs, the observed bands are originated from different
emitters from pCNBCzoCF3 (in case of PL spectrum of the pCNBCzoCF3:mCP film) and from
TFB (in case of PTZTPE-3-based OLEDs). Mainly, it is explained by the fact that there is no
pCNBCzoCF3:mCP film in the studied device structure. In the device structure, there is three
component-based film PTZTPE-3:pCNBCzoCF3:mCP which emission is resulted from the
emitter PTZTPE-3 but not from pCNBCzoCF3 (Figure 8a). It is also evident from comparison of
EL spectra of the studied devices with PL spectrum of TFB and pCNBCzoCF3:mCP films since
1) the shape of the blue band of EL spectra is very similar to the shape of PL spectrum of TFB
film but not to that of pCNBCzoCF3:mCP film; and 2) the set-on of PL spectrum of
pCNBCzoCF3:mCP film and EL spectra of PTZTPE-based devices as well as the set-on of PL
spectrum of TFB film are at different wavelengths (Figure S10c).
Because of electron-blocking properties of TFB layer and hole blocking properties of TSPO1 layer
(Figure 7a), the injected hole-electron pairs recombine within the orange light-emitting layer on
mCP and pCNBCzoCF3 co-hosts forming both singlet and triplet excitons. Triplets can be
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Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/D0TC03136D
converted to singlets due to RISC of TADF co-host pCNBCzoCF3. At the same time, singlets can
be transferred from co-host mCP to blue TFB emitter and from co-host pCNBCzoCF3 to orange
emitter PTZTPE-3 via FRET mechanism (Figure 1b, 7a). Stable color of white
hyperelectroluminescence is expected in such case. However, intensity of blue band of EL spectra
of OLEDs based on the system PTZTPE-3:pCNBCzoCF3:mCP-based OLEDs increased with
increasing of applied voltages (Figure 9d). This observation can apparently be attributed to direct
hole-electron recombination within TFB layer which lead to slight instability of white color of the
devices based on the system PTZTPE-3:pCNBCzoCF3:mCP (Figure 7a). More stable colors of
electroluminescence were obtained for OLEDs based on the systems PTZTPE-
1:pCNBCzoCF3:mCP and PTZTPE-4:pCNBCzoCF3:mCP (Figure 9d). However, their EL
spectra were far from white since emission spectra of PTZTPE-1 and PTZTPE-4 in the co-hosted
films were close to emission spectrum of the layer of TFB.
29