A thermally activated delayed blue fluorescent emitter with reversible externally tunable emission

Article abstract of DOI:10.1039/c5tc03943f

A thermally activated delayed fluorescent (TADF) material containing two meta carbazolyl groups on the phenyl ring of the benzoyl-4-pyridine moiety was prepared and its luminescence showed a colour change from blue to green by external stimuli. The organi

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A thermally activated delayed blue fluorescence emitter with
reversible externally tunable emission
Pachaiyappan Rajamalli,a Natarajan Senthilkumar,a Parthasarathy Gandeepan,a Chen‐Zheng Ren‐
Received 00th January 20xx,
Accepted 00th January 20xx
Wu,b Hao‐Wu Linb and Chien‐Hong Cheng*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
A thermally activated delayed fluorescence (TADF) material compounds have been designed and synthesized.7 However,
containing two meta carbazolyl groups on the phenyl ring of the developed blue TADF emitter to achieve efficient OLEDs are
benzoyl‐4‐pyridine moiety was prepared and its luminescence limited. Therefore, there is
a strong demand for the
shows colour change from blue to green by external stimuli. The development of high efficiency blue TADF OLEDs to replace the
organic‐blue‐light‐emitting device using this TADF dopant provided expensive and less stable blue phosphorescent materials.
a high external quantum efficiency of 18.4%.
Although conventional fluorescent materials and metal
complexes have been reported with mechanoluminescent
properties, TADF materials with reversible tunable emission
have not been reported so far. Hence, the development of new
organic TADF materials that can be applied in efficient organic
light‐emitting diodes (OLEDs) and also exhibit solid state
enhanced emission and mechanoluminescence is of great
importance both scientifically and practically.
Tuning and controlling the solid‐state emission of π‐conjugated
organic materials constitute an attractive area of research for
both fundamental interest and as a key component of many
advanced applications in the fields of lighting, data storage,
security inks, sensors, and re‐writable media.1 Re‐writable
materials would significantly reduce printer paper usage and
save millions of trees.2 The promising features of these
materials has attracted significant attention and a number of
studies have been reported on vapo, mechano/piezo, and
acido‐responsive π‐conjugated organic fluorescent materials.3
Nevertheless, materials with easy access, multiple applications,
and high‐contrast reversible‐switching luminescence remain
rare. Very recently, thermally activated delayed fluorescence
(TADF) materials have attracted much attention due to their
wide applicability in areas such as display, lighting, and
biotechnology.4 Mainly, TADF materials shed light on OLED
development due to 100% internal quantum efficiency can be
achieved by harvesting both singlet and triplet excitons by
reverse intersystem crossing (RISC) from the lowest triplet (T1)
state to lowest singlet (S1) state.5 A very small energy gap
between the S1 and T1 states (ΔEST) is required to activate TADF
because the rate of RISC is inversely proportional to the
exponent of ΔEST.6 Based on this required criteria, TADF
Here, we have synthesized a benzoylpyridine‐based blue
TADF
emitter,
(3,5‐di(9H‐carbazol‐9‐yl)phenyl)(pyridin‐4‐
yl)methanone (mDCBP) (Scheme 1) which exhibits highly
enhanced solid state emission compared to the solution
emission. Simultaneously, this molecule exhibits reversible
switchable emission via alternating applied mechanical force
and solvent fuming. This is a development over most reported
mechanoluminescence (ML) materials that exhibited weak
solid‐state emission. mDCBP also exhibited a very small ΔEST of
0.06 eV, with small spatial overlap between highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO). Transient PL measurements revealed the TADF
property clearly. The use of this TADF emitter for blue and green
OLEDs gave high external quantum efficiency (EQE) of 18.4%
and 14.7%, respectively.
The mDCBP can be easily synthesized in two steps as shown
in Scheme 1, with the detailed synthetic procedure given in the
Supporting Information. The compound was purified by
temperature‐gradient vacuum sublimation and characterized
aDepartment of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan.
E‐mail: chcheng@mx.nthu.edu.tw
1
by H and 13C NMR, and high‐resolution mass spectrometry.
Further, the structure was confirmed by single‐crystal X‐ray
analysis. As depicted in Fig. 1a, the HOMO of mDCBP is
distributed over one of the carbazolyl groups, with the HOMO+1
localized over the other carbazolyl group (Fig. S1). Conversely,
bDepartment of Materials Science and Engineering, National Tsing Hua University,
Hsinchu 30013, Taiwan
Electronic Supplementary Information (ESI) available: General information,
computational details, UV‐Vis, Pl, CV, TGA, DSC, current density and luminance vs
driving voltage and current efficiency vs luminance . See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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time‐dependent DFT (TDDFT) revealed that the LUMO is mostly
The electrochemical properties of mDCBP were investigated
localized on the benzoyl pyridine (BP) core. There is only a small by cyclic voltammetry (Fig. S3). From the onset of oxidation
overlap of the HOMO and LUMO at the centre phenyl group. In potential (Fig. S3), the HOMO level was calculated to be 5.72 eV.
addition, the calculation showed a small singlet and triplet The LUMO energy level as estimated from HOMO ‐ ES is 2.72 eV.
energy gap (ΔEST) of 0.15 eV; this value is comparable to In deoxygenated benzene a quantum yield of 6% is found; this
reported TADF emitters.5
decreases to 1.7% in the presence of oxygen. This efficiency
decrease in the presence of oxygen also supports assignment of
TADF5a and suggests that the T1 states are readily quenched by
the oxygen triplet ground state.
O
N
O
Br
In addition, an absolute photoluminescence quantum yield
was measured in a co‐doped film (DPEPO:mDCBP (10%)) to be
90%, which is much higher than in solution. This arises from the
absence of intramolecular rotation and collisional quenching in
the solid state. Notably, the PL quantum yield decreased from
90% under nitrogen to 73% under oxygen atmosphere revealing
the formation of triplet via intersystem crossing in the
DPEPO:mDCBP thin film during the PL process (vide supra). It is
known that a triplet excited state can be readily quenching by
oxygen.5a The transient PL decay characteristic of the material
in toluene was measured under vacuum and is shown in Fig. 2.
As shown in Fig. 2, transient decay curves can be divided into
two components. The first represents the prompt emission
decay curve from S1 to the singlet ground state (S0); the
Br
Cu, K2CO3
DCB, 180 o
i) n-BuLi
N
C
ii) 4-Cyanopyridine
N
Br
Br
N
Br
mDBBP
mDCBP
Scheme 1 Synthesis of mDCBP.
The absorption and emission spectra of mDCBP in various
solvents are shown in Fig. 1 and Fig. S2 and are summarized in
Table 1. In these solutions mDCBP exhibits a broad absorption
at 372 nm, attributed to the charge transfer (CT) associated with
the electron transfer from the carbazole groups to benzoyl
pyridine. No significant change was observed in the absorption
spectra in various solvents. Conversely, mDCBP displays a
significant solvatochromic effect of its fluorescent emission,
from deep blue (422 nm) in n‐hexane to green (536 nm) in
dichloromethane (Fig. 1c). The phosphorescent emission
spectrum of mDCBP, taken at 77 K in toluene (10‐5 M), is also
shown in Fig. 1b; the observed peaks were centred at 467 nm.
The triplet energy levels were calculated to be 2.94 eV from the
onset of the phosphorescence spectrum. The singlet energy
gaps calculated from the onset of fluorescence spectrum was
3.0 eV as summarized in Table 1. The ΔEST was estimated to be
0.06 eV based on the onset of emission in the fluorescence and
phosphorescence spectra; these values are even smaller than
that obtained from the TDDFT calculation. The small ΔEST
indicate that this molecule plausibly possess TADF properties
with efficient up‐conversion from T1 to S1.
emission lifetime (
plot to be 6.2 ns. The second component represents a delayed
emission component with = 0.2 μs; the delayed emission may
) was calculated from the intensity vs time

be rationalized as the thermal up‐conversion of T1 to S1,
followed by fluorescent relaxation from S1 to the S0. The short
excited‐state lifetime of this molecule is lower than the highly
emissive phosphorescent iridium complexes.8 This is an
important feature of TADF emitters that realizes high quantum
efficiency and reduces non‐radiative decay.
The transient Pl characteristics were measured at
temperature from 200‐300 K and shown in Figure S4a. The
delayed emission component gradually increases with
increasing temperature from 200 K to 300 K, due to RISC is
increased when the temperature increased. The results further
support that mDCBP is TADF material.7c The delayed emission
component intensity at 300 K slightly decreased compared to
that at 250 K, due to the fact that the T1 to S0 non‐radiative
decay increased at higher temperature. The prompt and decay
PL spectra of the co‐doped film are shown in Figure S4b. The
delayed PL component can be ascribed to the TADF emission
because the prompt and delayed emission spectra coincide with
each other. The thermal properties of these emitters were
mDCBP
1
Prompt
0.1
0.01
Delay
1E-3
Fig. 1 a) Optimized structure and calculated HOMO and LUMO of mDCBP; b) Absorption
(Abs.) and emission (Fl.) spectra of mDCBP (10‐5 M) were measured at room temperature
in toluene and the phosphorescence (phos.) spectrum measured at 77 K; c) emission
spectra of mDCBP (10‐5 M) in various solvents at room temperature.
1
2
Time, s
Fig. 2 Transient PL trace of mDCBP.
2 | J. Name., 2012, 00, 1‐3
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Table 1. Physical properties of mDCBP
λabs
Compound
λem
λem
HOMO
(eV)e
LUMO
(eV)f
Thin film
Tg (°C)c
105
Td (°C)d
394
Eg (eV)g
3.0
ET (eV)h
(nm)a
(nm)a
(nm)b
(%)i
mDCBP
334, 372
467
467
5.72
2.72
2.94
90
a
Measured in toluene (1×10‐5 M) at room temperature. bPhosphorescence measured in toluene (1×10‐5 M) at 77 K. cObtained from DSC measurements. dObtained from
TGA measurement. eMeasured from the oxidation potential in 10‐3 M DCM solution by cyclic voltammetry. fMeasured from HOMO ‐ Eg. gEstimated from the onset of
fluorescence spectrum. hEstimated from the onset of phosphorescence spectrum. iAbsolute total PL quantum yield evaluated for 10 wt% mDCBP doped in DPEPO film
using an integrating sphere.
investigated by thermogravimetric analysis (TGA) and nm after grinding. This is likely due to an increase in
differential scanning calorimetry (DSC) under a nitrogen intermolecular interactions resulting from the external
atmosphere; results are shown in Fig. S5 and S6. The compound pressure. As shown in Fig. S6, the powder X‐ray diffraction
possesses very high thermal stability with a decomposition (PXRD) pattern of mDCBP shows intense, sharp peaks for the
temperature (Td) of 394 °C and a glass transition temperature crystalline form, with peak intensity drastically reduced after
(Tg) of 105 °C. This high thermal stability endows high grinding. The intensity decrease indicates that the grinding
morphological stability in the film.
process converted the crystalline mDCBP into an amorphous
It is interesting to mention that mDCBP exhibits blue state. The observed small peaks in the PXRD pattern indicated
emission (460 nm) in the crystalline phase and green emission that a small part of the crystalline sample remained after
(510 nm) when in the amorphous glassy form. This tunable grinding, as can be seen from the emission spectra.
emission is shown in Fig. 3. These two different forms were
Interestingly, the green colour reverts to the blue colour
obtained by controlling the cooling rate of the vacuum after solvent diffusion. To demonstrate a practical application
sublimation process, with slow cooling yielding the crystalline of mDCBP, ‘NTHU’ was written on the ground sample using a
form and fast cooling yielding the glassy form. Further, the glass rod, with the letters showing high‐contrast green
crystalline form shows reversible high‐contrast piezochromism. emission. The letters were erased upon exposure to DCM
As a crystalline sample was ground with a mortar and pestle, a solvent vapour for 3 min followed by writing ‘TADF’. This
drastic colour change from blue to green was observed (Fig. S7). reversibility of emission can be repeated many times without
Importantly, the absolute fluorescence quantum yield was fatigue.
enhanced from 55 to 82% by grinding the crystalline sample into
an amorphous state. Grinding of such crystalline solids induces
a remarkable emission colour change from blue to green, a
spectral emission shift up to 40 nm, as well as a noticeable 27%
increase in emission quantum yield.
1.0
Fig. 4 Photograph of a) crystalline, b) after grinding, c) after solvent diffusion, d) writing
of ‘NTHU’ e) letter erasing by solvent diffusion and f) writing of ‘TADF’ on the material.
Crystal
Crystal grin
Glassy form
0.8
0.6
0.4
0.2
0.0
Single‐crystal XRD was used to understand the crystalline
form molecular packing (structure shown in Fig. 5). The single‐
crystal analysis reveals that the molecules form dimers via
intermolecular C=O‐‐‐H interactions. This analysis also revealed
that the pyridine ring interacts with a carbazole group of
450
500
550
600
650
Wavelength, nm
Fig. 3 Emission spectra of crystalline, amorphous, and glassy forms of mDCBP (insets:
corresponding photograph under UV light)
This mechanochromism appears to be reversible as
indicated by exposure of the ground sample to DCM solvent
vapour for 3 min, with the emission colour reverting to that of
the crystalline sample (Fig. 4). As indicated in Fig. 4, the
emission maximum of mDCBP at 460 nm was red‐shifted to 500
Fig. 5 Molecular packing of mDCBP.
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Table 2. The EL performances of the device A‐D using mDCBP as a dopanta,b
Device
A
Vd (V)
3.6
L (cd/m2, V)
1870, 14.5
EQE (%, V)
18.4, 3.0
CE (cd/A, V)
34.0, 4.0
PE (lm/W, V)
26.5, 4.0

max (nm)
474
CIE (x,y), 8V
(0.16;0.25)
37.5, 3.5
33.5, 3.5
B
C
D
3.2
3.0
2.8
3400, 12.0
5010, 13.5
8900, 18.0
17.9, 3.5
17.0, 3.5
14.7, 3.5
483
488
500
(0.16;0.30)
(0.17;0.33)
(0.22;0.44)
38.0, 3.5
42.8, 3.5
33.9, 3.5
38.2, 3.5
aDevice configuration for A‐D: ITO/NPB (40 nm)/mCP (10 nm)/DPEPO: mDCBP (10‐30 wt%) (30 nm)/ PPT (5 nm)/ TmPyPb (60 nm)/LiF (1 nm)/Al (100 nm); bVd, The
operating voltage at a brightness of 1 cd/m2; L, maximum luminance; EQE, maximum external quantum efficiency; CE, maximum current efficiency; PE, maximum power
efficiency; and λmax, the wavelength where the EL spectrum has the highest intensity.
another molecule. This interaction leads to blue emission, with
this arrangement being destroyed by external pressure. The
increase in intermolecular interaction then leads to red‐shifted
b)
emission.
To investigate the electroluminescence properties of
mDCBP, a multilayer OLEDs was fabricated using mDCBP doped
films as the emitting layer (EML). The device structure and
A
B
C
D
10
molecular structure used in the devices are shown in Fig. S8.
Devices A‐D is constructed with various concentration of
mDCBP with the following device structure: ITO/NPB (40
nm)/mCP (10 nm)/DPEPO: mDCBP (X wt%) (30 nm)/PPT (5
nm)/TmPyPb (60 nm)/LiF (1 nm)/Al (100 nm), where x = 10, 15,
20, 30 and the corresponding devices are referred to as devices
A, B, C and D, respectively. In this device, N,N′‐bis(1‐naphthyl)‐
N,N′‐diphenyl‐1,1′‐biphenyl‐4,4′‐diamine (NPB) was used as a
hole injection material, 1,3‐bis(9‐carbazolyl)benzene (mCP) as a
hole‐transporting material and also as an exciton blocker to
prevent exciton diffusion to the NPB layer, while oxybis(2,1‐
phenylene))bis(diphenylphosphine oxide (DPEPO) was used as
1
10
100
1000
10000
Luminance, cd/m2
Fig. 6 a) External quantum efficiency of device A‐D (inset: electroluminescent
spectra of device A‐D) and, b) Current efficiency vs luminance of device A‐D.
Electroluminescence performance of the devices were
shown in Fig.6, Fig.S9 and summarised in Table 2. It suggest that
when the emitter concentration increased from 10‐30% turn‐on
voltage of the devices are decreased 3.6 V to 2.8 V. EL spectra
of the DPEPO:mDCBP devices in Fig. 6a show a gradual shift of
the emission peak to long wavelength by increase of doping
concentration because of intense intermolecular interaction.
The colour coordinate of the devices (A‐D) was shifted from
(0.16, 0.25) at 10% doping to (0.22, 0.44) at 30% doping.
However, the EQE of the device gradually increased from D to
A, when the concentration of the emitter decreased from 30‐
10% (Table 2). Electroluminescence spectra of devices A‐D
remain relatively stable with increase in driving voltages, as
shown in Fig. S10. Device A gave blue electroluminescence with
a turn on voltage of 3.6 V and CIE (0.18, 0.25).10 A maximum
external quantum efficiency, current efficiency, and power
efficiency of 18.4%, 34.0 cd/A, and 26.5 lm/W, respectively, was
achieved. These efficiencies are comparable to those of blue
phosphorescent OLEDs10 and much higher than the
conventional fluorescent devices. Device D showed a maximum
luminance up to 8900 cd/m2 at 18 V without any light out‐
coupling enhancement. The observed high device efficiencies
suggest the existence of very efficient thermal up‐conversion of
a
host
material,
dibenzo[b,d]thiophene‐2,8‐
diylbis(diphenylphosphine oxide) (PPT) was used as an exciton
blockers and 1,3,5‐tri(m‐pyrid‐3‐yl‐phenyl)benzene (TmPyPb)
used as the electron transporting material.9
a)
A
B
C
D
10
1.0
0.8
0.6
0.4
0.2
0.0
A
B
C
D
1
400
500
600
Wavelength, nm
10
100
Luminance, cd/m2
1000
10000
4 | J. Name., 2012, 00, 1‐3
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Fig. 7 Transient electroluminescence characteristics of device B measured at 3.5V.
triplet excitons (T1) to S1 for this material. This result agrees well
with the results of transient PL measurements that consisted of
both fast and delayed fluorescent components (Fig. 2). Further,
In summary, we have successfully developed a TADF
emitter, mDCBP, bearing a benzoylpyridine core as an electron‐
accepting unit and two meta carbazolyl groups as electron‐
donating units. This TADF emitter exhibits high‐contrast
reversible tunable emission and can be utilized in rewritable
media. Blue and green OLEDs using mDCBP as the emitter
achieved a maximum EQE value of 18.4 to 14.7%, a brightness
reached up to 8900 cd/m2. Therefore, this multifunctional TADF
fluorescent emitter can be practically applied in OLED lighting
panels and rewritable media.
to
confirm
the
TADF
property,
the
transient
electroluminescence decay was measured for device B at 3.5V.
Fig. 7 shows that the delayed electroluminescence component
lasts for several tens of microseconds. The result indicates that
the EL efficiency is mostly contributed by the delayed
fluorescence, supporting the existence of TADF process in the
device.
1
We thank the Ministry of Science and Technology of Republic of
China (MOST 103‐2633‐M‐007‐001) for support of this research
and the National Center for High‐Performance Computing
(account number: u32chc04) of Taiwan for providing computing
time.
0.1
0
50
100
150
Time, s
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Journal of Materials Chemistry C
Page 6 of 6
View Article Online
DOI: 10.1039/C5TC03943F
A thermally activated delayed blue fluorescence emitter with
reversible externally tunable emission
Pachaiyappan Rajamalli,a Natarajan Senthilkumar,a Parthasarathy Gandeepan,a Chen-Zheng Ren-Wu,b
Hao-Wu Linb and Chien-Hong Cheng*a
aDepartment of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan.
E-mail: chcheng@mx.nthu.edu.tw
bDepartment of Materials Science and Engineering, National Tsing Hua University,
Hsinchu 30013, Taiwan
A TADF material containing a benzoylpyridine core and two meta carbazolyl groups shows externally
switchable emission in the solid states and a high external quantum efficiency of 18.4% when used as a
blue dopant in OLED.