1. Highly efficient organic light-emitting
diodes from delayed fluorescence
~ 3rd Generation OLED ~
Hiroki et al. Nature 492, 234-238 (2012)
Chris Huang
2. General Idea of OLED
Cathode
Electron Injection Layer (EIL)
Electron Transport Layer (ETL)
Emission Layer (EML)
Hole Transport Layer (HTL)
Hole Injection Layer (HIL)
Anode
HOMO
LUMO
e-
h+
Electro-luminescence
(EL)
Device Structure
LUMO : Lowest Unoccupied Molecular Orbital
HOMO : Highest Occupied Molecular Orbital
3. Three Generations of OLED
HOMO
LUMO
S0
S1
T1
25%
75%
e-
S : Singlet state
T : Triplet state
Fluorescence Phosphorescence
1st Gen.
2nd Gen.
3rd Gen.
ISC
RISC
Low efficiency
(25% upper limit)
Heavy atom
requirement
(toxic & pricy)
Special
Molecular Design
(difficult)“TADF”
4. Spin-Orbital Coupling
(R) ISC = (Reverse) Inter-System Crossing
(R) ISC & Phosphorescence both come from S-O coupling
Due to S-O coupling, ml, ms are not constants,
mj becomes “good” quantum number.
1
Ψ HSO
3
Ψ ∝ 1
φ
Zµ
riµ
3
i
n
∑
µ
N
∑
!
Li
3
φ ⋅
<
1
2
αβ − βα( )
!
S
αα
ββ
1
2
αβ + βα( )
⎛
⎝
⎜
⎜
⎜
⎜
⎜
⎜
⎞
⎠
⎟
⎟
⎟
⎟
⎟
⎟
>
Ψ = φ(r)⋅σ (α,β) = (spatial)⋅(spin)
Beljonne et al. J. Phys. Chem. A 105, 3899-3907 (2001)
Fermi’s Golden Rule :
kISC =
2π
!
1
Ψ HSO
3
Ψ
2 1
4πλRT
exp −
ΔE + λ( )2
4λRT
⎛
⎝
⎜
⎞
⎠
⎟
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
∆E : energy gap between the initial and final state
λ : Marcus reorganization energy
Tricky part !
Tricky part !
!
j =
!
l +
!
s
Z axis
j
s
l
5. TADF Materials
TADF = Thermal Activated Delayed Fluorescence
any molecular
pidly, yielding
t excitons can
t annihilation,
fluorescence).
sphorescence,
conventional
n of phospho-
tives in 199013
.
erved only at
y useless even
o involve both
1999, efficient
idium phenyl-
decay rate of
it coupling of
almost 100%
ED technology
44 Motooka, Nishi, Fukuoka 819-0395, Japan. 2
International Institute for Carbon Neutral Energy Research (WPI-I2CNER),
b
Phosphorescence
S0
TADF
4CzPN: R = carbazolyl
2CzPN: R = H
4CzIPN 4CzTPN: R = H
4CzTPN-Me: R = Me
4CzTPN-Ph: R = Ph
NC CN
RR
NN
N
N
N
N
NC CN
R
R
R
R
R
R
R
R
CN
CN
N
N N
N
Figure 1 | Energy diagram and molecular structures of CDCBs. a, Energy
diagram of a conventional organic molecule. b, Molecular structures of CDCBs.
Me, methyl; Ph, phenyl.
millan Publishers Limited. All rights reserved
Electron Withdrawing Group (EWG):
a. Cyano groups (CN) suppress geometrical change.
Electron Donating Group (EDG):
b. Bulky Carbazoyl groups break molecular symmetry.
Molecular Geometry :
c. Twisted structure separates the electron density
between HOMO and LUMO.
A
B C
6. of transient states such as T1. Studies of the T1
states by IR in rare-gas matrices have in fact been
plate was cooled by a closed cycle helium refriger-
ator (CTI Cryogenics, Model M-22) to about 16 K.
Infrared spectra of the matrix samples were mea-
sured with an FTIR spectrophotometer (JEOL,
Model JIR-7000). The spectral resolution was
0:5 cmÀ1
, and the number of accumulation was 64.
Other experimental details were reported elsewhere
[4,14]. UV light coming from a superhigh-pressure
mercury lamp (500 W) was focused on the matrix
sample through a quartz lens to increase popula-
tions of the T1 state, where a water filter was used
to remove thermal radiation.
The DFT calculations were performed by us-
ing the GAUSSIANAUSSIAN 98 program [16] with the
6-31++G** basis set, where BeckeÕs three-
parameter hybrid density functional [17], in
combination with the Lee–Yang–Parr correlation
functional (B3LYP) [18], was used to optimize
geometrical structures and estimate vibrational
Fig. 1. Numbering of atoms: (a) 1,2-dicyanobenzene and (b)
1,4-dicyanobenzene.
656 N. Akai et al. / Chemical Physics Letters 371 (2003) 655–661
A. Cyano Group
1,2-dicyanobenzene
Photo-excitation
Akai et al. Chem. Phys. Lett. 371, 655-661 (2003)
Acknowledgements
The authors thank Professors Kozo Kuchitsu
[6] E.T. Harrigan, T.C. Wong, N. Hirota, Chem. Phys. Let
14 (1972) 549.
[7] A.G. Merzlikine, S.V. Voskresensky, E.O. Danilov, M.A.J
Rodgers,D.C.Neckers,J. Am.Chem.Soc.124 (2002)14532
[8] H. Krumschmidt, C. Kryschi, Chem. Phys. 154 (1991) 459
[9] R.H. Clarke, P.A. Kosen, M.A. Lowe, R.H. Mann, R
Mushlin, J. Chem. Soc. Chem. Commun. (1973) 528.
[10] K. Nishikida, Y. Kamura, K. Seki, N. Iwasaki, M
Kinoshita, Mol. Phys. 49 (1983) 1505.
[11] J. Baiardo, R. Mukherjee, M. Vala, J. Mol. Struct. 8
(1982) 109.
[12] M.B. Mitchell, G.R. Smith, W.A. Guillory, J. Chem. Phy
75 (1981) 44.
[13] B. Hoestrey, M.B. Mitchell, W.A. Guillrory, Chem. Phy
Lett. 142 (1987) 261.
[14] S. Kudoh, M. Takayanagi, M. Nakata, J. Mol. Struct. 47
(1999) 253.
[15] S. Aich, S. Basu, Chem. Phys. Lett. 281 (1997) 247.
[16] M.J. Frisch et al., GAUSSIANAUSSIAN 98, Revision A.6, Gaussian
Inc., Pittsburgh, PA, 1998.
[17] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.
[18] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785
[19] C.G. Barraclough, H. Bisset, P. Pitman, P.J. Thistlethwai
Aust. J. Chem. 30 (1977) 753.
[20] M.A.C. Castro-Pedrozo, G.W. King, J. Mol. Spectrosc. 7
Fig. 4. Mulliken spin density distributions in the T1 state: (a)
1,2-dicyanobenzene and (b) 1,4-dicyanobenzene.
N. Akai et al. / Chemical Physics Letters 371 (2003) 655–661 66
Table 3
Calculated geometry parameters of 1,2-dicyanobenzene in the S0 and T1 states
Parametera
S0 state T1 state
Calc. Obs.b
Calc.
Bond length (AA)
C1–C2 1.416 1.395(5) 1.532
C2–C3 1.403 1.395(5) 1.424
C3–C4 1.394 1.395(5) 1.367
C4–C5 1.398 1.395(5) 1.482
C1–C7 1.434 1.444(11) 1.391
C7–N1 1.163 1.161(2) 1.178
C3–H1 1.084 1.087(5) 1.085
C4–H2 1.085 1.087(5) 1.084
Bond angle (°)
C1–C2–C3 119.5 120.2(5) 118.2
C2–C3–C4 120.3 119.7(8) 121.1
C3–C4–C5 120.2 120.2(5) 120.7
C1–C2–C8 121.1 120.0(15) 120.6
C1–C7–N1 178.3 179.4
C2–C3–H1 119.0 118.2
C3–C4–H2 119.6 120.4
a
Numbering of atoms is defined in Fig. 1.
b
Electron diffraction data [29]; averaged values for the C–C and C–H lengths of the benzene ring are given.
Table 4
Calculated geometry parameters of 1,4-dicyanobenzene in the S0 and T1 states
Parametera
S0 state T1 state
Calc. Obs.b
Calc.
Bond length (AA)
C1–C2 1.406 1.397(3) 1.472
C2–C3 1.390 1.397(3) 1.349
C1–C7 1.435 1.454(5) 1.388
C7–N1 1.164 1.167(2) 1.180
C2–H1 1.084 1.084
660 N. Akai et al. / Chemical Physics Letters 371 (2003) 655–661
In excited state, C=C would be made,
which suppress geometrical change, and
lower the “reorganization energy” λ.
C − C ≡ N ⎯ →⎯ C = C = N
kISC =
2π
!
1
Ψ HSO
3
Ψ
2 1
4πλRT
exp −
ΔE + λ( )2
4λRT
⎛
⎝
⎜
⎞
⎠
⎟
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
Smaller λ, larger kISC kRISC
7. B. Carbazoyl Group
Beljonne et al. J. Phys. Chem. A 105, 3899-3907 (2001)
lanar
D2h
or an
er of
es for
metry
ween
plane
. Of
pin-
ronic
weak
e the
from
small
tures
th of
ower
ither
cited
for this observation, because twisted structures are characterized
by higher excitation energies and the IC rate decreases with
increasing energy separation.) Molecular disorder thus appears
as a key parameter in the control of the nonradiative decay rates
and the singlet emission quantum efficiencies. In that respect,
the very high photoluminescence efficiency of ladder-type poly-
(paraphenylene)s (LPPP) in solution (on the order of 80%33)
appears to be related to the particularly low intrachain disorder
in this polymer. Using femtosecond time-resolved spectroscopy,
Rentsch and co-workers have demonstrated that the high
generation of triplets in Th2 and Th3 arises because of a very
efficient ISC channel involving the unrelaxed, nonplanar singlet
S1 excited state and a closely lying triplet state.10 This supports
the second scenario described above as a possible mechanism
for the intersystem crossing process in unsubstituted oligothio-
phenes.
In our approach, the SOC expectation values have been
computed for a series of model compounds, where we impose
a twist of the aromatic rings along the conjugation path
following an helical conformation. This is depicted below for
the thiophene trimer (θ is the interannular twist angle, taken
here as a free parameter): Since internal conversion is usually
a very fast process (the IC decay rates are on the order of 1012-
1013 s-1), intersystem crossing is likely to take place from the
lowest singlet excited state in its relaxed geometry. Note,
however that, upon excitation in the high-energy domain of the
optical spectrum of polythiophene (around 6 eV), a new efficient
channel for intersystem crossing opens up, which involves high-
lying singlet and triplet excited states most likely localized on
the thiophene aromatic rings.34 Here, all spin-orbit coupling
elements have been computed with the lowest singlet excited
state, S1, as the initial state. Note that all valence molecular
phene
xis)
xis)
xis)
the symmetry selection rules for spin-orbit coupling. In planar
conformations, the oligo(phenylene ethynylene)s have D2h
symmetry, whereas the oligothiophenes have either C2h (for an
even number of aromatic rings) or C2V (for an odd number of
aromatic rings) symmetry. Inspection of the character tables for
these point groups indicates that, depending on the symmetry
of the initial and final excited states (see Table 1), ISC between
π-π* excited states is forbidden, except in the out-of-plane
direction (and hence negligible for planar compounds). Of
course, as for optical transitions, the selection rules for spin-
orbit mixing can be somewhat relaxed through vibronic
couplings, though this second-order effect is expected to be weak
in most cases.21 As both ISC and phosphorescence involve the
SOC expectation values, these processes are predicted, from
simple symmetry arguments, to occur with a very small
crossing channel due to the enhanced SOC in those conjugated
chains that likely keeps a nonplanar conformation in the excited
state. (Note that internal conversion is not a likely explanation
for this observation, because twisted structures are characterized
by higher excitation energies and the IC rate decreases with
increasing energy separation.) Molecular disorder thus appears
as a key parameter in the control of the nonradiative decay rates
and the singlet emission quantum efficiencies. In that respect,
the very high photoluminescence efficiency of ladder-type poly-
(paraphenylene)s (LPPP) in solution (on the order of 80%33)
appears to be related to the particularly low intrachain disorder
in this polymer. Using femtosecond time-resolved spectroscopy,
Rentsch and co-workers have demonstrated that the high
generation of triplets in Th2 and Th3 arises because of a very
efficient ISC channel involving the unrelaxed, nonplanar singlet
S1 excited state and a closely lying triplet state.10 This supports
the second scenario described above as a possible mechanism
for the intersystem crossing process in unsubstituted oligothio-
phenes.
In our approach, the SOC expectation values have been
computed for a series of model compounds, where we impose
a twist of the aromatic rings along the conjugation path
following an helical conformation. This is depicted below for
the thiophene trimer (θ is the interannular twist angle, taken
here as a free parameter): Since internal conversion is usually
a very fast process (the IC decay rates are on the order of 1012-
1013 s-1), intersystem crossing is likely to take place from the
lowest singlet excited state in its relaxed geometry. Note,
however that, upon excitation in the high-energy domain of the
Figure 2. Energy diagram for the lowest singlet and triplet excited
states in (a) the phenylene ethynylene trimer, Ph3, and (b) the thiophene
trimer, Th3. Coplanar conformations are considered.
TABLE 1: Symmetry Selection Rules for Intersystem
Crossing
symmetry
group
initial state
symmetry
final state
symmetry polarization
D2h B3u B3u forbidden
B2u B2u forbidden
B2u B1u out-of-plane
C2h Bu Bu out-of-plane
Ag Ag out-of-plane
Ag Bu forbidden
C2V B1 B1 forbidden
A1 A1 forbidden
A1 B1 out-of-plane
C2 B B in-plane (short axis)
A A in-plane (short axis)
A B in-plane (long axis)
1
Ψ HSO
3
Ψ ∝ 1
φ
Zµ
riµ
3
i
n
∑
µ
N
∑
!
Li
3
φ
∝ 1
φ
i
n
∑
Lxi
Lyi
Lzi
⎛
⎝
⎜
⎜
⎜
⎞
⎠
⎟
⎟
⎟
3
φ
∝ 1
φ
i
n
∑
Rxi
Ryi
Rzi
⎛
⎝
⎜
⎜
⎜
⎞
⎠
⎟
⎟
⎟
3
φConsider
Selection Rule
kISC =
2π
!
1
Ψ HSO
3
Ψ
2 1
4πλRT
exp −
ΔE + λ( )2
4λRT
⎛
⎝
⎜
⎞
⎠
⎟
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
Due to steric hindrance, bulky group would
rotate and show a large dihedral angle between
molecular main plane and its plane, which break
the molecular symmetry and make S-O coupling
allowed.
Larger HSO, larger kISC kRISC
8. C. Twisted Molecular Structure
carbazole as a donor and
Fig. 1b). Because the carba-
dicyanobenzene plane by
cular orbital and the lowest
mitters are localized on the
y, leading to a small DEST.
zolyl groups are important
ficiency and various emis-
e derivatives are known to
s, changing their electronic
DFT) calculations predicted
ing advantages. The cyano
ivation and changes in the
, leading to a high quantum
engths of CDCBs should be
ting ability of the peripheral
g the number of carbazolyl
molecular design should
ent TADF but also a wide
mercially available starting
m or other rare-earth-metal
CBs cost effective. Nucleo-
anions generated by treat-
room temperature (300 K)
ned in high yields of .79%,
btained in lower yield (38%
ication problems. CDCBs
frared spectroscopy, high-
ental analysis (Methods).
mple, in thermogravimetric
flow conditions, 4CzIPN
decomposing.
toluminescence spectra of
b c
d
S0
S1
Energy
ΔES ΔES@S1
S*
S
300 400 500 600 700
Wavelength (nm)
0
(a.u.)
ΔES@S0
Figure 2 | Photoluminescence characteristics of 4CzIPN. a, Ultraviolet–
visible absorption and photoluminescence spectra of 4CzIPN in toluene at a
concentration of 1025
mol l21
. a.u., arbitrary units. b, c, Highest occupied NTO
(b) and lowest unoccupied NTO (c) according to the results of time-dependent
ΔEST = ES1
− ET1
∝ φH
*
(r1)φL
*
(r2 )∫
1
r1 − r2
φH (r2 )φL (r1)dr1dr2
HOMO LUMO
In the excited state, EDG and EWG could achieve
an “intramolecular charge transfer” (ICT) behavior,
and separate the charge more completely if the
structure is twisted (TICT state).
PACS: 82.50, 85.60, 42.55M
Twisted Intramolecular Charge Transfer States
(TICT) were first introduced by Grabowski, Rot-
kiewicz et al. [1, 2] to account for the anomalous dual
fluorescence of dimethylaminobenzonitrile DMABN
(1) observed by Lippert et al. [3] in polar solvents.
According to this model, TICT states are accessible in
multichromophoric systems possessing an electron
donor D and an electron acceptor A only if they are
weakly coupled. The classical TICT arrangement is
to twist the n-systems D and A against each other
around a common single bond [2] but spatial sepa-
ration of D and A is also effective [-4].
In systems like DMABN which are flexible but
planar in the ground state, the formation of the TICT
state involves intramolecular twisting in the excited
state which can be viewed as an adiabatic photoreac-
tion proceeding on the $1 hypersurface [-4] (Fig. 1).
The originally reached locally excited LE state
with planar conformation has only partial CT charac-
6- 6+ e
~ i f~/phoforeacfion ~--i~/~ 7 u ILI
LE sfafe '
TICT sfafe
($1, ptonar,parfia[ ET)
($I, fwisfed, full CT)
Fig. 1. Schematic formation of a TICT state from the locally
excited (LE) precursor state by an adiabatic photoreaction
ter (large mesomeric interaction between D and A
results in uncomplete charge separation) whereas in
the twisted TICT conformation, either a full or no
electronic charge is transferred from D to A, at least in
the simple n model, because the mesomeric interaction
between D and A is blocked.
Experimentally, the large charge separation of
TICT states manifests itself by a strong redshift of the
emitted TICT fluorescence in more polar solvents
(positive solvatochromism) [24] or by its sizeable
response to applied electric fields (electrooptical emis-
sion measurements) [5]. The twisting hypothesis could
be shown by chemical means to be true, namely by
comparing the bridged model compounds 2_ and 3
(only LE fluorescence band) and the twisted com-
pound 4 (only TICT fluorescence band) to 1 (both LE
and TICT fluorescence bands).
TICT states a populated by many bi- and multi-
chromophoric systems [2, 4] ranging from a multitude
of aromatic amines to biaromatic compounds, laser
dyes,liquid crystalsand biologicallyimportant systems.
The systems can be arranged into two groups (Fig. 2):
Those which undergo an intramolecular twisting re-
W. Rettig Appl. Phys. B 45, 145-149 (1988)
kISC =
2π
!
1
Ψ HSO
3
Ψ
2 1
4πλRT
exp −
ΔE + λ( )2
4λRT
⎛
⎝
⎜
⎞
⎠
⎟
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
Smaller ΔE, larger kISC kRISC
Density Distribution
Difference between
HOMO and LUMO
Ex. 4CzIPN :
9. Verification
11.2 6
of conv
Fina
ISC wit
of spin
ISC and
heavy m
10
10
10
10
Externalelectroluminescencequantumefficiency(%)
Figure
electrolu
OLEDs
triangle
emitters
accordin
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Photoluminescencequantumefficiency
0 50 100 150 200 250 300
Temperature (K)
Combined
Prompt
Delayed
0 10 20 30 40
Time (μs)
300 K
200 K
100 K
100
10–1
10–2
10–3
Normalizedphotoluminescenceintensity
Prompt
Delayed
400 500 600
1
0
3.5 4.0 4.5 5.0
10–3/T (K–1)
14.2
14.0
13.8
13.6
13.4
13.2
13.0
12.8
12.6
12.4
In(kRISC
)
a b
c d
300 K
ΔEST = 83 meV
Figure 4 | Temperature dependence of photoluminescence characteristics
of a 5 6 1 wt% 4CzIPN:CBP film. a, Photoluminescence decay curves of a
6 wt% 4CzIPN:CBP film at 300K (black line), 200 K (red line) and 100 K (blue
line). The photoluminescence decay curves show integrated 4CzIPN emission.
The excitation wavelength of the films was 337nm. b, Photoluminescence
spectrum resolved into prompt and delayed components. c, Temperature
luminescent TADF materials. Because these two properties conflict
with each other, the overlap of the highest occupied molecular orbital
and the lowest unoccupied molecular orbital needs to be carefully
a
b
Fluorescence
Phosphorescence
Electrical
excitationS1
S0
25%
75%
T1
~0.5–1.0 eV
TADF
e h
4CzPN: R = carbazolyl
2CzPN: R = H
4CzIPN 4CzTPN: R = H
4CzTPN-Me: R = Me
4CzTPN-Ph: R = Ph
NC CN
RR
NN
N
N
N
N
NC CN
R
R
R
R
R
R
R
R
CN
CN
N
N N
N
Figure 1 | Energy diagram and molecular structures of CDCBs. a, Energy
diagram of a conventional organic molecule. b, Molecular structures of CDCBs.
Me, methyl; Ph, phenyl.
ΔEST
TADF = Thermal Activated Delayed Fluorescence
me competitive non-radiative decay pathways, leading to highly
escent TADF materials. Because these two properties conflict
ach other, the overlap of the highest occupied molecular orbital
e lowest unoccupied molecular orbital needs to be carefully
Fluorescence
Phosphorescence
Electrical
excitationS1
S0
25%
75%
T1
~0.5–1.0 eV
TADF
e h
CN
N
R R
CN
Prompt
∵ Small HSO
Vibration level
Figure info :
a. Transient emission spectra with temperature
dependence.
b. The emission spectra of prompt and delayed
fluorescence are the same.
c. Temperature dependence of emission
quantum efficiency.
d. Arrhenius plot of kRISC, ∆EST = 83meV can be
obtained.
Temperature dependence of
emission characteristics of 4CzIPN
Figure :
10. Result Performance
groups on the geometry relaxation of the S1 and T1 states of 4CzIPN, we
evaluated vertical transition energies (DES@S0 and DES@S1) and relaxa-
tion energies (lS and lS*) for the S1 state (Fig. 2d), and compared them
with those of the molecule in which the two cyano groups of 4CzIPN are
replaced with hydrogen atoms (4CzBz; Supplementary Information).
The reorganization energies lS for 4CzIPN and 4CzBz were calculated
to be 0.27 and 0.83 eV, respectively. This energy is greatly reduced by
introducing cyano groups into the electron-accepting unit. Because lS
represents the degree of geometry relaxation of the S1 state to the S0
state, this result suggests that the cyano groups are important in sup-
pressing geometry relaxation in the fluorescent state of 4CzIPN. In
addition, lS* is also reduced by the presence of cyano groups.
Torsional angles of the carbazolyl groups are calculated to be small in
the presence of the cyano groups. This limited torsional flexibility can be
a major factor in reducing the non-radioactive decay of 4CzIPN. Like
that for the S1 state, the relaxation energy for the T1 state (lT) is mark-
edly reduced by the cyano groups. Thus, it is probable that the cyano
groups suppress non-radiative deactivation from the S1 and T1 states,
leading to the high photoluminescence quantum efficiency of 4CzIPN.
Photoluminescence spectra of the CDCBs in toluene are presented
in Fig. 3. The series of CDCBs yielded a wide range of emission colours
ranging from sky blue (473 nm) to orange (577 nm). The emission
wavelength depends on the electron-donating and -accepting abilities
of the peripheral carbazolyl groups and the central dicyanobenzene
unit, respectively. Introduction of methyl or phenyl substituents at the
3- and 6- positions of the carbazolyl groups of 4CzTPN induces a shift
of the emission maximum to longer wavelengths. Conversely, in the
case of 2CzPN, the presence of fewer carbazolyl groups reduces its
electron-donating ability and produces a shift of the emission maxi-
mum to shorter wavelengths. We measured the photoluminescence
quantum yield and transient photoluminescence of CDCBs in toluene
under a nitrogen atmosphere, and are summarized in Supplementary
Information. For 4CzPN and 4CzTPN W is high (74 6 3% and
72 6 3%, respectively), whereas for 4CzTPN-Me, 4CzTPN-Ph and
2CzPN it is lower (47 6 2%, 26 6 1% and 47 6 2%, respectively)
because of substituent effects or fewer carbazolyl groups. Because
the transient photoluminescence of all CDCBs showed both a nano-
second-scale prompt component and a microsecond-scale delayed
component, the CDCBs were confirmed to be TADF materials.
Figure 4a shows the photoluminescence decay curves for emission
of 4CzIPN at 100, 200 and 300 K in a 5 6 1 wt% 4CzIPN:4,49-bis
(carbazol-9-yl)biphenyl (CBP) film. The triplet excitons of 4CzIPN
are well confined using a CBP host because the T1 state of CBP is
higher in energy than the S1 state of 4CzIPN. In addition, the fluo-
integrated-sphere photoluminescence measurement system and the
temperature dependence of the photoluminescence decay curves
(Supplementary Fig. 2). The prompt component increases very slightly
as the temperature decreases, indicating the suppression of non-
radiative decay from the S1 state. Conversely, the delayed component
decreases monotonically as the temperature decreases because reverse
ISC becomes the rate-determining step, similar to the temperature
dependence of tin IV fluoride/porphyrin complexes, which are typical
TADF emitters14
. At room temperature (300 K), a high W value, of
83 6 2%, was observed. To evaluate DEST quantitatively, we estimated
the activation energy of the reverse ISC rate constant (kRISC) from
exp(2DEST/kBT), where kB is the Boltzmann constant and T is tem-
perature. This rate constant can be estimated from experimentally
determined rate constants and the W values of the prompt and delayed
components at each temperature using24
kRISC~
kpkd
kISC
Wd
Wp
ð1Þ
b
2CzPN
4CzIPN
4CzPN
4CzTPN
4CzTPN-Me
4CzTPN-Ph
400 500 600 700
Wavelength (nm)
Normalize
Figure 3 | Photoluminescence of the CDCB series. a, Photoluminescence
spectra measured in toluene. b, Photograph under irradiation at 365nm.
c hindrance
uses a large
carbazolyl
cupied and
ng toa small
PN is small
gesting that
e molecular
y related to
of the cyano
4CzIPN, we
and relaxa-
pared them
4CzIPN are
formation).
e calculated
reduced by
Because lS
te to the S0
ant in sup-
4CzIPN. In
no groups.
be small in
bility can be
CzIPN. Like
lT) is mark-
t the cyano
d T1 states,
of 4CzIPN.
e presented
ion colours
he emission
ing abilities
anobenzene
uents at the
a
b
2CzPN
4CzIPN
4CzPN
4CzTPN
4CzTPN-Me
4CzTPN-Ph
400 500 600 700
Wavelength (nm)
Normalizedphotoluminescenceintensity
2CzPN
4CzIPN
4CzPN
4CzTPN
4CzTPN-Me
4CzTPN-Ph
Figure 3 | Photoluminescence of the CDCB series. a, Photoluminescence
spectra measured in toluene. b, Photograph under irradiation at 365nm.
Sky Blue Green Orange
TADF Material 2CzPN 4CzIPN 4CzTPN
Internal Quantum Efficiency 26.7 ~ 40.0% 64.3 ~ 96.5% 37.3 ~ 56.0%
11. • TADF material is a kind of pure organic
material, but can achieve the quantum
efficiency of phosphorescent OLED.
• By engineered molecular design, other
metal-free OLED materials could be
developed in the future.
• Unfortunately, TADF technology is
exclusive to Adachi’s group and his
collaborator so far ….
Conclusion
Chihaya Adachi
