3rd gen. oled -- tadf

Highly efficient organic light-emitting diodes from delayed fluorescence

~ 3rd Generation OLED ~

Hiroki et al. Nature 492, 234-238 (2012)

Chris Huang

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

Three Generations of OLED

HOMO

LUMO

S0

S1T1

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”

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.

1Ψ HSO3Ψ ∝ 1φ

Zµ

riµ3

i

n

∑µ

N

∑!Li

3φ ⋅

< 12

αβ − βα( )!S

ααββ

12

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!

1Ψ HSO3Ψ

2 14πλRT

exp −ΔE + λ( )24λRT

⎛

⎝⎜⎞

⎠⎟⎡

⎣⎢⎢

⎤

⎦⎥⎥

∆E : energy gap between the initial and final state λ : Marcus reorganization energy

Tricky part !

Tricky part !

!j =!l + !s

Z axis

js

l

TADF MaterialsTADF = Thermal Activated Delayed Fluorescence

LETTERdoi:10.1038/nature11687

Highly efficient organic light-emitting diodes fromdelayed fluorescenceHiroki Uoyama1, Kenichi Goushi1,2, Katsuyuki Shizu1, Hiroko Nomura1 & Chihaya Adachi1,2

The inherent flexibility afforded by molecular design has accele-rated the development of a wide variety of organic semiconductorsover the past two decades. In particular, great advances have beenmade in the development of materials for organic light-emittingdiodes (OLEDs), from early devices based on fluorescent mole-cules1 to those using phosphorescent molecules2,3. In OLEDs, elec-trically injected charge carriers recombine to form singlet andtriplet excitons in a 1:3 ratio1; the use of phosphorescent metal–organic complexes exploits the normally non-radiative triplet exci-tons and so enhances the overall electroluminescence efficiency2,3.Here we report a class of metal-free organic electroluminescentmolecules in which the energy gap between the singlet and tripletexcited states is minimized by design4, thereby promoting highlyefficient spin up-conversion from non-radiative triplet states toradiative singlet states while maintaining high radiative decay rates,of more than 106 decays per second. In other words, these mole-cules harness both singlet and triplet excitons for light emissionthrough fluorescence decay channels, leading to an intrinsic fluor-escence efficiency in excess of 90 per cent and a very high externalelectroluminescence efficiency, of more than 19 per cent, which iscomparable to that achieved in high-efficiency phosphorescence-based OLEDs3.

The recombination of holes and electrons can produce light, in aprocess referred to as electroluminescence. Electroluminescence inorganic materials was first discovered in 1953 using a cellulose filmdoped with acridine orange5, and was developed in 1963 using ananthracene single crystal connected to high-field carrier injection elec-trodes1. Electrical charge carriers of both polarities were injected intothe organic layers, and the subsequent carrier transport and recom-bination produced blue electroluminescence originating from singletexcitons; that is, fluorescence. According to spin statistics, carrierrecombination is expected to produce singlet and triplet excitons ina 1:3 ratio6,7, and this ratio has been examined for many molecularsystems8–12. The singlet excitons produced decay rapidly, yieldingprompt electroluminescence (fluorescence). Two triplet excitons cancombine to form a singlet exciton through triplet–triplet annihilation,which results in delayed electroluminescence (delayed fluorescence).Direct radiative decay of triplet excitons results in phosphorescence,but usually occurs only at very low temperatures in conventionalorganic aromatic compounds. The first demonstration of phospho-rescent electroluminescence using ketocoumarin derivatives in 199013.However, the very faint electroluminescence was observed only at77 K, and with difficulty, and was assumed to be virtually useless evenif included in rare-earth complexes, which should also involve bothsinglet and triplet excitons in electrical excitation14. In 1999, efficientelectrophosphorescence was first demonstrated using iridium phenyl-pyridine complexes that achieve an efficient radiative decay rate of,106 s21 by taking advantage of the strong spin–orbit coupling ofiridium2. An internal electroluminescence efficiency of almost 100%was achieved3, providing convincing evidence that OLED technologycan be useful for display and lighting applications.

In the work reported here, we used a novel pathway to attain thegreatest possible electroluminescence efficiency from simple aromaticcompounds that exhibit efficient thermally activated delayed fluo-rescence (TADF) with high photoluminescence efficiency. Figure 1ashows the energy diagram of a conventional organic molecule, depictingsinglet (S1) and triplet (T1) excited states and a ground state (S0). It waspreviously assumed that the S1 level was considerably higher in energythan the T1 level, by 0.5–1.0 eV, because of the electron exchangeenergy between these levels. However, we found that careful designof organic molecules can lead to a small energy gap (DEST) between S1

and T1 levels4,15. Correspondingly, a molecule with efficient TADFrequires a very small DEST between its S1 and T1 excited states, whichenhances T1 R S1 reverse intersystem crossing (ISC). Such excitedstates are attainable by intramolecular charge transfer within systemscontaining spatially separated donor and acceptor moieties4. The cri-tical point of this molecular design is the combination of a small DEST,of = 100 meV, with a reasonable radiative decay rate, of .106 s21, toovercome competitive non-radiative decay pathways, leading to highlyluminescent TADF materials. Because these two properties conflictwith each other, the overlap of the highest occupied molecular orbitaland the lowest unoccupied molecular orbital needs to be carefully

1Center for Organic Photonics and Electronics Research, Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan. 2International Institute for Carbon Neutral Energy Research (WPI-I2CNER),Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan.

a

b

Fluorescence

Phosphorescence

Electrical excitationS1

S0

25%

75%

T1~0.5–1.0 eV

TADF

e h

4CzPN: R = carbazolyl2CzPN: R = H

4CzIPN 4CzTPN: R = H4CzTPN-Me: R = Me4CzTPN-Ph: R = Ph

NC CN

RR

NN

N

NN

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, Energydiagram of a conventional organic molecule. b, Molecular structures of CDCBs.Me, methyl; Ph, phenyl.

2 3 4 | N A T U R E | V O L 4 9 2 | 1 3 D E C E M B E R 2 0 1 2

Macmillan Publishers Limited. All rights reserved©2012

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

of transient states such as T1. Studies of the T1

states by IR in rare-gas matrices have in fact beenreported for several aromatic molecules such asnaphthalene and triphenylene [4,8–13]. The appli-cation of this method to larger molecules has beengreatly facilitated in recent years by the helpof density-functional-theory (DFT) calculations,which provides plausible estimates for spectralpatterns of the T1 states as well as the S0 states[14].

We report here the IR spectra of transientspecies of 1,2- and 1,4-dicyanobenzenes, Fig. 1, inlow-temperature argon matrices produced duringUV irradiation. These molecules are groupedamong electron acceptors in charge-transfer com-plexes [15]. A comparison of the observed spectrawith the IR patterns derived from DFT calculationhas resulted in the assignment of the transientspecies to their T1 states.

2. Experimental and calculation methods

A small amount of 1,2- or 1,4-dicyanobenzene(Tokyo Chemical Industry) was placed in a depo-sition nozzle with a heating system, on which argongas (Nippon Sanso, 99.9999% purity) was flowed toachieve sufficient isolation of the sample. The de-position time for 1,2-dicyanobenzene was about 7 hat 305 K, and about 2 h at 310 K for 1,4-dicy-anobenzene. The mixed gas deposited on a CsI

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 was0:5 cm!1, and the number of accumulation was 64.Other experimental details were reported elsewhere[4,14]. UV light coming from a superhigh-pressuremercury lamp (500 W) was focused on the matrixsample through a quartz lens to increase popula-tions of the T1 state, where a water filter was usedto remove thermal radiation.

The DFT calculations were performed by us-ing the GAUSSIANAUSSIAN 98 program [16] with the6-31++G** basis set, where Becke!s three-parameter hybrid density functional [17], incombination with the Lee–Yang–Parr correlationfunctional (B3LYP) [18], was used to optimizegeometrical structures and estimate vibrationalwavenumbers.

Fig. 1. Numbering of atoms: (a) 1,2-dicyanobenzene and (b)1,4-dicyanobenzene.

Fig. 2. Observed difference spectra and calculated spectralpatterns of 1,2-dicyanobenzene: (a) observed difference spec-trum between those measured after and during UV irradiation.Decreasing and increasing bands represent the reactant, S0, andtransient species, T1. Bands marked with * represent CO2. Vi-brational rotational lines of atmospheric H2O appear aroundthe 1600 cm!1, while noise appearing below 800 cm!1 origi-nates from low sensitivity of the detector. (b) Calculated spec-tral patterns obtained by the DFT/B3LYP/6-31++G** method.The upper and lower patterns refer to the S0 and T1 states,respectively, where a scaling factor of 0.98 is used.

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(Faculty of Science, Josai University) and MasaoTakayanagi (BASE, Tokyo University A&T) fortheir helpful discussions.

References

[1] R.J. Kessler, M.R. Fisher, G.N.R. Tripathi, Chem. Phys.Lett. 112 (1984) 575.

[2] F. Negri, G. Orlandi, A.M. Brouwer, F.W. Langkilde, R.Wilbrandt, J. Chem. Phys. 90 (1989) 5944.

[3] M. Puranik, J. Chandrasekhar, S. Umapathy, Chem. Phys.Lett. 337 (2001) 224.

[4] M. Nakata, S. Kudoh, M. Takayanagi, T. Ishibashi, C.Kato, J. Phys. Chem. A 104 (2000) 11304.

[5] P.J. Wagner, M.L. May, J. Phys. Chem. 95 (1991) 10317.

[6] E.T. Harrigan, T.C. Wong, N. Hirota, Chem. Phys. Lett.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. 80

(1982) 109.[12] M.B. Mitchell, G.R. Smith, W.A. Guillory, J. Chem. Phys.

75 (1981) 44.[13] B. Hoestrey, M.B. Mitchell, W.A. Guillrory, Chem. Phys.

Lett. 142 (1987) 261.[14] S. Kudoh, M. Takayanagi, M. Nakata, J. Mol. Struct. 475

(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. Thistlethwait,

Aust. J. Chem. 30 (1977) 753.[20] M.A.C. Castro-Pedrozo, G.W. King, J. Mol. Spectrosc. 73

(1978) 386.[21] J.F. Arenas, J.I. Marcos, F.J. Ramirez, Spectrochim. Acta

A 44 (1988) 1045.[22] A.P. Kumar, G.R. Rao, Spectrochim. Acta A 53 (1997)

2023.[23] A.P. Kumar, G.R. Rao, Spectrochim. Acta A 53 (1997)

2033.[24] J.T.L. Navarrete, J.J. Quirante, M.A.G. Aranda, V.

Hernandez, F.J. Ramirez, J. Phys. Chem. 97 (1993) 10561.[25] J. Higgins, X. Zhou, R. Liu, Spectrochim. Acta A 53 (1997)

721.[26] H. Hayashi, S. Nagakura, Mol. Phys. 19 (1970) 45.[27] H. Morita, S. Matsumoto, S. Nagakura, Bull. Chem. Soc.

Jpn. 47 (1975) 420.[28] Y. Achiba, K. Kimura, Chem. Phys. Lett. 48 (1977) 107.[29] G. Schultz, J. Brunvoll, A. Almenningen, Acta Chem.

Scand. Ser. A 40 (1986) 77.[30] G. Schultz, Kem. Kozl. 66 (1986) 291.

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) 655–661 661

1,2,4,5-tetracyanopyrazine, etc. by the presentmethod. However, we have not been able tomeasure that for 1,3-dicyanobenzene, thoughits lifetime is longer than those for 1,2- and

1,4-dicyanobenzenes, which are the main subjectof the present Letter. The details of theresults for other species will be reported else-where.

Table 3Calculated 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) 1.532C2–C3 1.403 1.395(5) 1.424C3–C4 1.394 1.395(5) 1.367C4–C5 1.398 1.395(5) 1.482C1–C7 1.434 1.444(11) 1.391C7–N1 1.163 1.161(2) 1.178C3–H1 1.084 1.087(5) 1.085C4–H2 1.085 1.087(5) 1.084

Bond angle (!)C1–C2–C3 119.5 120.2(5) 118.2C2–C3–C4 120.3 119.7(8) 121.1C3–C4–C5 120.2 120.2(5) 120.7C1–C2–C8 121.1 120.0(15) 120.6C1–C7–N1 178.3 179.4C2–C3–H1 119.0 118.2C3–C4–H2 119.6 120.4

aNumbering 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 4Calculated 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) 1.472C2–C3 1.390 1.397(3) 1.349C1–C7 1.435 1.454(5) 1.388C7–N1 1.164 1.167(2) 1.180C2–H1 1.084 1.084

Bond angle (!)C6–C1–C2 120.2 122.1(1) 118.8C1–C2–C3 119.9 120.6C6–C1–C7 119.9 120.6C1–C7–N1 180.0 180.0C1–C2–H1 119.8 118.4

aNumbering of atoms is defined in Fig. 1.b Electron diffraction data [30].

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!

1Ψ HSO3Ψ

2 14πλRT


⎛

⎝⎜⎞

⎠⎟⎡

⎣⎢⎢

⎤

⎦⎥⎥

Smaller λ, larger kISC & kRISC

B. Carbazoyl Group

Beljonne et al. J. Phys. Chem. A 105, 3899-3907 (2001)

the symmetry selection rules for spin-orbit coupling. In planarconformations, the oligo(phenylene ethynylene)s have D2hsymmetry, whereas the oligothiophenes have either C2h (for aneven number of aromatic rings) or C2V (for an odd number ofaromatic rings) symmetry. Inspection of the character tables forthese point groups indicates that, depending on the symmetryof the initial and final excited states (see Table 1), ISC betweenπ-π* excited states is forbidden, except in the out-of-planedirection (and hence negligible for planar compounds). Ofcourse, as for optical transitions, the selection rules for spin-orbit mixing can be somewhat relaxed through vibroniccouplings, though this second-order effect is expected to be weakin most cases.21 As both ISC and phosphorescence involve theSOC expectation values, these processes are predicted, fromsimple symmetry arguments, to occur with a very smallprobability in highly symmetrical, planar, conjugated structures(even if the conjugated backbone includes heavy atoms).If we now impose a twist angle along the conjugated path of

the molecules, the symmetry is lowered to C2 (or lowersymmetry), for which spin-orbit coupling is allowed eitheralong the C2 rotation axis (short axis of the molecule) for excited

states belonging to the same irreducible representation or alongthe main chain axis for excited states belonging to differentrepresentations (Table 1). Therefore, it is likely that rotation ofthe aromatic rings along the conjugated segments will consider-ably enhance the spin-orbit couplings and hence the transitionprobabilities for intersystem crossing and triplet light emission.Note, however, that geometry relaxation in the excited state isexpected to lead to more planar conformations, as a result ofthe increased quinoid character within the thiophene9 orphenylene rings. Two scenarios can then be invoked to explainefficient ISC: either torsional relaxation to a fully planarconformation is impeded by steric hindrance or the process takesplace from an unrelaxed nonplanar singlet excited state con-formation.In the case of polythiophenes, the conformations adopted by

the chains depend on the size and nature of the substituentgroups in positions. Recent experiments by Theander et al. 31

have demonstrated a decrease in fluorescence quantum yieldin solution when grafting bulky groups on the polythiophenechains, resulting mainly from more efficient nonradiative decaychannels; similar findings have been reported by Lanzani et al.32

From the above considerations, these data can be interpretedwithin the first scenario as resulting from a faster intersystemcrossing channel due to the enhanced SOC in those conjugatedchains that likely keeps a nonplanar conformation in the excitedstate. (Note that internal conversion is not a likely explanationfor this observation, because twisted structures are characterizedby higher excitation energies and the IC rate decreases withincreasing energy separation.) Molecular disorder thus appearsas a key parameter in the control of the nonradiative decay ratesand 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 disorderin this polymer. Using femtosecond time-resolved spectroscopy,Rentsch and co-workers have demonstrated that the highgeneration of triplets in Th2 and Th3 arises because of a veryefficient ISC channel involving the unrelaxed, nonplanar singletS1 excited state and a closely lying triplet state.10 This supportsthe second scenario described above as a possible mechanismfor 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 imposea twist of the aromatic rings along the conjugation pathfollowing an helical conformation. This is depicted below forthe thiophene trimer (θ is the interannular twist angle, takenhere 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 thelowest singlet excited state in its relaxed geometry. Note,however that, upon excitation in the high-energy domain of theoptical spectrum of polythiophene (around 6 eV), a new efficientchannel for intersystem crossing opens up, which involves high-lying singlet and triplet excited states most likely localized onthe thiophene aromatic rings.34 Here, all spin-orbit couplingelements have been computed with the lowest singlet excitedstate, S1, as the initial state. Note that all valence molecular

Figure 2. Energy diagram for the lowest singlet and triplet excitedstates in (a) the phenylene ethynylene trimer, Ph3, and (b) the thiophenetrimer, Th3. Coplanar conformations are considered.

TABLE 1: Symmetry Selection Rules for IntersystemCrossing

symmetrygroup

initial statesymmetry

final statesymmetry polarization

D2h B3u B3u forbiddenB2u B2u forbiddenB2u B1u out-of-plane

C2h Bu Bu out-of-planeAg Ag out-of-planeAg Bu forbidden

C2V B1 B1 forbiddenA1 A1 forbiddenA1 B1 out-of-plane

C2 B B in-plane (short axis)A A in-plane (short axis)A B in-plane (long axis)

3902 J. Phys. Chem. A, Vol. 105, No. 15, 2001 Beljonne et al.

the symmetry selection rules for spin-orbit coupling. In planarconformations, the oligo(phenylene ethynylene)s have D2hsymmetry, whereas the oligothiophenes have either C2h (for aneven number of aromatic rings) or C2V (for an odd number ofaromatic rings) symmetry. Inspection of the character tables forthese point groups indicates that, depending on the symmetryof the initial and final excited states (see Table 1), ISC betweenπ-π* excited states is forbidden, except in the out-of-planedirection (and hence negligible for planar compounds). Ofcourse, as for optical transitions, the selection rules for spin-orbit mixing can be somewhat relaxed through vibroniccouplings, though this second-order effect is expected to be weakin most cases.21 As both ISC and phosphorescence involve theSOC expectation values, these processes are predicted, fromsimple symmetry arguments, to occur with a very smallprobability in highly symmetrical, planar, conjugated structures(even if the conjugated backbone includes heavy atoms).If we now impose a twist angle along the conjugated path of

the molecules, the symmetry is lowered to C2 (or lowersymmetry), for which spin-orbit coupling is allowed eitheralong the C2 rotation axis (short axis of the molecule) for excited

states belonging to the same irreducible representation or alongthe main chain axis for excited states belonging to differentrepresentations (Table 1). Therefore, it is likely that rotation ofthe aromatic rings along the conjugated segments will consider-ably enhance the spin-orbit couplings and hence the transitionprobabilities for intersystem crossing and triplet light emission.Note, however, that geometry relaxation in the excited state isexpected to lead to more planar conformations, as a result ofthe increased quinoid character within the thiophene9 orphenylene rings. Two scenarios can then be invoked to explainefficient ISC: either torsional relaxation to a fully planarconformation is impeded by steric hindrance or the process takesplace from an unrelaxed nonplanar singlet excited state con-formation.In the case of polythiophenes, the conformations adopted by

the chains depend on the size and nature of the substituentgroups in positions. Recent experiments by Theander et al. 31

have demonstrated a decrease in fluorescence quantum yieldin solution when grafting bulky groups on the polythiophenechains, resulting mainly from more efficient nonradiative decaychannels; similar findings have been reported by Lanzani et al.32

From the above considerations, these data can be interpretedwithin the first scenario as resulting from a faster intersystemcrossing channel due to the enhanced SOC in those conjugatedchains that likely keeps a nonplanar conformation in the excitedstate. (Note that internal conversion is not a likely explanationfor this observation, because twisted structures are characterizedby higher excitation energies and the IC rate decreases withincreasing energy separation.) Molecular disorder thus appearsas a key parameter in the control of the nonradiative decay ratesand 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 disorderin this polymer. Using femtosecond time-resolved spectroscopy,Rentsch and co-workers have demonstrated that the highgeneration of triplets in Th2 and Th3 arises because of a veryefficient ISC channel involving the unrelaxed, nonplanar singletS1 excited state and a closely lying triplet state.10 This supportsthe second scenario described above as a possible mechanismfor 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 imposea twist of the aromatic rings along the conjugation pathfollowing an helical conformation. This is depicted below forthe thiophene trimer (θ is the interannular twist angle, takenhere 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 thelowest singlet excited state in its relaxed geometry. Note,however that, upon excitation in the high-energy domain of theoptical spectrum of polythiophene (around 6 eV), a new efficientchannel for intersystem crossing opens up, which involves high-lying singlet and triplet excited states most likely localized onthe thiophene aromatic rings.34 Here, all spin-orbit couplingelements have been computed with the lowest singlet excitedstate, S1, as the initial state. Note that all valence molecular

Figure 2. Energy diagram for the lowest singlet and triplet excitedstates in (a) the phenylene ethynylene trimer, Ph3, and (b) the thiophenetrimer, Th3. Coplanar conformations are considered.

TABLE 1: Symmetry Selection Rules for IntersystemCrossing

symmetrygroup

initial statesymmetry

final statesymmetry polarization

D2h B3u B3u forbiddenB2u B2u forbiddenB2u B1u out-of-plane

C2h Bu Bu out-of-planeAg Ag out-of-planeAg Bu forbidden

C2V B1 B1 forbiddenA1 A1 forbiddenA1 B1 out-of-plane

C2 B B in-plane (short axis)A A in-plane (short axis)A B in-plane (long axis)

3902 J. Phys. Chem. A, Vol. 105, No. 15, 2001 Beljonne et al.

1Ψ HSO3Ψ ∝ 1φ

Zµ

riµ3

i

n

∑µ

N

∑!Li

3φ

∝ 1φi

n

∑LxiLyiLzi

⎛

⎝

⎜⎜⎜

⎞

⎠

⎟⎟⎟

3φ

∝ 1φi

n

∑Rxi

Ryi

Rzi

⎛

⎝

⎜⎜⎜

⎞

⎠

⎟⎟⎟

3φConsider Selection Rule

kISC = 2π!

1Ψ HSO3Ψ

2 14πλRT


⎛

⎝⎜⎞

⎠⎟⎡

⎣⎢⎢

⎤

⎦⎥⎥

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

C. Twisted Molecular Structure

balanced. Furthermore, to enhance the photoluminescence efficiencyof a TADF material, the geometrical change in molecular conforma-tion between its S0 and S1 states should be restrained to suppress non-radiative decay. Limited orbital overlap generally results in virtually noemission, as has been shown in benzophenone derivatives. Therefore,it was previously assumed that a high photoluminescence efficiencycould never be obtained from molecules with a small DEST. Here wedemonstrate that it is possible to realize a high photoluminescenceefficiency and a small DEST simultaneously.

We designed a series of highly efficient TADF emitters based oncarbazolyl dicyanobenzene (CDCB), with carbazole as a donor anddicyanobenzene as an electron acceptor (Fig. 1b). Because the carba-zolyl unit is markedly distorted from the dicyanobenzene plane bysteric hindrance, the highest occupied molecular orbital and the lowestunoccupied molecular orbital of these emitters are localized on thedonor and acceptor moieties, respectively, leading to a small DEST.Moreover, the dicyanobenzene and carbazolyl groups are importantin obtaining a high photoluminescence efficiency and various emis-sion colours, respectively. Dicyanobenzene derivatives are known toalter their chemical bonds in excited states, changing their electronicproperties16. Density functional theory (DFT) calculations predictedthat using CDCBs would have the following advantages. The cyanogroups suppress both non-radiative deactivation and changes in thegeometries of the S1 and T1 states of CDCBs, leading to a high quantumefficiency. Conversely, the emission wavelengths of CDCBs should beeasy to tune by changing the electron-donating ability of the peripheralgroups, which can be altered by changing the number of carbazolylgroups or introduced substituents. Such molecular design shouldallow us to achieve not only highly efficient TADF but also a widerange of emission colours.

We synthesized CDCBs from commercially available startingmaterials in a one-step reaction. Palladium or other rare-earth-metalcatalysts were not required, making CDCBs cost effective. Nucleo-philic aromatic substitution of carbazole anions generated by treat-ment with NaH and dicyanobenzenes at room temperature (300 K)yielded the CDCBs. All CDCBs were obtained in high yields of .79%,except 4CzPN and 2CzPN, which were obtained in lower yield (38%and 9%, respectively) because of purification problems. CDCBswere fully characterized by NMR and infrared spectroscopy, high-resolution mass spectrometry and elemental analysis (Methods).They had high thermal stability; for example, in thermogravimetricanalysis measurements under nitrogen-flow conditions, 4CzIPNbegan to sublime at around 450 uC before decomposing.

Ultraviolet–visible absorption and photoluminescence spectra of4CzIPN in toluene are presented in Fig. 2a. 4CzIPN has intense greenemission with a maximum at 507 nm and a high photoluminescencequantum yield (W) of 94 6 2%. The Stokes shift of 4CzIPN is verysmall; generally, the emission produced by intramolecular chargetransfer between donor and accepter units shows a large Stokesshift17. Under nitrogen-saturated conditions, the delayed component(TADF), which had a lifetime of t < 5.1 6 0.5ms, was more than twoorders of magnitude longer than the prompt component, for whicht < 17.8 6 1 ns (Supplementary Information). To determine whetherthe T1 state was involved in luminescence, the transient photolu-minescence and photoluminescence quantum yield of 4CzIPN weremeasured in toluene under an oxygen atmosphere. When oxygen wasbubbled through the solution of 4CzIPN in toluene for 10 min, thelifetime of the delayed component became very short (t < 91 6 3 ns),the prompt component became t < 6.9 6 0.5 ns and W decreased to10%. These results suggest that 4CzIPN is a TADF material because thedelayed fluorescence was substantially quenched by oxygen4.

The geometry of the S0 state of 4CzIPN in the gas phase was opti-mized using DFT and the 6-31G(d) basis set18. Geometry optimiza-tions of the S1 and T1 states were carried out using time-dependentDFT. All ab initio calculations were performed using Gaussian 09software19 without symmetry constraints. For 4CzIPN, generalized

gradient approximation functionals underestimated DEST (the experi-mental value is 83 meV), whereas long-range corrected functionalsoverestimated it (Supplementary Information). The M06-2X func-tional20, a hybrid meta-generalized gradient-approximation functional,yields an intermediate value for DEST and, moreover, by taking intoaccount solvent effects using the polarizable continuum model21,22,predicts an emission wavelength of 460 nm, which is reasonably closeto the experimentally obtained photoluminescence peak at 507 nm intoluene solution. In the following discussion, we use the results cal-culated for M06-2X/6-31G(d). The highest occupied and lowestunoccupied natural transition orbitals23 (NTOs) based on the resultsof time-dependent DFT for the S1 state of 4CzIPN using the optimizedstructure of the S0 state are depicted in Fig. 2b and Fig. 2c, respectively.The highest occupied NTO is delocalized over the four carbazolylmoieties, whereas the lowest unoccupied NTO is centred on the dicya-nobenzene moiety, suggesting that the S1 state has charge transfercharacter; the carbazolyl groups act as electron donors and the

a

b c

d

S0

S1

Ener

gy

ΔES ΔES@S1

OS*

OS

300 400 500 600 700

Wavelength (nm)

5

4

3

2

1

0

Mol

ar e

xtin

ctio

n co

effic

ient

(105

l mol

–1 c

m–1

)P

hotoluminescence intensity (a.u.)

ΔES@S0

Figure 2 | Photoluminescence characteristics of 4CzIPN. a, Ultraviolet–visible absorption and photoluminescence spectra of 4CzIPN in toluene at aconcentration 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-dependentDFT for the S1 state of 4CzIPN using the optimized structure of the S0 state.Colours indicate the different phases of the natural transition orbitals.d, Potential energy surfaces, vertical transition energies (DES@S0 and DES@S1)and relaxation energies (lS and lS*) for the S1 state.

LETTER RESEARCH

1 3 D E C E M B E R 2 0 1 2 | V O L 4 9 2 | N A T U R E | 2 3 5


ΔEST = ES1− ET1

∝ φH* (r1)φL

* (r2 )∫1

r1 − r2φH (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).

Appl. Phys. B 45, 145 149 (1988) Applied .ho,o- physics Physics B and Laser Chemistry 9 Springer-Verlag 1988

Photophysical and Photochemical Switches Based on Twisted Intramolecular Charge Transfer (TICT) States W. Rettig

Iwan-N.-Stranski-Institut, Technische Universit/it Berlin, Strasse des 17. Juni 112, D- 1000 Berlin 12

Received 12 November/987/Accepted 31 December 1987

Abstract. TICT states, accessible in twisted multichromophoric systems, are nonradiative funnels to the ground state in many dyes. By controlling these funnels, faster saturable absorbers for subpicosecond laser pulses can be developed. Oriented assemblies of TICT molecules, as in liquid crystalline polymers, are expected to exhibit light-induced macroscopic charge separation. Chemical approaches to supramolecular bistable species are also shown.

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 separation 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 photoreaction 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~/" phoforeacfio n ~ - - i ~ / ~ 7 u ILI LE s f a f e ' "

TICT s f a f e ($1, ptonar,parfia[ ET) ($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) [ 2 4 ] or by its sizeable response to applied electric fields (electrooptical emission 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 multichromophoric systems [2, 4] ranging from a multitude of aromatic amines to biaromatic compounds, laser dyes, liquid crystals and biologically important 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!

1Ψ HSO3Ψ

2 14πλRT


⎛

⎝⎜⎞

⎠⎟⎡

⎣⎢⎢

⎤

⎦⎥⎥

Smaller ΔE, larger kISC & kRISC

Density Distribution Difference between HOMO and LUMO

Ex. 4CzIPN :

Verification

that kRISC is higher than the non-radiative rate constant of the T1 state.It should be noted that for temperatures less than 200 K the decaycurves do not agree with the double-exponential decay model, butare described well by a multi-exponential decay model. This can beexplained by the widened DEST distribution caused by inhomogeneousmolecular environments at lower temperatures.

We then evaluated the performance of OLEDs containing theCDCB derivatives 4CzIPN (green emission), 4CzTPN-Ph (orangeemission) and 2CzPN (sky-blue emission) as emitters. Figure 5 showsthe external electroluminescence quantum efficiency of OLEDs con-taining the CDCB derivatives. To achieve high electroluminescenceefficiency in these OLEDs, the T1 state of the CDCB derivatives mustbe confined using a host material with a higher triplet energy level.Therefore, we used CBP as a host material in the green and orangeOLEDs and used 2,8-bis(diphenylphosphoryl)dibenzo[b,d]thiophene(PPT) as a host material in the sky-blue OLED. The structures of theOLEDs were composed of multiple layers of indium tin oxide (ITO),4,4-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl (a-NPD, 35 nm),5 6 1 wt% 4CzIPN or 5 6 1 wt% 4CzTPN-Ph:CBP (15 nm), TPBi(65 nm), LiF (0.8 nm), Al (70 nm) and ITO (100 nm), a-NPD(40 nm), 1,3-bis(9-carbazolyl)benzene (mCP, 10 nm), 5 6 1 wt%2CzPN:PPT (20 nm), PPT (40 nm), LiF and Al. For the green OLED, avery high external electroluminescence quantum efficiency, of19.3 6 1.5%, was achieved, which is equivalent to an internal electro-luminescence quantum efficiency of 64.3–96.5% assuming a light out-coupling efficiency of 20–30% (refs 25, 26). The orange and sky-blueOLEDs had external electroluminescence quantum efficiencies of

11.2 6 1% and 8.0 6 1%, respectively, which are also higher than thoseof conventional fluorescence-based OLEDs.

Finally, we consider the mechanism that drives such efficient reverseISC without heavy metals. It is generally accepted that the introductionof spin–orbit coupling provided by heavy atoms is required for bothISC and reverse ISC to be efficient. Thus, metal complexes containingheavy metals and aromatic compounds with halogens and carbonylscan promote efficient spin conversion. However, our novel moleculardesign produces highly efficient spin conversion without needingsuch atoms. This is because the first-order mixing coefficient betweensinglet and triplet states (l) is inversely proportional to DEST (ref. 27):

l!HSO

DESTð2Þ

Here HSO is the spin–orbit interaction. It follows from equation (2)that heavy atoms are not required to achieve efficient spin conversionwhen a molecule possesses a small DEST and HSO is not vanishinglysmall. This broadens the scope for the molecular design of TADFmaterials. A detailed strategy for large l will be clarified on the basisof correlated quantum-chemical calculations28.

METHODS SUMMARYSynthesis of carbazolyl dicyanobenzene. The synthesis of CDCB derivatives wasperformed according to the synthetic method reported previously for pyrrole andfluorobenzene derivatives29,30. CDCBs were synthesized by reaction of a carbazolylanion with a fluorinated dicyanobenzene at room temperature (300 K) for 10 hunder a nitrogen atmosphere. CDCBs were purified by column chromatographyon silica gel, by reprecipitation or both. CDCBs were further purified by sublima-tion before photoluminescence and electroluminescence spectra were measured.Photoluminescence measurements. Organic films for optical measurementswere fabricated by thermal evaporation onto clean quartz and silicon substratesunder high vacuum (,7 3 1024 Pa). The photoluminescence spectra of thesefilms were recorded with a spectrofluorometer (FluoroMax-4, Horiba JobinYvon), and the photoluminescence quantum efficiencies were measured usingan absolute photoluminescence quantum yield measurement system (C9920-02,Hamamatsu Photonics). The transient photoluminescence characteristics weremeasured (in terms of photon number) under vacuum using a streak camera(C4334, Hamamatsu Photonics). A nitrogen gas laser with a wavelength of337 nm and a pulse width of approximately 500 ps (MNL200, Lasertechnik) wasused as an excitation source. Low-temperature measurements were made using acryostat (CRT-006-2000, Iwatani Industrial Gases). InGa alloy was applied as anadhesive to ensure good thermal conductivity between the silicon substrate andsample holder.

10–3 10–2 10–1 100 101 102

Current density (mA cm–2)

4CzIPN4CzTPN-Ph2CzPN

101

100

10–1

10–2

Exte

rnal

ele

ctro

lum

ines

cenc

e qu

antu

m e

ffici

ency

(%)

400 500 600 700Wavelength (nm)

1

0

Nor

mal

ized

ele

ctro

lum

ines

cenc

ein

tens

ity (a

.u.)

Figure 5 | Performance of OLEDs containing CDCB derivatives. Externalelectroluminescence quantum efficiency as a function of current density forOLEDs containing 4CzIPN (green circles; error within 1.5%), 4CzTPN-Ph (redtriangles; error within 1.0%) and 2CzPN (blue triangles; error within 1.0%) asemitters. Inset, electroluminescence spectra of the same OLEDs (colouredaccordingly) at a current density of 10 mA cm22.

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0Pho

tolu

min

esce

nce

quan

tum

effi

cien

cy

0 50 100 150 200 250 300Temperature (K)

CombinedPromptDelayed

0 10 20 30 40Time (μs)

300 K200 K100 K

100

10–1

10–2

10–3

Nor

mal

ized

pho

tolu

min

esce

nce

inte

nsity

PromptDelayed

400 500 600

1

0

3.5 4.0 4.5 5.010–3/T (K–1)

14.2

14.0

13.8

13.6

13.4

13.2

13.0

12.8

12.6

12.4

In(k

RIS

C)

a b

c d

300 K

ΔEST = 83 meV

Figure 4 | Temperature dependence of photoluminescence characteristicsof a 5 6 1 wt% 4CzIPN:CBP film. a, Photoluminescence decay curves of a6 wt% 4CzIPN:CBP film at 300 K (black line), 200 K (red line) and 100 K (blueline). The photoluminescence decay curves show integrated 4CzIPN emission.The excitation wavelength of the films was 337 nm. b, Photoluminescencespectrum resolved into prompt and delayed components. c, Temperaturedependence of photoluminescence quantum efficiencies (errors are within 2%)for combined (prompt plus delayed; black squares), prompt (red circles) anddelayed (blue triangles) components of 4CzIPN emission for 5 6 1 wt%4CzIPN:CBP film. The straight lines are guides for the eye. d, Arrhenius plot ofthe reverse ISC rate from the triplet state to the singlet state of 4CzIPN with kISC

set to 4 3 107 s21. The straight line (least-squares regression) is used todetermine the activation energy. The ln(kRISC) errors are within 0.2.

LETTER RESEARCH

1 3 D E C E M B E R 2 0 1 2 | V O L 4 9 2 | N A T U R E | 2 3 7









a

b

Fluorescence

Phosphorescence


S0

25%

75%

T1~0.5–1.0 eV

TADF

e h



NC CN

RR

NN

N

NN

N

NC CN

R

R

R

R

R

R

R

R

CN

CN

N

N N

N




ΔEST

TADF = Thermal Activated Delayed Fluorescence








a

b

Fluorescence

Phosphorescence


S0

25%

75%

T1~0.5–1.0 eV

TADF

e h



NC CN

RR

NN

N

NN

N

NC CN

R

R

R

R

R

R

R

R

CN

CN

N

N N

N




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 4CzIPNFigure :

Result & Performance

dicyanobenzene group acts as an electron acceptor. Steric hindrancebetween the carbazolyl and dicyanobenzene moieties causes a largedihedral angle of about 60u between the planes of the carbazolyland dicyanobenzene groups. Consequently, the highest occupied andlowest unoccupied NTOs are spatially well separated, leading to a smallDEST and enhanced T1 R S1 reverse ISC.

As stated above, the Stokes shift of the S1 state of 4CzIPN is smallcompared with those for typical charge transfer states, suggesting thatthe S1 r S0 excitation results in limited relaxation of the moleculargeometry. We believe that this small Stokes shift is partly related tothe presence of the cyano groups. To investigate the effect of the cyanogroups on the geometry relaxation of the S1 and T1 states of 4CzIPN, weevaluated vertical transition energies (DES@S0 and DES@S1) and relaxa-tion energies (lS and lS*) for the S1 state (Fig. 2d), and compared themwith those of the molecule in which the two cyano groups of 4CzIPN arereplaced with hydrogen atoms (4CzBz; Supplementary Information).The reorganization energies lS for 4CzIPN and 4CzBz were calculatedto be 0.27 and 0.83 eV, respectively. This energy is greatly reduced byintroducing cyano groups into the electron-accepting unit. Because lSrepresents the degree of geometry relaxation of the S1 state to the S0state, this result suggests that the cyano groups are important in sup-pressing geometry relaxation in the fluorescent state of 4CzIPN. Inaddition, lS* is also reduced by the presence of cyano groups.Torsional angles of the carbazolyl groups are calculated to be small inthe presence of the cyano groups. This limited torsional flexibility can bea major factor in reducing the non-radioactive decay of 4CzIPN. Likethat 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 cyanogroups 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 presentedin Fig. 3. The series of CDCBs yielded a wide range of emission coloursranging from sky blue (473 nm) to orange (577 nm). The emissionwavelength depends on the electron-donating and -accepting abilitiesof the peripheral carbazolyl groups and the central dicyanobenzeneunit, respectively. Introduction of methyl or phenyl substituents at the3- and 6- positions of the carbazolyl groups of 4CzTPN induces a shiftof the emission maximum to longer wavelengths. Conversely, in thecase of 2CzPN, the presence of fewer carbazolyl groups reduces itselectron-donating ability and produces a shift of the emission maxi-mum to shorter wavelengths. We measured the photoluminescencequantum yield and transient photoluminescence of CDCBs in tolueneunder a nitrogen atmosphere, and are summarized in SupplementaryInformation. For 4CzPN and 4CzTPN W is high (74 6 3% and72 6 3%, respectively), whereas for 4CzTPN-Me, 4CzTPN-Ph and2CzPN it is lower (47 6 2%, 26 6 1% and 47 6 2%, respectively)because of substituent effects or fewer carbazolyl groups. Becausethe transient photoluminescence of all CDCBs showed both a nano-second-scale prompt component and a microsecond-scale delayedcomponent, the CDCBs were confirmed to be TADF materials.

Figure 4a shows the photoluminescence decay curves for emissionof 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 4CzIPNare well confined using a CBP host because the T1 state of CBP ishigher in energy than the S1 state of 4CzIPN. In addition, the fluo-rescence of CBP is completely quenched by efficient energy transferbetween the guest and host molecules. The intense emission observedaround t 5 0 s corresponds to the prompt component, and the long tailis the delayed fluorescence. The prompt component is assigned to thefluorescence of 4CzIPN. The delayed component is attributed todelayed fluorescence occurring via reverse ISC, that is, TADF, becausethe photoluminescence spectrum of the delayed fluorescence is iden-tical to that of the prompt fluorescence (Fig. 4b).

Figure 4c shows the temperature dependence of W for the promptand delayed components of the film. The two components wereresolved by combining the absolute value of W estimated using an

integrated-sphere photoluminescence measurement system and thetemperature dependence of the photoluminescence decay curves(Supplementary Fig. 2). The prompt component increases very slightlyas the temperature decreases, indicating the suppression of non-radiative decay from the S1 state. Conversely, the delayed componentdecreases monotonically as the temperature decreases because reverseISC becomes the rate-determining step, similar to the temperaturedependence of tin IV fluoride/porphyrin complexes, which are typicalTADF emitters14. At room temperature (300 K), a high W value, of83 6 2%, was observed. To evaluate DEST quantitatively, we estimatedthe activation energy of the reverse ISC rate constant (kRISC) fromexp(2DEST/kBT), where kB is the Boltzmann constant and T is tem-perature. This rate constant can be estimated from experimentallydetermined rate constants and the W values of the prompt and delayedcomponents at each temperature using24

kRISC~kpkd

kISC

Wd

Wpð1Þ

where kp and kd are the rate constants of the prompt and delayedfluorescence components, respectively; kISC is the ISC rate constantfrom S1 to T1 states; and Wp and Wd are the photoluminescencequantum yields of the prompt and delayed components, respectively.In Fig. 4d, the values of kRISC calculated from equation (1), assumingthat kISC was independent of temperature, are plotted against 1/T forT 5 200–300 K. From the Arrhenius plot (Fig. 4d), we estimate anactivation energy of 83 meV. Therefore, kRISC would be suppressedconsiderably at low temperatures. However, even at low temperatures,the W value of the delayed component is still high, at .40%, implying

a

b2CzPN

4CzIPN

4CzPN

4CzTPN

4CzTPN-Me

4CzTPN-Ph

400 500 600 700

Wavelength (nm)

Nor

mal

ized

pho

tolu

min

esce

nce

inte

nsity

2CzPN4CzIPN4CzPN4CzTPN4CzTPN-Me4CzTPN-Ph

Figure 3 | Photoluminescence of the CDCB series. a, Photoluminescencespectra measured in toluene. b, Photograph under irradiation at 365 nm.

RESEARCH LETTER



dicyanobenzene group acts as an electron acceptor. Steric hindrancebetween the carbazolyl and dicyanobenzene moieties causes a largedihedral angle of about 60u between the planes of the carbazolyland dicyanobenzene groups. Consequently, the highest occupied andlowest unoccupied NTOs are spatially well separated, leading to a smallDEST and enhanced T1 R S1 reverse ISC.

As stated above, the Stokes shift of the S1 state of 4CzIPN is smallcompared with those for typical charge transfer states, suggesting thatthe S1 r S0 excitation results in limited relaxation of the moleculargeometry. We believe that this small Stokes shift is partly related tothe presence of the cyano groups. To investigate the effect of the cyanogroups on the geometry relaxation of the S1 and T1 states of 4CzIPN, weevaluated vertical transition energies (DES@S0 and DES@S1) and relaxa-tion energies (lS and lS*) for the S1 state (Fig. 2d), and compared themwith those of the molecule in which the two cyano groups of 4CzIPN arereplaced with hydrogen atoms (4CzBz; Supplementary Information).The reorganization energies lS for 4CzIPN and 4CzBz were calculatedto be 0.27 and 0.83 eV, respectively. This energy is greatly reduced byintroducing cyano groups into the electron-accepting unit. Because lSrepresents the degree of geometry relaxation of the S1 state to the S0state, this result suggests that the cyano groups are important in sup-pressing geometry relaxation in the fluorescent state of 4CzIPN. Inaddition, lS* is also reduced by the presence of cyano groups.Torsional angles of the carbazolyl groups are calculated to be small inthe presence of the cyano groups. This limited torsional flexibility can bea major factor in reducing the non-radioactive decay of 4CzIPN. Likethat 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 cyanogroups 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 presentedin Fig. 3. The series of CDCBs yielded a wide range of emission coloursranging from sky blue (473 nm) to orange (577 nm). The emissionwavelength depends on the electron-donating and -accepting abilitiesof the peripheral carbazolyl groups and the central dicyanobenzeneunit, respectively. Introduction of methyl or phenyl substituents at the3- and 6- positions of the carbazolyl groups of 4CzTPN induces a shiftof the emission maximum to longer wavelengths. Conversely, in thecase of 2CzPN, the presence of fewer carbazolyl groups reduces itselectron-donating ability and produces a shift of the emission maxi-mum to shorter wavelengths. We measured the photoluminescencequantum yield and transient photoluminescence of CDCBs in tolueneunder a nitrogen atmosphere, and are summarized in SupplementaryInformation. For 4CzPN and 4CzTPN W is high (74 6 3% and72 6 3%, respectively), whereas for 4CzTPN-Me, 4CzTPN-Ph and2CzPN it is lower (47 6 2%, 26 6 1% and 47 6 2%, respectively)because of substituent effects or fewer carbazolyl groups. Becausethe transient photoluminescence of all CDCBs showed both a nano-second-scale prompt component and a microsecond-scale delayedcomponent, the CDCBs were confirmed to be TADF materials.

Figure 4a shows the photoluminescence decay curves for emissionof 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 4CzIPNare well confined using a CBP host because the T1 state of CBP ishigher in energy than the S1 state of 4CzIPN. In addition, the fluo-rescence of CBP is completely quenched by efficient energy transferbetween the guest and host molecules. The intense emission observedaround t 5 0 s corresponds to the prompt component, and the long tailis the delayed fluorescence. The prompt component is assigned to thefluorescence of 4CzIPN. The delayed component is attributed todelayed fluorescence occurring via reverse ISC, that is, TADF, becausethe photoluminescence spectrum of the delayed fluorescence is iden-tical to that of the prompt fluorescence (Fig. 4b).

Figure 4c shows the temperature dependence of W for the promptand delayed components of the film. The two components wereresolved by combining the absolute value of W estimated using an

integrated-sphere photoluminescence measurement system and thetemperature dependence of the photoluminescence decay curves(Supplementary Fig. 2). The prompt component increases very slightlyas the temperature decreases, indicating the suppression of non-radiative decay from the S1 state. Conversely, the delayed componentdecreases monotonically as the temperature decreases because reverseISC becomes the rate-determining step, similar to the temperaturedependence of tin IV fluoride/porphyrin complexes, which are typicalTADF emitters14. At room temperature (300 K), a high W value, of83 6 2%, was observed. To evaluate DEST quantitatively, we estimatedthe activation energy of the reverse ISC rate constant (kRISC) fromexp(2DEST/kBT), where kB is the Boltzmann constant and T is tem-perature. This rate constant can be estimated from experimentallydetermined rate constants and the W values of the prompt and delayedcomponents at each temperature using24

kRISC~kpkd

kISC

Wd

Wpð1Þ

where kp and kd are the rate constants of the prompt and delayedfluorescence components, respectively; kISC is the ISC rate constantfrom S1 to T1 states; and Wp and Wd are the photoluminescencequantum yields of the prompt and delayed components, respectively.In Fig. 4d, the values of kRISC calculated from equation (1), assumingthat kISC was independent of temperature, are plotted against 1/T forT 5 200–300 K. From the Arrhenius plot (Fig. 4d), we estimate anactivation energy of 83 meV. Therefore, kRISC would be suppressedconsiderably at low temperatures. However, even at low temperatures,the W value of the delayed component is still high, at .40%, implying

a

b2CzPN

4CzIPN

4CzPN

4CzTPN

4CzTPN-Me

4CzTPN-Ph

400 500 600 700

Wavelength (nm)

Nor

mal

ized

pho

tolu

min

esce

nce

inte

nsity

2CzPN4CzIPN4CzPN4CzTPN4CzTPN-Me4CzTPN-Ph

Figure 3 | Photoluminescence of the CDCB series. a, Photoluminescencespectra measured in toluene. b, Photograph under irradiation at 365 nm.

RESEARCH LETTER



Sky Blue Green Orange

TADF Material 2CzPN 4CzIPN 4CzTPN

Internal Quantum Efficiency 26.7 ~ 40.0% 64.3 ~ 96.5% 37.3 ~ 56.0%

• 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

Thank You for Your Attention

3rd gen. oled -- tadf

Science

singlet state t

singlet andtriplet excitons

intothe organic layers

energy gap

iscrisclow efficiency

tripletexcited states

so couplingdue

development of materials