Photophysics of Cu(I) and Ag(I) compounds showing

efficient thermally activated delayed fluorescence.

Strategies for material design.

Hartmut Yersin

Universität Regensburg, Germany

Outline

Focus: Design of new emitter molecules for OLEDs

Short introduction to Singlet Harvesting for 100% exciton use

- based on TADF

Strategies how to develop materials with short-lived emission

Important for:

- high emission quantum yield

- increase of device stability

- decrease of roll-off

Several case studies:

- First: focus on Cu(I) complexes

- Then: presentation of extraordinarily efficient Ag(I) materials

Conclusion

t(T )1

Spins and electron-hole recombination

3 -1DE(S -T ) < 10 cm (0.12 eV)1 1

threetriplet paths 75 % 25 %

singlet

up-/down-ISC

S0

S1

T1

path

k TB

t(TADF)k(S )1

TADF and Singlet Harvesting in OLEDs for 100% exciton use

TADF: Parker 1961OLEDs: Yersin 2006

e

DE(S -T )1 1

k TB

-

Case Study

Blue-light emitting Cu(I) complex

weak SOC

T = 300K l max = 464 nm F PL = 90 % t = 13 ms

Cu(pop)(pz2Bph2) powder

NN

Cu2P P

2

O

NNB

MLCT LUMO

HOMO

T = 300K l max = 464 nm F PL = 90 % t = 13 ms

Cu(pop)(pz2Bph2) powder

NN

Cu2P P

2

O

NNB

MLCT 1MLCT

3MLCT

S0

T1

S1 small E(S1-T1)

k TB

474 nm

l max l max

464 nm

S 1

T 1

S 0

Cu(pop)(pz BPh ) (powder) - TADF material2 2

25000 20000

400

Em

issi

on in

tensi

ty

450 500 550l

n

nm

30 K 300 K

464800

474 nm

-1cm

-1cm

Czerwieniec R., Yu J., Yersin H.Inorg. Chem. 2011, 50, 8293

t(TADF)13 ms

t(T )1

480 ms

rk increasefactor 35

13 480 ms

Cu(pop)(pz Bph ) (powder) - Singlet harvesting based on TADF 2 2

B

CuP

22P

N N

N N

O

480 ms 13 ms

-1650 cm

S 1

T 1

S 0

(fit)k TB

0

Em

issi

on d

eca

y tim

e

50 100 150 200 250 K 300

Czerwieniec R., Yu J., Yersin H.Inorg. Chem. 2011, 50, 8293

0

100

200

300

400

500

T emission1

S emission1

480 ms

ms

t(TADF)

13 ms

DE(ZFS)-1<1 cm

(80 meV)

t(phos)

t(T) = 3 + exp [-DE(S -T ) / (k T)] 1 1 B

3k(T →S ) + k(S →S ) exp [-DE(S -T ) / (k T)]1 0 1 0 1 1 B

rk (S →S )1 0

6 -15.3∙10 s (fit)

DE(S -T )1 1

rate increasefactor 35

For OLED applications

τ (T) =

3 + exp [ E(S1 T1) / (kBT)]

3 k(T1→S0) + k(S1→S0) exp [ E(S1 T1) / (kBT)]

t(300 K) should be as short as possible high emission quantum yield increase of device stability decrease of roll-off

For OLED applications

τ (T) =

3 + exp [ E(S1 T1) / (kBT)]

3 k(T1→S0) + k(S1→S0) exp [ E(S1 T1) / (kBT)]

t(300 K) should be as short as possible high emission quantum yield increase of device stability decrease of roll-off

How to realize?: DE(S1-T1) as small as possible k(T1→S0) as large as possible k(S1→S0) as large as possible

Summary of requirements

S1

T1

S0

E k(S1↔S0) large

k(S1)

challenge

S0

T1

S1

k(T1→S0) large

phos k(T1)

High SOC

S1

T1

S0

pre-exponential factor

ΔE small

next Case Study

S 1

T 1

S 0

TADF

k TB

Combined TADF

and phosphorescence

phosk(T )1

k(T →S ) large1 0

SOC governed by HOMO – (HOMO-1) energy difference

Energy difference from

simple DFT calculations

Energy difference

governs SOC

LUMO

HOMO

HOMO-1

MLCT 1 MLCT 2

*

d1

d2

1,3MLCT 1 1,3MLCT 2

SOC

S0

S1

S2

T1

T2

1MLCT2

3MLCT2

1MLCT1

3MLCT1

SOC SOC

Energy State Diagram

rk (T -S )1 0 borrowsallowedness from

S ↔S2 0

HOMO-1 d , p2 2

HOMO d , p1 1

LUMO p*

MLCT 1 MLCT 2

DE

Orbital Diagram

SOC routes for energy states

0 0.5 1.0 1.5 2.0

0

1

2

3

4

5

4-1

k(

) =

1/t

(T)

[10

s]

1

exp

eri

men

tal

T1

D(HOMO - (HOMO-1)) [eV] from DFT

Triplet decay rate vs. (HOMO) - (HOMO-1) energy for different Cu(I) complexes

Cu Cl (P^N)2 , strong SOC2 2

Cu(POP)(pz Bph ), weak SOC2 2

Case Study

Di-nuclear Cu(I) complex with

very strong SOC

NPh2P

Cu Cu

N PPh2

ClCl

T = 300K l max = 510 nm F PL = 92 % t = 8 ms

Cu2Cl2(N^P)2 powder

Emission decay of Cu Cl (N^P) powder2 2 2

Results

· Broad unstructured emission

· Mono-exponential decay Þ fast equilibration· Different states involved

Boltzmann-like distribution

T. Hofbeck, U. Monkowius, H. YersinJACS 2015, 137, 399

Þ

Fit data Þ

1000

100

10

Deca

y tim

e [µ

s]

Temperature [K]

101 100

TADFT Phos.1ZFS of T1

N P

Cu Cu

P2

2

Cl Cl

N

13

T. Hofbeck, U. Monkowius, H. YersinJACS 2015, 137, 399

From the fit:Energy level scheme and decayconstants of Cu Cl (N^P) powder2 2 2

Results· t(S ) = 1

·

· TADF effective

Þ emission effective

40 ns-1

DE(S -T ) = 930 cm1 1

· SOC largeT 1

S0

42 µs

-1930 cm

S1

T1 II

3.5 ms

III

I

T1}7

10 µs

26 µs

30 µs

-1 cm

15

tav TADF

fastISC

ZFS

New harvesting mechanism beingeffective for high SOC compounds

JACS 2014, 136, 16032JACS 2015, 137, 399

S0

k TB

TADF + tripletemitter

S0

S1

T1

TADFpath

k TB

Conventional TADF-onlyemitter

DE(S -T )1 1

T1

S1

T + TADF pathscombined

1

42 µs 10 µs

8 µs

Highlights

· two radiative decay paths· Þ shorter overall emission decay· Þ new strategy to reduce roll-off effects

Summary of requirements

S1

T1

S0

E k(S1↔S0) large

k(S1)

challenge

S0

T1

S1

k(T1→S0) large

phos k(T1)

High SOC

S1

T1

S0

pre-exponential factor

ΔE small

S 1

T 1

S 0

TADF

k TBDE(S -T )1 1

Case Study

Cu(I) compound with very

small DE(S -T ), and very weak SOC1 1

T = 300K l max = 535 nm F PL = 70 %

t(TADF) = 3.3 ms

Case Study: Cu(dppb)(pz2Bph2) powder

NN

Cu2P P

2

NNB

S 1

T 1

S 0

DE(S -T )1 1

1MLCT

3MLCT

Cu(dppb)(pz Bph )2 2

T geometry, B3LYP/def2-svp1

NN

Cu2P P

2

NNB

LUMO

HOMO

MLCT

1200 ms

3.3 ms

time [ms]wavelength [nm]

inte

nsi

tyin

tensity

counts

counts

300 K

30 K

S 1

T 1

S 0

TADF

k TB

548 nm1200 msphos

535 nm3.3 ms

R. Czerwieniec, H. Yersin; Inorg. Chem. 2015, 54, 4322

13 nm

Cu(dppb)(pz Bph ) powder - Emission spectra and decay2 2

Cu(dppb)(pz Bph ) powder2 2

t(T) = 3 + exp [-DE(S -T ) / (k T)] 1 1 B

3k(T →S ) + k(S →S ) exp [-DE(S -T ) / (k T)]1 0 1 0 1 1 B

1200 ms 3.3 ms

S 1

T 1

k TB

t(TADF)t(phos)r

k (S →S )1 06 -1

3.9·10 sfit

DE(S -T )1 1

rate increase

factor ≈ 250

1200 ms

phos mainly TADF TADF

Complecx 2

t(TADF) 3.3 ms

0 50 100 150 200 250

300 K

deca

y tim

e [m

s]

temperature [K]

300

600

900

1200

80 K30 K

fit

DE(S -T )1 1-1

370 cm(46 meV)

-1370 cm46 meV

Remarkable results for Cu(dppb)(pz2Bph2)

Very small E(S1-T1) = 370 cm

-1

short (TADF) = 3.3 s

Very long (T1→S0) = 1200 s

SOC weak, not induced by S1

Obviously, no significant SOC between S1 and T1

Increase of the decay rate by the TADF effect

250phosk

TADFk

We found: small E(S1-T1)

However, combined with small k(S1→S0)

Challenge

small E(S1-T1)

small kr(S1→S0)

further reduction of (TADF) possible?

Relation between kr(S1-S0) and DE(S1-T1)

2

LH01r r rr const)SS(k

radiative rate

1L2H12

2L1H

11

rrr

1rrkonst

TSE

D

exchange interaction

S0

T1

S1

T1

S1

S0

HOMO HOMO LUMOLUMO

DE(S -T )1 1

small

rk (S -S )1 0

small

rk (S -S )1 0

large

DE(S -T )1 1

large

Schematic illustration

rRelation between k (S -S ) and DE(S -T ) 1 0 1 1

200 400 600 800 1000 1200 1400

0

10

20

30

40

50r

6-1

k(S

) [1

0 s

]1

-1ΔE(S -T ) [cm ]1 1

1

2

3

4

5

6

7

8

9

10

11

12

13

14 16

17

18

19

15

Relation between and r

k (S -S )1 0 DE(S -T )1 1

Radiative rate versus

for Cu(I) complexes. Exponential fit function.

rk (S →S )1 0 DE(S -T )1 1

Relation between kr(S1-S0) and DE(S1-T1)

Compound ΔE(S1-T1)

[cm1]

t(S1)

[ns]

ΦPL

(300 K)

kr(S1)

[106 s1]

1 Cu2I2[MePyrPHOS)(Pph3)2 270 570 0.97 1.7

2 Cu(dppb)(pz2Bph2) 370 180 0.70 3.9

3 [Cu(µ-Cl)(PNMe2)]2 460 210 0.45 2.1

4 [Cu(µ-Br)(PNMe2)]2 510 110 0.65 5.9

5 [Cu(µ-I)(PNMe2)]2 570 90 0.65 7.2

6 Cu2Cl2(dppb)2 600 70 0.35 5.0

7 [Cu(µ-I)(PNpy)]2 630 100 0.65 6.5

8 Cu(pop)(pz2BPh2) 650 170 0.9 5.3

9 Cu(pop)(tmbpy)+ 720 160 0.55 3.4

10 (IPr)Cu(py2-BMe2) 740 160 0.76 4.8

11 [Cu(PNPtBu)]2 786 138 0.57 4.1

12 Cu2I2(MePyrPHOS)(dpph) 830 190 0.88 4.6

13 Cu2Cl2(N^P)2 930 40 0.92 23

14 CuCl(Pph3)2(4-Mepy) 940 47 0.99 21

15 Cu(dmp)(phanephos)+ 1000 40 0.80 20

16 Cu(pop)(pz4B) 1000 80 0.9 11

17 CuBr(Pph3)2(4-Mepy) 1070 41 0.95 23

18 CuI(Pph3)2(4-Mepy) 1170 14 0.66 47

19 Cu(pop)(pz2BH2) 1300 10 0.45 45

Summary of requirements

S1

T1

S0

E k(S1↔S0) large

k(S1)

challenge

S0

T1

S1

k(T1→S0) large

phos k(T1)

High SOC

S1

T1

S0

pre-exponential factor

ΔE small

S0

k TB

- New TADF materials required- Ag(I) complexes suited?

S0

S1

T1

t(TADF)„long“

k TB

Traditional TADFcomplexes

T1

S1

t(TADF)short

Sn

DE(S -T )1 1

small

CI of states

Efficient configuration interaction to increase r

k (S →S )1 0

DE(S -T )1 1

small

p* (ligand)

p (ligand)

4d (Ag)

1 3small DE( MLCT- MLCT)

Frequent material properties:

Cu(I) complexes Ag(I) complexes

LC

p* (ligand)

p (ligand)

3d (Cu)

MLCT

TADF

1 3large DE( LC- LC)

No TADF

p* (ligand)

p (ligand)

4d (Ag)

SuggestionUse of electrondonating ligand

Material design - TADF for Ag(I) complexes

LC

p* (ligand)

p (ligand)

4d (Ag)

MLCT

ResultAg(I) complex with TADF

S 1

T 1

S 0

TADF

k TB

Case Study

Ag(I) compounds with very

rhigh k (S →S )1 0

rk (S -S )1 0

large

Ag(phen)(P2-nCB)

Calculation: MO62X/def2-SVP, T1 optimized, gas phase

strongly electron donating ligand

F PL = 36 %

tr(TADF) = 5.3 ms

Ag(phen)(P2-nCB) powder

F PL = 36 %

tr(TADF) = 5.3 ms

Ag(phen)(P2-nCB) powder

Why only 36 % ?

Fundamental relations

radiative rate

fSrSk

k

1

kkk

k

2

10r

r

r

r

nrr

r

PL

oscillator strength

2

21nrk

vibrational wavefunctions

Important messages for large PL

• knr: as small as possible• f, kr(S1-S0): as large as possible

Extensive flattening distortion upon excitation

ground state S0 geometry excited state T1 geometry

Ag(phen)(P2-nCB)

F PL = 36 %

tr(TADF) = 5.3 ms

lmax = 575 nm

Ag(phen)(P2-nCB) Ag(mbp)(P2-nCB) Ag(dmp)(P2-nCB) Ag(dbp)(P2-nCB)

70 %

2.9 ms

535 nm

78 %

3.2 ms

537 nm

100 %

1.4 ms

526 nm

Quantum Yield

Smaller flattening distortion

ground state S0 excited state T1

Ag(dbp)(P2-nCB)

F PL 36 %

tr(TADF) 5.3 ms

krexp.(S1→S0)

fTD-DFT(S1→S0) 0.024

Ag(phen)(P2-nCB) Ag(mbp)(P2-nCB) Ag(dmp)(P2-nCB) Ag(dbp)(P2-nCB)

70 %

2.9 ms

2.2·107 s-1

0.048

78 %

3.2 ms

2.2·107 s-1

0.042

100 %

1.4 ms

5.6·107 s-1

0.0536

S1↔S0 allowedness

very short t(TADF) = 1.4 msrelated to the high oscillator strength

Geometry and oscillator strength

A Ag(dbp)(P2-nCB) TD-DFT: f = 0.0536calculated geometry

B Ag(phen)(P2-nCB) TD-DFT: f = 0.0687geometry fixedto geometry of A

The geometry determines the allowedness and not the phen-substitutionsLevel of theory: MO62X/def2-svp

ground state S0 geometry excited state T1 geometry

200 400 600 800 1000 1200 1400

0

10

20

30

40

50r

6-1

k(S

) [1

0 s

]1

-1ΔE(S -T ) [cm ]1 1

1

2

3

4

5

6

7

8

9

10

11

12

13

14 16

17

18

19

15

Relation between and r

k (S -S )1 0 DE(S -T )1 1

Radiative rate versus r

k (S →S )1 0 DE(S -T )1 1

for Cu(I) complexes. Exponential fit function.

Ag(dbp)(P -nCB)2

Case Study

Ag(dbp)(P2-nCB)

Ag(dbp)(P2-nCB) powder

Shafikov, Suleymanova,Czerwieniec, and YersinChem. Mater. 2017, 29, 1708

DE 10 2016 115 633.7

Ag(dbp)(P2-nCB) powder

Shafikov, Suleymanova,Czerwieniec, and YersinChem. Mater. 2017, 29, 1708

DE 10 2016 115 633.7

Summary and guidelines for short (TADF)

Extremely small E(S1-T1) is not required

High allowedness of the S1→S0 transition is more

important

The S1 state must experience mixings with

higher lying Sn states

Conclusion

Systematic understanding of photophysical properties

design of new and efficient materials

TADF optimization

- E(S1-T1) moderately small

- S1→S0 allowedness as high as possible

Material record: PL(TADF) = 100 %, (TADF) = 1.4 s

Thanks to my group

Dr. Rafal Czerwieniec

Dr. Thomas Hofbeck

Dr. Markus Leitl

Dr. Larisa Mataranga-Popa

Alexander Schinabeck, M. Sc.

Alfiya Suleymanova, M. Sc.

Marsel Shafikov, M. Sc.

We acknowledge the financial

support by the BMBF