The organic (optoelectronic) revolution

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The organic (optoelectronic) revolution
What is optoelectronics? The study and
application of electronic devices that source,
detect and control light
solar cells
LEDs
lasers
PM
CRT

• the classical devices use inorganic materials
  Si, GaN, Y2O2SEu, YAGNd

• 1987 Tang and van Slyke demonstrate the first
  organic optoelectronic device

• nowadays

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Advantages of organic versus inorganic LEDs

• synthetic flexibility

• tuning of chemical structure ? different optical
  and electronic properties

• (potentially) very cheap production
• - low temperature
• - scalable to large area

• (potentially) very energy efficient

• new paradigm in the field

• ultra-thin and lightweight
• self-luminescent ? no backlighting
• the substrates can be flexible or transparent

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Classes of organic emitters for OLEDs

• purely organic dyes
• - fluorescent (limited to 25 efficiency)
• - broad emission bands
• - photo-bleaching

• organometallic complexes
• - phosphorescent
• (theoretical 100 efficiency)
• - broad emission bands
• - sensitivity to oxygen

• lanthanide complexes with organic ligands
• - first example Kido, 1990

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Properties of lanthanide ions
LnIII ground state Xe4fn, n 0..14
Lu
Yb
Tm
Er
Ho
Dy
Tb
Gd
Eu
Sm
Pm
Nd
Pr
Ce
La

• Shielding of 4f orbitals ?
• similar chemical properties
• electrostatic bonding
• variable geometry and CNs
• hard acid behaviour

blue ? NIR

• Fascinating optical properties
• luminescence from f-f transitions
• characteristic emission for each ion
• narrow emission bands
• long excited-states lifetimes

Applications in optoelectronics and bio-medicine
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Advantages of lanthanide complexes in
optoelectronics

• sharp emission ? pure colors (no filters)

• one ligand, different emission colors (even NIR)

• no oxygen sensitivity and no photo-bleaching

• easier coordination chemistry

f-f transitions are forbidden the excited states
cannot be efficiently populated directly
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Sensitization of lanthanide ions
Indirect excitation by energy transfer from a
suitable antenna to the lanthanide ion
Antenna excitation
Light emission
Energy transfer

• Antenna requirements
• excellent energy harvester
• efficient inter-system crossing
• matching electronic levels

• Deactivation
• radiative processes (fluorescence,
  phosphorescence)
• non-radiative processes (vibration-induced)
• electronic processes (energy back-transfer)

1S
ISC
3T
ET
absorption
Antenna
LnIII
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Antennas for lanthanides
organic chromophores (pyridines, phenantroline)
d-metal complexes (RuII, PtII, IrIII)
matrixes (PVK, CBP)
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The design of lanthanide complexes
Connecting the antenna to negatively charged
groups (carboxylate)
Grenthe, J. Am. Chem. Soc. 1961 Bunzli,
Spectrosc. Lett. 2007
Bunzli, Dalton Trans. 2000
Latva, J. Lumin. 1997
Mazzanti, Angew. Chem. Int. Ed. 2005

• Associating the antenna to diketonate complexes
• low stability
• few structure-property relationships
• difficult optimization

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Luminescent lanthanide architectures for
optoelectronics

• synthesize new stable lanthanide architectures
• tuned absorption and emission properties by
  ligand design
• investigate their potential for applications in
  optoelectronics

• high denticity ligands with negatively charged
  groups
• sensitizing antenna - organic chromophores
• - d-metal complexes

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The tetrazole motif in coordination chemistry

• carboxylate often used for lanthanide
  coordination

• tetrazole - highly acidic, aromatic

• tetrazolate could replace carboxylate
• tuning of absorption wavelength

• Tetrazole-based complexes of d-metals
• high thermodynamic stability
• interesting properties

Very few examples in lanthanide coordination
chemistry!

• no luminescent lanthanides
• no comparative studies

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Lanthanide complexes based on pyridine-tetrazolate
s
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Design of tetrazole-based ligands
terpyridine ligands pentadentate
bipyridine ligands tetradentate
pyridine ligands tridentate

• influence of tetrazolate on the properties of
  the complexes
• direct comparison with carboxylate analogues

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Organic synthesis of terpyridine-based ligands
Andreiadis et al, submitted patent pending
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Organic synthesis of bipyridine-based ligands
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Organic synthesis of pyridine-based ligands
Easy access to tetrazole-based ligands
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Lanthanide complexes with terpyridine-based
ligands
Ln(L)2-, Ln Nd, Eu, Tb
the tetrazole-based ligands are well adapted to
lanthanide complexation
Giraud, Inorg. Chem. 2008, 47, 3952-3954
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Lanthanide complexes with bipyridine-based ligands
Ln(L)2-, Ln Eu, Tb
Andreiadis et al, submitted
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Lanthanide complexes with pyridine-based ligands
Ln(L)33-, Ln Nd, Eu, Tb
Andreiadis et al, submitted
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Increasing the solubility in chlorinated solvents
Solubility strong advantage for the
applications in OLED devices (wet process)

• ligand functionalization

• change of counterion

isolated as an oil
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Stability of tetrazolate-based complexes

• stable without dissociation in air and wet
  methanol solutions
• quantitative study by UV titration

L2-
L2-
EuL2-
EuL
EuL2-
EuL
logß2 11.8(4)
logß2 10.5(5)
Comparable stability to carboxylate analogues
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Absorption properties of pyridine-based complexes
Ln(L)33-
4
3
e / 104 cm-1M-1
2
1
0
250
275
300
325
350
Wavelength / nm
aromatic tetrazolate ? increase of absorption
wavelength and intensity
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Absorption properties of bipyridine-based
complexes
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Ln(L)2-
3
2
e / 104 cm-1M-1
1
0
250
275
300
325
350
375
400
Wavelength / nm
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Absorption properties of terpyridine-based
complexes
10
8
6
e / 104 cm-1M-1
4
Ln(L)2-
2
0
250
300
350
400
450
500
Wavelength / nm
substituents ? tuning of absorption wavelength
and intensity
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Photophysical properties of terpyridine-based
complexes
Ligand triplet states
Modulation of ligand triplet state
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Photophysical properties of terpyridine-based
complexes
Emission quantum yields
Ln(L)2-
Eu 36 Tb 35 Nd 0.09
Eu 35 Tb 6 Nd 0.22
Eu 29 Tb 0.1
Eu 28
Eu 5 Nd 0.29
Nd 0.19
Modulation of ligand triplet state
? Tuning of emission quantum yields
Very good QY for Eu (35) and Nd (0.29)
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Photophysical properties of terpyridine-based
complexes
Emission quantum yields
Terbium QY function of triplet state
Ln(L)2-
Eu 36 Tb 35 Nd 0.09
Eu 35 Tb 6 Nd 0.22
Eu 29 Tb 0.1
Eu 28
Eu 5 Nd 0.29
Nd 0.19
Latva, J. Lumin. 1997
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Photophysical properties of bipyridine-based
complexes
Ln(L)2-
Eu 45 Tb 27
Eu 54 Tb 13
Eu 63 Tb 6
Measured after drying
Similar tuning of emission quantum yields
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Photophysical properties of pyridine-based
complexes
Ln(L)33-
Eu 61 Tb 65 Nd 0.21
Eu 39
Eu 24 Tb 22
Chauvin, Spectr. Lett. 2007, 40, 193

• excellent quantum yields
• for pyridine-tetrazole complexes

• solubility in chlorinated solvents

Possible applications in OLEDs
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Neutral lanthanide diketonate complexes
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New approach towards neutral lanthanide complexes

• Lanthanide complexes employed in optoelectronics
• neutral (vacuum processing)
• based on the ß-diketonate motif
• additional soft, neutral ligands

• low stability
• dissociation during processing

Replacing neutral chromophores with negatively
charged ones for increasing the stability of the
complex
Preliminary testing in OLED devices
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The terpyridine-monocarboxylate ligand
Terpyridine carboxylic acid leads to stable
homoleptic mono- or poly-metallic complexes
Ln Eu, Gd, Tb, Nd
Ln(L)2(OTf)
n
Ln (LnL2)6(OTf)9
Bretonnière, J. Am. Chem. Soc., 2002, 124,
9012 Chen, Inorg. Chem., 2007, 46, 625
formation of heteroleptic complexes with
ß-diketonate units
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Synthesis and properties of the complexes
QY 41
QY 13

• complexes stable in air and solution
• good quantum yields

Investigate potential applications in OLED devices
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Preliminary testing in OLED devices
Excellent film-forming properties (doping in PVK
matrix)

• Collaboration Dr. Pascal Viville (Univ. Mons)
• testing in OLED devices (spin-coating)
• classical device architecture

• the OLED devices display promising results
• rather low current intensities 5.4 mA/cm2 at
  25V (Eu)
• 45 mA/cm2 at 20V (Tb)

device optimization in progress
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Heterometallic iridium-europium complexes
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Sensitization of europium by d-metals
Indirect excitation using d-transitional metals
by inter-metallic communication

• absorption at visible wavelength
• sensitization of NIR emitting lanthanides
• europium sensitization requires high energy

IrIII complexes - modulation of emission energy
by the coordinated ligands
use blue-emitting Ir complexes
Thompson et al. Inorg Chem 2005, 44, 7992
Coppo, Angew. Chem. Int. Ed., 2005, 44, 1806
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Heterometallic complex - strategy and ligand
design
Connecting the metal ions by a completely
covalent structure (stability)

• terpyridine-tetrazolate motif for lanthanide
  complexation

• several target ligands investigated

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Synthesis of iridium-based ligand
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Synthesis of iridium-based ligand

• 1H NMR and X-ray diffraction studies prove the
  retention of Ir conformation during the synthesis

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Synthesis of the heterometallic complex
Eu(L)2-
1H NMR indicates a similar structure to the
mono-metallic lanthanide complexes
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Protophysical properties of the heterometallic
complex
QY 0.96
ex 400 nm
2,5
2,0
?Ir-Eu 85-90
1,5
intensity / a.u.

• iridium ? europium energy transfer

• residual emission from iridium

1,0

• Eu emission due exclusively to Ir

• very good energy transfer efficiency

selective excitation of Ir moiety
0,5
0,0
300
400
500
600
700
800
promising architecture
wavelength / nm
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Final conclusions and perspectives

• tetrazole-based antennas for lanthanide

• combining stability with tuning
• of absorption and emission properties

? extending the work to other architectures
(podates)
? applications in OLEDs

• improving the stability of neutral diketonate
• complexes by using charged chromophores

? applications in OLEDs and surface grafting

• polyvalent stable heterometallic architecture
• with very high Ir ? Eu transfer efficiency

? improving europium emission efficiency
? extending the chemistry to other metals
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Acknowledgements
Prof. Luisa DE COLA, Prof. Jean WEISS, Prof.
Muriel HISSLER, Dr. Guy ROYAL
Dr. Daniel IMBERT Dr. Jacques PECAUT
Dr. Marinella MAZZANTI Dr. Renaud DEMADRILLE
Yann KERVELLA, Dr. Bruno JOUSSELME, Prof.
Alexander FISYUK Colette LEBRUN, Pierre-Alain
BAYLE
Dr. Pascal VIVILLE (Mons University), Prof.
Jean-Claude BUNZLI (EPFL)
my colleagues and friends
European Community Marie Curie EST CHEMTRONICS
MEST-CT-2005-020513