The main objective of my PhD research at CEA Grenoble (M. Mazzanti, R. Demadrille) was related to a better understanding of the structure-property relationship in emissive lanthanide complexes with potential applications in opto-electronic devices. This was achieved by a careful design of lanthanide antennas based on either organic chromophores or transition metals as ligands, followed by a study of the structural and photophysical properties of the resulting complexes, in order to estimate and further predict the sensitization efficiencies.
In a first line of research, we have described and patented the incorporation of tetrazole groups as carboxylic acid replacements for the sensitization of lanthanide emission. We were able to show how the variation of ligand substituents influences the photophysical properties, allowing us to draw predictions and to adapt the structures for improving the emission efficiency. Some of the compounds have been successfully tested in OLED devices.
We also became interested in designing and studying new types of polymetallic architectures based on iridium complexes for the sensitization of lanthanide emission, as well as preliminary investigating the grafting of lanthanide complexes on silicon surfaces.
I will start by presenting some of the remarkable achievements in the field of organic optoelectronics. first of all, what is optoelectronics? it is the study… well-known examples of such devices are the LED, CRT or the laser, which all convert an electric signal into light. Conversely, solar cells or the PM converts the light into an electric signal. all these devices are normally based on inorganic materials, among which inorganic lanthanide compounds based on Eu or Nd and used for example in the CRT or in lasers. it was only in 1987 that T and vS… green OLED using as emitting layer Alq3 since then, considerable progress has been achieved in the field of organic optoelectronics. The most successful story is that of the OLEDs, which have now found applications in several commercial products.
But what are the advantages of the organic versus the purely inorganic materials? first of all, there is the synthetic flexibility with which new types of materials can be prepared by organic synthesis moreover, tuning of the chemical structure…
There are several classes of organic materials used as active layers in the OLED structure … A very interesting class of emitters are the lanthanide complexes with organic ligands. Lanthanide complexes have been used in many applications for a long time, but it was only in 1990 that Kido prepared the first lanthanide OLED using a terbium acetylacetonate complex, which was later followed by an europium complex containing also the phenantroline ligand.
lanthanides are a series of 15 elements in the f block of the periodic system. They range from La to Lu. They are most stable in the 3+ oxydation state, in which their ions have a GROUND STATE electronic configuration [Xe]4fn. in the same time, they have fascinating optical properties… the most interesting ions are Eu and Tb (visible) and Nd, Yb and Er (NIR)
-sharp emission -due to their similar chemical properties, one single ligand can be used for the complexation of several lanthanides, each with its own characteristic emission wavelength, ranging from blue to the near-IR
1942- weissman optimization of the process is crucial for obtaining good emission efficiencies the luminescence efficiency is measured by the absolute quantum yield
Most of the antennas used for lanthanide sensitization are based on organic chromophores… more recently, d-metal complexes have been demonstrated especially for the sensitization of NIR emitting Ln when the lanthanide complex is dispersed in an organic matrix, as in the emissive layer of an OLED device, the matrix itself acts as an antenna.
The sensitizers have been included in lanthanide complexes by using two main approaches - in the first, the chromophore has been connected to negatively charged donor groups, such as the carboxylate, to afford stable chelates. - in the second approach, the chromophore, most often a phenantroline unit, has been associated to beta-diketonate-based lanthanide complexes. This approach is especially used in the field of optoelectronics. The resulting chelates display however a low stability, resulting sometimes in the dissociation of the phenantroline upon device processing. Consequently, it is difficult to relate the structure of the complexes to the properties of the final device, and to predict better types of lanthanide architectures.
In this context,… having tuned absorption and emission properties by appropriate ligand design
The carboxylate unit is often used for lanthanide coordination due to the strong electrostatic bond formed by the negatively charged O with the metal ion. Most nitrogen-based ligands are too soft to afford enough stability. However, the tetrazole molecule is highly acidic, with a pKa similar to that of acetic acid. Upon deprotonation, the tetrazolate ring becomes aromatic. These features suggest that the tetrazolate motif could be used to replace the carboxylate, leading to stable lanthanide complexes having new electronic properties due to the extended conjugation of the ligand with the aromatic tetrazolate. In the transition metal coordin chemistry… Surprisingly, there are very few examples of the use of tetrazole in lanthanide coordination chemistry. Moreover, no
In a first axis of research, we have designed, synthesized and studied a series of lanthanide complexes based on the tetrazolate motif
For this, we have introduced the tetrazole motif in a series of ligands based on the terpy, bipy and py chromophores penta-, tetra- and tridentate ligands having a strong chelating effect in addition, the terpyridine-tetrazole ligand has been functionalized by a series of substituents having various electronic effects.
the key intermediate in the synthesis is the terpy-dicarbonitrile. a cycloaddition reaction with NaN3 in the presence of NH4Cl in DMF results in the high-yield formation of the tetrazole ligands. In the same time, hydrolysis of the carbonitrile in basic medium affords the carboxylate-based ligand. the substituents R have been added either during the synthesis of the terpy ring, or later, by the Stille coupling of the bromo-functionalized terpyridine-carbonitriles.
the synthesis of the symmetric bipyridine tetrazole and carboxylate ligands proceeds similarly, while in the synthesis of the dissymetric tetrazole-carboxylate ligand a different strategy has been followed. The bromo-functionalized methyl-bipyridine has been oxidized to carboxylic acid and then protected by esterification. The carbonitrile substituent has been introduced by microwave-assisted cyanation.
The described reactions show that different types of tetrazole-based ligands can be obtained with ease starting from the carbonitrile precursors.
The synthesis of the corresponding lanthanide complexes based on the ligand series presented has been carried out in methanol and the complexes have been isolated by crystallization. The terpyridine-based ligands lead to the formation of homoleptic bis-ligand complexes, as exemplified for the parent terpyridine-tetrazole and the terpyridine-carboxylate ligands. In the resulting crystal structures, the Ln ion is 10-coordinated and is completely encapsulated by the two helically-wrapped ligands. Study of a series of crystal structures in the series of Nd, Eu and Tb complexes indicate that the tetrazole-based ligands are well adapted to Ln complexation
The bipyridine-tetrazole ligand leads to the formation of heteroleptic complexes containing two bipyridine ligands and one water molecule coordinated to the lanthanide ion, as indicated by the crystal structure. By contrast, The dissymmetric bipyridine ligand forms homoleptic bis-ligand complexes without a coordinated solvent molecule, in which the Ln ion is octa-dentate. One possible interpretation is that both complexes can crystallize in two different forms (with or without a coordinated water molecule) although only one type of crystals was isolated.
The pyridine-based ligands lead to the formation of homoleptic tris-ligand complexes. In the resulting crystal structures, the Ln ion is nona-coordinated and is completely encapsulated by the three ligands hellically arranged around it.
The lanthanide complexes presented so far display solubility only in methanol. However, for the manufacturing of OLED devices by the spin-coating, solubility in chlorinated solvents such as chloroform of chlorobenzene is essential. Therefore, we have devised two approaches for increasing the solubility of the tetrazole complexes. The first is based on the initial functionalization of the ligand… The second is based on a change of counterion. Using trioctyl amine instead of triethylamine for the deprotonation of the pyridine-tetrazole ligand results in the introduction of 9 octyl chains in the structure of the final complex. Consequently, the product was isolated as a highly viscous oil showing very high solubility in chlorinated solvents.
modulation of ligand triplet state => what is the influence on the photophysical properties?
the quantum yields of the unsubstituted complexes have also been measured in methanol solutions, and are similar compared to the solid state values. The QY of the parent tetrazolate and carboxylate europium complexes are identical, in agreement with similar triplet states optimally positioned with respect to the 5D0 emissive level of europium.
In the pyridine series, we have observed excellent emission efficiencies for the pyridine-tetrazole complexes. Eu QY of xxx and Tb xxx are among the highest reported in literature.
After seeing how we can improve the absorption and emission properties of the classical carboxylate-based complexes, by introducing the tetrazolate motif in complexes having similar stability, we have turned our attention to the neutral lanthanide complexes often used in optoelectronics
These complexes are based on the beta-diketonate unit and additional neutral ligands such as Phen. Due to the weak bonding between the soft N atoms in the phenantroline and the Ln ion, the resulting complexes display low stability and often dissociate upon device processing. Therefore, we decided to replace the neutral phen chromophore with a neg charged ligand… For this, we have designed two types of architectures, both based on the terpyridine chromophore. In the first, we have connected the terpyridine to one anionic carboxylate group, and in the second, we have used the previously studied terpyridine-bis-tetrazolate ligand. The coordination sphere of the lanthanide ions is completed by 2, and respectively 1 beta-dik unit. In the following, I will present only our first approach, together with the preliminary testing of the complexes in OLED devices.
in our laboratory, we have previously shown… for the present purpose, we have inckuded the terpy in heteroleptic complexes containing also dik ligands. The dik have been chosen function of their TS in order to afford luminescent europium and terbium complexes.
homogeneous films, no aggregation)
after seeing how we can improve the photophysical properties and the stability of the lanthanide complexes based on organic chromophores, we have turned our attention to the second sensitization process = intermetallic transfer from d-metal complexes.
In our design of the heterometallic complex, we have decided to connect the metal ions…
above 800 nm, the emission is due only to the europium ion. Therefore, recording the excitation spectrum of the molecule upon monitoring this wavelength will allow us to determine which group is responsible for the sensitization of europium. The resulting excitation spectrum displays one broad band centered at 400 nm which has been assigned to the MLCT of Ir, by comparison with the absorption spectrum. these features indicate that this type of architecture is very promising for the sensitization of lanthanide ions by iridium or other transition metal complexes.
Andreiadis PhD Presentation
The organic (optoelectronic) revolution What is optoelectronics? The study and application of electronic devices that source, detect and control light LEDs solar cells lasers PM <ul><li>the classical devices use inorganic materials: Si, GaN, Y 2 O 2 S:Eu, YAG:Nd </li></ul>CRT <ul><li>1987: Tang and van Slyke demonstrate the first organic optoelectronic device </li></ul><ul><li>nowadays: </li></ul>
Advantages of organic versus inorganic LEDs <ul><li>tuning of chemical structure different optical and electronic properties </li></ul><ul><li>(potentially) very cheap production </li></ul><ul><ul><li>- low temperature </li></ul></ul><ul><ul><li>- scalable to large area </li></ul></ul><ul><li>(potentially) very energy efficient </li></ul><ul><li>synthetic flexibility </li></ul><ul><ul><li>ultra-thin and lightweight </li></ul></ul><ul><ul><li>self-luminescent no backlighting </li></ul></ul><ul><ul><li>the substrates can be flexible or transparent </li></ul></ul><ul><li>new paradigm in the field </li></ul>
Classes of organic emitters for OLEDs <ul><li>purely organic dyes </li></ul><ul><ul><li>- fluorescent (limited to 25% efficiency) </li></ul></ul><ul><ul><li>- broad emission bands </li></ul></ul><ul><ul><li>- photo-bleaching </li></ul></ul><ul><li>organometallic complexes </li></ul><ul><ul><li>- phosphorescent </li></ul></ul><ul><ul><li>(theoretical 100% efficiency) </li></ul></ul><ul><ul><li>- broad emission bands </li></ul></ul><ul><ul><li>- sensitivity to oxygen </li></ul></ul><ul><li>lanthanide complexes with organic ligands </li></ul><ul><ul><li>- first example: Kido, 1990 </li></ul></ul>
Properties of lanthanide ions <ul><li>Shielding of 4f orbitals </li></ul><ul><ul><li>similar chemical properties </li></ul></ul><ul><ul><li>electrostatic bonding </li></ul></ul><ul><ul><li>variable geometry and CNs </li></ul></ul><ul><ul><li>hard acid behaviour </li></ul></ul>Lu Yb Tm Er Ho Dy Tb Gd Eu Sm Pm Nd Pr Ce La <ul><li>Fascinating optical properties: </li></ul><ul><ul><li>luminescence from f-f transitions </li></ul></ul><ul><ul><li>characteristic emission for each ion </li></ul></ul><ul><ul><li>narrow emission bands </li></ul></ul><ul><ul><li>long excited-states lifetimes </li></ul></ul>Ln III ground state [Xe]4f n , n = 0..14 blue NIR Applications in optoelectronics and bio-medicine
Advantages of lanthanide complexes in optoelectronics <ul><li>one ligand, different emission colors (even NIR) </li></ul><ul><li>no oxygen sensitivity and no photo-bleaching </li></ul><ul><li>sharp emission pure colors (no filters) </li></ul><ul><li>easier coordination chemistry </li></ul>f-f transitions are forbidden the excited states cannot be efficiently populated directly
Sensitization of lanthanide ions Indirect excitation by energy transfer from a suitable antenna to the lanthanide ion <ul><li>Antenna requirements: </li></ul><ul><li>excellent energy harvester </li></ul><ul><li>efficient inter-system crossing </li></ul><ul><li>matching electronic levels </li></ul><ul><li>Deactivation: </li></ul><ul><li>radiative processes ( fluorescence , phosphorescence ) </li></ul><ul><li>non-radiative processes ( vibration-induced ) </li></ul><ul><li>electronic processes ( energy back-transfer ) </li></ul>Antenna Ln III 1 S 3 T absorption ISC ET Antenna excitation Energy transfer Light emission
Antennas for lanthanides organic chromophores (pyridines, phenantroline) d-metal complexes (Ru II , Pt II , Ir III ) matrixes (PVK, CBP)
The design of lanthanide complexes <ul><li>Connecting the antenna to negatively charged groups (carboxylate) </li></ul><ul><li>Associating the antenna to diketonate complexes </li></ul><ul><li>low stability </li></ul><ul><li>few structure-property relationships </li></ul><ul><li>difficult optimization </li></ul>Grenthe, J. Am. Chem. Soc. 1961 Bunzli, Spectrosc. Lett. 2007 Bunzli, Dalton Trans. 2000 Latva, J. Lumin. 1997 Mazzanti, Angew. Chem. Int. Ed. 2005
Luminescent lanthanide architectures for optoelectronics <ul><li>synthesize new stable lanthanide architectures </li></ul><ul><li>tuned absorption and emission properties by ligand design </li></ul><ul><li>investigate their potential for applications in optoelectronics </li></ul><ul><li>high denticity ligands with negatively charged groups </li></ul><ul><li>sensitizing antenna: - organic chromophores </li></ul><ul><li> - d-metal complexes </li></ul>
The tetrazole motif in coordination chemistry <ul><li>carboxylate often used for lanthanide coordination </li></ul><ul><li>no luminescent lanthanides </li></ul><ul><li>no comparative studies </li></ul><ul><li>Tetrazole-based complexes of d-metals: </li></ul><ul><li>high thermodynamic stability </li></ul><ul><li>interesting properties </li></ul>Very few examples in lanthanide coordination chemistry! <ul><li>tetrazole - highly acidic, aromatic </li></ul><ul><li>tetrazolate could replace carboxylate </li></ul><ul><li>tuning of absorption wavelength </li></ul>Aime, Tetrahedron Lett. 2002 , 43 , 783 Facchetti, Chem. Commun. 2004 , 1770
Lanthanide complexes based on pyridine-tetrazolates
Design of tetrazole-based ligands terpyridine ligands – pentadentate bipyridine ligands – tetradentate pyridine ligands – tridentate <ul><li>influence of tetrazolate on the properties of the complexes </li></ul><ul><li>direct comparison with carboxylate analogues </li></ul>
Organic synthesis of terpyridine-based ligands Andreiadis et al, submitted ; patent pending
Organic synthesis of pyridine-based ligands Easy access to tetrazole-based ligands
Lanthanide complexes with terpyridine-based ligands Giraud, Inorg. Chem. 2008 , 47 , 3952-3954 the tetrazole-based ligands are well adapted to lanthanide complexation [Ln( L ) 2 ] - , Ln = Nd, Eu, Tb
Lanthanide complexes with bipyridine-based ligands Andreiadis et al, submitted [Ln( L ) 2 ] - , Ln = Eu, Tb
Lanthanide complexes with pyridine-based ligands Andreiadis et al, submitted [Ln( L ) 3 ] 3- , Ln = Nd, Eu, Tb
Increasing the solubility in chlorinated solvents isolated as an oil <ul><li>ligand functionalization </li></ul><ul><li>change of counterion </li></ul>Solubility – strong advantage for the applications in OLED devices (wet process)
Stability of tetrazolate-based complexes logβ 2 = 10.5(5) logβ 2 = 11.8(4) [Eu L ] + [Eu L 2 ] - L 2- [Eu L ] + [Eu L 2 ] - L 2- Comparable stability to carboxylate analogues <ul><li>stable without dissociation in air and wet methanol solutions </li></ul><ul><li>quantitative study by UV titration </li></ul>
Absorption properties of pyridine-based complexes 250 275 300 325 350 0 1 2 3 4 Wavelength / nm ε / 10 4 cm -1 M -1 aromatic tetrazolate increase of absorption wavelength and intensity [Ln( L ) 3 ] 3-
Absorption properties of bipyridine-based complexes 250 275 300 325 350 375 400 0 1 2 3 4 Wavelength / nm ε / 10 4 cm -1 M -1 [Ln( L ) 2 ] -
Absorption properties of terpyridine-based complexes 250 300 350 400 450 500 0 2 4 6 8 10 Wavelength / nm ε / 10 4 cm -1 M -1 substituents tuning of absorption wavelength and intensity [Ln( L ) 2 ] -
Photophysical properties of terpyridine-based complexes Modulation of ligand triplet state Ligand triplet states
Photophysical properties of terpyridine-based complexes Emission quantum yields Eu: 35% Tb: 6% Nd: 0.22% Eu: 36% Tb: 35% Nd: 0.09 % Eu: 29% Tb: 0.1% Eu: 28% Eu: 5% Nd: 0.29% Nd: 0.19% Tuning of emission quantum yields Modulation of ligand triplet state [Ln( L ) 2 ] - Very good QY for Eu (35%) and Nd (0.29%)
Photophysical properties of terpyridine-based complexes Emission quantum yields Eu: 35% Tb: 6% Nd: 0.22% Eu: 36% Tb: 35% Nd: 0.09 % Eu: 29% Tb: 0.1% Eu: 28% Eu: 5% Nd: 0.29% Nd: 0.19% Terbium QY function of triplet state Latva, J. Lumin. 1997 [Ln( L ) 2 ] -
Photophysical properties of bipyridine-based complexes Eu: 45% Tb: 27% Eu: 54% Tb: 13% Eu: 63% Tb: 6% Measured after drying [Ln( L ) 2 ] - Similar tuning of emission quantum yields
Photophysical properties of pyridine-based complexes Eu: 61% Tb: 65% Nd: 0.21% Eu: 39% Eu: 24% * Tb: 22% * * Chauvin, Spectr. Lett. 2007 , 40, 193 <ul><li>excellent quantum yields </li></ul><ul><li>for pyridine-tetrazole complexes </li></ul>[Ln( L ) 3 ] 3- <ul><li>solubility in chlorinated solvents </li></ul>Possible applications in OLEDs
New approach towards neutral lanthanide complexes <ul><li>Lanthanide complexes employed in optoelectronics </li></ul><ul><li>neutral (vacuum processing) </li></ul><ul><li>based on the β -diketonate motif </li></ul><ul><li>additional soft, neutral ligands </li></ul>Replacing neutral chromophores with negatively charged ones for increasing the stability of the complex Preliminary testing in OLED devices <ul><li>low stability </li></ul><ul><li>dissociation during processing </li></ul>
Terpyridine carboxylic acid leads to stable homoleptic mono- or poly-metallic complexes [Ln (LnL 2 ) 6 ](OTf) 9 ∩ [Ln(L) 2 ](OTf) Ln= Eu, Gd, Tb, Nd The terpyridine-monocarboxylate ligand Bretonnière, J. Am. Chem. Soc., 2002 , 124 , 9012 Chen, Inorg. Chem ., 2007 , 46 , 625 formation of heteroleptic complexes with β -diketonate units:
Synthesis and properties of the complexes QY = 41% QY = 13% Investigate potential applications in OLED devices <ul><li>complexes stable in air and solution </li></ul><ul><li>good quantum yields </li></ul>
Preliminary testing in OLED devices Excellent film-forming properties (doping in PVK matrix) <ul><li>Collaboration Dr. Pascal Viville (Univ. Mons) </li></ul><ul><li>testing in OLED devices (spin-coating) </li></ul><ul><li>classical device architecture </li></ul><ul><li>the OLED devices display promising results </li></ul><ul><li>rather low current intensities: 5.4 mA/cm 2 at 25V (Eu) </li></ul><ul><ul><ul><li>45 mA/cm 2 at 20V (Tb) </li></ul></ul></ul>device optimization in progress + – – Al (cathode) ITO (anode ) glass substrate Cs 2 CO 3 PVK : Ln complex PEDOT:PPS
Indirect excitation using d-transitional metals by inter-metallic communication Sensitization of europium by d-metals Ir III complexes - modulation of emission energy by the coordinated ligands Thompson et al. Inorg Chem 2005, 44 , 7992 <ul><li>absorption at visible wavelength </li></ul><ul><li>sensitization of NIR emitting lanthanides </li></ul><ul><li>europium sensitization requires high energy </li></ul>Coppo, Angew. Chem. Int. Ed., 2005 , 44 , 1806 use blue-emitting Ir complexes
Heterometallic complex - strategy and ligand design <ul><li>terpyridine-tetrazolate motif for lanthanide complexation </li></ul><ul><li>several target ligands investigated </li></ul>Connecting the metal ions by a completely covalent structure (stability)
<ul><li>1 H NMR and X-ray diffraction studies prove the retention of Ir conformation during the synthesis </li></ul>Synthesis of iridium-based ligand
Synthesis of the heterometallic complex 1 H NMR indicates a similar structure to the mono-metallic lanthanide complexes [Eu( L ) 2 ] -
Protophysical properties of the heterometallic complex 300 400 500 600 700 800 0,0 0,5 1,0 1,5 2,0 2,5 intensity / a.u. wavelength / nm η Ir-Eu = 85-90% QY = 0.96% ex 400 nm selective excitation of Ir moiety <ul><li>iridium europium energy transfer </li></ul><ul><li>residual emission from iridium </li></ul><ul><li>very good energy transfer efficiency </li></ul><ul><li>Eu emission due exclusively to Ir </li></ul>promising architecture
Final conclusions and perspectives <ul><li>combining stability with tuning </li></ul><ul><li>of absorption and emission properties </li></ul><ul><li>improving the stability of neutral diketonate </li></ul><ul><li>complexes by using charged chromophores </li></ul><ul><li>polyvalent stable heterometallic architecture </li></ul><ul><li>with very high Ir Eu transfer efficiency </li></ul> extending the work to other architectures (podates) <ul><li>tetrazole-based antennas for lanthanide </li></ul> applications in OLEDs and surface grafting applications in OLEDs improving europium emission efficiency extending the chemistry to other metals
Acknowledgements Dr. Marinella MAZZANTI Dr. Renaud DEMADRILLE Dr. Daniel IMBERT Dr. Jacques PECAUT Prof. Luisa DE COLA, Prof. Jean WEISS, Prof. Muriel HISSLER, Dr. Guy ROYAL Yann KERVELLA, Dr. Bruno JOUSSELME, Prof. Alexander FISYUK Colette LEBRUN, Pierre-Alain BAYLE European Community Marie Curie EST “CHEMTRONICS” MEST-CT-2005-020513 Dr. Pascal VIVILLE (Mons University), Prof. Jean-Claude BUNZLI (EPFL) my colleagues and friends