Proposal
Introduction: Previous Work and Motivation
Bearing in mind the shortage of fossil fuels and therewith the energy problem, new concepts
and materials with higher energy efficiencies have to be developed. Organic light emitting
diodes (OLEDs) are considered to hold one answer to the problem of more efficient lighting.
Compared to nowadays common LEDs, which make use of high cost and less
environmentally friendly semiconductor materials, OLEDs gain high quantum yields at low
energy consumption and reduce unit costs due to inexpensive emitting materials and largescale production.
So far, usually iridium(III)-[1], osmium(II)-[2] and platinum(II)-complexes[3] were used as
phosphorescent emitters, which can theoretically reach photoluminescence quantum yields
of almost 100%, for OLED applications. In contrast to these cost intensive metals, copper
presents a cheap and easily accessible alternative and offers a new opportunity to develop
blue emitting materials. Copper(I)-complexes with a d10 electronic configuration lack
unoccupied low-lying metal-d-orbitals, which can quench emission, i.e. dd*-quenching does
not occur.[4] Furthermore a new emission principle, so-called “Singlet Harvesting“[1][5], was
ascribed to copper(I)-complexes. This phenomenon can be explained according to figure 1:
After excitation of the emitting molecule from the singlet ground state to higher states, the
excited molecule can relax back to the ground state via different paths.
Figure 1: Photophysical processes of different emission systems.[1]
In transition metal complexes such as iridium, platinum, and osmium, the central metal atom
induces spin-orbit coupling, which allows transition pathways with spin-flip, so that
intersystem crossing populates the triplet state and phosphorescence can occur (“Triplet
Harvesting“). In transition metal complexes with small spin-orbit coupling and small singlettriplet splitting, e.g. copper(I)-complexes, the singlet state S1 can be re-populated by
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thermal activation and fluorescence occurs from this singlet to the ground state. Due to the
larger energy difference between the singlet states S1 and S0 compared to T1 and S0, a blueshift of the emission maximum is observed. Additionally, by using fluorescence as the main
radiative pathway, this new emission system shows short emission decay times which are
crucial for the efficiency of an OLED at high current flow.
The aim of my Ph. D. project is to develop new emitting materials for OLEDs on the basis of
copper(I)-complexes. For material and device stability as well as for processing
requirements, the complexes need to be in a neutral state since counter ions present
potential charge carrier traps which may perturb charge transport in the device[6][7] and also
influence aggregation in solution and film negatively. Only few examples of neutral,
mononuclear copper(I)-complexes are known so far[8][9][10], but they gain more and more
interest as attractive emitting materials.
Planned Experiments:
My Ph. D. thesis deals with the development and synthesis of novel luminescent, neutral,
mononuclear copper(I)-complexes. These complexes can be realized by linking a copper(I)
atom with one negatively charged ligand and additional neutral ligands. In order to
energetically favor the complexation behavior as well as to stiffen the complex structure
and therefore enhance the quantum yields, chelating ligands are used with coordinating
atoms like nitrogen, phosphorus, arsene or oxygen. Possible coordination structures are
shown in figure 2, where the central metal atom forms a tetrahedral or trigonal planar
structure.
Figure 2: Possible copper(I)-complex structures, D = donor atom.
The anionic ligands can be divided into three different ligand classes depending on their
coordination atoms: N^N, N^P und N^O. For enhanced complexation stability, the negative
charge should not be localized on one of the donor atoms. Bis(phosphines) are used as
neutral ligands due to their good luminophore properties, which cause a blue-shift of the
emission maxima and can enhance quantum yields[11].
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 For all three named ligand classes various examples will be synthesized and
subsequently complexed with copper(I) salts.
Scheme 1: Exemplary synthesis of ligand 2-(Benzimidazol-2-yl)quinoline[10]
On the basis of experiments during my diploma thesis a complexation route shown in
scheme 2 is followed:
Scheme 2: Complexation of 2-(1H-benzo[d]imidazol-2-yl)quinoline[10]
 The synthesized complexes will be examined on their photophysical properties to
gain new knowledge about the structure, stability and luminescence behavior.

The next step is to modify ligands for better luminescence and solubility.
 Further steps are the evaluation of the processing properties of the synthesized
complexes for vacuum deposition, coating or printing techniques. This will be done in
a close cooperation with the KIT’s Institut für Thermische Verfahrenstechnik (W.
SCHABEL, Thin Film Technology).
 The properties of OLED test devices made with these emitter materials will be
examined in cooperation partners, e.g. the KITs Lichttechnisches Institut (U.
Lemmer).

While the synthetic work will be done at the laboratory of a close collaborative
partner, the Start-Up cynora GmbH at the KIT Campus North, my Ph. D. project will
be scientifically supervised by Prof. Dr. Stefan Bräse at the Institute of Organic
Chemistry, who is also member of the KSOP.
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List of Literature:
[1]
H. Yersin, A. F. Rausch, R. Czerwieniec, T. Hofbeck, T. Fischer, Coord. Chem. Rev. 2011, in
press. The Triplet State of Organo-Transition Metal Compunds. Triplet Harvesting and
Singlet Harvesting for Efficient OLEDs.
[2]
D. Kumaresan, K. Shankar, S. Vaidya, R. H. Schmehl, Top. Curr. Chem. 2007, 281, 101 – 42.
Photochemistry and Photophysics of Coordination Compounds: Osmium.
[3]
J. A. G. Williams, Top. Curr. Chem. 2007, 281, 205 – 68. Photochemistry and Photophysics
of Coordination Compounds: Platinum.
[4]
C. W. Hsu, C. C. Lin, M. W. Chung, Y. Chi, G. H. Lee, P.T. Chou, C. H. Chang, P. Y. Chen, J.
Am. Chem. Soc. 2011, in press. Systematic Investigation of the Metal-Structure–
Photophysics Relationship of Emissive d10-Complexes of Group 11 Elements: The
Prospect of Application in Organic Light Emitting Devices.
[5]
J. C. Deaton, S. C. Switalski, D. Y. Kondakov, R. H. Young, T. D. Pawlik, D. J. Giesen, S. B.
Harkins, A. J. M. Miller, S. F. Mickenberg, J. C. Peters, J. Am. Chem. Soc. 2010, 132,
9499 – 508. E-Type Delayed Fluorescence of a Phosphine-Supported Cu2(μ-NAr2)2
Diamond Core: Harvesting Singlet and Triplet Excitons in OLEDs.
[6]
F. So, D. Kondakov, Adv. Mater. 2010, 22, 3762 – 77. Degradation Mechanisms in SmallMolecule and Polymer Organic Light-Emitting Diodes.
[7]
N. Armaroli, G. Accorsi, M. Holler, O. Moudam, J. F. Nierengarten, Z. Zhou, R. T. Wegh, R.
Welter, Adv. Mater. 2006, 18, 1313 – 6. Highly Luminescent Cu(I) Complexes for LightEmitting Electrochemical Cells.
[8]
R. Czerwieniec, J. Yu, H. Yersin, Inorg. Chem. 2011, in press. Blue-Light Emission of Cu(I)
Complexes and Singlet Harvesting.
[9]
T. McCormick, W. L. Jia, S. Wang, Inorg. Chem. 2006, 45, 147 – 55. Phosphorescent
Cu(I)Complexes of 2-(2´-pyridylbenzimidazolyl)benzene: Impact of Phosphine Ancillary
Ligands on Electronic and Photophysical Properties of the Cu(I)Complexes.
[10]
J. Min, Q. Zhang, W. Sun, Y. Cheng, L. Wang, Dalton Trans. 2011, 40, 686 – 93. Neutral
copper(I) phosphorescent complexes from their ionic counterparts with 2-(2´quinolyl)benzimidazole and phosphine mixed ligands.
[11]
N. Armaroli, G. Accorsi, F. Cardinali, A. Listorti, Top. Curr. Chem. 2007, 280, 69 – 115.
Photochemistry and Photophysics of Coordination Compounds: Copper.
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