Luminescent Re(I) and Ir(III) complexes: from design to application in light-emitting devices

In the past two decades, luminescent materials have attracted enormous interest due to their great photophysical and electrochemical properties. Such compounds have shown potential in biological imaging and, most important, as photoactive components for electroluminescent materials (i.e. OLEDs and LEECs), as well as sensitizers in solar cells[3]. In some case they became a real market technology to date. The most powerful driving forces for the research in such field are the pursuit of renewable non-oil based energy sources (solar cells application) and of more efficient lightening devices (OLED). The development of organic light-emitting devices (OLEDs) started with the discovery of the electroluminescence from polymer OLEDs 25 years ago. Afterwards, the development of thin-film heterojunction devices based on Alq3 (Aluminum tris-quinolinate) by Tang and Van Slyke in the 80s opened the commercial interest to such a device in display and lighting technology.[4] Indeed, the optical stimulation is not the only way to generate an excited state. In some case, the charge recombination reaction between the electrically-generated reduced and oxidized forms of the same compound may lead to the formation of an excited state which could radiatively relax, with emission of a photon. In this respect, complexes containing transitions metals such as Ru(II), Re(I) and Ir(III) are particularly appealing since they are able to exhibit luminescence upon electrical stimulation. The presence of a heavy metal atom in transition metal complexes could induce a considerable degree of spin-orbit coupling to such an extent that spin-forbidden electronic transitions may become sufficiently allowed. This effect is particularly evident for complexes of metals belonging to the second and third row of the transition metals block (mainly Pt, Ir, Re, Os and Ru). Differently to the fluorescent-based OLEDs, the phosphorescent-based ones can efficiently make use also of the electrically generated exciton with triplet spin, increasing the theoretical efficiency limit up to 100%.[5] In this Ph.D. thesis we have focused our attention on luminescent Re and Ir complexes able to act as phosphorescent emitters in electroluminescent devices. In collaboration with other universities and industrial research groups (Prof. L. De Cola, Universität Münster and Center for Nanotechnology, Münster (D); Ciba Inc., Basel (CH) and National Chiao Tung University, Hsinchu (Taiwan)), we have designed, synthesized and characterized several highly emitting Re and Ir complexes in order to develop molecules possessing the desired properties. Our aim is to find new classes of stable phosphorescent emitters, suitable for monochromatic and/or white-light OLEDs (i.e. Re complexes) as well as compounds containing multi-redox centres with externally and deliberately adjustable photophysical properties (i.e. Ir complexes), which can be useful for reduction of small molecules (H2O, CO2, ...). In this project, we have carried out tailored synthesis with joint experimental and theoretical studies on a series of Re and Ir complexes in order to gain a deeper insight into the electronic processes involved in these classes of compounds. Some of these materials were also successfully tested as emitting dopants in electroluminescent devices. This thesis is basically divided in four main sections: 1. New class of Re complexes and their application in OLED; 2. Re-based LEECs; 3. A new class of porous materials consisting of charged iridium complexes; 4. New amphiphilic luminescent iridium complexes able to self-organize. Section 1. Several parameters are important for describing the efficiency of an OLED, and one of these is the emission quantum efficiency of the materials, em. Although the photophysics of Re(I) complexes occupy a prominent position among the transition metal complexes (TMC), such compounds are usually considered poor emitters and then not much used as emitting species in light-emitting device, that to date mainly based on Ir and Pt complexes. Nonetheless, we are able to prepare a new family of dinuclear Re complexes, namely [Re2(-halogen)2(CO)6(-diazine)] (Figure S1), designed for enhancing the spin-orbit coupling effect, by linking two heavy metal atoms on the same chromophoric ligand, differently to the commonly employed strategies which make use of only one metal. Moreover, the optimization of the interaction energy allowed to predict the decrease of the radiationless pathways. Our strategy resulted in easily to make, air stable and green-yellow highly emitting compounds. The em are up to 52% in solution, values ten times higher than observed for the before best reported neutral Re(I) complex. Much more important, em values are kept very high even in solid state (>50%), where generally quenching effects take place. Figure S1. General chemical structure of the new class of luminescent neutral dinuclear rhenium complex described in this thesis. These properties encouraged the possible testing in OLED fabricated, for the first time, by both solution processing and sublimation of the metal complex. Figure S2. Inside cover dedicated to our Adv. Funct. Mater. paper concerning the use of the dinuclear rhenium complexes in both PLED and OLED. Employing a compound showing only em = 10% in solid state, the vacuum-processed device had a turn-on voltage of 4.1 V and at a maximum luminance of 2000 cd m-2, its driving voltage was merely 12.5 V. In the meanwhile, with an emission at 512 nm, it represents the bluest Re-based OLED. The EL efficiencies of this Re complex are amongst the highest reported for Re-based OLEDs. Notably, the current efficiency of the vacuum-deposited device was 5.5 times higher than that of its spin-coated counterpart. These results show that this class of triplet emitters can be processed in different ways and give similar emission energies for both types of devices. The electroluminescence obtained with the sublimed devices is one of the most efficient ever reported and such findings suggest that Re complexes could play an important role for the development of new triplet emitters for OLEDs. Section 2. Light-emitting electrochemical cells (LEECs) represent the most promising alternative to OLED technology. Differently from an OLED, a LEEC usually consists of a solution-processed single-layer of an ionic light-emitting TMC and makes use of air-stable metal used as cathode, eliminating the need of encapsulation. All of these feature make LEEC a cheaper and easier to make light-emitting device. Figure S3. Chemical structure of the cationic rhenium complexes for LEEC. In this section, the photophysical properties of pristine cationic Re complexes, namely [Re(CO)3(phenantroline)(pyridine)]X, where X is a small counter-ion, are investigated (Figure S2). In particular, carefully attention was given to the properties in solid state, where the investigated complexes showed high em. Figure 4. Semplified representation of a LEEC device and its operating mechanism. Also for the first time, the suitability of the use of cationic Re complexes as active component when blended with ionic liquid in LEEC is demonstrated and the performances in such devices are described (Figure S3). Section 3. Here a new class of non-covalently linked crystalline porous materials based on luminescent metal complexes is described. The idea consists in the use of a pair of luminescent complexes, possessing different emission colors, and complementary charges which are employed to form complex salts of general formula [Ir(C^N)2(CN)2]-[Ir(C^N)2(N^N)]+, where C^N and N^N are the cyclometalating and chelating ligands, respectively. Figure S5. Crystal packing of the double complex salt 1. Four unit cells are represented. The picture highlights the channels running along c. The crystalline materials formed fascinating motifs, 3D porous networks, as demonstrated by the single-crystal structure (Figure S4). Such a feature is explored to modulate the photophysical properties of the assemblies. We demonstrated that the emission color can be tuned by intermolecular interaction, inclusion of a solvent or even more dramatically, by selective quenching of one of the component of the crystal by insertion of appropriate quenchers. The concept can be extended to many classes of photo- and electroresponsive ionic TMC and the selective quenching or enhancement of one of the component properties open fascinating routes for the realization of light modulators and novel molecular devices. Also, the presence of multiredox metallic centers in these new porous materials could play extremely important role for reduction of small molecules as H2O and CO2. Section 4. New advanced functionalities can be introduced when TMC are incorporated into the molecular structure of the surfactants leading to metallosurfactants. Thus, this amphiphilic compounds have recently found important potential applications in several areas such as templating of mesoporous materials, thin-film optoelectronics and most importantly for building nano-scale reactors useful for catalysis and redox reactions. It is know that the photophysical properties have demonstrated an exceptional sensitivity to the environment experienced by the molecule. Our aim was to use the change in the photophysical behavior of the simple components versus the aggregates in order to monitor the self-assembly process. Figure S6. SEM image on glass of Ir12_Na. In this view, a new class of amphiphilic self-assembling luminescent complexes of general formula [Ir(C^N)2(C^O–alkyl–SO4)]Na were prepared and characterized. Moreover, these measurements were complemented by dynamic light scattering and microscopy techniques (SEM and AFM) (Figure S5). References 1. R. H. Friend, et al., Nature 1990, 347, 539-541; 2. A. J. Heeger et al. Science 1995, 269, 1086-1088; 3. M. Gratzel, Nature, 2001, 414, 338; 4. K. Leo et al. Nature, 2009, 549, 234; 5. H. Yersin, Top. Curr. Chem. 2004, 241, 1.