Phosphorescent systems based on iridium(III) complexes

Phosphorescent iridium(III)-based complexes are experiencing a growing interest in a number of research fields. Aside from lighting and display technologies (i.e. OLEDs and LECs), they find use in various applications such as biolabeling, sensors, solar cells and water splitting. In particular, the opportunity to tune the electro-optical properties by adapting the coordinated ligand sets makes these transition metal complexes very attractive. Emission colors covering the whole spectrum of the visible light can be realized. High luminescence efficiencies and lifetimes in the range of nano- to micro¬seconds are further promissing features. The rich coordination chemistry of the iridium(III) ion provides immense freedom for the design of new systems. Besides the wide scope of options to modify the electro-optical characteristics, the various possibilities to introduce active sites and functionalities are highly appealing. The most common iridium(III) emitters are bis- and tris-cyclometallated species based on 2-phenylpyridine-derived ligands. In particular, facial tris-cyclometallated emitters are considered to be highly efficient. However, a broad spectrum of ancillary ligand structures can be used in the formation of bis-cyclometallated species offering additional options to tune the electro-optical properties and to introduce lateral functionalities. Dimeric µ-chloro-bridged precursor complexes, easily accessible by the reaction of iridium(III) chloride hydrate and the designated cyclometallating ligands, are the basis for most synthetic approaches towards iridium(III) phosphors. In this thesis, a set of precursors based on "classical" ligands (i.e. 2-phenylpyridinato, 2-(4-formylphenyl)-pyridinato and coumarinato-6) have been synthesized and applied in the formation of various bis-cyclometallated emitters. Depending on the ancillary ligand charged and neutral species can be formed. In the first part, several S-shaped terpyridines have been used to synthesize a series of cationic emitters. The characteristics of the new species were investigated by NMR, UV-vis absorption and photoluminescence spectroscopy as well as elemental analysis, MALDI-TOF mass spectrometry, thermogravimetric analysis and cyclic voltammetry. In particular, the emission behavior was found to be influenced by variations of the coordinated ligands. Aside from the selection of the cyclometallating ligands, some of the employed S-terpyridine ligand caused distinct alterations; while modifications at the periphery of the S-terpyridine (i.e. an arylic substituent) led only to marginal changes in the optical properties, the exchange of a central substituent (aryl towards alkyl) and the modification of the conjunction of the terpyridine rings (from ( CH2-)2 to (-CH2-)1) clearly affected the emission. The synthesis of "classical" cyclometallating compounds exhibiting pyridine motifs commonly involves cross-coupling methods such as the Stille and the Suzuki reaction. In the second part of this thesis, an approach towards new cyclometallating ligands was investigated. The highly efficient "click" reaction of acetylene- and azide-functions was utilized to synthesize two isomeric phenyl-1H-[1,2,3]-triazole derivatives, which were employed in the formation of dimeric µ-chloro-bridged precursor complexes. Based on 1 decyl-4-phenyl-1H-[1,2,3]triazole, a whole set of charged and neutral bis-cyclo¬metallated emitters was synthesized and investigated in detail. The properties of the new species were compared with 2-phenylpyridine-based complexes possessing the same ancillary ligands. More insights into the photophysical properties could be gained by theoretical DFT calculations. Aside from expanding the toolbox for the tuning of the electro-optical properties this coordination approach appears suitable to easily introduce further functionalities. A convenient way to introduce functional groups was followed by utilizing acetoacetate derivatives as ancillary ligand. By performing simple transesterification reactions at commercial tert-butyl acetoacetate, a broad variety of functional derivatives is accessible. Butyl acetoacetate, as well as oxetane- and methacrylate-equipped acetoacetate, have been employed in the formation of emissive bis-cyclometallated species. The oxetane- as well as methacrylate functionality were used to incorporate the phosphorescent emitters into polymeric assemblies. There are two main procedures to fabricate organic light emitting devices: vacuum deposition, and solution processing. While the vacuum deposition of small and thermal stable molecules easily allows the realization of elaborated device designs by applying mask techniques, the assembly of multilayer devices by solution processing techniques such as spin-coating and inkjet-printing is often limited. However, the spectrum of materials which can be processed from solution is much broader, and the processing from solution is cost-effective. One approach to overcome the limitations is the crosslinking of the deposited layers, which prevents redissolving in the following steps. In particular, the cationic ring opening polymerization of oxetane-units was demonstrated to be an effective tool to form crosslinked device layers. One part of this thesis was dedicated to the design of oxetane-equipped iridium(III) phosphors, which can be incorporated into oxetane-based crosslinkable matrixes. Thus, the approach could be expanded to the fabrication of crosslinked emissive layers doped with phosphorescent emitters. After synthesis and characterization of the emitters, their incorporation and performance in an oxetane-equipped matrix were investigated. Although the host and the guest were found to be not a good match, employing an optimized multilayered device assembly including a hole-blocking/electron-conducting layer, a fully solution processed, highly efficient OLED could be fabricated. However, considering the restricted stability of the applied complex (in particular under acidic conditions), the adjustment of the emitter design could lead to further improvements. Due to their film-forming abilities, polymeric materials are highly suitable for processing from solution. The doping of phosphors into polymeric hosts became a common approach for the fabrication of emissive layers. However, the efficiency of such systems can suffer from undesired phase separation and crystallization. By combining the different species for charge transport and emission within one polymer, these problems can potentially be overcome. Moreover, a better performance due to improved charged transport and/or energy transfer to the emitter can be expected. The last part of this thesis deals with two approaches to synthesize polymeric chains with attached iridium(III) complexes. The first approach utilizes a reactive µ-hydroxy-bridged dimeric iridium(III) precursor complex to coordinate polymer-bound ligand sites. The defined polymer was obtained by the copolymerization of methylmethacrylate and acetoacetoxyethyl meth¬acrylate under reversible addition fragmentation chain transfer (RAFT) polymerization conditions. The successful formation of a coordinated polymer could be confirmed by size exclusion chromatography (SEC) in combination with a photodiode array detector. Further evidence was obtained by the comparison of NMR, UV-vis and photo¬luminescence spectra with the data of an analogous butyl acetoacetate coordinated model complex. By this synthetic approach, the complex content within a polymer can be defined by the number of ligand sites and the feed of the coordinating species. For the second approach, a complex monomer (methacrylate-functionalized) was synthesized, which could be successfully applied in radical polymerizations with meth¬acrylate-based comonomers. Also here the formation of polymers containing iridium(III) emitters could be demonstrated by PDA-supported SEC-experiments. Investigations by NMR, UV-vis and photoluminescence spectroscopy provided information about their composition. Moreover, indications for highly efficient host-guest behavior were found for a carbazole-based copolymer