From STM to LEECs. syntheses and applications of multifunctional bipyridine ligands and their iridium(III) complexes

Abstract

The theoretical background for this thesis is given in Chapter 1. It covers the field of supramolecular chemistry including the phenomena of self-assembly, the history and synthesis of dendrimers, the concept of coordination chemistry and the chemistry of iridium, the history and principles of the scanning tunnelling microscope (STM), and the theory and applications of solid state lighting, especially of the light-emitting electrochemical cells (LEECs). The background chapter is followed by a short introduction to the materials, methods, and instruments used in this thesis (Chapter 2). In the following two chapters, the syntheses of achiral and chiral Frechet dendrimers (Chapter 3) and the subsequent reactions to the achiral and chiral Frechet dendronised 2,2'-bipyridine ligands (Chapter 4) are described. Additionally, for most of the compounds presented in these chapters, the monolayer behaviour on graphite was studied with STM. For example, for 3,5-bis(dodecyloxy)phenylmethanol, a very highly resolved image could be detected and detailed considerations of the adopted monolayer could be performed. Chirality was introduced into the molecules for the purpose of altering the preference for a particular conformation, as it has been shown before by L. Scherer[1] that these type of ligands tend to adopt different conformations when adsorbed on graphite. Unfortunately, the measurements of the chiral ligands did not reveal any significant information. Therefore, no detailed discussion of the conformations in the monolayer could be given. Nevertheless, in a monolayer of the diastereomeric mixture of 4,4'-bis(1-(3,5-bis(dodecyloxy)phenyl)propoxy)-2,2'-bipyridine, two clearly differing patterns could be observed which were attributed to different stereoisomers. Chapter 5 deals with the synthesis of dendrons decorated with perfluorinated alkyl chains and their use in the functionalisation of 2,2'-bipyridine ligands. Adsorbed monolayers on graphite of such a ligand were studied with STM. Due to a, apparently, lower propensity to establish monolayers, only few examples of visualised patterns could be observed. The following three chapters cover the synthesis and STM-visualisation of 2,2'-bipyridine-based ligands (Chapter 6), their iridium(III) complexes (Chapter 7), and the use thereof in LEEC devices (Chapter 8). In Chapter 6, simple and more advanced ligands were synthesised and characterised. In the case of the ligands which were functionalised with dendrons presented in Chapter 2, STM studies of monolayers on graphite are discussed. Chapter 7 presents the synthesis and characterisation of iridium(III) complexes obtained from ligands described in the previous chapter. The characterisation comprises measurements of NMR, MS, UV-Vis, photoluminescence, electrochemistry, and, where single crystals could be obtained, their solid state structures. For the complexes bearing dendronised ligands, STM measurements were performed which revealed highly resolved patterns. In the last chapter (Chapter 8), results from LEEC devices fabricated with complexes described in Chapter 7 are shown. The device preparation and the measurement of their characteristics were performed by the group of H. Bolink who kindly allowed the publication of their results in this thesis. It could be shown that for all complexes exhibiting an intramolecular π-π stacking, the stability of their devices was increased dramatically. This thesis has brought together the realms of chemical design with, firstly, studies of the physical behaviour of the envisioned molecules on the surface and, secondly, systematic structural optimisation of iridium(III) complexes for the application in solid state lighting. With the work presented in this thesis, a major breakthrough for long-lived LEECs has been achieved allowing lifetimes of several thousands of hours, an increase of several orders of magnitude compared to the best-performing devices reported to date (see Chapter 1 and Chapter 8).