Rational ligand design in heteroleptic copper(I) complexes for application in light-emitting electrochemical cells

Light-emitting electrochemical cells are an emerging class of devices based on solid state lighting technology. The utilization of charged molecules as the main active component gives these devices inherent advantages over other modern lighting technologies such as low turn on voltage, easy device fabrication and the applicability on flexible substrates. These benefits make LECs an excellent candidate for display and large area low-tech lighting applications. Iridium based LECs are well established but face the problem of high material cost due to the low abundance of iridium. Using copper(I) based complexes as active material lower the production cost and therefore make LEC fabrication more affordable. This thesis takes a systematic approach to alter the luminescent properties of [Cu(N^N)(P^P)][PF6] complexes, where N^N is a chelating diimine and P^P is a chelating bisphosphane ligand, and optimize them for LEC application. Chapter 1 gives a brief history of artificial lighting technology, describes the rise of solid-state lighting and gives an introduction to modern OLED and LEC technology. Chapter 2 describes the preparation of 2,2'-bipyridine ligands bearing large aromatic substituents in the 6-position. Complexation with copper(I) and suitable P^P ligands yields new heteroleptic complexes and their structural and photophysical properties are investigated in depth. Chapter 3 presents the preparation of a new asymmetric methyl functionalized 2,2'-bipyridine ligand and the properties of the corresponding [Cu(N^N)(P^P)][PF6] complexes. The particular substitution pattern is designed to optimize the steric demand of the N^N ligand and therefore increase LEC device performance. After in depth investigation of the N^N ligand in [Cu(N^N)(P^P)][PF6] complexes, the development of new P^P ligands is describes in Chapter 4. Altering the bisphosphane ligand allows to influence the HOMO energy level, as well as the coordination sphere around the copper(I) centre. Five new complexes and their structural, photophysical and device properties are described. Chapter 5 combines the previous findings in attempting to prepare a tetradentate N^N^P^P type ligand. Such a ligand is expected to give more stable copper(I) complexes and therefore increase the device lifetime of a corresponding light-emitting cell. Six synthetic approaches to prepare such a tetradentate ligand are described. The results of this thesis and a brief outlook on the project are given in Chapter 6.