The effects of sulfur on heteroleptic copper (I) complexes for the potential application in light emitting electrochemical cells

The aim of this project was the synthesis and characterisation of heteroleptic copper(I) compounds that emit light in the visible spectrum (400–800 nm), with the goal to incorporate these complexes as emitting species in light emitting electrochemical cells (LECs). LECs have several advantages when compared to organic light emitting diodes (OLEDs) and light emitting diodes (LEDs), and some challenges that need to be overcome before large scale applications can be considered. These advantages and challenges will be discussed in Chapter 1: Introduction, together with a brief overview over the history of lighting and an introduction to LEDs and OLEDs.
Typically, the cation in such a light emitting copper(I) complex consists of the copper centre, a diphosphane – in the case of this project the commercially available bis(2-(diphenylphosphano)phenyl) (POP) and 4,5-bis(diphenylphosphano)-9,9-dimethylxanthene (xantphos) – and a diimine such as 2,2’-bipyridine (bpy) or 1,10-phenantroline (phen). Often, the bpy or phen carries alkylic or aromatic substituents.
The overarching theme of this project was the incorporation of heteroatom substituents into these complexes. In Chapter 2 sulfur replaces one of the chelating nitrogens, and thereby takes an active role in binding to the Cu(I) centre. The first series of complexes with only one aromatic ring (pyridine), did not show significant luminescence. The second series, which used a 2-(thiophen-2-yl)pyridine as a chelating ligand, featured a yellow emission in solid-state with photoluminescence quantum yields (PLQYs) up to 10.8% and a blue emission, with significant ligand-based contributions, in solution with PLQYs up to 33.2%.
Chapter 3 focuses on classical [Cu(P^P)(N^N)][PF6] compounds, with N^N being a dibromo-1,10-phenanthroline. In this series of complexes, the influence of substituents in the 2- and 9-positions of the phen on the emission wavelength and PLQY are clearly visible. The complexes do not feature a PLQY >1% in deaerated CH2Cl2 solution, but in the solid state they are yellow to orange emitters with a PLQY up to 45%.
Chapter 4 investigates the influence of chalcogen substituents in the 1,10-phenanthroline backbone. The introduction of either alkylsulfanyl or alkoxy substituents has significant influence on the photophysical properties of the complexes. The complexes are yellow to orange emitters with a PLQY of up to 9.4% in deaerated CH2Cl2 solution and up to 60% in the solid state. 2,9-Alkylsufanyl substituted phenanthrolines feature a notable blue shift compared to 3,8- and 4,7-subtituted ones with a significantly higher PLQY. In the solid state these complexes exhibit excited state lifetimes in the μs regime with the longest being 19 μs. To investigate the nature of these emissions further measurement in a frozen matrix of 2-methyltetrahydrofuran at 77 K were carried out, where most of the complexes exhibited a considerable blue-shift and dual emission.
At the end of this work a short summary of the work at hand and a glance into the future of light emitting Cu(I) complexes and LECs awaits.