New electrochemiluminescent horizons: novel complexes of platinum, iridium and cadmium

Submission note: A thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy to the Department of Chemistry, Faculty of Science, Technology and Engineering, La Trobe University, Bundoora.

Electrogenerated chemiluminescence (ECL), the generation of light emitting states by means of electron transfer between electrochemically generated precursors continues to grow in importance as the basis for highly sensitive analytical techniques. Studies of ECL systems also provide valuable insights into the principles underpinning the operation of organic light-emitting devices (OLEDs) and light emitting electrochemical cells (LEECs) which operate under a similar mechanism. Commercial ECL systems are all based on tris(2,2'-bipyridine)ruthenium(II) ([Ru(bpy)3] 2+) or closely related derivatives. Moreover, the vast majority of research in this area has focused on ruthenium complexes of this type. This is surprising considering the relatively low luminescent quantum yields and limited wavelength tuneability of such complexes, (eg. [Ru(bpy)3] 2+ shows only ~4% quantum efficiency in aqueous media). New materials possessing tunable emission wavelengths and higher quantum efficiencies would significantly enhance the sensitivity and selectivity of current ECL detection systems. The broad objective of the work presented in this thesis is to discover new ECL luminophores outside the realm of ruthenium, evaluate their properties and suggest new avenues of exploration for ECL systems. The novel complexes explored in this work are based on the metal atoms of platinum, iridium and cadmium. Through variation of metal centres and coordinating ligands, we demonstrate the ability to tune the energetics of the reaction leading to the emitting state, tune the energy of the excited state itself and most importantly, increase our understanding of these systems and their potential for ECL application. A range of photophysical and electrochemical techniques have been used to explore the relationship between the photophysical and redox properties of the new transition metal complexes. The findings provide insights into the characteristics required for efficient ECL and suggest new directions in the search for novel ECL luminophores.