Tunable Electrochemiluminescence of iridium(III) complexes

Submission note: A thesis submitted in total fulfillment of the requirements for the degree of Doctor of Philosophy to the department of Chemistry and Physics, La Trobe University, Melbourne.

Electrochemiluminescence (ECL), the phenomenon where luminescent emission is generated following electron-transfer reactions between electrochemically generated radicals, has drawn a growing amount of attention in the past decade. This is due to its potential as the basis for a range of interesting applications, particularly in the areas of lighting technology and detection science. This thesis, while focusing primarily on fundamental aspects of ECL also explores applications in the area of low-cost sensing. The unusual ECL properties of a homoleptic 1,2,4-triazole iridium(III) complex were initially investigated. Intriguingly, it was found that a range of colours apart from the original blue phosphorescence of the complex, were obtainable depending on the applied potential. It was also shown the different emission colours were obtainable depending on which mode of ECL experiment was performed; only red emission was attained for the co-reactant mode while a range of assorted colours were attained using annihilation ECL. To explore the mechanism leading to this ECL emission, a 3D-ECL technique was employed; the first demonstration of the use of such methodology for this type of investigation. It was shown that the electrochemical generation of two emissive by-products at different potentials is responsible for the convolved ECL emission. Bulk electrolysis studies coupled with mass spectrometry suggest dissociation of the methyl group from the triazole moiety of the ligands, leading to more stabilised LUMO and consequently lower energy band-gap. To achieve a better understanding of the ECL behaviour of this class of compounds, the effects of attaching an electron-withdrawing groups to the phenyl moiety of the ligands were investigated in a family of fac-tris 1-methyl-5-(4,5-disubstitutedphenyl)-3-n-propyl-[1,2,4]- triazoly iridium(III) complexes. A detailed study of the photophysical and electrochemical properties were investigated and the experimental results were rationalised using theoretical calculations. It was shown that similar ECL behaviour to the complex studied in the previous chapter was only observed for some of the complexes in family. It is indicated that the emissive products generated during the electrolysis of the complexes were responsible for the convoluted ECL signals obtained. It was also shown that electrochemically generated products originated from the nitrogens (N1 and N2) in the 1,2,4-triazole ring being either electrochemically or chemically oxidised during the voltammetric scans. The results obtained from the theoretical natural bond orbital (NBO) calculations and normalised 3D-ECL also supported this hypothesis. In the last chapter of this thesis we have focused on a practical aspect of ECL. A miniaturized, stand-alone, low-cost ECL detection instrument, based on single board computers and employing custom-made inexpensive ITO sensors. The device was successfully used for analytical ECL measurements using the Ru(bpy)3 2+ DBAE and Ru(bpy)3 2+ L-proline systems as proofs-of-concept. We have successfully shown the possibility of employing ECL in an extremely low-cost sensing device while maintaining excellent sensitivity and precision by utilising inexpensive silicon-photodiodes as the photo-sensing module, single board computers as the processing unit and disposable low-cost paper-based sensors.