Synthesis and evaluation of novel and existing water-soluble iridium chelates for use in aqueous electrogenerated chemiluminescence systems

The photophysical properties and photochemical utility of cyclometalated transition metal complexes have been well documented. Owing largely to their structure – viz. a transition metal core chelated to three ligands, with significant ligand and metal molecular orbital overlap – these compounds find use in myriad applications across both the synthetic and the analytical chemistry fields, particularly in photoredox catalysis and electrogenerated chemiluminescence (ECL). Whilst ruthenium-centred chelates have, historically, been the gold standard of ECL analysis – particularly enjoying use in clinical diagnostics and bioassays owing to their solubility in aqueous environments – their relatively low quantum yields and singular emission maxima have driven scientific interest in developing iridium-based complexes due to their emissive tunability across the visible spectrum via ligand substitution. However, their lack of solubility in aqueous solvents compared to ruthenium-centred chelates has proven problematic when seeking to harness their power for both syntheses and analyses which require aqueous systems, such as in the case of biochemical and bioanalytical assays.It is thus that this project sought to adapt the synthetic routes for both reported and novel water-soluble iridium complexes, with particular emphasis on ligand sulfonation. Whilst the synthetic design used was already reported, this project sought to optimise the isolation and purification of the chelates synthesised. In so doing, a novel [Ir(sppz)3]3- chelate was successfully synthesised and characterised, as were the literature reported [Ir(sppy)3]3- and [Ir(dFsppy)3]3- chelates. Following synthesis, the analytical utility of these chelates was assessed with special attention paid to the spectroscopic and co-reactant ECL characterisation of such compounds. Particularly, noting previous literature surrounding its non-sulfonated parent analogue, the suitability of the [Ir(sppy)3]3- chelate in detection systems was examined by way of ECL signal enhancement of a [Ru(bpy)2(mbpy-NHS)]2+ label within a bioanalytical system. To this end, a 2.7-fold enhancement on the ECL signal detected for the ruthenium label, where tri-n-propylamine was the co-reactant, was observed. This offers promising results and exciting possibilities for the use of these water-soluble iridium chelates as potential ECL biosensor enhancers