In this study, we describe, for the first time, the synthesis and photophysical and microbiological investigation of ruthenium trischelate diimine complexes designed so as to possess properties specifically suited for use in Photodynamic antimicrobial chemotherapy (PACT). Of the three compounds investigated, one ([Ru(dmob) 3 ]Cl 2 ) has demonstrated considerable promise as a photosensitiser for use in PACT. As a result, this compound is now the subject of comprehensive chemical, toxicological and formulation studies.