TLD1433 is the first Ru(II) complex to be tested as a photodynamic therapy agent in a clinical trial. In this contribution we study TLD1433 in the context of structurally-related Ru(II)- imidozo[4,5-f][1,10]phenanthroline (ip) complexes appended with thiophene rings to decipher the unique photophysical properties which are associated with increasing oligothiophene chain length. Substitution of the ip ligand with ter- or quaterthiophene changes the nature of the long-lived triplet state from metal-to-ligand charge-transfer to 3 ππ* character. The addition of the third thiophene thus presents a critical juncture which not only determines the photophysics of the complex but most importantly its capacity for 1 O2 generation and hence the potential of the complex to be used as a photocytotoxic agent.