Mitochondria generate energy but malfunction in many cancer cells, hence targeting mitochondrial metabolism is a promising approach for cancer therapy. Here we have designed cyclometallated iridium(iii) complexes, containing one TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) spin label [C43H43N6O2Ir1·PF6] (IrO1) and two TEMPO spin labels [C52H58N8O4Ir1·PF6] (IrO2). Electron paramagnetic resonance (EPR) spectroscopy revealed spin-spin interactions between the TEMPO units in IrO2. Both IrO1 and IrO2 showed bright luminescence with long lifetimes (ca. 35-160 ns); while IrO1 displayed monoexponential decay kinetics, the biexponential decays measured for IrO2 indicated the presence of more than one energetically-accessible conformation. This observation was further supported by density functional theory (DFT) calculations. The antiproliferative activity of IrO2 towards a range of cancer cells was much greater than that of IrO1, and also the antioxidant activity of IrO2 is much higher against A2780 ovarian cancer cells when compared with IrO1. Most notably IrO2 was particularly potent towards PC3 human prostate cancer cells (IC50 = 0.53 μM), being ca. 8× more active than the clinical drug cisplatin, and ca. 15× more selective towards cancer cells versus normal cells. Confocal microscopy showed that both IrO1 and IrO2 localise in the mitochondria of cancer cells.