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We have synthesised ptⅣ-diazido complexes that are inert in the dark, but can be activated by selective irradiation with light to become potently cytotoxic towards a number of cancer cell lines.1,2,3 The complexes trans, trans, trans-[Pt(N3)2(OH)2(L)(L)] where L, L=imine/amine exhibit intense azide-to-PtⅣ ligand-to-metal charge-transfer (LMCT) absorption bands at ca.289 nm, with the absorption tails extending up to ca.500 nm.When irradiated with UVA (λmnax =365 nm) or blue light (λmax =420 nm, 450 nm), they undergo photo-decomposition, releasing the azido ligands, as monitored by UV-Vis and 14N-NMR spectroscopy.These complexes are non-toxic in the dark but exhibit potent photocytotoxicity towards HaCaT, A2780, A2780cis (A2780 cisplatin-resistant subline) and OE19 cell cultures when irradiated with UVA, as well as with blue light (λmax =420 nm).Importantly, these complexes show ca.5× lower cross-resistance to cisplatin towards A2780cis ovarian cancer cells than our previously reported1,2 complexes trans,trans,trans-[Pt(N3)2(OH)2(NH3)(py)] and trans,trans,trans-[Pt(N3)2(OH)2(py)2].The photoreactions of the new mixed ligand complexes with 5-GMP upon irradiation with blue light (420 or 450 nm) rapidly formed mono-5-GMP PtⅡ adducts [ptⅡ(N3)(L)(L)(5-GMP)]+, which slowly underwent further photoactivation to give bis-5-GMP PtⅡ adducts [PtⅡ(L)(L)(5-GMP)2]2+.Their cellular mechanism of action may in part involve the formation of DNA crosslinks on photoactivation (comet assay).The results indicate that these complexes are promising candidates for use in the cancer photochemotherapy of thin-walled organs.