Photoactive Platinum(II) Azopyridine Complexes†

Platinum(II) complexes containing the strong pi-acceptor N,N-chelating ligand phenylazopyridine (Ph-azpy) [Pt(p-R-Ph-azpy)X-2], R = H, NMe2 or OH, X = Cl or N-3, have been synthesized and characterized to explore the effects of monodentate ligands and phenyl substituents on their absorption spectra and photoactivation. Time-dependent density functional theory calculations showed that the complexes have a low-lying unoccupied orbital with strong sigma-antibonding character toward the majority of the coordination bonds. The UV-visible absorption bands were assigned as mainly ligand-centered or metal-to-ligand charge-transfer transitions, with strong contributions from the chlorido and azido groups. In complexes with substituted Ph-azpy ligands, sigma-donation from NMe2 and OH/O- groups results in a redshift of the main absorption bands compared with unsubstituted Ph-azpy complexes. The diazido complexes are photoactive in solution upon irradiation with either UVA or visible light for R = H or NMe2, or UVA only when R = OH/O-. Intriguingly, the phenolate group of the latter complex undergoes very slow protonation in solution. Biological screening was limited by poor solubility; however, initial tests showed that the phenolato diazido complex is rapidly taken up into the nuclei of HaCaT keratinocytes, which are stained intensely blue, and its cytotoxicity is increased upon irradiation with UVA light.