Heteronuclear d-d and d-f Ru(ii)/M complexes [M = Gd(iii), Yb(iii), Nd(iii), Zn(ii) or Mn(ii)] of ligands combining phenanthroline and aminocarboxylate binding sites: combined relaxivity, cell imaging and photophysical studies

A ligand skeleton combining a 1,10-phenanthroline (phen) binding site and one or two heptadentate N3O4 aminocarboxylate binding sites, connected via alkyne spacers to the phen C-3 or C-3/C-8 positions, has been used to prepare a range of heteronuclear RuM and RuM2 complexes which have been evaluated for their cell imaging, relaxivity, and photophysical properties. In all cases the phen unit is bound to a {Ru(bipy)(2)}(2+) unit to give a phosphorescent {Ru(bipy)(2)(phen)}(2+) luminophore, and the pendant aminocarboxylate sites are occupied by a secondary metal ion M which is either a lanthanide [Gd(iii), Nd(iii), Yb(iii)] or another d-block ion [Zn(ii), Mn(ii)]. When M = Gd(iii) or Mn(ii) these ions provide the complexes with a high relaxivity for water; in the case of RuGd and RuGd2 the combination of high water relaxivity and (MLCT)-M-3 phosphorescence from the Ru(ii) unit provides the possibility of two different types of imaging modality in a single molecular probe. In the case of RuMn and RuMn2 the Ru(ii)-based phosphorescence is substantially reduced compared to the control complexes RuZn and RuZn2 due to the quenching effect of the Mn(ii) centres. Ultrafast transient absorption spectroscopy studies on RuMn (and RuZn as a non-quenched control) reveal the occurrence of fast (<1 ns) PET in RuMn, from the Mn(ii) ion to the Ru(ii)-based (MLCT)-M-3 state, i.e. Mn-II-(phen(-))-Ru-III Mn-III-(phen(-))-Ru-II; the resulting Mn-III-(phen(-)) state decays with approximate to 5 ns and is non-luminescent. This occurs in conformers when an ET pathway is facilitated by a planar, conjugated bridging ligand conformation connecting the two units across the alkyne bridge but does not occur in conformers where the two units are electronically decoupled by a twisted conformation of the bridging ligand. Computational studies (DFT) on RuMn confirmed both the occurrence of the PET quenching pathway and its dependence on molecular conformation. In the complexes RuLn and RuLn(2) (Ln = Nd, Yb), sensitised near-infrared luminescence from Nd(iii) or Yb(iii) is observed following photoinduced energy-transfer from the Ru(ii) core, with Ru Nd energy-transfer being faster than Ru Yb energy-transfer due to the higher density of energy-accepting states on Nd(iii).