Synthesis and Photophysical Properties of LnIII-DOTA-Bipy Complexes and LnIII-DOTA-Bipy-RuII Coordination Conjugates

The synthesis and the systematic and comparative photo-physical study of a series of visible (Eu-III, Tb-III) and NIR-emitting (Nd-III, Yb-III) lanthanide complexes (Ln(2)L) and ruthenium lanthanide coordination conjugates (Ln(2)LRu) are reported. The Gd-III complex, the Gd-III-Ru-II coordination conjugate, as well as the Ru-II complex of the ligand H6L have also been synthesized and photophysically studied as control systems. The ligand H6L, composed of a central bipyridine binding unit and functionalized on each 5'-position with a DOTA macrocycle, has been successfully synthesized from cyclen, 5,5'-dimethyl-2,2'-bipyridine and 1,2-ethylendiamine in a nine-step process. Detailed luminescence studies of all complexes, including the determination of the quantum yield and lifetime, were carried out on finely powdered microcrystalline samples as well as in water, deuterated water and [D-6]DMSO at ambient (295 K) and low temperature (77 K). The photophysical data corroborate the existence of energy transfer in the Ln(2)L. complexes and in the Nd2LRu coordination conjugate. However, no (or at most, very little) energy transfer is takes place from the Ru(bipy)(3) chromophore to the Ln(III) ion in the other Ln(2)LRu heteropolymetallic complexes. Moreover, the photophysical studies reveal that all the complexes and coordination conjugates adopt different conformations and hydration states in solution and in the solid state, which influences the efficiency of the energy transfer between the bipy and/or Ru(bipy)(3) antennae and the Ln(III) ions.