A ditopic chelating ligand (H64) that bears catechol and diethylenetriamine-N,N,N′,N′′,N′′-pentaacetate (DTPA) has been designed and shown to specifically bind lanthanide(III) ions at the DTPA core ([Ln(H24)(H2O)]−) and further self-assemble with titanium(IV), thereby giving rise to the formation of a supramolecular metallostar complex with a lanthanide(III)-to-titanium(IV) ratio of 3:1, [(Ln4)3Ti(H2O)3]5− (Ln=La, Eu, Gd). The efficacy of the metallostar complex as a potential bimodal optical/magnetic resonance imaging (MRI) agent has been evaluated. Nuclear magnetic relaxation dispersion (NMRD) measurements for the [(Gd4)3Ti(H2O)3]5− complex have demonstrated an enhanced r1 relaxivity that corresponds to 36.9 s−1 mM−1 per metallostar molecule at 20 MHz and 310 K, which is a result of a decreased tumbling rate. The ability of the complex to bind to human serum albumin (HSA) was also examined by relaxometric measurements. In addition, upon UV irradiation the [(Gd4)3Ti(H2O)3]5− complex exhibits broad-band green emission in the range 400–750 nm with a maximum at 490 nm. Taking into account the high relaxivity and luminescence properties, the [(Gd4)3Ti(H2O)3]5− complex is a good lead compound for the development of efficient bimodal contrast agents.
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