Remarkable Tuning of the Photophysical Properties of Bifunctional Lanthanide tris(Dipicolinates) and its Consequence on the Design of Bioprobes

Derivatives of dipicolinic acid with a polyoxyethylene pendant arm at the pyridine 4-position have been functionalized for potential grafting with biological material. Four ligands with different terminal functions (alcohol, methoxy, phtalimide and amine) have been synthesized, which react with trivalent lanthanide ions $Ln^{III}$ to yield triple helical $[Ln(L)_3]^{3-}$ complexes, as shown by NMR and UV-vis titrations. The tris chelates display large thermodynamic stability with $log \beta_{13} \approx 19-20$ for all $Eu^{III}$ complexes for instance. Photophysical measurements reveal adequate sensitization of the metal-centered luminescence in the europium ($\eta_{sens} = 33-72\%$) and terbium complexes, which is modulated by the nature of the terminal function. The lifetimes of the metal-centered excited states are long, up to 1.4 ms for $[Eu(L)_3]^{3-}$ and 1.6 ms for $[Tb(L)_3]^{3-}$ at room temperature, in line with hydration numbers essentially equal to zero. Quantum yields are as high as 29% for the $[Eu(L^{NH2})_3]^{3-}$ and 18% for the $[Tb(L^{OH})_3]^{3-}$ tris chelates in water at physiological pH. These series of complexes demonstrate the extent of fine-tuning achievable for lanthanide luminescent probes and are simple models for investigating the effect of binding to biological molecules on the metal-centered luminescent properties.