Functional luminescent lanthanide complexes: modulation of visible luminescence from europium complexes

The syntheses of three new ligands (L1–3), which are based upon a DO3A core and appended with additional receptor sites for metal cations, are described, together with their corresponding Eu(III) complexes (Eu–L1–3). The complexes are visibly luminescent in aqueous solution, following sensitization via the pyridine chromophore, showing characteristic narrow line-like emission from Eu(III). The luminescence properties show that water is effectively excluded from the inner coordination sphere of europium (q = 0). Each of the complexes showed perturbed luminescence properties upon addition of a variety of d-block metal ions. For example, emission quenching was observed for each complex following addition of Cr(III) and Cu(II). Selectivity towards Hg(II) (over Cd(II), Cu(II) and Zn(II)) was demonstrated with Eu– L3, which possesses a receptor site incorporating a softer thiophene moiety. More specifically, Hg(II) binding resulted in changes in the form of the steady state emission spectrum, together with a corresponding reduction of the luminescence lifetime in water, which can be attributed to an increase in inner sphere hydration (q = 2) and thus enhanced non-radiative deactivation of the 5D0 state by proximate O–H oscillators