title: Self-aggregated dinuclear lanthanide(III) complexes as potential bimodal probes for magnetic resonance and optical imaging.
abstract: Homodinuclear lanthanide complexes (Ln = La, Eu, Gd, Tb, Yb and Lu) derived from a bis-macrocyclic ligand featuring two 2,2',2''-(1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid chelating sites linked by a 2,6-bis(pyrazol-1-yl)pyridine spacer (H2L(3)) were prepared and characterized. Luminescence lifetime measurements recorded on solutions of the Eu(III) and Tb(III) complexes indicate the presence of one inner-sphere water molecule coordinated to each metal ion in these complexes. The overall luminescence quantum yields were determined (ϕ H2O = 0.01 for [Eu2(L(3))] and 0.50 for [Tb2(L(3))] in 0.01 M TRIS/HCl, pH 7.4; TRIS = tris(hydroxymethyl)aminomethane), pointing to an effective sensitization of the metal ion by the bispyrazolylpyridyl unit of the ligand, especially with Tb. The nuclear magnetic relaxation dispersion (NMRD) profiles recorded for [Gd2(L(3))] are characteristic of slowly tumbling systems, showing a low-field plateau and a broad maximum around 30 MHz. This suggests the occurrence of aggregation of the complexes giving rise to slowly rotating species. A similar behavior is observed for the analogous Gd(III) complex containing a 4,4'-dimethyl-2,2'-bipyridyl spacer ([Gd2(L(1))]). The relaxivity of [Gd2(L(3))] recorded at 0.5 T and 298 K (pH 6.9) amounts to 13.7 mM(-1)  s(-1). The formation of aggregates has been confirmed by dynamic light scattering (DLS) experiments, which provided mean particle sizes of 114 and 38 nm for [Gd2(L(1))] and [Gd2(L(3))], respectively. TEM images of [Gd2(L(3))] indicate the formation of nearly spherical nanosized aggregates with a mean diameter of about 41 nm, together with some nonspherical particles with larger size.
authors.affiliation: Departamento de Química Fundamental, Universidade da Coruña, Campus da Zapateira, Rúa da Fraga 10, 15008A Coruña, Spain.
authors.forename: Martín
authors.initials: M
authors.lastname: Regueiro-Figueroa
authors.forename: Aline
authors.initials: A
authors.lastname: Nonat
authors.forename: Gabriele A
authors.initials: GA
authors.lastname: Rolla
authors.forename: David
authors.initials: D
authors.lastname: Esteban-Gómez
authors.forename: Andrés
authors.initials: A
authors.lastname: de Blas
authors.forename: Teresa
authors.initials: T
authors.lastname: Rodríguez-Blas
authors.forename: Loïc J
authors.initials: LJ
authors.lastname: Charbonnière
authors.forename: Mauro
authors.initials: M
authors.lastname: Botta
authors.forename: Carlos
authors.initials: C
authors.lastname: Platas-Iglesias
keywords: europium; gadolinium; lanthanides; luminescence; magnetic resonance imaging