Multiphoton-excited luminescent lanthanide bioprobes: two- and three-photon cross sections of dipicolinate derivatives and binuclear helicates.

Multiphoton excited luminescent properties of water-soluble Eu(III) and Tb(III) complexes with derivatives of dipicolinic acid functionalized with a polyoxyethylene pendant arm and terminal groups, [Eu(L(OMe))(3)](3-), [Eu(L(NH2))(3)](3-), and [Tb(L(OH))(3)](3-), as well as of binuclear helicates with overall composition [Ln(2)(L(CX))(3)] (X = 2, 5) are investigated. Characteristic emission from the (5)D(0) and (5)D(4) excited levels of Eu(III) and Tb(III), respectively, upon approximately 800 nm excitation results from three-photon absorption (3PA) for [Eu(L(OMe))(3)](3-), [Eu(L(NH2))(3)](3-), [Tb(L(OH))(3)](3-), and [Ln(2)(L(C2))(3)], while luminescence from [Eu(2)(L(C5))(3)] is induced by two-photon absorption (2PA) owing to its 1PA spectrum extending further into the visible. The 3PA cross sections have been determined and are the first ones reported for lanthanide complexes: (i) those of Eu(III) and Tb(III) bimetallic helicates [Ln(2)(L(C2))(3)] are 20 times larger compared to the corresponding values for tris(dipicolinates); (ii) derivatization of dipicolinic acid for Tb(III) complexes has almost no influence on the 3PA cross section; however, for Eu(III) complexes a approximately 2 times decrease is observed. The feasibility of [Eu(2)(L(C5))(3)] as multiphoton luminescence bioprobe is demonstrated by two-photon scanning microscopy imaging experiments on HeLa cells incubated with this bimetallic helicate.