Multiphoton-Excited Luminescent Lanthanide Bioprobes: Two- and Three-Photon Cross Sections of Dipicolinate Derivatives and Binuclear Helicates

Multiphoton excited luminescent properties or 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 D-5(0) and D-5(4) excited levels of Eu-III and Tb-III, respectively, upon approximate to 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 IPA 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 similar to 2 times decrease is observed. The feasibility of [Eu-2(L-C5)(3)] its multiphoton luminescence bioprobe is demonstrated by two-photon scanning microscopy imaging experiments on HeLa cells incubated with this bimetallic helicate.