Multiphoton-excited luminescent lathanide bioprobes: two-and three-photon cross sections of dipicolinates derivatives and binuclear helicates
Multiphoton excited luminescent properties of water-soluble EuIII and TbIII complexes with derivatives of dipicolinic acid functionalized with a polyoxyethylene pendant arm and terminal groups, [Eu(LOMe)3]3−, [Eu(LNH2)3]3−, and [Tb(LOH)3]3−, as well as of binuclear helicates with overall composition [Ln2(LCX)3] (X = 2, 5) are investigated. Characteristic emission from the 5D0 and 5D4 excited levels of EuIII and TbIII, respectively, upon ≈800 nm excitation results from three-photon absorption (3PA) for [Eu(LOMe)3]3−, [Eu(LNH2)3]3−, [Tb(LOH)3]3−, and [Ln2(LC2)3], while luminescence from [Eu2(LC5)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 EuIII and TbIII bimetallic helicates [Ln2(LC2)3] are 20 times larger compared to the corresponding values for tris(dipicolinates); (ii) derivatization of dipicolinic acid for TbIII complexes has almost no influence on the 3PA cross section; however, for EuIII complexes a 2 times decrease is observed. The feasibility of [Eu2(LC5)3] as multiphoton luminescence bioprobe is demonstrated by two-photon scanning microscopy imaging experiments on HeLa cells incubated with this bimetallic helicate.