ABSTRACT:Two europium complexes with bis(bipyridine) azamacrocyclic ligands featuring pendant arms with or without π-conjugated donor groups are synthesized and fully characterized by theoretical calculations and NMR spectroscopy. Their photophysical properties, including two-photon absorption, are investigated in water and in various organic solvents. The nonfunctionalized ligand gives highly water-stable europium complexes featuring bright luminescence properties but poor two-photon absorption cross sections. On the other hand, the europium complex with an extended conjugated antenna ligand presents a two-photon absorption cross section of 45 GM at 720 nm but is poorly luminescent in water. A detailed solvent-dependent photophysical study indicates that this luminescence quenching is not due to the direct coordination of OÀH vibrators to the metal center but to the increase of nonradiative processes in a protic solvent induced by an internal isomerization equilibrium.
' INTRODUCTIONThe sensitization of europium or terbium luminescence by nonlinear two-photon excitation is recently becoming a promising strategy for the design of a new generation of bioprobes for multiphoton microscopy imaging techniques. 1,2 These new probes should combine the intrinsic advantages of two-photon excitation, 3 such as 3D resolution, low-energy excitation localized in the biological window, reduced photodamage, and photobleaching, with those of lanthanide luminescence, namely, sharp emission bands with large apparent Stokes' shifts, long excited-state lifetimes, and sensitivity to the local environment. 1b,4 The proof-of-concept of the biphotonic sensitization of lanthanide luminescence by organic ligands was established in the early 2000s by Lakowicz and co-workers. 5 The two-photon absorption efficiency estimated by the measure of the two-photon cross section (σ 2 ) was then improved using ligands featuring charge-transfer transitions like Michler's ketone, 6 2-(diethylanilin-4-yl)-4,6-bis(3,5-dimethylpyrazolyl)triazine, 7 or functionalized pyridinedicarboxamide. 8 Significant σ 2 values (between 100 and 300 GM) were obtained, but the poor solubility and stability of these complexes in water preclude any practical applications as bioprobes. These limitations were partially overcome by the design of tris(dipicolinate) lanthanide complexes [Ln(DPA) 3 ] 3À (DPA = pyridinedicarboxylate), where the DPA ligand was functionalized by π-conjugated donor moieties decorated with hydrosolubilizing poly(ethylene glycol) (PEG) end groups (Chart 1), leading to complexes soluble enough in water so that the first two-photon scanning microscopy experiment on fixed human cancer cells could be successfully performed. 2b Moreover, fine tuning of the π-conjugated antenna allowed us to reach very important two-photon absorption cross sections, up to 775 GM. 9 However, the stability of this class of complexes in water remains moderate, and the complexes tend to dissociate in dilute solutions (30% of dissociation in the 10 À4 À10 À5
