bS Supporting Information

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 OH vibrators to the metal center but to the increase of nonradiative processes in a protic solvent induced by an internal isomerization equilibrium. The sensitization of europium or terbium luminescence by nonlinear two-photon excitation is recently becoming a promis-ing strategy for the design of a new generation of bioprobes for multiphoton microscopy imaging techniques.1,2 These ne