ANTENNA EFFECT IN LUMINESCENT LANTHANIDE CRYPTATES: A PHOTOPHYSICAL STUDY

Abstract

Abstract— Excited state emission and absorption decay measurements have been made on the cage‐type cryptate complexes [M bpy.bpy.bpy] n+ , where M n+ = Na + , La 3+ , Eu 3+ , Gd 3+ or Tb 3 + and [bpy.bpy.bpy] is a tris‐bipyridine macrobicyclic cryptand. Excitation has been performed in the high intensity 1 π‐π* cryptand band with maximum at about 300 nm. Experiments have been carried out in H 2 O or D 2 O solutions and at 300 and 77 K to evaluate the rate constants of radiative and nonradiative decay processes. For M n+ = Na + , La 3+ and Gd 3+ the lowest excited state of the cryptate is a 3 ππ* level of the cryptand which decays in the microsecond time scale at room temperature in H 2 O solution and in the second‐millisecond time scale at 77 K in MeOH‐EtOH. For M n+ = Eu 3+ , the lowest excited state is the luminescent 5 D 0 Eu 3+ level which in H 2 O solution is populated with 10% efficiency and decays to the ground state with rate constants 2.9 × 10 3 s _1 at room temperature and 1.2 × 10 3 s − ′ at 77 K. The relatively low efficiency of 5 D 0 population upon 1 ππ* excitation is attributed to the presence of a ligand‐to‐metal charge transfer level through which 1 ππ* decays directly to the ground state. For M n+ = Tb 3+ the lowest excited state is the luminescent 5 D 4 Tb 3+ level. The process of 5 D 4 population upon 1 ππ* excitation is ˜100% efficient, but at room temperature it is followed by a high‐efficiency, activated back energy transfer from the 5 D 4 Tb 3+ level to the 3 ππ* ligand level because of the relatively small energy gap between the two levels (1200 cm _1 ) and the intrinsically long lifetime of 5 D 4 . At 77 K back energy transfer cannot take place and the 5 D 4 Tb 3* level deactivates to the ground state with rate constant 5.9 × 10 2 s ‐ ′ (H 2 O solution). The relevance of these results toward the optimization of Eu 3+ and Tb 3+ cryptates as luminescent probes is discussed.