Sensitized luminescence of Eu and Tb macrocyclic complexes bearing benzophenone antennae

Sensitized luminescence behavior of lanthanide (Ln=Eu 3+ , Tb 3+ ) macrocyclic cyclen (1,4,7,10-tetraazacyclododecane) complexes bearing one or four benzophenone (BP) moieties as antenna (Ln L1 and Ln L4 ) has been studied in water. Despite higher molar extinction coefficient of Eu L4 owing to four antennae, it shows only one-thirtieth the luminescence intensity of Eu L1 . Energy level of triplet excited–state of BP antenna ( E T ) is only a few kJ mol −1 higher than that of 5 D 2 excited–state of Eu 3+ , thus promoting a back energy transfer (BET) from 5 D 2 of Eu 3+ to ground-state BP antennae. On Eu L4 bearing four antennae, BET occurs more rapidly than that on Eu L1 , thus exhibiting much weaker luminescence. For Tb complexes, the energy gap between E T of BP antenna and 5 D 4 excited state of Tb 3+ is large enough (>13 kJ mol −1 ), such that practically no BET occurs. The luminescence intensity of Tb L4 is, however, lower (two-third) than that of Tb L1 . Time-resolved luminescence measurement reveals that hydration number of Tb 3+ within Tb L4 is twice that within Tb L1 . This is because the structural distortion of ligands on Tb L4 , caused by an intramolecular dipole–dipole interaction among the BP antennae, allows coordination of higher number of H 2 O molecules to Tb 3+ , thus leading to a strong Tb luminescence quenching via O–H oscillators.