Sensitized luminescence of Eu and Tb macrocyclic complexes bearing benzophenone antennae

Sensitized luminescence behavior of lanthanide (Ln=Eu3+, Tb3+) macrocyclic cyclen (1,4,7,10-tetraazacyclododecane) complexes bearing one or four benzophenone (BP) moieties as antenna (LnL1 and LnL4) has been studied in water. Despite higher molar extinction coefficient of EuL4 owing to four antennae, it shows only one-thirtieth the luminescence intensity of EuL1. Energy level of triplet excited-state of BP antenna (ET) is only a few kJ mol−1 higher than that of 5D2 excited-state of Eu3+, thus promoting a back energy transfer (BET) from 5D2 of Eu3+ to ground-state BP antennae. On EuL4 bearing four antennae, BET occurs more rapidly than that on EuL1, thus exhibiting much weaker luminescence. For Tb complexes, the energy gap between ET of BP antenna and 5D4 excited state of Tb3+ is large enough (>13 kJ mol−1), such that practically no BET occurs. The luminescence intensity of TbL4 is, however, lower (two-third) than that of TbL1. Time-resolved luminescence measurement reveals that hydration number of Tb3+ within TbL4 is twice that within TbL1. This is because the structural distortion of ligands on TbL4, caused by an intramolecular dipole-dipole interaction among the BP antennae, allows coordination of higher number of H2O molecules to Tb3+, thus leading to a strong Tb luminescence quenching via O-H oscillators.