Time resolved studies of dual emission and photoinduced energy transfer in a Tris methoxy coumarin derivative of a cryptand and its complex with Tb(NO3)3

The paper reports time resolved emission studies in different solvents of the dual emission observed in the macrotricyclic cryptand (L) where the three secondary amino nitrogen have been derivatized with methoxy coumarin at room temperature and at 77 K. The emission from the 'locally excited monomer state' has a lifetime less than 1 ns while the other emitting state is an exciplex state with a lifetime of 4-5 ns depending on the solvent. The lifetime is found to increase significantly in the presence of protons and at 77 K exhibiting photoinduced electron transfer (PET) in the system L. The system exhibits photoinduced energy transfer (ET) in its Tb(III) complex using NO3- ion as counteranion at room temperature as well as at 77 K. The rate constants for energy transfer from coumarin moiety to Tb(III) have been evaluated at room temperature and at 77 K following the decay of 5D4 → 7F5 emission of Tb(III). The results indicate that energy transfer takes place from the lowest triplet state of coumarin moiety to Tb(III) by exchange mechanism. The energy transfer (ET) rate constants at room temperature and at 77 K have been evaluated and interpreted using the geometry of L obtained by theoretical calculation.