Photoinduced energy transfer from the triplet state of naphthalene moiety in the Tb3+/Eu3+ complexes of a specially designed naphthalene cryptand

The paper reports time-resolved emission and energy transfer (ET) studies of metal ion complexes of a specially designed rigid macrocyclic naphthalene cryptand (L) under different conditions. Complex formation of L with Li+ and H+ causes an appreciable increase in singlet state quantum yield and lifetime of L implying photoinduced electron transfer (PET) from the cryptand moiety to naphthalene unit in the free L. The system exhibits photoinduced ET at 77 K in its Tb3+ and Eu3+ complexes with either NO3−1 or Cl−1 as counter-anion. The extent of ET is higher for the Tb3+ complex as compared to that for the Eu3+ complex. In both Tb3+ and Eu3+ complex, the NO3−1 ions influence the relative orientation of donor (L) and acceptor (Ln3+) more in favour of ET than the Cl−1 ions. The rate constants for the ET from the naphthalene moiety of L to the acceptor (Ln3+) have been evaluated at 77 K. The results suggest ET from the triplet state of naphthalene using an exchange mechanism. The ground state geometries of the system L and its complexes with Li+, Cs+ and Tb3+ have been determined using DFT methods to interpret our results.