Photoinduced energy transfer from the triplet state of naphthalene moiety in the Tb 3+/Eu 3+ complexes of a specially designed naphthalene cryptand

Abstract

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 Tb 3+ and Eu 3+ complexes with either NO 3 −1 or Cl −1 as counter-anion. The extent of ET is higher for the Tb 3+ complex as compared to that for the Eu 3+ complex. In both Tb 3+ and Eu 3+ complex, the NO 3 −1 ions influence the relative orientation of donor (L) and acceptor (Ln 3+) 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 (Ln 3+) 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 Tb 3+ have been determined using DFT methods to interpret our results.