Excited-State Dynamics of Crossing-Controlled Energy Transfer in Europium Complexes.

Photosensitized energy transfer (EnT) phenomena occur frequently in a variety of photophysical and photochemical processes and have traditionally been treated with the donor-acceptor distance-dependent Förster and Dexter models. However, incorrect arguments and formulae were employed by ignoring energy resonance conditions and the selection rules of the state-to-state transition in special cases, especially for the sensitive intramolecular EnT of lanthanide complexes. Herein, we proposed an innovative model of energy-degeneracy-crossing-controlled EnT, which can be experimentally confirmed by time-resolved two-dimensional photoluminescence measurements. The computationally determined energy resonance region provides the most effective channel to achieve metal-to-ligand EnT beyond the distance-dependent model and sensitively bifurcates into symmetry-allowed or -forbidden channels for some representative europium antenna complexes. The outcomes of the multidisciplinary treatment contribute to a complementary EnT model that can be tuned by introducing a phosphorescence modulator and altering the antenna-related parameters of the ligand-centered energy level of the 3 ππ* state and its spin-orbit coupling for the 3 ππ* → S 0 * transition through mechanism-guided crystal engineering and should motivate further development of mechanistic models and applications.