Homo-FRET in π-Conjugated Polygons: Intermediate-Strength Dipole-Dipole Coupling Makes Energy Transfer Reversible

The concept of homo-FRET is often used to describe energy transfer between like chromophores of molecular aggregates such as in π-conjugated polymers. Homo-FRET is revealed by a dynamic depolarization in fluorescence but strictly only applies to the limit of weak dipole-dipole coupling, where energy transfer occurs on time scales much longer than those of nuclear relaxation. By considering the polarization anisotropy of photoluminescence emission and excitation of model multichromophoric aggregates on the single-molecule level, we demonstrate the transition of energy-transfer dynamics from the case of weak coupling to that of strong coupling, revealing the elusive regime of intermediate-strength coupling where energy transfer between degenerate donor and acceptor chromophores becomes reversible so that information on the excitation route of the emitting chromophore is lost.