title: Resonance energy transfer: when a dipole fails.
abstract: The Coulombic coupling of electric dipole (E1) transition moments is the most commonly studied and widely operative mechanism for energy migration in multichromophore systems. However a significant number of exceptions exist, in which donor decay and/or acceptor excitation processes are E1-forbidden. The alternative transfer mechanisms that can apply in such cases include roles for higher multipole transitions, exciton- or phonon-assisted interactions, and non-Coulombic interactions based on electron exchange. A quantum electrodynamical formulation provides a rigorous basis to assess the first of these, specifically addressing the relative significance of higher multipole contributions to the process of energy transfer in donor-acceptor systems where electric dipole transitions are precluded by symmetry. Working within the near-zone limit, where donor-acceptor separations are small in comparison to the chromophore scale, the analysis highlights the contributions of both electric quadrupole-electric quadrupole (E2-E2) coupling and the seldom considered second-order electric dipole-electric dipole (E1(2)-E1(2)) coupling. For both forms of interaction, experimentally meaningful rate equations are secured by the use of orientational averaging, and the mechanisms are analyzed with reference to systems in which E1-forbidden transitions are commonly reported.
authors.affiliation: Nanostructures and Photomolecular Systems, School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, United Kingdom. d.l.andrews@uea.ac.uk
authors.forename: David L
authors.initials: DL
authors.lastname: Andrews
authors.forename: Jamie M
authors.initials: JM
authors.lastname: Leeder