Resonance energy transfer: when a dipole fails

Area of Science:

• Quantum mechanics
• Spectroscopy
• Materials science

Background:

• Coulombic coupling of electric dipole (E1) transition moments is the primary mechanism for energy migration in multichromophore systems.
• Exceptions arise when donor decay or acceptor excitation processes are E1-forbidden.
• Alternative mechanisms include higher multipole transitions, exciton/phonon-assisted interactions, and electron exchange.

Purpose of the Study:

• To rigorously assess higher multipole contributions to energy transfer in systems with E1-forbidden transitions.
• To analyze the significance of electric quadrupole-electric quadrupole (E2-E2) and second-order electric dipole-electric dipole (E1(2)-E1(2)) couplings.
• To provide a quantum electrodynamical framework for understanding these alternative energy transfer pathways.

Main Methods:

• Quantum electrodynamical formulation applied to donor-acceptor systems.
• Analysis within the near-zone limit (donor-acceptor separations small relative to chromophore scale).
• Development of experimentally meaningful rate equations using orientational averaging.

Main Results:

• Identified electric quadrupole-electric quadrupole (E2-E2) coupling as a significant mechanism for E1-forbidden energy transfer.
• Highlighted the role of second-order electric dipole-electric dipole (E1(2)-E1(2)) coupling.
• Demonstrated the applicability of these mechanisms in systems where E1 transitions are precluded by symmetry.

Conclusions:

• Higher multipole transitions, particularly E2-E2 and E1(2)-E1(2) couplings, are crucial for energy migration when E1 transitions are forbidden.
• The quantum electrodynamical approach provides a robust framework for studying these non-standard energy transfer mechanisms.
• This research offers insights into energy transfer dynamics in complex molecular systems with symmetry-imposed selection rules.