Resonance energy transfer: The unified theory revisited

Resonanceenergy transfer (RET) is the principal mechanism for the intermolecular or intramolecular redistribution of electronic energy following molecular excitation. In terms of fundamental quantum interactions, the process is properly described in terms of a virtual photon transit between the pre-excited donor and a lower energy (usually ground-state) acceptor. The detailed quantum amplitude for RET is calculated by molecular quantum electrodynamical techniques with the observable, the transfer rate, derived via application of the Fermi golden rule. In the treatment reported here, recently devised state-sequence techniques and a novel calculational protocol is applied to RET and shown to circumvent problems associated with the usual method. The second-rank tensor describing virtual photon behavior evolves from a Green’s function solution to the Helmholtz equation, and special functions are employed to realize the coupling tensor. The method is used to derive a new result for energy transfer systems sensitive to both magnetic- and electric-dipole transitions. The ensuing result is compared to that of pure electric-dipole–electric-dipole coupling and is analyzed with regard to acceptable transfer separations. Systems are proposed where the electric-dipole–magnetic-dipole term is the leading contribution to the overall rate

Conformational rearrangements in and twisting of a single molecule

Single molecule spectroscopy is used to obtain detailed information on the photophysical properties of immobilized perylenediimide-based molecules, substituted in the bay positions. The fluorescence spectra recorded for numerous single molecules show a clear bimodal distribution of the peak position. Within the low energy component of the distribution, two different vibronic shapes of the emission spectrum can be seen, which can be correlated to different decay times. We show that former observation can be explained by conformational changes of the bay substituents while the latter are related to twisting of the single molecule around the central perylenediimide long axis