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

Polarisation Sensitive Single Molecule Fluorescence Detection with Linear Polarised Excitation Light and Modulated Polarisation Direction Applied to Multichromophoric Entities

Recently, investigations of the fluorescence properties of a multichromophoric dendritic entity at the single molecule level have revealed multiple fluorescence levels, collective off-states, variations of the polarisation, large shifts in the spectral position and changes in the fluorescence decay time. In order to further elucidate the multiple processes taking place in this entity, measurements were done in which the polarisation direction of the linear polarised excitation light was modulated. The detection was sensitive for the s- and p-components of the emitted light. The patterns of modulation and relative intensity in the acquired traces reflect the energy transfer processes occurring in this multichromophoric molecule. In-phase modulation and no modulation are the typical modulation patterns that were observed. Simulations involving several models for energy transfer between the chromophores have been carried out taking into account identical conditions as for the performed measurements. The comparison of the modulation patterns and polarisation histograms to the measured data rules out certain models and refines the photophysical model for the multichromophoric entity

Intramolecular evolution from a locally excited state to an excimer-like state in a multichromophoric dendrimer evidenced by a femtosecond fluorescence upconversion study

A time-resolved fluorescence upconversion study on a polyphenylene dendrimer with eight peryleneimide chromophores on the surface and on a monochromophoric model compound is reported. The time-dependent fluorescence spectra of the dendrimer show that the initial excitation is into a locally excited chromophore. They further indicate the existence of a decay channel that leads to excited state interaction between chromophores in one dendrimer which takes place on a 5 ps timescale

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