Excitation Energy Transfer between Closely Spaced Multichromophoric Systems:  Effects of Band Mixing and Intraband Relaxation

Abstract

We theoretically analyze the excitation energy transfer between two closely spaced linear molecular J-aggregates, whose excited states are Frenkel excitons. The aggregate with the higher (lower) exciton band edge energy is considered as the donor (acceptor). The celebrated theory of Förster resonance energy transfer (FRET), which relates the transfer rate to the overlap integral of optical spectra, fails in this situation. We point out that in addition to the well-known fact that the pointdipole approximation breaks down (enabling energy transfer between optically forbidden states), also the perturbative treatment of the electronic interactions between donor and acceptor system, which underlies the Förster approach, in general loses its validity due to overlap of the exciton bands. We therefore propose a nonperturbative method, in which donor and acceptor bands are mixed and the energy transfer is described in terms of a phonon-assisted energy relaxation process between the two new (renormalized) bands. The validity of the conventional perturbative approach is investigated by comparing to the nonperturbative one; in general this validity improves for lower temperature and larger distances (weaker interactions) between the aggregates. We also demonstrate that the interference between intraband relaxation and energy transfer renders the proper definition of the transfer rate and its evaluation from experiment a complicated issue, which involves the initial excitation condition.

Anomalous reflection properties in high density limit fibril shaped PIC‐J aggregates in microcavity structure

“…Theoretically, the ET of this type is also indicated e.g. by Knoester et al [9]. Since the density of coherent J segments in our fibril-shaped PIC-J is critically high, there is a possibility that the ET rate becomes high enough to disturb the formation of polariton coherence.…”

Exciton dynamics in molecular aggregates

“…These intermediate states are not observed in the transient spectra because they are optically inactive. As was recently demonstrated for two closely spaced linear aggregates, the excitation ET may occur from or towards a dipole forbidden (optically dark) exciton state, implying that overlap of optical spectra is not a requirement for ET [20]. Since a double-wall tubular aggregate is qualitatively equivalent system to two closely spaced linear aggregates, similar ET pathways can be considered.…”

Two-Dimensional Spectroscopy of Extended Molecular Systems: Applications to Energy Transport and Relaxation in an α-Helix

“…Disentangling intraband dynamics from interband dynamics and defining an operational way to measure the interband energy transfer rate is a hard problem. 16 In this article we will show that using the combined information from several polarization dependent 2D spectra taken as a function of the waiting time, it is in fact possible to disentangle intraband and interband energy transport and relaxation, and to obtain the separate time scales for these types of dynamics from experiment. We will do this by simulating the coupled dynamics of the amide I and amide II vibrations in model R-helices dissolved in water (see Figure 1) and calculating the 2DIR spectra as a function of t 2 .…”