Excitation Energy Transfer in an Intermediate Regime: A Multiconfigurational Gaussian Wavepacket Study of a Light-Harvesting Supramolecular Dyad.

Ultrafast excitation energy transfer (EET) is studied for a supramolecular rhodamine-BODIPY dyad, which exemplifies EET systems that fall into a non-Förster regime where coherent effects are important. A key question that arises concerns the transition between coherent and kinetic transfer regimes, which is addressed here based on real-time quantum dynamics and the time-evolving state-to-state flux that transitions from early time transients to a quasi-stationary regime. Multiconfigurational wavepacket calculations are carried out using the two-layer Gaussian-based multiconfiguration time-dependent Hartree (2L-GMCTDH) method, in conjunction with the thermofield dynamics method in order to include thermalization of low-frequency modes. Several characteristic time scales are identified that are intimately connected to the flux evolution and decoherence phenomena. An almost fully decoherent state is reached at around 75 fs, but the purity is restored to a large extent as the transfer to the acceptor state proceeds. It is found that the ultrafast EET step that is almost complete at around 200 fs is mediated by vibronic resonance effects, which lead to an athermal nonequilibrium state of the donor moiety, exhibiting mode-selective vibrational excitation following the EET transfer. A slower time scale associated with a kinetic regime shows a non-negligible temperature dependence.