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

## TL;DR

This paper studies how energy moves quickly between molecules in a light-harvesting system, revealing how coherent and kinetic processes interact.

## Contribution

The study introduces a new method to analyze energy transfer in a non-Förster regime using multiconfigurational wavepacket dynamics.

## Key findings

- An almost fully decoherent state is reached at around 75 fs, followed by purity restoration during energy transfer.
- Ultrafast energy transfer is mediated by vibronic resonance effects, causing mode-selective vibrational excitation.
- A slower kinetic process shows significant temperature dependence.

## Abstract

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.