Vibrational damping effects on electronic energy relaxation in molecular aggregates

TL;DR

This paper extends the theoretical modeling of molecular vibrational effects on electronic energy relaxation by including vibrational-phonon couplings, revealing how damping influences energy transfer rates in pigment-protein systems.

Contribution

It introduces a model that incorporates vibrational-phonon interactions, demonstrating their impact on vibrational mode quenching and energy relaxation pathways in molecular aggregates.

Findings

Intermode coupling quenches vibrational modes.

Vibrational-phonon interactions redistribute spectral density.

Damping accelerates electronic excited state relaxation.

Abstract

Representation of molecular vibrational degrees of freedom by independent harmonic oscillators, when describing electronic spectra or electronic excitation energy transport, raises unfavourable effects as vibrational energy relaxation becomes inaccessible. A standard theoretical description is extended in this paper by including both electronic-phonon and vibrational-phonon couplings. Using this approach we have simulated a model pigment-protein system and have shown that intermode coupling leads to the quenching of pigment vibrational modes, and to the redistribution of fluctuation spectral density with respect to the electronic excitations. Moreover, new energy relaxation pathways, opened by the vibrational-phonon interaction, allow to reach the electronic excited state equilibrium quicker in the naturally occurring water soluble chlorophyll binding protein (WSCP) aggregate, demonstrating the significance that the damping of molecular vibrations has for the intrarmolecular energy relaxation process rate.