Intramolecular vibrations enhance the\xa0quantum efficiency of excitonic energy transfer

Hong-Guang Duan,Michael Thorwart,Peter Nalbach,R. J. Dwayne Miller

doi:10.1007/s11120-020-00742-x

Abstract

We study the impact of underdamped intramolecular vibrational modes on the efficiency of the excitation energy transfer in a dimer in which each state is coupled to its own underdamped vibrational mode and, in addition, to a continuous background of environmental modes. For this, we use the numerically exact hierarchy equation of motion approach. We determine the quantum yield and the transfer time in dependence of the vibronic coupling strength, and in dependence of the damping of the incoherent background. Moreover, we tune the vibrational frequencies out of resonance with the excitonic energy gap. We show that the quantum yield is enhanced by up to 10% when the vibrational frequency of the donor is larger than at the acceptor. The vibronic energy eigenstates of the acceptor acquire then an increased density of states, which leads to a higher occupation probability of the acceptor in thermal equilibrium. We can conclude that an underdamped vibrational mode which is weakly coupled to the dimer fuels a faster transfer of excitation energy, illustrating that long-lived vibrations can, in principle, enhance energy transfer, without involving long-lived electronic coherence.

Highlights

The first steps of photosynthesis involve a quantum mechanical transfer of excitation energy between different spatial regions of a molecular complex
An interesting question how electronic and vibrational coherence collude and whether vibrational coherence is in principle able to influence the efficiency of the transfer of electronic excitation energy, i.e., the quantum yield or the transfer times, and whether the interplay is significant for biological functionality
Since electronic dephasing is much faster than the exciton transfer time, dynamic electronic coherence is considered irrelevant for the transfer and only vibrational coherence is a candidate for influencing the transfer efficiency which occurs on time scales similar to the life time of vibrational coherence of a few picoseconds

Summary

Method

We calculate the population dynamics of the dimer model by using the hierarchy equation of motion (HEOM) for the Hamiltonian of Eq (4). Smallamplitude but long-lived oscillations are present in the occupations of both the donor and the acceptor up to ~ 1000 fs They reflect weak vibrational coherence which survives longer on the picosecond time scale. D of the intramolecular vibrational mode at the donor (at a fixed acceptor frequency) increases the final occupation of the acceptor because it is unfavorable to excite vibrational donor states in comparison to decay downhill to the acceptor state This observation is in agreement with the analysis in the section “Mapping to a dissipative dimer-plus-oscillator system” and with Fig. 1, where the energy level scheme is shown. Resonances between vibrational modes and excitonic transition energies are not relevant here

Compliance with ethical standards

Findings

Conclusion