Elucidation of near-resonance vibronic coherence lifetimes by nonadiabatic electronic-vibrational state character mixing

Abstract

Recent work suggests that the long-lived coherences observed in both natural and artificial light-harvesting systems (such as the Fenna-Matthews-Olson complex) could be attributed to the mixing of the pigments' electronic and vibrational degrees of freedom. To investigate the underlying mechanism of these long coherence lifetimes, a sophisticated description of interactions between the molecular aggregates and the nonequilibrium fluctuations in the surrounding environment is necessary. This is done by implementing the hierarchical equations of motion approach on model homodimers, a method used in the intermediate coupling regime for many molecular aggregates wherein the nonequilibrium environment phonons play nontrivial roles in exciton dynamics. Here we report a character change in the vibronic states-reflective of property mixing between the electronic and vibrational states-induced by an interplay between system coupling parameters within the exciton-vibrational near-resonance regime. This mixing dictates vital aspects of coherence lifetime; by tracking the degree of mixing, we are able to elucidate the relationship between coherence lifetime and both the electronic energy fluctuation and the vibrational relaxation dephasing pathways.

Does electronic coherence enhance anticorrelated pigment vibrations under realistic conditions?

“…Indeed, for weak electronic dephasing, we find an increase of the anticorrelated vibrational amplitude which confirms the picture of Refs. 32,35 . However, for the more realistic case of stronger electronic dephasing, the amplitude of the anticorrelated vibration depends only weakly on the vibronic coupling, since the coherent electron-vibrational mixing is dephased very rapidly and does not influence the vibration at later times.…”

“…This second concept does not involve long-lived electronic coherence and is conceptually in agreement with the observation that a strong vibronic coupling produces large-amplitude coherent oscillations of the electronic component with a usual short lifetime and a long-lived vibrational coherence, but with a rather small amplitude 33,34 . This scenario was reexamined again recently 35 with an explicit coupling to an electronic and a vibrational bath. The result that an increased vibronic coupling survives weak electronic dephasing at short times and induces a resonantly enhanced long-lived vibrational coherence of the anticorrelated mode was presented as an explanation of the long-lived coherence signals observed in Ref.…”

“…Most importantly, although the frequencies indeed mix and additional peaks are generated, the line widths of the peaks at 200 cm −1 , 300 cm −1 , 500 cm −1 , and 500 cm −1 are 35 cm −1 , 40 cm −1 , 20 cm −1 , and 45 cm −1 , and are thus all comparable. This proves that the lifetime of the electronic coherence is not affected by vibronic coupling to a vibrational mode.Impact of coherent vibronic coupling on anticorrelated vibrations Finally, we address the possibility that a strong coherent vibronic coupling could enhance the amplitude of the anticorrelated component of the vibrational dynamics32,35 . The latter is given by the magnitude of the vibrational peak in the Fourier spectrum of the wave-packet dynamics, as, e.g., shown in Figs.…”

“…,35 , but does not play a role in realistic physical systems, the reason being that the required coherent vibronic mixing is rapidly destroyed by fast electronic dephasing.42. Gelin MF, Egorova D, Domcke W (2005) Efficient method for the calculation of time-andfrequency-resolved four-wave mixing signals and its application to photon-echo spectroscopy.…”

Two-Dimensional Electronic–Vibrational Spectroscopy of Coupled Molecular Complexes: A Near-Analytical Approach

“…17 Another rationale proposes that the long oscillation lifetimes arise due to Vibrationally Assisted Electronic Energy Transfer (VA-EET). [18][19][20][21][22][23][24][25][26][27][28] Here, the nuclear degrees of freedom are hypothesized to contribute a discrete, sharp vibrational/nuclear mode that is resonant/quasi-resonant with a pair of electronic excitons.…”