Abstract
We introduce a heterodimer model in which multiple mechanisms of vibronic coupling and their impact on energy transfer can be explicitly studied. We consider vibronic coupling that arises through either Franck-Condon activity in which each site in the heterodimer has a local electron-phonon coupling or Herzberg-Teller activity in which the transition dipole moment coupling the sites has an explicit vibrational mode-dependence. We have computed two-dimensional electronic-vibrational (2DEV) spectra for this model while varying the magnitude of these two effects and find that 2DEV spectra contain static and dynamic signatures of both types of vibronic coupling. Franck-Condon activity emerges through a change in the observed excitonic structure, while Herzberg-Teller activity is evident in the appearance of significant side-band transitions that mimic the lower-energy excitonic structure. A comparison of quantum beating patterns obtained from analysis of the simulated 2DEV spectra shows that this technique can report on the mechanism of energy transfer, elucidating a means of experimentally determining the role of specific vibronic coupling mechanisms in such processes.
Highlights
Elucidating the mechanisms of quantum mechanical energy transfer has fundamental implications for the way we understand natural light-harvesting and develop artificial analogs.1 Previous experimental studies on natural systems2–4 have been unable, to clearly establish the mechanism of energy transfer that leads to quantum efficiencies approaching unity5 and have launched long-standing debates obfuscating the role of observed electronically and/or vibrationally coherent phenomena in the transfer process.6–23 It has been postulated that these coherent processes may not serve any purpose in the overall energy transfer mechanism.24,25 This ambiguity largely surrounds the lack of consistent treatment of electronic–vibrational coupling in energy transfer models, which we address through a simplified heterodimer model in this paper
By utilizing a model system, we are able to isolate the role that different vibronic coupling mechanisms have on the structure of the excitonic states that are electronically excited in typical experiments and show how that structure is identifiable in 2DEV spectroscopy both statically and dynamically
We have introduced a minimal model for a vibronically coupled heterodimer for which two distinct mechanisms of vibronic coupling can be systematically tuned. This model adequately describes the coupling of a low-frequency nuclear mode to site-exciton states in a multichromophoric system and introduces a set of local high-frequency modes to report on the vibronic coupling in 2DEV spectroscopy
Summary
Subsequent 2DEV measurements have shown evidence of vibronic mixing and its facilitation of ultrafast energy transfer in LHCII.18 In the latter, the 2DEV spectra showed rich vibrational structure corresponding to the dominant electronic excitations which exhibited oscillatory dynamics reminiscent of non-Condon effects found in previous transient absorption measurements.. By utilizing a model system, we are able to isolate the role that different vibronic coupling mechanisms have on the structure of the excitonic states that are electronically excited in typical experiments and show how that structure is identifiable in 2DEV spectroscopy both statically and dynamically We further compare these signatures to the diabatic population dynamics, which demonstrates the ability to directly link the mechanism of energy transfer with spectral observables and connects model systems to potential ab initio simulations for which only simple observables such as the populations are available.
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