Abstract
We investigate how collective behaviors of vibrations such as cooperativity and interference can enhance energy transfer in a nontrivial way, focusing on an example of a donor–bridge–acceptor trimeric chromophore system coupled to two vibrational degrees of freedom. Employing parameters selected to provide an overall uphill energy transfer from donor to acceptor, we use numerical calculations of dynamics in a coupled exciton–vibration basis, together with perturbation-based analytics and calculation of vibronic spectra, to identify clear spectral features of single- and multi-phonon vibrationally-assisted energy transfer (VAET) dynamics, where the latter include up to six-phonon contributions. We identify signatures of vibrational cooperation and interference that provide enhancement of energy transfer relative to that obtained from VAET with a single vibrational mode. We observe a phononic analogue of two-photon absorption, as well as a novel heteroexcitation mechanism in which a single phonon gives rise to simultaneous excitation of both the trimeric system and the vibrational degrees of freedom. The impacts of vibrations and of the one- and two-phonon VAET processes on the energy transfer are seen to be quite different in the weak and strong site–vibration coupling regimes. In the weak coupling regime, two-phonon processes dominate, whereas in the strong coupling regime up to six-phonon VAET processes can be induced. The VAET features are seen to be enhanced with increasing temperature and site–vibration coupling strength, and are reduced in the presence of dissipation. We analyze the dependence of these phenomena on the explicit form of the chromophore–vibration couplings, with comparison of VAET spectra for local and non-local couplings.
Highlights
Recent experimental and theoretical studies of the molecular structures present in biological light harvesting complexes have revealed the delicate interplay of electronic and vibrational degrees of freedom, and how these come together to orchestrate efficient transfer of photoexcitations in such systems [1–8]
We investigate how collective behaviors of vibrations such as cooperativity and interference can enhance energy transfer in a nontrivial way, focusing on an example of a donor-bridge-acceptor trimeric chromophore system coupled to two vibrational degrees of freedom
We focus first on the behavior when the vibrations are coupled locally to individual chromophore sites, as in the case of trapped ions coupled to transverse modes.The model trimeric system of primary interest in this work is generalized from a dimeric system studied previously with an experimental ion trap emulator [20] and starts from a Hamiltonian in which individual vibrational modes are coupled to Frenkel excitons on specific sites
Summary
Recent experimental and theoretical studies of the molecular structures present in biological light harvesting complexes have revealed the delicate interplay of electronic and vibrational degrees of freedom, and how these come together to orchestrate efficient transfer of photoexcitations in such systems [1–8]. Coherent beating patterns in nonlinear spectroscopy signals initially ascribed to long-lived electronic coherence in such systems [9] are generally agreed to be a due to combination of electronic and vibrational coherence, with a key role played by coupling of the relevant electronic degrees of freedom to long-lived, underdamped vibrational modes of molecules [10–14] This revelation has brought to light the subtle ways in which vibrational dynamics in molecular complexes can influence electronic and excitonic properties [15–19]. It is of interest to explore whether cooperative or interference effects might play a role in the vibrationally enhanced energy transfer These systems offer the possibility of finding both the phononic analog of the well-known two-photon absorption [23–25], and the inverse phenomenon [26], in which one phonon might simultaneously excite two excitonic transitions, where the latter could be of different frequencies. VIII provides a summary and outlook for observation of the predicted VAET phenomena in trapped ion experiments, together with a discussion of the implications of this VAET study for understanding excitonic energy transfer in molecular systems
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