In this work, we discuss the scope of vibrational modes as “quantum ratchets”. The vibrational modes/ratchets serve as effective non-thermal routes that enable excitation transport in light-harvesting devices where energy transport is otherwise energetically unfavorable. While we restrict our discussion to model D1-D2-A systems, the principles can easily be extended to formulate the design principles of multi-level synthetic devices for efficient energy-harvesting.

In our studies, the influence of a vibrational mode is quantified through its (a) Huang-Rhys mixing, and (b) the resonance/non-resonance of the frequency Ω of the vibrational mode with the energy gap between the pair of electronic excitons (│ε₁-ε₂│) formed by the D1-D2chromophores. We demonstrate that the system needs to achieve an optimum balance between the D1-D2electronic coupling (J), the Huang Rhys mixing and resonance of the vibrational mode to maximize transport to the acceptor. While a large electronic coupling results in delocalized excitations which reduces unidirectional transport, a resonant vibrational mode provides a means to circumvent this. We find this is an effective strategy for cases where │E_D₁-E_D2│>> J. On the other hand, a “traditionally” resonant vibrational mode is ineffective for strongly delocalized excitations where │E_D₁-E_D2│≈ J. For such cases, we propose a special resonance regime, the “local-excitonic” (LE) resonance, that accomplishes transport.

The excitation decay from D2to A is considered unidirectional and is seen to have a profound effect on the ratcheting dynamics, with a stronger decay rate resulting in faster ratcheting. We also propose future directions where we aim to explore non-Condon coupling such as in Hertzberg-Teller active vibrational modes and their ability to influence transport at shorter timescales.

Direct excitation transport from D1 to A is energetically unfavorable. We, therefore, place an intermediate molecule D2 and use a vibrational mode as a ratchet to achieve nonthermal transport from D1 to D2, followed by a unidirectional decay to A.

Pallavi Bhattacharyya

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