An important goal for solar energy conversion is efficient light harvesting. For efficient quantum energy transport, it is critical to find the right balance between (a) inter-chromophore electronic coupling, resulting in coherences and (b) coupling to the environment causing decoherences, and (c) vibrations resonant to excitonic energy gaps. Theoretical calculations by Fleming and co-workers suggest that the resonant vibrations could lead to ‘ratcheted’ excitation energy transport by overcoming local energy traps.
In this work, our goal is to engineer a synthetic molecular system (D₁D₂A, as shown in Figure) that facilitates uphill energy flow from donor D₁ to D₂, followed by unidirectional irreversible electron transfer to an acceptor (A) spatially distant from primary donor D₁, exploiting vibronic coherence to access the D₁D₂⁺A⁻ charge-separated state, which is otherwise inaccessible from the D₁ singlet excited state (D₁*). We construct such a vibronic quantum ‘ratcheted’ molecular system, D₁D₂A and demonstrate an experimental characterization of ‘ratcheting’ process by fluorescence, transient infrared (IR) and two-dimensional electronic-vibrational (2DEV) spectroscopies supported by quantum mechanical calculations. The transient IR spectra after selective excitation of D₁ in D₁D₂A show the appearance (<200 fs) of a pair of photo-induced absorption bands at 1630 and 1596 cm^-1, which are well-known characteristic IR bands of NDI^- radical anion, provides the direct evidence of quantum ratcheted electron transfer. Furthermore, the 2DEV spectra illustrate that excitation of higher energy vibronic bands of D₁ results in higher yield of NDI^- radical anion, indicating an involvement of a vibronic coupling between D₁ and D₂ that mediates the uphill energy transfer followed by a rapid electron transfer from D₂ to A.

Schematic representation of vibronic quantum ratcheted electron transfer.

Sutirtha Chowdhury

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