Effect of geometrical parameters of dyad D-A and triad D-A1-A2 on the efficiency of ultrafast intramolecular charge separation from the second excited state

- Photoinduced charge separation in D-A dyads and D-A1-A2 triads.
- Methods of suppression for ultrafast charge recombination.
- How to maximize photoinduced intramolecular charge separation yield.

Ultrafast intramolecular charge transfer induced by the Soret-band excitation of molecular dyads, donor-acceptor, and triads, donor-acceptor1-acceptor2, has been explored within the stochastic point-transition model. The influence of the effective radii of the donor, acceptors, distances between them, and the angle between the directions of two sequential electron transfers on the quantum yield of thermalized charge separated states has been studied. A bell-shaped dependence of the quantum yield of the charge separated state on the center-to-center distance between the donor and acceptor is revealed for dyads. This dependence is a result of competition between two processes suppressing formation of the charge separated state. At short distances the electronic coupling is strong and the non-equilibrium charge recombination into the first excited state is very effective. At large distances the charge separation is slow and can not compete with internal conversion. This is the reason why the quantum yield is expected to be small in both limits. In triads the non-equilibrium electron transfer from the primary to the secondary acceptor can effectively compete with ultrafast back electron transfer into the first excited state of the donor and considerably increase the quantum yield of thermalized charge separated state. The strategy of maximizing the quantum yield is discussed. The regions of spatial parameters of triads and dyads maximizing the quantum yield of the charge separated states have been estimated for typical charge transfer parameters.

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