Fast photodriven electron spin coherence transfer: a quantum gate based on a spin exchange J-jump.

Photoexcitation of the electron donor (D) within a linear, covalent donor-acceptor-acceptor molecule (D-A(1)-A(2)) in which A(1) = A(2) results in sub-nanosecond formation of a spin-coherent singlet radical ion pair state, (1)(D(+•)-A(1)(-•)-A(2)), for which the spin-spin exchange interaction is large: 2J = 79 ± 1 mT. Subsequent laser excitation of A(1)(-•) during the lifetime of (1)(D(+•)-A(1)(-•)-A(2)) rapidly produces (1)(D(+•)-A(1)-A(2)(-•)), which abruptly decreases 2J 3600-fold. Subsequent coherent spin evolution mixes (1)(D(+•)-A(1)-A(2)(-•)) with (3)(D(+•)-A(1)-A(2)(-•)), resulting in mixed states which display transient spin-polarized EPR transitions characteristic of a spin-correlated radical ion pair. These photodriven J-jump experiments show that it is possible to use fast laser pulses to transfer electron spin coherence between organic radical ion pairs and observe the results using an essentially background-free time-resolved EPR experiment.