Fast photo-driven electron spin coherence transfer: the effect of electron-nuclear hyperfine coupling on coherence dephasing

Selective photoexcitation of the donor in an electron donor-acceptor(1)-acceptor(2) (D-A(1)-A(2)) molecule, in which D = perylene and both A(1) and A(2) = naphthalene-1,8: 4,5-bis(dicarboximide), results in sub-nanosecond formation of a spin-correlated singlet radical pair (1)(D+center dot-A(1)(-center dot)-A(2)) having a large electron spin-spin exchange interaction, 2J, which precludes its observation by transient EPR spectroscopy. Subsequent selective photoexcitation of A(1)(-center dot) rapidly produces (1)(D+center dot-A(1)-A(2)(-center dot)), resulting in a dramatic decrease in 2J, which allows coherent spin evolution to mix the singlet (S) radical pair state (1)(D+center dot-A(1)-A(2)(-center dot)) with the T-0 triplet sublevel of (3)(D+center dot-A(1)-A(2)(-center dot)) in an applied magnetic field, where B >> 2J. A spin-polarized transient EPR spectrum characteristic of the spin-correlated radical pair D+center dot-A(1)-A(2)(-center dot) is then observed. The time delay between the two laser pulses was incremented to measure the rate of decoherence in (1)(D+center dot-A(1)(-center dot)-A(2)) in toluene at 295 K, which was found to be 8.1 x 10(7) s(-1). Deuteration of the perylene donor or the toluene solvent decreases the decoherence rate constant of (1)(D+center dot-A(1)(-center dot)-A(2)) to 4.3 x 10(7) s(-1) and 4.6 x 10(7) s(-1), respectively, while deuteration of both the perylene donor and the toluene solvent reduced the decoherence rate constant by more than half to 3.4 x 10(7) s(-1). The data show that decreasing electron-nuclear hyperfine interactions significantly increases the zero quantum coherence lifetime of the spin-correlated radical pair.