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–acceptor₁–acceptor₂ (D–A₁–A₂) molecule, in which D = perylene and both A₁ and A₂ = naphthalene-1,8:4,5-bis(dicarboximide), results in sub-nanosecond formation of a spin-correlated singlet radical pair ¹(D⁺˙–A₁⁻˙–A₂) having a large electron spin–spin exchange interaction, 2J, which precludes its observation by transient EPR spectroscopy. Subsequent selective photoexcitation of A₁⁻˙ rapidly produces ¹(D⁺˙–A₁–A₂⁻˙), resulting in a dramatic decrease in 2J, which allows coherent spin evolution to mix the singlet (S) radical pair state ¹(D⁺˙–A₁–A₂⁻˙) with the T₀ triplet sublevel of ³(D⁺˙–A₁–A₂⁻˙) in an applied magnetic field, where B ≫ 2J. A spin-polarized transient EPR spectrum characteristic of the spin-correlated radical pair D⁺˙–A₁–A₂⁻˙ is then observed. The time delay between the two laser pulses was incremented to measure the rate of decoherence in ¹(D⁺˙–A₁⁻˙–A₂) in toluene at 295 K, which was found to be 8.1 × 10⁷ s⁻¹. Deuteration of the perylene donor or the toluene solvent decreases the decoherence rate constant of ¹(D⁺˙–A₁⁻˙–A₂) to 4.3 × 10⁷ s⁻¹ and 4.6 × 10⁷ 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 × 10⁷ s⁻¹. The data show that decreasing electron-nuclear hyperfine interactions significantly increases the zero quantum coherence lifetime of the spin-correlated radical pair.