Manipulating photogenerated radical ion pair lifetimes in wirelike molecules using microwave pulses: Molecular spintronic gates

We have studied spin-dependent charge transfer dynamics in wirelike donor-bridge-acceptor (D-B-A) molecules comprising a phenothiazine (PTZ) donor, an oligo(2,7-fluorene) (FL_n) bridge, and a perylene-3,4:9,10- bis(dicarboximide) (PDI) acceptor, PTZ-FL₃-PDI (1) and PTZ-FL ₄-PDI (2), dissolved in the magnetic field-aligned nematic phase of 4-cyano-4′-n-pentylbiphenyl (5CB) at 295 K. Time-resolved EPR spectroscopy using both continuous wave and pulsed microwaves shows that the photogenerated radical pairs (RPs), PTZ^+•-FL₃-PDI^-• and PTZ^+•-FL₄-PDI^-•, recombine much faster from the singlet RP manifold than the triplet RP manifold. When a strong resonant microwave π pulse is applied following RP photogeneration in 1 and 2, the RP lifetimes increase about 50-fold as indicated by electron spin-echo detection. This result shows that the RP lifetime can be greatly extended by rapidly switching off fast triplet RP recombination.