| Abstract
| - A t-butylphenylnitroxide (BPNO•) stable radical is attached to an electron donor−bridge−acceptor (D−B−A) system having well-defined distances between the components: MeOAn−6ANI−Ph(BPNO•)−NI, whereMeOAn = p-methoxyaniline, 6ANI = 4-(N-piperidinyl)naphthalene-1,8-dicarboximide, Ph = phenyl, andNI = naphthalene-1,8:4,5-bis(dicarboximide). MeOAn−6ANI, BPNO•, and NI are attached to the 1, 3, and5 positions of the Ph bridge, respectively. Time-resolved optical and EPR spectroscopy show that BPNO•influences the spin dynamics of the photogenerated triradical states 2,4(MeOAn+•−6ANI−Ph(BPNO•)−NI-•),resulting in slower charge recombination within the triradical, as compared to the corresponding biradicallacking BPNO•. The observed spin−spin exchange interaction between the photogenerated radicals MeOAn+•and NI-• is not altered by the presence of BPNO•. However, the increased spin density on the bridge greatlyincreases radical pair (RP) intersystem crossing from the photogenerated singlet RP to the triplet RP. Rapidformation of the triplet RP makes it possible to observe a biexponential decay of the total RP population withcomponents of τ = 740 ps (0.75) and 104 ns (0.25). Kinetic modeling shows that the faster decay rate is dueto rapid establishment of an equilibrium between the triplet RP and the neutral triplet state resulting fromcharge recombination, whereas the slower rate monitors recombination of the singlet RP to ground state.
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