Electron donor-bridge-acceptor molecules with bridging nitronyl nitroxide radicals: Influence of a third spin on charge- and spin-transfer dynamics

Appending a stable radical to the bridge molecule in a donor-bridge-acceptor system (D-B-A) is potentially an important way to control charge- and spin-transfer dynamics through D-B-A. We have attached a nitronyl nitroxide (NNcenter dot) stable radical to a D-B-A system having well-defined distances between the components: MeOAn-6ANl-Ph(NNcenter dot)-Nl, where MeOAn = p-methoxyaniline, 6ANl = 4-(N-piperidinyl)naphthalene-1,8-dicarboximide, Ph = phenyl, and NI = naphthalene-1,8:4,5-bis(dicarboximide). MeOAn-6ANI, NNcenter dot, and NI are attached to the 1, 3, and 5 positions of the Ph bridge. Using both time-resolved optical and EPR spectroscopy, we show that NNcenter dot influences the spin dynamics of the photogenerated triradical states (2.4)(MeOAn+(center dot)6ANI-Ph(NNcenter dot)-NI-center dot) resulting in slower charge recombination within the trilradical compared to the corresponding biradical lacking NNcenter dot. The observed spin-spin exchange interaction between the photogenerated radicals MeOAn(+center dot) and Nl(-center dot) is not altered by the presence of NNcenter dot, which only accelerates radical pair intersystem crossing. Charge recombination within the triradical results in the formation of (2.4)(MeOAn(+center dot)-6ANl-Ph(NNcenter dot)(-3*)Nl), in which NN, is strongly spin-polarized. Normally, the spin dynamics of correlated radical pairs do not produce a net spin polarization; however, net spin polarization appears on NNcenter dot with the same time constant as describes the photogenerated radical ion pair decay. This effect is attributed to antiferromagnetic coupling between NNcenter dot and the local triplet state 3(*)Nl, which is populated following charge recombination. This requires an effective switch in the spin basis set between the triradical and the three-spin charge recombination product having both NNcenter dot and 3(*)Nl present.