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 (NN^.) stable radical to a D-B-A system having well-defined distances between the components: MeOAn-6ANl-Ph(NN^.)-Nl, where MeOAn = p-methoxyaniline, 6ANl = 4-(N-piperidinyl)-naphthalene-1,8-dicarboximide, Ph = phenyl, and Nl = naphthalene-1,8:4,5-bis(dicarboximide). MeOAn-6ANl, NN ^., and Nl are attached to the 1, 3, and 5 positions of the Ph bridge. Using both time-resolved optical and EPR spectroscopy, we show that NN ^. influences the spin dynamics of the photogenerated triradical states ^2,4(MeOAn^+.-6ANl-Ph(NN^.)-Nl ^-.), resulting in slower charge recombination within the triradical compared to the corresponding biradical lacking NN^.. The observed spin-spin exchange interaction between the photogenerated radicals MeOAn ^+. and Nl^-. is not altered by the presence of NN ^., which only accelerates radical pair intersystem crossing. Charge recombination within the triradical results in the formation of ^2,4(MeOAn-6ANl-Ph(NN^.)-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 NN^. with the same time constant as describes the photogenerated radical ion pair decay. This effect is attributed to antiferromagnetic coupling between NN^. 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 NN^. and ³*Nl present.