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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry