Spin dynamics of photogenerated triradicals in fixed distance electron donor - Chromophore - Acceptor - TEMPO molecules

The stable free radical 2,2,6,6-tetramethylpiperidinoxyl (TEMPO, T ^.) was covalently attached to the electron acceptor in a donor-chromophore-acceptor (D-C-A) system, MeOAn-6ANI-Ph _n-A-T ^., having well-defined distances between each component, where MeOAn = p-methoxyaniline, 6ANI = 4-(N-piperidinyl)-naphthalene-1,8-dicarboximide, Ph = 2,5-dimethylphenyl (n = 0,1), and A = naphthalene-1,8:4,5-bis(dicarboximide) (NI) or pyromellitimide (PI). Using both time-resolved optical and EPR spectroscopy, we show that T ^. influences the spin dynamics of the photogenerated triradical states ^2,4(MeOAn ^+.-6ANI-Ph _n- A- ^.T ^.), resulting in modulation of the charge recombination rate within the triradical compared with the corresponding biradical lacking T ^.. The observed spin-spin exchange interaction between the photogenerated radicals MeOAn ^+. and A ^-. is not altered by the presence of T ^., which interacts most strongly with A ^-. and accelerates radical pair intersystem crossing. Charge recombination within the triradicals results in the formation of ^2,4(MeOAn-6ANI-Ph _n- ³*NI-T ^.) or ^2,4(MeOAn- ³*6ANI-Ph _n-PI-T ^.) in which T ^. is strongly spin polarized in emission. Normally, the spin dynamics of correlated radical pairs do not produce a net spin polarization; however, the rate at which the net spin polarization appears on T ^. closely follows the photogenerated radical ion pair decay rate. This effect is attributed to antiferromagnetic coupling between T ^. and the local triplet state ³*NI, which is populated following charge recombination. These results are explained using a switch in the spin basis set between the triradical and the three-spin charge recombination product having both T ^. and ³*NI or ³*6ANI present.