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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry