Mapping the influence of molecular structure on rates of electron transfer using direct measurements of the electron spin-spin exchange interaction

The spin-spin exchange interaction, 2J, in a radical ion pair produced by a photoinduced electron transfer reaction can provide a direct measure of the electronic coupling matrix element, V, for the subsequent charge recombination reaction. We have developed a series of dyad and triad donor-acceptor molecules in which 2J is measured directly as a function of incremental changes in their structures. In the dyads the chromophoric electron donors 4-(N-pyrrolidinyl)- and 4-(N-piperidinyl)naphthalene-1,8-dicarboximide, 5ANI and 6ANI, respectively, and a naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor are linked to the meta positions of a phenyl spacer to yield 5ANI-Ph-NI and 6ANI-Ph-NI. In the triads the same structure is used, except that the piperidine in 6ANI is replaced by a piperazine in which a para-X-phenyl, where X = H, F, Cl, MeO, and Me₂N, is attached to the N' nitrogen to form a para-X-aniline (XAn) donor to give XAn-6ANI-Ph-NI. Photoexcitation yields the respective 5ANI⁺-Ph-NI^-, 6ANI⁺-Ph-NI^-, and XAn⁺-6ANI-Ph-NI^- singlet radical ion pair states, which undergo subsequent radical pair intersystem crossing followed by charge recombination to yield ³*NI. The radical ion pair distances within the dyads are about 11-12 Å, whereas those in the triads are about ∼16-19 Å. The degree of delocalization of charge (and spin) density onto the aniline, and therefore the average distance between the radical ion pairs, is modulated by the para substituent. The ³*NI yields monitored spectroscopically exhibit resonances as a function of magnetic field, which directly yield 2J for the radical ion pairs. A plot of In 2J versus r_DA, the distance between the centroids of the spin distributions of the two radicals that comprise the pair, yields a slope of -0.5 ± 0.1. Since both 2J and k_CR, the rate of radical ion pair recombination, are directly proportional to V², the observed distance dependence of 2J shows directly that the recombination rates in these molecules obey an exponential distance dependence with β = 0.5 ± 0.1 Å^-1. This technique is very sensitive to small changes in the electronic interaction between the two radicals and can be used to probe subtle structural differences between radical ion pairs produced from photoinduced electron transfer reactions.