Novel Mechanism for Triplet State Formation in Short Distance Covalently Linked Radical Ion Pairs

We report on a series of electron donor−acceptor (D-A) dyads that undergo singlet-initiated charge separation to produce a strongly spin coupled radical ion pair that subsequently undergoes charge recombination to produce a triplet state with unusual spin polarization. The molecules consist of either a 4-(N-piperidinyl)naphthalene-1,8-imide (6P) or 4-(N-pyrrolidinyl)naphthalene-1,8-imide (5P) donor and a 1,8:4,5-naphthalenediimide (NI) or pyromellitimide (PI) acceptor. Selective photoexcitation of D within D-−A produces the radical ion pair 1[D•+-A•-] quantitatively. This is followed by the formation of 3[D•+-A•-] via singlet−triplet mixing within the radical pair. Radical pair intersystem crossing (RP-ISC) leads to charge recombination to yield [D-3*A] or [3*D-A]. Time-resolved optical absorption and emission spectroscopy is coupled with EPR spectroscopy to characterize the mechanism of the nearly quantitative initial charge separation reaction and the subsequent radical ion pair recombination reaction leading to the unusually spin polarized triplet state. These radical pairs also possess charge transfer emission bands that aid in the data analysis. The small number of previously reported covalent donor−acceptor systems that yield a triplet state from radical ion pair recombination use multistep charge separation reactions to achieve a ≥20 Å spacing between the oxidized donor and reduced acceptor. These examples have small exchange couplings, J, within the radical pair, so that S-T0 mixing between the radical pair energy levels occurs. In the strongly coupled systems described here, we show that the triplet states are formed by means of both S-T0 and S-T- 1 mixing, producing novel spin-polarized EPR spectra characterized by anisotropic spin lattice relaxation.