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 s inglet-initiated charge separation to produce a strongly spin coupled radic al ion pair that subsequently undergoes charge recombination to produce a t riplet state with unusual spin polarization. The molecules consist of eithe r a 4-(N-piperidinyl)naphthalene-1,8-imide (6P) or 4-(N-pyrrolidinyl)-napht halene-1,8-imide (5P) donor and a 1,8:4,5-naphthalenediimide (NI) or pyrome llitimide (PI) acceptor. Selective photoexcitation of D within D--A produce s 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 recombina tion 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 an d the subsequent radical ion pair recombination reaction leading to the unu sually spin polarized triplet state. These radical pairs also possess charg e transfer emission bands that aid in the data analysis. The small number o f previously reported covalent donor-acceptor systems that yield a triplet state from radical ion pair recombination use multistep charge separation r eactions to achieve a greater than or equal to 20 Angstrom spacing between the oxidized donor and reduced acceptor. These examples have small exchange couplings, J, within the radical pair, so that S-To mixing between the rad ical pair energy levels occurs. In the strongly coupled systems described h ere, we show that the triplet states are formed by means of both S-T-0 and S-T-1 mixing, producing novel spin-polarized EPR spectra characterized by a nisotropic spin lattice relaxation.