Factoring through-space and through-bond contributions to rates of photoinduced electron transfer in donor-spacer-acceptor molecules

Contributions from direct orbital overlap (through-space interactions) and superexchange (through-bond interactions) to the electronic coupling matrix elements for photoinduced charge separation and recombination in a series of linked donor-spacer-acceptor molecules were studied. The molecules consisted of a 4-piperidinyl-naphthalene-1,8-dicarboximide (ANI) electron donor and a N-(n-octyl)pyromellitimide (PI) electron acceptor attached to the 1,5- and 1,8-positions of either anthracene (ANC) or dibenzobicyclo(2.2.2)octatriene (DBO) spacers. For the 1,8-disubstituted compounds, ANI and PI are held approximately cofacial with a center-to-center distance of 5.3 Å, whereas for the 1,5-disubstituted compounds, the center-to-center distance increases to 13.5 Å. The through-bond interaction was investigated by replacing the ANC spacer with DBO. The charge separation and recombination reactions were examined in both toluene and tetrahydrofuran (THF). The results show that for the 1,8-disubstituted DBO system in both toluene and THF, charge separation and recombination is dominated by through-space interactions. For the 1,8-disubstituted ANC system in both toluene and THF, charge separation occurs by means of a direct, through-space interaction, while charge recombination occurs through the intermediacy of the ANI - ANC⁺ - PI^- ion pair in toluene and directly from ANI⁺ - ANC - PI^- to ground state in THF. For the 1,5-disubstituted molecules, possessing either the ANC or the DBO spacers, electron transfer from ^1*ANI to PI was not kinetically competitive with the decay of ^1*ANI to ground state in either toluene or THF.