Triplet recombination of radical ion pairs: CIDNP effects and DFT calculations on 1,2-dicyanoethylene

Radical ion pairs generated by electron transfer from photo-excited aromatic hydrocarbons to maleo- and fumaronitrile (cis- and trans-1,2-dicyanoethylene, 1) undergo back electron transfer from singlet and triplet pairs. The pair energies relative to the reactant ground states and to the triplet state, respectively, determine the competition between the recombination pathways. Cross sections through the potential surfaces of the radical anion and the triplet state of 1 have been examined by density functional theory calculations. The radical anion surface has minima in which the carbon and nitrogen skeleton is essentially planar; the central C-C bond is lengthened (weakened), but the barrier to geometric isomerization is still sizeable (31.2 kcal mol(-1)). The triplet energy surface has a minimum at a bisected geometry (plus its enantiomer); rotation of the H-C-CN segments of the triplet yields a syn-periplanar and an anti-periplanar point 9.3 kcal mol(-1) and 8.2 kcal mol(-1) above the minimum, respectively.

A metallofullerene electron donor that powers an efficient spin flip in a linear electron donor-acceptor conjugate

The dream target of artificial photosynthesis is the realization of long-lived radical ion pair states that power catalytic centers and, consequently, the production of solar fuels. Notably, magnetic field effects, especially internal magnetic field effects, are rarely employed in this context. Here, we report on a linear Lu3N@Ih-C80-PDI electron donor-acceptor conjugate, in which the presence of the Lu3N cluster exerts an appreciable electron nuclear hyperfine coupling on the charge transfer dynamics. As such, a fairly efficient radical ion pair intersystem crossing converts the initially formed singlet radical ion pair state, (1)[(Lu3N@Ih-C80)(•+)-PDI(•-)], to the corresponding triplet radical ion pair state, (3)[(Lu3N@Ih-C80)(•+)-PDI(•-)]. Most notably, the radical ion pair state lifetime of the latter is nearly 1000 times longer than that of the former.