Singlet Fission and Aromaticity

Qualitative aromaticity-based arguments are often used to explain singlet fission (SF) properties in polycyclic conjugated systems. It has been shown by Fowler and collaborators that magnetically induced ring currents are associated with transitions between occupied and unoccupied molecular orbitals (MOs). Since SF has to do with relative energies of electronic states, it was hypothesized that induced currents may indicate SF properties. The quantitative aromaticity of linear oligoacene and several doubly boron-doped anthracenes and phenanthrenes, in their closed-shell singlet, open-shell singlet (where applicable), and triplet electronic states, has been studied using nucleus-independent chemical shift (NICS)-XY scan methods. It is shown that quantitative magnetic aromaticity can be used to identify SF compounds, at least for initial screening. Thus, the induced current features that are indicative of singlet fission (SF) ability have been found to be global current and local current at each ring of the systems. This conclusion was verified for very different systems─a tetracyclic, nitrogen-containing quinone (6), diphenyl benzofurane (7), and cibalackrot (8), all reported to be SF systems. Finally, it is predicted that some isomers of the doubly boron-doped linear[4]phenylene should show SF properties.