Effect of radical groups on the magnetic properties of [7]helicene derivatives: a DFT computational study

Bi- and polyradical organic compounds are of significant interest due to their outstanding magnetic properties. A special attention is paid to the investigation of open-shell helicenes combining in one molecule chirality and paramagnetic centers. With the aim to elucidate the effect of insertion of radical groups on the magnetic behavior of [7]helicene, a series of helicene-based molecules functionalized with 1,2,3,5-dithiadiazolyl, 1,5-dimethyl-6-oxoverdazyl (verdazyl), 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl-3-oxide (nitronyl nitroxide), fluorenyl and tert-butyl nitroxide radicals has been investigated by a comprehensive quantum-chemical modeling of their electronic structure and magnetic properties. The choice of a suitable density functional theory approximation has been made, and its reasonability check was proved by multi-configurational CASSCF calculations. It was shown that fluorenyl- and tert-butyl nitroxide-bearing compounds were stabilized in the closed-shell states, while dithiadiazolyl, verdazyl, and nitronyl nitroxide derivatives were represented by tetraradicals. For the tetraradical molecules, different in nature exchange interactions have been predicted, an important role in the formation of which is played by an extended π-conjugated system of [7]helicene moiety and stacking between terminal six-membered cycles. Considering the presence of two types of paramagnetic centers and unusual optical properties of the helicene skeleton, the proposed systems can be used as promising building blocks for the molecular photonics and electronics devices.