Open-Shell States in Dynamic Diradicaloids.

The open-shell organic and carbon-based systems, with either a doublet, triplet or higher spin-states, play a key role in contemporary research, opening potential applicability for several crucial fields. Among those derivatives, specific attention has been given to p-phenylene-based systems derived from the original Thiele hydrocarbon. These systems stabilize an open-shell diradicaloid resonance structure with a thermally accessible triplet state and are derived from a quinone-benzene (Clar's sextet) equilibrium. In our discussion, we very carefully choose examples which focus on fundamental derivatives that merge diatropic subunits, ready to stabilize two unpaired electrons via a dynamic modulation of geometry. This process provides an additional factor to the resonance energy of aromatics, mostly responsible for stabilization of two unpaired electrons.