Leap from Diradicals to Tetraradicals by Topological Control of π-Conjugation

In this work, we explore the series of diradical(oid)s based on 2,2'-(5,11-dihydroindolo[3,2-b]carbazole-3,9-diyl)dimalononitrile (further referred to as PH). Hydrogen atoms in the central benzenoid (CB) ring of PH are substituted by the series of substituents with various lengths of π-conjugated chain and electron-donating or electron-withdrawing properties to study how they modulate the diradical character of the parent compound. The diradical character of molecules increases up to 88-89% by two groups doubly bonded to both sides of the CB ring of PH in para relative positions. This breaks the direct π-conjugation between unpaired electrons that gives rise to two radical centers and restricts the minimal polyradical identity of the compound to diradical. We show that diradicals and tetraradicals can be designed, and their polyradical character can be modulated by controlling the topology of π-conjugation as long as there is sufficient aromatic stabilization. Henceforth, the bridge between diradicals and tetraradicals is established, leading to the tetraradical(oid) molecule, which has been predicted to have narrow low-spin to high-spin energy gaps in our recent Letter.