The odd electron density is the guide toward achieving organic molecules with gigantic third-order nonlinear optical responses

Using density functional theory, the central role of the odd electron density for designing open-shell singlet compounds with enhanced second hyperpolarizabilities [third-order nonlinear optical (NLO) properties at the molecular level] is evidenced. Its amplitude, corresponding to the diradical character, gives the potential of enhancement, whereas its spatial distribution indicates where the donor and acceptor substituents should be placed to further increase this third-order NLO response. This is illustrated by comparing hexacenes, open-shell singlet graphene nanoflakes with intermediate diradical character, with reference closed-shell terrylenes. Enhancements up to 2 and 3 orders of magnitude are achieved when the OH/CN and NH/NO substituents are placed in the middle of the zigzag edge region, which corresponds to the largest odd electron density amplitude.