Third‐Order Nonlinear Optical Properties of One‐Dimensional Quinoidal Oligothiophene Derivatives Involving Phenoxyl Groups

The diradical characters (y) and third‐order nonlinear optical (NLO) properties of open‐shell quinoidal oligothiophene derivatives with phenoxyl groups, and the corresponding reduced (hydrogenated)‐state oligomers, are investigated by using the broken‐symmetry density functional theory method. The oxidized (dehydrogenated) states are predicted to have an open‐shell singlet ground state and their y values increase with the number of units. Static second hyperpolarizabilities (γ) of the open‐shell oligomers with intermediate y are shown to be enhanced significantly compared with those of the closed‐shell analogues. Furthermore, owing to the effective diradical distances, the γ values of open‐shell oligomers are found to exceed that of s‐indaceno[1,2,3‐cd;5,6,7‐c′d′]diphenalene, which is known as an organic molecule with the largest two‐photon absorption cross‐section in this size of the pure hydrocarbons. This feature extends the range of efficient open‐shell third‐order NLO materials to a novel class of one‐dimensional conjugated oligomers with redox‐based high tunability of third‐order NLO properties.