Theoretical Study on the Electronic Structure and Third-Order Nonlinear Optical Properties of Open-Shell Quinoidal Oligothiophenes

The chain-length dependences of geometric and electronic structures and third-order nonlinear optical (NLO) properties of open-shell quinoidal oligothiophenes (QTs) are investigated from the viewpoint of diradical character (gamma) using the broken-symmetry density functional theory method. The results are compared with those of closed-shell oligothiophenes. The bond-length alternation and the nucleus-independent chemical shift values are evaluated to clarify the chain-length dependence of aromaticities of the terminal and middle thiophene rings in the oligomer chain, which is closely related to the variation in the diradical character through the quinoidal/aromatic resonance structure. The y values of these QTs ranging from monomer to hexamer are found to have a wide variety of y values [0 (dosed-shell state) <= y <= 1 (pure diradical state)]. The gamma values per unit of the open-shell QTs having intermediate gamma are found to be significantly enhanced as compared to those of the dosed-shell counterparts. The feature of y dependence of gamma in open-shell QTs is found to be in good agreement with so-called "y-gamma correlation", which has been proposed on the basis of the simple two-site diradical model using the valence configuration interaction method. These results reveal the relationships among the terminal substituent group, molecular structure and size, and diradical character, which contribute to building design guidelines for highly efficient third-order NLO materials based on the open-shell one-dimensional oligomers.