Open-Shell Character Dependences of the Second Hyperpolarizability in Two-Dimensional Tetraradicaloids

The open-shell character dependences of the second hyperpolarizability, gamma, are investigated for rectangular-shaped tetraradicaloid models, such as diradical dimers, using numerically exact solutions of the extended Hubbard model. The newly defined local diradical characters for intra- and intermolecular interactions (referred to as y(intra) and y(inter); respectively) are compared to conventional global ones (y(0) and y(1)) and provide a comprehensive understanding of the electronic structure of the system. The system shows two kinds of enhancements of the gamma components, gamma(intra) and gamma(intra) (caused, respectively, by y(intra)- and intermolecular diradical interactions): (i) a system with a large y(intra) (Y-inter) (>similar to 0.4) exhibits the enhancement of a single component of gamma(inter) (gamma(intra)), at the intermediate (inter) (gamma(inter) region (similar to 0.3-0.4), and (ii) in contrast to conventional diradical systems, the system exhibits a further enhancement of both components of gamma (gamma(intra) and gamma(inter)) at the region where y(intra) similar to y(inter) with small values (<=similar to 0.3). The obtained relationships are verified by using ab initio quantum chemical calculations of the realistic tertraradical models of a 4,4'-bis(1,2,3,5-dithiazdiazolyl) (BDTDA) dimer and a disilene dimer. The present results are expected to pioneer an alternative class of two-dimensional multiradical NLO systems, which potentially cause further enhancement of gamma as compared with conventional intermediate diradical NLO systems.