Theoretical Study on Third-Order Nonlinear Optical Properties for One-Hole-Doped Diradicaloids

[Image: see text] We investigate the relationships between open-shell character and longitudinal static second hyperpolarizabilities γ for one-hole-doped diradicaloids using the strong-correlated ab initio molecular orbital methods and simple one-dimensional (1D) three-site two-electron (3s-2e) models. As examples of one-hole-doped diradicaloids, we examine H(3)(+), methyl radical trimer cation ((CH(3))(3)(+)), silyl radical trimer cation ((SiH(3))(3)(+)), and 1,2,3,5-dithiadizolyl trimer cation (DTDA(3)(+)). For H(3)(+), the static γ exhibits negative values and shows a monotonic increase in amplitude with an increase in the open-shell character defined by a neighbor-site interaction (y(S)). On the other hand, it is found for (CH(3))(3)(+), (SiH(3))(3)(+), and DTDA(3)(+) that the static γ value exhibits similar behavior to that for H(3)(+) up to an intermediate y(S) value, while it takes the negative maximum at a large y(S) value, followed by a decrease in γ amplitude, and subsequently, γ changes to positive values with a drastic increase for larger y(S) values. For example, in DTDA(3)(+), the negative/positive γ values, −69 × 10(5)/700 × 10(5) au at y(S) = 0.75/0.87, exhibit significant enhancements in amplitude, 2.4/24 times as large as that (−29 × 10(5) au) at intermediate y(S) = 0.59 as is often the case in DTDA(2). Using the 1D 3s-2e valence-bond configuration interaction model, these sign inversions and drastic increase in the amplitude of γ are found to originate in the differences in Coulomb interactions between valence electrons, between valence and core electrons, and between valence electrons and nuclei. These results contribute to pave the way for the construction of novel control guidelines for the amplitude and sign of γ for one-hole-doped diradicaloids.