Spin Multiplicity Effects on the Second Hyperpolarizability of an Open-Shell Neutral π-Conjugated System

The spin multiplicity effects on the second hyperpolarizability (γ) are investigated for a small-size open-shell neutral conjugated model, the C 5 H 7 radical, in the doublet, quartet, and sextet states by using several ab initio molecular orbital and density functional theory methods. The spatial contributions of total, α- and β-electrons to γ are examined to characterize the spin polarization and electron correlation effects on γ. It turns out that the second hyperpolarizability increases with the spin multiplicity, suggesting the interest of designing spin-enhanced nonlinear optical (NLO) systems based on open-shell neutral organic compounds, which also present the possibility of spin control of the NLO properties.