Second hyperpolarizabilities of singlet diradical compounds and their radical ions

We investigate the second hyperpolarizabilities (γ) of singlet diradical s-indacene and its radical anion. It is found that the neutral s-indacene, which has an intermediate diradical character, exibits a positive γ value, while its radical anion does a negative γ value. Moreover, the magnitude of the γ value of s-indacene radical anion turns out to be significantly larger than that of neutral s-indacene. We also examine several singlet diradical 1,3-dipoles and their radical cations. On the basis of these results, we discuss the charging effects on γ in singlet diradical systems.