In order to use the light efficiently both in engineering and science, it is necessary to design the optical functional materials interacting with light having desired properties and functionals. For this purpose, it is important to understand the mechanisms how the materials interact with light, such as absorption and/or emission of the light, and photochemical reactions. Combining these mechanisms provides a variety of optical functionals. In conventional designs for enhancing second hyperpolarizabilities, γ, one adopted an elongation of pi-conjugation, introducing donor-acceptor substituents on the both side of molecules, and so on. On the other hand, Open-shell singlet diradical molecules have been widely investigated because they are key to understanding the nature of chemical bonds. We have proposed a design principle for γ, by referring a diradical character, y (0<y<1), which an index of the instability of chemical bonds. In our design principle, γ becomes large at intermediate y region. This y-γ correlation has been theoretically confirmed for various molecules such as H2 molecule and p-quinodimethane. This theoretical prediction has subsequently been verified by the experimental results of gigantic two-photon absorption cross section, which is proportional to the imaginary part of γ, of the polycyclic aromatic hydrocarbons with intermediate y. We here propose a new concept for reversible switching of diradical characterof a molecule by photochromic reaction. Photochromic diarylethene derivatives with various open-shell singlet diradical characters are theoretically designed, and their photochromic diradical character switching behaviors are clarified. These results contribute to designing highly efficient third-order nonlinear optical switching substances based on the correlation between the diradical character, y, and second hyperpolarizability, γ. With this idea, we achieve switching ratio γc/γo ~ 25 for model compounds with intermediate y for a closed-ring isomer and y=1 for open-ring isomer