Third-order nonlinear optical properties of a π-conjugated biradical molecule investigated by third-harmonic generation spectroscopy

Third-order optical nonlinearity of a biradical molecule with open-shell π-electron system, 1,4-bis- (4,5-diphenylimidazole-2-ylidene)-cyclohexa-2,5-diene (BDPI-2Y) was investigated by the third-harmonic generation method. The figure of merit for third-order optical nonlinearity in BDPI-2Y is comparable to those for conjugated polymers, which have elongated closed π-conjugated systems. This indicates that the biradical nature of BDPI-2Y enhances third-order optical nonlinearity.