Thin
films of a thiazyl biradical, naphtho[2,1-d:6,5-d′]bis([1,2,3]dithiazole), abbreviated as NT, exhibit
nonlinear optical properties with a large third-order nonlinear susceptibility
of 2.1 × 10–11 esu. The electroabsorption and
low-temperature-absorption measurements reveal a series of degenerate
electronic states, which is probably caused by the biradical character
of NT. It is concluded that this degeneracy should enhance the third-order
nonlinear optical properties.