Third-Order Nonlinear Optical Properties and Electroabsorption Spectra of an Organic Biradical, [Naphtho[2,1‑<i>d</i>:6,5‑<i>d</i>′]bis([1,2,3]dithiazole)]

Thin films of a thiazyl biradical, naphtho­[2,1-<i>d</i>:6,5-<i>d</i>′]­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.