Exceptional second‐order nonlinear optical susceptibilities of quinoid systems

A new class of organic quinoid systems possessing intrinsically large, nonlinear second‐order optical susceptibilities has been developed based on theoretical calculations and dc‐induced second‐harmonic generation measurements of the molecular second‐order optical susceptibilities β. For one example, 2‐(4‐dicyanomethylenecyclohexa‐2, 5‐dienylidine)‐imidazolidine (DCNQI), βx = −240±60×10−30 cm5/esu. The unusually large magnitude and sign originate from certain charge‐correlated features in the quinoid ground and excited states.