Syntheses of Photochromic Dihetarylethenes

This chapter focuses on the methods that are used for the synthesis of photochromic dihetarylethenes, in which both heterocycles are in vicinal positions at the double bond of cyclopentene, perfluorocyclopentene, and various heterocyclic moieties. The photocyclization of stilbenes and dihetarylethenes (DHEs) is accompanied by cis–trans isomerization, and the trans isomers do not give a cyclized form. Thus, products with a fixed cis conformation, in which the thienyl or benzothienyl moieties are linked to the double bonds of perfluorocyclopentene, maleic anhydride, and maleimide, are synthesized. Photochromes containing such “bridges” possess not only thermal irreversibility but also high fatigue resistance, amounting in some cases, to 10,000 cycles. These properties combined with other useful physicochemical characteristics offer ample opportunities for the use of dihetarylethenes in nonlinear optics, optical memory devices, as photoswitches, linkers, display elements, etc. Some dihetarylethenes, such as bis(indolyl)furan-2,5-diones that were covered by patents as typical photochromes have a broad spectrum of biological activities. This chapter discusses the synthesis of perfluorocyclopentene-bridged dihetarylethenes, dithienylperhydrocyclopentenes, and their heteroanalogs, synthesis of dithienyl photochromes containing a maleic anhydride fragment as the ethene-bridge and diarylmaleimides containing optically active dihetarylethenes.