Photochromic materials exhibit light-induced color change essentially due to the reversible structure change such as ring-open to ring-closed states upon photoirradiation, resulting in the distinct difference in photo-induced chemical reactions/physical properties. These photochromic properties have attracted much attention on the applications of ophthalmic lenses, photo-switches, molecular actuators, logic gates, and information storages. Among various photochromic systems, organic diarylethenes have become increasing interest because of their high thermal stability. Particularly, the ethene bridge with perfluorocyclopentene can bring excellent fatigue resistance and bistability to the diarylethene family, thereby nowadays it is adopted for developing diarylethene derivatives. The ethene bridges have very great effects on photochromic properties since they are located exactly in the photoreaction center. Given the difficulty in structurally modifying the classic ethene bridge of perfluorocyclopentene owing to its chemical inertness, several other ethene bridges have emerged to achieve higher performance and more applications. This review covers the latest research progresses on the important ethene bridges and their corresponding diarylethene derivatives, and their application prospects.

Contents
1 Introduction
2 Five-membered-ring bridges
2.1 Cyclopentene derivative bridges
2.2 Thiophene derivative bridges
2.3 Thiazole derivative bridges
2.4 Imidazole derivative bridges
2.5 Other five-membered-ring bridges
3 Six-membered-ring bridges
3.1 Non-aromatic bridges
3.2 Aromatic bridges
4 Miscellaneous bridges
5 Conclusion and outlook