Hemiindigoids as Prominent Photoswitch Scaffolds

Hemiindigo photoswitches, a class of chromophores derived from the well-known indigo dye, have attracted considerable interest over the past two decades. These compounds are unique in their ability to absorb light in the visible region of the spectrum in both isomeric states, making them ideal for applications where high-energy UV light is not tolerated. A particularly attractive feature of hemiindigoids is that, despite their rigid structures, they undergo significant and controllable changes during the photoisomerisation process. Moreover, they exhibit a combination of high thermal bistability, pronounced photochromism, and fast, efficient photoisomerisation, positioning hemiindigoid photoswitches as versatile chromophores. This review summarises the current developments and advances in hemiindigoid photoswitches, including their synthesis, photoswitching performance, and applications, with a strong focus on their practical use as reversible triggers in fields ranging from photopharmacology to advanced materials.