Learning from Spiropyrans: How to Make Further Developments of Donor-Acceptor Stenhouse Adducts

Donor-acceptor Stenhouse adducts (DASAs) as a species of novel photochromic molecules, have been developing rapidly and attracting broad researchers' sights since 2014. DASAs show visible/near-infrared (NIR) light induced linear-to-cyclic isomerization and heat induced cyclic-to-linear isomerization, therefore they are attractive in photoresponsive hydrogels, drug delivery, cell culturing and tissue engineering. As a series of well-known photoresponsive molecules, spiropyrans (SPs) show similar molecular properties and comparable photoswitching with DASAs. UV light triggers closed-to-open (also known as spirocyclic (SC)-to-merocyanine (MC)) isomerization of SPs, while the reversed open-to-closed isomerization occurs under visible light or heat. Light irradiation switches the molecular color, scale, geometry, and polarity of SPs and DASAs reversibly. Since the researches on DASAs are still in the infancy, the chemical structures, isomerization mechanisms and potential applications need to be further investigated and explored. The well-studied SPs have important reference values to the comprehensive developments of DASAs. In the present review, we summarized, compared and discussed SPs and DASAs with regards to their molecular structures, synthesis, photoswitching and applications. We also make our perspective on the developments of DASA in future.