How do donor and acceptor substituents change the photophysical and photochemical behavior of dithienylethenes? The search for a water-soluble visible-light photoswitch

Dithienylethenes are a type of diarylethene and they constitute the most widely studied class of photoswitches, yet there have been no systematic studies of how electron-donor or -acceptor substituents affect their properties. Here we report eight dithienylethenes bearing push-push,pull-pull and push-pull substitution patterns with different lengths of conjugation in the backbone and investigate their photophysical andphotochemical properties. Donor-acceptor interactions in the closed forms of push-pull dithienylethenes shift their absorption spectra into the near-infrared (lmax 800 nm). The push-pull systems also exhibit low quantum yields for photochemical electrocyclization, and computational studies indicate that this can be attributed to stabilization of the parallel, rather than anti-parallel, conformations. The pull-pull systems have the highest quantum yields for switching in both directions, ring-closure and ring-opening. The chloride salt of a pull-pull DTE, with alkynes on both arms, is the first water-soluble dithienylethene that can achieve >95% photostationary state distribution in both directions with visible light. It has good fatigue resistance: in aqueous solution on irradiation at 365 nm, the photochemical quantum yields for switching and decomposition are 0.15 and 2.6 x 10–5 respectively, indicating that the molecule could undergo >5000 switching cycles before decomposition. These properties make it a promising candidate for biological applications such as super-resolution microscopy and photopharmacology.