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 one of the most widely studied classes of photoswitch, 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 and photochemical properties. Donor–acceptor interactions in the closed forms of push–pull dithienylethenes shift their absorption spectra into the near-infrared region (λ(max) ≈ 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 excellent fatigue resistance: in aqueous solution on irradiation at 365 nm, the photochemical quantum yields for switching and decomposition are 0.15 and 2.6 × 10(−5) respectively, i.e. decomposition is more than 5000 times slower than photoswitching. These properties make it a promising candidate for biological applications such as super-resolution microscopy and photopharmacology.