The photoisomerization of azobenzenes provides a general means for the photocontrol of molecular structure and function. For applications in vivo, however, the wavelength of irradiation required for trans-to-cis isomerization of azobenzenes is critical since UV and most visible wavelengths are strongly scattered by cells and tissues. We report here that azobenzene compounds in which all four positions ortho to the azo group are substituted with bulky electron-rich substituents can be effectively isomerized with red light (630-660 nm), a wavelength range that is orders of magnitude more penetrating through tissue than other parts of the visible spectrum. When the ortho substituent is chloro, the compounds also exhibit stability to reduction by glutathione, enabling their use in intracellular environments in vivo.

Switching zebrafish: A fluorescent reporter peptide permits imaging of azobenzene photoisomerization in vivo (see picture), which indicates that azobenzene‐based photochemical switches may be generally useful for spatiotemporal control in living systems.

Zebrafisch mit Schalter: Mithilfe eines fluoreszierenden Reporterpeptids gelingt es, die Photoisomerisierung von Azobenzol in vivo abzubilden (siehe Bild). Daraus folgt, dass photochemische Schalter auf Azobenzolbasis allgemein für die räumlich‐zeitliche Kontrolle in lebenden Systemen eingesetzt werden können.