This large absorption shift leads to an uniquely pronounced photochromism, which facilitates isomer accumulation in the photostationary states. As a consequence of the highly red-shifted absorptions in general and the additional very strong photochromism, efficient all-red-light photoswitching is achievable with PAT 1. The isomer conversion after illumination to the photostationary state (pss) was quantified with the aid of 1 H NMR spectroscopy by integrating suitable signals of both isomers (Figure ). Quantitative E to Z photoisomerization is induced by 730 nm or 780 nm red-light irradiation and reversely 73% E isomer can be obtained after Z to E photoisomerization with 617 nm red-light. Further, the yield of E isomer can be increased to 87% (NMR)/91%(UV/Vis) when using 590 nm orange light. Quantum yields were measured