Synthesis of PAT 1 was developed with the requirement to introduce a mesitylene residue as solubilizing group because of the extended conjugated p-system, which is expected to lead to pronounced aromatic interactions (Scheme 1). For this reason 9-iodo-10-bromoanthracene 2 was chosen as starting material to selectively introduce first methyl 2-mercaptoacetate at the 9-position followed by substation with mesitylene at the 10-position. After synthesis photoswitching and thermal stability of PAT 1 isomers were scrutinized in detail in benzene solution (Figure ). The E isomer is the most stable isomer and is populated quantitatively after heating similar to the related thioindigo photoswitch. E-1 possesses an absorption with its lowest energy maximum at 701 nm tailing out at 850 nm, which leads to a green color of the corresponding benzene solution (Figure ). High molar absorption coefficients are measured, e.g. 24,800 molL -1 cm -1 at the 701 nm maximum. This absorption thus fully resides within the biooptical window and makes PAT a very interesting NIR chromophore. Irradiation with 730 nm or 780 nm red light leads to pronounced changes in the absorption and an overall blue-shift of the spectrum caused by E to Z photoisomerization. The lowest energy absorption maximum of the isolated metastable Z-1 isomer is shifted more than 100 nm to 591 nm, which makes the benzene solution appear blue in color. Again substantial molar absorption coefficients are observed, e.g. 22,000 molL -1 cm -1 at the 591 nm maximum.