Fatigue cycles were measured to test the reversibility of photoinduced E-Z isomerization of 7. By switching the azobenzene moiety 20 times consecutively back and forth using a 505 nm and a 400 nm LED, we investigated to which degree irreversible processes like photobleaching occur. Figure shows the fatigue cycles of 7 in Milli-Q ® water at 37 °C. After irradiation with light of 505 nm, the absorbance of 7 at 335 nm decreases, compared to the initial thermal equilibrium (shown in blue). Irradiation with a 400 nm LED increases the absorbance at 335 nm. Further, the absorbances after irradiation with each wavelength are restored throughout all consecutive switching cycles. Hence, no irreversible, competing side processes like photobleaching or hydrolysis take place. Data in Figure reveals that compound 7 is interconverted by light between its E and Z isomers in a fully reversible fashion. This enables the use of the photochromic cAMP derivative in living cells over a long period of time and several switching cycles. The observed resistance towards photobleaching is in accordance with the literature and emphasizes the benefits of tetra-ortho-fluorination of the azobenzene moiety. Figure . Absorbances of a solution of 8-azo-cAMP in Milli-Q ® water at 37 °C at the initial thermal equilibrium (indicated in blue), directly after 1 min irradiation with =505 nm (data points at the bottom) or =400 nm (data points at the top). The absorbances at a wavelength of 335 nm are shown since they exhibit the most pronounced differences for the two photoisomers (cf. Fig. ). Consecutive irradiation with 505 nm and 400 nm is considered as one cycle.