Isomers AR-1, CT-1 and CR-1 as well as DR-1 could be used directly as starting points for thermal and photoirradiation experiments to decipher the function and working mechanism of 1. Heating a solution of AR-1 to 120 °C in (CDCl2)2 led to population of CT-1 and CR-1 until an equilibrium distribution with 12% AR-1, 19% CT-1, and 69% CR-1 was established. This distribution could directly be translated into the relative free energy differences of the three isomers. Thus, the global minimum in this solvent was found to be CR-1, followed by CT-1 (1.0 kcal×mol -1 higher in energy), and then AR-1 (1.3 kcal×mol -1 higher in energy). Analysis of the corresponding kinetic measurements using COPASI ). In this sequence of isomer interconversions, the same (Sa)-configuration of the biaryl axis is retained. Isomer DT-1 was then scrutinized for its thermally induced motions in CD2Cl2 solution at -90 °C, which revealed exclusive and complete thermal atropisomerization populating only DR-1 (Figure ). Kinetic analysis of this process corresponded to an energy barrier of DG ¹ = 13.11 kcal×mol -1 . When irradiating a solution of pure DR-1 at -90 °C in CD2Cl2 solution, only light induced photoisomerization to Z-isomeric CR-1 was observed establishing a photoequilibrium between just these two isomers (Figure ). Allowing the sample to warm to 25 °C resulted in thermal conversion of CR-1 to CT-1 in up to 22% (Figure ) via a thermally activated atropisomerization of the biaryl moiety. Although CR-1 is still the dominant isomer in the final mixture in polar CD2Cl2 solution, this last transformation directly evidences completion of a four-step interconversion cycle. When moving to an apolar solvent like CS2 the relative stability of CR-1 to CT-1 is inverted and CT-1 becomes the dominant isomer in up to 73% (Figure ). In this case the last thermal atropisomerization step also becomes dominant and so does the entire four-step cycle.