Facilitating different chemistries between the rare earth (RE = La-Lu, Sc, Y) ions is of significant interest for their separations. While the bulk of attention has been on maximizing the small differences in their ground state chemistry, interest is beginning to shift toward the differences in their electronic excited states. In this work, we demonstrate modulation of the photostationary state of an azobenzene derivative, Na1, via chelation to a series of RE^IIIDO3A (DO3A = 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid) complexes. The extent of photoisomerization of 1^– follows the trend in RE^III Lewis acidity with two exceptions: Sm^III and Er^III. UV-vis spectroscopy, titration experiments, and computational analysis show that these exceptions are a result of energy transfer rather than differences in ground state chemistry. These results open a pathway to differentiate REs by new photochemical means.