Herein, we provide some structural evidence of the complexation color-change of murexide solutions in presence of lanthanide, which has been used for decades in complexometric studies. For Ln=Sm to Lu and Y, the compounds crystallize as monomeric [Ln(Murex)3 ]⋅11 H2 O with an N3O6 tricapped square-antiprism environment, which are stable up to 250 °C. Single-ion magnet (SIM) behavior is then observed on the Yb(III) derivative in an original nine-coordinated environment. In-field slow relaxation (Δ=(15.6±1) K; τ0 =2.73×10(-6) s) is observed with a very narrow distribution of the relaxation time (αmax =0.09). Magnetic and photophysical properties can be correlated. On one hand the analysis of NIR emission spectrum permits to have access to crystal field parameters and to compare them with those extracted from dc measurements. On the other hand, magnetic measurements permit to identify the nature of the MJ states involved in the (2) F5/2 →(2) F7/2 luminescence spectrum. The gap between the low-lying states is in agreement with the energy barrier obtained from magnetic slow-relaxation measurement.