In the case of 1, 1UV and 1vis , the temperature dependence of the molar magnetic susceptibility achieves a plateau in the 100-140 K range with values close to 34.4 cm 3 Kmol -1 expected for three noninteracting Er III ions in the free-ion approximation ( 4 I15/2, gJ = 6/5): 33.9 cm 3 Kmol -1 for 1, 34.2 cm 3 Kmol -1 for 1UV and 33.7 cm 3 Kmol -1 for 1vis. The χT(T) gradually decreases on cooling and reaches 27.1 for 1, 27.5 cm 3 Kmol -1 for 1UV and 26.5 cm 3 Kmol -1 for 1vis at 1.8 K. Compound 2 and its UV irradiated form 2UV exhibit a very similar χT(T) behavior. The observed χT(T) decrease is ascribed to the thermal depopulation of the excited mJ sublevels. Interestingly the χT(T) traces for 1 and 1vis overlap while that for 1UV shows a slightly different shape in the 1.8-100 K range. This might indicate that the photocyclization affects the mJ energy diagram for 1. The χT(T) for 2 and 2UV overlap almost perfectly, indicating negligible influence of the UV irradiation on the magnetic properties of 2. This is consistent with results of the UV-vis and IR experiments.