The lnτ(T -1 ) dependencies indicate that the slow magnetization dynamics in both compounds are dominated by the Raman relaxation process -all curves presented in Figures and could be fitted without the contribution of the Orbach relaxation mechanism. The direct relaxation process operates only at very low temperatures. This is similar to the behaviour of the parent compound [Er III (BHT)3(THF)]. The analysis of the lnτ(T -1 ) dependencies before irradiation (2) and after 365 nm irradiation (2UV) suggest, that the UV light has basically no effect on this chain system -in line with other results. This can be rationalized by the chain structure of 2 which probably prevents the structural reorganization of the azopy ligand required by its photoisomerization. The behaviour of 1, on the other hand, deserves a more detailed discussion as the shape of the lnτ(T -1 ) for the pristine 1 is different from that of the UV irradiated sample 1UV (Figure ). The closer look at the lnτ(T -1 ) fitting parameters presented in Table for 1 and Table for 1UV leads to the conclusion that the structural changes induced by light affect mainly the Raman exponent (4.92(4) for 1 and 4.3(3) for 1UV). However, despite the observed reversibility of the photochromic behaviour of 1 confirmed by UV-vis and IR spectroscopy, the AC magnetic susceptibility measurements reveal that after red light irradiation does not allow the recovery of the initial SMM behaviour. This is revealed by the shape of the τ -1 (H) and lnτ(T -1 ) dependencies for 1vis similar to 1UV rather than to the pristine 1. The possible explanation for this behaviour is the collapse of the structural order in 1UV during photocyclization of the dtepy (amorphization caused by UV light irradiation), which affects the AC magnetic properties.