where A stands for QTM, B2T describes the temperature dependence of the direct relaxation process and B2 = B1H 4 /T, CT n described the Raman process and τ0 -1 exp(-Ueff/kBT)) -1 is the Orbach relaxation process with τ0 -attempt time of relaxation, Ueff -effective energy barrier for the thermally activated magnetization reversal, kB -Boltzmann constant. The values of the best fit parameters to Eqs. 1 and 2 are gathered in separate Tables: Table for 1, Table for 1UV , Table for 1vis , Table for 2 and Table for 2UV. Careful analysis of the τ -1 (H) curves show important differences between the slow relaxation of the magnetization in 1 and 2. The behaviour of 1 is typical for isolated Er III species with low coordination numbers, where the small applied magnetic field quenches the quantum tunnelling of magnetization (QTM) and does not induce fast direct relaxation. Compound 2, on the other hand, shows splitting of the χ"(ν) maxima above 1000 Oe resulting in two τ -1 (H) branches in Figure . Such a behaviour could be associated with the chain character of the compound. The influence of the UV light irradiation on the τ - 1 (H) curves is evident only in the case of compound 1 -the minimum of the τ -1 (H) dependence after UV irradiation (sample 1UV) moves towards slightly higher magnetic fields, which indicates that the QTM and direct relaxation processes are affected by the photocyclization of the dtepy ligands. However, the consecutive red light irradiation of 1UV, which results in the opening of the dtepy as evidenced by the UVvis and IR studies, does not result in the recovery of the original τ -1 (H) shape in 1vis. In other words the slow relaxation of the magnetization in the pristine 1 and 1vis are not identical from the point of view of τ -1 (H) dependence which means that the observed reversible photocyclization of the dtepy ligand is not transferred onto the magnetic properties.