AC (alternating current) magnetic susceptibility measurements were performed at various external DC magnetic fields and temperatures as a function of the AC magnetic field frequency in the 1-1000 Hz range. Both compounds exhibit slow magnetic relaxation in the presence of a small external DC field typical for single molecule magnets. In the case of 1, the relaxation of the magnetization can be mildly altered by light irradiation due to the photochromic switching of the bridging dtepy ligand. All frequency dependencies were carefully analyzed by applying the Debye model. All details of the respective Debye fittings of the AC magnetic susceptibility data in the form of Tables and Figures can be found in the Electronic Supplementary Information: 1 (Figure and Tables ), 1UV (Figure and Table -S5), 1vis (Figure and Table -S7), 2 (Figure and Tables ) and 2UV (Figure and Tables ). For both compounds in different photo-excited/ photo-deexcited states, the respective magnetic field and temperature dependencies of the relaxation times were obtained and analyzed using the following equations (Eq. 1 and 2) taking into account four different relaxation mechanisms possible for the Kramers ion Er III . The magnetic field dependence of the relaxation time was fitted using Eq. 1 and is presented in Figure for 1 (black) 1UV (blue) and 1vis (red) and in Figure for 2 (black) and 2UV (blue):