The white light irradiation (180 min, power 0.5 mW/cm 2 ) of 1•2DMF at 10 K clearly shows a significant increase in the absolute reflectivity spectra (Figure , left), even if it is less intense than for the modification with temperature. With a 0.5 mW/cm² light power, the photo-excited state was not saturated even after 180 min (Figure ) light irradiation. Furthermore, light-irradiation experiments were performed using 14 different LEDs ranging from 1050 nm to 365 nm which showed the maximum photoexcitation effect using the 530 nm LED light (Figure ). After irradiating the sample with 530 nm for 10 min (16 mW/cm 2 ), an increase in the reflectivity value was observed, indicating the light-induced LS to HS conversion of Fe(II) center in 1•2DMF at 10 K (Figure ). When the sample was heated keeping the light on, it showed a gradual thermal de-excitation corresponding to the relaxation of the photo-induced metastable Fe(II) HS state into the diamagnetic Fe(II) LS ground state at 70 K (TLIESST) (Figure , right).