To corroborate the findings from the photo-induced surface reflectivity study and investigate the photo-induced spin-state switching of the iron center, the photo-sensitive behavior of complex 1 was examined by measuring the magnetic susceptibility under light irradiation by applying various To explore thermal stability and relaxation of the photo-excited HS state, T vs T was measured in the dark after T value gained saturation (ca. 25 min at 635 nm; Figure ) with a sweep rate of 0.4 K min -1 . The T value shows an increase to attain a maximum with 3.40 cm 3 mol -1 K in between 15 and 2 K, confirming a complete photo-switching from the LS state to the photo-excited HS state. Upon further increasing temperature, the photo-excited HS state relaxes back to the LS ground state at 56 K (TLIESST = 56 K) which is in close range to the relaxation temperature obtained from the surface reflectivity measurements (TLIESST = 60 K). The absence of a clear plateau suggests fast relaxation of the photoinduced excited state to LS ground state, which is consistent with the optical reflectivity studies. With increasing temperature, T vs T curves follow the same curves as obtained before light irradiation confirming the complete relaxation to the ground LS state upon heating.