Phase change materials (PCMs) exhibiting large changes in the optical properties are of great interest due to their application towards next-generation photonic devices such as optical modulators, smart windows, optical switches, optical memories, and photonic integrated circuits , etc. Transition metal oxides with metal-insulator transition, chalcogenide alloys with crystalline-amorphous transition, and liquid crystals with order-disorder transition are the most studied examples of PCMs. In this perspective, spin crossover (SCO) systems can be used as a PCM due to their reversible change of spin-state between high-spin (HS) and low-spin (LS) states along with their exciting photophysical properties. For last few decades, a lot of effort towards photonic application has been devoted to the SCO systems accompanied by optical property change arising from spin-state switching. In 1998, Kahn et al. developed the first SCO molecule-based optical device for information storage and device application where the data writing and erasing were associated with the change of optical properties (violet (LS) to white (HS)) due to the thermal effect. However, one of the major disadvantages of SCO molecules is having low absorbance coefficient in the visible and near infra-red (Vis-NIR) region which results in a low signal-to-noise ratio for detection. Therefore, the optical contrast of the two spin states of SCO systems becomes very important for the nano-photonic and display devices. Optical absorbance change in any system is directly related to its refractive index; therefore, it can be a useful tool to study the phase change in a material and its application toward new technological devices. In contrast, most of the metal-to-metal electron-transfer (MMET) systems show a broad and high intense absorbance-based charge-transfer band. MMET-based bistable materials are also one type of PCMs in which the diamagnetic ground state (Fe II LS-CN-Co III LS) transforms into a paramagnetic metastable state (Fe III LS-CN-Co II HS) associated with distinctly different optical, electrical, and magnetic properties due to the application of external stimuli such as light, temperature, pressure, pH, etc. In 2020, a