The first report in 1999 by Fabbrizzi and co-workers outlined a modular approach to design a two-component luminescent sensor for temperature using spin-state switching of the Ni II ion in a tetra-aza coordinative environment (Fig. ) in square planar (LS) and octahedra (HS) geometry (Fig. ). The high-spin/low-spin conversion in [Ni II (L28)] 2+ ( ) is endothermic, where increasing temperature favors the LS square planar form of the complex, and the emission spectra recorded over the temperature range established the applicability of [Ni II (L28)] 2+ as a thermometric probe operating at the molecular level (Fig. ) . In 2008, Matsuda and co-workers reported the first observation of thermo-induced ON/OFF switching of the electroluminescent (EL) emission in an organic thin film device spin-coated with SCO active [Fe(dpp)2](BF4)2 ( ) (dpp=2,6-di(pyrazol-1-yl)pyridine) . The ease of formulating a thin film of 42, owing to its solubility in a polar solvent, with retention of SCO property, facilitated the fabrication of ITO/chlorophyll a-[Fe(dpp)2](BF4)2/Al . Further investigation of this led to the conclusion that 42 acts as the electron injection material, leading to the formation of electron-hole pairs, which recombine with electrons to give radiative luminous emission in OLEDs. The quenching of EL results from low-temperature LS Fe(II) form, where the excited state of the luminophore is responsible for electroluminescence beyond Tc does not occur due to a change in the efficiency of the charge injection process .