It is important to mention that most of the aforementioned bidentate nitrogen donor coligands belong to the category of α-diimine ligands (Scheme 1). In contrast, β-diimine ligands are rarely studied in compounds of Lanthanides. However, the β-diimine ligands like bik-R (R = Et, Me, etc.) (Schemes 1, right) have not been explored yet in lanthanide chemistry. The more flexible βdiimine ligands form a six-membered chelating ring with the metal centre in comparison to less flexible α-diimine ligands, which form a five-membered chelating ring. Importantly, bik-R ligands have been widely used to study various switchable molecular magnetic materials with 3d-5d transition metals in the recent past. We have lately reported the reversible switching of photo- physical properties of SCO, MMET systems using bik-R ligands where we have shown that the chelating and flexible nature along with the electronic effect of these bik-R ligands can significantly affect the switching behaviour of the associated materials. Thus, we for the first time used the -diimine ligands bik-R (R = Et, Me) in tuning the magnetic anisotropy of the Dy(III) mononuclear complexes, the visualization of their diverse electronic nature is evident from the distinct luminescence and magnetic properties discussed below. The tuning of SMM and stark sublevel-based luminescence properties together within a compound is a challenging task since both properties have opposing requirements. The slow dynamics in magnetization relaxation in an oblate top like Dy(III) is achieved by enhancing the axial symmetry and lowering the equatorial symmetry elements to enhance the axial anisotropy and the energy gap between the ±mj levels of the Kramers doublet which again reduces the charge cloud repulsion of the f-electron density and the electron density of the ligand. On the other hand, the energy difference between the mj levels of the emitter level is lowered by a distorted pseudosymmetry favouring partial admixture of orbitals of opposite parity which favours symmetry forbidden (Laporte forbidden) ff transitions and hence the luminescence (Stark level) thermometric features. In order to observe luminescence, the selection of the ligand system should be made judiciously so that the energy transfer can take place by the 'antenna effect' from the ligand to the emitter level of the Dy(III) centre alongside curbing the thermo-induced back electron transfer. The amalgamation of the luminescence thermometric property with the SMM feature is desirable as it can act as an insitu thermometer, thereby self-monitoring the temperature of the operation and giving an insight into the fundamentals of heat generation which can act as the potential solution of the increasing need for device miniaturization and multifunctionality. This property is highly desirable in modern optoelectronic devices. The speciality of Stark-level thermometry is that its dimension can be reduced to a submicrometric scale; it is contactless and remote sensing, which is essential in the realization of microdevices in the diverse fields of material science, biology and quantum technologies. Moreover, these thermometers are self-calibrated following a ratiometric approach where the anomalies due to intensity fluctuations, i.e. variation of concentration or excitation power, are nullified and thereby the requirement of any external reference is redundant. 48 Luminescent thermometric properties in Dy SMMs are majorly studied with diimine ligand (Table ); in this work, we have explored this optical thermometric property in diimine mononuclear Dy systems.