The syntheses, characterisations, photophysical properties and their applications in organic light emitting devices of a series of 2-phenylbenzothiazolato (bt)/substituted 2-phenylbenzothiazolato (X-bt) platinum(II) based square-planar complexes [(X-bt)Pt(acac); acac = acetylacetonate; X = unsubstituted (1), F (2), OMe (3) and CF3 (4)] are discussed. Reaction of K2PtCl4 with btH/X-btH in glacial acetic acid for a few days results in the dinuclear chloro-bridged Pt(II) complex, (bt/X-bt)Pt(μ-Cl)Pt(bt/X-bt) which is cleaved with acetylacetone to give the corresponding mononuclear (bt/X-bt)Pt(acac) complex. The (MeO-bt)Pt(acac) complex has been characterized by X-ray single crystal structure analysis. The packing diagram shows the Pt-Pt distances and the intermolecular spacings are 3.369 and 3.360 Å, respectively, which is consistent with excimer formation. It has also been supported by time-resolved photoluminescence (PL) measurements. The nature of the lowest emitting states are the triplet MLCT as well as triplet π-π* states and it has been tuned according to the electronic properties of the substituents. The electroluminescent (EL) devices were fabricated by doping platinum complexes 1 and 2 (the corresponding devices denoted as D-1 and D-2, respectively) into the host CBP (4,4′-(N,N′-dicarbazole)biphenyl), in the emitting zone with a doping content of 5%, 7% and 9%. The EL performances for these devices are exceptionally high (14.3 and 16.0 cd A-1 @2 mA cm-2 and luminances 10 550 and 11 320 cd m-2 @100 mA cm-2 for D-1 and D-2, respectively).