We have used density functional theory (DFT) and time-dependent DFT to investigate the geometric and electronic structure and the optical properties of the phosphorescent platinum compounds: Pt(II) (2-(4',6'-difluorophenyl)pyridinato-NC2')(2,4-pentanedionato-O,O) (FPt1) and Pt(II) (2-(4',6'-difluorophenyl)pyridinato-N,C-2')(1,3-propanedionato-O,O) (FPt0). We first examined isolated compounds (monomers) and evaluated their photophysical properties at the ground-state and lowest triplet excited-state (T-1) geometries; the characteristics of the S-0 -> T-1 transitions are nearly identical in both compounds. Dimers of FPt0 and of FPt1 were then studied in order to shed light, at least qualitatively, on the respective role of Pt-Pt bimetallic interactions and interligand pi-pi interactions in the formation of excimer structures. While the Pt-Pt interactions are critical for excimer formation, the interligand pi-pi interactions also play a significant role in determining the optimal excimer geometry and the magnitude of the phosphorescence energy lowering in going from the monomer to the aggregated dimer. The distorted cofacial-type excimer structures found for FPt1, with a Pt-Pt distance around 2.9 angstrom and interligand distances around 3.5-3.8 angstrom, lead to phosphorescence energy lowerings with respect to the monomer on the order of 0.7-0.96 eV, in very good agreement with experiment.