Triplet excimer formation in platinum-based phosphors: A theoretical study of the roles of Pt-Pt bimetallic interactions and interligand π-π interactions

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-N,C^2′)(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 (T₁) geometries; the characteristics of the S₀ → T₁ 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 π-π interactions in the formation of excimer structures. While the Pt-Pt interactions are critical for excimer formation, the interligand π-π 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 Å and interligand distances around 3.5-3.8 Å, 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.