Photophysics and electroluminescence of organometallic compounds with d-electrons

Platinum octaethyl porphyrin (PtOEP) shows much lower electroluminescence (EL) than tris(2-phenylpyridine) iridium [Ir(ppy)3]. The low EL intensity is due to weaker spin-orbit coupling in PtOEP than in Ir(ppy)3 by small overlap of 5d-orbital of Pt2+ with π-orbital of porphyrin ligand. It is suggested that lower symmetry around Pt2+ enhances the spin-orbit coupling by mixing of the 5d-π-orbitals, giving rise to high luminance. Numerical analysis is made for the photoluminescence (PL) lifetimes and intensity of a low-symmetry cyclometalated Pt2+-compound NcircumflexNcircumflexC-Pt(Cl) which emits much intense phosphorescent luminance than PtOEP, by taking into account non-radiative relaxations among three zero-field splitting sublevels of triplet T1 level and energy dissipation from the sublevels to unexcited neighboring NcircumflexNcircumflexC-Pt(Cl) and by solving the rate equations for the three sublevels. From the fitting between the calculated and observed temperature dependences of PL lifetime and intensity, large zero-field splitting of 90 cm-1 and short radiative lifetime of 1.9 μs were obtained for the upper sublevel, indicating strong spin-orbit coupling as the case of Ir(ppy)3