Higher fluorescence in platinum(iv) orthometallated complexes of perylene imine compared with their platinum(ii) or palladium(ii) analogues

Abstract

The reaction of 3-perylenylmethylen-4′-ethylaniline (1) with [Pt₂Me₄(μ-SMe₂)₂] (and subsequent addition of PPh₃) or with [Pt₂(η³-C₄H₇)₂(μ-Cl)₂] produced cyclometallated Pt^II complexes [Pt(C^N)Me(PPh₃)] (2) and, respectively, [Pt₂(C^N)₂(μ-Cl)₂] (3) (HC^N = 3-C₂₀H₁₁CHNC₆H₄-p-C₂H₅), with Pt bound to the ortho site of the perylenyl fragment. From 3 the mononuclear complexes [Pt(C^N)L₂] (L₂ = acac (4); S₂COMe (5); S₂CNEt₂ (6) are easily formed. Oxidative addition of methyl iodide to the square-planar Pt^II complexes 2, 4, and 6 gave the corresponding cyclometallated Pt^IV compounds [Pt(C^N)L₂MeI] 7, 8 and 9. The X-ray structures of 4, 6, and 7 show that the perylenyl fragment remains essentially flat in 4 and 6 and slightly twisted in 7. Comparison of the optical properties of these Pt^II complexes with those reported for similar Pd^II derivatives reveals that the change of metal exerts a notable influence on the UV-vis spectra. In solution at room temperature, all the Pt complexes exhibit fluorescence associated with the perylene fragment with low emission quantum yields for the Pt^II complexes (<1%) and remarkably higher emission values for the Pt^IV complexes: up to 29%, with emission lifetimes of 1–5 ns. Time-dependent density functional theory (TD-DFT) calculations were performed on the perylene imine and on representative complexes [M(C^N)(acac)] (M = Pd, Pt) and [Pt(C^N)(acac)MeI] to analyse the absorption spectra. These calculations support a perylene-dominated intraligand π–π*emissive state based on the HOMO and LUMO orbitals of the perylene chromophore, and a ligand-to-ligand charge-transfer (more intense for the Pt^II complex) that explains the observed influence of the metal on the absorption properties.