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

The reaction of 3-perylenylmethylen-4'-ethylaniline (1) with [Pt2Me4(mu-SMe2)(2)] (and subsequent addition of PPh3) or with [Pt-2(eta(3)-C4H7)(2)(mu-Cl)(2)] produced cyclometallated PtII complexes [Pt(C boolean AND N)Me(PPh3)] (2) and, respectively, [Pt-2(C boolean AND N)(2)(mu-Cl)(2)] (3) (HC boolean AND N = 3-C20H11CH-NC6H4-p-C2H5), with Pt bound to the ortho site of the perylenyl fragment. From 3 the mononuclear complexes [Pt(C boolean AND N)L-2] (L-2 = 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 boolean AND 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 PtII 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 PtII 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 boolean AND N)(acac)] (M = Pd, Pt) and [Pt(C boolean AND N)(acac)MeI] to analyse the absorption spectra. These calculations support a perylene-dominated intraligand pi-pi* 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.