Highly luminescent mixed-metal Pt(II)/Ir(III) complexes: Bis-cyclometalation of 4,6-diphenylpyrimidine as a versatile route to rigid multimetallic assemblies

Article abstract of DOI:10.1021/ic200706e

The proligand 4,6-di-(4-tert-butylphenyl)pyrimidine LH₂ can undergo cycloplatination with K₂PtCl₄ at one of the two aryl rings to give, after treatment with sodium acetylacetonate, a mononuclear complex Pt(N^{^}C-LH)(acac) (denoted Pt). If an excess of K ₂PtCl₄ is used, a dinuclear complex of the form [Pt(acac)]₂{μ-(N^{^}C-L-N^{^}C)} (Pt₂) is obtained instead, where the pyrimidine ring acts as a bridging unit. Alternatively, the mononuclear complex can undergo cyclometalation with a different metal ion. Thus, reaction of Pt with IrCl₃?3H ₂O (2:1 ratio) leads, after treatment with sodium acetylacetonate, to an unprecedented mixed-metal complex of the form Ir{μ-(N^{^}C-L- N^{^}C)Pt(acac)}₂(acac) (Pt₂Ir). The mononuclear iridium complex Ir(N^{^}C-LH)₂(acac) (Ir) has also been prepared for comparison. The UV-visible absorption and photoluminesence properties of the four complexes and of the proligand have been investigated. The complexes are all highly luminescent, with quantum yields of around 0.5 in solution at room temperature. The introduction of the additional metal centers is found to lead to a substantial red-shift in absorption and emission, with λ_max in the order Pt < Pt₂ < Ir < Pt ₂Ir. The trend is interpreted with the aid of electrochemical data and density functional theory calculations, which suggest that the red-shift is due primarily to a progressive stabilization of the lowest unoccupied molecular orbital (LUMO). The radiative decay constant is also increased. This versatile design strategy may offer a new approach for tuning and optimizing the luminescence properties of d-block metal complexes for contemporary applications.