Highly Luminescent Mixed-Metal Pt(II)/Ir(III) Complexes: Bis-Cyclometalation of 4,6-Diphenylpyrimidine As a Versatile Route to Rigid Multimetallic Assemblies

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 boolean AND C-LH)(acac) (denoted Pt). If an excess of K2PtCl4 is used, a dinuclear complex of the form [Pt(acac)](2){mu-(N boolean AND C-L-N boolean AND C)} (Pt-2) 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 center dot 3H(2)O (2:1 ratio) leads, after treatment with sodium acetylacetonate, to an unprecedented mixed-metal complex of the form Ir{mu-(N boolean AND C-L-N boolean AND C)Pt(acac)}(2)(acac) (Pt2Ir). The mononuclear iridium complex Ir(N boolean AND 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 lambda(max) in the order Pt < Pt-2 < In < Pt2Ir. 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.