A "flexible" Rigid Rod, trans-Pt(PMe3)2(C-CC6H4CN)2 (L1), to Form 2D [{Cu2(μ2-X)2}2(μ4-L1)]n Polymers (X = Br, I) Exhibiting the Largest Bathochromic Emissions

Abstract

The trans-Pt(PMe3)2(C-CC6H4CN)2 organometallic ligand L1, which is prepared from 4-ethynylbenzonitrile and cis-Pt(PMe3)2Cl2, binds CuX salts to form two strongly luminescent two-dimensional coordination polymers (CPs) [{Cu2(μ2-X)2}2(μ4-L1)]n (X = I, CP1; X = Br, CP2). The emission quantum yields, φe ≈ 30% at 298 K, are the largest ones for all CPs built upon the trans-Pt(PMe3)2(C-CC6H4X)2 motifs (X = SMe, CN). X-ray crystallography reveals that, to accommodate these layered CPs, L1 must undergo major distortions of the C-C-C angles (∼159°) and significant rotations about the Pt-CC bonds, so that the dihedral angles made by the two aromatic planes is 90° in a quasi-identical manner for both CPs. Together, these two features represent the largest distortion for trans-Pt(PMe3)2(C-CC6H4X)2 complexes among all of the CPs built upon this type of ligand (2 of 16 entries). Concurrently, CP1 and CP2 also exhibit the most red-shifted emissions (λmax = 650 and 640 nm, respectively) known for this type of chromophore at room temperature. The {Cu2(μ2-X)2} rhomboids adopt the trans- (X = I, common) and cis-geometries (X = Br, extremely rare) making them "isomers" if excluding the fact that the halides are different. Density functional theory (DFT) and time-dependent DFT suggest that the triplet emissive excited state is metal/halide-to-ligand charge transfer in both cases despite this difference in rhomboid geometry.