Double-Decker Platinum Complexes: From Visible to NIR-II Luminescence

The Pt(II) dinuclear compounds [{Pt(C^N)(μ-S^N)}₂] [S^NH: 2-mercapto-1-methylimidazol; HC^N= 1-naphthalen-2-yl-1H-pyrazole (naph-pz, 2a); benzo[h]quinoline (bzq, 2b)] were obtained by reaction of the corresponding mononuclear precursor [Pt(C^N)Cl(S^NH)] (C^N: naph-pz 1a, bzq 1b) with NEt₃. Then, 2a and 2b were reacted with aqueous HX (X: Cl, Br, I) in molar ratio 1:2 to give the corresponding oxidized derivatives [{Pt(C^N)(μ-S^N)X}₂] (X = Cl 3a/b-Cl, Br 3a/b-Br, I 3a/b-I). Their X-ray structures showed shorter Pt-Pt distances in the Pt₂(III) complexes (ca. 2.7 Å) than in the Pt₂(II) ones (ca. 3.0 Å) in agreement with the existence of a Pt-Pt bond. The photophysical properties were studied experimentally and theoretically by DFT and TD-DFT. The Pt₂(II) complexes exhibit an emission in the visible region (640 nm 2a, 685 nm 2b) of mostly excimeric ³ππ* and ³MMLCT [dσ*(Pt₂) → π* (C^N)] character respectively, while the Pt₂(III) complexes emit in the NIR-II region with maxima reaching up to 1215 nm. The ³XMMCT nature of these emissions justify the influence of both the axial ligand (X) and the overlap of the dz² orbitals on their energies; the latter being affected by the nature of the C^N and the bridging ligand.