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-1<i>H</i>-pyrazole (naph-pz, <b>2a</b>); benzo[<i>h</i>]quinoline (bzq, <b>2b</b>)] were obtained by reaction of the corresponding mononuclear precursor [Pt(C^N)Cl(S^NH)] (C^N: naph-pz <b>1a</b>, bzq <b>1b</b>) with NEt₃. Then, <b>2a</b> and <b>2b</b> 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 <b>3a/b-Cl</b>, Br <b>3a/b-Br</b>, I <b>3a/b-I</b>). Their X-ray structures showed shorter Pt-Pt distances in the Pt₂(III) complexes (<i>ca</i>. 2.7 Å) than in the Pt₂(II) ones (<i>ca</i>. 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 <b>2a</b>, 685 nm <b>2b</b>) 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.