Cyclometalated NHCs Pt(II) compounds with chelating P^P and S^S ligands: From blue to white luminescence

Abstract

Ionic [Pt(C^C*A/B)(P^P)]PF6 and neutral [Pt(C^C*A/B)(S^S)] complexes were designed and synthesized containing cyclometalated N-heterocyclic carbenes (C^C*A/B) and diphosphines (P^P: dpfppe, dcypm) or dithiocarbamates (S^S: dmdtc, pdtc) as chelating ligands. Their structural and spectroscopic properties were investigated and found to be dependent on both cyclometalated (C^C*A/B) and ancillary (P^P, S^S) ligands. The photophysical and computational studies for the [Pt(C^C*A/B)(P^P)]+complexes disclose the nature of the low-lying electronic transitions to be mainly intraligand charge transfer [ILCT (C^C*A/B)] with some contribution of ligand-to-ligand charge transfer (LL’CT) or ligand-to metal charge transfer (LMCT) for the dpfppe or dcypm derivatives, respectively. The blue and cyan emissions of PMMA films doped with the [Pt(C^C*A/B)(P^P)]+ complexes exhibited very high quantum yields (QYs) reaching up to ∼90%. However, the low-energy absorptions and emissions of the [Pt(C^C*A/B)(S^S)] complexes in solution (rt or 77 K) arise from mixed ILCT [C^C*A/B]/MLCT [dπ(Pt) → π*(C^C*A/B)] excited states, showing no change with the different S^S ligands. In solid-state and in doped films, these dithiocarbamate complexes, excluding 8B, display dual emissions with the high-energy vibronic band (3ILCT /3MLCT) appearing together with an additional low-energy structureless band. The latter is attributed to 3ππ* transitions originated from π-stacked aggregates, as reported in the X-ray structure of 7A. Thus, white light emissions can be obtained with photo- and colorimetric values lying inside the stipulated limits for general lighting applications; yet, they display low QY.