A double rollover cycloplatinated(ii) skeleton: a versatile platform for tuning emission by chelating and non-chelating ancillary ligand systems

Abstract

Described here is the synthesis and characterization of heteroleptic binuclear platinum(II) complexes of the type [Pt₂(μ-bpy-2H)(S^S)₂] and [Pt₂(μ-bpy-2H)(L)₂(X)₂], containing a 2,2′-bipyridine-based double rollover cycloplatinated core (Pt(μ-bpy-2H)Pt), in combination with the anionic S^S⁻ chelate ligands di(ethyl)dithiocarbamate (dedtc) and O,O′-di(cyclohexyl)dithiophosphate (dcdtp) or non-chelating L/X ancillary ligands (PPh₃/Me, t-BuNC/Me, PPh₃/SCN and PPh₃/N₃). The new complexes were characterized using multinuclear (¹H, ³¹P and ¹⁹⁵Pt) NMR spectroscopy and some of them additionally using single crystal X-ray diffraction. The absorption and photoluminescence of the complexes show a strong dependence on the ancillary ligands. Upon excitation at 365 nm, in a CH₂Cl₂ rigid matrix (77 K), the complexes exhibit structured emission bands with λ_max between 488 nm and 525 nm and vibrational spacing around 1350 cm⁻¹, indicating the excited states centered on the cyclometalated ligand (³ILCT) with some mixing ³MLCT characteristics. In the case of the PPh₃/N₃ complex, a dual emission band (orange color) is observed in the solid state at 298 K for which the low energy band arises from an aggregation-induced emission (AIE). Upon lowering the temperature (77 K), thermochromism is observed (orange to yellow) which is accompanied by the intensification of the high energy band (ligand-centered structured band). Finally, in order to rationalize the obtained photophysical data, complete DFT (density functional theory) and TD-DFT (time-dependent DFT) calculations were performed on the selected complexes.