Cationic or Neutral: Dependence of Photophysical Properties of Bis-Alkynylphosphonium Pt(II) Complexes on Ancillary Ligand.

A series of D-π-A alkynylphosphonium salts with different linker between donor and acceptor groups was used to synthesize two series of trans-bis-alkynylphosphonium Pt(II) complexes with different ancillary ligands (triphenylphosphine, P series, and cyanide, CN series). The nature of the ancillary ligand manages the overall charge and emission properties of the complexes obtained. In addition, the variation of the linker in alkynylphosphonium ligands allows fine-tuning the luminescence wavelength. Dicationic series P is unstable in solution under UV excitation, whereas in the solid state, these complexes are the first example of phosphorescent trans-phosphine-bis-alkynyl Pt(II) compounds. Neutral series CN demonstrates bright emission in solution, including dual emission for 2CN complex with biphenyl linker in alkynylphosphonium ligand. However, in the solid state for the CN series drastic decrease in the emission quantum yield compared to the P series was observed. DFT calculations reveal the complicated emission nature for the both P and CN series with various contributions of ILCT, LLCT and MLCT states. However, in the naphthyl-containing derivatives 3P and 3CN, the dominating LC character with some admixture of CT states is postulated.