The impact of cyclometalated and phosphine ligands on the luminescence properties of cycloplatinated(ii) complexes: photophysical and theoretical investigations

Abstract

The present work investigates the synthesis and characterization of a series of cycloplatinated(II) complexes, bearing the dfppy (2-(2,4-difluorophenyl)pyridine) cyclometalated ligand with the general formula of [Pt(dfppy)(Cl)(L)], where L = dmso, dimethyl sulfoxide (A), PPh₂py, diphenyl-2-pyridylphosphine (1a), and PPh₃, triphenylphosphine (1b). The phosphine-containing dfppy derivatives are photophysically compared with their previously reported ppy (2-phenylpyridine) counterparts. UV-Vis absorption spectra of these complexes were assigned using TD-DFT (time-dependent density functional theory) calculations. It is observed that the wavelengths (low-energy region) used for excitation are related to the mixed ¹ILCT/¹MLCT/¹XLCT electronic transitions. Theoretical investigations on the frontier molecular orbitals of the T₁ states confirmed that these transitions are also kept in the triplet excited states and lead to the emissive states with ³ILCT/³MLCT/³XLCT character. This would be experimentally verified by the appearance of the structured emission bands and the ³ILCT being mainly present in the emissive state. However, the dfppy derivatives show dual emission bands (high energy as a structured emission band and low energy as an unstructured emission band) in the solid state, which are not observed in the ppy analogues. The dfppy complexes exhibit spectral blue shifts and have higher quantum yield values compared with the ppy analogs. In this regard, the complexes with PPh₂py show stronger emissions than those containing PPh₃.