Influence of Functional Groups on Solution and Solid-State Emission in Planar Pt(II) Complexes: Synthesis,...

Pt(II)-based complexes bearing functional groups can promote intermolecular interactions, which play an important role in tuning the emission wavelength toward the deep-red (DR) and near-infrared (NIR) regions. In this work, we report a series of platinum complexes (1-4) functionalized with various electron-withdrawing (-F) and electron-donating (tert-butyl and diphenylamine) groups that are synthesized from pyridazine-based organic bidentate ligands L¹-L⁴ and K₂PtCl₄ with the aid of an ancillary ligand (isoquinoline-1-carboxylic acid). All complexes were comprehensively characterized by spectroscopic and analytical techniques. The complexes (1-4) exhibit strong emission in the 500-800 nm range in solution, along with enhanced photoluminescence quantum yields (PLQY). Notably, in the solid state, these molecules display significantly red-shifted emission, which is attributed to their highly ordered stacking with shortened Pt···Pt interactions. The introduction of an isoquinoline unit into the molecules allows it to contribute to the LUMO, which facilitates emission wavelength tuning and enhances emission efficiency, as confirmed by DFT and TD-DFT calculations. A transition from metal-to-ligand charge transfer (MLCT) to MMLCT character is observed from solution to the solid state. These results demonstrate their potential as efficient NIR emitters for OLED applications.