Tuning the Terminal N* and C^ Moieties Toward Tailored Pt(II) Complexes with Thiazole-Based N*N^C Luminophores

Abstract

The synthesis, characterization, cyclovoltammetric and photophysical properties of 11 new d⁸-configured Pt(II) complexes with N*N^C coordinated ligands, alternatively involving N*N six-ring and N^C five-ring chelates, are presented. By using various boronic acids, variation of the cyclometalating aryl units was achieved. The DFT-calculated HOMOs are localized on the metal with contributions from the Cl^– coligand and either the phenyl/thiophenyl unit or the thiazolyl moiety, depending on the substitution pattern. The LUMOs have phenyl-pyridine π*-character. Both calculated orbital sets agree well with the redox potentials from cyclic voltammetry. The TD-DFT calculated absorption spectra are in agreement with experimental data showing long-wavelength bands in the range from 400 to 500 nm, which matches the yellow color of the complexes. The ligand variation enabled a fine-tuning of the emissive properties related to the resulting complexes, going from greenish-blue (471 nm) to red (617 nm) phosphorescence. The position of the substituent affects the excited state properties, which is attributed to mesomeric and inductive effects on the Pt–C bond and the adjacent pyridine ring. In general, modulation of the excited state character can be achieved by variation of the cyclometalating unit, thus affecting the excited state energy as well as the radiative and radiationless deactivation rates.