Correlating the Structural and Photophysical Features of Pincer Luminophores and Monodentate Ancillary Ligands in Pt-II Phosphors

Phosphorescent PtII complexes featuring pincer luminophores of 2,6-bis(1,2,4-triazolyl)pyridine (H2L1) and 2,6-bis(pyrazolyl)pyridine (H2L3) with a bulky adamantyl or tolyl substituent (H2L4) are systematically compared, and their structural features are correlated with their photophysical properties. The combination with 4-amylpyridine (Py), triphenylphosphine (P) or benzimidazol-2-ylidene (N-heterocyclic carbene, NHC) donors as monodentate ancillary ligands gave a series of highly luminescent triplet emitters with variable aggregation properties. The molecular structures of four of these complexes, namely, Pt-L1-P, Pt-L1-NHC, Pt-L3-P, and Pt-L4-P were garnered from single-crystal X-ray diffraction analysis. The coordination complexes displayed green phosphorescence in solution and in the solid state. In doped poly(methyl methacrylate) (PMMA) matrices, most of the complexes exhibited high phosphorescence quantum yields, which reached 59 % for Pt-L3-P. A comparative analysis between the spectroscopic data and the computed parameters derived from time-dependent density functional theory (TD-DFT) calculations suggests that the emission originates from metal-perturbed ligand-centered excited triplet states (3MP-LC). The radiationless deactivation rate constants of the emissive states can be correlated with the aggregation properties derived from the substitution pattern at the tridentate luminophores and the ancillary ligands, whereas the radiative rate constants are determined by the electronic structures of the complexes. We found that PtII complexes containing pyrazolate donors showed an enhanced charge-transfer character in the excited state, whereas bulky adamantyl moieties and triphenylphosphine ancillary ligands suppress bimolecular aggregation and quenching phenomena.