Phosphorescent Tris-cyclometalated Pt(IV) Complexes with Mesoionic N-Heterocyclic Carbene and 2-Arylpyridine Ligands

The synthesis, structure, photophysical properties, and electrochemistry of the first series of Pt(IV) tris-chelates bearing cyclometalated aryl-NHC ligands are reported. The complexes have the general formula [Pt(trz)(2)(C & BOTTOM;N)](+), combining two units of the cyclometalated, mesoionic aryl-NHC ligand 4-butyl-3-methyl-1-phenyl-1H-1,2,3-triazol-5-ylidene (trz) with a cyclometalated 2-arylpyridine [C & BOTTOM;N = 2-(2,4-difluorophenyl)-pyridine (dfppy), 2-phenylpyridine (ppy), 2-(p-tolyl)pyridine (tpy), 2-(2-thienyl)pyridine (thpy), 2-(9,9-dimethylfluoren-2-yl)-pyridine (flpy)], and presenting a mer arrangement or metalated aryls. They exhibit a significant photostability under UV irradiation and long-lived phosphorescence in the blue to yellow color range, arising from (LC)-L-3 excited states involving the C & BOTTOM;N ligands, with quantum yields of up to 0.34 in fluid solution and 0.77 in the rigid matrix at 298 K. The time-dependent density functional theory (TD-DFT) calculations reveal that nonemissive, deactivating excited states of ligand-to-metal charge-transfer (LMCT) character are pushed to high energies as a consequence of the strong sigma-donating ability of the carbenic moieties, making the Pt(trz)(2) subunit an essential structural component that enables efficient emissions from the chromophoric C & BOTTOM;N ligands, with potential application for the development of different Pt(IV) emitters with tunable properties.

Automated summary

This study reports the synthesis, structure, photophysical properties, and electrochemistry of the first series of Pt(IV) tris-chelates bearing cyclometalated aryl-NHC ligands. The complexes exhibit significant photostability, long-lived phosphorescence, and potential application for Pt(IV) emitters with tunable properties.