Luminescent Organoplatinum(II) Complexes with Functionalized Cyclometalated C^N^C Ligands: Structures, Photophysical Properties, and Material Applications

A series of [(R′-C^N^C-R′′)Pt(L)] complexes with doubly deprotonated cyclometalated R′-C^N^C-R′′ ligands (R′-C^N^C-R′′=2,6-diphenylpyridine derivatives) functionalized with carbazole, fluorene, or thiophene unit(s) have been synthesized and their photophysical properties studied. The X-ray crystal structures reveal extensive intermolecular π⋅⋅⋅π and CH⋅⋅⋅π interactions between the cyclometalated C^N^C ligands. Compared to previously reported cyclometalated platinum(II) complexes [(C^N^C)Pt(L)], which are non-emissive in solution at room temperature, the carbazole-, fluorene- and thiophene-functionalized [(R′-C^N^C-R′′)Pt(L)] (L=DMSO 1–9, CNAr, 1 a–9 a) complexes are emissive in solution at room temperature with λmax at 564–619 nm and Φ=0.02–0.26. The emissions of the [(R′-C^N^C-R′′)Pt(L)] complexes are attributed to electronic excited states with mixed 3MLCT and 3IL character. The carbazole/fluorene/thiophene unit(s) allow the tuning of the electronic properties of the [(R′-C^N^C-R′′)Pt] moiety, with the emission maxima in a range of 564–619 nm. These are the first examples of organoplatinum(II) complexes bearing doubly deprotonated cyclometalated C^N^C ligands that are emissive in solution at room temperature. In non-degassed DMSO, the emission intensities of 6 a–9 a are enhanced upon exposure to ambient light. This phenomenon is caused by reacting photogenerated 1O2 with a DMSO molecule to form dimethyl sulfone, leading to the removal of dissolved oxygen in solution. Self-assembled nanowires and nanorods are obtained from precipitation of 3 a in THF/H2O and 8 a in DMSO/Et2O, respectively. The [(R′-C^N^C-R′′)Pt(L)] complexes are soluble in common organic solvents with a high thermal stability (>300 °C), rendering them as phosphorescent dopants for organic light-emitting diode (OLEDs) applications. Red OLEDs with CIE coordinates of (0.65±0.01, 0.35±0.01) were fabricated from 7 a or 8 a. A maximum external efficiency (ηExt) of 12.6 % was obtained for the device using 8 a as emitter.