Tuning the Luminescent Properties of Pt(II) Acetylide Complexes through Varying the Electronic Properties of N-Heterocyclic Carbene Ligands

This Article reports the synthesis, structural characterization, electrochemistry, and photophysical investigations of five groups of luminescent Pt(II) alkynyl complexes bearing N-heterocyclic carbene (NHC) ligands with varying electronic properties. Complexes of the type [Pt(pmdb)(C equivalent to CR)(2)] 1a-c, [Pt(pm2tz)(C equivalent to CR)(2)] 2a-d, [Pt(pm3tz)(C equivalent to CR)(2)] 3a-c, [Pt(ppim)(C equivalent to CR)(2)] 4(a, b, e), and [Pt(pphim)(C equivalent to CR)(2)] 5(a, b, e), where pmdb =1,1'-dipenty1-3,3'-methylene-dibenzimidazoline-2,2'-diylidene, pm2tz = 1,1'-dipenty1-3,3'-methylene-di-1,2,4-triazoline-5,5'-diylidene, pm3tz = 1,1'-dipenty1-3,3'-methylene-di-1,3,4-triazoline-5,5'-diylidene, ppim = 3-pentyl-1-picolylimidazoline-2-ylidene, and ppbim = 3-pentyl-1-picolylbenzimidazoline-2-ylidene, and R = 4-C6H4F, C6H5, 4-C6H4OMe, SiMe3, and 4-C6H4N(C6H5)(2), were prepared, and the consequences of the electronic properties of the NHC ligands on the phosphorescent emission efficiencies were studied. Moreover, the emission quantum efficiencies of the previously reported complexes [Pt(pmim)(C equivalent to CR)2] where pmim 1,1'-dipenty1-3,3'-methylene-diimidazoline-2,2'-diylidene and R = 4-C6H4F 6a, C6H5 6b, and 4-C6H4OMe 6c were also recorded in neat solid and in 10 wt % PMMA film. The square planar coordination geometry with the alkynyl ligands in cis configuration was corroborated for selected complexes by single crystal X-ray diffraction studies. The observed moderate difference in emission efficiencies of the bis-carbene complexes 6a-c, 1a-c, 2a-c, and 3a-c in conjunction with the decreasing electron-donating nature of the NHC ligands, pmim > pmdb > pm2tz pm3tz, can be attributed to the slight modification of the triplet emission parentage among the different complexes. The quantum efficiencies of complexes 4(a, b) and 5(a, b) bearing pyridyl-NHC ligand were significantly low in comparison to the bis-carbene complexes owing to the significant change in the charge transfer character of the triplet manifold. Complexes 4e and 5e bearing diarylamine phenylacetylenes display high phi(em) of 27% and 33% in 10 wt % PMMA film, respectively.