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(<b>pmdb</b>)­(CCR)₂] <b>1a</b>–<b>c</b>, [Pt­(<b>pm2tz</b>)­(CCR)₂] <b>2a</b>–<b>d</b>, [Pt­(<b>pm3tz</b>)­(CCR)₂] <b>3a</b>–<b>c</b>, [Pt­(<b>ppim</b>)­(CCR)₂] <b>4</b>(<b>a</b>, <b>b</b>, <b>e</b>), and [Pt­(<b>ppbim</b>)­(CCR)₂] <b>5</b>(<b>a</b>, <b>b</b>, <b>e</b>), where pmdb =1,1′-dipentyl-3,3′-methylene-dibenzimidazoline-2,2′-diylidene, pm2tz = 1,1′-dipentyl-3,3′-methylene-di-1,2,4-triazoline-5,5′-diylidene, pm3tz = 1,1′-dipentyl-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(<b>pmim</b>)­(CCR)₂] where <b>pmim</b> = 1,1′-dipentyl-3,3′-methylene-diimidazoline-2,2′-diylidene and R = 4-C₆H₄F <b>6a</b>, C₆H₅ <b>6b</b>, and 4-C₆H₄OMe <b>6c</b> were also recorded in neat solid and in 10 wt % PMMA film. The square planar coordination geometry with the alkynyl ligands in <i>cis</i> 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 <b>6a</b>–<b>c</b>, <b>1a</b>–<b>c</b>, <b>2a</b>–<b>c</b>, and <b>3a</b>–<b>c</b> in conjunction with the decreasing electron-donating nature of the NHC ligands, <b>pmim</b> > <b>pmdb</b> > <b>pm2tz</b> ≈ <b>pm3tz</b>, can be attributed to the slight modification of the triplet emission parentage among the different complexes. The quantum efficiencies of complexes <b>4</b>(<b>a</b>, <b>b</b>) and <b>5</b>(<b>a</b>, <b>b</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 <b>4e</b> and <b>5e</b> bearing diarylamine phenylacetylenes display high ϕ_em of 27% and 33% in 10 wt % PMMA film, respectively