Phosphorescence Properties of Discrete Platinum(II) Complex Anions Bearing N-Heterocyclic Carbenes in the Solid State

The synthesis, crystal structures, and photophysical properties of four anionic platinum complexes bearing N-heterocyclic carbenes (NHCs), n-Bu₄N[Pt(CN)₂(NHC)] (H₂NHC⁺ = 1-methyl-3-phenyl-1 H-imidazolium (1), 1-methyl-3-phenyl-1 H-benzimidazolium (2), 1-methyl-3-(naphthalen-2-yl)-1 H-imidazolium (3), 1-methyl-3-(naphthalen-1-yl)-1 H-imidazolium (4)), are reported. The tetra- n-butylammonium salts afforded discrete Pt(II) complexes surrounded by the bulky cations in the crystalline states with no Pt-Pt, π-π, or solute-solvent interactions. As a result, the crystals exhibited strong phosphorescence with quantum yields of 0.24-0.72 at 298 K. The three isomeric complexes 2-4 with π-extended structures of 1 exhibited luminescence with different colors depending on the π-extension site. These complexes in a discrete and restricted space enabled the comparative investigation of the luminescent states and regioselective benzannulation effect using computational methods and luminescence spectroscopy. Blue-emissive 1 and 2 exhibited lifetime changes at temperatures of 77-298 K, suggesting the influence of dark states located 2000-3000 cm^-1 above the emissive state. In contrast, yellow- and orange-emissive 3 and 4 showed no thermal deactivation effect at ambient temperature. The temperature dependence of the emission lifetimes in the low-temperature region of 4-77 K showed that blue-emissive 1 and 2 had a relatively large zero-field splittings (ZFSs) of 35 and 28 cm^-1, respectively, indicating the significant contribution of triplet metal-to-ligand charge transfer character to the ³π-π* emission. On the other hand, the ZFS of 4 was less than 2 cm^-1, suggesting purer ³π-π* character.