High-Purity and Saturated Deep-Blue Luminescence from trans-NHC Platinum(II) Butadiyne Complexes: Properties and Organic Light Emitting Diode Application

Two new platinum(II) compounds with trans-(NHC)2Pt(C≡C–C≡C–R)2 (where NHC = N-heterocyclic carbene and R = phenyl or trimethylsilyl) architecture exhibit sharp blue-green or saturated deep-blue phosphorescence with high color purity. The photoluminescence of both compounds is dominated by an intense 0–0 band with distinct but weaker vibronic progressions in both tetrahydrofuran (THF) and poly(methyl methacrylate) (PMMA) matrix. The full width at half-maximum (fwhm) of the photoluminescence of trans-(NHC)2Pt(C≡C–C≡C-trimethylsilyl)2 are 10 nm at room temperature and 4 nm at 77 K, while the trans-(NHC)2Pt(C≡C–C≡C-phenyl)2 shows a fwhm of 14 nm at room temperature and 8 nm at 77 K. The Commission International de L’Eclairage (CIE) coordinates of trans-(NHC)2Pt(C≡C–C≡C-phenyl)2 are (0.222, 0.429) in PMMA, and trans-(NHC)2Pt(C≡C–C≡C-trimethylsilyl)2 has a deep-blue CIE of (0.163, 0.077) in PMMA. When doped into PMMA, the phosphorescence quantum yield of the complex with trimethylsilyl-butadiyne ligand increases dramatically to 57% from 0.25% in THF, while the complex with phenyl-butadiyne ligand has similar quantum yields in PMMA (32%) and THF (37%). Organic light-emitting diodes (OLEDs) employing these two complexes as the emitters were successfully fabricated with electroluminescence that closely matches the corresponding photoluminescence. The OLEDs based on trans-(NHC)2Pt(C≡C–C≡C-trimethylsilyl)2 display highly pure deep-blue electroluminescence (fwhm = 12 nm) with CIE coordinates of (0.172, 0.086), approaching the most stringent National Television System Committee (NTSC) coordinates for “pure” blue of (0.14, 0.08).