Double decker NHC-cyclometallated Pt complexes for highly-efficient vacuum-deposited green OLEDs and NIR emission

Abstract

The dinuclear compound [{Pt^II(C^C*)(µ-N^N)}₂] (HN^N: diphenylformamidine; HC^C* = 3-methyl-1-(naphthalen-2-yl)-1H-imidazol-2-ylidene, 3) was obtained as a pure single anti-isomer, with a Pt–Pt distance of 2.8645 Å. Under UV light, 3 reacted with haloforms to obtain the corresponding oxidized species [{Pt^III(C^C*)(µ-N^N)X}₂] (4-X). X-ray analysis of 4-Cl confirmed the retention of the anti-conformation and revealed a shortened Pt–Pt distance (2.6070 Å) consistent with metal–metal bonding in the Pt₂(III,III) system. In addition, methylation of the electron-rich complex 3 with [Me₃O]BF₄ provided the mixed-valence dinuclear species [(C^C*)Pt^II(µ-N^N)₂Pt^IV(C^C*)(CH₃)]BF₄ (5), which exhibits two distinct ¹⁹⁵Pt NMR resonances (δ: −2373 and −3264 ppm) and a large Pt–Pt coupling constant (2155 Hz). Compound 3 shows a structureless emission band at 520 nm in 2 wt% PMMA (poly(methyl methacrylate)) film, with a photoluminescence decay of 1.80 µs and quantum yield of 0.90, typical of triplet metal–metal–to-ligand charge transfer (³MMLCT) [dσ*(Pt–Pt) → π*(C^C*)] excited states. The oxidized counterparts, 4-X, display phosphorescent emissions with maxima at 960, 970 and 1055 nm in solid state at 77 K. These are tentatively assigned to triplet axial ligand-to-metal–metal charge transfer (³XMMCT) [σ(X) → dσ*(Pt–Pt)]. Whilst complex 5 shows no emission in either the visible or near-infrared regions at 298 or 77 K. Organic light-emitting diodes (OLEDs) incorporating 3 as a green emitter demonstrate excellent device performance, achieving a turn-on voltage of 3 V, an external quantum efficiency (EQE) reaching 22% and a maximum luminance exceeding 22 000 cd m⁻².