The Use of Cyclometalated NHCs and Pyrazoles for the Development of Fully Efficient Blue PtII Emitters and Pt/Ag Clusters

Abstract

New bis-pyrazole complexes [Pt(C^C*)(RpzH)₂ ]X, containing a cyclometalated N-heterocyclic carbene ligand (HC^C*=1-(4-(ethoxycarbonyl)phenyl)-3-methyl-1H-imidazol-2-ylidene) were prepared as chloride (X=Cl^- , RpzH: 3,5-Me₂ pzH 1 a, 4-MepzH 2 a, pzH 3 a), perchlorate (X=ClO₄^- , 1 b-3 b), or hexafluorophosphate (X=PF₆^- , RpzH: 3,5-Me₂ pzH 1 c) salts. The X-ray structure of 1 a showed that the Cl^- anion is trapped by the cation through two N-H⋅⋅⋅Cl bonds. In solution of methanol, acetone and THF at RT, 1 a-3 a coexist in equilibrium with the corresponding [PtCl(C^C*)(RpzH)] (B) and RpzH species. In CH₂ Cl₂ , this equilibrium takes place just for 2 a and 3 a, but it is completely shifted to the left at 243 and 223 K for 2 a and 3 a, respectively. The low-lying absorption and emission bands were assigned to intraligand (ILCT) charge transfer on the NHC group. Quantum yield measurements in PMMA films revealed that 1 b, 2 b and 1 c are amongst the most efficient blue-light emitters, with values up to 100 %. Proton abstraction from the coordinated 3,5-Me₂ pzH in 1 b by NEt₃ and replacement by Ag⁺ afforded a neutral [Pt₂ Ag₂ ] cluster containing Pt→Ag dative bonds.

Keywords: N-heterocyclic carbenes; luminescence; metal-metal interactions; platinum; pyrazole.