Luminescent Pt(ii) complexes featuring imidazolylidene–pyridylidene and dianionic bipyrazolate: from fundamentals to OLED fabrications

Abstract

Pt(II) complexes bearing imidazolylidene–pyridylidene (impy) and dianionic biazolate chelates were synthesized, for which the end products depend on the alkyl substituents of the impy chelate. Treatment of Pt(DMSO)₂Cl₂ with dimethyl substituted imidazolium–pyridinium pro-ligand Me₂impy(PF₆)₂, followed by addition of 5,5′-(1-methylethylidene)-bis-(3-trifluoromethyl-1H-pyrazole) (mepzH₂), 5,5′-di(trifluoromethyl)-3,3′-bis-pyrazole (bipzH₂), and 5,5′-di(pentafluoroethyl)-3,3′-bis-pyrazole (biepzH₂), afforded Pt(II) complexes [Pt(Me₂impy)(mepz)] (1), [Pt(Me₂impy)(bipz)] (2) and [Pt(Me₂impy)(biepz)] (3), respectively. In contrast, reactions with ethyl and isopropyl substituted Et₂impy(PF₆)₂ and Pr₂impy(PF₆)₂ and with bipzH₂ gave [Pt(EtimHpy)(bipz)] (4) and [Pt(PrimHpy)(bipz)] (5) respectively, where notable alkyl-to-hydrogen transformations on the pyridylidene fragment took place. The reaction of Pt(DMSO)₂Cl₂ with Et₂impy(PF₆)₂ followed by addition of (biepzH₂) gave two products [Pt(Et₂impy)(biepz)] (6) and [Pt(EtimHpy)(biepz)] (7). Single crystal X-ray diffraction analyses of 1, 2 and 5 revealed negligible intermolecular Pt⋯Pt interactions. Hybrid-DFT and TD-DFT computations were carried out on 1, 2 and 5 to model the observed crystal structures and explain the photophysical data successfully. Organic light emitting diodes (OLEDs) were fabricated from complexes 4 or 5 using a multiple layered device architecture. The associated OLED performances (i.e. η_max = 12.5%, 11.2%, η_L = 44.0 cd A⁻¹, 40.6 cd A⁻¹, and η_P = 28.0 lm W⁻¹, 25.8 lm W⁻¹ for 4 and 5) confirmed their suitability in serving as potential OLED phosphors.