Homoleptic platinum(ii) complexes with pyridyltriazole ligands: excimer-forming phosphorescent emitters for solution-processed OLEDs

Abstract

Two new homoleptic platinum(II) complexes are reported that feature aryl-appended 5-(2-pyridyl)-1,2,4-triazole chelates acting as N^N⁻ ligating ions, PtL¹₂ and PtL²₂. Readily prepared from easily accessible proligands, they offer good solubility in organic solvents, allowing them to be incorporated into OLEDs through solution processing. Crystal structures reveal staggered, face-to-face packing of the π systems in adjacent complexes, but with no close Pt⋯Pt interactions. The complexes display bright unimolecular phosphorescence: for PtL¹₂ and PtL²₂ respectively, λ_max = 502 and 514 nm; Φ = 0.21 and 0.48; τ = 5.1 and 4.6 μs in deoxygenated CH₂Cl₂ at 295 K. Both complexes show a strong propensity to form intensely emissive excimers at higher concentrations: λ_max = 585 and 625 nm for PtL¹₂ and PtL²₂. The photophysical properties in doped and neat thin films have been investigated using steady-state and time-resolved methods. These studies highlight the presence of different environments of bimolecular excited states with different lifetimes, those emitting at lowest energy apparently having the longest lifetimes, contrary to what is normally found for unimolecular emitters through the effects of vibrational deactivation. The prototype solution-processed OLEDs gave EQEs of 9.6–12.5% for PtL¹₂ and 8.8–11.4% for PtL²₂, impressive values for solution-processed devices incorporating such simple complexes and only a little inferior to the EQE of 15% achieved using PtL¹₂ in a device prepared by evaporation. Compounds of this type have potential to provide the red and green components for white light OLEDs, due to their tunable, uni- and bimolecular excited state emission.