Homoleptic purine-based NHC iridium(iii) complexes for blue OLED application: impact of isomerism on photophysical properties

Abstract

With a motivation to develop deep-blue emitters for organic light-emitting diodes (OLEDs) stereoisomers of a homoleptic N-heterocyclic carbene (NHC) iridium(III) complex bearing 7,9-dihydro-8H-purin-8-ylidene ligands (mer-PhP and fac-PhP) have been synthesized and their photophysical, electrical and electroluminescent properties investigated. The studied complexes only transport holes with the highest hole mobility value of 2 × 10⁻⁴ cm² V⁻¹ s⁻¹ at an electric field of 4.9 × 10⁵ V cm⁻¹ in the case of fac-PhP. When molecularly dispersed in PMMA films, the compounds show bright blue phosphorescence (466 and 437 nm) with photoluminescence quantum yield (Φ_PL) values of 0.99 and 0.77. The complexes also possess exceptional thermal stability with a 5% mass loss point at up to 480 °C. While the mer-isomer shows more efficient photoluminescence and a twofold higher radiative rate, it is more sensitive to external stimulus: its Φ_PL significantly drops and an emission redshift takes place upon transfer from a rigid to liquid surrounding medium or under exposure to a temperature increase. DFT calculations relate this effect to a weakened metal–ligand bonding at the lowest energy excited triplet state (T₁) of the compound, which promotes the extent of non-radiative relaxation processes. On the basis of mer-PhP an efficient blue OLED was prepared with an emission maximum wavelength of 467 nm and a maximum external quantum efficiency of 16.1%.