Enhancing the photoluminescence quantum yields of blue-emitting cationic iridium(iii) complexes bearing bisphosphine ligands

Abstract

Herein we present a structure–property relationship study of thirteen cationic iridium(III) complexes of the form of [Ir(C^N)₂(P^P)]PF₆ in both solution and the solid state through systematic evaluation of six bisphosphine (P^P) ligands (xantphos, dpephos, dppe, Dppe, nixantphos and isopropxantphos). All of the complexes are sky-blue emissive, but their photoluminescence quantum yields (Φ_PL) are generally low. However, strong and long-lived blue luminescence (λ_em = 471 nm; Φ_PL = 52%; τ_e = 13.5 μs) can be obtained by combining the reduced bite angle of the 1,2-bis-diphenylphosphinoethene (dppe) chelate with the bulky 2-(4,6-difluorophenyl)-4-mesitylpyridinato (dFmesppy) cyclometalating ligand. To the best of our knowledge this is the highest Φ_PL and the longest τ_e reported for cyclometalated iridium(III) complexes bearing bisphosphine ligands. Light-emitting electrochemical cells (LEECs) were fabricated using lead complexes from this study, however due in part to the irreversible electrochemistry, no functional LEEC was achieved. Organic light-emitting diodes were successfully fabricated but only attained maximum external quantum efficiencies of 0.25%.