Fluorine-free blue-emitting cationic iridium complexes with oxadiazole-type cyclometalating ligands and their use in light-emitting electrochemical cells

Abstract

Fluorine-free, blue-emitting cationic iridium complexes, namely [Ir(Tipr-dphoxd)2(bpy)]PF6 (1) and [Ir(Tipr-dphoxd)2(pzpy)]PF6 (2), which use 2-phenyl-5-(2,4,6-triisopropylphenyl)-1,3,4-oxadiazole (Tipr-dphoxd) as the cyclometalating ligand (CΛN) and 2,2’-bipyridine (bpy) or 2-(1H-pyrazol-1-yl)pyridine (pzpy) as the ancillary ligands (NΛN), have been synthesized and fully characterized. In solution, complex 1 emits efficient blue-green light centered at 505 nm with notable charge-transfer (CT) (Ir/CΛN → NΛN) character; while in the lightly doped film, it emits efficient sky-blue light peaked at 474 and 502 nm with much CΛN-centered 3π-π∗ character. In both solution and films, complex 2 always emits sky-blue light peaked around 468 nm and 500 nm with dominant CΛN-centered 3π-π∗ character. The sky-blue light emission from complexes 1 and 2 is among the bluest reported for fluorine-free, blue-emitting cationic iridium complexes. Theoretical calculations reveal that complex 1 bears energetically close-lying CT and CΛN-centered 3π-π∗ states, and its emission can arise from either or both of them; while for complex 2, its emission always comes from the lowest-lying CΛN-centered 3π-π∗ state. Because the CT state contributes considerably to the emission of complex 1, complex 1 exhibits more rapid radiative decays, higher luminescent efficiencies and shorter excited-state lifetimes than complex 2 in both the solution and lightly doped film. Complexes 1 and 2 have been tested as emitting materials in solid-state light-emitting electrochemical cells (LECs), with red-shifted electroluminescence compared to the photoluminescence. The green-yellow LEC based on complex 1 and the blue-green LEC based on complex 2 show peak current efficiencies of 12.4 and 5.0 cd A−1, respectively.