•Cationic iridium complexes with phenyl-imidazole type CΛN ligands were developed.•The CΛN ligands destabilize both the HOMO and LUMO levels of the complexes.•Orange-red LEC using the complex gives a maximum current efficiency of 14.3 cd A−1.Phosphorescent cationic iridium complexes with phenyl-imidazole type cyclometalating ligands have been synthesized for the first time and their photophysical, electrochemical properties have been comprehensively investigated. By changing the ancillary ligands, the complexes give orange-red or green-blue light. Compared to 2-phenylpyridine (ppy), the phenyl-imidazole ligands destabilize simultaneously the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals of the complexes. Their emitting triplet states show dominant charge-transfer (iridium/cyclometalating ligands→ancillary ligands) character. The complexes have been used to fabricate solid-state light-emitting electrochemical cells (LECs). The orange-red LEC gives a high peak current efficiency of 14.3 cd A−1, which is among the highest reported for orange-red LECs; the green-blue LEC gives a peak current efficiency of 6.3 cd A−1. It is shown that the phenyl-imidazole cyclometalating ligands hold promise for the invention of iridium-based cationic phosphorescent dyes with tunable energy levels and emission properties.

A route toward fluorine-free blue-emitting cationic iridium complexes, to generate emission from the cyclometalating ligands with enhanced triplet energy, has been proposed and demonstrated. Attaching electron-donating groups to the pyridine moieties of the ppy-type cyclometalating ligands (Hppy is 2-phenylpyridine) enhances the triplet (3π–π*) energy of the ligand, and the use of electron-rich or non-conjugated ancillary ligands ensures that the emission is generated from the 3π–π* states of cyclometalating ligands. By this molecular design, [Ir(buoppy)2(pzpy)]PF6 (1) and [Ir(buoppy)2(bim-cb)]PF6 (2) have been developed, with 4-butoxy-2-phenylpyridine (buoppy) as the cyclometalating ligand and electron-rich 2-(1H-pyrazol-1-yl)pyridine (pzpy) or non-conjugated N-heterocyclic dicarbene (bim-cb) as the ancillary ligands. Complexes 1 and 2 give emission with major emission peaks around 465 nm, which is among the bluest reported for fluorine-free cationic iridium complexes. For both complexes, the emission is generated from the 3π–π* states centered on buoppy. For complex 1, the charge-transfer (Ir/buoppy → pzpy) state is dominated by non-radiative deactivation and it behaves as a non-radiative deactivation channel for the emissive buoppy-centered 3π–π* states which lies close to the charge-transfer state in energy. Such a non-radiative deactivation channel is largely suppressed in the rigid matrix, and is eliminated in complex 2 with a non-conjugated dicarbene ancillary ligand.

Two cationic iridium complexes, namely [Ir(dph-oxd)2(bpy)]PF6 (1) and [Ir(dph-oxd)2(pzpy)]PF6 (2), using 2,5-diphenyl-1,3,4-oxadiazole (dph-oxd) as the cyclometallating ligand and 2,2′-bipyridine (bpy) or 2-(1H-pyrazol-1-yl)pyridine (pzpy) as the ancillary ligands, have been synthesized, and their photophysical and electrochemical properties have been comprehensively investigated. In solution, both complexes emit efficient blue-green light. For complex 1, the light emission in a neat film is remarkably red-shifted; in solid state, it gives an intriguing piezochromic phenomenon. Compared with archetype [Ir(ppy)2(bpy)]PF6 (ppy is 2-phenylpyridine), complex 1 shows a largely stabilized HOMO (highest occupied molecular orbital) level, induced by the electron-deficient 1,3,4-oxadiazole (oxd) heterocycle of dph-oxd, which results in an enlarged energy gap and blue-shifted emission. Compared with complex 1, complex 2 shows an enhanced LUMO (lowest unoccupied molecular orbital) level, caused by the electron-rich pzpy ancillary ligand, but they exhibit similar emission energy in solution. For both complexes, theoretical calculations reveal that their blue-green emission in solution arises primarily from the 3π–π* states centered on dph-oxd; moreover, complex 1 bears close-lying 3π–π* and 3CT (charge-transfer) states, underlying its remarkably red-shifted emission in the neat film and unique piezochromic behavior in the solid state. Solid state light emitting electrochemical cells (LECs) based on complexes 1 and 2 give efficient yellow and green-blue light, with peak current efficiencies of 18.3 and 5.2 cd A−1, respectively. It is demonstrated that oxd-type cyclometallating ligands are promising as an avenue to stabilize the HOMOs and tune emission properties of cationic iridium complexes to a large extent.

An electron-transporting counter-anion, OXD-7-SO3−, was developed for a blue-green-emitting cationic iridium complex, which showed superior device performance in solution-processed phosphorescent light-emitting diodes over its counterpart complex with a PF6− counter-anion, due to the facilitated electron-injection/transport and more balanced carrier recombination imparted by OXD-7-SO3−.

A route toward fluorine-free blue-emitting cationic iridium complexes, to generate emission from the cyclometalating ligands with enhanced triplet energy, has been proposed and demonstrated. Attaching electron-donating groups to the pyridine moieties of the ppy-type cyclometalating ligands (Hppy is 2-phenylpyridine) enhances the triplet (3π–π*) energy of the ligand, and the use of electron-rich or non-conjugated ancillary ligands ensures that the emission is generated from the 3π–π* states of cyclometalating ligands. By this molecular design, [Ir(buoppy)2(pzpy)]PF6 (1) and [Ir(buoppy)2(bim-cb)]PF6 (2) have been developed, with 4-butoxy-2-phenylpyridine (buoppy) as the cyclometalating ligand and electron-rich 2-(1H-pyrazol-1-yl)pyridine (pzpy) or non-conjugated N-heterocyclic dicarbene (bim-cb) as the ancillary ligands. Complexes 1 and 2 give emission with major emission peaks around 465 nm, which is among the bluest reported for fluorine-free cationic iridium complexes. For both complexes, the emission is generated from the 3π–π* states centered on buoppy. For complex 1, the charge-transfer (Ir/buoppy → pzpy) state is dominated by non-radiative deactivation and it behaves as a non-radiative deactivation channel for the emissive buoppy-centered 3π–π* states which lies close to the charge-transfer state in energy. Such a non-radiative deactivation channel is largely suppressed in the rigid matrix, and is eliminated in complex 2 with a non-conjugated dicarbene ancillary ligand.

Two cationic iridium complexes, namely [Ir(dph-oxd)2(bpy)]PF6 (1) and [Ir(dph-oxd)2(pzpy)]PF6 (2), using 2,5-diphenyl-1,3,4-oxadiazole (dph-oxd) as the cyclometallating ligand and 2,2′-bipyridine (bpy) or 2-(1H-pyrazol-1-yl)pyridine (pzpy) as the ancillary ligands, have been synthesized, and their photophysical and electrochemical properties have been comprehensively investigated. In solution, both complexes emit efficient blue-green light. For complex 1, the light emission in a neat film is remarkably red-shifted; in solid state, it gives an intriguing piezochromic phenomenon. Compared with archetype [Ir(ppy)2(bpy)]PF6 (ppy is 2-phenylpyridine), complex 1 shows a largely stabilized HOMO (highest occupied molecular orbital) level, induced by the electron-deficient 1,3,4-oxadiazole (oxd) heterocycle of dph-oxd, which results in an enlarged energy gap and blue-shifted emission. Compared with complex 1, complex 2 shows an enhanced LUMO (lowest unoccupied molecular orbital) level, caused by the electron-rich pzpy ancillary ligand, but they exhibit similar emission energy in solution. For both complexes, theoretical calculations reveal that their blue-green emission in solution arises primarily from the 3π–π* states centered on dph-oxd; moreover, complex 1 bears close-lying 3π–π* and 3CT (charge-transfer) states, underlying its remarkably red-shifted emission in the neat film and unique piezochromic behavior in the solid state. Solid state light emitting electrochemical cells (LECs) based on complexes 1 and 2 give efficient yellow and green-blue light, with peak current efficiencies of 18.3 and 5.2 cd A−1, respectively. It is demonstrated that oxd-type cyclometallating ligands are promising as an avenue to stabilize the HOMOs and tune emission properties of cationic iridium complexes to a large extent.