Blue-green emitting cationic iridium complexes with 1,3,4-oxadiazole cyclometallating ligands: synthesis, photophysical and electrochemical properties, theoretical investigation and electroluminescent devices

Abstract

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 (3)π-π* states centered on dph-oxd; moreover, complex 1 bears close-lying (3)π-π* and (3)CT (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.

Phosphorescent Cationic Iridium(III) Complexes with 1,3,4-Oxadiazole Cyclometalating Ligands: Solvent-Dependent Excited-State Dynamics

“…At 298 K, their emission peaks show slight red-shifts relative to those at 77 K. The structured phosphorescence emission curves with weak solvatochromism and rigidochromism indicate that 3 π-π * character dominates the lowest emissive triplet states [12]. The sim- ilar emission curves of complexes 1 and 2 indicate that their emission properties are dominated by the cyclometalating ligands of dph-oxd, which bears an electron-withdrawing group, forming the emissive dphoxd-centered ( 3 π-π * ) states [10,42,43]. For general iridium(III) complexes, on account of the spin-flip exchange energy, the energetic order of these transition is 1 LC> 1 MLCT> 3 MLCT> 3 LC [12].…”

“…The synthesis of [Ir(dph-oxd) 2 (bpy)]PF 6 (complex 1) and [Ir(dph-oxd) 2 (pzpy)]PF 6 (complex 2) has been described in detail elsewhere [10]. Briefly, for the synthesis of complex 1, [Ir(dph-oxd) 2 Cl] 2 reacted with bpy in CH 2 Cl 2 /CH 3 OH.…”

“…As a third-row transition metal cation with an electron configuration of 5d 6 , iridium(III) is capable of forming cyclometalated cationic complexes, referred as [Ir(C ∧ N) 2 (N ∧ N)] + A − , with the cyclometalating ligands (C ∧ N), the ancillary ligand (N ∧ N) and the counter an-ion (A − ) [9,10]. For this type of iridium(III) complexes, the frontier molecular orbitals are delocalized over the entire molecule with remarkable charge-transfer characteristics.…”

“…Currently, much attention concentrates on the structure-property relationship of heavy metal complexes to design, modify and optimize their photoluminescence and photosensitization properties, where ligands are the key factors to control the relative energies of frontier orbitals and excited-state relaxation dynamics [34]. We recently reported the synthesis of [Ir(dphoxd) 2 (bpy)]PF 6 (1) and [Ir(dph-oxd) 2 (pzpy)]PF 6 (2) (their molecular structures are shown in Scheme 1), with the cyclometalating ligand of 2,5-diphenyl-1,3,4oxadiazole (dph-oxd) and the ancillary ligands of 2,2 ′ -bipyridine (bpy) and 2-(1H-pyrazol-1-yl)pyridine (pzpy), respectively [10]. Due to the stabilization of HOMO levels induced by the electron-deficient oxadiazole heterocycle in dph-oxd, complexes 1 and 2 have wide energy gaps (∼2.8−3.5 eV) and efficient blue-green (480−580 nm) phosphorescent emissions [10,34].…”

“…We recently reported the synthesis of [Ir(dphoxd) 2 (bpy)]PF 6 (1) and [Ir(dph-oxd) 2 (pzpy)]PF 6 (2) (their molecular structures are shown in Scheme 1), with the cyclometalating ligand of 2,5-diphenyl-1,3,4oxadiazole (dph-oxd) and the ancillary ligands of 2,2 ′ -bipyridine (bpy) and 2-(1H-pyrazol-1-yl)pyridine (pzpy), respectively [10]. Due to the stabilization of HOMO levels induced by the electron-deficient oxadiazole heterocycle in dph-oxd, complexes 1 and 2 have wide energy gaps (∼2.8−3.5 eV) and efficient blue-green (480−580 nm) phosphorescent emissions [10,34]. In addition, complex 2 in which the electron-rich pzpy substitutes the bpy, presents a largely enhanced LUMO level relative to complex 1.…”