Phosphorescent Ionic Iridium(III) Complexes Displaying Counterion‐Dependent Emission Colors for Flexible Electrochromic Luminescence Device

Abstract

Phosphorescent transition-metal complexes, typically exhibiting various chargetransfer excited states, such as triplet metal-to-ligand charge transfer ( 3 MLCT), ligand-to-ligand charge transfer ( 3 LLCT), intraligand charge transfer ( 3 ILCT), and metal-to-ligand-ligand charge transfer ( 3 MLLCT) states, [33][34][35][36][37] are ideal candidates for stimuli-responsive luminescent materials. These complexes can show highly sensitive optical properties toward surrounding microenvironment or external stimuli. In the previous work, it was reported that the emission color of a cationic iridium(III) complex that contains a NH group in the N^N ligand can be varied by changing its counterion. [32] Additionally, the emission color of this complex can also be changed by applying a voltage, and an information recording device has been successfully constructed on the basis of this interesting ECL property. Nevertheless, there are still several critical drawbacks in this system, restricting its real applications: (i) the emission range tuned by the counterions or electric field is narrow; (ii) the fabricated information recording device consists of ionic liquid BMIM + PF 6 − and silica nanoparticles, and the consequent quasi-solid film device suffers from the low thermal stability and mechanical strength. Thus, it is of great importance to develop excellent stimuli-responsive luminescent materials with a wide wavelength tuning range, and electrolytes with high stability and mechanical strength to solve the existing problems.In this present study, a series of iridium(III) complexes [(CZPY) 2 IrNH] + X − (CZPY = 9-hexyl-3-pyridylcarbazole, NH = 2,2′-dibenzimidazole, X − = PF 6 − , I − , Br − , or Cl − ) that contain two NH units in their N^N ligand (NH) have been designed and synthesized to address the abovementioned limits. It is known that the formation of hydrogen bond increases the electron cloud density on the nitrogen atom of the NH moiety and thus elevates the lowest enoccupied molecular orbital (LUMO) energy level. [38] Compared with the complexes containing only one NH moiety in N^N ligand, the formation of two hydrogen bonds might further elevate the LUMO energy level and increase the energy gap between highest occupied molecular orbital (HOMO) and LUMO, thus widening the emission wavelength tuning range. The tuning range of emission wavelength for these complexes can be extended to nearly 100 nm, and [(CZPY) 2 IrNH] + PF 6 − (Ir1) exhibited an electric field induced Stimulus-responsive luminescent materials have drawn considerable interest because they are ideal candidates for optoelectronic applications. Herein, a series of phosphorescent iridium(III) complexes [(CZPY) 2 IrNH] + X − (X − = PF 6 − , I − , Br − , Cl − ) containing two NH moieties in the N^N ligand is reported. These complexes exhibit significantly counterion-dependent emission color change from green (493 nm) to orange (590 nm) in CH 3 CN. Besides, the emission color of [(CZPY) 2 IrNH] + PF 6 − is sensitive to the external electric field. Upon a...

Recent Progress in Sublimable Cationic Iridium(III) Complexes for Organic Light‐Emitting Diodes

“…Besides, deeper investigation is also imperative referring the underlying sublimation mechanism, for instance, how the cations and anions ultimately separate by heat, travel under vacuum and finally deposit onto the substrates during device preparation. Moreover, sublimable cationic iridium(III) complexes also show great promise in other organic semiconductors, since their solution‐processable analogues have been widely used in light‐emitting electrochemical cells (LEECs) and optical devices for data record, storage and security . Therefore we believe this paper will attract more research interest and stimulate greater progress in the future.…”

Electroluminochromic Materials: From Molecules to Polymers

“…Zhao et al recently studied the effects of counter anions on the emission colour tuning and switching properties of the Ir(III) complex containing 9-hexyl-3-pyridylcarbazole and 2,2′-dibenzimidazole ligands ( C8a – d ) [46]. Changing the counter-anion from Cl − to Br − , then I − and PF 6 − produced red-shifting of emission with wavelengths from 493 to 590 nm and a corresponding change in emission colour from green to orange, due to the extent of anion-ligand interactions.…”

“…The application of 5 V voltage in solution of C8a in acetonitrile led to the orange emission colour of solution around the cathode changing to green with a clear boundary observed. This is due to the ionic migration leading to non-homogenous distribution of cationic complex and PF 6 − counter-anions over the two electrodes, thus, the extent of hydrogen bond formation between 2,2′-dibenzimidazole ligands and PF 6 − which was translated to the charge cloud polarization of the ligand [46].…”

Electroluminochromic Materials and Devices Based on Metal Complexes

“…[45][46][47][48][49] Compared to neutralo nes, ionic Ir III complexesh ave unique advantages such as milder synthesis conditions and stable redox reversibility.M oreover,t hey also have varieties of charge-transfer excited states, including intraligand charget ransfer ( 3 ILCT), ligand-to-ligand charge transfer ( 3 LLCT), metal-to-ligand-ligand charge transfer ( 3 MLLCT) and triplet metal-to-ligand charge transfer ( 3 MLCT) states. [50][51][52][53][54][55] These excited states are sensitive to externale nvironment, which makes ionic Ir III complexess uitable for constructing an excellent class of electrical stimuli-responsivematerials.…”

“…In order to obtain aw ider emission range tuning by the electric stimulus and improve the performance of the ELC devices, an ionic Ir III complex Ir-6 ( Figure 2) with two NÀHm oieties in the N^N ligand was designed and synthesized. [55] Under electricals timulus, the formation of two hydrogen bondings can further promote the LUMO energy level and increase the E g value, thereby widening the tuning range of emission wavelength. The ELC properties of complex Ir-6 in acetonitrile solution were depicteda sf ollows.T he solution showed an orange emission color before the voltage was applied.…”