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Two blue‐emitting cationic iridium complexes with 2‐(1H‐pyrazol‐1‐yl)pyridine (pzpy) as the ancillary ligands, namely, [Ir(ppy)2(pzpy)]PF6 and [Ir(dfppy)2(pzpy)]PF6 (ppy is 2‐phenylpyridine, dfppy is 2‐(2,4‐difluorophenyl) pyridine, and PF6− is hexafluorophosphate), have been prepared, and their photophysical and electrochemical properties have been investigated. In CH3CN solutions, [Ir(ppy)2(pzpy)]PF6 emits blue‐green light (475 nm), which is blue‐shifted by more than 100 nm with respect to the typical cationic iridium complex [Ir(ppy)2(dtb‐bpy)]PF6 (dtb‐bpy is 4,4′‐di‐tert‐butyl‐2,2′‐bipyridine); [Ir(dfppy)2(pzpy)]PF6 with fluorine‐substituted cyclometalated ligands shows further blue‐shifted light emission (451 nm). Quantum chemical calculations reveal that the emissions are mainly from the ligand‐centered 3π–π* states of the cyclometalated ligands (ppy or dfppy). Light‐emitting electrochemical cells (LECs) based on [Ir(ppy)2(pzpy)]PF6 gave green‐blue electroluminescence (486 nm) and had a relatively high efficiency of 4.3 cd A−1 when an ionic liquid 1‐butyl‐3‐methylimidazolium hexafluorophosphate was added into the light‐emitting layer. LECs based on [Ir(dfppy)2(pzpy)]PF6 gave blue electroluminescence (460 nm) with CIE (Commission Internationale de L'Eclairage) coordinates of (0.20, 0.28), which is the bluest light emission for iTMCs‐based LECs reported so far. Our work suggests that using diimine ancillary ligands involving electron‐donating nitrogen atoms (like pzpy) is an efficient strategy to turn the light emission of cationic iridium complexes to the blue region.

Solid-state light-emitting electrochemical cells (LECs) have aroused great interest in recent years as novel organic light-emitting devices and a promising lighting solution. LECs have many advantages over multilayered organic lighting-emitting diodes (OLEDs), such as single layer, solution process and air-stable cathodes. In recent years, ionic iridium complexes are widely used in LECs for their high efficiencies and tunable emission colours. In this feature article, we will review the proposed operation mechanisms of LECs, the development of ionic iridium complexes and the progress in improving colour, efficiency, stability and response time of the LECs. Finally, the prospects and remaining problems will be discussed.

Using imidazole‐type ancillary ligands, a new class of cationic iridium complexes (1–6) is prepared, and photophysical and electrochemical studies and theoretical calculations are performed. Compared with the widely used bpy (2,2′‐bipyridine)‐type ancillary ligands, imidazole‐type ancillary ligands can be prepared and modified with ease, and are capable of blueshifting the emission spectra of cationic iridium complexes. By tuning the conjugation length of the ancillary ligands, blue‐green to red emitting cationic iridium complexes are obtained. Single‐layer light‐emitting electrochemical cells (LECs) based on cationic iridium complexes show blue‐green to red electroluminescence. High efficiencies of 8.4, 18.6, and 13.2 cd A−1 are achieved for the blue‐green‐emitting, yellow‐emitting, and orange‐emitting devices, respectively. By doping the red‐emitting complex into the blue‐green LEC, white LECs are realized, which give warm‐white light with Commission Internationale de L'Eclairage (CIE) coordinates of (0.42, 0.44) and color‐rendering indexes (CRI) of up to 81. The peak external quantum efficiency, current efficiency, and power efficiency of the white LECs reach 5.2%, 11.2 cd A−1, and 10 lm W−1, respectively, which are the highest for white LECs reported so far, and indicate the great potential for the use of these cationic iridium complexes in white LECs.

Organic light-emitting diodes (OLEDs) have attracted great attention because of their potential applications in full-color displays and solid-state lights. In the continual effort to search for ideal materials for OLEDs, small molecules with bipolar transporting character are extremely attractive as they offer the possibility to achieve efficient and stable OLEDs even in a simple single-layer device. In this Research News, we review the two design strategies of bipolar materials for OLEDs: molecules with or without donor-acceptor structures. The correlation between the experimental results and theoretical calculations of some of the materials is also discussed.