A comprehensive review of tuneable polypyridine complexes as the emissive components of OLED and LEC devices is presented, with a view to bridging the gap between molecular design and commercialization.

The development of near‐infrared (NIR) luminescent materials has emerged as a promising research field with important applications in solid‐state lighting (SSL), night‐vision‐readable displays, and the telecommunication industry. Over the past two decades, remarkable advances in the development of light‐emitting electrochemical cells (LECs) have stunned the SSL community, which has in turn driven the quest for new classes of stable, more efficient NIR emissive molecules. In this review, an overview of the state of the art in the field of near‐infrared light‐emitting electrochemical cells (NIR‐LEC) is provided based on three families of emissive compounds developed over the past 25 years: i) transition metal complexes, ii) ionic polymers, and iii) host–guest materials. In this context, ionic and conductive emitters are particularly attractive since their emission can be tuned via molecular design, which involves varying the chemical nature and substitution pattern of their ancillary ligands. Herein, the challenges and current limitations of the latter approach are highlighted, particularly with respect to developing NIR‐LECs with high external quantum efficiencies. Finally, useful guidelines for the discovery of new, efficient emitters for tailored NIR‐LEC applications are presented, together with an outlook towards the design of new NIR‐SSL materials.

Light-emitting electrochemical cells (LECs) based on ionic transition metal complexes (iTMCs) represent a cost-effective solidstate lighting technology compatible with large-area and industrial-scale manufacturing. To improve the current LEC performance and compete with rivaling light-emitting diode (LED) devices, it is pivotal to design efficient iTMCs/counterion couples that combine high photoluminescence efficiency with optimized ionic and electron carrier transport. Despite the continuous proposal of novel iTMCs, the investigated counterions are typically limited to the traditional ones, including PF 6 − and BF 4 − . In this work, we introduce both rigid and flexible LEC architectures based on a novel single active layer of [Ir-(ppy) 2 (phtz)] − [Et 3 NH] + + X (ppy = 2-phenylpyridine, phtz = 5-phenyl-1H-tetrazole and X = lithium bis(trifluoromethane)sulfoneimide (LiTFSI), tetrabutylammonium perchlorate (TBAP), or sodium perchlorate (NaClO 4)) sandwiched between a FTOcoated glass or ITO-coated polyethylene terephthalate (PET) anode and Ga:In cathode. Our new Ir-cyclometaled complex with a tetrazole ligand, without salt additives or polymers, shows a bright green electroluminescence emission at 508 nm. The LECs based on the synthesized iTMC and TBAP additive show a current efficiency as high as 1.44 cd/A, a luminance of 503.82 cd/m 2 , and an external quantum efficiency of 1.73% at 3.7 V. By using a dual salt additive made of TBAP:LiTFSI (1:1), the LECs further improve the performance of the single salt-based devices, exhibiting a current efficiency of 1.72 cd/A, a luminance of 603.14 cd/m 2 , and an external quantum efficiency of 2.06% at 3.6 V. Such improvement of the LEC performance is attributed to the combination of the TBAP anion−iTMC cation size matching and the peculiar electrical properties of the LiTFSI-based solid electrolytes (i.e., high TFSI − mobility), leading to a compact space charge region near the electrodes and low turn-on voltage, respectively.

The color-tuning strategies of solid-state light-emitting devices (ss-LEDs) are mainly focused on engineering molecular structures. In this paper, for the first time, we developed a facile strategy for tuning the electroluminescence (EL) color from orange to green through the addition of the ionic additive TBAP (tetrabutylammonium perchlorate). To achieve the active ionic emissive compound for use in a light-emitting electrochemical cell (LEC), the neutral biscyclometalated bromo tetrazole iridium(III) [Ir(ppy) 2 (BrTz)] was exchanged to its cationic complex, [Ir(ppy) 2 (BrTz-Me)]ClO 4 (ppy = 2-phenyl pyridine, BrTz = 4-bromo-2-pyridine tetrazole, BrTz-Me = 4-bromo-2pyridine methyl tetrazole) with a new synthetic strategy. This method allows employing neutral Ir-cyclometalated complexes, which are ruled out for use in LECs because of their non-ionic behaviors. In the following, an LEC based on the new cationic [Ir(ppy) 2 (BrTz-Me)]ClO 4 as the emissive layer was fabricated between the FTO (fluorine-doped tin oxide) anode and Ga:In alloy cathode without using any additive or polymers, which makes this configuration the simplest ss-LED so far. By adding the ionic additives, the electroluminescence characteristics of [Ir(ppy) 2 (BrTz-Me)]ClO 4 were dramatically increased, including luminance (L) from 162.8 cd/m 2 for the device with an additive to 212.9 and 355.9 cd/m 2 for devices containing LiTFSI (bis(trifluoromethane)sulfonamide lithium salt) and TBAP, respectively. In particular, when TBAP was added to the [Ir(ppy) 2 (BrTz-Me)]ClO 4 complex, the irradiance was significantly increased from 166.4 to 220.8 μW/cm 2 with an efficacy of 1.78 cd/A and external quantum efficiency (EQE) value of 2.14%. The obtained EL results clearly showed that adding TBAP and LiTFSI significantly improved the electroluminescence characteristics and tuned the electroluminescence color.

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