The development process, molecular design principles, material systems, structure–property relationships and OLED applications of hot exciton materials are comprehensively summarized.

Excited state characters and components play a decisive role in photoluminescence (PL) and electroluminescence (EL) properties of organic light‐emitting materials (OLEDS). Charge‐transfer (CT) state is beneficial to enhance the singlet exciton utilizations in fluorescent OLEDs by an activated reverse intersystem crossing process, due to the minimized singlet and triplet energy splitting in CT excitons. However, the dominant CT component in the emissive state significantly reduces the PL efficiency in such materials. Here, the strategy is to carry out a fine excited state modulation, aiming to reach a golden combination of the high PL efficiency locally emissive (LE) component and the high exciton utilizing CT component in one excited state. As a result, a quasi‐equivalent hybridization of LE and CT components is obtained in the emissive state upon the addition of only an extra phenyl ring in the newly synthesized material 4‐[2‐(4′‐diphenylamino‐biphenyl‐4‐yl)‐phenanthro[9,10‐d]imidazol‐1‐yl]‐benzonitrile (TBPMCN), and the nondoped OLED of TBPMCN exhibited a record‐setting performance: a pure blue emission with a Commission Internationale de L'Eclairage coordinate of (0.16, 0.16), a high external quantum efficiency of 7.8%, and a high yield of singlet exciton of 97% without delayed fluorescence phenomenon. The excited state modulation could be a practical way to design low‐cost, high‐efficiency fluorescent OLED materials.

Blue emissions in organic light-emitting devices (OLEDs) are of great significance for their application in full color flat-panel displays and white lightings. [1] However, high-performance blue emitters are still relatively rare. In OLEDs, the injected electrons and holes recombine to form singlet and triplet excitons in the ratio of 1:3, according to the spin statistics, whereas only singlet exciton can decay radiatively in fluorescent materials. [2] Approximately 75% of the triplet excitons are wasted in nonradiative processes, leading to an upper limit of the internal quantum efficiency (IQE) of only 25% in conventional fluorescent devices. One of the methods to enhance the efficiency of OLEDs is to make use of the nonemissive triplet excitons. [3] Phosphorescent OLEDs (PhOLEDs) based on Ir, Pt, and Os organic-metal complexes can approach 100% IQE, which is attributed to the heavy-atom effect. [4] Yet, pure-blue and deep-blue phosphors with Commission Internationale de l'Eclairage (CIE) y values smaller than 0.15 are particularly scarce due to the inherently great challenge in their molecular design; similarly, proper host materials with a large band gap that allows for the refinement of the triplet excitons in devices are also rare. Therefore, it is important to find a way to develop efficient, stable, pure-and deep-blue fluorescent materials. In principle, new-generation, purely organic fluorescent materials can also utilize the nonemissive triplet excitons and achieve high efficiency by converting triplet excitons into singlet excitons. The main mechanisms involve triplet-triplet annihilation (TTA), thermally activated delayed fluorescence (TADF) and the "hot exciton" channel. [5] Essentially, both the TTA and TADF processes can promote the external quantum efficiency (EQE) of the devices by converting excitons from the lowest triplet excited state (T 1 ) to the lowest singlet excited state (S 1 ). Experimental results have confirmed that devices based on TTA and TADF materials can realize a high EQE with a breakthrough of the spin statistical limitation. [6] Although a high EQE has been obtained in TTA and TADF materials, pure-and deep-blue emitters with high efficiency and stability are still exiguous. Unlike TTA and TADF materials, the "hot exciton" materials reported by our group highlight the reverse intersystem crossing from Purely organic electroluminescent materials, such as thermally activated delayed fluorescent (TADF) and triplet-triplet annihilation (TTA) materials, basically harness triplet excitons from the lowest triplet excited state (T 1 ) to realize high efficiency. Here, a fluorescent material that can convert triplet excitons into singlet excitons from the high-lying excited state (T 2 ), referred to here as a "hot exciton" path, is reported. The energy levels of this compound are determined from the sensitization and nanosecond transient absorption spectroscopy measurements, i.e., small splitting energy between S 1 and T 2 and rather large T 2 -T 1 energy gap, which are expected to...

A series of new mixed-ligand copper(I) complexes [Cu(NN)-POP]BF 4 , where NN = 1,10-phenanthroline (phen; 1a), 2,9-dimethyl-phen (DMphen; 1b), 4,7-diphenyl-phen (DPpehn; 1c) and 2,2Ј-bipyridine (bpy; 2a), have been synthesized. Density functional theory (DFT) was applied to study the ground-and excited-state properties of these copper(I) complexes. The electronic structure variation is obtained by changing the substituted positions on the phenanthroline ligand. A time-dependent-DFT approach (TDDFT) was used to interpret the absorption and emission spectra in this system based on the optimized geometries at the B3LYP/ LANL2DZ and CIS/LANL2DZ levels of theory, respectively. The results show that the lowest-energy excitations of all