Rational Design of Coumarin-Based Hybridized Local and Charge-Transfer Blue Emitters for Solution-Processed Organic Light-Emitting Diodes.

Hybridized local and charge-transfer (HLCT) with the utilization of both singlet and triplet excitons through the "hot excitons" channel have great application potential in highly efficient blue organic light-emitting diodes (OLEDs). The proportion of charge-transfer (CT) and locally excited (LE) components in the relevant singlet and triplet states makes a big difference for the high-lying reverse intersystem crossing process. Herein, three novel donor (D)-acceptor (A) type HLCT materials, 7-([1,1'-biphenyl]-4-yl(9,9-dimethyl-9H-fluoren-2-yl)amino)-3-phenyl-1H-isochromen-1-one (pPh-7P), 7-([1,1'-biphenyl]-4-yl(9,9-dimethyl-9H-fluoren-2-yl)amino)-3-methyl-1H-isochromen-1-one (pPh-7M), and 6-([1,1'-biphenyl]-4-yl(9,9-dimethyl-9H-fluoren-2-yl)amino)-3-methyl-1H-isochromen-1-one (pPh-6M), were rationally designed and synthesized with diphenylamine derivative as donor and oxygen heterocyclic coumarin moiety as acceptors. The proportions of CT and LE components were fine controlled by changing the connection site of diphenylamine derivative at C6/C7-position and the substituent at C3-position of coumarin moiety. The HLCT characteristics of pPh-7P, pPh-7M, and pPh-6M were systematically demonstrated through photophysical properties and density functional theory calculations. The solution-processed doped OLEDs based on pPh-6M exhibited deep-blue electroluminescence with the maximum emission wavelength of 446 nm, maximum luminance of 8755 cd m-2, maximum current efficiency of 5.83 cd A-1, and maximum external quantum efficiency of 6.54 %. The results reveal that pPh-6M with dominant 1LE and 3CT components has better OLED performance.

Hybrid Local and Charge Transfer Emitters Utilizing Hyperconjugation Effect Towards Solution-Processed Ultra-Deep-Blue OLEDs with External Quantum Efficiency Approaching 12 .

Hybrid local and charge transfer (HLCT) excited state materials, which possess weak donor-acceptor (D-A) pure organic structures, deserve one of the most promising efficient and stable blue emitters. Through high-lying reverse intersystem crossing (hRISC) process, 75 % triplet excitons generated by electrical excitation could be harvested and utilized in organic light-emitting diodes (OLEDs). However, there are still significant challenges to achieve high-efficiency ultra-deep-blue HLCT emitters with low Commission Internationale de l'Eclairage (CIE) 1931 chromaticity coordinate y values. Here, a series of novel blue HLCT emitters based on spiro[1,8-diazafluorene-9,2'-imidazole] structure were designed and synthesized by fine-tuning the spiro[fluorene-9,2'-imidazole] core structure in our previous work through heteroatom substitution and hyperconjugation effect. The target emitters were endowed with excellent photophysical and electrochemical merits, thermal stability and solution processibility. The solution-processed OLED based on 4',5'-bis(4-(9H-carbazol-9-yl)phenyl)spiro[1,8-diazafluorene-9,2'-imidazole] (NFIP-CZ) achieved efficient ultra-deep-blue emission (CIEx,y=0.1581, 0.0422) with the maximum external quantum efficiency (EQEmax), maximum current efficiency (CEmax) and maximum power efficiency (PEmax) of 11.94 %, 4.07 cd ⋅ A-1 and 2.56 lm ⋅ W-1. The record EQE is a breakthrough in both solution-processed and vacuum vapor deposition ultra-deep-blue HLCT OLEDs currently.

A Breakthrough in Solution-Processed Ultra-Deep-Blue HLCT OLEDs: A Record External Quantum Efficiency Exceeding 10% Based on Novel V-Shaped Emitters.

It is always a great challenge to achieve high-efficiency solution-processed ultra-deep-blue organic light-emitting diodes (OLEDs) with the Commission Internationale de l'Eclairage (CIE) 1931 chromaticity coordinates matching the blue primary of Rec. International Telecommunication Union-Radiocommunication BT.2100, which specifies high dynamic range television image parameters. Inspired by hybrid local and charge transfer (HLCT) excited state emitters improving exciton utilization through high-lying reverse intersystem crossing, here, a series of high-performance blue emitters by a V-shaped symmetric donor (D)-π-acceptor (A)-π-D design strategy are developed. Here, the large torsions and unstable bonds of D-A structures can be improved through π bridges, and also the conjugation length and donor groups can be easily adjusted. The obtained emitters merit excellent photophysical and electrochemical properties, thermal stability, solution processibility, and HLCT excited state excellence. Results suggest that the OLEDs based on the obtained blue emitters all achieve high maximum external quantum efficiency (EQEmax ) of more than 8% with very low efficiency roll-off. In particular, the device based on 4',5'-bis(4-(9H-carbazol-9-yl)phenyl)spiro[fluorene-9,2'-imidazole] exhibits a satisfactory ultra-deep-blue emission (CIEx , y = 0.1579, 0.0387) and a record-high EQEmax (10.40%) among solution-processed HLCT OLEDs, which is very close to the record EQEmax of devices by vacuum vapor deposition technology.