Two new orange–red thermally activated delayed fluorescence (TADF) materials, PzTDBA and PzDBA, are reported. These materials are designed based on the acceptor–donor–acceptor (A–D–A) configuration, containing rigid boron acceptors and dihydrophenazine donor moieties. These materials exhibit a small ΔEST of 0.05–0.06 eV, photoluminescence quantum yield (PLQY) as high as near unity, and short delayed exciton lifetime (τd) of less than 2.63 µs in 5 wt% doped film. Further, these materials show a high reverse intersystem crossing rate (krisc) on the order of 106 s−1. The TADF devices fabricated with 5 wt% PzTDBA and PzDBA as emitting dopants show maximum EQE of 30.3% and 21.8% with extremely low roll‐off of 3.6% and 3.2% at 1000 cd m−2 and electroluminescence (EL) maxima at 576 nm and 595 nm, respectively. The low roll‐off character of these materials is analyzed by using a roll‐off model and the exciton annihilation quenching rates are found to be suppressed by the fast krisc and short delayed exciton lifetime. These devices show operating device lifetimes (LT50) of 159 and 193 h at 1000 cd m−2 for PzTDBA and PzDBA, respectively. The high efficiency and low roll‐off of these materials are attributed to the good electronic properties originatng from the A–D–A molecular configuration.
Bifunctional star-burst amorphous molecular materials displaying both efficient solid-state luminescence and high hole-transport properties are developed in this study. A high external electroluminescence quantum efficiency up to 5.9% is attained in OLEDs employing the developed amorphous materials. It is revealed that the spontaneous horizontal orientation of these light-emitting molecules in their molecular-condensed states leads to a remarkable enhancement of the electroluminescence efficiencies and carrier-transport properties.
Functional organic materials that display reversible changes in fluorescence in response to external stimuli are of immense interest owing to their potential applications in sensors, probes, and security links. While earlier studies mainly focused on changes in photoluminescence (PL) color in response to external stimuli, stimuli-responsive electroluminescence (EL) has not yet been explored for color-tunable emitters in organic light-emitting diodes (OLEDs). Here a stimuli-responsive fluorophoric molecular system is reported that is capable of switching its emission color between green and orange in the solid state upon grinding, heating, and exposure to chemical vapor. A mechanistic study combining X-ray diffraction analysis and quantum chemical calculations reveals that the tunable green/orange emissions originate from the fluorophore's alternating excited-state conformers formed in the crystalline and amorphous phases. By taking advantage of this stimuli-responsive fluorescence behavior, two-color emissive OLEDs were produced using the same fluorophore in different solid phases.
A series of green-emitting fluorophores based on a tetra-azaacene core is synthesized by introducing nitrile substituents at different positions. Their molecular structure−optical property relationship [i.e., vibronic transitions in photoluminescence (PL) and electroluminescence (EL) spectra] is investigated to obtain a sharp emission where the vibronic peak ν 0−0 should be intensified by suppressing ν 0−n (n = 1, 2, 3...) transitions. The intensity ratios (I 0−1 /I 0−0 ) of the ν 0−1 and ν 0−0 vibronic transitions in the PL spectra of DBBNP, DBBNP2CN1, and DBBNP2CN2 in hexane are 1.13, 0.80, and 0.67, respectively. Theoretical calculations explain that the CN substitution at positions 8 and 13 in DBBNP2CN2 induces a uniform charge distribution and reduces the Huang−Rhys factors (HRFs) of the vibrational normal modes coupled to the electronic transition. The organic light-emitting diode (OLED) fabricated with DBBNP2CN2 shows a narrower green EL emission at 518 nm with a smaller bandwidth (50 nm) than those of devices adopting DBBNP or DBBNP2CN1. The careful modification of the molecular structures and positions of substituents enables us to reduce the HRFs of vibrations to achieve a narrow emission bandwidth with decreased I 0−1 /I 0−0 , which suggests a design strategy to develop narrowband organic fluorophores to improve the color purity for wide-gamut OLED displays.
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