Thermally activated delayed fluorescence from 3nπ* to 1nπ* up-conversion and its application to organic light-emitting diodes

Abstract

Intense nπ* fluorescence from a nitrogen-rich heterocyclic compound, 2,5,8-tris(4-fluoro-3-methylphenyl)-1,3,4,6,7,9,9b-heptaazaphenalene (HAP-3MF), is demonstrated. The overlap-forbidden nature of the nπ* transition and the higher energy of the 3ππ* state than the 3nπ* one lead to a small energy difference between the lowest singlet (S1) and triplet (T1) excited states of HAP-3MF. Green-emitting HAP-3MF has a moderate photoluminescence quantum yield of 0.26 in both toluene and doped film. However, an organic light-emitting diode containing HAP-3MF achieved a high external quantum efficiency of 6.0%, indicating that HAP-3MF harvests singlet excitons through a thermally activated T1 → S1 pathway in the electroluminescent process.

Fluorescence and physical properties of the organic salt 2-chloro-4-nitrobenzoate–3-ammonium-phenol

“…Stable organic molecules with high electroluminescence efficiency have been very attractive for fabricating OLED materials [29]. For materials possessing a narrow energy gap between its singlet and triplet excited states, ∆E ST , leads to reverse intersystem crossing, T 1 →S 1 wherein thermal energy is harvested from the surrounding [1][2][3][4][5][6]. Therefore, a molecular design with a narrow ∆E ST (≤ 100 meV) and a reasonable decay rate of ˃10 6 s -1 , leads to highly efficient TADF materials.…”

“…In addition, thermally activated delayed fluorescence (TADF) is a promising property that merits further development in the field of organic electronics and photonics [1]. One of the key approach to achieve TADF in organic materials is to reduce the spatial overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), which produces a narrow singlet to triplet (∆E ST ) energy gap suitable for reverse intersystem crossing (T 1 →S 1 ) [2][3][4][5][6]. Donor and acceptor interaction in a single molecule and ionic or electrostatic interaction between them in a salt can give rise to a narrow ∆E ST for molecular electronics.…”

“…To the best of our knowledge, the fluorescence properties of CNBA have not been investigated. HOMO-LUMO analysis can be used to study luminescence properties of molecules facilitating the design of materials for OLED applications [1][2][3][4][5][6][26][27][28]. Stable organic molecules with high electroluminescence efficiency have been very attractive for fabricating OLED materials [29].…”

A computational study on a series of phenanthrene and phenanthroline based potential organic photovoltaics

“…A molecule meets this requirement only when its lowest-energy transition has low singlet-triplet exchange energy [36]. Current trends in the research into novel TADF emitters are mainly focused on intramolecular donoracceptor (D-A)-type molecules, as in the present case [37]. Therefore, appropriate donor-acceptor units have to be selected carefully to obtain full-color TADF molecules, with their HOMO and LUMO being localized on different constituents.…”

Recent progress of green thermally activated delayed fluorescent emitters

“…Other than the TADF materials shown above, heptaazaphenalene-based 2,5,8-tris(4-fluoro-3-methylphenyl)-1,3,3a1,4,6,7,9-heptaazaphenalene (HAP-3MF) [75], phthalimide-based 5,6-bis(4-(9,9-dimethylacridin-10 (9H)-yl)phenyl)-2-methylisoindoline-1,3-dione (AcPI), and maleimide-based 3,4-bis(4 -(9,9-dimethylacridin-10(9H)-yl)-[1,1 -biphenyl]-4-yl)-1-methylpyrrolidine-2, 5-dione (AcMI) also emitted green light through a TADF emission process involving the triplet-to-singlet up-conversion channel [76]. The chemical structures, photophysical parameters, and device performances of the green TADF emitters based on acceptors, except for the CN-modified aromatics and triazine, are summarized in Tables 5 and 6.…”