An Organic Emitter Displaying Dual Emissions and Efficient Delayed Fluorescence White OLEDs

Abstract

number of one-compound-based efficient white light photoluminescence (PL) have been reported. [3] The performance of the single-emitter-based WOLEDs should be improved. [4] As a consequence, highly efficient organic white electroluminescence (EL) based on one emissive compound is still a great challenge. A fundamental issue is to construct organic molecules that can form two kinds of electron-injection-induced excited states with blue and yellow emission characteristics. Furthermore, elucidating and understanding the mechanism of organic molecules with dual emission behaviors is also very important, which can promote to establish a molecular design strategy for this class of molecules. For ideal WOLEDs, the yellow emissive excited states should have thermally activated delayed fluorescence (TADF) [5] or phosphorescence [6] properties to harvest the electro-generated excited states and reach the highest internal quantum efficiency of 100%.In this contribution, we report an organic molecule PTZ-BP composed of phenothiazine (PTZ) as an electron donor and benzophenone (BP) as an electron acceptor (Scheme S1, Supporting Information). The model compound phenyl-phenothiazine (PhPTZ) was employed as reference in this study. PTZ-BP-based solid or solution displays dual emissive property and the luminescent spectra are constituted by a prompt fluorescence at around 440 nm and a delayed fluorescence at around 550 nm resulting in the generation of white light. The very simple molecular structure of PTZ-BP provides an ideal model for understanding the mechanism of organic molecules with dual emission feature. More importantly, PTZ-BP has been employed to fabricate efficient WOLEDs based on a single emitter. So far, a few white organic compounds with prompt and delayed fluorescence have been reported. [3a] However, only PL was achieved and efficient EL remains a great challenge for this kind of system. The EL property, presented in this study, provides a promising strategy for developing high-performance WOLEDs based on pure organic luminescent materials.The molecular structures of PTZ-BP and PhPTZ are presented in Figure 1a. by nucleophile substitution reaction in high yield and fully characterized by 1 H and 13 C NMR, mass spectrometry, and elemental analysis (see the Supporting Information). Finally, the product was purified two times by vacuum sublimation approach. The obtained PTZ-BP sample displayed very high purity of 99.92%The design and synthesis of one-compound-based white light organic solids that can be employed to fabricate efficient organic white light-emitting devices (OLEDs) still remains a challenge. In this contribution, a classic organic donor-acceptor (D-A) type molecule (PTZ-BP) is presented, which is composed of phenothiazine (PTZ) as an electron donor and benzophenone (BP) as an electron acceptor. Intriguingly, PTZ-BP displays dual emission and thermally activated delayed fluorescence properties. Its dual emission originates from locally excited state and intramolecular charge transfer excited stat...

Dual emission in purely organic materials for optoelectronic applications

“…The combination of blue and yellow emissions from a semi-crystalline film of 24 resulted in WOLEDs with a maximum EQE of 6.2% and CIE (0.34, 0.46). 11 The existence of two ground-state conformers, with different singlet-triplet energy gaps (1.14 and 0.18 eV) arising from the distortion of the phenothiazine unit, was confirmed by both theoretical and experimental results for the triazine derivative 25. Interestingly, both ax and eq excited state conformers of the PTZ unit exhibited CT emission.…”

“…10 In contrast to systems composed of several fluorescent molecules, single-molecular systems with white emission through DE have several advantages: they can avoid the drawbacks of phase separation, colour aging, and degradation; moreover, they lead to enhanced reproducibility and stability, as well as simpler device fabrication. 11,12 However, the design of purely organic molecules capable of blue/yellow DE suitable for efficient WOLEDs requires in-depth understanding of the different DE mechanisms. As an additional complication, 11 ideal WOLEDs should also harvest the electro-generated excited triplet states in order to reach the highest internal quantum efficiency of 100% by utilizing thermally activated delayed fluorescence (TADF) 13 or phosphorescence (PH) 14 most commonly for the yellow-emitting excited state.…”

“…11,12 However, the design of purely organic molecules capable of blue/yellow DE suitable for efficient WOLEDs requires in-depth understanding of the different DE mechanisms. As an additional complication, 11 ideal WOLEDs should also harvest the electro-generated excited triplet states in order to reach the highest internal quantum efficiency of 100% by utilizing thermally activated delayed fluorescence (TADF) 13 or phosphorescence (PH) 14 most commonly for the yellow-emitting excited state.…”

Recent progress of sulphur-containing high-efficiency organic light-emitting diodes (OLEDs)

“…29) consisting of a PTZ donor and a BP acceptor. 100 The PL spectra of PTZ-BP in diluted toluene solution at room temperature and low temperature showed two maximum emission values at 440 nm and 550 nm, respectively, and the emission at 550 nm showed typical TADF characteristics. The X-ray structure of the single crystal and PL spectra showed that the two emission peaks were derived from the H-extra and H-intra conformations of PTZ-BP, respectively.…”

2-Hydroxybenzophenone Derivatives: ESIPT Fluorophores Based on Switchable Intramolecular Hydrogen Bonds and Excitation Energy–Dependent Emission

“…To achieve dual emission, a series of strategies have been reported, such as by forming excimers ( Lee et al, 2014 ; Chen et al, 2016 ), combining intra- and intermolecular charge transfer ( Li et al, 2019 ; Wen et al, 2020 ), and constructing supramolecular self-assembles ( Pal et al, 2017 ; Li et al, 2018 ; Wang et al, 2020 ) and excited-state intramolecular proton transfer (ESIPT) emission ( Sakai et al, 2016 ; He et al, 2018 ; Li et al, 2018 ; Sedgwick et al, 2018 ; Suzuki et al, 2018 ; Wu et al, 2018 ). As a good approach that can rationally tune the dual-emission property, ESIPT involves a rapid photoinduced tautomerization of a molecule in its electronic excited state, in which the protons are prone to transfer to adjacent heteroatoms through intramolecular hydrogen bonding forming excited-state isomers, i.e., from the enol (E*) to keto (K*) formula ( Figure 1A ).…”