Isobenzofuranone- and Chromone-Based Blue Delayed Fluorescence Emitters with Low Efficiency Roll-Off in Organic Light-Emitting Diodes

Significant efforts have been devoted to the development of novel efficient blue-emitting molecules for organic light-emitting diode (OLED) applications. Blue organic emitters exhibiting thermally activated delayed fluorescence (TADF) have the potential to achieve ∼100% internal electroluminescence quantum efficiency in OLEDs. In this paper, we report a promising molecular design strategy for obtaining appropriate high singlet and triplet excited energies, short exciton lifetimes, and high quantum efficiencies in blue TADF emitters. We introduce isobenzofuranone and chromone containing a cyclic ketone or lactone moiety as effective acceptor building units to construct donor–acceptor TADF emitters. Owing to their small singlet–triplet energy splitting, properly contracted π-conjugation, and weakened intramolecular charge-transfer character, these new emitters display strong blue TADF emissions with high photoluminescence quantum yields (53–92%) and notably short TADF emission lifetimes (2.8–4.3 μs) in thin films. Blue TADF-OLEDs utilizing these emitters exhibit external electroluminescence quantum efficiencies of up to 16.2% and extremely low efficiency roll-off even at practical high luminance. The current findings open new avenues for designing practically usable high-performance blue TADF emitters with simple molecular structures

Spin-Dependent Exciton Funneling to a Dendritic Fluorophore Mediated by a Thermally Activated Delayed Fluorescence Material as an Exciton-Harvesting Host

Thermally activated delayed fluorescence (TADF) materials generate energetically equivalent spin-singlet and spin-triplet excited states. In the presence of an energy acceptor, each excited state undergoes energy transfer on different length scales. However, the lack of quantitative understanding of the length dependence of the excited energy-transfer processes hampers the rational design of molecular systems that control exciton transport in organic light-emitting diodes (OLEDs) using TADF. We herein utilize a dendritic fluorophore G1, which consists of an anthracene-based fluorescent core encapsulated by four insulating tris­(4-<i>tert</i>-butylphenyl)­methyl groups as an energy acceptor. By combining transient photoluminescence measurements and kinetic modeling, we demonstrate the spin-dependent energy transfer in a binary host–guest system composed of a TADF material as the exciton-harvesting host and G1 as the guest fluorophore. The encapsulated structure with the dendritic shell effectively inhibits triplet excitons on the TADF host from funneling to the fluorescent core, thus allowing efficient reverse intersystem crossing and singlet energy transfer. The utilization of G1 in solution-processed OLEDs leads to a maximum external electroluminescence quantum efficiency as high as 5.2%, which is equivalent to an enhancement by a factor of 1.6 over the corresponding nondendritic fluorophore

Tunable Full-Color Electroluminescence from All-Organic Optical Upconversion Devices by Near-Infrared Sensing

Full-color all-organic optical upconversion devices that can directly convert incident near-infrared (NIR) light into tunable visible light were developed by integrating an organic light-emitting diode (OLED) on an NIR-sensitizing organic photodetector. Thermally activated delayed fluorescence (TADF) emitters were utilized for the first time in the upconversion devices for achieving high electroluminescence (EL) efficiency in the OLED unit and high overall upconversion efficiency. The emission color of upconversion EL can be varied across the entire visible region ranging from blue to red and white by judicious selection of the incorporating TADF emitters. These all-organic optical upconversion devices have a great potential for low-cost, large-area, pixelless NIR imaging applications

Possibilities and Limitations in Monomer Combinations for Ternary Two-Dimensional Covalent Organic Frameworks

The diversity and complexity of covalent organic frameworks (COFs) can be largely increased by incorporating multiple types of monomers with different topologies or sizes. However, an increase in the number of monomer types significantly complicates the COF formation process. Accordingly, much remains unclear regarding the viability of monomer combinations for ternary or higher-arity COFs. Herein, we show that, through an extensive examination of 12 two-nodes-one-linker ([2 + 1]) combinations, monomer-set viability is determined primarily by the conformational strain originating from disordered monomer arrangements, rather than other factors such as the difference in COF formation kinetics between monomers. When monomers cannot accommodate the strain associated with the formation of a locally disordered, yet crystalline framework, the corresponding [2 + 1] condensation yields a mixture of different COFs or an amorphous polymer. We also demonstrate that a node–linker pair that does not form a binary COF can be integrated to generate a single-phase framework upon addition of a small amount of the third component. These results will clarify the factors behind the successful formation of multicomponent COFs and refine their design by enabling accurate differentiation between allowed and disallowed monomer combinations