Shorter Exciton Lifetimes via an External Heavy-Atom Effect: Alleviating the Effects of Bimolecular Processes in Organic Light-Emitting Diodes

Multiexcited‐state phenomena are believed to be the root cause of two exigent challenges in organic light‐emitting diodes; namely, efficiency roll‐off and degradation. The development of novel strategies to reduce exciton densities under heavy load is therefore highly desirable. Here, it is shown that triplet exciton lifetimes of thermally activated delayed‐fluorescence‐emitter molecules can be manipulated in the solid state by exploiting intermolecular interactions. The external heavy‐atom effect of brominated host molecules leads to increased spin–orbit coupling, which in turn enhances intersystem crossing rates in the guest molecule. Wave function overlap between the host and the guest is confirmed by combined molecular dynamics and density functional theory calculations. Shorter triplet exciton lifetimes are observed, while high photoluminescence quantum yields and essentially unaltered emission spectra are maintained. A change in the intersystem crossing rate ratio due to increased dielectric constants leads to almost 50% lower triplet exciton densities in the emissive layer in the steady state and results in an improved onset of the photoluminescence quantum yield roll‐off at high excitation densities. Efficient organic light‐emitting diodes with better roll‐off behavior based on these novel hosts are fabricated, demonstrating the suitability of this concept for real‐world applications.United States. Department of Energy (Grant DE‐FG02‐07ER46474

Donor-Acceptor Iptycenes with Thermally Activated Delayed Fluorescence

A new donor–acceptor iptycene containing carbazole donors and a thiadiazoloquinoxaline acceptor was synthesized and its photo‐ and electrochemical properties evaluated. The key intermediate 1 allows a lateral modification through cross‐coupling, and the (triisopropylsilyl)acetylene product 2 exhibits bright yellow fluorescence with emission lifetimes of 2.42 µs in deoxygenated hexane. The long lifetime and high quantum efficiency (73 %) is quenched by O₂ and therefore attributed to thermally activated delayed fluorescence (TADF). This approach allows functionalization through cross‐coupling reactions and depicts a promising scaffold for the synthesis of TADF‐active molecules.Keywords: Functional organic materials; Fluorescence; Donor–acceptor systems; Iptycenes; Cross-couplin