Structure–Property Correlation in Luminescent Indolo[3,2-b]indole (IDID) Derivatives: Unraveling the Mechanism of High Efficiency Thermally Activated Delayed Fluorescence (TADF)

Abstract

A series of indolo[3,2-b]indole (IDID) derivatives are designed as a novel structural platform for thermally activated delayed fluorescence (TADF) emitters. Intramolecular charge transfer (ICT)-type molecules consisting of IDID donor (D) and various acceptor (A) moieties are synthesized and characterized in the protocol of the systematical structure-property correlation. IDID derivatives exhibit high efficiency, prompt fluorescence as well as TADF with emission ranges tuned by the chemical structure of the acceptor units. Interestingly, almost all of the IDID derivatives show an identical energy level of the lowest triplet excited state (T) attributed to the locally excited triplet state of the IDID backbone (LE), while that of their lowest singlet excited state (S) is largely tuned by varying the acceptor units. Thus, we demonstrate the underlying mechanism in terms of the molecular engineering. Among the compounds, Tria-phIDID and BP-phIDID generate efficient delayed fluorescence based on the small energy gap between the lowest singlet and triplet excited states (ΔE) and mediation of the LE state. Organic light-emitting diodes with these Tria-phIDID and BP-phIDID as a dopant in the emitting layer show highly efficient electroluminescence with maximum external quantum efficiencies of 20.8% and 13.9%, respectively.

Triggering Thermally Activated Delayed Fluorescence by Managing the Heteroatom in Donor Scaffolds: Intriguing Photophysical and Electroluminescence Properties

“…Intriguingly, in contrast to the fluorescence spectra, the phosphorescence spectrum of BFICNTrz is well resolved and shows a clear vibrational pattern for BFI, which indicates that its triplet state originates from the local excited state ( 3 LE). Conversely, BTICNTrz exhibited a rather broad and redshifted phosphorescence spectrum compared with the BTI donor, which indicated that its triplet state can be characterized as a charge‐transfer ( 3 CT) state . This difference in the origin of T 1 states can be explained by the nature of the heteroatom in the donor scaffold.…”

“…For example, Park et al. established detailed structure–activity relationships of indolo[3,2‐ b ]indole‐based TADF emitters with different acceptor units . They found that the nature of the acceptor unit significantly affected the excited‐state properties and consequently the Δ E ST value and device performance of the materials.…”

“…In this regard, the development of donor–acceptor (DA)‐type bipolar systems with or without π‐conjugation linkers have been demonstrated as the most promising approach to ensure spatially partitioned HOMO and LUMO orbitals on the donor and acceptor units, respectively . It has been demonstrated that the nature of donor and acceptor units plays a vital role in determining the functional properties of the materials . For example, Park et al.…”

“…[2,8] It has been demonstrated that the nature of donora nd acceptor units plays av ital role in determining the functional properties of the materials. [9][10][11][12] For example, Park et al established detailed structure-activity relationshipso fi ndolo [3,2-b]indolebased TADF emittersw ith different acceptor units. [10] They found that the nature of the acceptor unit significantly affected the excited-state properties and consequently the DE ST value and device performance of the materials.…”

“…[9][10][11][12] For example, Park et al established detailed structure-activity relationshipso fi ndolo [3,2-b]indolebased TADF emittersw ith different acceptor units. [10] They found that the nature of the acceptor unit significantly affected the excited-state properties and consequently the DE ST value and device performance of the materials. Similarly, Adachi et al reported as eries of carbazole donors with methyl substituents on peripheral nuclear positions to develop efficient deep-blue TADF emitters.…”