Fused‐Nonacyclic Multi‐Resonance Delayed Fluorescence Emitter Based on Ladder‐Thiaborin Exhibiting Narrowband Sky‐Blue Emission with Accelerated Reverse Intersystem Crossing

Abstract

Developing organic luminophores with unique capability of strong narrowband emission is both crucial and challenging for the further advancement of organic lightemitting diodes (OLEDs). Herein, a nanographitic fusednonacyclic p-system (BSBS-N1), which was strategically embedded with multiple boron, nitrogen, and sulfur atoms, was developed as a new multi-resonance thermally activated delayed fluorescence (MR-TADF) emitter. Narrowband skyblue emission with a peak at 478 nm, full width at half maximum of 24 nm, and photoluminescence quantum yield of 89 % was obtained with BSBS-N1. Additionally, the spin-orbit coupling was enhanced by incorporating two sulfur atoms, thereby facilitating the spin-flipping process between the excited triplet and singlet states. OLEDs based on BSBS-N1 as a sky-blue MR-TADF emitter achieved a high maximum external electroluminescence quantum efficiency of 21.0 %, with improved efficiency roll-off.

Photophysical Properties of Boron‐Based Chromophores as Effective Moieties in TADF Devices: A Computational Study**

“…One of the main strategies which can be used to accelerate RISC rate is to provide a high density of close lying 3 CT states, which can be achieved through peripheral substitution [30] . Alternatively the inclusion of heavy main group atoms such as S or Se to the molecular skeleton can be considered [31] . Finally, the inclusion of heavy metals is a suitable strategy to increase SOC values and improve RISC rates in MR‐emitters, although this reduces the economic and ecological sustainability of the devices [32] …”

“…On the other hand, it has been reported that increasing the molecular rigidity and suppressing large excited state structural changes represents an efficient approach to achieve high color purity [31] . Therefore, design strategies focused on rigidity of the scaffold have been implemented.…”

“…[30] Alternatively the inclusion of heavy main group atoms such as S or Se to the molecular skeleton can be considered. [31] Finally, the inclusion of heavy metals is a suitable strategy to increase SOC values and improve RISC rates in MR-emitters, although this reduces the economic and ecological sustainability of the devices. [32] On the other hand, it has been reported that increasing the molecular rigidity and suppressing large excited state structural changes represents an efficient approach to achieve high color purity.…”

“…[32] On the other hand, it has been reported that increasing the molecular rigidity and suppressing large excited state structural changes represents an efficient approach to achieve high color purity. [31] Therefore, design strategies focused on rigidity of the scaffold have been implemented. As an alternative to fused π systems, ortho-substituted DÀ A structures that suppresses excited state structural changes can also be used to obtain devices with high color purity.…”

A Design Strategy for Multiple Resonance‐Induced Pure Violet Thermally Activated Delayed Fluorescence Emitters with a Narrow Emission Band

“…Some of the more important studies based on boron‐based core structures [ 2–19 ] suggested that the 5,9‐dioxa‐13bboranaphtho[3,2,1‐de]anthracene (DOBNA) is the one well‐known core for MR‐TADF emitters. It has a boron and oxygen‐based fused aromatic structure due to MR effect and rigid structural properties.…”