Neoteric Advances in Oxygen Bridged Triaryl Boron-based Delayed Fluorescent Materials for Organic Light Emitting Diodes

Understanding the Properties of Donor-Acceptor Substituted Boron Difluoride 3-Cyanoformazanate Dyes

π-Conjugated materials offer attractive traits including semiconductivity, low-energy light absorption/photoluminescence, and solution processability that render them ubiquitous within the organic electronics field. Among many strategies for property tuning, the creation of asymmetric electronic structures through the installation of donor and acceptor substituents commonly results in low-energy absorption/photoluminescence bands. Boron difluoride formazanate dyes are readily synthesized, can be asymmetrically substituted with donor and acceptor groups, and have unexpectedly low-energy absorption/photoluminescence bands that extend into the near-infrared. In this study, we prepared a series of donor-acceptor substituted boron difluoride 3-cyanoformazanate dyes and compared their properties to symmetric analogues. Our findings suggest that donor-acceptor derivatives are highly delocalized with properties intermediate of their symmetric counterparts. Furthermore, the data obtained suggest that the N-aryl substituents act as donors to the strongly accepting boron difluoride formazanate core, regardless of the functional groups appended to them. These properties were reproduced computationally, and while the frontier orbitals calculated for donor-acceptor dyes were modestly asymmetric, there was no evidence of charge-transfer character. This work provides significant insight into the unexpected properties of boron difluoride formazanates and reveals that their strongly accepting nature circumvents the predicted augmentation of electronic structure commonly observed for donor-accepter substituted dyes.

Facile Synthetic Access Towards Sulfur- and Selenium-Functionalized Boron-Based Multiresonance TADF Emitters

Thermally activated delayed fluorescence (TADF) materials with high photoluminescence quantum yields and a fast reverse intersystem crossing (RISC) are of the highest interest for organic light-emitting diodes (OLEDs). In the past decade, triaryl boranes with multiple resonance effect (MR) have captured significant attention. The efficiency of MR-TADF emitters strongly depends on small singlet-triplet energy gaps (ΔEST), but also on large reverse intersystem crossing (RISC) rate constants (kRISC). The latter effect has strongly been focused on very recently and has drawn attention to heavier elements, including sulfur and selenium, the large spin-orbit coupling (SOC) of which accelerates RISC effects. Within the context of MR-TADF emitters, the 5,9-X2-13b-boranaphtho [3,2,1-de]anthracene scaffold (X-B-X, X = donor heteroatom, e.g., N, O, S, Se) has been recognized as a promising narrowband-emissive TADF material. However, the incorporation of sulfur and selenium as highly SOC-inducing elements has proven to be difficult. Most synthetic strategies apply protocols initially suggested by Hatakeyama to obtain nitrogen- and oxygen-doped materials. We present an alternative route over the established methodology, which affords highly sought-after sulfur- and selenium-doped materials with a high yield and purity.