Organic light-emitting diodes (OLEDs) are surging in use across multiple markets such as those of wearable electronics, smartphones, flat displays, and solid-state lighting. This is largely driven by their numerous promising advantages compared to conventional liquid crystal displays (LCDs), such as lower cost, greater flexibility, better picture quality and fast response time. Fabrication of highperformance OLEDs is paramount to the success of these applications. Fluorescent OLEDs can only convert 25% of singlet excitons into emission, with the loss of 75% triplet excitons as heat. In the past decade, thermally activated delayed fluorescence (TADF) materials have emerged as emitters for the third-generation OLED, able to harvest 100% of excitons to generate light, at comparable exciton utilization efficiencies to phosphorescent OLEDs. Generally, TADF emitters are based on a strongly twisted donor-acceptor (D-A) design motif to electronically separate the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) that results in a small energy gap between S 1 and T 1 states (DE S1T1 ). However, a relatively large nuclear reorganization and chargetransfer (CT) transition in the excited state unavoidably produce a broad emission band, reflected by a large full width at high maximum (FWHM) of around 100 nm. These undesired properties in D-A TADF emitters restrict their practical application in high-resolution displays where the color point requirements demanded by industry are difficult to obtain using materials showing broad emission.