Design of High-Performance Thermally Activated Delayed Fluorescence Emitters Containing <i>s</i>-Triazine and <i>s</i>-Heptazine with Molecular Orbital Visualization by STM
Don M. Mayder, Christopher M. Tonge, Giang D. Nguyen, Ryoga Hojo, Nathan R. Paisley, Jiabin Yu, Gary Tom, Sarah A. Burke, Zachary M. Hudson
Abstract
Materials exhibiting thermally activated delayed fluorescence (TADF) are now key components of some of the most advanced organic light-emitting diodes, photocatalysts, and bioimaging probes. Designing a TADF emitter requires a precise understanding of its frontier molecular orbitals (FMOs), yet rarely are these orbitals visualized experimentally. Here, we use scanning tunneling microscopy on Ag(111) to probe the electronic structures of high-performance TADF materials with different orbital landscapes based on s-triazine and s-heptazine acceptors. These materials exhibit room-temperature phosphorescence or thermally activated delayed fluorescence, deep-blue (452 nm) to red (615 nm) emission, near-unity photoluminescence quantum yields, exceptional photostability, and two-photon absorption cross sections as high as 2098 GM. Overall, this work demonstrates the potential of s-heptazines as optoelectronic materials, as well as the utility of direct FMO visualization in material design.