Visualization of Frontier Molecular Orbital Separation of a Single Thermally Activated Delayed Fluorescence Emitter by STM

Abstract

Because the spatial distribution of frontier molecular orbitals (FMOs) regulates the thermally activated delayed fluorescence (TADF) property, researchers synthesize TADF emitters by designing their FMO distribution. However, it remains challenging to clarify how the FMO distribution is altered at molecular interfaces. Thus, visualizing the FMOs at molecular interfaces helps us to understand the working behavior of TADF emitters. Using scanning tunneling microscopy (STM), we investigated the electronic structure of a single TADF emitter, hexamethylazatriangulene-triazine, at molecule–metal and molecule–insulating film interfaces. FMOs at the molecule–metal interface were not spatially confined to the donor–acceptor moieties because of hybridization. Meanwhile, FMOs at the molecule–insulator interface exhibited spatially separated filled and empty states confined to each moiety, similar to the calculated gas-phase FMOs. These observations illustrate that the molecule–environment interaction alters the spatial distribution of FMOs, proving that STM is a powerful tool for studying TADF molecules.

Carbazole-substituted benzobisoxazoles: near-UV fluorescent emitters and ambipolar hosts for organic light-emitting diodes

“…8 This is typically accomplished by spatially separating the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) within a flurophore. 9…”

Design of High-Performance Thermally Activated Delayed Fluorescence Emitters Containing s-Triazine and s-Heptazine with Molecular Orbital Visualization by STM

“…This technique also allows one to probe the dependence of a molecule’s electronic structure on its local environment, for example, by depositing it on different substrate materials (e.g., metals vs. dielectric thin films), and investigating the influence of the local heterogeneities . This provides powerful information about the working behavior of TADF materials at organic–metal or organic–dielectric interfaces, where the molecules are likely to be found in a device. ,− To maximize resolution and simplify correlation between a molecule’s atomic and electronic structures, this technique is best suited to highly planar materials that lie flat on the substrate to maximize the surface area of the molecule accessible to the tip and also minimize the travel distance of the STM tip. With strong evidence for the power of STM to inform material design, we sought to employ FMO visualization in the design of high-performance de novo TADF materials.…”

“…Instead, computational modeling is typically used to simulate their spatial distributions, and ensemble measurements such as cyclic voltammetry and UV–visible spectroscopy are used to infer FMO energy levels. While scanning tunneling microscopy (STM) provides a well-established means of visualizing FMOs, − this has only recently been applied to TADF materials . This technique also allows one to probe the dependence of a molecule’s electronic structure on its local environment, for example, by depositing it on different substrate materials (e.g., metals vs. dielectric thin films), and investigating the influence of the local heterogeneities .…”

“…While scanning tunneling microscopy (STM) provides a well-established means of visualizing FMOs, 16−19 this has only recently been applied to TADF materials. 20 This technique also allows one to probe the dependence of a molecule's electronic structure on its local environment, for example, by depositing it on different substrate materials (e.g., metals vs. dielectric thin films), and investigating the influence of the local heterogeneities. 21 This provides powerful information about the working behavior of TADF materials at organic−metal or organic−dielectric interfaces, where the molecules are likely to be found in a device.…”

Uncovering the Mechanism of Thermally Activated Delayed Fluorescence in Coplanar Emitters Using Potential Energy Surface Analysis

“…Intramolecular hydrogen bonding is reported to occur between the donor and acceptor moieties of HMAT-TRZ, forcing planarity and imposing rigidity into its structure. 22,23 Following this discovery, there have been several reports of hydrogen bond induced coplanar emitters, 24,25 some of which appear to exhibit TADF despite having a large ΔE ST . 26−28 This class of TADF emitters has several advantages, including improved photostability, enhanced photoluminescence quantum yield (PLQY), and high color purity as a result of their rigid structures.…”