Excellence in Research: Structure and Dynamics of Exciplexes in Thermally Activated Delayed Fluorescence

In this project, jointly funded by the Office of Integrative Activities (OIA), the Chemical Structure, Dynamics, and Mechanisms-A (CSDM-A) program in the Division of Chemistry, and the Electronic and Photonic Materials (EPM) program in the Division of Materials Research, Seyhan Salman and her research group at Clark Atlanta University are using theoretical models and computational tools to investigate the electronic structure and optical properties of organic light-emitting diode (OLED) materials that exhibit thermally activated delayed fluorescence (TADF). The project seeks to significantly advance the understanding of excited-state complexes (exciplexes) that are responsible for TADF based on their electronic structures and optical properties, as well as the various transition processes associated with the excited complexes. Better understanding of the fundamental structure and dynamics of such species will help guide the development of efficient new materials for applications in optical and electronic devices, including OLEDs, solar cells, field-effect transistors, (bio)chemical sensors and storage devices. The project also provides students from groups that are underrepresented in science with a challenging research experience and professional training in computational materials chemistry that will help prepare them for the STEM (science, technology, engineering and mathematics) workforce. The main goal of this project is to elucidate the excited-state dynamics of thermally activated delayed fluorescence (TADF) in exciplex-based OLED materials. Such systems represent one of the most promising classes of materials for realizing highly efficient OLEDs using organic molecules and could lead to a significant reduction in fabrication costs associated with this technology. The electronic structure and properties of exciplexes are influenced by the intermolecular interactions in the exciplex state, which are markedly different than that in the ground state and are relatively weak, leading to various structural conformations. Further studies of exciplexes, including their electronic structures, properties, and dynamics of charge and energy transfer processes, are needed for their application in OLEDs. This project uses state-of-the-art computational approaches with the goal of quantifying microscopic parameters and establishing structure-property relationships in these materials. The research team uses an integrated approach combining all-atom molecular dynamics (AA-MD) simulations with density functional theory (DFT) calculations to study the effect of the solid matrix environment on the emissive properties of exciplexes. They are also examining the impact of energetic disorder (static and dynamic components) on the efficiency of radiative decay processes. The impacts of host-guest interactions and the dynamics of charge- and energy-transfer processes are also included in the modeling scheme in order to account for intersystem crossing and other radiative and non-radiative transitions. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.