Exploring the Theoretical Foundations of Thermally Activated Delayed Fluorescence (TADF) Emission: A Comprehensive TD‐DFT Study on Phenothiazine Systems

Abstract

This study conducts a thorough theoretical investigation of Thermally Activated Delayed Fluorescence (TADF) in phenothiazine‐based systems, examining ten molecular configurations recognized experimentally as TADF‐active. Employing Time‐Dependent Density Functional Theory (TD‐DFT), our analysis spans the investigation of singlet‐triplet energy gaps ($\Delta E_{ST}$), spin‐orbit coupling, and excitation characteristics using Multiwfn. This approach validates the adherence to El Sayed's rule across these systems but also provides a detailed understanding of charge transfer dynamics, as visualized through heat maps. A significant aspect of our study is the exploration of different oxidation states of sulfur and site substitutions on phenothiazine. This systematic variation aims to identify additional TADF‐active compounds, drawing parallels with properties characterizing other known TADF emitters. Our investigation into Reverse Intersystem Crossing (rISC) rates and the analysis of dihedral angles in relation to $\Delta E_{ST}$ values offer nuanced insights into the TADF behaviours of these molecules. By integrating rigorous computational analysis, we provide a foundational understanding that enhances the design and optimization of phenothiazine‐based materials for optoelectronic applications. This work not only advances our theoretical understanding of TADF in phenothiazine derivatives but also serves as a guide for experimentalists and industry professionals in the strategic design of new TADF materials.