An In-Depth Quantum Chemical Investigation of INVEST TADF Dyes

The design of organic emitters with inverted singlet-triplet (INVEST) gaps presents a promising route to enhance triplet harvesting in optoelectronic applications, yet reliable descriptors for predicting this phenomenon remain scarce. Here, we introduce a wave function-based descriptor, <i>Q</i>_<i>a</i>^<i>t</i>, derived from the transition density matrix, to quantify the short-range charge transfer (SRCT) character and transition charges localization on atomic-sites, critical for singlet-triplet inversion. Through a systematic computational study of N-triangulenes, extended π-systems, and nonalternant hydrocarbons, we demonstrate that <i>Q</i>_<i>a</i>^<i>t</i> correlates with negative singlet-triplet gap values computed at the SCS-CC2 level. The trend in the <i>Q</i>_<i>a</i>^<i>t</i> values is also found at the TDA-DFT level, offering a reliable and universal metric for inversion prediction and enabling efficient screening with a cost-effective procedure. Our findings underscore <i>Q</i>_<i>a</i>^<i>t</i> is a versatile tool for INVEST design, bypassing the need for high-level correlated methods while providing insights into the relationship between electronic structure and inversion. We further analyze the excited state decay kinetics of INVEST dyes by computing their fluorescence (<i>k</i>_f), intersystem crossing (<i>k</i>_ISC), and reverse intersystem crossing (<i>k</i>_RISC) rate constants, revealing that Herzberg-Teller effects dominate the spin conversion processes. For the investigated INVEST dyes, larger <i>k</i>_RISC values are systematically calculated with respect to the <i>k</i>_ISC values with several dyes exhibit <i>k</i>_RISC values exceeding 10⁶ s^-1, indicating promising potential for efficient delayed fluorescence. This work advances both the theoretical framework of excited-state dynamics in INVEST systems and the design of efficient organic emitters.