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, Qat, 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 Qat correlates with negative singlet-triplet gap values computed at the SCS-CC2 level. The trend in the Qat 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 Qat 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 (kf), intersystem crossing (kISC), and reverse intersystem crossing (kRISC) rate constants, revealing that Herzberg-Teller effects dominate the spin conversion processes. For the investigated INVEST dyes, larger kRISC values are systematically calculated with respect to the kISC values with several dyes exhibit kRISC 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.