RESEARCH

Organic molecules have unlimited potential as functional materials due to the enormous diversity in chemical structures and molecular conformations. Yet, electronic and optical properties of organic photo functional molecules have significantly remained unexplored. Thermally activated delayed fluorescence (TADF), a new optoelectronic property, has recently opened an active organic electronics research area. TADF has unique capabilities to harvest triplet exciton through the reverse intersystem crossing (T1→S1) process depending on the singlet-triplet gap. It could directly affect their properties and performances, which is attractive for many low‐cost optoelectronic devices. Recent experimental reports have shown potential TADF applications for a set of closed-shell organic chromophores (cyclazine and heptazine derivative), where first excited singlet (S1) states are lower than first excited triplet (T1) states (ΔEST 0). Due to the violation of Hund's rule, it is thought that the triangular topology of these organic chromophores could undoubtedly play a role in their electronic structures. In the first paper (CPL, 2021, 779, 138827), I have reported a detailed analysis of this unusual electronic structure properties and inabilities to capture the inverted singlet-triplet gap by linear-response TDDFT method. Instead, DLPNO-STEOM-CCSD method can accurately capture this inverted singlet-triplet gap. From the analysis of the DLPNO-STEOM-CCSD results, we found that missing of doubles correlation in the DFT functional is the reason for failure. Hence, we further extended this work where we addressed the inclusion of doubles-corrected TDDFT with proper choice of double-hybrid functional could also capture the inverted singlet-triplet gap from LR-TDDFT.