Does BODIPY Drive\nthe “Hot-Exciton”\nTADF Mechanism? A Detailed Computational Study

According to Kasha’s rule, high-lying excited\nstates (S<sub><i>n</i></sub> or T<sub><i>n</i></sub>, <i>n</i> > 1) usually have little effect on\nfluorescence or phosphorescence\nemissions. However, in certain molecular systems, these higher energy\nstates play a substantial role in determining photophysical properties,\nparticularly harnessing nonradiative triplet excitons in OLEDs. Hence,\nthe exploration of a new molecular design strategy for the development\nof novel “hot-exciton” materials remains a challenging\nyet controversial endeavor. Here, we present an innovative approach\nusing the BODIPY acceptor unit to develop new hot exciton materials,\nemploying density functional theory (DFT) and time-dependent DFT computational\nmethods. To achieve this, we substituted phenylcarbazole (P-CBZ) donor\nand phenylBODIPY (P-BODIPY) acceptor units with diverse configuration\ntriads on 7 nonconjugated and 9 conjugated cores and investigated\ntheir ground and excited state properties. Unexpectedly, molecules\nwith nonconjugated core units upconvert triplet excitons from T<sub>2</sub> to S<sub>1</sub> pathways, whereas molecules with conjugated\ncore units follow T<sub>3</sub> to S<sub>1</sub> state mechanisms\nin the unsymmetrical triad substitution. Moreover, the degenerate\ntriplet states T<sub>3</sub> to T<sub>10</sub>, combined with singlet\nstates S<sub>1</sub> to S<sub>4</sub>, revealed multiple hot exciton\nchannels, enabling the upconversion of nonradiative triplet excitons\nin the symmetrical substitution of P-BODIPY on nonconjugated cores.\nThis study unveils the regulatory mechanism of photophysical properties\ninfluenced by high-lying excited states, providing a substantial theoretical\nfoundation for modulating excited state energies in the future design\nof hot exciton-based emitters employing BODIPY acceptor units.