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According to Kasha's rule, high-lying excited states (Sn or Tn, n > 1) usually have little effect on fluorescence or phosphorescence emissions. However, in certain molecular systems, these higher energy states play a substantial role in determining photophysical properties, particularly harnessing nonradiative triplet excitons in OLEDs. Hence, the exploration of a new molecular design strategy for the development of novel "hot-exciton" materials remains a challenging yet controversial endeavor. Here, we present an innovative approach using the BODIPY acceptor unit to develop new hot exciton materials, employing density functional theory (DFT) and time-dependent DFT computational methods. To achieve this, we substituted phenylcarbazole (P-CBZ) donor and phenylBODIPY (P-BODIPY) acceptor units with diverse configuration triads on 7 nonconjugated and 9 conjugated cores and investigated their ground and excited state properties. Unexpectedly, molecules with nonconjugated core units upconvert triplet excitons from T2 to S1 pathways, whereas molecules with conjugated core units follow T3 to S1 state mechanisms in the unsymmetrical triad substitution. Moreover, the degenerate triplet states T3 to T10, combined with singlet states S1 to S4, revealed multiple hot exciton channels, enabling the upconversion of nonradiative triplet excitons in the symmetrical substitution of P-BODIPY on nonconjugated cores. This study unveils the regulatory mechanism of photophysical properties influenced by high-lying excited states, providing a substantial theoretical foundation for modulating excited state energies in the future design of hot exciton-based emitters employing BODIPY acceptor units.
Mahaan et al. (Mon,) studied this question.