Design of Anti-Hund Organic Emitters Based on Heptazine

Hund's rule, which is powerful in governing the first excited states of closed-shell organic materials, can hardly be violated to get inverted singlet-triplet gap (INVEST) molecules with negative singlet-triplet energy gaps (Δ<i>E</i>_ST), although INVEST materials have shown extraordinary photophysical properties and promising device performance especially in light-emitting diodes. Here, we propose a facile strategy to construct emissive INVEST molecules by introducing different types of substituents to heptazine in various modes, which can effectively tune the Δ<i>E</i>_ST to be negative with the enlarged oscillator strength (<i>f</i>) for the high fluorescence rate of the heptazine derivatives. Systematic computational studies show that the double substitution of electron-donating units with another nonconjugated substituent in hybrid substitution mode is the most favorable way in achieving slightly negative Δ<i>E</i>_ST and large <i>f</i> values; the conjugated substituent will compete with heptazine to make the molecule deviate from the INVEST feature. Especially, a series of high-performance heptazine-based INVEST emitters were constructed, exhibiting Δ<i>E</i>_ST low to -0.362 eV, <i>f</i> up to 0.0436, as well as a wide range emission color from 339 to 716 nm. Also, the designed molecules were predicted to have fluorescence radiative rates up to 10⁶ s^-1, along with efficient reverse intersystem crossing rates reaching 10⁸ s^-1. Importantly, the figure of merit (FM) was first proposed as a parameter to wholly evaluate the performance of INVEST emitters, and the highest FM of 0.198 was found in the triazine and double nonconjugated amine-substituted heptazine. These results highlight the great potential of the heptazine chromophore in constructing INVEST emitters, revealing fundamental structure-property understandings for the material design of efficient anti-Hund organic molecules with improved emission properties.