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 E 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 E ST to be negative with the enlarged oscillator strength ( f ) 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 E ST and large f 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 E ST low to −0.362 eV, f 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 10 6 s –1, along with efficient reverse intersystem crossing rates reaching 10 8 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.