Design of Anti-Hund\nOrganic Emitters Based on Heptazine

Hund’s rule, which is powerful in governing the\nfirst excited\nstates of closed-shell organic materials, can hardly be violated to\nget inverted singlet–triplet gap (INVEST) molecules with negative\nsinglet–triplet energy gaps (Δ<i>E</i><sub>ST</sub>), although INVEST materials have shown extraordinary photophysical\nproperties and promising device performance especially in light-emitting\ndiodes. Here, we propose a facile strategy to construct emissive INVEST\nmolecules by introducing different types of substituents to heptazine\nin various modes, which can effectively tune the Δ<i>E</i><sub>ST</sub> to be negative with the enlarged oscillator strength\n(<i>f</i>) for the high fluorescence rate of the heptazine\nderivatives. Systematic computational studies show that the double\nsubstitution of electron-donating units with another nonconjugated\nsubstituent in hybrid substitution mode is the most favorable way\nin achieving slightly negative Δ<i>E</i><sub>ST</sub> and large <i>f</i> values; the conjugated substituent\nwill compete with heptazine to make the molecule deviate from the\nINVEST feature. Especially, a series of high-performance heptazine-based\nINVEST emitters were constructed, exhibiting Δ<i>E</i><sub>ST</sub> low to −0.362 eV, <i>f</i> up to 0.0436,\nas well as a wide range emission color from 339 to 716 nm. Also, the\ndesigned molecules were predicted to have fluorescence radiative rates\nup to 10<sup>6</sup> s<sup>–1</sup>, along with efficient reverse\nintersystem crossing rates reaching 10<sup>8</sup> s<sup>–1</sup>. Importantly, the figure of merit (FM) was first proposed as a parameter\nto wholly evaluate the performance of INVEST emitters, and the highest\nFM of 0.198 was found in the triazine and double nonconjugated amine-substituted\nheptazine. These results highlight the great potential of the heptazine\nchromophore in constructing INVEST emitters, revealing fundamental\nstructure–property understandings for the material design of\nefficient anti-Hund organic molecules with improved emission properties.