Taming boroloborinines: toward photostable polycyclic antiaromatic hydrocarbons

Boron-containing heterocycles are attracting growing attention due to their unique electronic structures and effectiveness as electron acceptors in functional organic materials. Among them, boroloborinines, a fused borole-borinine scaffold, constitute a rare class of 8π-electron antiaromatic systems with potential applications in organic electronics. However, their inherent antiaromatic instability has limited synthetic exploration and practical deployment. In this work, we employ density functional theory (DFT) along with wavefunction-based methods to systematically investigate strategies for stabilizing boroloborinine derivatives via nitrogen incorporation and selective benzannulation. Photochemical stability is evaluated using HOMO-LUMO and singlet-triplet (S-T) energy gaps, while aromaticity is assessed through three classes of indices: the multicenter index (MCI), the Harmonic Oscillator Model of Aromaticity (HOMA), and magnetically induced current density (MICD). Our findings show that both the position and nature of substitution critically influence the electronic structure, with the formation of Clar's sextets correlating strongly with increased stability. Molecules that exhibit reduced antiaromaticity in the singlet state and minimal aromaticity in the triplet state tend to possess the largest S-T gaps. Besides photostability, we examined proton affinity of our model systems to verify which molecules can remain stable in acidic environments. Our modeling suggests that nitrogen atoms in the vicinity of boron have higher proton affinity; therefore, they are potentially more prone to acid catalyzed reactions. These insights provide guiding principles for designing antiaromatic chromophores with enhanced stability and tunable optoelectronic properties, potentially enabling the development of novel organic emitters with distinct emission wavelengths and improved quantum yields.