Revival of Huckel Aromatic (Poly)benzenoid Subunits in Triplet State Polycyclic Aromatic Hydrocarbons by Silicon Substitution

By employing density functional theory (DFT) calculations we show that mono- and disilicon substitution in polycyclic aromatic hydrocarbons, having two to four benzene units, quenches their triplet state antiaromaticity by creating Huckel aromatic (poly)benzenoid subunit(s) and weakly antiaromatic, or almost nonaromatic silacycle. Therefore, such systems are predicted to be globally aromatic in both the ground state and the first excited triplet state. Putting the silicon atom(s) into various positions of a hydrocarbon provides an opportunity to tune the singlet-triplet energy gaps. They depend on the global aromaticity degree which, in turn, depends on the type of aromatic carbocyclic subunit(s) and the extent of their aromaticity. On the basis of the set of studied compounds, some preliminary rules on how to regulate the extent of global, semiglobal and local aromaticity are proposed. The results of this work extend the importance of Huckel aromaticity concept to excited triplet states which are usually characterized by the Baird type of (anti)aromaticity.

Automated summary

By employing density functional theory calculations, this study investigates the effect of silicon substitution on the aromaticity of polycyclic aromatic hydrocarbons. The results show that the placement of silicon atoms can tune the singlet-triplet energy gaps and regulate the extent of global, semiglobal, and local aromaticity. This work extends the importance of Huckel aromaticity to excited triplet states.