Triplet state (anti)aromaticity of some monoheterocyclic analogues of benzene, naphthalene and anthracene

Abstract

Aromaticity–antiaromaticity switch upon singlet–triplet transition of some biologically and synthetically important monoheterocycles (heteroatom = N, O, Si, P, and S) derived from benzene, naphthalene and anthracene was studied by employing energetic, magnetic and structural aromaticity criteria, at the density functional theory (DFT) level. The relationship between spin density distribution, (anti)aromaticity and singlet–triplet energy gaps, in the studied molecules, was found. In general, spin delocalization results in antiaromaticity, spin density localization to one ring in bi- and tricycles localizes antiaromaticity and spin localization on a heteroatom reduces global and local antiaromaticity. The latter reaches nonaromaticity in the case of silicon atoms which have larger orbitals and show more tendency to accept unpaired electrons. Spin density localization in bi- and tricycles allows benzene subunit(s) to develop local aromaticity, which, when combined with nonaromatic silacycle and weak global antiaromaticity, results in overall triplet state weak aromaticity. The singlet–triplet energy gaps decrease with a decrease in the triplet state antiaromaticity and are the lowest for silicon-containing compounds.