Theoretical Study On Excited State Aromaticity And Antiaromaticity Via Energetic Criteria

As an important concept in organic chemistry,aromaticity is used to explain the extra stability of cyclic compounds.Huckel's rule defines that 4n+2 ?-electron planar conjugated cyclic molecule is aromatic whereas 4n ?-electron compound is the opposite.With the development of aromaticity,various aromatic concepts have been proposed.Lots of criteria based on geometric,energetic,magnetic,electronic properties and reactivity have also been employed to evaluate aromaticity.Baird's rule states that excited state aromaticity is reversed compared to ground state aromaticity,which is essential for understanding and conducting photochemical reactions.However,the excited state aromaticity is quite less developed,and only a few work mainly focus on magnetic and electronic properties.The initial essence of aromaticity is related to unique stability,so it's necessary to explore the energetic criteria in excited state.Here we employ the theoretical calculations to investigate the energetic criteria suitable for excited state aromaticity.The main contents are summarized as follows:1.Study the lowest triplet state(T1)aromaticity via methyl-methylene isomerization stabilization energy(ISEi)method.The results suggest that this method can not only evaluate excited state aromaticity,but further confirm Baird's rule.The strong correlation with NICS values also shows the reliability of this method.2.Study the excited state aromaticity by indene-isoindene isomerization stabilization energy(ISE?)method.The results show that this method agrees well with the ISEi and NICS values,indicating the reliability of estimating excited state aromaticity with ISE?method.3.Study the aromaticity in both ground and excited states by directly cleaving the cyclic rings.The results demonstrate that this cleaved method is able to evaluate the ground and excited states aromaticity of the compounds with small ring strain energies.Moreover,this method can also be used in the aromatic evaluation of bicyclic naphthalene,metallabenzenes and even metallanaphthalene.4.Study the first singlet excited state aromaticity via two isomerization stabilization energy methods.The results indicate that 4n+2 ?-electron compounds lose stability and aromaticity while 4n ?-electron systems obtain stability and aromaticity.