Theoretical Study On Triplet Ground State Aromaticity In Organometallics

Aromaticity is an important concept in chemistry.Aromatic compounds have unique properties in their structure,energy,electronic structure and reactivity and have enhanced thermodynamic stability due to the delocalization of the ?electrons.Baird in 1972 proposed that the annulene with 4n ?-electrons is aromatic in the lowest ??*triplet state,whereas the annulene with[4n+2]?-electrons is antiaromatic,which is opposite to the ground state.Thus,aromaticity can be used to design the triplet ground state species.Understanding the excited state aromaticity is essential for photochemical reactions.Here,we employ theoretical calculations to investigate the connection between aromaticity and triplet ground state,and provide some useful information to design photochemical materials and reactions.The main contents are summarized as following:1.With the help of theoretical calculations,we study the relative stability and aromaticity of the silapentafulvene isomers and investigate the substitution effect on the relative stability and aromaticity.And we tune the singlet-triplet energy gap with different substitutions.The results show that the magnitude of the aromaticity can follow this order:6-silapentafulvenes>pentafulvenes>5-silapentafulvenes.Furthermore,the relative stability of the isomers of silapentafulvenes can be rationalized by the bond dissociation energy of the exocyclic Si/C-X bonds,whereas the aromaticity only has a small contribution.In addition,four triplet ground state species are predicted by introducing amino group on the ring.2.Through theoretical calculations,we investigate the aromaticity of 2H-benzo[c]borole with antiaromatic borolc and benzene.And we study how the substitritions affect the aromaticity and singlet-triplet energy gap.The result suggested that the benzene ring becomes antiaromatic by the influence of the borole ring.The substitutions at the 5-memebered ring can significantly affect the singlet triplet energy.And we predict three triplet ground state species by utilizing substitutions at the 5-memebered ring.3.We investigate the aromaticity of the disilver[8]annulene and digold[8]annulene in both the lowest singlet and triplet states.The results demonstrate that their derivatives and non-substituted digold[8]annulene have triplet ground state.