Electronic structure study of the triplet azulene-like molecules

In this Letter a detailed electronic structure study of singlet and triplet states of a series of azulene-like molecules was performed using several DFT and complete active space (CAS) methods. According to the results obtained at the B3LYP level of theory using the unrestricted symmetry-broken method, the singlet–triplet splitting for the members of the series with three and more hexagons is quite small, and in addition, four members of the series were found to be triplet diradicals. The CASSCF calculations revealed that higher members of the series of the azulene-like molecules with six and more hexagons are triplet diradicals.