Electronic structure study of the biradical pleiadene-like molecules

Abstract

A detailed electronic structure study of singlet and triplet states of a series of biradicaloid pleiadene-like molecules was performed using the DFT and complete active space methods. It was shown that the singlet-triplet gaps obtained with the B3LYP, B2PLYP, and CASSCF methods are not a monotone function of the number of annelated benzene rings. According to the CASSCF results, 9,10-naphthopleiadene is a triplet biradical, whereas all other members of the series are found to be singlet biradicaloids exhibiting very pronounced biradical character. On the other hand, the B2PLYP calculations revealed that 9,10-anthracenopleiadene is a triplet biradical molecule. A detailed analysis of the local aromaticity of singlet and triplet states of the examined molecules was done by means of the multi-centre delocalization indices, harmonic oscillator model of aromaticity index, and nucleus independent chemical shifts. It was shown that the triplet states are more aromatic than the corresponding singlet electronic states. The difference in aromaticity of the singlet and triplet electronic states becomes smaller along the series of benzannelated pleiadenes. The more pronounced aromatic character of the triplet states can compensate the instability coming from the presence of unpaired electrons, which can explain the fact that the mid-size members of the examined series are found to be triplet biradicals. Graphical abstract: [Figure not available: see fulltext.] © 2013 Springer-Verlag Wien.