Biradicaloid character of phenalenyl-based aromatic compounds with a small HOMO–LUMO gap

Abstract

Abstract Biradicaloid character of three Kekule aromatic compounds containing two phenalenyl moieties is discussed on the basis of the theoretical and experimental results. DFT calculation of the compounds reveals a small HOMO–LUMO gap with a large spatial overlap between them, leading to a singlet biradical character in a ground state and an excited triplet biradical state with a small ΔES–T. Singlet biradical character for 1 (see Fig. 1 ) is indicated by the X-ray crystallographic analysis, which shows dimeric pairs with substantially short non-bonding contacts of ∼3.1 A. The ESR measurements for 1 and 3 give typical spectra for triplet species and the temperature dependence of the half-field signal indicates the thermal excitation to the triplet states.