Interplay of Biradicaloid Character and Singlet/Triplet Energy Splitting for cis-/ trans-Diindenoacenes and Related Benzothiophene-Capped Oligomers as Revealed by Extended Multireference Calculations

The biradicaloid character of different types of polycyclic aromatic hydrocarbons is an important quality that guides the development of new materials with interesting magneto-optical properties. Diindenoacene-based systems represent such a class of promising compounds. In this work, the three types trans-diindenoacenes, cis-diindenoacenes, and trans-dicyclopentaacenobis(benzothiophenes) have been studied by means of advanced ab initio methods. The descriptors singlet/triplet splitting energy (?E_S-T), effective number of unpaired electrons (N_U), unpaired electron density, and the harmonic oscillator measure of aromaticity have been used to characterize the biradicaloid properties of these species. For all trans structures, low-spin singlet ground states were found, in agreement with previous investigations. Increasing the length of the inner acene chain decreased the ?E_S-T and strongly increased the N_U values of the singlet state. The cis-diindenoacenes displayed a greatly increased biradicaloid character, indicating enhanced chemical instability. The thiophene rings in the trans-dicyclopentaacenobis(benzothiophenes) structures were found to simultaneously restrict the unpaired electron density from extending into the terminal six-membered rings and confine the unpaired electron density found in the core benzene rings.