Invited talk

Diradicaloids: The role of non-bonding states
Juan Casado

Department of Physical Chemistry, Faculty of Science, University of Malaga 29071 Málaga, Spain

Diradicaloid molecules [1] represent cornerstone systems to explore the nature of the chemical bond and the validity of its theories. Recently, they have also gained relevance as multipurpose substrates for organic electronic applications [2]. There are several approaches to design these molecules: (i) diradicaloids made of non-alternant antiaromatic [4n]annulene hydrocarbons, (ii) those developed from quinoidal or pro-aromatic cores, and (iii) those made of a combination of (i) and (ii).

The main observable physical and chemical properties of polycyclic aromatic and diradicaloid molecules are dictated by the states and molecular orbitals around the gap. The midgap positions are dominated by states with non-bonding character which separates the chirally symmetric bonding and antibonding orbitals. The frontier orbitals of π-conjugated molecules, aside of their aromatic or antiaromatic characters, aside of the shapes and aside of the number of π-electrons in delocalization are characterized by fractions of non-bonding orbitals. The existence of such non-bonding orbitals is usually paralleled by their disjoint character which makes them ideally suited to minimize electron correlation / repulsion energy, E_corr. With these non-bonding orbitals, it is also possible to construct the bonding and antibonding orbitals form which the hybridization energy, E_hyb, is deduced. Many of the observed properties of these molecules are dictated by the balance between E_hyb and E_corr.

The Hückel molecular orbital theory (more familiar to chemists) or the tight-binding model (more familiar to physicists) provide the ideal framework to approach the electronic, optical and magnetic properties of many polycyclic organic molecules. In this contribution, all these aspects of the impact of the E_hyb and E_corr in the formation of open-shell molecules and the relevance and role of the non-bonding states is discussed.

Figure 1. Representation on the bonding, anti-bonding and non-bonding states of a chain of CH units, or (CH)_n. Also, the frontier molecular orbitals of some molecules such as benzene, naphthalene and azulene are shown.

[1] M. Abe, Chem. Rev. 2013, 113, 7011–7088.
[2] S. Moles Quintero, M. M. Haley, M. Kerstez, J. Casado, Angew. Chem. Int. Ed. 2022, 61, e202209138.
[3] G. E. Rudebusch, J. L. Zafra, K. Jorner, K. Fukuda, J. L. Marshall, C. J. Gómez-García, L. N. Zakharov, M. Nakano, H. Ottosson, J. Casado, M. M. Haley, Nat. Chem. 2016, 8, 753-759.