Are Fully Conjugated Expanded Indenofluorenes Analogues and Diindeno[n]thiophene Derivatives Diradicals? A Simplified (Spin- Flip) Time-Dependent Density Functional Theory [(SF-)sTD-DFT] Study

de Wergifosse, Marc [UCL] Seibert, Jakob [Mulliken Center for Theoretical Chemistry, Institut für Physikalische und Theoretische Chemie, Beringstrasse 4, 53115 Bonn, Germany] Champagne, Benoît [Laboratoire de Chimie Théorique, Département de Chimie, Namur Institute of Structured Matter, 5000 Namur, Belgium] Grimme, Stefan [Mulliken Center for Theoretical Chemistry, Institut für Physikalische und Theoretische Chemie, Beringstrasse 4, 53115 Bonn, Germany]

Polycyclic hydrocarbons are often used to understand the electronic structure of nanographene systems. Among them, indeno-[1,2b]fluorene and indeno[1,2c]fluorene isomers present a central pquinodimethane unit leading to unique optical properties. In this work, we characterized the absorption spectra of indeno[1,2b]fluorene and [2,1-c]diindeno[n]thiophene derivatives with (spin-flip) simplified timedependent density functional theory [(SF-)sTD-DFT] methods. Note that the SF-sTD-DFT level of theory allows a computationally efficient treatment for large diradicals. To interpret spectra, we implemented natural transition orbitals (NTOs) at both SF-sTD-DFT and sTD-DFT levels. This compact and method-independent representation of the electronic excitation provides a simple interpretation for the low-lying excited states of this set of molecules in terms of three different types of NTOs: “quinoid”, “aromatic”, and “π-bonded”. When comparing with experiment, we found that only one molecule of this set is actually a high-spin triplet diradical. Others are almost closed-shell molecules with a very small contribution from a doubly excited configuration that only the spin-flip method could capture. The small amount of static correlation recovered by the spin-flip active space provides a linear relation between the first visible theoretical and experimental excitation energies among this set.