### Résumé

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 p-quinodimethane 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 time-dependent 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.

langue originale | Anglais |
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Pages (de - à) | 9828-9839 |

Nombre de pages | 12 |

journal | Journal of physical chemistry A |

Volume | 123 |

Numéro de publication | 45 |

Les DOIs | |

Etat de la publication | Publié - 14 nov. 2019 |

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## Contient cette citation

*Journal of physical chemistry A*,

*123*(45), 9828-9839. https://doi.org/10.1021/acs.jpca.9b08474