Effect of Exocyclic Substituents and π-System Length on the Electronic Structure of Chichibabin Diradical(oid)s

The ground state (GS) of Chichibabin's polycyclic hydrocarbons (CPHs) can be singlet (open- or closed-shell (OSS or CS)) or triplet (T), depending on the elongation of the π-system and the exocyclic substituents. CPHs with either a small singlet-triplet energy gap (ΔEST) or even a triplet GS have potential applications in optoelectronics. To analyze the effect of the size and exocy-clic substituents on the nature of the GS of CPHs, we have selected a number of them with different substituents in the exocyclic carbon atoms and different ring chain lengths. OPBE/cc-pVTZ level of theory was used for the optimization of the systems. The aromaticity of the resulting electronic structures was evaluated with HOMA, NICS, FLU, PDI, Iring, and MCI aromaticity indices. Our results show that the shortest π-systems (one or two rings) have a singlet GS. However, systems with three to five rings favor OSS GSs. Electron-withdrawing groups (EWGs) and aromatic substituents in the exocyclic carbons tend to stabilize the OSS and T states, while electron-donating groups slightly destabilize them. For CS, OSS, and T states, aromaticity measures indicate a gain of aromaticity of the six-membered rings of the CPHs with the increase in their size and when CPHs incorporate EWGs or aromatic substituents. In general, the CPHs analyzed present small singlet-triplet energy gaps, and in particular, the ones contain-ing EWGs or aromatic substituents present the smallest singlet-triplet energy gaps A.P. and M.S. thank the ́ Ministerio de Economi ́ a y Competitividad (MINECO) of Spain for projects CTQ2014-59832-JIN and CTQ2017-85341- P, Generalitat de Catalunya for project 2017SGR39, Xarxa de Referencia en Qui ̀ mica Teo ́ rica i Computacional, the ICREA ̀ Academia prize 2014 awarded to M.S., and European Fund for Regional Development (FEDER) grant UNGI10-4E-801