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 exocyclic 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. The
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, whereas electron-donating groups slightly destabilize them.
For CS, OSS, and T states, aromaticity measures indicate a gain of
aromaticity of the 6-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 containing EWGs or aromatic
substituents present the smallest singlet–triplet energy gaps.