Low band gap polycyclic aromatic hydrocarbons (PAH) attracted tremendous attention because of their wide-spread applications in the field of optoelectronics. PAHs can either have closed shell electron configuration wherein the π electrons are populated only in bonding orbitals or open-shell configuration in which the molecule has one or more radicals present. The two conceptually different approaches to conjugated diradicals are based on Kekulé vs. non-Kekulé structures. The non-Kekulé class of structures already have open-shell character because of their high-spin ground state which follows from the bonding pattern where radicals cannot couple via resonance. In contrast, the open-shell character in the Kekulé diradicals is generated because of the competition between the closed-shell quinoid and the open-shell singlet diradical resonance structures. Here in this project, we outline a conceptually new way of design and synthesis of Kekulé and Non-Kekulé diradicaloids by maximum utilization of Clar’s π sextets. The ΔES-T values were calculated to be -0.20 kcal/mol for Kekulé diradicaloid and +0.20 kcal/mol for non-Kekulé diradicaloid. It is also found that Kekule diradicaloid has a very large singlet diradical character with y0 of 0.98 to represent its diradical nature between the closed-shell and pure diradical state (0 to 1) and experimental results also suggest large diradical character.

Design and synthesis of Kekulé and non-Kekulé diradicaloids utilizing radical peri-annulation strategy

ACS Spring 2022

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