Multi-Zinc-Expanded Oligoacenes: An Intriguing Class of Well-Defined Open-Shell Singlet Diradicals

Abstract

Two classes of multi-Zn-expanded oligoacenes from benzene to pentacene are computationally designed through introducing a Zn array into acene rings in two ways: acene-chain axial versus single-ring quasi-transversal direction. Combined density functional theory, CASSCF, and CCSD calculations predict that all these multi-Zn-expanded oligoacenes have the open-shell singlet diradical ground states, in contrast with the common fact that their parent oligoacenes are closed-shell systems or may have a triplet ground state and only acenes larger than octacene have open-shell singlet diradical ground states. These results offer the first theoretical prediction that the multi-Zn introduction into the acene ring(s), forming the Zn-expanded oligoacenes, can lead them to diradical structures. The diradical character of the ground states of these molecules arises from the Zn-participation-induced disjoint nature of the nonbonding molecular orbitals that are singly occupied in the diradicals. This work provides a strategy to design perfect and stable singlet diradicals from oligoacenes or their derivatives.