We studied the electronic structure and (anti)aromatic character of 3,4-dimethylenecyclobutene (DMCB) and of macrocycles composed of fused DMCB units in their lowest-lying states of different multiplicities. The designed macrocycles exhibit very stable low-lying nonet states, which can be even thermally populated as result of local Hückel- and Baird-aromaticity.

Abstract

3,4-Dimethylenecyclobutene (DMCB) is an unusual isomer of benzene. Motivated by recent synthetic progress to substituted derivatives of this scaffold, we carried out a theoretical and computational analysis with a particular focus on the extent of (anti)aromatic character in the lowest excited states of different multiplicities. We found that the parent DMCB is non-aromatic in its singlet ground state (S₀), lowest triplet state (T₁), and lowest singlet excited state (S₁), while it is aromatic in its lowest quintet state (Q₁) as this state is represented by a triplet multiplicity cyclobutadiene (CBD) ring and two uncoupled same-spin methylene radicals. Interestingly, the Q₁ state, despite having four unpaired electrons, is placed merely 4.8 eV above S₀, and there is a corresponding singlet tetraradical 0.16 eV above. The DMCB is potentially a highly useful structural motif for the design of larger molecular entities with interesting optoelectronic properties. Here, we designed macrocycles composed of fused DMCB units, and according to our computations, two of these have low-lying nonet states (i. e., octaradical states) at energies merely 2.40 and 0.37 eV above their S₀ states as a result of local Hückel- and Baird-aromatic character of individual 6π- and 4π-electron monocycles.