Linearly Expanded Azulenes in the S 0 , T 1 , and Qu 1 States: A Unified View of Electronic Structure and Aromaticity.

Linearly expanded azulenes (LEAs) are isomers of polyacenes (PAs) in which the two terminal benzene rings are replaced by a five- and seven-membered ring, respectively. Despite this seemingly minor structural perturbation, LEAs exhibit electronic behavior fundamentally distinct from their PA counterparts. Herein, we systematically investigate the electronic structure and magnetic aromaticity of a series of LEAs in their S 0 , T 1 , and Qu 1  states. We find that LEAs begin to exhibit biradical character at smaller molecular sizes than the corresponding PAs, driven by preferential localization of unpaired electrons on the terminal rings, which allows for the formation of a migrating Clar sextet in the central benzenoid chain and results in a distinctive pattern of paratropic terminal domains and diatropic central domains. This same preference causes the two families to show opposite locations of unpaired electrons in T 1 . In Qu 1 , it results in a superposition of the two T 1 -state patterns, whereby LEAs localize unpaired electrons at the terminal rings while the benzenoid core adopts the PA-like arrangement. Together, these results deepen our understanding of non-benzenoid open-shell molecules and underscore the critical role of ring topology in governing electronic structure and aromatic character.