Exploring the Boundaries of Aromaticity in Complex Structures and Excited States Through Computational Analysis.
Abstract
Aromaticity is a central concept for understanding and predicting molecular structure, reactivity, and properties. Commonly associated with cyclic or 3D systems, it originates from electron delocalization along a closed pathway, producing characteristic effects such as enhanced thermodynamic stability, magnetic anisotropy, and unusual chemical shifts. Although aromaticity is not a directly measurable physical observable, it has remained a key concept in chemistry for almost two centuries. Its counterpart, antiaromaticity, is generally linked to reduced stability and increased reactivity, further emphasizing the importance of electron delocalization in molecular behavior. In recent years, aromaticity has gained particular relevance in the study of electronically excited states, where it helps explain photochemical behavior and guides the development of advanced materials and reactions. The extension of ground-state aromaticity principles to excited-state chemistry has proven promising, but uncertainties remain regarding the applicability of specific aromaticity rules and descriptors in complex excited systems. At the same time, the concept of aromaticity has expanded far beyond classical annulenes. Modern examples include molecules with multiple delocalization pathways, aromatic character involving σ-, π-, δ-, or ϕ-electrons, and nonplanar or 3D architectures, challenging conventional definitions and motivating renewed conceptual reassessment. These advances continue reshaping theoretical perspectives on molecular delocalization phenomena worldwide.