Examination of the Aromatic vs. Coupled Polymethine Character in Dicationic and Neutral Acceptor–Donor–Acceptor Dyes

A series of twenty-two acceptor–donor–acceptor chromophores, comprising both dicationic and neutral dyes, were synthesized and characterized. These compounds were designed with a wide range of electron-withdrawing end groups to explore the balance between acceptor–donor–acceptor aromatic structures and hypothetical nonaromatic coupled polymethine electronic structures within the central six-membered ring. Structural, electrochemical, photophysical, and computational investigations were conducted to gain insights into the structure–property relationships of these systems. The experimental data revealed that the redox and optical properties were most consistent with acceptor–donor–acceptor aromatic structures, significantly impacted by the nature of the end groups. Solvatochromic studies further indicated that the dyes were unlikely to lose their aromatic character in favor of a coupled polymethine structure. Theoretical calculations suggested that a few dicationic derivatives exhibited a slightly reduced aromatic character, hinting at minor coupled polymethine contributions. However, the majority of the chromophores retained a predominantly aromatic central ring.