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.