Synthesis and properties of quinoidal (di-)anionic coupled polymethine-oxonol dyes and their aromatic counterparts

Abstract

We report the synthesis of a new family of chromophores based on benzothiazole, indandione, and tricyanofuran moieties, forming aromatic neutral precursors or negatively charged quinoidal counterparts. Upon deprotonation, these dyes undergo marked bathochromic shifts, leading to highly delocalized coupled polymethine structures, particularly in their moderatly antiaromatic dianionic forms. The evolution of optical properties was systematically studied through pH-dependent absorption measurements and supported by TD-DFT and CC2 calculations. Distinct behaviors were identified across the series: ESIPT in benzothiazole dyes, photoacidity in indandione derivatives, and spirocyclization in tricyanofuran-containing dyes. These findings provide insights into structure–property relationships in anionic polymethine systems.