Triarylmethane dye ethynologue with a fused julolidine motif as a compact dye in the near infrared range†

Near-infrared (NIR) dyes have garnered significant attention for applications in bioimaging and optoelectronics due to their low phototoxicity and excellent tissue penetration. However, many conventional NIR dyes rely on extended π-conjugated systems and require complex multi-step syntheses. In this study, we report the development of compact NIR-absorbing dyes based on triarylmethane dye ethynologues. A series of compounds was synthesized by incorporating a julolidine unit as a strong electron-donating group and modifying either electron-donating or electron-withdrawing substituents on the aromatic rings adjacent to the carbocation center. The julolidine moiety effectively red-shifted the absorption into the NIR region, while electron-donating groups enhanced photostability and resistance under acidic conditions. Nevertheless, the presence of alkyne linkages makes the dyes susceptible to degradation under basic conditions, as well as to oxidation and thermal decomposition, limiting their long-term durability. Ongoing efforts focus on overcoming these drawbacks to advance the practical applications of these dyes in photocatalysis, photonic devices, and solar energy conversion systems. This work offers a promising approach toward the sustainable design of compact, efficient, and synthetically accessible NIR dyes.