Squaraine (SQ) dyes that feature structures with central electron‐deficient squaric acid moieties and electron‐donating groups at both ends are known for their high molar absorption coefficients. While modifying the structures of the donor sites reportedly enables absorption and fluorescence in the visible‐to‐near‐infrared range, insufficient photostability remains a pressing issue. In this study, we used boron complexation to improve the photostability of a norSQ dye with a benzoindolenine skeleton. The norSQ dye was successfully boron‐complexed with the benzoindolenine skeleton by reacting a boron trifluoride diethyl ether complex in the presence of lithium bis(trimethylsilyl)amide. The structure of the synthesized norSQ‐dye–monoboron complex was determined by single‐crystal X‐ray analysis. A comparison of the optical properties of the norSQ dye and the norSQ‐dye–monoboron complex revealed the following: 1) Boron complexation results in shifts in the maximum absorption and fluorescence wavelengths to shorter values in various solvents and in polymethyl methacrylate (PMMA) films, while also 2) lowering the fluorescence quantum yield. Notably, the norSQ‐dye–monoboron complex is significantly more photostable than the norSQ dye in dichloromethane.
Oyama et al. (Wed,) studied this question.