Hydroxy‐Functionalized Near‐Infrared Squaraine Dyes for Advanced Organic Conjugates and Hybrid Materials

Near‐infrared squaraine dyes absorbing beyond 900 nm are promising for use in sensing and imaging, as photothermal additives or as complementary materials for energy conversion applications. Here, the synthesis of alkyl hydroxy‐functionalized, benz[ cd ]indole‐donor capped squaraine dyes using the squaric acid and the dicyanomethylene‐substituted squaraine acceptor unit is reported. In parallel synthesis, the hydroxy group is introduced either directly on the dye‐solubilizing alkyl side chains or coupled to the aryl part of the donor, and one or two functional linkers are attached. The hydroxy‐functionalized dyes are then esterified with carboxyl groups attached to silica nanoparticles, a meltable polymer and a fullerene electron acceptor. Properties of the resulting hybrid material and organic conjugates demonstrate application merits of covalent bonding over blending for targeted photoinduced emission, heat and charge transfer reactions using near‐infrared squaraine dyes.