Synthesis of Soluble Nickel–Salphen Ligand Complexes as Near‐Infrared Dyes

Salphen metal complexes have the advantages of simple synthesis, high stability, and excellent optical properties. However, their poor solubility, severe aggregation, and short absorption wavelengths limit their application as near‐infrared dyes. In this study, five soluble Ni–salphen complexes with strong NIR absorption are synthesized by introducing alkyl chains and extending conjugated cores. The results show that the introduction of thiophene and electron‐withdrawing end‐groups has resulted in the red‐shifted absorption of these complexes, with the absorption edge reaching 800 nm. These compounds have well solubility in organic solvents, more soluble than 8 mg mL −1 in chloroform, tetrahydrofuran, toluene, etc. The target molecules also have excellent photostability and thermal stability (T d > 325 °C).