The influence of N-alkyl groups on the properties of squarylium cyanine dyes

Dyes with strong intermolecular interactions often exhibit low solubility, which restricts their application in some areas. Solubility can be improved by introducing substituent groups with a steric hindrance capacity. By enlarging the intermolecular distance, the interaction can be reduced, and so the solubility is higher. In this report, bis(2,3,3-trimethylindolenium-2-ylidene) squaraines with different branch chain alkyl groups on the nitrogen atom were synthesized and the relationship between substituents and solubility in different solvents, melting point and photophysical properties in solution are discussed.