Highly efficient crystal red fluorescent 1,2-squaraine dyes with excellent biocompatibility and bioimaging

A new family of asymmetrical 1,2-squaraine dyes (1,2-SQs) were designed and synthesized in high yields. The photophysical analysis presented that these 1,2-SQs showed strong red fluorescence in crystal state but weak fluorescence in solution, suggesting their aggregation-induced emission enhancement (AIEE) properties. Single crystals X-ray diffraction revealed that the intense fluorescence in the crystal state can be attributed to the effects of long alkyl chains or large group substituents in the molecules, which efficiently prevented the tight intermolecular contact and reduced π–π interactions. The biocompatibility and cell imaging applications of SQ4 were further investigated. The results demonstrated that SMMC-7721 cell can be clearly imaged by SQ4, which inferred SQ4 had excellent biocompatibility and potential value for bioimaging.