The effect of substituents on the stability of the central four-member ring of unsymmetrical squaraine derivatives

Unsymmetrical squaraine dyes SQ-OPA can effectively identify Cys, Hcy and GSH through two response processes. [Display omitted] •The property of unsymmetrical squaraine dyes is variable.•Acrylate groups changed the nature of the four-member ring of squaraine.•The central of squaraines plays a role in the identification of thiols. Two unsymmetrical squaraines (SQ-OH and SQ-OPA) have been synthesized. SQ-OPA was gained by esterification of acryloyl chloride and hydroxyl group on SQ-OH. Based on the investigation of the UV–vis, fluorescent, time-dependent 1H NMR and mass spectra, SQ-OPA was demonstrated to have two response sites to biothiols: the central four-member ring and acryloyl group. SQ-OPA is the first “turn-on” fluorescent squaraine thiol probe, and can probe the biothiols (Cys, Hcy, and GSH) simultaneously by naked-eye. However, SQ-OH bearing similar squaraine molecular framework with SQ-OPA, showed good chemical stability during the addition of biothiols. Therefore, for unsymmetrical squaraines, the substituent attached in benzene ring, far away from the central four-member ring, can signficantly affect the stability of squaraine central four-member ring against the nucleophilic attack, which could provide a new strategy for designing “turn-on” fluorescent SQ probes.