Macrocycle threading using solvatochromic squaraine dyes

ABSTRACT

Non-covalent assembly is an attractive way to fabricate deep-red fluorescent probes for biological imaging. Four new solvatochromic squaraine dyes with unsymmetric structures are prepared and shown to exhibit macrocyclic encapsulation by tetralactam macrocycle in chloroform solvent or in water. Macrocycle threading in chloroform produces a relatively small 20 nm red-shift in absorption wavelength. In contrast, macrocycle threading in water produces ~80 nm red-shift in absorption wavelength, indicating a large change in polarity surrounding the solvatochromic squaraine as it transfers from bulk water solution to the relatively non-polar macrocycle interior. The large red-shift in absorption maxima translates into a very large 70-fold enhancement of squaraine fluorescence. Macrocycle threading in water by two homologous squaraines was studied and there was a large difference in threading kinetics. Macrocycle threading by a squaraine with an N-methyl substituent at the end of the structure was 5 million times faster than threading by a squaraine with an N-propyl substituent.

Graphical abstract

KEYWORDS:

• Squaraine dye
• macrocycle
• negative solvatochromism
• fluorescence
• association constant
• kinetics
• host-guest complex
• supramolecular complex

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary material

Supplementary data for this article can be accessed here.

Additional information

Funding

This work was supported by the National Science Foundation [CHE1708240].