New tetralactam hosts for squaraine dyes

Abstract

The photophysical properties of a deep-red fluorescent squaraine dye can be improved by encapsulating it within a tetralactam macrocycle. Three new tetralactams are described with different substituents (methyl, methoxy, methylenedioxy) on the macrocycle aromatic sidewalls. The capability of each tetralactam to encapsulate a squaraine dye in chloroform solution was determined experimentally using absorption, fluorescence, and NMR spectroscopy. Two of the tetralactams were found to thread a squaraine dye with association constants on the order of 10⁶ M⁻¹, while a third macrocycle exhibited no squaraine affinity. An X-ray crystal structure of the third tetralactam showed that the substituents sterically blocked squaraine association. Of the two tetralactams that encapsulate a squaraine, one induces an increase in squaraine fluorescence quantum yield, while the other quenches the squaraine fluorescence. The results suggest that these new squaraine binding systems will be useful for biological imaging and diagnostics applications.