Ice Confinement-Induced Solubilization and Aggregation of Cyanonaphthol Revealed by Fluorescence Spectroscopy and Lifetime Measurements

When an aqueous salt solution freezes, a freeze-concentrated solution (FCS) separates from the ice. The properties of the FCS may differ from those of a supercooled bulk solution of the same ionic strength at the same temperature. The fluorescence and lifetime characteristics of 6-cyano-2-naphthol (6CN) were studied in frozen NaCl solutions in order to provide insight into the solution properties of the FCS. While the photoacidity of 6CN in an FCS is similar to that in solution, several anomalous behaviors are observed. Fluorescence spectra indicate that the solubility of 6CN is significantly enhanced in the FCS (50 mM or higher) compared to that in the bulk NaCl solution where the solubility limit is 250 μM. The high solubility induces the aggregation of 6CN in the FCS, which is not detected in bulk solutions. This trend becomes marked as the initial NaCl concentration decreases and the FCS is confined in a small space. The fluorescence lifetimes of 6CN in the FCS support the spectroscopy results. In addition to the species identified by fluorescence spectroscopy, excimers are assigned from lifetime measurements in the FCS. The excimer formation is also a result of the enhanced solubility of 6CN in the FCS.