Adsorption-Induced Fluorescence of Pseudoisocyanine Monomers in Systems with Layered Silicates

Abstract

Interaction of organic dyes with layered silicates can cause various changes in dye properties, such as metachromasia (color change), fluorescence quenching, resonance energy transfer, or even fluorescence enhancement. These phenomena are caused by the molecular aggregation or adsorption of dye molecules on silicate particles. The present work studied the photophysical properties of pseudoisocyanine (PIC) in colloidal dispersions of five samples of layered silicates (smectites) at variable PIC/smectite loadings. The silicates of variable composition and layer charge included one synthetic saponite, one synthetic hectorite, two synthetic Laponites, and one montmorillonite. Chemometric analysis of the absorption spectra revealed the formation of four spectral species: monomers and three types of molecular aggregates exhibiting light absorption in both H- and J-band regions. In addition, the samples of hybrid systems with silicates of lower layer charge under the condition of the lowest PIC/silicate ratio (0.01 mmol g–1) exhibited strong fluorescence surpassing that of J-aggregates. The luminescent species achieving quantum yields of about 50% were assigned to the adsorbed PIC monomers, a phenomenon that has never been observed for this dye. Theoretical calculations found that in water solution bending of the molecule supports nonradiative deactivation of the excited state of the free molecule. Adsorption of PIC molecules on the surface of silicate particles caused the change of the dihedral angle between pyridine subunits, which hinders the nonradiative deactivation channel and favors emission. Fluorescence from adsorbed monomers exhibited a maximum at ≈535 nm, which is at significantly lower wavelengths with respect to the J-aggregates emitting at ≈570 nm and thus cannot be related to dye aggregation as supported by the fact that this phenomenon occurs only at the lowest surface concentrations of the dye.