Pyrazinacene luminescence enhancement by heat-activated surface adsorption and de-aggregation in a saponite colloidal system

This work presents heat-activated luminescence enhancement of butterfly-shaped pyrazinacene dye (BP) in an aqueous hybrid colloidal solution containing synthetic silicate, clay mineral saponite (Sap). The nitrogen-rich BP molecule consists of a quadruply benzo-fused fluorubine core N-alkylated at 6,13-positions with six hydrophilic [2-[2-(2-methoxyethoxy)ethoxy]ethoxy] chains to promote its solubility in polar solvents. BP exhibits strong luminescence emission especially in organic solvents. However, in aqueous solutions, BP dye alone usually aggregates, which leads to significantly weaker fluorescence. This effect can be overcome by using a mixed aqueous colloidal system containing Sap particles and BP dye molecules. This process is further promoted by heating the colloidal BP/Sap system to 70 °C, where the BP aggregates are irreversibly dispersed as monomers subsequently adsorbed at the surfaces of Sap particles. Following cooling to room temperature, the luminescence emission undergoes a several-fold increase in intensity. Such an improvement was not observed in aqueous solutions without the silicate or in hybrid colloids without thermal treatment. The heat-assisted irreversible adsorption of BP on Sap particles represents a simple stabilization method for fluorescence enhancement that can be applied to various other photoactive dyes in aqueous systems. This is essential to many applications, especially those involving biological systems, such as photosensitizers, luminescent sensors, or probes.