Ring Formation in Evaporating Porphyrin Derivative Solutions

The formation of micrometer-size, ring-shaped structures is studied in evaporating solutions of porphyrin-based molecules on glass. Solute concentration, solution and substrate temperature, vapor pressure, and solvent are varied. The molecular arrangements on the substrate are monitored by confocal fluorescence microscopy (CFM), atomic force microscopy (AFM), and near-field scanning optical microscopy (NSOM). From experiment, a nonequilibrium morphology diagram for the observed structures is deduced as a function of solute concentration and evaporation time. The mechanisms involved in ring formation are discussed on the basis of solvent and solute dynamics.