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The formation mechanism of ring-shaped assemblies (wheels) obtained from the evaporation of solutions of bis(21H,23H-5(4-pyrydyl)-10,15,20-tris(4-hexadecyloxyphenyl)porphyrin)platinum dichloride (PtP) has been studied by a variety of spectroscopic and microscopic techniques, including confocal fluorescence microscopy (CFM), atomic force microscopy (AFM), and near-field scanning optical microscopy (NSOM). Ring-shaped structures have been obtained by deposition of CHCl solutions on glass, and a strong dependence of ring shape and size on the initial PtP concentration has been observed. Addition of methanol (MeOH) to the solution inhibited ring formation if the content of MeOH was higher than 10% in volume. Depositions of CHCl solutions on graphite instead of on glass exhibited more perfect circular ring structures. Polarization and local time-resolved measurements of the fluorescence at the edge of the rings demonstrated, however, that the rings have similar spectroscopic properties on both substrates. Scanning probe microscopy techniques (AFM and NSOM) gave detailed information on the morphology of the ring. The size of the porphyrin wheels varied from 10 nm to several μm in diameter and between 10 and 200 nm in height. NSOM experiments on the nanoscale optical properties of the samples indicated that the assemblies are organized on the nanometer scale due to small molecular aggregates. Additionally, the effect of the porphyrin (PtP) concentration on the spectroscopic and scattering properties of the PtP solutions revealed that molecular aggregates are formed prior to evaporation. The implications of these results on the ring formation mechanism are discussed in this paper.

Mesostructure of Evaporated Porphyrin Thin Films: Porphyrin Wheel Formation