Macroscopic Films of Porphyrin Nanowell-Arrays via Solvent Diffusion-Induced Self-Assembly

The self-assembly of neutral chiral porphyrins in aqueous solution was studied by absorption, fluorescence, circular dichroism (CD) spectroscopy, and optical and electron microscopy. Different supramolecular structures of the porphyrins were obtained by changing the diffusion rate of water into the porphyrin organic solution. When water was mixed and homogenized with the porphyrin solution immediately, the absorbance at the both sides of absorption Soret band increased along with hypochromism, indicating that J- and H-type self-assembly occurred simultaneously, and AFM measurements showed that porphyrin nanoparticles approximately 250 nm formed. On the other hand, the significant red-shift of the absorption Soret band and very long porphyrin microtubes observed by optical microscopy clearly demonstrated that J-type intermolecular packing increased significantly when porphyrin solution was slowly diffused into water. These observations indicate that the J- and H-type self-assembly pathways are kinetic and thermodynamic, respectively, and different self-assembly structures could be obtained by controlling the kinetic and thermodynamic pathways, which was also supported by theoretical calculations. In particular, porphyrin macroscopic nanowell-array films, which morphology is ideal for organic photovoltaic devices and biosensors, were obtained by diffusing water vapor into porphyrin organic solution. The methodology and results reported here may provide a novel, simple, and efficient approach to obtain well-defined self-assembly nanostructures.