Chirality amplification of porphyrin assemblies exclusively constructed from achiral porphyrin derivatives

Two achiral porphyrin derivatives, 5,10, 15, 20-tetrakis(4-methoxyphenyl)-21H,23H-porphine (TPPOMe) and 5,10,15,20-tetrakis(4-hydroxyphenyl)-21H,23H porphine (TPPOH), were spread onto an air/water interface. The spreading films were transferred onto solid substrates by the Langmir-Schaefer (LS) method. Although both of the porphyrin derivatives are achiral species, the transferred LS multilayer films shows macroscopic supramolecular chirality, which is suggested to be due to the spontaneous symmetry breaking that occurs at the air/water interface. A strong CD signal is observed from the as-deposited TPPOH LS film, while a relatively weak CD signal is detected from that of TPPOMe. Interestingly, when the TPPOMe LS film was annealed in high vacuum, a significant amplification of the supramolecular chirality is observed. Atomic force microscopy observations confirm that TPPOMe form more ordered aggregates upon annealing. It is suggested that the small amount of chiral assemblies formed in the as-deposited LS film grow into larger ones following the "sergeants and soldiers" principle during the annealing process.