Research On The LB Films And Supramolecular Chirality Of Several Achiral Porphyrins

We herein studied the chiral LB assemblies of several achiral porphyrins, mainlyinvestigate the generation, amplification and control of supramolecular chirality, andsimply expound the factors of chirality amplification, together with a kind of simplemethod to control supramolecular chirality mechanically.To begin with, molecular assemblies of two achiral porphyrins with differentsubstituents,5-(4-methoxycarbonylphenyl)-10,15,20-triphenyl-21H,23H-porphine(TPPCOOMe) and5-(4-carboxyphenyl)-10,15,20-triphenyl-21H,23H-porphine(TPPCOOH), have been fabricated by LB technique. It is disclosed that although onlyslight differences exist in the molecular skeleton of these two compounds, theirinterfacial assemblies display distinct chiroptical properties. It is found that weak CDsignals are observed from the TPPCOOH assemblies, while in the case of theTPPCOOMe assemblies, only negligible CD signals could be detected. Interestingly,after the assemblies are subjected to a thermal annealing treatment, TPPCOOHassemblies show a distinct amplification of CD signals, while those of TPPCOOMe donot. An explanation in terms of the effect of substituents on the spreading properties ofthe compounds, and the effect of intermolecular hydrogen-bonds on the cooperativestacking of the building blocks, is proposed to explain these new findings. Theinvestigation suggests that, during the assembling progress, in the present porphyrinsystems besides a nice spreading property, the cooperative interaction of variousnon-covalent interactions, including hydrogen-bond, π-π stacking and hydrophobicinteraction, is essentially required for the occurrence of symmetry breaking at theair/water interface.Besides, a new method is described through which the macroscopic chirality ofinterfacial molecular assemblies of an achiral porphyrin, can be mechanicallycontrolled using an original yet efficient LB technique. By using the unilateralcompression geometry of5,10,15,20-tetra-p-tolyl-21H,23H-porphine (H2TPPMe), wefind that the assemblies deposited from the mirror regions of the LB trough display mirror macroscopic chirality. Moreover, the standard sample-fabrication method withbilateral compression geometry is reformed, and we find that the samples formulatedaround the left-hand-and right-hand-side Langmuir barriers display oppositemacroscopic chiralities. Combining experimental results and computer simulations, weproved that vortex-like flows could be generated during compression, and that it is thedirection of this compression-generated vortex-like flow that determine themacroscopic chirality of the formed assemblies. The results suggest that mechanicallycontrolled supramolecular chirogenesis could be efficiently realized through such anLB technique. The investigation establishes a new forum for further investigation ofthe mechanically induced preferred supramolecular chirality in terms of interfacialorganization, and provides the old LB technique with new opportunities for controllingthe macroscopic chirality of a supramolecular system that is wholly composed ofachiral units.