Chirality Amplification of Porphyrin Assemblies Exclusively Constructed from Achiral Porphyrin Derivatives

Abstract

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 Langmuir-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.

A General Method for Constructing Optically Active Supramolecular Assemblies from Intrinsically Achiral Water‐Insoluble Free‐Base Porphyrins

“…If the sample intrinsically had genuine chirality, both the methods would give the same results. [10,11] We measured the CD spectra of the LS film of H 2 OEP deposited from the hydrochloric acid subphase according to the above-described methods as an example to illustrate the authenticity of the observed macroscopic chirality. As shown in Figure 3A, the angle dependence of the CD amplitude was determined by the difference between the maximum value at 351 nm and minimum value at 411 nm, and the corresponding angle dependence of the background was determined by the difference between the values at the lower edge at 250 nm and upper wavelength edge at 600 nm, which are displayed as filled squares and circles, respectively.…”

“…[8][9][10][11]19] Authenticity of the observed macroscopic chirality: It has been suggested that the authenticity of the observed chirality through conventional CD measurement could not be exclusively determined for solid-state samples, because the detected CD spectra are often accompanied by artifacts which usually originate from the interaction between the macroscopic anisotropies of a sample such as birefringence and linear dichroism (LD). [20] Effective strategies for the CD measurement of thin solid films have been proposed by Spitz and co-workers, [21] and used by us previously, [10,11] to validate the authenticity of the chirality detected from the LB or LS films. This can be achieved in two ways.…”

“…For the measurement of the CD spectra, the samples were placed perpendicular to the light path and rotated within the film plane to avoid the polarization-dependent reflections and eliminate the possible angle dependence of the CD signal. [10,11,21] The AFM height images without any image processing except flattening were recorded on a Digital Instrument Nanoscope IIIa Multimode system (Santa Barbara, CA) with a silicon cantilever by using the tapping mode.…”

“…[10,11] Herein, we describe how we have established a general way to obtain chiral porphyrin assemblies from achiral porphyrins through interfacial assembly. The method involves the in situ diprotonation of the porphyrins at the air-water interface and subsequent assembly under compression.…”

Chiral assembly of achiral pseudoisocyanine with D- and L-phenylalanine

“…Recently, there has been growing interest in the supramolecular assembly of achiral molecules into chiral nanostructures with a controlled helical sense [1][2][3][4] . Among them, chiral J-aggregates of achiral cyanine dyes [5][6][7][8] and porphyrins [9][10][11][12] are particularly interesting because of their potential applications to optoelectronic materials and photosynthesis, as well as the possible explanation for the origin of biomolecular homochirality [13,14] .…”