20-Substitution effect on self-aggregation of synthetic zinc bacteriochlorophyll-d analogs

Self-aggregation of synthetic zinc 3-hydroxymethyl-chlorophyll-a derivatives possessing electron-withdrawing groups at the 20-position in aqueous micelle solution

Zinc methyl 3-hydroxymethyl-pyropheophorbides-[Formula: see text] bearing an acetyl or nitro group at the 20-position were prepared by chemically modifying natural chlorophyll-[Formula: see text]. The synthetic zinc 20-substituted bacteriochlorophyll-[Formula: see text] analogs self-aggregated in an aqueous Triton X-100 micelle solution to give red-shifted and broadened electronic absorption bands, similarly as in one of the photosynthetic light-harvesting antennas, chlorosomes. Although the sterically bulky 20-substituents of the semisynthetic zinc 31-hydroxy-131-oxo-chlorins partially disturbed the chlorosomal self-aggregates which would be produced by the [Formula: see text]–[Formula: see text] stacking of the chlorin chromophores, their electron-withdrawing abilities should enhance the specific, intermolecular interaction of Zn[Formula: see text]O32–H[Formula: see text]O=C-13 which was essential for the chlorosomal self-aggregation. Such self-assemblies using these model pigments would provide a new clue for constructing of artificial photosynthetic systems.

Spectral and Structural Variations of Biomimetic Light-Harvesting Nanotubes

Bioinspired, self-assembled nanotubes have been investigated by low-temperature, polarization-resolved single-tube spectroscopy. These assemblies are based on zinc chlorin monomers and are considered as model systems that resemble the secondary structural elements in the natural light-harvesting systems of green (non)sulfur bacteria. Compared to the natural systems, the spectral parameters extracted from the single-nanotube spectra feature distributions with significantly smaller widths, which is ascribed to a tremendous reduction of structural heterogeneity in the artificial systems. Employing quantum chemical molecular modeling the spectra of individual nanotubes can be explained consistently only for a molecular packing model that is fundamentally different from those considered so far for the natural systems. Subsequent theoretical simulations reveal that the remaining spectral variations between single nanotubes can be traced back to small variations of the mutual orientations of the monomer transition dipole moments that are far beyond the resolving power of high-resolution electron microscopy imaging techniques.