Synthesis and Self-Assembly of Zinc Methyl Bacteriopheophorbide-f and its Homolog

Zinc methyl bacteriopheophorbide-f, zinc 3-(1-hydroxyethyl)-7-formyl-chlorin, and its 31-demethyl derivative were prepared by modification of naturally occurring chlorophyll-b. The former complex was a 31-epimeric mixture and separated to pure stereoisomers by HPLC, while such a separation was unnecessary for the latter possessing the 3-hydroxymethyl group. The absolute configuration at the 31-position of the separated samples was determined by transformation of the 7-formyl to the 7-methyl group because the 31-stereochemistry of zinc methyl bacteriopheophorbide-d (3-(1-hydroxyethyl)-7-methyl-chlorin) has been confirmed. All the synthetic zinc chlorins self-aggregated in 0.5% (v/v) THF–hexane as well as in 6% (v/v) THF–water to form oligomers which absorbed longer-wavelength lights by the J-aggregation than the monomers in THF. Diastereomeric control in the self-aggregation of the 31-epimers was observed. The 31-demethyl compound interacted more tightly in the self-aggregates than the 31-epimers. Comparing with the corresponding 7-methyl analogs, zinc 7-formyl-chlorins have red-shifted Soret and blue-shifted Qy bands in both the monomeric and aggregated states. Visible, circular dichroism and resonance Raman spectra showed that the supramolecular structures of self-aggregates of the 7-formyl and 7-methyl derivatives were similar and highly ordered, and were built-up by 13-CO⋯H(3-CHR)O⋯Zn and π–π interaction of the chlorin chromophores. The coordinatable 7-formyl group did not disturb the special bondings of the composite molecules in the self-aggregates.