Synthesis of zinc 13-oxime-functionalized chlorophyll-a derivatives and their (131E/Z)-dependent self-aggregation

Abstract

Zinc methyl 3-hydroxymethyl-pyropheophorbides-a possessing hydroxyimino, acetoxyimino, and benzoyloxyimino groups at the 131-position were prepared by chemical modification of naturally occurring chlorophyll-a. The (131E)- and (131Z)-stereoisomers of the obtained unsymmetric oximes were readily separated by column chromatography, and their stereochemistry was determined by 1H NMR spectroscopy. The synthetic O-(un)substituted oximes were monomeric in tetrahydrofuran and their electronic absorption spectra were dependent on the stereochemistry as well as the O-substituents in the oxime moiety. These chlorophyll-a derivatives self-aggregated in an aqueous Triton X-100 micellar solution to give red-shifted absorption bands. The J-aggregation ability of the (131E)-isomers was higher than that of the corresponding the (131Z)-isomers. Such stereoselective self-aggregation was comparable to that of bacteriochlorophylls-c, d, e, and f in the main light-harvesting antennas of green photosynthetic bacteria.