Effect of additional hydroxy group on self-aggregation of synthetic zinc bacteriochlorophyll-c analogs

Zinc methyl 3-hydroxymethyl-pyropheophorbides-a possessing a hydroxymethyl or 2-hydroxyethyl group at the 20-position were prepared by chemical modification of naturally occurring chlorophyll-a. The synthetic bacterio-chlorophyll-c analog bearing the 20(1)-hydroxy group self-aggregated in an aqueous Triton X-100 micellar solution to give large oligomers with red-shifted and broadened electronic absorption bands, which was similar to the chlorophyllous J-aggregates in the main light-harvesting antennas of green photosynthetic bacteria. By contrast, the other 20(2)-hydroxylated analog did not aggregate under the same conditions to remain its monomeric species. The red shift in the longest wavelength (Qy) band by the self-aggregation of the former analog was greater than that of the corresponding 20(1)-methyl substitute with a similar size (CH3 approximate to OH). These results could be ascribable to the less sterically demanding and more electron-withdrawing 20-hydroxymethyl group, compared with more interacting 2-hydroxyethyl and more electron-donating ethyl groups at the 20-position.