Self-aggregation of synthetic protobacteriochlorophyll-d derivatives.

3(1)-Racemically pure zinc 3(1)-hydroxy-13(1)-oxo-porphyrins (zinc methyl 17,18-dehydro-bacteriopheophorbides-d) as well as their 3(1)-demethyl form were prepared by modifying chlorophyll-a through oxidation by 2,3-dichloro-5,6-dicyano-benzoquinone. From visible, circular dichroism and infrared spectral analyses, these synthetic pigments self-aggregated in 1%(vol/vol) tetrahydrofuran and cyclohexane to give large oligomers by an intermolecular bonding of 13-C=O...H-O(3(1))...Zn(central) and pi-pi interaction of the porphyrin chromophores. The supramolecular structures are similar to those of the corresponding chlorins and a core part of extramembranous light-harvesting antennas of photosynthetic green bacteria. The 17,18-dehydrogenation of a chlorin to porphyrin moiety did not disturb its self-aggregation and the synthetic zinc porphyrins are good models for naturally occurring self-aggregative bacteriochlorophylls.