Self‐aggregation of Synthetic Protobacteriochlorophyll‐d Derivatives^¶

Abstract

31‐Racemically pure zinc 31‐hydroxy‐131‐oxo‐porphyrins (zinc methyl 17,18‐dehydro‐bacteriopheophorbides‐d) as well as their 31‐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(31) … Zn(central) and π‐π interaction of the porphyrin chromophores. The supramolecular structures are similar to those of the corresponding chlorins and a core part of extra‐membranous 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.