Self‐aggregation of Synthetic Zinc Chlorophyll Derivatives Possessing 3 1 ‐Hydroxy or Methoxy Group and 13 1 ‐Mono‐ or Dicyanomethylene Moiety in Nonpolar Organic Solvents as Models of Chlorosomal Bacteriochlorophyll‐ d Aggregates

Methyl 13 1 ‐(di)cyanomethylene‐pyropheophorbides were synthesized by Knoevenagel reactions of the corresponding 13 1 ‐oxo‐chlorins prepared from modifying chlorophyll‐ a with malononitrile or cyanoacetic acid. Alternatively, methyl 13 1 ‐cyanomethylene‐pyropheophorbides were produced by Wittig reactions of 13 1 ‐oxo‐chlorins with Ph 3 P= CHCN . Self‐aggregation of zinc complexes of the semi‐synthetic chlorophyll derivatives possessing a hydroxy or methoxy group at the 3 1 ‐position was examined in 1%(v/v) tetrahydrofuran or dichloromethane and hexane by electronic absorption and circular dichroism spectroscopy. Although intermolecular hydrogen‐bonding between the 3 1 ‐hydroxy and 13 1 ‐oxo groups of bacteriochlorophylls‐ c / d / e / f was essential for their self‐aggregation in natural light‐harvesting antenna systems (=chlorosomes), zinc 3 1 ‐hydroxy‐13 1 ‐di/monocyanomethylene‐chlorins self‐aggregated in the less/lesser polar organic solvents to form chlorosome‐like large oligomers in spite of lacking the 13 1 ‐oxo moiety as the hydrogen‐bonding acceptor. Zinc 3 1 ‐methoxy‐13 1 ‐dicyanomethylene‐chlorin gave similar self‐aggregates regardless of lack of both the 3 1 ‐hydroxy and 13 1 ‐oxo groups. The present self‐aggregation was ascribable to stronger coordination of the 3 1 ‐oxygen atom to the central zinc than the conventional systems, where the electron‐withdrawing cyano group(s) increased the coordinative ability of the central zinc through the chlorin π ‐system.