Synthesis of Zinc Chlorophyll Homo/Hetero‐Dyads and their Folded Conformers with Porphyrin, Chlorin, and Bacteriochlorin π‐Systems

Abstract

Zinc complex of pyropheophorbide-b, a derivative of chlorophyll-b, was covalently dimerized through ethylene glycol diester. The synthetic homo-dyad was axially ligated with two methanol molecules from the β-face and both the diastereomerically coordinating methanol species were hydrogen bonded with the keto-carbonyl groups of the neighboring chlorin in a complex. The resulting folded conformer in a solution was confirmed by visible, (1) H NMR and IR spectra. All the synthetic zinc chlorin homo- and hetero-dyads consisting of pyropheophorbides-a, b and/or d took the above methanol-locked and π-π stacked supramolecules in 1% (v/v) methanol and benzene to give redmost (Qy) electronic absorption band(s) at longer wavelengths than those of the corresponding monomeric chlorin composites. The other zinc chlorin and bacteriochlorin homo-dyads completely formed similar folded conformers in the same solution, while zinc inverse chlorin and porphyrin homo-dyads partially took such supramolecules. The J-type aggregation to folded conformers and the redshift values of composite Qy bands were dependent on the electronic and steric factors of porphyrinoid moieties in dyads.