A heterodimer, where zinc pyropheophorbide-a was linked with zinc pyropheophorbide-d through ethylene glycol diester, was prepared, as well as the corresponding homodimers. The synthetic dimers were complexed with methanol in benzene to give folded dimers by mutual Zn...O(Me)-H...O=C13(1) bonding. Such complexes had furthest red (Qy) absorption bands at longer wavelengths than the monomeric species. These red-shifts were ascribable to excitonic coupling of the Qy transition states in the chlorin pi-pi stacking conformer. In the heterodimeric system, a minor band was observed at the shorter wavelength side of the main Qy band. This observation can be explained by an additional contribution of Qy vibronic state to the exciton-coupled states. Based on the experimental results, a pair of chlorophyll(Chl)-d with Chl-a as well as a Chl-d homopair were proposed as dimers in reaction centers of Chl-d dominating cyanobacteria.

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.

Bacteriochlorophyll(BChl)-f which has not yet been found in natural phototrophs was prepared by chemically modifying chlorophyll-b. The retention time of reverse-phase high-performance liquid chromatography of the synthetic monomeric BChl-f as well as its visible absorption and fluorescence emission spectra in a solution were identified and compared with other naturally occurring chlorophyll pigments obtained from the main light-harvesting antenna systems of green sulfur bacteria, BChls-c/d/e. Based on the above data, BChl-f was below the level of detection in three strains of green photosynthetic bacteria producing BChl-e.

Covalently linked zinc chlorin and zinc porphyrin/bacteriochlorin heterodyads formed methanol-locked conformers with red-shifted Qy bands as models of Chl-a and Chl-c/BChl-a(or g) dimeric species, respectively, in photosynthetic apparatuses.

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