Synthesis of bacteriochlorophyll-d analogs with peripheral 7,172-bis(alkoxycarbonyl) groups and their substituent-dependent self-aggregation in lipophilic alkanes

Four zinc 3 1 -demethyl-bacteriopheophorbides- d possessing methoxy/hexyloxycarbonyl groups at the 7- and 17 2 -positions along the molecular x-axis were prepared by chemical modification of naturally occurring chlorophyll- b . The synthetic compounds were monomeric states in tetrahydrofuran (THF) to give Soret and Qy bands at longer and shorter wavelengths, respectively, than those of the 7-methyl counterpart. The changes in the electronic absorption spectra were ascribable to the electronegativity of the 7-substituents. In 1% THF and decane, the zinc 3 1 -hydroxy-13 1 -oxo-chlorins self-aggregated along the molecular y-axis to exhibit broadened and red-shifted Soret and Qy bands along with intense circular dichroism bands, compared to their monomeric bands. These spectral behaviors were comparable to those of bacteriochlorophyll- d self-aggregates in the major light-harvesting antenna of photosynthetic green bacteria (chlorosomes). After standing the decane solution at 25 °C, the supramolecular structure of the self-aggregate of the 7-hexyl/17 2 -methyl-esterified homolog was transformed into another assembly with more tolerant to deaggregation, whereas the other three homologs did not alter their self-aggregate supramolecules. This specific transformation was due to the hydrophobic interaction of the hexyl group in the 7-substituent with the lipophilic decane solvent and less interaction of the methyl group in the 17-propionate residue with decane. The supramolecular transformation was dependent on the solvent hydrophobicity and viscosity and occurred through an autocatalytic process.