Self-aggregation of synthetic 7- or 8-unsubstituted bacteriochlorophyll-d analogs

Abstract

Zinc methyl 3¹-demethyl-bacteriopheophorbides-d lacking the 7-methyl or 8-ethyl group were prepared from chlorophyll-b or a, respectively, via Pinnick oxidation of a formyl to carboxy group at the 7- or 8-position followed by its pyrolysis. The substitution effects of the 7-methyl and 8-ethyl groups on the optical properties of the zinc chlorophyll derivatives in THF and in an aqueous Triton X-100 micellar solution were investigated. In tetrahydrofuran (THF), all the synthetic zinc complexes were monomeric to give sharp electronic absorption bands and highly fluorescent emission bands. The Qy absorption and fluorescence emission maxima were hypsochromically shifted in the order of zinc 7-methyl-8-ethyl-, 7-methyl-8-deethyl-, and 7-demethyl-8-ethyl-chlorins, but no significant effects of the 7- or 8-dealkylation were observed in the Qx and Soret bands. In the aqueous micelles, the three zinc 3-hydroxymethyl-13¹-oxo-chlorins self-aggregated to give broadened and red-shifted absorption bands, which were similar as in the self-aggregates of bacteriochlorophylls-c/d/e in the major light-harvesting antennas of green photosynthetic bacteria (chlorosomes). The dealkylation moved the red-shifted Qy maxima of the self-aggregates to shorter wavelengths than that of the alkylated compound. The substitution effect was comparable to the bathochromic shifts in the Qy bands of the bacteriochlorophyll self-aggregates in chlorosomes by the sequential methylation at the 8-ethyl terminal. The dealkylation slightly suppressed the self-aggregation ability, revealing the CH–π interaction of the alkyl groups with the chlorin chromophore in the self-aggregated supramolecules, to affect their photophysics through the excitonic interaction dependent on geometries of the chlorin macrocycles.