No abstract

C3‐vinyl and C13‐keto‐carbonyl groups of methyl pyropheophorbide‐a, a chlorophyll‐a derivative, were systematically modified to investigate the substituent effects along the y‐axis of chlorin macrocycles. C131 deoxygenation resulted in blueshifts of the Qy absorption maxima and a large reduction of their redox potentials, whereas introduction of the dicyanomethylene group at the 131‐position caused the opposite effect on both optical and electrochemical properties. Among the nine chlorins examined, the Qy peak positions could be varied from 647 to 736 nm by the cooperative effects of the diagonal substituents' combinations.

We prepared novel zinc 8-ethyl-8-methyl-7-oxo- and 7-ethyl-7-methyl-8-oxo-bacteriochlorins 1 and 2 possessing substituents characteristic of chlorosomal chlorophylls, exclusively observed in extramembraneous light-harvesting antennas of photosynthetic green bacteria. The electronic absorption spectra of monomeric 1 and 2 in THF were obviously different: the Q(y) maximum of the former was 724 and that of the latter was 683 nm. This observed spectral difference was clearly explained by theoretical ZINDO/S calculation of their energetically minimized molecules. The optical properties of monomeric 1/2 were controlled by the electronic effect of the 7/8-oxo groups. Specific spectral changes in the electronic, CD, and FT-IR absorption spectra by dilution of the monomeric THF solutions of 1/2 with a 100/200-fold volume of cyclohexane showed the formation of chlorosomal self-aggregation species constructed by 13-C=O...H-O(3(1))...Zn and pi-pi stacking. Especially, the red-shift values in the Q(y) band of 1/2 by self-aggregation were 2450/1970 cm(-1), indicating that exciton interaction among the composite molecules in the self-aggregation of 1 was stronger than in those of 2. Molecular model calculations for dodecamers of 1/2 based on a parallel chain arrangement gave partially different supramolecular structures; the specific hydrogen-bonding distances in 2-dodecamer were larger than those of 1-dodecamer, while both coordinations gave the same Zn-O distance. These modeling results showed that 1 was more tightly packed in the self-aggregates to give a larger red-shift value in the Q(y) band by self-aggregation than 2. The difference in the supramolecular structures is mainly ascribable to the steric effect of 8/7-dialkyl groups in self-aggregates of 1/2.

We have synthesized zinc 31‐hydroxychlorins 1–4 possessing a methoxycarbonyl group at the 13‐position by modifying naturally occurring chlorophyll‐a. Synthetic zinc chlorins 1–4 have three specific substituents, namely 31‐OH, central Zn and 13‐C=O moieties, along the Qy axis, and 1–3 self‐aggregate in an aqueous medium containing Triton X‐100, as do natural bacteriochlorophylls in the main light‐harvesting antennas (chlorosome) of green photosynthetic bacteria. The 13‐methoxycarbonyl group in 1–3 is a prerequisite for such self‐aggregation, while the other methoxycarbonyl groups at the 151‐ and/or 172‐positions are not. This indicates that such a linear situation of these substituents is significant for chlorosomal self‐aggregation of chlorophyllous pigments and also that the 13‐methoxycarbonyl group is an alternative to the 13‐keto‐carbonyl group in natural pigments. Furthermore, the resulting oligomers of 1–3 have fluorescence emission peaks at less than 700 nm, which are considerably more blue‐shifted than previously prepared self‐aggregates of chlorins possessing the 13‐keto‐carbonyl group fixed onto an exo five‐membered ring, and are a new class of artificial light‐harvesting systems. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

3-Deacetyl-3-(1-hydroxyethyl)bacteriopyrochlorophyll-a (1), 7,8-dihydrobacteriochlorophyll-d possessing 8-ethyl, 12-methyl and 17(4)-phytyl groups, was prepared by modifying naturally occurring bacteriochlorophyll-a. The synthetic 3(1)-epimers were separated by high-performance liquid chromotagraphy, and the absolute configuration at the 3(1)-position was determined by derivatization of 1 to a structurally determined chlorin. A dichloromethane solution of 3(1)R-1 or 3(1)S-1 was diluted with a 1000-fold volume of cyclohexane to give self-aggregation species absorbing light at a near-infrared (NIR) region (<910 nm). The resulting Qy maximum in 3(1)R-1 was 860 nm and redshifted by 2170 cm(-1) from the monomeric one, whereas epimeric 3(1)S-1 showed a less redshifted peak at ca 800 nm, with a small dimeric band around 740 nm. Such visible spectra indicated that 3(1)R/S-1 formed different supramolecular structures in the self-aggregates. In contrast, self-aggregation of the 7,8-dehydro-compound 2, bacteriochlorophyll-dP, found in natural antennas of photosynthetic green bacteria showed much smaller diastereomeric control. The self-aggregates of 3(1)R-1 absorbing light in the NIR region would be models of intrinsic membraneous light-harvesting complexes 1 in photosynthetic purple bacteria as well as extramembranous antennas in green bacteria.