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Self-Aggregation of Synthetic Zinc Oxo-Bacteriochlorins Bearing Substituents Characteristic of Chlorosomal Chlorophylls

2004; American Chemical Society; Volume: 70; Issue: 3 Linguagem: Inglês

10.1021/jo048263p

1520-6904

Michio Kunieda, Hitoshi Tamiaki,

Photoreceptor and optogenetics research

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 Qy 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-CO···H−O(31)···Zn and π−π stacking. Especially, the red-shift values in the Qy 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 Qy 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.