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Issue 11, 2011
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De novo synthesis and properties of analogues of the self-assembling chlorosomal bacteriochlorophylls

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Abstract

Natural photosynthetic pigments bacteriochlorophyllsc, d and e in green bacteria undergo self-assembly to create an organized antenna system known as the chlorosome, which collects photons and funnels the resulting excitation energy toward the reaction centers. Mimicry of chlorosome function is a central problem in supramolecular chemistry and artificial photosynthesis, and may have relevance for the design of photosynthesis-inspired solar cells. The main challenge in preparing artificial chlorosomes remains the synthesis of the appropriate pigment (chlorin) equipped with a set of functional groups suitable to direct the assembly and assure efficient energy transfer. Prior approaches have entailed derivatization of porphyrins or semisynthesis beginning with chlorophylls. This paper reports a third approach, the de novo synthesis of macrocycles that contain the same hydrocarbon skeleton as chlorosomal bacteriochlorophylls. The synthesis here of Zn(II) 3-(1-hydroxyethyl)-10-aryl-131-oxophorbines (the aryl group consists of phenyl, mesityl, or pentafluorophenyl) entails selective bromination of a 3,13-diacetyl-10-arylchlorin, palladium-catalyzed 131-oxophorbine formation, and selective reduction of the 3-acetyl group using BH3·tBuNH2. Each macrocycle contains a geminal dimethyl group in the pyrroline ring to provide stability toward adventitious dehydrogenation. A Zn(II) 7-(1-hydroxyethyl)-10-phenyl-17-oxochlorin also has been prepared. Altogether, 30 new hydroporphyrins were synthesized. The UV-Vis absorption spectra of the new chlorosomal bacteriochlorophyll mimics reveal a bathochromic shift of ∼1800 cm−1 of the Qy band in nonpolar solvent, indicating extensive assembly in solution. The Zn(II) 3-(1-hydroxyethyl)-10-aryl-131-oxophorbines differ in the propensity to form assemblies based on the 10-substituent in the following order: mesityl<phenyl<pentafluorophenyl. Infrared spectra show that assemblies also can be formed in solid media and likely involve hydrogen-bonding (or other) interactions of the ring E keto group.

Graphical abstract: De novo synthesis and properties of analogues of the self-assembling chlorosomal bacteriochlorophylls

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Publication details

The article was received on 12 Jul 2011, accepted on 09 Aug 2011 and first published on 19 Sep 2011


Article type: Paper
DOI: 10.1039/C1NJ20611G
Citation: New J. Chem., 2011,35, 2671-2690
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    De novo synthesis and properties of analogues of the self-assembling chlorosomal bacteriochlorophylls

    O. Mass, D. R. Pandithavidana, M. Ptaszek, K. Santiago, J. W. Springer, J. Jiao, Q. Tang, C. Kirmaier, D. F. Bocian, D. Holten and J. S. Lindsey, New J. Chem., 2011, 35, 2671
    DOI: 10.1039/C1NJ20611G

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