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Title: Chiroptical properties of an alternatingly functionalized cellotriose bearing two porphyrin groups

item SAKAKIBARA, KEITA - University Of Natural Resources & Applied Life Sciences - Austria
item NAKATSUBO, FUMIAKI - Kyoto University
item French, Alfred - Al
item ROSENAU, THOMAS - University Of Natural Resources & Applied Life Sciences - Austria

Submitted to: Journal of the Chemical Society Chemical Communications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/21/2012
Publication Date: 3/22/2012
Citation: Sakakibara, K., Nakatsubo, F., French, A.D., Rosenau, T. 2012. Chiroptical properties of an alternatingly functionalized cellotriose bearing two porphyrin groups . Journal of the Chemical Society Chemical Communications. 48: 7672-7674.

Interpretive Summary: Because the cellulose molecule has a fairly rigid linear shape, chemical modifications can be imagined that would provide a linear array of the added chemical groups. These groups could add varying functionality, including the ability to act as the current-generating part of a solar cell. This paper adds to knowledge about these modified molecules, and how they absorb light. A short fragment of the long cellulose molecule was chemically synthesized and studied, among other analytical techniques, with ultraviolet light and a technique called circular dichroism. It is easier and more rigorous to study short fragments by both experimental and theoretical methods. A mystery that was solved in this work is that while cellulose itself tends to have a somewhat left-handed twist, the circular dichroism work showed a right-handed twist. This was resolved by careful consideration of how helices are defined. The synthesized molecule was also found to make a non-bonded complex with a fullerene molecule commonly known as a buckyball. This research is of interest to scientists developing new and improved uses for materials based on cellulose, such as cotton fabrics or paper.

Technical Abstract: Alternatingly functionalized cellulose molecules have potential applications in optoelectronics and molecular receptors. For example, cellulose-based solar cells have been proposed. As a prototype for such molecules, the trisaccharide fragment of cellulose was modified by attachment of porphyrin groups on the first and third glucose residues. This compound was characterized by NMR, IR, UV-VIS and CD spectroscopy, with indications of interactions between the pi-electrons on the two prophyrin rings allowed by their regular separation along one side of a ribbon-like cellulose molecule. The complexation with the C60 fullerene molecule was also studied. An interesting finding is that while the cellulose backbone tends to take a somewhat left-handed twist, the CD study of the alternatingly substituted cellotriose showed a right-handed arrangement. The two chiralities are in agreement, however, because of the alternating substitution. The repeating unit of the cellulose molecule is the glucose residue, but the repeating unit of an alternatingly substituted cellulose helix is the substituted cellobiose residue. In a left-handed, three-fold cellulose helix, each successive glucose unit is translated about 5 angstroms from its predecessor and rotated -120 degrees. Following that same backbone, a substituted cellobiose unit could be translated 10 angstroms and rotated -240 degrees, or rotated +120. Since that is the shorter rotation, the helix of cellobiose units is considered right handed.