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Methods for engineering catenane and rotaxane-based materials

Methods have been developed to enable attachment of rotaxanes to various surfaces, such as self-assembled monolayers on silicon, terminated with carboxylic acids, and on glass. We have adapted the synthesis procedures for covalent attachment of alkenes to Si(111) or Si(110).

When simple alkenes were used, densely packed monolayers could be easily formed, in agreement with the literature. In order to obtain a carboxylic acid terminated monolayer, procedures were followed in which alkene esters (R=Me or R =CH2CF3) were attached to the surface and subsequently hydrolyzed.
In this case, it was found that the published procedures do not work well. Complete hydrolysis of all ester groups could not be accomplished without damaging the monolayer, as demonstrated by XPS and contact angle measurements in collaboration with the Groningen group. For the purpose of attaching rotaxanes using hydrogen bonding to the COOH-groups it is, luckily, sufficient to hydrolyse only a part of the ester groups.

Furthermore, procedures were established to immobilize rotaxanes on glass, starting with the condensation of aminopropyltriethoxysilane to the SiOH groups on the surface. This method may be less attractive for creating monolayers because it is known to produce less ordered materials than the procedure described above, but for immobilization of single rotaxane molecular switches it will be particularly suitable.
Methods for detecting time-resolved fluorescence of rotaxane monolayers have been established. New rotaxanes have become available which can be attached to surfaces, and measurements will be carried out in the near future.

The effect of thickness of the alkane spacer on the photophysical properties was investigated for the rotaxane that was studied previously on a self-assembled monolayer of mercaptoundecanoic acid on gold (F. Cecchet, P. Rudolf, S. Rapino, M. Margotti, F. Paolucci, J. Baggerman, A. M. Brouwer, E. R. Kay, J. K. Y. Wong, D. A. Leigh, J. Phys. Chem. B 2004, 108, 15192.). Surprisingly, the same molecule on a SAM of hexadecanoic acid showed a spectrum that was blue-shifted compared with that on the thinner C11 spacer, and similar to that in solution. Apparently, when the SAM is thin enough, the metal not only influences the fluorescence intensity, but also the fluorescence spectrum.

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University of Edinburgh
Forbes Chair of Organic Chemistry West Main's Road
EH9 3JJ Edinburgh
United Kingdom
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