sujets memoire 2014_Duwez

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Nanochimie et Systèmes Moléculaires

Prof. A.-S. Duwez

Sujets de mémoire

1. Etude de nano-architectures non conventionnelles: les oligorotaxanes

Les foldamères sont des architectures moléculaires synthétiques repliées, inspirées des structures et

fonctions des biopolymères naturels. Le repliement est un processus sélectionné par la nature pour

pouvoir réaliser des tâches chimiques ou mécaniques, comme la catalyse enzymatique, le stockage

d'information, la duplication des acides nucléiques, le transport,...

Etant donné que les repliements des biomolécules sous-tendent leur fonction, nous pouvons imaginer

qu'en conférant aux molécules synthétiques des structures secondaires repliées, un vaste éventail de

matériaux fonctionnels avec de nouvelles propriétés émergentes serait potentiellement accessible. La

synthèse de molécules artificielles adoptant des structures secondaires repliées est en effet un

domaine de recherche très actif et très riche ces dernières années.

Très récemment, une classe d'oligomères synthétiques se trouvant à l'intersection entre le domaine

des molécules mécaniquement entrelacées et les foldamères synthétiques a émergé: les

oligorotaxanes. Ce système est le tout premier à combiner un motif de repliement et une structure

entrelacée mécaniquement liée. La structure secondaire à l'état solide a été résolue par

cristallographie aux rayons X. Les caractéristiques structurales des molécules en solution ont été

étudiées par spectroscopie RMN. Cependant, la structure détaillée et la dynamique du système en

solution restent controversées et l'idée même d'une structure repliée en solution est un sujet à débat.

Nous proposons ici d'étudier des foldamères d'oligorotaxanes par spectroscopie de force AFM afin de

résoudre les études contradictoires (ou incomplètes) obtenues par des techniques d'ensemble, et

ainsi obtenir des informations détaillées sur la structure et les propriétés mécano-chimiques.

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observations, we can infer the following empirical selection

rule: the most stable co-conformation

in any of the oligorotaxanes

will be one in which no two CBPQT

rings reside on adjacent DNP

sites. This rule simultaneously explains the presence of multiple

translational isomers in the Confused series and the line

broadening observed in the Frustrated series, since multiple co-

conformations can be identied to satisfy the rule in the former,

while no translational isomers will satisfy the selection rule in

the case of the latter. Thus, we use the term Confused to imply a

rotaxane that expresses multiple stable translational isomers

that are slow to equilibrate on the

H NMR timescale. A Frus-

trated rotaxane, on the other hand, equilibrates more rapidly

between multiple higher-energy states. It seems that the Happy

series exhibits a ‘Goldilocks’

eﬀect, where the number of rings

and recognition units are perfectly matched so as to make a

single ‘happy’translational isomer far more stable than any

alternative. These anthropomorphic concepts are illustrated

graphically using the 7NP oligorotaxanes (the smallest set we

isolated with members in all three ‘moods’) as examples in

Fig. 2. A selection rule that forbids the CBPQT

rings to encircle

adjacent DNP sites also agrees well with a secondary structure

that is stabilized by continuous D–A–D–A stacking, as

observed

in the donor–acceptor pseudorotaxanes with 50%

recognition-site occupancy in the solid state.

Although the Confused oligorotaxanes adopt multiple trans-

lational isomers, the species with the longest D–A stacking

motif will be the most highly stabilized and therefore the

dominant isomer. In the simplest Confused rotaxane [2]3NPR

for example, the most stable translational isomer –that with the

CBPQT

ring encircling the central DNP unit –is adopted by

80% of the molecular population (see Section 3B of the ESI†).

The remaining 20% of the molecules adopt a co-conformation

with the CBPQT

on the periphery, where the stabilizing D–A

interactions are inevitably weaker on account of the ring's

access to half as many ‘alongside’DNP units. This 4 : 1 distri-

bution indicates that there is only a small diﬀerence between

the free energies (<1 kcal mol

) of the various rule-compliant

isomers of the Confused oligorotaxanes. There is no indication

that Happy oligorotaxanes are signicantly more stabilized than

the dominant Confused rotaxanes, however, despite the bias in

favor of Happy oligorotaxanes in the product distributions.

Rather, the preference for Happy oligorotaxanes during the

stoppering protocol is most likely a result of using CBPQT

nearly 1 : 1 stoichiometry with the DNP recognition units,

which amounts to an excess with respect to the half-occupancy

foldamers. Indeed, the entropic penalty for adding more rings

onto the dumbbells of Happy rotaxanes may compete with the

enthalpic gains earned from the extra D–A interactions enabled

by the BIPY

units of the terminal cyclophanes. In any event,

only the Happy family of oligorotaxanes (Fig. 3) are conducive to

detailed solution-state analysis by high eld

H NMR

Fig. 2 A Goldilocks eﬀect in donor–acceptor oligorotaxanes, exempliﬁed by the

7NP-based compounds. The [0.5(n!1) + 1]rotaxane [4]7NPR

12+

has up to six

translational isomers that satisfy the selection rule forbidding CBPQT

rings to

encircle adjacent DNP sites, eliciting the ‘Confused’designation. On the other

hand, it is impossible to draw a structure that obeys the rule for the [0.5(n!1) +

3]rotaxane [6]7NPR

20+

, hence it is ‘Frustrated’. Only the [0.5(n!1) + 2]rotaxane

[5]7NPR

16+

has a single ‘Happy’translational isomer that satisﬁes the rule.

Fig. 3 Molecular structures of the oligomers in the Happy series of donor–acceptor oligorotaxanes.

This journal is ªThe Royal Society of Chemistry 2013 Chem. Sci.,2013,4,1470–1483 | 1475

Edge Article Chemical Science

Downloaded by Universite De Liege on 17/04/2013 14:48:12.

Published on 04 February 2013 on http://pubs.rsc.org | doi:10.1039/C3SC00015J

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2. Etude de nano-architectures hélicoidales non conventionnelles: les foldamères

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