PhotoTRAP · Photo-Triggered Reversible Assembly of Polymers in Water

Photo-Triggered Reversible Assembly of Polymers in Water

The precision with which nature creates rich functionality through the hierarchical self-organisation of multiple components is fascinating. A major challenge in chemistry is to produce and regulate artificial functional supramolecular structures in a similar manner. Supramolecular self-assembly offers a variety of elegant and energy-efficient bottom-up strategies for the preparation of complex functional materials showing controlled actuation in aqueous media. A promising self-assembly motif to address the challenges of molecular recognition in water is based on the barrel-shaped cyclic oligomer cucurbit[n]uril (CB[n]) (1). This family of macrocyclic host molecules shows strong and selective binding properties on account of both rigidity and multiple non-covalent interactions. Particularly, CB[8] is a remarkable host molecule as it is able to bind two organic guests simultaneously with a high association constant. Kim et al. have shown that a stable ternary complex between methyl viologen (MV), as a first guest, and 2,6-dihydroxynaphthalene (DHN), as a second guest, readily forms inside the cavity of CB[8] (2). Furthermore, the ternary complex is known to form in a stepwise manner with two subsequent equilibria; a typical first guest for CB[8] is an electron deficient moiety, often MV, while there are a wide variety of possible second guests that can be employed, including photoisomerizable moieties such as azobenzenes. The photoinduced isomerisation of azobenzene derivatives is at the origin of a variety of photosensitive materials and non-covalent systems; and the inclusion of azobenzene compounds as guests in different macrocyclic host molecules (including cyclodextrins, calixarenes, pillarenes and, CB[n]s) can give rise to photosensitive host-guest complexes. The photocontrolled molecular recognition of hosts with azobenzenes has been used to develop hydrogels, micelles, molecular shuttles, ion channels, and drug-delivery vehicles (3). With respect to the combination of azobenzene compounds and CB[n]s Isaacs reported in 2009 that the inclusion of 4,4'-diaminoazobenzene in CB[7] (aqueous solution at pH 3–6) can promote the trans-to-cis isomerization of the guest as the high binding energy of the complex overcompensates the thermodynamic cost for populating the trans isomer (4). The host group at the University of Cambridge has recently developed a methodology to control the formation of azobenzene-containing ternary CB[8] complexes by light irradiation and exploited this phenomena in the preparation of 'smart surfaces' (5). This methodology exploits neutral azobenzene compounds as second guests for the complex; which may narrow its applicability due the low water solubility of the azobenzene-containing materials. Therefore, the main goal of the PHOTOTRAP project was to development of a light-triggered stimuli responsive supramolecular complex from fully water-soluble building blocks and its utilisation in the reversible controlled assembly of macromolecular architectures in aqueous media. The fellow, in close collaboration with the scientist in charge and colleagues at the host group, has investigated a variety of cationic small-molecule azobenzene-containing guests for CB[8] and the control of the stoichiometry of these complexes by means of light irradiation. In comparison to previous works, the new photosensitive guests exhibit both excellent water solubility and high binding association. The type of mechanism for CB[8] complexation / decomplexation under light irradiation had never been reported before and allowed for the development of new supramolecular polymers and host-guest inclusion complexes. The fellow's work has had a large impact in the field and his contribution has certainly advanced the state-of-the-art of stimuli-responsive non-covalent materials. The most important results of PHOTOTRAP are summarised as follows: - Synthesis and characterisation of a series of azobenzene-containing CB[8] complexes which enable reversible control over the stoichiometry of complexes. - The development of versatile methods to anchor CB[n]-binding moieties to a variety of preformed polymeric architectures. - The preparation and characterisation of stimuli-responsive compartmentalised structures in water which are based on a mixture of CB[8] and two complementary end-functionalised homopolymers. The application of these block copolymer micelles as substances delivery vehicles has also been investigated. - The preparation of unique single-chain polymeric nanoparticles showing hierarchical self-organised structures in water. - The preparation and full characterization of new photoresponsive supramolecular polymer from low molecular weight building blocks. - The solid state characterisation of a series of CB[8] host-guest complexes by means of X-ray diffraction. The methodologies developed by the fellow allow triggering and reversibly controlling ternary complex formation by light and represent a fundamental advance in the research on supramolecular materials in water. The project has made outstanding contributions not only in the area of synthetic procedures in polymer chemistry and the preparation of well-defined macromolecular architectures but also in the structure-activity research on stimuli-responsive polymeric micelles and supramolecular polymers. References 1. Lagona, J.; Mukhopadhyay, P.; Chakrabarti, S.; Isaacs, L. Angew. Chem. Int. Ed. 2005, 44, 4844 2. Kim, H. J.; Heo, J.; Jeon, W. S.; Lee, E.; Kim, J.; Sakamoto, S.; Yamaguchi, K.; Kim, K. Angew. Chem. Int. Ed. 2001, 40, 1526 3. Yagai, S.; Kitamura, A. Chem. Soc. Rev., 2008, 37, 1520, and references therein 4. Wu, J.; Isaacs, L. Chem. Eur. J. 2009, 15, 11675 5. Tian, F.; Jiao, D.; Biedermann, F.; Scherman, O. A. Nat. Commun. 2012, 3, 1207

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