Host-guest-complexes with cyclodextrins - structural characteristics and didactical potential
Hotspots with particularly relevant contents for "Chemistry with Light" can be found in this thesis on the following pages with the respective contents :
- p. 72 to p. 86: Fluorescence enhancement by complexation with β-cyclodextrin
- p. 87 to p. 101: Photochromism and thermochromism of spiropyrane-γ-cyclodextrin complexes
Get the thesis (in German): urn:nbn:de:hbz:468-20100124
Based on the status quo of scientific findings host-guest-complexes with cyclodextrins were investigated in order to demonstrate the essential structural characteristics of cyclodextrins and their supramolecular compounds.
The results of the scientific research are as follows:
1. The iodine-β-cyclodextrin-complex demonstrates exemplarily that the formation of the complex is attended by modification and masking of some typical material properties of the guest. The newly developed experiment with the iodine-β-cyclodextrin-complex provides the opportunity to prove the major role of water for the formation and dissociation of cyclodextrin-complexes.
2. The phenomenologically and didactically fruitful experiment with the phenolphthalein-β-cyclodextrin-complex plays a decisive role for the comprehension of the bonding relationship between host- and guest-molecules. 1H-NMR studies were carried out to understand the unambiguous observation of the decolouration of an alkaline phenolphthalein solution on the molecular level. The 1H-NMR measurements with phenolphthalein-solutions at different pH values in combination with the detection of Nuclear-Overhauser-Effects lead to the conclusion that phenolphthalein-dianions are existent as lactonic dianions and penetrate the cavity of β-cyclodextrin-molecules with the aromatic ring bearing the lacton group.
The results of the investigation of the phenolphthalein-β-cyclodextrin-complex underline the hypothesis that complex formation can cause conformational modification and even valence isomerisation from the guest-molecule.
3. The phenomenon of the fluorescence enhancement of 8 anilino-1 naphthalinsulfonic acid-ammonium salt and fluorescein disodium salt in solutions is shown and quantified in an experimental setup.
4. For the complexation of spiropyran in γ-cyclodextrin the following reaction conditions turn out to be best: use of a solvent-mixture of ethanol with 5% water, ratio of the amount of substances of 1 : 1.
The spiropyran-γ-cyclodextrin-complex accumulates in powder form which is a smart material with the function of a molecular switch. The findings about the photochromic and thermochromic behaviour of this material are summarised in an energy diagram with energy profile curves of the ground state and the excited state of the host-guest-complex.
5. A convincing pearlescent is created in a shampoo formulation by addition of β-cyclodextrin. The pearlescent is attributed to the formation of crystalline domains of surfactant-β-cyclodextrin-complexes distributed in the solution which causes a selective reflexion and refraction of light.
The scientific investigations about the host-guest-complexes provide the basis for research in chemical education and the following aspects clarify the didactical capability of this topic:
1. The developed experiments are predicated on current scientific findings and perform the postulation of scientific consistency. With the selection and the didactical rework of the experiments great importance to distinctive observations and didactical conciseness is attached.
2. A further important criterion for the choice of experiments is the possibility for anchoring to topics from the curriculum. For the experiments there are multiple possibilities from the scientific point of view and also from the point of view of motivating contexts in different educational stages.
3. Due to the versatile applications of cyclodextrins and their host-guest-complexes the topic can be integrated in a meaningful context from the pupils' everyday life, which meets with the curricular requirements of context orientation. The realised Flash-animation on the host-guest-complex of phenolphthalein with β-cyclodextrin enables the dynamic representation of the complex formation on the molecular level. This electronic medium is an important support for teaching.