Coal-tar dyes are artificial colouring agents, derived from coal. Germany’s early chemical industry, mainly centered in North Rhine-Wesphalia, began manufacturing synthetic dyes already in the 19th century for colouring textiles, paper and cosmetics – as well as filling your ball-point pens. More advanced applications include the use as pH-indicators and photosentitizers. Now, researchers at TU Dortmund University have found new promising capabilities for some members of this dye family, namely Michler’s Ketone (yellow), Methylene Blue, Rhodamine B (bright pink), and Crystal Violet. Solvation scientist Guido Clever and his team were able, for the first time, to integrate these chromophores into self-assembled nano cages while retaining their characteristics, i.e. their colour. The obtained structures resemble either a hollow lantern or a twisted helix. Assemblies based on these or similar chromophores may find application as photo-redox catalysts, optical materials and diagnostic tools. The results have been recently published open access in the renowned journal ‘Angewandte Chemie International Edition’. The journal has selected the publication as "Hot Paper".
In the following interview, Irene Regeni, newly minted PhD and first author of the publication, explains the research’s findings and why they are important.
What is the new discovery that you made - and why is it exciting?
We selected these specific chromophores that have been very popular products of the chemical industry ever since. Yet they didn’t find any integration into self-assembled supramolecular structures like cages with accessible cavity. So we incorporated them into banana-shaped organic molecules that are able to form 3-dimensional objects in the presence of a metal like palladium. Depending on the connecting group attached to these ligands, pyridine or quinoline, we obtained either lantern-like cages or twisted helicates in solution. In the end we could synthesise nine new molecules that keep the original properties of the dyes and show colours from yellow to deep violet. We tested them with spectroscopic methods such as NMR, circular dichroism (CD) and mass spectrometry, obtained some crystal structures and studied their ability to interact with guest molecules. The latter experiments showed that we really made something new we now call ‘3D chromophores’!
Why is it important?
First of all, we were able to demonstrate that it is feasible to implement classic coal-tar dyes into coordination cages and helicates. One very promising feature is the ability of the new supramolecular structures to recognise a range of chiral molecules (i.e. compounds that exist in two forms with mirror-imaged shapes). The lanterns can encapsulate smaller chiral guests that imprint their mirror image character on the whole system by inducing a twist in clockwise or counter-clockwise direction, a process called ‘chirality transfer’. Larger chiral ions also induce chirality transfer onto the helicates, but they bind weakly on the outside of the structure. We plan to exploit these features for some fancy applications: For example, photocatalytic reactions can be carried out in the cages, serving as small reaction chambers that offer a special environment, separated from the surrounding solution. Furthermore, we study their use as detection tools for the recognition of biopolymers and biomolecules in water. Our new compounds may also be attractive as light-harvesting units in electron- and energy-transfer nano devices and materials.
What’s the role of the solvent, if any?
By changing the solvent, we can fine tune the properties of the system. For example, by going from DMSO to acetonitrile and water, we can modulate a characteristic equilibrium process known for the Rhodamine B structure as part of the cage system, allowing a stepwise transition from an almost colourless form to its intense pink state. The solvent water can also speed up the pace at which the helical cages twist, thereby offering control over the chirality transfer phenomenon.
Most interesting, however, is the prospect that the 3-dimensional shape of the chromophore-based assemblies allows to separate a nano-sized cavity from the surrounding bulk solution environment. Apertures in the structure allow analytes, reaction substrates and solvent molecules to enter the confined interior space, yet these guests have to obey certain size criteria. This can have a particular influence on the few solvent molecules that find room inside the nano cavity, as their properties such as acidity/basicity and participation in chemical reactions can change dramatically when torn away from the bulk solvent environment – all effects that may be reminiscent of the roles that solvents play in enzyme pockets. We can now combine these features with the light-addressable character of our dye-based compounds. Most pleasingly, many of them are indeed soluble in water, giving us a lot of ideas for upcoming studies and collaborative projects within RESOLV!
Original Publication: I. Regeni, B. Chen, M. Frank, A. Baksi, J. J. Holstein, and G. H. Clever: Coal-Tar Dye-based Coordination Cages and Helicates, in Angew. Chem. Int. Ed., 2020, DOI: 10.1002/anie.202015246 ( German version here)