Chemical Science Blog

As children, we learn very early on about the concept of shape and size complementarity. No matter how many times you try, the square peg doesn’t fit in the round hole, and that second (or third!) piece of cake just wasn’t a good idea. This same concept also extends to supramolecular interactions, especially when we consider the arena of host–guest chemistry.

Generally speaking, the conformation of a macrocyclic host is relatively rigid, which means that the scope of host molecules it can encase is also somewhat limited. Whilst this feature of host–guest chemistry and molecular recognition is the basis for a range of catalytic events and the template-directed synthesis of mechanically interlocked molecules, it would be advantageous in expanding the scope of this field if a macrocycle existed that could happily host a wide range of guest molecules.

Wei Jiang and his team from the South University of Science and Technology of China have achieved this feat in the synthesis of a naphthalene-based macrocycle, oxatub[4]arene, that has the rigidity required of host–guest interactions, but, in addition, the flexibility necessary to adapt to and accommodate the chemical shape and size of a variety of guest molecules. Its naphthalene units are able to rotate, and in doing so generate four predominant conformers, each with different cavity depths and diameters, as well as different binding affinities for molecular guest molecules.

The dynamic nature of this unique macrocycle is an important step forward in the construction of host–guest complexes, especially as we look to introduce further complexity into the arsenal of supramolecular interactions we have at our disposal, and particularly in the development of increasingly multifaceted stimuli-responsive and molecular machines.

Read this hot ChemSci article in full – it’s open access and free to download:

Oxatub[4]arene: A smart macrocyclic receptor with multiple interconvertible cavities
Fei Jia, Zhenfeng He, Liu-Pan Yang, Zhi-Sheng Pan, Min Yi, Ren-Wang Jiang and Wei Jiang
Chem. Sci., 2015, Advance Article.
DOI: 10.1039/C5SC03251B, Edge Article

About the Writer:

Anthea Blackburn is a guest web writer for Chemical Science. Anthea is a recent graduate student hailing from New Zealand. She studied at Northwestern University in the US under the tutelage of Prof. Fraser Stoddart (a Scot), where she exploited supramolecular chemistry to develop multidimensional systems and study the emergent properties that arise in these superstructures. When time and money allowed, she ambitiously attempted to visit all 50 US states.

The exploitation of porous frameworks, both metal-organic and wholly organic in nature, for their ability to take up and store gases and small molecules, has been well reported over the past few decades. The modularity of these frameworks with respect to their size, flexibility and chemical functionality gives rise to their propensity to adsorb a myriad of chemicals; however, this process is not always selective and the chemical instability of these frameworks can cause difficulties in their extension towards real-life applications. A framework that a) is able to favourably and preferentially adsorb chemicals with high sensitivity, and b) exhibits chemical stability under a wide range of conditions, remains somewhat elusive and is thus a hot topic of investigation in many research groups.

Whilst the uptake and sensing properties of two-dimensional porous polymers have been well studied, the bulk nature of these materials often results in the decrease of their properties on account of the extensive aggregation of the polymer layers and resulting reduction of electrons available to interact with guest molecules. Rahul Banerjee and researchers from the Council of Scientific and Industrial Research – National Chemical Laboratory set out to address this limitation in the field of porous polymers through the Liquid Phase Exfoliation of an imide-based covalent organic framework to its covalent organic nanosheet (CON) analogue. Such a CON, which contains between five and fifteen layers of the material, has chemical and thermal stability equal to that of its bulk counterpart; however, its sensing ability down to 10^–5 M towards 2,4,6-trinitrophenol (TNP) and other nitroaromatic explosive chemicals is far superior. Furthermore, this reversible detection upon uptake can be seen both spectroscopically in solution and visually in the solid state via the turning off and on of the CON luminescence respectively.

If you have ever been stuck in a crowd, you may have noticed that your range of motion and the speed at which you can move is highly dependent not only on whether you are leaving a sports game or a pop concert, but also on where you are positioned in the mass of people. The same is true of a solution of molecules – the molecules that are located in the bulk of the solution would be expected to have different properties from those that are surface-immobilised. This is especially true if we consider the supramolecular association of a guest within a host, where thermodynamics and kinetics play an important role in whether a complex will form or not.

Pablo Ballester and his team from the Institute of Chemical Research of Catalonia (ICIQ) set out to study this phenomenon and prove whether or not there was a difference in the binding of a guest, pyridine N-oxide derivatives, with a host molecule, α,α,α,α-calix[4]pyrrole, in bulk solution or tethered to a gold surface. To achieve this goal, the team employed a surface plasmon resonance (SPR) technique, a technique that is sensitive to the accumulation or release of mass, and has been used previously to study large biomolecular systems in real time.

It was found that thermodynamically, binding events between two molecules are similar in bulk solution and at an interface. This is perhaps not surprising, as the changes in enthalpy and entropy to a calix[4]pyrrole in bulk solution or tethered to a surface will be similar; therefore so too will be the binding event. Differences were observed, however, when considering the kinetic aspect of binding, such that binding was much slower when the molecule was on a surface than when it was in bulk solution. This was attributed to the presence of a matrix hindering the motion of the surface-bound calix[4]pyrrole, thereby providing a barrier to complexation.

This work presents an interesting method of studying the binding events that occur in the different regions of a solution. It also shows that the events that occur on the macroscale, such as in a crowd of people, can, in some cases, be analogous to those that occur on the molecular level.

Read this HOT ChemSci article in full!

Binding of calix[4]pyrroles to pyridine N-oxides probed with surface plasmon resonance
Louis Adriaenssens, Josep Lluís Acero Sánchez, Xavier Barril, Ciara K. O’Sullivan and Pablo Ballester
Chem. Sci., 2014, Edge Article
DOI: 10.1039/C4SC01745E

Biography

Anthea Blackburn is a guest web writer for Chemical Science. Anthea is a graduate student hailing from New Zealand, studying at Northwestern University in the US under the tutelage of Prof. Fraser Stoddart (a Scot), where she is exploiting supramolecular chemistry to develop multidimensional systems and study the emergent properties that arise in these superstructures. When time and money allow, she is ambitiously attempting to visit all 50 US states before graduation.

Fluorescent imaging is one of the most useful techniques in modern day medicine, as it provides us with a method of visualizing the inside of a body to allow for diagnosis and treatment of disease. Typically, an organic fluorophore is attached to a biomolecule that, upon introduction to the body, is able to interact with a specific type of molecule in the body, such as a cancerous cell or a molecule that carries out a particular and important function. More recently, in addition to the use of biomolecules, fluorescent labeling has turned to the polymer world for applications in targeted drug delivery or cell patterning to name but a few examples. Fluorescence becomes important in these applications, as the introduction of non-biological molecules to the body requires a method of locating them to track and monitor their activity.

There are a number of methods of introducing organic fluorophores to polymers, for example, polymerization of fluorescently labeled monomer units, end-group attachment, or post-synthetic modification, all of which offer advantages and disadvantages. The one factor that all of these approaches have in common however, is that one needs to beware of how the attachment of fluorophores, which are typically large, will change the chemistry of the polymer. It would therefore be advantageous if small, yet fluorescent, groups could be attached to polymers without otherwise changing their properties.

It is this interest in synthesising fluorescent polymers using small molecules that Mathew Robin and Rachel O’Reilly from the University of Warwick sought to tackle. They were able to demonstrate that the introduction of dithiomaleimide functional groups, which have a large Stokes shift (250 nm) and bright emission, to acrylate or methacrylate polymers did not change the properties of the polymer itself. Perhaps even more interestingly is that it was demonstrated that the functional groups could be introduced both pre-synthetically and post-synthetically. In this way, polymer fluorescence could be both turned on in a profluorescent polymer that contained a reactive dibromomaleimide monomer unit, as well as reversibly turned off through a dithiol exchange reaction to a non-fluorescent dithiomaleimide monomer unit.

The development of a relatively simple system whose fluorescence can be reversible turned on and off is an exciting step forward in developing polymers, especially since the end groups of this functionalised polymer allows for its further incorporation into more complex polymeric systems. These polymers could have applications in not only the biomedical uses discussed, but also in a numerous other applications such as organic electronic devices, sensing materials and polymer materials like nanoparticles and hydrogels.

Read this HOT Chemical Science Edge article in full for free*!

Fluorescent and chemico-fluorescent responsive polymers from dithiomaleimide and dibromomaleimide functional monomers

Mathew P. Robin and Rachel K. O’Reilly
Chem. Sci., 2014, 5, 2717.
DOI: 10.1039/C4SC00753K, Edge Article

About the Writer

Anthea Blackburn is a guest web writer for Chemical Science. Anthea is a graduate student hailing from New Zealand, studying at Northwestern University in the US under the tutelage of Prof. Fraser Stoddart (a Scot), where she is exploiting supramolecular chemistry to develop multidimensional systems and study the emergent properties that arise in these superstructures. When time and money allow, she is ambitiously attempting to visit all 50 US states before graduation.

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