Our research is primarily in the field of supramolecular chemistry. This is the study of non-covalent interactions and their role in the reversible association of two or more molecular species (also known as molecular recognition or host–guest chemistry). Specific themes within our research include molecular machines, sensing and electronic materials.
The world’s tiniest machines are interlocked molecules such as rotaxanes and catenanes that produce well-defined motions of their molecular components.
The supramolecular chemistry of organic dye molecules can be used as a mechanism to sense biological substrates such as anions and neurotransmitters.
Mechanically interlocked moolecules (MIMs) such as catenanes and rotaxanes provide ideal candidates as molecular machines since their degrees of freedom are restricted by the nature of their inherent mechanical bond. In these systems, an external stimulus may be used to promote macrocycle (blue) motion between recognition stations (green and yellow) in a rotaxane (Figure 1, left) or a catenane (Figue 1, right).
In previous work the recognition of an anion governs these dynamic processes.
For example, the incorporation of a naphthalene diimide–triazolium derivative into an axle component produced a two-station rotaxane in which the position of the macrocyclic wheel could be controlled upon binding of an anion within the rotaxane’s cavity (Figure 2).
The system incorporating a XB donor anion recognition site was demonstrated to exhibit superior macrocycle shuttling relative to the HB analogue courtesy of strong XB–chloride anion binding interactions. Full details of this work may be found in Chemical Science whilst further reading on halogen bonding in supramolecular chemistry may be found in this publication in Chemical Reviews.
This concept was extended to a halogen bonding rotaxane four-station molecular shuttle that is capable of the colourimetric sensing of oxoanions, in particular nitrate, courtesy of pincer-like motion of the two macrocycle components upon guest binding (Figure 3).
Photoactive MIMs with dyanamic, chiroptical properties are now being targeted.
The development of selective supramolecular sensors that undergo a macroscopic, detectable response upon binding a particular substrate is an intense area of interest.
Previous work has used the recognition and dynamic properties of MIMs for sensing. For example, rotary motion in a catenane was used as a mechanism to perform colourimetric and fluorescence anion sensing (Figure 4). The co-conformation was also found to be sensitive to solvent. This work has been published in the Journal of the American Chemical Society.
Current molecular targets for our organic-dye based sensors include biomarkers such as neurotransmitters or environmental pollutants such as polyaromatic hydrocarbons.
Our work aims to provide a more comprehensive understanding of the connection between molecular structure (self-assembly) and function (electron transfer) in supramolecular donor–acceptor systems.
Molecular motion has been explored as a means to control photoinduced electron transfer in a rotaxane molecular shuttle. Altering the relative positions of the electron donor (ferrocene) and acceptor (naphthalene diimide or C60 fullerene) motifs of the rotaxane alters the pathways of electronic communication in the excited state. This is manifested in a change in fluorescence quantum yield (Figure 5). Further details maybe be found in the Journal of the American Chemical Society.
Further work describes a rare example of electron transfer to C60 without the need for photoexcitation, by encapsulating the fullerene as a guest inside the electron rich cavity of a macrocyclic host (Figure 6). Under different conditions, the macrocycle also enhances photoinduced electron transfer because, like chlorophyll in a leaf, it harvests light from a region where a significant fraction of the solar spectrum occurs (red region). The macrocycle is nicknamed the “Green Box” and is reported in the Journal of the American Chemical Society.
Future work targets new organic semiconducting materials that can be used in electronic devices such as transistors (OFETs) and displays (OLEDs). These materials are comprised of self-assembled organic building blocks such as those in the supramolecular assemblies above.
A recent example is reported in Chemistry, A European Journal in which a macrocycle host–fullerene guest complex is shown to enhance electron mobility when used in a transistor device (Figure 7). These organic semiconductors possess economical, practical and chemical advantages over conventional electronic materials.