Small molecule and polymer functional materials
Chirality is a fundamental symmetry property; chiral objects, such as chiral small molecules, exist as a pair of non-superimposable mirror images. Although small-molecule chirality is routinely considered in biologically focused application areas (such as drug discovery and chemical biology), other areas of scientific development have not considered small-molecule chirality to be central to their approach. We are actively exploring ways in which chiral materials (principally small molecules) can be used to enable/improve function in a range of technological applications, particularly organic electronic devices. These new applications often go beyond exploiting the interactions between two different chiral molecules — the mechanism underlying chiral biological recognition processes and sensors — and include organisation dependent on chiral composition, circularly polarised (CP) light and the spin of moving electrons (spintronics).
Polymer blend materials
Circularly polarised (CP) light is a chiral electromagnetic radiation and is central to a large range of current and future display and photonic technologies, including highly efficient displays, optical quantum information processing and communication, and optical spintronics. There is therefore high interest in constructing CP-light-emitting devices. Together with Professor Alasdair Campbell, we have been exploring the development of CP-organic light emitting diodes (CP-OLEDs) based on organic blend materials consisting of a chiral small molecule and a non-chiral polymer. We have previously showed that through simple doping a conventional light emitting polymer (F8BT) with a small amount of a single handed helically chiral aromatic (a helicene), we are able to generate substantial levels of CP-electroluminescence from the polymer. The sign of the CP emission was directly determined by the handedness of the helicene dopant — demonstrating the small molecule to be responsible for the induced chiral response in the non-chiral polymer — and the magnitude of the response competitive with or better than the state of the art. In ongoing work we are elucidating the mechanisms at play that underpin this induced CP-emission and applying our approach to other polymeric systems. Representative publication: Adv. Mater. 2013, 25, 2624. DOI.
The work above combines a non-emissive chiral small molecule additive with an emissive polymer. A conceptually similar but distinct approach would be to combine an emissive chiral small molecule with a non-emissive polymer host. We are therefore developing useful chiral small molecule emitters to validate this methodology for CP-OLED applications. One option is the use of phosphorescent chiral small molecule materials. Phosphorescent emitters in OLEDs (PHOLEDs) can emit light from triplet excited states and can therefore achieve very high efficiencies. However, CP-PHOLEDs are significantly understudied, and the limited previous reports suffered from very low brightness or low levels of circular polarisation. We have recently reported the use of a platinahelicene complex to construct a CP-PHOLED that achieves a display level brightness. The dissymmetry of CP emission reached with this proof-of-concept single-layer helicene-based device is sufficient to provide real-world benefits over nonpolarised emission and paves the way toward chiral metal complex-based CP-PHOLED displays. Representative publication: J. Am. Chem. Soc. 2016, 138, 9743. DOI.
Chiral small molecules
Circularly polarised (CP) light detecting phototransistors. The development of CP-based technologies to their full potential requires the realisation of miniature, integrated devices that are capable of detecting the chirality or ‘handedness’ of CP light. Organic field-effect transistors, in which the active semiconducting layer is an organic material, allow the simple fabrication of ultrathin, compact devices. Together with Professor Alasdair Campbell, we have previously shown, for the first time, that an organic field-effect transitor (OFET) based on enantiomerically pure helical aromatic (a helicene) can detect and differentiate CP-light, acting as a CP-electrical switch. A highly specific and reversible photo-response to CP light was observed, which is directly related to the handedness of the helicene molecule. We believe this opens up a unique possibility for CP light sensing in highly integrated photonic technologies. Representative publication: Nature Photon. 2013, 7, 634. DOI.
Tuning charge transport mobility using chiral composition. It is well-known that the key to successful and high-performance organic devices is the need to link well-understood characteristics of an isolated molecule (so-called “molecular” properties: HOMO/LUMO energy levels, molecular optical properties, etc.) into the collective behavior of multiple units in thin films (“material” properties). When employing a chiral organic semiconducting material in a given organic electronic application, it is possible to employ a range of chiral compositions using differing mixtures of the left- and right-handed structures; the most common being a racemate (a 50:50 mixture of left-handed and right-handed molecules) and an enantiopure (single handedness) composition. As the right- and left-handed enantiomers of a given chiral material have identical “molecular” properties, such properties would not be expected to change when comparing an enantiopure composition to a racemate. However, just like for shaking hands, the interactions between, e.g., two right-handed molecules and a right- and a left-handed molecule are not equivalent, meaning an enantiopure substance and a racemate have different bulk packing and, ultimately, different bulk material properties. This possibility to exploit chirality to alter the “material” properties without affecting the “molecular” properties is a fascinating concept which could lead to wide ranging and currently unexploited possibilities in the area of organic semiconductor design and optimisation, particularly for usage in devices. In collaboration with Professor Alasdair Campbell, Professor Jenny Nelson and Dr Kim Jelfs we have recently shown that organic field-effect transistors (OFETs) constructed from the helically chiral molecule 1-azahelicene can display up to an 80-fold difference in hole mobility, together with differences in thin-film photophysics and morphology, solely depending on whether enantiopure or racemic chiral compositions are employed under analogous fabrication conditions. We investigated the bulk structures underpinning this effect using crystal structure prediction, a computational methodology rarely applied to molecular materials, and linked the results to the differences in charge transport observed. We continue to explore the opportunities presented by chiral composition in technological applications. Representative publication: ACS Nano 2017, 11, 8329. DOI.
Chiroptical switches. Molecular chiroptical switches that exploit the chirality of circularly polarised light as read-out parameter have been shown to change their chiroptical properties upon exposure to stimuli such as pH-level, electrochemical potential or light. In many cases — especially of relevance to electronic applications — the switching occurs through changes in electrochemical potential (redox switching), which drives simple, reversible redox reactions with little or no significant structural change of the chiral molecular switch. In collaboration with Dr Lubomír Pospíšil, Professor Tadashi Mori, and Dr Filip Teplý, we have recently reported a new approach based on the intermolecular dimerization of a pyridinium helicene that achieves strong, reversible chiroptical redox switching with a prominent >500 mV hysteresis. This hysteresis creates a very large potential range of bistability, where the structure of the molecule, and thereby the chiroptical readout, is determined solely by the previous redox history. This memory effect, combined with the pronounced, fully reversible redox-triggered chiroptical switching could inspire the development of future chiroptical switches for molecular electronic applications. Representative publication: Chem.Commun. 2017, 53, 9059. DOI.