The interaction of light with molecules continues to be a key area of research for the group. For example, photoswitchable molecules are photochemically active compounds that can be switched between (at least) two states by irradiation with light. There is high interest in the development of new and improved photoswitches due to the high spatial and temporal control that is possible using light as an external stimulus. As such, photoswitchable molecules are being reported in a number of applications, from photopharmacology – where light switches between two states of a molecule with different pharmacologies – to optical data storage.
Novel organic photoswitches
Azobenzenes represent a privileged scaffold for photoswitches, since they are easily synthesized, have high extinction coefficients and quantum yields (allowing photoswitching with low-intensity light), and are stable to repeated switching. We continue to explore the replacement of one or both of the benzene rings present in azobenzenes with an alternative heteroaromatic ring. For example, we have reported a new class of highly promising compounds, the arylazopyrazoles. The arylazopyrazoles show high thermal stability in solution at room temperature (half-life of up to 1000 days) and good to quantitative two-way photoswitching. By virtue of their five-membered rather than six-membered aromatic ring, such compounds can access a conformation in the Z isomer that is not accessible to conventional azobenzenes; a conformation that can be sterically tuned by substitution on the heterocyclic ring. Such conformational properties significantly affect the intensity of the n-π* absorbance in these molecules, thus offering photophysical and photochemical properties not achievable with azobenzenes. In collaboration with Professor Julia Contreras-García, our recent computational and experimental studies have elucidated the origin of the long thermal half-lives and excellent addressability of the arylazopyrazoles, applying this understanding to determine important structure-property relationships for a wide array of comparable azoheteroaryl photoswitches. Representative publications: J. Am. Chem. Soc. 2017, 139, 1261. DOI; J. Am. Chem. Soc. 2014, 136, 11878. DOI.
Leveraging our discovery of novel heteroaromatic azo photoswitches, we continue to explore the use of these molecules to a range of optically-addressable applications. In particular, we are interested in applications where the presence of heteroatoms on the heteroaryl ring system present other functional opportunities. For example, we have recently reported the first photoswitchable Brønsted base of this class, utilising an azobisimidazole. The protonation event occurs on one of the imidazole rings and is stabilised by the neighbouring ring, rather than through participation of the azo moiety. This results in a compound in which the Z isomer is over an order of magnitude more basic than the E isomer. Furthermore, by avoiding protonation of the azo linkage, the thermal half-life of the protonated Z isomer is increased relative to the neutral form. Representative publication: Chem. Commun. 2016, 52, 4521. DOI.
Circularly polarised light (CPL) is a form of chiral electromagnetic radiation, which in theory should be able to induce absolute asymmetric synthesis via photochemical reactions. Indeed, the interaction of CPL with interstellar primordial molecules (combined with some amplification mechanism) is one hypothesis for the evolution of homochirality on Earth. As early as the 19th century van’t Hoff and Le Bel pointed out that CPL might be used to induce asymmetric photochemistry, however despite extensive effort in the subsequent century, only a few examples exist where moderate levels of asymmetric induction have been achieved, and no examples exist for highly enantioselective reactions. Nonetheless, CPL-mediated photochemistry still continues to draw significant interest in asymmetric synthesis, origins of life science, asymmetric photoswitching of chiral molecules, in molecular motors, and in liquid crystalline materials.
Building on our work studying the interaction of CPL with homochiral helicenes in photo-transistors, we have revisited the asymmetric photochemical synthesis of helicenes from a prochiral precursor using CPL in collaboration with Dr Marina Kuimova. We have demonstrated the principle that two wavelengths of circularly polarized (CP) light can be used to control separate reactions simultaneously. In doing so, a photostationary state (PSS) can be set up in such a way that the enantiomeric induction intrinsic to each step can combine additively, significantly increasing the asymmetric induction possible in these reactions. Representative publications: Chem. Sci. 2015, 6, 3853. DOI.