Responsive Liquid Crystalline Materials Based on Interfacial Photocatalytic Processes

Abstract

Here we address if it is possible to couple changes in liquid crystal (LC) ordering with photocatalytic processes occurring on surfaces to enable the design of photoresponsive materials. We report that ultraviolet (UV) illumination of anatase (101) supporting 4′-n-pentyl-4-biphenylcarbonitrile (5CB) leads to a change in LC ordering driven by photo-oxidation of 5CB to 4′-cyano-4-biphenylcarboxylic acid (CBCA), as confirmed by infrared spectroscopy and mass spectrometry. Specifically, we find that a 0.09 monolayer (ML) surface coverage of the product CBCA on anatase (101) is sufficient to trigger the ordering transition of the LC from planar to homeotropic, thus reporting the transformation with high sensitivity. Additionally, we observe that the LC ordering serves as a reporter of the amount of adsorbed water on anatase (101), a key molecular species involved in the photocatalytic transformation of 5CB. We also demonstrate that the LC film alters the local concentration and structure of water adsorbed on titania, as characterized using infrared spectroscopy and electronic structure calculations. Lastly, the competitive adsorption of water and LC on anatase (101) is reported as a nonmonotonic trend in the rate of photocatalytic transformation of LC as a function of RH levels, a phenomenon captured by the rate of change of LC order. In summary, these results demonstrate that LCs couple to photocatalytic transformations on titania, offering designs of photoresponsive LCs, a novel readout of photocatalytic molecular events and the ability to tune interfacial photocatalytic processes.