Multi-stimuli actuation of a photoresponsive azobenzene based molecular switch

Abstract

There has been considerable interest in building switching functions into self-assembled monolayers with switching actuated by external stimuli such as light, electrical current, heat, pressure or chemical changes. In this study, dual switching functionality has been built into azobenzene based molecular monolayers. Switching behaviour has been compared for unsubstituted azobenzene monolayer adsorbates and two other monolayers whose ortho position on the terminal phenyl group is substituted by ethyl and isopropyl chains, respectively. The dual molecular switching functionality with light or protonation actuation is compared. EGaIn contacts to the monolayers have been used to record the J–V curves and characterize the on/off switching. This is complemented with further characterization by transition voltage spectroscopy (TVS), ultraviolet photoelectron spectroscopy (UPS), water contact angle determination, atomic force microscopy (AFM) and theoretical computations. It is concluded that side chains (the ethyl and isopropyl groups) are able to decouple neighbouring azobenzene adsorbates which promotes the photo-efficiency of isomerisation and switching. In addition, acid treatment is also applied to these three molecular layers to try to achieve dual stimuli actuation. The absorption wavelength of the azobenzene moiety red shifts by ∼100 nm for all the three protonated molecules. In the case of the unsubstituted azobenzene, its triggering wavelength is totally reversed once it is protonated. A logic truth table has been constructed for the SAM device, which shows that the simple azobenzene molecular layers exhibit the behaviour of an ‘AND’ logic gate which uses blue light and acid as two inputs.