Molecular switching on surfaces

Molecular switching has established itself as a key functionality of building blocks developed for addressable materials and surfaces over the last two decades. Many challenges in their use and characterisation have been presented by the wide variation in interfaces studied, these ranging from truly single-molecule devices to two-dimensional self-assembled monolayers and thin films that bridge the gap between surface and macroscopically bulk materials (polymers, MOFs, COFs), and further still to other interfaces (solid–liquid, liquid–air, etc.). The low number density of molecules on monolayer-coated interfaces as well as in thin films, for example, presents substantial challenges in the characterisation of the composition of modified interfaces. The switching of molecular structure with external stimuli such as light and electrode potential adds a further layer of complexity in the characterisation of function. Such characterisation “in action” is necessary to correlate macroscopic phenomena with changes in molecular structure. In this review, key classes of molecular switches that have been applied frequently to interfaces will be discussed in the context of the techniques and approaches used for their operando characterisation. In particular, we will address issues surrounding the non-innocence of otherwise information-rich techniques and show how model – non-switching – compounds are often helpful in confirming and understanding the limitations and quirks of specific techniques.