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Synthetic molecular systems undergoing structure and property changes as a response to external stimuli offer a series of relevant functions. Their direct use in solution, immobilized at surfaces, or integrated into adequate matrices, allow for the elaboration of nano-carriers, molecular electronic memory circuits, adaptive systems, artificial muscles, or healable materials, among others. Reversible systems are commonly generated by utilizing molecular switches as responsive bi-stable molecules. Of particular interest are switches controlled by non-invasive triggers, especially light, which can display high spatial and temporal resolution. Photoswitches were used to change properties, such as conductance and basicity, based on light-induced alteration of the molecular geometry, that is, their dimensions and shape. In nature, light-induced geometrical modifications of molecular systems lead to primary biological functions, such as the visual perception in vertebrates, or proton pumping in some bacteria, two examples that nicely illustrate the potential of photo-inducing structural modifications in organic systems.
Multiple timescales in the photoswitching kinetics of crystalline thin films of azobenzene-trimers
(2017)
Functional materials that exhibit photoinduced structural phase transitions are highly interesting for applications in optomechanics and mechanochemistry. It is, however, still not fully understood how photochemical reactions, which are often accompanied by molecular motion, proceed in confined and crystalline environments. Here we show that thin films of azobenzene trimers exhibit high structural order and determine the crystallographic unit cell. We demonstrate that thin film can be switched partially reversibly between a crystalline and an amorphous phase. The time constant of the photoinduced amorphisation as measured with real-time x-ray diffraction ($\approx $ 220 s) lies between the two time constants (120 s and 2870 s) of the ensemble photoisomerisation processes that are measured via optical spectroscopy. Our observation of a photoinduced shrinking of the crystalline domains indicates a cascading process, in which photoisomerisation starts at the surface of the thin film and propagates deeper into the crystalline layer by introducing disorder and generating free volume. This finding is important for the rapidly evolving research field of photoresponsive thin films and smart crystalline materials in general.