Azobenzenes as energy transducers in dynamic supramolecular systems

Abstract

Azobenzenes are molecular photo-switches that are widely used as photo-responsive building blocks for dynamic and responsive materials. However, while the thermal relaxation of the cis-isomer provides spontaneous reversibility that can be useful for specific applications, in other situations, the lack of thermal stability remains a limitation in the function and performance of functional materials, and in particular, a continuously energy input is required to sustain the photo-generated state. In this chapter, we review different design strategies to develop azobenzenes with long-lived cis-isomers, and we discuss theirperformance as photo-responsive building blocks in supramolecular systems. 9 IntroductionWe define smart materials as materials that respond to a specific stimulus by changing a chemical or physical property and thus generating new functions, e.g. reactivity for certain chemical reactions [1] , geometric conformation and mechanic motion [2, 3] , electric properties [4, 5] or optical properties [6, 7] . Supramolecular chemistry is a useful tool to develop such materials, by building highly ordered functional architectures from small molecular building blocks through non-covalent interactions. Compared to covalent chemical bonds, with a typical energy ranging from 150 kJ mol -1 to 450 kJ mol -1 for a single bond, the non-covalent interactions are weaker, e.g. the energy for π-π stacking lies between 0-50 kJ mol -1 , and van der Waals interactions are associated with an energy that is below 5 kJ mol -1 [8] . The weak interaction allows a relative low energy barrier for the material to generate responsiveness to the stimulus as well as low energy input requirements, meanwhile the side-reaction are minimized in the absence of covalent bond formation.We focus on light as a stimulus, as a clean and precisely controllable input source that provides both command and energy to the supramolecular system, and wherein the desired function or properties are generated without chemical waste [9] . The receptor of light is a molecular photo-switch, which is defined as a molecule that undergoes reversible transformation by absorption of light between two forms, A and B, having different absorption spectra [10] . The transformation process is called photo-isomerization, or photoswitching [11] . In a supramolecular system incorporating molecular switches in its design, the input energy from light illumination is firstly converted into chemical energy, induces changes on the structure of the molecular switch itself, and consequently on its physical and 10 chemical properties as well, which is followed by the interaction and alignment between the switch and its neighbor molecules, and is further transmitted to the whole system [12,13] .In this thesis, we focus on azobenzenes as archetypal building blocks for dynamic supramolecular systems [14] , in view of their easy synthesis and functionalization [15] , their typically high extinction coefficient, and their quantum yields [16] . The simplest azobenzene...