Dynamic clicked surfaces based on functionalised pillar[5]arene.

A dynamic smart surface which was constructed by the self-assembly of an azobenzene-functionalised pillar[5]arene exhibits reversibly responsive morphologies towards UV and visible light as observed by TEM, SEM and AFM.

Mechanically interlocked mechanophores by living-radical polymerization from rotaxane initiators.

Two [2]rotaxane initiators for single-electron-transfer living-radical-polymerization were synthesized and used for the controlled polymerization of methyl acrylate. The mechanically interlocked polymers exhibited distinct responses to mechanical activation by ultrasound. Monitoring the fate of the rotaxanes' charge transfer absorption bands provides evidence for preferential mechanical degradation of a midsection rotaxane unit as compared to a terminal rotaxane entity as a consequence of mechanical forces accumulating in the central region of the polymer chain.

Artificial light-gated catalyst systems.

Having control over an entity or even an entire process is arguably the ultimate demonstration of its understanding and it will enable its potential to be fully exploited. With this in mind, chemists have not only been creating and optimizing a myriad of different catalysts for most (relevant) chemical reactions over the past decades, but have recently started to implement controlling elements into the catalyst design. These incorporated gates operate upon exposure to suitable control stimuli, and light represents perhaps the scientifically and technologically most attractive stimulus. In principle, irradiation can thereby induce activity and selectivity in a given catalyst system with high spatial and temporal control, leading to an overall localization and amplification of an optical signal and translation into chemical action. While nature has developed and utilized this concept, in particular in the processes of vision and photomovement, such artificial photocontrolled catalyst systems offer unique opportunities and have high potential for future applications. In this Review, we outline the general concept of light-gated catalysis based on photocaged and also photoswitchable systems, and discuss relevant examples of the past and recent literature.

Immobilization of a photoswitchable piperidine base.

The synthesis of a photoswitchable piperidine base, carrying a monochlorosilane anchoring group, and its immobilization on silica gel, mimicking an oxide surface, is reported. Efficient photoswitching between the E and Z isomers of the azobenzene photochrome was demonstrated for the immobilization precursor in solution and the immobilized piperidine base in suspension of the functionalized silica gel.

Photoswitchable catalysts: correlating structure and conformational dynamics with reactivity by a combined experimental and computational approach.

Photocontrol of a piperidine's Brønsted basicity was achieved by incorporation of a bulky azobenzene group and could be translated into pronounced reactivity differences between ON- and OFF-states in general base catalysis. This enabled successful photomodulation of the catalyst's activity in the nitroaldol reaction (Henry reaction). A modular synthetic route to the photoswitchable catalysts was developed and allowed for preparation and characterization of three azobenzene-derived bases as well as one stilbene-derived base. Solid-state structures obtained by X-ray crystal structure analysis confirmed efficient blocking of the active site in the E isomer representing the OFF-states, whereas a freely accessible active site was revealed for a representative Z isomer in the crystal. To correlate structure with reactivity of the catalysts, conformational dynamics were thoroughly studied in solution by NMR spectroscopy, taking advantage of residual dipolar couplings (RDCs), in combination with comprehensive DFT computational investigations of conformations and proton affinities.

On the illusive nature of o-formylazobenzenes: exploiting the nucleophilicity of the azo group for cyclization to indazole derivatives.

Facile rearrangement of azobenzenes is shown to occur in cases where the azo group is placed in the ortho position to carbonyl electrophiles to furnish the indazole skeleton. While this study demonstrates the illusive nature of o-formylazobenzenes, it offers potential for the synthesis of indazoles and related heterocycles.