Light-controlled assembly and disassembly of cyclodextrin-bisazobenzene supramolecular complexes

This is the authors' abstract. We don't add key points for this paper.

• Bis-azo (2):CyD 2 complexes serve as prototypes for light-responsive self-assemblies. • 2 shows a high affinity for α-CyD in the E-form, but none upon photoisomerization. • Both E- and Z-isomers of 2 fit in the β-CyD cavity, with a moderate preference for E. • 2 forms pseudopolyrotaxanes at high γ-CyD concentrations, disrupted by irradiation. • α-CyD is an excellent selective host for making azobenzene light-sensitive complexes. The inclusion complexation of a water soluble bis-azobenzene derivative (bis-Azo) and its monotopic analog (mono-Azo) with α-, β-, and γ-cyclodextrins (CyDs) was investigated as a prototype for light-responsive self-assembling systems. Using spectroscopic techniques (UV–vis, induced circular dichroism, 1 H NMR), computational methods (molecular mechanics and dynamics), and thermodynamic analyses, we examined the photoswitching properties, stability, and structural dynamics of these systems. The azobenzene moieties in the E -isomer of mono-Azo and bis-Azo consistently showed strong affinity for the αCyD cavity, characterized by high association constants. In contrast, no complex formation was observed upon photoinduced E -to- Z isomerization. For bis-Azo, this implies the formation of supramolecular αCyD dimers, with the spatial separation between the oppositely oriented host components determined by the connector linking the two azobenzene moieties in the E -configured ditopic guest. This complex disassembles upon photoswitching, driven by the structural disruption associated with the Z -form. Both the E -and Z -isomers fitted in the cavity of βCyD, with moderate selectivity towards the E -form. A similar scenario was found for complexes with γCyD when using low concentrations of the host. Interestingly, at high concentrations γCyD formed low-solubility pseudopolyrotaxane-type supramolecular architectures with bis-Azo, which were disrupted upon Z -isomer photoisomerization. All the complexes demonstrated high fatigue resistance, maintaining structural integrity after multiple isomerization cycles. This work advances the design of stimuli-responsive preorganized supramolecular systems, with potential applications in nucleic acid delivery through dual pH/light-sensitive mechanisms.