A photoswitchable strapped calix[4]pyrrole receptor: highly effective chloride binding and release

A stiff-stilbene strapped calix[4]pyrrole receptor can be reversibly switched by light between a strong chloride-binding Z-isomer and a very weakly binding E-isomer. The light-induced switching process is monitored by UV-Vis and H-1 NMR spectroscopy and chloride binding is studied in detail using both H-1 NMR and ITC titrations in DMSO and MeCN. In DMSO, at millimolar concentrations, switching from a fully bound to an almost fully unbound state can be triggered. Quantification of the binding constants in MeCN reveals an extraordinary 8000-fold affinity difference between the Z- and E-isomer. Single crystal X-ray crystallographic analysis gives insight into the structure of the photogenerated E-isomer and the geometry of the chloride-bound receptors is optimized by DFT calculations. The highly effective control of binding affinity demonstrated in this work opens up new prospects for on demand binding and release in extractions and photocontrol of membrane transport processes, among other applications.

Automated summary

This study presents a Calix[4]pyrrole receptor that can be reversibly switched by light between a strong chloride-binding Z-isomer and a weakly binding E-isomer. The significant difference in binding affinity between the Z- and E-isomer is quantified using H-1 NMR and ITC titrations in MeCN. Structural insights into the photogenerated E-isomer and the chloride-bound receptors are provided through DFT calculations and X-ray crystallographic analysis, offering new prospects for on-demand binding and release in extractions and membrane transport processes.