A study of azopyrimidine photoswitches and their interactions with cyclodextrins: When the guest governs the type of accommodation at the host

Azopyrimidines have recently emerged as a new class of photoswitches structurally similar to azobenzenes switching between trans and cis isomers. These compounds have a wide range of potential applications in photopharmacology, catalysis or as molecular devices, but their low solubility in water limits their biological ap-plications. Nevertheless, this drawback could be overcome by their inclusion into supramolecular structures. To test this hypothesis, we prepared a series of azo-pyrimidines and investigated their structure, cis-trans isomerization and host-guest chemistry with cyclodextrins (CDs) using an experimental-theoretical approach combining capillary electrophoresis (CE), Nuclear Magnetic Resonance (NMR) spectroscopy and computational models. The results revealed that beta-CD exhibits high selectivity over several CDs tested for the accommodation of the studied compounds. Orientation of guests in inclusion complexes was experimentally assessed by Nuclear Overhauser Effect (NOE) contacts and supported by quantum-chemical calculations. Moreover, investigation of noncovalent interactions of azopyrimidines with beta-CD by CE showed that azopyrimidines with suitable structural modification can form strong complexes with high values of binding constants (over 17 000 L mol- 1). For such complexes, trans-cis isomerization proceeds directly inside the beta-CD cavity with azopyrimidine remaining encapsulated through its narrower primary portal. By contrast, in weakly interacting complexes, the trans isomer is released, and the cis isomer is generated outside the cavity. In subsequent re-complexation, the cis isomer is then re-captured through the wider secondary portal. Therefore, our findings point to two significantly different complexation processes depending on substitutional pattern and binding constant values of azopyrimidines; a feature which may lead to future promising applications.