Phosphorescence Induction by Host‐Guest Complexation with Cyclodextrins – The Role of Regioisomerism and Affinity

• Published in: Chemistry : a European journal Vol. 28; no. 51; pp. e202201081 - n/a
• Main Authors: Hayduk, Matthias, Schaller, Torsten, Niemeyer, Felix C., Rudolph, Kevin, Clever, Guido H., Rizzo, Fabio, Voskuhl, Jens
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 12.09.2022
• Subjects: Aqueous solutions; Calorimetry; Chemistry; Complexation; Conformation; Cyclodextrin; Cyclodextrins; host-guest systems; Inclusion complexes; isothermal titration calorimetry; Mass spectrometry; Mass spectroscopy; NMR; Nuclear magnetic resonance; Phosphorescence; Room temperature; room temperature phosphorescence; supramolecular chemistry; Titration; Titration calorimetry; Cyclodextrins; host-guest systems; isothermal titration calorimetry; supramolecular chemistry; Room Temperature Phosphorescence
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.202201081

We present an in‐depth investigation of cyclodextrin complexes with guest compounds featuring complexation‐induced room temperature phosphorescence (RTP) in aqueous solution. Very interestingly, only the complexed regioisomers bearing lateral substituents on meta‐position show RTP, whereas the stronger host‐guest systems with para‐substituted dyes show no RTP features. The reported systems were investigated regarding their complexation behavior in water using isothermal titration calorimetry and mass spectrometry. In the case of γ‐CD very strong 1 : 1 inclusion complexes (Ka up to 5.13×105 M−1) were unexpectedly observed. It was found that not only a strong binding to the cyclodextrin cavity is needed to restrict motion, inducing the emission, but also the conformation inside the cavity plays a pivotal role – as supported by an extensive NMR study and MD simulations. Water‐soluble aromatic thioethers reveal orange phosphorescence emission upon the formation of 1 : 1 inclusion complexes with γ‐cyclodextrin, which was proven by state of the art photophysical characterisations as well as ITC‐, NMR‐ and fluorescence titration‐measurements supported by molecular dynamics simulations. It was found that not only the binding affinity but also the orientation of the guest in and around the cavity is crucial to restrict motion able to induce the emission.