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

Hayduk M., Schaller T., Niemeyer F.C., Rudolph K., Clever G.H., Rizzo F., Voskuhl J.

Research article (journal) | Peer reviewed

Abstract

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.