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

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 are showing 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 (K a up to 5.13 x 10 5 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.