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

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 (K a up to 5.13 × 10 5 M À 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.

Regioisomers of Aromatic Thioethers with Aggregation‐Induced Phosphorescence

“…Substituting one or more thiophenyl groups with electron‐withdrawing substituents, such as nitriles, forms highly efficient push‐pull systems. As we have shown before, para ‐nitrile substituted tetrathiophenols are still capable of AIP with lifetimes in the range of microseconds [12,18,26] . Apart from the number and type of functional groups on the peripheral rings, substitution pattern plays a decisive role in the emission properties as well.…”

“…As we have shown before, paranitrile substituted tetrathiophenols are still capable of AIP with lifetimes in the range of microseconds. [12,18,26] Apart from the number and type of functional groups on the peripheral rings, substitution pattern plays a decisive role in the emission properties as well. Feng and co-workers demonstrated that by varying the substitution pattern of the peripheral mercaptophenol in a nitrile-substituted asterisk, the emission could be tuned from AIP to fluorescence occurring both in solution and in the solid-state (SSSE).…”

“…As coined by Tang and co‐workers in 2001, [10] AIE describes compounds that show no emission in solution but become strongly emissive once they aggregate. However, emission can be triggered not only by aggregation but also by different processes such as crystallisation, [11] host‐guest complexation [12] or other supramolecular interactions [13] . In these cases, the observed behaviour is mainly attributed to the restriction of intramolecular motion (RIM) of monomers in sterically congested media, thus favouring spontaneous photon emission over radiationless decay pathways [14] .…”

Design Concepts for Solution and Solid‐State Emitters – A Modern Viewpoint on Classical and Non‐Classical Approaches

“…For instance, the previously mentioned selfassembled J-aggregates show a distinct emission band that has been the subject of several studies regarding their interesting intermolecular interactions. [22] Besides viscosity increase or temperature lowering, molecular rigidification inducing emerging emission can appear as a consequence of host-guest inclusion with cyclodextrins (CDs) [23,24] or after DNA hybridization. [25] Because in many cases, emission is not induced solely upon aggregation but through restriction of motion, Gierschner et al proposed the more proper name "solid-state luminescence enhancement" (SLE).…”