Complexation and Chiral Recognition of Chiral Binaphthyl Derivatives and β-Cyclodextrins in Solution Probed by Triplet Excited State Relaxation

Abstract

Abstract It is well known in supramolecular chemistry that cyclodextrin host molecules (CDs) are capable of including and binding guest molecules in their hydrophobic cavities via non-covalent interactions. The unique recognition capability of CDs depends on their inherent asymmetric cavities. We explored here the impacts of the chiral recognition of excited triplet state of guest 1,1′–Binaphthyl–2,2′–diylhydrogenphosphate (BNP) enantiomers by host native β–cyclodextrin (β–CD) and chemically modified Heptakis(2,3,6–tri–O–methyl)–β–cyclodextrin (TMe–β–CD). For this regard, UV-Vis, fluorescence, and laser flash photolysis spectroscopy were performed and our experimental results were flanked by density functional theory (DFT) calculations. Triplet decay dynamics of BNP enantiomers were investigated in the absence and presence of cyclodextrins under different conditions and the chiral recognition behavior is discussed based on the obtained kinetic and spectral data of excited triplet state. Here we show that the long lived triplet excited states (as opposed to the singlet states) and its relaxation in solution may be a sensitive probe to detect chiral recognition. The ability of TMe–β–CD to recognize axial chirality is superior to that of β–CD. These findings may be attributed to the asymmetric cavities of cyclodextrins, which determines the degree of interaction. The differences in the triplet behavior of R– and S–BNP(T1)–TMe–β–CD may arise from the different orientations of the guest molecule within the twisted and restricted TMe–β–CD cavity. We assume here that the orientation of the more favorable enantiomer (S–BNP(T1)) is geometrically preferable, while a similar orientation of the less favorable enantiomer (R–BNP(T1)) might be hindered by steric reasons. In contrast, the triplet dynamics of BNP enantiomers within the unrestricted macrocyclic β–CD ring are similar. This might be ascribed to a similar orientation and interaction and ultimately relaxation of both enantiomers within a round and undistorted β–CD cavity, suggesting the formation of similar inclusion complexes. These findings are evidence for the possibility to detect chiral recognition and interaction as well as tiny structural differences in the BNP complexes within cyclodextrin cavities via laser flash photolysis technique.