CPL on/off switching by enantiomer encapsulation in TPE heterochiral molecular cages

Abstract

Chiral molecular cages have exhibited potential in chiral recognition, chiroptical materials, etc. They are generally obtained by introduction of chiral groups into linkers. Chiral cages derived from chiral lids are very rare. Here, enantiomerically pure hindered tetraphenylethene (hTPE) was used as a lid to synthesize chiral TPE cages. While the same helical-handed hTPE units are exploited as lids to give homochiral cages, heterochiral cages with two lids having inverse helical directions are obtained when one hTPE unit and one simple TPE unit are used as lids. Due to the homochirality of the two lids, the homochiral cages display 3-fold stronger circularly polarized luminescence (CPL) than the heterochiral cages. However, the heterochiral cages can adaptively include aromatic guest molecules whereas the homochiral cages fail to do so, thanks to the greater flexibility of TPE compared to the hTPE unit. Very exceptionally, one enantiomer of the chiral guest induces opposite helical chirality between the TPE lid and hTPE lid, turning off the CPL whereas the other enantiomer induces the same helical chirality of the two lids, maintaining or enhancing the CPL signal and furnishing a novel CPL switch. This discrimination between two enantiomers can be carried out using fluorescence spectra, and can even be applied to enantiomer excess (ee%) determination of chiral diacids. Furthermore, through adaptive inclusion, chiral energy transfer between the heterochiral cages and achiral dyes such as Eosin Y occurs, leading to CPL multi-color emission from the achiral dyes.