Endohedral functionalization of chiral metal-organic cages for encapsulating achiral dyes to induce circularly polarized luminescence

Circularly polarized luminescence (CPL) is useful for 3D optical imaging and optical storage, but it remains a big challenge to prepare chiral compounds with both strong emission and high-dissymmetry factor (glum). Here, we demonstrate that encapsulation of achiral dyes into hollow cages can inhibit the aggregation-caused quenching (ACQ) behavior and promote CPL induction and amplification. This leads to the synthesis of two chiral hexahedral Pd6L12 cages with crown ether or methoxymethyl moieties. Complexation of K+ with crown ethers allows inclusion of anionic achiral boron-dipyrromethene (BODIPY) dyes into the cage cavities to form stable host-guest complexes by cation-anion and host-guest interactions. Although both the cage and dyes exhibit no CPL, their host-guest complexes display strong CPL emissions with the glum factors in the order of 10−3 in solution. Upon crystallization, one order of magnitude amplification of the glum values in the cage-dye adducts (up to 2.4 × 10−2) is obtained.