Keto-form directed hierarchical chiral self-assembly of Schiff base derivatives with amplified circularly polarized luminescence

Abstract

While enol-keto tautomerism has attracted great interest in Schiff bases and related compounds in solution and crystal states, the self-assembly of energy-unfavored keto form were scarcely investigated. Here, we report a keto-form directed self-assembly of a naphthalene-attached enantiomeric N-salicylideneanil analog L/DGG-Nap accompanied with a significantly amplified circularly polarized luminescence (CPL). It was found that LGG-Nap exists as a mixture of enol and keto form in monomer at a diluted toluene solution. The increment of the concentrations leads to the formation of predominated keto form, which subsequently triggers the self-assembly. Cryo-transmission electron microscopy (Cryo-TEM) revealed that a hierarchical assembly process happened upon increasing the concentration of LGG-Nap in toluene. Individual nanofibers formed at 1 × 10−4 mol/L and transferred into helical nanofiber bundles in 5 × 10−3 mol/L. Interestingly, while these is nearly no circular dichroism (CD) or CPL in the monomeric solution, the assembly showed strong CD and CPL. Remarkably, the dissymmetry factor (glum) was significantly amplified from zero in solution through the 0.005 in individual nanofiber to 0.1 in nanofiber bundles. This work demonstrates that the enol-keto tautomerism can be broken and trigger the self-assembly upon increasing the concentration, which can subsequently direct the chiral self-assembly and significantly amplify the dissymmetry factor of assembled CPL materials.