Asymmetric Noncovalent Synthesis of Self-Assembled One-Dimensional Stacks by a Chiral Supramolecular Auxiliary Approach

Abstract

Stereoselective noncovalent synthesis of one-dimensional helical self-assembled stacks of achiral oligo(p-phenylenevinylene) ureidotriazine (AOPV₃) monomers is obtained by a chiral supramolecular auxiliary approach. The racemic mixture of helical stacks of achiral AOPV₃ molecules is converted into homochiral helical stacks, as shown by both spectroscopic measurements and molecular modeling simulations. The conversion is promoted by an orthogonal two-point ion-pair interaction with the chiral auxiliary dibenzoyl tartaric acid (d- or l-TA) molecules, which biases the angle population distribution and thereby the stack helicity. The induced preferred helicity is maintained by the OPV stacks even after the removal of the chiral auxiliary by extraction with ethylenediamine (EDA), due to the kinetic stability of the OPV stacks at room temperature. Spectroscopic probing of the helical self-assembly and the racemization process of these π-conjugated OPV chromophores shed further light into the mechanistic pathways of this chiral asymmetric noncovalent synthesis and the kinetic stability of the stacks produced. The racemization of the stacks follows first-order kinetics and no switch in mechanism is observed as a result of a temperature change; therefore, a racemization via disassembly assembly is proposed. Remarkably, the preferred helicity of the stacks of achiral AOPV₃ can be retained almost completely after a heating–cooling cycle where the stacks first partially depolymerize and then polymerize again with the still existing stacks being the seeds for self-assembly of achiral AOPV₃. Only after a fully dissociated state is obtained at high temperatures, the optical activity of the supramolecular stack self-assembled at room temperature is lost.