Structural Origin of Mono- and Bisignate Circular Dichroism Signals in Phenyleneethynylene Assemblies

Abstract

The influence of the positional placement of chiral handles on a molecular core upon self-organization and chiroptical properties is investigated using phenylalanine-substituted oligo(p-phenyleneethynylene)s (OPEs) through joint chiroptical and molecular dynamics simulation studies. The linear OPE systems with d- and l-phenylalanine units appended at the meta- and para-positions of the terminal benzene ring exhibit exciton-coupled and induced circular dichroism (CD), respectively, upon self-assembly under identical experimental conditions. Meta-substituted OPEs form ordered helical stacks in 50% (v/v) water in methanol, facilitating on-resonance intermolecular transition dipole moment coupling that produces bisignate CD signals and strong circularly polarized luminescence (CPL). In contrast, para-substituted OPEs assemble into ladder-type structures under the same conditions, leading to off-resonance coupling between the terminal chiral handles and the OPE core, resulting in weak monosignate CD signals that are devoid of CPL. The potential of meta-substituted OPEs as temperature-dependent chiroptical switches is also demonstrated.