A chiral and configurationally stable bis-perylene diimide macrocycle is reported, which exhibits switchable circular dichroism (CD) and multiphoton circularly polarized luminescence (MP-CPL). This behavior is realized due to the macrocycle's tuneable chiral conformation, which impacts the intrinsic and supramolecular helicity of the perylene diimide dimer.

Abstract

Chiroptical switches exhibit stimuli-induced changes in the sign or strength of their circular dichroism (CD) or circularly polarized luminescence (CPL) and are materials with significant potential for chiroptical sensing and biological imaging. However, the chromophores used in chiroptical materials usually require excitation by ultraviolet or visible light, which results in higher cytotoxicity and lower penetration compared to near-infrared (NIR) photoexcitation. This can be overcome by multiphoton excitation, yet multiphoton CPL (MP-CPL) has been challenging to measure in organic emitters. Core-twisted perylene diimides (PDIs) are excellent chromophores with which to develop MP-CPL emitters due to a chiral π-system capable of two-photon absorption. Furthermore, the integration of these chiral PDIs into stimuli-responsive macrocycles affords configurational stability and the potential for chiroptical switching, due to the chromophore's tuneable chiral conformation. In this work we report a chiral bis-PDI macrocycle with stable, yet conformationally dynamic, enantiomers, and in doing so realize a novel chiroptical material that combines switchable CD and MP-CPL for the first time.