[2.2]Paracyclophane–dicyanorhodanine conjugates as planar chiral molecular photoswitches

Abstract

(Chir)optical molecular photoswitches have garnered significant attention for their applications in asymmetric synthesis, supramolecular chemistry, and the materials sciences. The planar chirality and associated photophysical and chiroptical properties of [2.2]paracyclophane ([2.2]pCp) derivatives have been long appreciated. Even so, chiral photoswitches incorporating the [2.2]pCp framework remain largely unexplored. Dicyanorhodanine (RCN), conjugated with oligothiophenes and pyrroles, has been recently shown as a highly competent photoisomerizable scaffold. The current work introduces a novel photoswitch system combining an electron-rich [2.2]pCp moiety with an electron-deficient RCN unit, as a planar chiral “push–pull” architecture. Single crystal X-ray study reveals the Z configuration for the as-synthesized RCN–pCp conjugate; upon visible light irradiation well-controlled and reversible Z/E photoisomerization is observed to achieve a photostationary state distribution (PSD) up to 40/60 (Z/E). The model chromophores exhibit negative solvatochromism across solvents of different polarity. Engineering the “push–pull” electronic structure by introducing an electron-donating methoxy group influences the photophysical properties and the photo- and thermal isomerization behavior. An enantioenriched planar chiral photoswitch based on the RCN–pCp scaffold is also introduced, that displays a reversible chiroptical response as evaluated by circular dichroism (CD) spectroscopy as a consequence of Z/E photoisomerization. Ground-state (DFT) and excited-state (TD-DFT) calculations correlate well with the experimental geometries and the spectral (UV–vis and CD) characteristics, respectively. We are optimistic that the reported planar chiral [2.2]pCp-based photoswitches will facilitate the design of next-generation photoresponsive organic functional materials.