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.