Self-similar chiral organic molecular cages

Abstract

The endeavor to enhance utility of organic molecular cages involves the evolution of them into higher-level chiral superstructures with self-similar, presenting a meaningful yet challenging. In this work, 2D tri-bladed propeller-shaped triphenylbenzene serves as building blocks to synthesize a racemic 3D tri-bladed propeller-shaped helical molecular cage. This cage, in turn, acts as a building block for a pair of higher-level 3D tri-bladed chiral helical molecular cages, featuring multilayer sandwich structures and displaying elegant characteristics with self-similarity in discrete superstructures at different levels. The evolutionary procession of higher-level cages reveals intramolecular self-shielding effects and exclusive chiral narcissistic self-sorting behaviors. Enantiomers higher-level cages can be interconverted by introducing an excess of corresponding chiral cyclohexanediamine. In the solid state, higher-level cages self-assemble into supramolecular architectures of L-helical or D-helical nanofibers, achieving the scale transformation of chiral characteristics from chiral atoms to microscopic and then to mesoscopic levels.

Fig. 1. Synthetic route of molecular cages CHO-TMC, 4P-HTMC, and 4M-HTMC.

a Racemic triphenylbenzene (TPB)-based [2 + 3] O-bridged oxacalixarene molecular cages 2P-CHO-TMC (blue) and 2M-CHO-TMC (red). b The enantiopure higher-level TPB-based molecular cages 4P-HTMC (blue) and 4M-HTMC (red). For clearly observing the structures, the molecular cages are schematically represented in blue for the (P)-enantiomer, in red for the (M)-enantiomer.

Fig. 2. Structures of molecular cage CHO-TMC.

The a chemical structure and b X-ray single crystal structures of molecular cage CHO-TMC from the side view and top view.

Fig. 3. Structural characterization of higher-level molecular cage 4P-HTMC and 4M-HTMC.

a The ¹H NMR molecular cages of CHO-TMC (black), 4P-HTMC (blue) and 4M-HTMC (red), and b the 2D ¹H,¹H-COSY spectrum of 4P-HTMC (600 MHz, CDCl₃). c X-ray single crystal structures of molecular cages 4P-HTMC and 4M-HTMC from side view and top view. (The hydrogen atom was omitted for clarity).

Fig. 4. The chiral properties of higher-level molecular cage 4P-HTMC and 4M-HTMC.

a The UV spectra and b circular dichroism (CD) spectra of 4P-HTMC, 4M-HTMC and 4MP-HTMC in dichloromethane DCM (c = 0.5 mM), and c the ¹H NMR (600 MHz, CDCl₃) of molecular cages 4MP-HTMC (black), 4P-HTMC (blue) and 4M-HTMC (red) in CDCl₃ upon adding an excess of the chiral shift reagent (S)-( + )−2,2,2-trifluoro-1-(9-anthryl)ethanol.

Fig. 5. The chiral interconversion properties of higher-level molecular cage 4P-HTMC and 4M-HTMC.

The circular dichroism (CD) spectra of a 4M-HTMC and b 4P-HTMC in dichloromethane (DCM) (V = 2 mL, c = 0.5 mM) upon adding different volumes of (R,R)-CHDA and (S,S)-CHDA in DCM (c = 10 mM), respectively. c The ¹H NMR (600 MHz, CDCl₃) of molecular cages 4M-HTMC with an excess of the chiral shift reagent (S)-( + )−2,2,2-trifluoro-1-(9-anthryl)ethanol in CDCl₃ upon adding different quantity of (R,R)-diaminocyclohexane CHDA.

Fig. 6. The self-assembly of higher-level molecular cage 4P-HTMC and 4M-HTMC.

The assembly of L- or D-helical structures by a 4P-HTMC (blue) and b 4M-HTMC (red), and the schemes of their assembly process. The transmission electron microscopy (TEM) images of c 4P-HTMC and e 4M-HTMC assembled L-helical or D-helical nanofibers and the scanning electron microscopy (SEM) images of d 4P-HTMC and f 4M-HTMC assembled L-helical or D-helical nanofibers.