Giant Crystalline Molecular Rotors that Operate in the Solid State

Area of Science:

• Solid-state chemistry
• Supramolecular chemistry
• Materials science

Background:

• Molecular motion in solids is typically restricted due to dense packing.
• Previous studies focused on smaller molecules as solid-state rotors.
• Triptycene was the largest known molecule exhibiting solid-state rotation.

Purpose of the Study:

• To demonstrate and characterize the solid-state rotational motion of exceptionally large molecules: triptycene and pentiptycene.
• To investigate the influence of molecular size on solid-state dynamics.
• To explore the potential of bulky N-heterocyclic carbene (NHC) Au(I) complexes as crystalline media for molecular rotors.

Main Methods:

• Synthesis and single-crystal X-ray diffraction of triptycene and pentiptycene encapsulated with an NHC Au(I) complex.
• Variable-temperature solid-state Deuterium (²H) spin-echo Nuclear Magnetic Resonance (NMR) spectroscopy to probe molecular motion.
• Analysis of crystal structures to assess free volume around the rotator molecules.

Main Results:

• Pentiptycene, with a rotational diameter of 13.0 Å, surpasses triptycene (9.5 Å) as the largest reported solid-state molecular rotor.
• Single-crystal X-ray diffraction confirmed sufficient free volume within the crystalline media for molecular rotation.
• Solid-state ²H NMR studies verified the rotational motion of both triptycene and pentiptycene.
• Triptycene exhibited three-fold rotation, while pentiptycene showed temperature-dependent changes in its rotational angle.

Conclusions:

• Giant molecules like triptycene and pentiptycene can undergo significant rotational motion in the solid state.
• The encapsulation strategy using NHC Au(I) complexes provides a viable method for achieving solid-state molecular rotation in large molecules.
• This work expands the size limitations for solid-state molecular rotors and opens new avenues for designing dynamic molecular materials.