Crystal Fluidity Reflected by Fast Rotational Motion at the Core, Branches, and Peripheral Aromatic Groups of a Dendrimeric Molecular Rotor

Abstract

Low packing densities are key structural features of amphidynamic crystals built with static and mobile components. Here we report a loosely packed crystal of dendrimeric rotor, 2, and the fast dynamics of all its aromatic groups, both resulting from the hyperbranched structure of the molecule. Compound 2 was synthesized with a convergent strategy to construct a central phenylene core with stators consisting of two layers of triarylmethyl groups. Single crystal X-ray diffraction analysis confirmed a low-density packing structure consisting of one molecule of 2 and approximately eight solvent molecules per unit cell. Three isotopologues of 2 were synthesized to study the motion of each segment in the molecule in the solid state using variable temperature quadrupolar echo 2H NMR spectroscopy. Line shape analysis of the spectra reveals that the central phenylene, the six branch phenylenes and the eighteen periphery phenyls all display megahertz rotational dynamics in the crystals at ambient temperature. Arrhenius analysis of the data gives similar activation energies and pre-exponential factors for different parts of the structure. The observed pre-exponential factors are 4–6 orders of magnitude greater than those of elementary site-exchange processes, indicating that the dynamics are not dictated by static energetic potentials. Instead, the activation energies for rotations in the crystals of 2 are controlled by temperature dependent local structural fluctuations and crystal fluidity.

Layered metal(IV) phosphonate materials: Solid‐state 1H, 13C, 31P NMR spectra and NMR relaxation

“…In fact, a τ 0 value determined for 2 from the treatment in Figure was found to be 3.3 × 10 −8 s, which is large compared to the ~10 −12 s expected for simple motions. Similarly, abnormal slow pre‐exponential factors (τ 0 ) between 10 −15 and 10 −18 s have been reported for rotation of aromatic rings in Jiang et al…”

Molecular rotor as a high-temperature Brownian motor

“…The last is also typical for chemisorbed polyatomic molecules or polyatomic groups tightly bound to a surface through one atom, whereas other atoms can have several equilibrium positions in the potential induced by the nearest substrate atoms [1,2]. Recently high interest has been attracted by so-called molecular rotors artificially formed on surfaces [3][4][5][6][7][8][9][10][11][12][13][14][15]. They are the special case of molecular motors.…”

“…Work [11] describes amphidynamic crystalsformations from a combination of static components, that determine the crystalline order. These elements are associated with dynamic elements that make conformational motion along a certain direction [11].…”

“…These elements are associated with dynamic elements that make conformational motion along a certain direction [11]. Arrhenius analysis of the data of such structures gives information about the activation energies and pre-exponential factor for different parts of the rotor.…”

“…The observed preexponential factors are 4-6 times greater than those of elementary site-exchange processes, indicating that the dynamics is not dictated by static energetic potentials. Instead, the activation energies for rotations in the crystals are controlled by temperature dependent local structural fluctuations and crystal fluidity [11].…”