Hydrogen‐Bonded Crystalline Molecular Machines with Ultrafast Rotation and Displacive Phase Transitions

Abstract

Two new crystalline rotors 1 and 2 assembled through N−H⋅⋅⋅N hydrogen bonds by using halogenated carbazole as stators and 1,4‐diaza[2.2.2]bicyclooctane (DABCO) as the rotator, are described. The dynamic characterization through 1H T1 relaxometry experiments indicate very low rotational activation barriers (Ea) of 0.67 kcal mol−1 for 1 and 0.26 kcal mol−1 for 2, indicating that DABCO can reach a THz frequency at room temperature in the latter. These Ea values are supported by solid‐state density functional theory computations. Interestingly, both supramolecular rotors show a phase transition between 298 and 250 K, revealed by differential scanning calorimetry and single‐crystal X‐ray diffraction. The subtle changes in the crystalline environment of these rotors that can alter the motion of an almost barrierless DABCO are discussed here.

Compression and Thermal Expansion in Organic and Metal–Organic Crystals: The Pressure–Temperature Correspondence Rule

“…Alterations in temperature and pressure can affect the molecular aggregation in crystals and lead to solid-state phase transitions or chemical reactions, − and formation of different polymorphs − or pseudopolymorphs. − Moreover, the pressure or temperature changes can cause visible macroscopic effects in the color and shape of the crystal − and thermosalient behavior, − or can alter the molecular motions in crystalline molecular machines. − Although pressure and temperature can lead to the same kinds of phenomena (phase transitions, shape changes, etc. ), it is natural to expect that response can differ, both qualitatively and quantitatively.…”

Tailoring the cavities of hydrogen-bonded amphidynamic crystals using weak contacts: towards faster molecular machines

“…For rotors 2 ( E a = 0.84 kcal mol −1 ) and 3I (0.80 kcal mol −1 ), their barriers to rotation were estimated from the slope of the T 1 data, as reported elsewhere. 33,34 Although the rotational barriers achieved from 1 H T 1 experiments are not identical to those from periodic DFT computations, there is an excellent agreement in the values and their relative position, given that the E a in rotor 1 three times larger than those of rotors 2 and 3I .…”

Dual-State Emission (DSE) in Organic Fluorophores: Design and Applications

“…Naturally, the frequency of these reorientations may be altered if they occur in solution 151,152 or the solid state. 153 This section includes representative examples that document the inner motion of conjugated molecules. It must be recognized that fast intramolecular dynamics may occur in DSE molecules, although studies dealing with this aspect are still very scarce.…”

“…Motions at the molecular level may occur at different time scales, depending on the size of the mobile component. Naturally, the frequency of these reorientations may be altered if they occur in solution , or the solid state . This section includes representative examples that document the inner motion of conjugated molecules.…”