Proton grease: an acid accelerated molecular rotor

Area of Science:

• Supramolecular Chemistry
• Organic Chemistry
• Materials Science

Background:

• Molecular rotors are crucial for developing advanced molecular machines and devices.
• Controlling rotor dynamics is key to their functional applications.
• Existing molecular rotors often lack efficient and reversible switching mechanisms.

Purpose of the Study:

• To design and synthesize a novel molecular rotor with significantly enhanced rotational dynamics.
• To investigate the mechanism of acid-induced acceleration of molecular rotation.
• To explore the reversibility of the rotor's switching behavior.

Main Methods:

• Synthesis of a quinoline-based N-arylimide molecular rotor.
• Kinetic studies at room temperature (23 °C) to determine baseline rotation speed.
• Protonation experiments using acid addition to measure changes in rotational dynamics.
• Mechanistic investigations, including computational studies, to elucidate the role of hydrogen bonding.

Main Results:

• The designed molecular rotor exhibits slow rotation at room temperature (t(1/2) = 26 min, ΔG(‡) = 22.2 kcal/mol).
• Upon addition of acid, the rotor's rotation accelerates by seven orders of magnitude (t(1/2) = 2.0 × 10(-4) s, ΔG(‡) = 12.9 kcal/mol).
• Mechanistic studies confirmed that protonation stabilizes the planar transition state via intramolecular hydrogen bonding, enabling rapid rotation.

Conclusions:

• A novel molecular rotor demonstrates a dramatic, acid-catalyzed increase in rotational speed.
• Intramolecular hydrogen bond formation upon protonation is the key mechanism for acceleration.
• The observed acceleration is reversible upon base addition, highlighting its potential as a molecular switch.