Temperature-dependent structural dynamics in covalent organic frameworks observed by cryogenic infrared spectroscopy

Area of Science:

• Materials Science
• Chemistry
• Spectroscopy

Background:

• Covalent organic frameworks (COFs) are advanced porous materials with tunable structures.
• Understanding COF behavior at cryogenic temperatures is essential for their practical applications.
• Previous studies have not fully elucidated the structural dynamics of COFs under extreme cold.

Purpose of the Study:

• To investigate the reversible structural dynamics of COFs at cryogenic temperatures (30 K).
• To analyze temperature-dependent changes in COF vibrational modes using infrared spectroscopy.
• To correlate observed spectral shifts with specific conformational changes via computational methods.

Main Methods:

• In situ cryogenic infrared (IR) spectroscopy was employed to study COFs (COF-300, COF-300-amine, COF-V) from 298 K down to 30 K.
• Fourier-transform infrared (FTIR) spectra were analyzed for temperature-induced peak shifts.
• Quantum-chemistry calculations were performed on model systems to understand the origin of spectral changes.

Main Results:

• Reversible peak shifts in FTIR spectra were observed as COFs were cooled and warmed.
• A general blue shift (higher frequency) in IR peaks occurred upon cooling.
• COF-300 showed quantitatively larger blue shifts compared to other frameworks, indicating distinct temperature-dependent dynamics.
• Quantum-chemistry calculations revealed that a 'pedal motion' within the framework structure is responsible for the observed IR shifts.

Conclusions:

• COFs exhibit significant, reversible structural dynamics in response to cryogenic temperatures.
• The 'pedal motion' is identified as a key conformational change influencing COF vibrational modes.
• This research provides critical insights for designing and applying COFs in extreme temperature environments.